detector signatures of - hyperons in the forward

Detector signatures of Ξ− - Ξ+ hyperons in the Forward Spectrometer
at PANDA
April 27, 2018
Abstract
A study of the Forward Tracking Spectrometer (FTS) at PANDA was done using the PandaRoot software. The FTS is designed to have six stations along the direction of the beam line, however a reduction to four stations and size changes have been considered. This project focuses on looking at the track reconstruction process in the case of using two of the reduced configurations or the full experimental set up. For this, simulations of 10,000 events of the reaction pp→ Ξ+Ξ− → pπ+π+pπ−π−, at beam momenta 4.6 GeV and 7 GeV, for the three different set ups were performed. The data is presented to show the behaviour of the final state particles in angle and momentum terms as well as the efficiency of the FTS for tracks reconstructed, in terms of production angle. It was concluded that in general the FTS is important to collect the information corresponding to small angles and both low and high momenta particles. There are some differences regarding number of reconstructed tracks per event, number of hits and efficiency between the three proposed set ups. The differences can be attributed not only to the way the FTS is configurated but also to changes in the central tracker for each case. It was also found that there is a high number of reconstructed tracks at larger than expected angles. It is worth to make a deeper study of these special cases. The present study is helpful to get familiarized with the way the products of a Ξ+ - Ξ− pair reaction interact with the FTS at PANDA and thus can give valuable input to the development of reconstruction algorithms.
1 Introduction
1.1 The Standard Model
The study of a system is based on the behavior of its fundamental components. Understanding the fundamental bulding blocks of the Universe and the interactions between them has represented different challenges throughout the history of science.
So far, the Standard Model (SM ) has succesfully described the way the Universe works. It divides the particles in three families: quarks, leptons and bosons. Leptons can be found free in nature, while quarks can only be found in bound states called hadrons. The SM also describes three of the four fundamental forces: electromagnetic force, strong force and weak force. Bosons are also called force carriers because they are in charge of mediating these forces. Since a gravitational force carrier has not yet been found, it is not included in the model [1].
Quarks forming hadrons are held together thanks to the strong force. Such force as it name implies, is the strongest among all, but its range is the lowest: 10−15m. At distances greater than its range, there are no obvious effects of its existence, but its strength increases with distance until it exceeds the energy required to create a new quark pair, making the existence of unbound quarks impossible. The strong force is also responsible of keeping the nucleons together inside the atoms.
1
Figure 1: Schematic representation of the fundamental particles according to the Standard Model. Image extracted from [2]
Protons and neutrons are examples of hadrons. Both are made of the lightest quarks: up and down. However, if we add up the masses of these components, only about 2% of the total mass of the corresponding nucleon is obtained. The rest is dynamically generated by the strong force. One way of understanding how this occurs, is to look up to a configuration similar to that of the nucleons, but involving one or more heavy quarks such as the strange quark, instead of the light ones. These particles which involve one or more strange quarks are called hyperons and their study to understand the strong interaction is one of the contemporary topics in hadron physics.
Hyperons are an important source of knoweledge, however they are unstable which make their study a challenge. The devices designed for their study have to focus not only on the physics behind their production, but also on the way they decay and the behaviour and properties of the decay products.
1.2 PANDA
The PANDA experiment, showed at Figure 2, is being built at the Facility for Antiproton and Ion Research (FAIR) of the GSI laboratory at Darmstadt, Germany. PANDA is focused in diverse physics topics and one of them is to study hyperon physics towards the understanding of strong force. It will be a fixed target experiment where a beam of antiprotons coming from the High-Energy Storage Ring will collide with the fixed proton target placed inside the detector. The energies at which the beam will be accelerated vary from 1.5 to 15 GeV [1].
Given the wide range of physics topics that PANDA will cover, it is necessary to have an accurate particle identification system and the biggest solid angle coverage possible. The latter is planned to be accomplished by combining charged particle tracking together with high-resolution electromagnetic calorimeters, Cherenkov detectors, time-of-flight walls and muon chambers.
After the beam-target collision, different resulting particles with different physical properties are expected. It is neccesary to have devices that allow the accurate detection of the particles which give information about the processes to be studied. PANDA will consist on a Target Spectrometer (TS) surrounding the interaction point covering scattering angles up to 140 in the vertical plane, added to a Forward Spectrometer (FS) for the detection of particles with angles between ±5 in the vertical direction and ±10 in the horizontal direction [3]. The modular structure of the detection system is advantageous for future convenient adaptations.
2
1.3 The tracking system at PANDA
This section is intended to have a brief description of the devices that make up the tracking system. PANDA will operate using a 2T solenoidal field in the target spectrometer and a 1T dipole field in the forward spectrometer [3]. There are four tracking systems that will perform the tracking tasks:
Micro Vertex Detector (MVD) This is a charged particle tracking system and it is the first detector positioned right after the interaction point. It is important for the determination of secondary decay vertices of short-lived particles (hyperons for instance). The MVD is divided in the central and the forward part. The geometry is shown in Figure 3 and it consists on 4 (1-4 in blue) half-shell layers in the barrel part, from which the first have pixel detectors and the last two are double sided silicon strip detectors (DSSD). On the other hand, 6 (1-6 in red) half-disks layers are placed in the forward part which consist in both pixels and silicon strips. At the begining of PANDA operation, the half-disks corresponding to the forward part will not have the pixels, which would be in the inner part of the disc and closer to the beam pipe.
Figure 3: Micro Vertex Detector (MVD) layout. Extracted from [3]
Straw Tube Tracker (STT) It consists of 12 double layers drift tubes filled with gas, each one with a
3
length of 150 cm. The tubes are arranged in a cylindrical volume surrounding the beam axis. Each time a charged particle passes through a tube, the gas inside ionizes and the liberated electrons drift towards a wire located at the middle of the tube. Multiplication and amplification turn into an electric signal giving rise to the detection. The STT layout is shown in Figure 4
Figure 4: Straw Tube Tracker (STT) layout. Extracted from [3]
Gaseous Electron Multipliers (GEM) These tracking devices are located after the STT. Each GEM has a disc geometry with a hole in the middle to allow the beam line to pass. A GEM disc is a device that includes components to perform the tasks of determining the track position in four projections, electron amplification, cooling, support, voltage supply and shielding. The full experimental set up of PANDA includes three GEM discs, however at the first stages only two of them will be available. The layout in the case of the full set up is shown in Figure 5.
Figure 5: Gaseous Electron Multipliers detectors forms a GEM-Tracker (in FTSFull case), which will be used as a first forward tracking detector after the central tracker. Extracted from [3]
Forward tracker (FT) This is designed to detect the particles that escape from the central part and are emmited at low angles. It consist on 6 rectangular stations, each one with four double layers of straw tubes similar to those in the STT. The first and last layer of straws are vertical, while the middle ones are tilted.
4
More specific information and the layout of the FS can be found in the next section since it is the device in which we are focusing.
1.4 The Forward Spectrometer
The Forward Spectrometer consists of three pairs of planar stations and a solenoid dipole providing the magnetic field inside of it. As it is showed in Figure 6, the first pair is placed in front the dipole magnet (FS1,FS2), the second (FS5,FS6) is placed after the dipole magnet and third is planned to be inside the dipole magnet (FS3,FS4). Each station will have four double layers of straw tubes. In the stations, the first and last double layers of straws tubes are placed at 0 angle (vertical) and the second and third layers are tilted to +5,−5 respectively.
Figure 6: Forward Spectrometer at PANDA.
Each straw tube is filled with gas and it has a wire (anode) along its vertical axis. The operating principle is based on generating an electric field in the gas when a high voltage is applied between the wire and the tube, this results in ionization of the gas everytime a charged particle passes through the tube. As a result of the ionization, the electrons drift towards the wire and the multiplication of the primary charge gives a signal. The tubes are made of 30 µm aluminized Mylar foil and the anodes are made of 20 µm gold plated tungsten. All wires are grounded.
More specific details about the FS is shown in Table 1, however these can change due to the ongoing implemen- tation of the experiment.
Table 1: Some specifics about the tracking stations belonging to the Forward Spectrometer [3].
Station #modules #straws z-pos. [mm]
Area x×y [mm2]
1 4×8 = 32
1024 2954 1298×640
2 4×8 = 32
1024 3274 1298×640
3 4×12 = 48
1536 3945 1944×690
4 4×12 = 48
1536 4385 1944×767
5 4×25 = 100
3200 6075 4045×1180
6 4×37 = 148
4736 7475 5984×1480
5
Although the FS is planned to have six stations, it is likely that in the first stage of operation at PANDA, not all of them will be available. There is the possibility of having reduced set ups, which take only four of the six stations. It is important to study the impact of changing the set ups, which is one of the aims of this project. To have a better understanding, from now on we will refer to each of the set up as: FTS1234, FTS1256 and FTSFull, where the numbers 1234 and 1256 represent the number of the stations present at each set up respectively (as in Figure 6), while Full means that all six stations are taken into account.
1.5 The reaction
In this project, we concentrate in one specific reaction involving Ξ+-Ξ− hyperons, shown in reaction 1. This is one of the reactions that will be studied at PANDA.
pp→ Ξ+Ξ− → pπ+π+pπ−π− (1)
The particles that we are interested in, are produced by colliding an antiproton with a proton and this interaction results in a Ξ− − Ξ+ pair. The baryon Ξ− is the combination of two strange quarks and one down, while its antiparticle Ξ+ is formed by the corresponding antiquarks. As we can see in Table 2, both Ξ− and Ξ+ baryons are unstable. They most probably decay into Λ + π− and Λ + π+ respectively [4]. The Λ and Λ hyperons are also unstable, and they decay as it is shown at table 2. This means that the particles that have to be detected are the final six shown in the reaction: pπ+π+pπ−π−. It is important to underline that except for the pions, primary decays are shown in Table 2 and only those cases are the ones we are interested in.
Table 2: Main properties of the involved particles of the reaction showed at 1 [4].
Name Symbol Quark Content Mass (MeV) Mean Life (10−10s) Decay Mode Xi Ξ− dss 1327.71± 0.07 1.639± 0.015 Λπ−
Anti-Xi Ξ+ dss 1321.71± 0.07 1.70± 0.08 Λ π+
Lambda Λ uds 1115.683± 0.006 2.632± 0.020 pπ−
Anti-Lambda Λ uds 1115.683± 0.006 2.632± 0.020 pπ+
Pion π− du 139.57061± 0.00024 260.33± 0.05 Anti-Pion π+ ud 139.57061± 0.00024 260.33± 0.05
To succesfully detect all the involved particles, the detectors at PANDA should be able to distinguish between particles with different properties such as mass, energy and momenta. To achieve it, it is helpful to study the way in which the particles behave in the different components of the detector. This is done through computer simulations using specific software tools. In the case of PANDA, the PandaRoot Software was developed to help all users to perform different kinds of tasks going from the simulation of events, to the physics analysis of the results.
1.6 PandaRoot
The PandaRoot framework is based on ROOT and Virtual Monte-Carlo [5]. It was designed specifically to perform the tasks related to the simulation and analysis of processes that will be possible at PANDA. When a simulation is performed, several stages are carried out. These are showed at Figure 7.
Figure 7: Data flow in PandaRoot.
6
First we have to know the reaction in which we are interested (section 1.5). Then we start by using an ideal event generator to simulate this reaction taking into account all the possible particles properties and their corresponding decays. In our case, one event is a single collision between a proton and an antiproton i.e. a single reaction process. In the simulation, each event will produce the final state particles corresponding to one reaction and its corresponding tracks. It is possible to choose the number of events we want.
After the simulation, we get MonteCarlo information in the form of MonteCarlo Tracks and MonteCarlo Points. These objects are helpful to understand how the particles behave in each one of the detectors. As it was mentioned before, the used event generator is ideal, so it is necessary to have something that reflects the effects of the interaction between the simulated particles and the detector that we are studying. The particles are taken through the detector devices using a transport model, where some effects corresponding to such interaction are added to make the data look more realistic. The aim of the digitizing stage is to make the data look as it was coming from the experiment. This means converting the hits into signals [6]. This is followed by the reconstruction of the track and its fitting (tracking), where each signal is translated into information that leads to identify the track with a specific particle. More about the traking process is explained in the next section.
After chossing a tracking strategy and relate the tracks to a particle at the particle identification stage, it is time to use the generated data to perform the physics analysis. This is the stage where it is possible to relate track candidates with particles corresponding to the reaction that is studied.
This Root-based type of framework is convenient since there is a lot of information available and accessing it is relatively easy. This is mainly because the type of structure in which the information is stored (called trees) has diverse branches to which one can access to obtain the required data to consult and use. There are 4-momenta information storaged for each one of the found particles.
1.7 Tracking
The tracking process is crucial to extract physical observables of the corresponding particles e.g. momentum. It is performed in two main steps: first a track is found and then it must be fitted. To find the track, the involved detector is divided into subsets which contain measurements that are believed to come from the same particle. These measurements will constitute the track candidates.
It is possible to perform online or offline tracking. For online tracking, in which the found tracks are used to choose events that are worth to be saved, the algorithms that perform the track finding are chosen considering that they must be computationally fast. On the other hand, accuracy is a key factor when offline tracking is performed. One could have a track model based on the way each one of the involved particles interact with the detector material according to their specific characteristics, but this could result on a high time consumption [9].
There are two types of track finding approaches: Those that divide the detector into small parts and find the information in each one are called local. While those which find the information in the whole detector at the same time are called global. For the former it is required to develop several algorithms for each section of the detector to then collect the complete information, while for the latter a single extensive algorithm is needed.
7
Figure 8: Scheme of the tracking process using Cell Track Finder. A Identification of straws with hits. B Classifi- cation according to number of neighbours: Blue means one neighbour, red means two neighbours and green means more than two neighbours (ambiguous cells). C Number assignment. D Exchange of the number by the lower value of the neighbors. E Ambiguous cells exchange their number for the lower value of the neighbors. F Ambiguous cells all the values of their neighboring cells [8].
We are focused on the forward spectrometer for which so far, an ideal track finder is used: it takes the models of the tracks and adds a small smearing to the hits. Afterwards it fits the tracks with a line connecting all the hits corresponding to the same track. There is interest in developing a cell track finder similar to that of the barrel part used in the target spectrometer.
For the Cell Track Finder, an input of hits is required. Among all the information linked to a hit, a timestamp and the ID of the specific tube that registered the signal are included [7]. For this type of reconstruction process, tracklets are generated by using the Cellular Automaton. The first step for this, is to group all the hits that are believed to correspond to a track (Fig 8 A). Each straw where a hit has been recorded will be identified as an active cell. Subsequently, the cells are classified according to the number of neighbors they have (Fig 8 B). Neighboring cells are chosen according to the distance from one hit to another. Those cells with more than two neighbours are called ambiguous and those with one or two neighbors are called unambiguous. Each unambiguous cell obtains an arbitrary but unique number called status (Fig 8 C), which will be the cell ID. The next step is to compare the ID of each cell with that of the neighbors. Each cell aquires the smallest value between itself and its neighbouring cells (Fig8 D). Finally, the ambiguous cells take the values of all their neighboring cells (Fig 8 E) and in this way they obtain a group of possible ID’s.
A set of unambiguous cells without ambiguous cells as neighbors form a tracklet which can be directly fitted with a curve. On the other hand, if there are ambiguous cells with four possible ID numbers, it means that there is overlaping of two tracks (8 F). In this case, a fitting algorithm has to be used on the four tracklet pair combinations. The two tracks are chosen according to the minimum error after fitting [8].
2 Method & Results
The main focus of the project was to use the PandaRoot software to simulate and study different characteristics related with the topology of the reaction 1 in the FTS. This was done by looking the characteristics of the generated
8
h!
Figure 9: Scheme showing the angle θ at which the particles are emmited: A is a particle created directly from the decay of Ξ± i.e. Λ, Λ, π± while B represents the decay product of Λ or Λ i.e. anti(proton) or π±.
and reconstructed tracks. The simulation file consists on a Macro in which the reaction is specified, it includes a file with the corresponding decaying chains and it is possible to use different beam momenta by just changing a line in the code. The energies available at PANDA can reach up to 15 GeV. However for this project, we only chose two beam momenta: 4.6 GeV and 7 GeV.
After the simulation, Macros corresponding to the transport, digitizing and tracking stage were used. The ideal track finder reconstrucks a track if it has at least 6 hits in the forward spectrometer. Finally, several analysis Macros were used to look at specific characteristics of the reaction. After the reconstruction, it is possible to identify the particles involved. Every Macro corresponding to a physics analysis makes use of all the previous information generated during the simulation.
As it was mentioned before, at the first stage of PANDA, not all 6 stations will be available. Therefore, some comparissons between the possible set ups are helpful to find out if there are considerable differences between them at the reconstruction stage. Here we present the results corresponding to both selected beam momenta and three different configurations: two where only four of the six available detector stations are taken into account and the full set up.
The different set ups of the Forward Tracker Spectrometer will be abreviated as FTS1234, FTS 1256 and FTSFull, where the numbers denote the stations present at the corresponding configuration and Full means the complete experimental set up. Although in this project we are only interested on looking at the reconstruction in the forward part, it is important to make clear that when we choose one specific forward configuration, we also choose the design for central tracker that belongs to this forward set up. The specific characteristics of the different set ups are shown at Table 3. The differences between the set ups in the central tracker can also affect the way the tracks are reconstructed for each case of FTS.
Table 3: Table showing the specifics of each partial and full set up
Set Up No. GEM MVD FS STATIONS FTS1234 2 Strips 1,2,3,4 FTS1256 2 Strips 1,2,5,6
FTSFULL 3 Strips + pixels 1,2,3,4,5,6
It is also important to set a reference system. This is shown at Figure 9 where it can be seen that angles of emmission are taken with respect to a line parallel to the beam line and we will refer to this direction as longitudinal or Z axis. On the other hand, the radial direction is taken on the x,y plane.
9
2.1 Reconstructed tracks
The used method only focuses on characteristics after the reconstruction stage in the forward spectrometer, where no other fits or cuts are done. The number of reconstructed tracks per event for each FTS configuration is shown at Figure 10 for beam momentum 4.6 GeV and Figure 11 for beam momentum 7 GeV. The three set ups are presented in different colors: magenta for FTSFull, red for FTS1256 and blue for FTS1234. There are some similarities between these figures. Almost half of the events don’t have reconstructed tracks in all cases and in both cases of beam momenta there seems to be overlaping between the lines representing the different set ups. Furthermore, we can see that around 33% of the events have one reconstructed track in all cases. Around 17% of the events have two reconstructed tracks in the case of beam momentum 4.6 GeV while this happens for nearly 10% of the events in the case of 7 GeV. In both cases less than 5% have three reconstructed tracks and less than 1% events have four or five reconstructed tracks. We can see also that there are no events for which all 6 final state particles tracks are reconstructed in all cases.
Figure 10: Tracks per event for beam momentum 4.6 GeV. All configurations of FTS: Full set up - magenta, FTS1234 - blue and FTS1256 red.
Figure 11: Tracks per event for beam momentum 7 GeV. All configurations of FTS: Full set up - magenta, FTS planes 1234 - blue and FTS 1256 red.
At Figures 12 and 13, a clearer comparisson between the partial and full set up is shown. It is important to remember however, that at this stage we are only looking at the reconstructed tracks without performing any fitting or further analysis. In these figures, the markers represent the ratio between the number of reconstructed tracks per event in each case: black for FTSFull/FTS1234, green for FTSFull/FTS1256 while blue for the ratio between the partial set ups FTS1234/FTS1256. The ratios are within a range of 0.6 to 1 and they are not far from each
10
other in all the cases of set ups, showing that at this stage the number of reconstructed tracks per event does not vary a lot.
Figure 12: Ratio between the number of reconstructed tracks per event for all set ups. Beam momentum 4.6 GeV.
Number of tracks 0 1 2 3 4 5 6 7
R at
1.6 Ratio between partial and full set ups, 4.6 GeV
Figure 13: Ratio between the number of reconstructed tracks per event for all set ups. Beam momentum 7 GeV.
Number of tracks 0 1 2 3 4 5 6 7
R at
1.6 Ratio between partial and full set ups, 7 GeV
On the other hand, Table 4 shows the number of reconstructed tracks corresponding to each final state particles in each case of set up. We can point out that the number of reconstructed tracks for protons and antiprotons do not vary for more than 22 between set ups, however there are considerably more pions reconstructed for the partial configurations than the full experimental set up. If we assume that the particles have a Poisson distribution, we can obtain the standard deviation σ for each case of final state particle. This is shown at Table 4 where for the case of pions, there is a difference of at least two standard deviations between the partial and the full set up. This is a big difference, however it does not mean that using the partial set ups is better since we have to consider that at at this point we are counting all those tracks which fulfill the ideal track finder requirement (≥ 6 hits), thus some
11
of them might have bad quality or even impossible to reconstruct in a realistic case.
Switching to beam momenta 7 GeV with its correspoinding information at Table 5, we can see a similar situation: proton and antiproton tracks have been reconstructed at the same rate in all cases of FTS, however the number of tracks belonging to pions is notably less in the case of the FTSFull and just as in the other case, there is a difference of at least two standard deviations between the partials and the full set up. Adding up all the final state particles tracks reconstructed in each case, we get the numbers shown in Table 6.
Table 4: Reconstructed particles for 4.6 GeV beam momentum.
Number of reconstructed tracks final state particles, beam momentum 4.6 GeV
Set up FTS 1234 σ FTS 1256 σ FTS- Full σ π− 1,409 38 1,419 38 1,336 37 π+ 1,400 37 1,426 38 1,326 36
From Λ p 732 27 754 27 735 27 π− 1,871 43 1,820 43 1,717 41
From Λ p− 707 27 722 27 715 27 π+ 1,833 43 1,864 43 1,694 41
Table 5: Reconstructed particles for 7 GeV beam momentum.
Number of reconstructed tracks final state particles, beam momentum 7 GeV
Set up FTS 1234 σ FTS 1256 σ FTS- Full σ π− 1,421 38 1,413 38 1,367 37 π+ 1,452 38 1,471 38 1,403 37
From Λ p 817 29 847 29 829 29 π− 1,515 29 1,466 38 1,438 38
From Λ p− 780 28 824 29 790 28 π+ 1,515 39 1,495 39 1,478 38
Table 6: Total number of reconstructed final state particles for both energies.
Total of reconstructed final state particles Set up 4.6 GeV 7 GeV
FTS1234 7,952 7,500 FTS1256 8,005 7,516 FTSFull 7,523 7,305
In Tables 7 and 8, the number of particles that were possible to reconstruct taking into acount the decay chains which lead to the six final state particles is shown. This means that for example, if in one event there are tracks that can be connected to a proton and a π−, then a Λ can be reconstructed. The numbers are very similar for all cases of set up and between beam momenta.
12
Table 7: Number of reconstructed particles taking into account final state particle reconstructed tracks. Beam momentum 4.6 GeV
Reconstructed tracks, beam momentum 4.6 GeV
FTS 1234 FTS 1256 FTS- Full No. Events 10,000 10,000 10,000
Reconstructed Ξ+ 56 62 52 Reconstructed Ξ− 64 62 51 Reconstructed Λ 239 251 240 Reconstructed Λ 251 252 225
Table 8: Number of reconstructed particles taking into account final state particle reconstructed tracks. Beam momentum 7 GeV
Reconstructed tracks, beam momentum 7 GeV
FTS 1234 FTS 1256 FTS- Full No. Events 10,000 10,000 10,000
Reconstructed Ξ+ 141 164 163 Reconstructed Ξ− 161 167 168 Reconstructed Λ 401 415 399 Reconstructed Λ 394 416 399
2.2 Hits distributions different configurations
Having in mind that changes in the configuration and shape of the FTS are possible, it is useful to know the hits distribution in each station. This is shown at Figures 14 and 15 for beam momentum 4.6 GeV and 7 GeV respectively. It is important to remember that each station consist on four layers and this is the reason why there are four peaks per station. Looking closely at the spatial position of the stations (Z direction), there seems to be a difference in the geometry depending on the set up used: Station 6 changes position from the FTS1256 set up (14 (b)) to the FTSFull configuration (14 (c)). This difference can be attributed to the geometry files but after consulting the most recent documentation for the forward spectrometer layout, we know that the correct position for station 6 is the one showed in the full set up [11].
In Tables 9 and 10, the exact number of hits in each station is shown. In general, the number of hits in the stations decreases with distance from the interaction point. However, stations 5 and 6 have still a big number of hits. It is natural to ask how the hits are distributed among the tracks. This is shown at Figures16 and 17 for beam momentum 4.6 GeV and 7 GeV respectively. The usual colors to distinguish between set ups are used: blue for FTS1234, red for FTS1256 and magenta for FTSFull. Remembering that each station has 4 double layers of straw tubes, it is most likely that a track traversing a complete station can leave 8 hits in the case of having one hit in each layer.
13
Z_plane Entries 212180 Mean 368.7 Std Dev 59.67
Z position [cm] 0 100 200 300 400 500 600 700
N um
be r
of h
FTS1234 Hits, 4.6 GeV
Z_plane1256 Entries 197728 Mean 463.8 Std Dev 183.9
Z position [cm] 0 100 200 300 400 500 600 700
N um
be r
of h
FTS1256 Hits, 4.6 GeV
Z_planeFull Entries 277007 Mean 440.6 Std Dev 127.5
Z position [cm] 0 100 200 300 400 500 600 700
N um
be r
of h
FTSFull Hits, 4.6 GeV
(c)
Figure 14: Number of hits in FTS for all configurations at beam momentum 4.6 GeV.
Table 9: Table showing the number of hits in each station corresponding to each of the three set ups. Beam momentum 4.6 GeV
Hits per station, at 4.6 GeV FTS1234 FTS1256 FTSFull
Station 1 60,003 61,185 56,691 Station 2 56,282 57,072 53,153 Station 3 48,680 ———– 46,094 Station 4 47,215 ———– 44,917 Station 5 ———– 41,719 39,244 Station 6 ———– 37,752 36,908 TOTAL 212,180 197,728 277,007
14
Figure 15: Number of hits FTS for all configurations at beam momentum 7 GeV.
(a)
Z_plane Entries 214820 Mean 371.7 Std Dev 59.99
Z position [cm] 0 100 200 300 400 500 600 700
N um
be r
of h
FTS1234 Hits, 7 GeV
Z_plane1256 Entries 189351 Mean 462.4 Std Dev 183.4
Z position [cm] 0 100 200 300 400 500 600 700
N um
be r
of h
FTS1256 Hits, 7 GeV
Z_planeFull Entries 278994 Mean 438.5 Std Dev 124.7
Z position [cm] 0 100 200 300 400 500 600 700
N um
be r
of h
FTSFull Hits, 7 GeV
Table 10: Table showing the number of hits in each station corresponding to each of the three set ups. Beam momentum 7 GeV
Hits per station, at 7 GeV FTS1234 FTS1256 FTSFull
Station 1 57,419 58,417 55,340 Station 2 55,161 55,556 53,179 Station 3 51,000 ———– 49,136 Station 4 51,240 ———– 48,993 Station 5 ———– 39,658 37,817 Station 6 ———– 35,720 34,529 TOTAL 214,820 189,351 278,994
In Figure 16 showing the hits per track belonging to beam momentum 4.6 GeV, we can distinguish some peaks. For FTS1234 (blue line) the first one at 8 hits, the second at 16 hits and the last one at 32 hits. The height of the peaks show that there are less than 100 tracks reconstructed using only the first station, around 600 stop or are deflected after the first two stations and most of the reconstructed tracks, around 3500, have hits in the four stations. In the case of FTS1256 (red line), close to 400 tracks are reconstructed with hits in the first station only, about 1600 have 16 hits meaning that they get to traverse only the first 2 stations, leading to the conclusion that a large amount of tracks get stuck or deviated before reaching Station 5, finally slightly more than 2000 tracks are reconstructed using hits in the four stations. In the case of FTSFull, there are three small peaks showing that
15
around 300 tracks use hits only in the first station, between 400 and 500 tracks have hits in stations 1 and 2, and close to 300 have hits in the first four stations. In this case we have another peak at 48 hits, corresponding to the about 1800 tracks that have hits in the 6 stations. A higher number of hits involved in the reconstruction of a track is good because it can decrease the error when certain fits are performed.
Figure 16: Hits per reconstructed track for beam momentum 4.6 GeV. Different configurations represented in different colors: blue for FTS1234, red for FTS1256 and magenta for FTSFull.
Number of hits 0 10 20 30 40 50 60 70 80 90 100
N um
be r
of tr
ac ks
Hits per track for all configurations of FTS, 4.6 GeV
What was presented for the case of 4.6 GeV, was also done for beam momentum 7 GeV. In general, the number hits look similar as in the case of less momentum. There is a bigger number of hits in the closest stations to the interaction point (Figure 15), and the biggest number of total hits corresponds to the full set up (Table 10).
In Figure 17 where we have the hits per track at 7 GeV, and as in the case for lower momenta, we can see three peaks for FTS1234. The first and smaller one is at 8 hits, meaning that less than 200 of the reconstructed tracks pass only through the first station. A second and slightly bigger peak at 16 hits, shows that around 400 of the reconstructed tracks pass through the first and second stations but then get deflected. The final and bigest peak, pointing at around 3500 tracks, is at 32 hits showing reconstruction using hits in the four stations. For FTS1256, about 200 of the reconstructed tracks have hits in the first station only. In this case about 1800 of the tracks are reconstructed with 16 hits, while only about 100 tracks more (i.e. around 1900) are reconstructed with 32 hits. Finally for the full set up, the peaks at 8, 16 and 32 are very small, showing that less than 500 are reconstructed with these numbers of hits. The tallest peak is around 1600 at 48 hits and it corresponds to tracks that pass through the 6 stations.
Figure 17: Hits per reconstructed track for beam momentum 7 GeV. Different configurations represented in different colors: blue for FTS1234, red for FTS1256 and magenta for FTSFull
Number of hits 0 10 20 30 40 50 60 70 80 90 100
N um
be r
of tr
ac ks
Hits per track for all configurations of FTS, 7 GeV
16
2.3 Illumination of the stations
Histograms showing the illumination of the FTS configurations are presented in Figure 18 for beam momentun 4.6 GeV and Figure 19 for beam momentum 7 GeV. The yellow zones mean bigger particle density, while the blue means less. The stations are more illuminated at the zones surrounding the beam line and leaving the edges with lower hits number. The latter means that if there is a size reduccion of the stations, the outer edges would be the candidate area to reduce since it would affect less the number of registered hits.
Z-Position [cm] 300 400 500 600 700 800
X -P
os iti
on [c
m ]
300−
200−
100−
0
100
200
300 FTS1234 Hits Entries 212180 Mean x 368.7 Mean y 0.06145 Std Dev x 59.67 Std Dev y 32.72
1
10
210
FTS1234 Hits Entries 212180 Mean x 368.7 Mean y 0.06145 Std Dev x 59.67 Std Dev y 32.72
FTS1234 Stations illumination, 4.6 GeV
(a)
X -P
os iti
on [c
m ]
300−
200−
100−
0
100
200
300 hHitsStation Entries 197728 Mean x 463.8 Mean y 1.293 Std Dev x 183.9 Std Dev y 70.07
1
10
210
hHitsStation Entries 197728 Mean x 463.8 Mean y 1.293 Std Dev x 183.9 Std Dev y 70.07
FTS1256 Stations illumination, 4.6 GeV
(b)
X -P
os iti
on [c
m ]
300−
200−
100−
0
100
200
300 hHitsStation Entries 277007 Mean x 440.6 Mean y 0.7251 Std Dev x 127.5 Std Dev y 51.16
1
10
210
hHitsStation Entries 277007 Mean x 440.6 Mean y 0.7251 Std Dev x 127.5 Std Dev y 51.16
FTSFULL Stations illumination, 4.6 GeV
(c)
Figure 18: Illumination of the stations of the three configurations for beam momentum 4.6 GeV. The colors of the axis are the number of hits in each station.
17
X -P
os iti
on [c
m ]
300−
200−
100−
0
100
200
300 FTS1234 Hits Entries 214820 Mean x 371.7 Mean y 0.05803− Std Dev x 59.99 Std Dev y 34.9
1
10
210
FTS1234 Hits Entries 214820 Mean x 371.7 Mean y 0.05803− Std Dev x 59.99 Std Dev y 34.9
FTS1234 Stations illumination, 7 GeV
(a)
X -P
os iti
on [c
m ]
300−
200−
100−
0
100
200
300 hHitsStation Entries 189351 Mean x 462.4 Mean y 0.1705− Std Dev x 183.4 Std Dev y 78.6
1
10
210
hHitsStation Entries 189351 Mean x 462.4 Mean y 0.1705− Std Dev x 183.4 Std Dev y 78.6
FTS1256 Stations illumination, 7 GeV
(b)
X -P
os iti
on [c
300 hHitsStation
Entries 278994 Mean x 438.5 Mean y 0.7773− Std Dev x 124.7 Std Dev y 56.76
1
10
210
hHitsStation Entries 278994 Mean x 438.5 Mean y 0.7773− Std Dev x 124.7 Std Dev y 56.76
FTSFULL Stations illumination, 7 GeV
(c)
Figure 19: Illumination of the stations of the three configurations for beam momentum 7 GeV. The colors of the axis are the number of hits in each station.
18
2.4 Study of GEM and FTS hits distributions
As it was mentioned before, there are different devices along the beamline for the particles detection and tracking. Being the GEM planes the first forward tracking detector after the central part, some hits registered there can belong to tracks that are reconstructed in the forward spectrometer, so we are interested in how often that happens. The geometry of the GEM planes is shown in Figure 5. Figure 20 shows a the number of hits registered in the GEM planes that are connected with a track in the FTS for the case of beam momentum 4.6 GeV. The corresponding plots for beam momentum 7 GeV are shown in Figure 21. At Table 11, the number of these hits are shown for each configuration. We can see that there are more hits in the case of FTSFull. This could be due to the extra GEM plane in the full set up. As it was mentioned before, different simulations done with each possible FTS set up contain differences in more than just the forward spectrometer. One of the changes is the number of GEM planes as it was pointed out in Table 1.
An even more interesting result could be to look at how many of the reconstructed tracks at the forward spectrometer have at least one hit in the GEM plane. This is good to know because it has been considered that the GEM could give some time signature to the tracks. The numbers are shown in Table 12 and if we compare this with Table 6, we can see that most of them fulfill the requirement.
Table 11: Number of hits belonging to tracks that hit FTS and GEM, for two beam momenta and three set ups.
FTS & GEM 4.6 GeV 7 GeV FTS1234 27724 25817 FTS1256 27824 25788 FTSFull 39668 38295
s
Table 12: Number of reconstructed tracks at the forward spectrometer that have at least 1 hit at any of the GEM planes. Percentage from total reconstructed tracks according to Table 6.
Beam mom FTS1234 % from total FTS1256 % from total FTSFULL % from total 4.6 GeV 7540 95 7583 95 7301 97 7 GeV 6951 93 6927 92 6974 96
19
X position 40− 30− 20− 10− 0 10 20 30 40
Y p
os iti
(a)
X position 40− 30− 20− 10− 0 10 20 30 40
Y p
os iti
(b)
X position 40− 30− 20− 10− 0 10 20 30 40
Y p
os iti
(c)
Figure 20: Hits overlap between GEM planes and different configurations of FTS for beam momentum 4.6 GeV.
20
X position 40− 30− 20− 10− 0 10 20 30 40
Y p
os iti
(a)
X position 40− 30− 20− 10− 0 10 20 30 40
Y p
os iti
(b)
X position 40− 30− 20− 10− 0 10 20 30 40
Y p
os iti
(c)
Figure 21: Illumination of the hits overlap between GEM planes and different configurations of FTS for beam momentum 7 GeV.
2.5 Momentum of reconstructed tracks
It is important to make a comparisson between the MonteCarlo and the reconstructed information. In this section, this is done by plotting the momenta of the simulated and reconstructed tracks in two directions: longitudinal (Z) and radial (reference system shown in Figure 9). The resulting plots are shown at Figure 22 in the case of the FTSFull set up. Figure 22 (a) shows all the final state particles simulated. On the other hand, at Figure 22 (b) only the reconstructed tracks are shown. In the latter we can see a separation of points in two groups. One of the groups has longitudinal momenta between 0 and 1 GeV and lower than 0.2 GeV radial momenta, while the other group, which is less crowded, has longitudinal momenta between 2 and 3.5 GeV and radial momenta from 0 to 0.6 GeV. Something to be noticed here, is that those MonteCarlo tracks which have radial momenta higher than 0.6 GeV are not reconstructed. If those particles dont reach the FS, then it means that either they are reconstructed at some of the devices belonging to the central tracker or they are not reconstructed at all.
The purpose of plots 22 (c) to (j) is to know which of the reconstructed tracks belong to each kind of final state particle. From figures 22 (c),(e) we can conclude that simulated protons and antiprotons have a wide range of longitudinal and radial momenta, varying from 0 to 3.5 GeV and from 0 to 1.2 GeV respectively. From Figures 22 (d),(f) we can see that only a few of those which have around 0.5 GeV are reconstructed, while a bigger amount of those which have longitudinal momenta from 0 to 3.5 GeV are reconstructed. From Figures 22 (g),(i) we conclude that pions have less longitudinal momentum, ranging from 0 to 1 GeV in the longitudinal direction and 0 to 0.3 GeV in radial direction. In this case, there are reconstructed tracks with longitudinal momenta of all the simulated
21
range, however only those tracks with radial momenta in the range from 0 to 0.2 GeV are reconstructed. From these plots, we can distinguish a region where the reconstruction at the FTS is important. It is important emphatize that here we are not separating beteween the pions that are created from to the Ξ− − Ξ+ pair decay and the ones coming from the Λ− Λ decay.
Z momentum [GeV] 0.5− 0 0.5 1 1.5 2 2.5 3 3.5
R ad
1.2 hMCMom_r_z
Entries 60000 Mean x 0.7667 Mean y 0.2732 Std Dev x 0.7647 Std Dev y 0.2253
1−10
1
10
hMCMom_r_z Entries 60000 Mean x 0.7667 Mean y 0.2732 Std Dev x 0.7647 Std Dev y 0.2253
MC tracks FTS Full set up, 4.6 GeV
(a)
Z momentum [GeV] 0.5− 0 0.5 1 1.5 2 2.5 3 3.5
R ad
1.2 hMom_r_z
1−10
1
10hMom_r_z Entries 7523 Mean x 0.8032 Mean y 0.111 Std Dev x 0.923 Std Dev y 0.101
Reconstructed tracks FTS Full set up, 4.6 GeV
(b)
Z momentum [GeV] 0.5− 0 0.5 1 1.5 2 2.5 3 3.5
R ad
1.2 hMCMom_z_r_protons
2−10
1−10
1
hMCMom_z_r_protons
Protons MC tracks FTS Full set up, 4.6 GeV
(c)
Z momentum [GeV] 0.5− 0 0.5 1 1.5 2 2.5 3 3.5
R ad
0
0.2
0.4
0.6
0.8
1
1.2
hMom_z_r_protons Entries 735 Mean x 2.583 Mean y 0.2759 Std Dev x 0.5501 Std Dev y 0.1198
2−10
1−10
1 hMom_z_r_protons Entries 735 Mean x 2.583 Mean y 0.2759 Std Dev x 0.5501 Std Dev y 0.1198
Protons Reconstructed tracks FTS Full set up, 4.6 GeV
(d)
22
Z momentum [GeV] 0.5− 0 0.5 1 1.5 2 2.5 3 3.5
R ad
1.2 hMCMom_z_r_antiprotons
2−10
1−10
1
hMCMom_z_r_antiprotons
Anti-Protons MC tracks FTS Full set up, 4.6 GeV
(e)
Z momentum [GeV] 0.5− 0 0.5 1 1.5 2 2.5 3 3.5
R ad
1.2 hMom_z_r_antiprotons
2−10
1−10
1
hMom_z_r_antiprotons
Anti-Protons Reconstructed tracks FTS Full set up, 4.6 GeV
(f)
Z momentum [GeV] 0.5− 0 0.5 1 1.5 2 2.5 3 3.5
R ad
1.2 hMCMom_z_r_pi_minus
1−10
10 hMCMom_z_r_pi_minus
Pi minus MC tracks FTS Full set up, 4.6 GeV
(g)
Z momentum [GeV] 0.5− 0 0.5 1 1.5 2 2.5 3 3.5
R ad
1.2 hMom_z_r_pi_minus
2−10
1−10
1
hMom_z_r_pi_minus
Pi Minus Reconstructed tracks FTS Full set up, 4.6 GeV
(h)
Z momentum [GeV] 0.5− 0 0.5 1 1.5 2 2.5 3 3.5
R ad
1.2 hMCMom_z_r_pi_plus
1−10
1
10hMCMom_z_r_pi_plus
Pi Plus MC tracks FTS Full set up, 4.6 GeV
(i)
Z momentum [GeV] 0.5− 0 0.5 1 1.5 2 2.5 3 3.5
R ad
0
0.2
0.4
0.6
0.8
1
1.2
hMom_z_r_pi_plus Entries 3020 Mean x 0.3777 Mean y 0.07175 Std Dev x 0.2171 Std Dev y 0.03708
1−10
1
10
Pi Plus Reconstructed tracks FTS Full set up, 4.6 GeV
(j)
23
The same analysis was done for the reaction 1 at beam momentum 7 GeV. In general, the plots look the same as in the case of 4.6 GeV, the difference only lies on the magnitude of the momentum: the pions and protons reach higher momenta, as it is expected. In this case, the MonteCarlo tracks longitudinal momenta ranges from 0 to 6 GeV, while the radial momenta vary from 0 to ∼1.4 GeV as it is shown in Figure 23 (a). In Figure 23 (b), we can see that only those tracks of particles with longitudinal momenta at the ranges 0 to ∼1.3 GeV and 3 to 5.3 GeV and radial momentum between 0 and ∼ 0.4 GeV are reconstructed. Once again we can see that the tracks with the highest transversal momenta are not reconstructed.
There is an evident separation between protons and pions in their momentum distributions. This may help with particle identification. The histograms corresponding to each of the possible configurations of FTS were done. The behaviour of the simulated and reconstructed tracks for the partial set ups is the same, changing only the number of tracks reconstructed as it was showed in table 4 and 5. The corresponding plots for set ups FTS1234 and FTS1256 for beam momentum 4.6 GeV and 7 GeV are presented can be consulted in appendix A.
Z momentum [GeV] 0 1 2 3 4 5 6
R ad
1.8 2
1−10
1
10
MC tracks FTS Full, 7 GeV
(a)
R ad
1.8 2
hMom_r_z Entries 7305 Mean x 1.42 Mean y 0.1649 Std Dev x 1.605 Std Dev y 0.1747
1−10
10 hMom_r_z
Reconstructed tracks FTS Full, 7 GeV
(b)
R ad
hMCMom_z_r_protons
2−10
1−10
1
hMCMom_z_r_protons
Protons MC tracks FTS Full, 7 GeV
(c)
R ad
2−10
1−10
1hMom_z_r_antiprotons
Anti-Protons Reconstructed tracks FTS Full, 7 GeV
(d)
24
R ad
2−10
1−10
1
Anti-Protons MC tracks FTS Full, 7 GeV
(e)
R ad
2−10
1−10
1hMom_z_r_antiprotons
Anti-Protons Reconstructed tracks FTS Full, 7 GeV
(f)
R ad
hMCMom_z_r_pi_plus
1−10
10 hMCMom_z_r_pi_plus
Pi Plus MC tracks FTS Full, 7 GeV
(g)
R ad
1.8 2
1−10
1
10
Pi Plus Reconstructed tracks FTS Full, 7 GeV
(h)
R ad
hMCMom_z_r_pi_minus
1−10
1
10hMCMom_z_r_pi_minus
Pi minus MC tracks FTS Full, 7 GeV
(i)
R ad
hMom_z_r_pi_minus
1−10
10 hMom_z_r_pi_minus
Pi minus Reconstructed tracks FTS Full, 7 GeV
(j)
25
2.6 Momentum vs Angle of reconstructed tracks
Another result obtained, involves the relation between angle at which the particles are created with respect of the beam line (polar angle θ as in Figure 9) and the total momenta of the tracks reconstructed. This is important because the FTS is designed to have angular acceptance defined by the aperture of the magnet equal to ±10
horizontally and ±5 vertically with respect the beam direction [3], so we would expect that only small angle particles tracks can be seen by the FTS. In this case, Figure 24 show plots of total momentum vs angle for the FTSFull set up. In this figure, (a) shows all the MonteCarlo tracks simulated, with angles going from 0 to 100 , while Figure 24 (b) shows all the reconstructed tracks, in which we can see a considerable decrease in the range of angle. Those particles which have angles bigger than 60 are not reconstructed at all. There previous seeing separation in regions of momentum is notable again. In the low momentum region (from 0 to 1 GeV) there are tracks with angles ranging from 0 to 60 . In the higuer momentum region (from 2 to 3.5 GeV) there are angles from 0 to 20 and momentum .
It is good to make a distintion between particles as it was done in the section 2.5. As we see in 22 (c), (e) the simulated protons and antiprotons have a wide range of total momentum going from 0.5 to 3.5 GeV, but their corresponding angles are not bigger than 45 . However, in 24 (d), (f) we can see that only the tracks belonging to particles that are emmited with angles lower than 25 are reconstructed. This reflects that the higher angle protons and antiprotons tracks are not reconstructed. This can be interpreted as expected, since the FTS is dessigned to work to detect ”low angle” particles. On the other hand, in figures 24 (g) and (i) we can see that the simulated pions have a wider range of angles going from 0 to 100 , but their momenta is no larger than 1 GeV. In figures 24 (h) and (j) we can see that mostly the particles with angles in the same range as (anti)protons are reconstructed. However, a significant number of pions with angles higher up to 60 are reconstructed too. One of the explanations can be that these pions come from the decay of the Λ or Λ, which means that they are generated very far from the interaction point and closer to one of the FTS stations so they can hit one of the stations. They could also be pions that after being generated, have an spiraling trajectory and they manage to hit the stations. It is worth to do a deeper study of this, taking a closer look to those large angle tracks which are being reconstructed.
26
Momentum [GeV] 0 0.5 1 1.5 2 2.5 3 3.5
T he
ta [D
0 10 20 30 40 50 60 70 80 90
100 MomMCTheta
1−10
1
10
MomMCTheta Entries 60000 Mean x 0.8314 Mean y 25.61 Std Dev x 0.7822 Std Dev y 15.53
Final state particles MCTracks FTS Full, 4.6 GeV
(a)
Momentum [GeV] 0 0.5 1 1.5 2 2.5 3 3.5
T he
ta [D
0 10 20 30 40 50 60 70 80 90
100 MomTheta
1−10
10 MomTheta
Final state particles Reco tracks FTS Full, 4.6 GeV
(b)
Momentum [GeV] 0 0.5 1 1.5 2 2.5 3 3.5
T he
ta [D
0 10 20 30 40 50 60 70 80 90
100 MomMCTheta_protons
1−10
1
10MomMCTheta_protons
FTS FULL MC Tracks Protons, 4.6 GeV
(c)
Momentum [GeV] 0 0.5 1 1.5 2 2.5 3 3.5
T he
ta [D
0 10 20 30 40 50 60 70 80 90
100 MomTheta_protons Entries 735 Mean x 2.601 Mean y 6.322 Std Dev x 0.5502 Std Dev y 2.851
2−10
1−10
1
MomTheta_protons Entries 735 Mean x 2.601 Mean y 6.322 Std Dev x 0.5502 Std Dev y 2.851
FTS Full Reco Protons, 4.6 GeV
(d)
27
Momentum [GeV] 0 0.5 1 1.5 2 2.5 3 3.5
T he
ta [D
0 10 20 30 40 50 60 70 80 90
100 MomMCTheta_antiprotons
1−10
1
10MomMCTheta_antiprotons
FTS FULL MC Tracks Antiprotons, 4.6 GeV
(e)
Momentum [GeV] 0 0.5 1 1.5 2 2.5 3 3.5
T he
ta [D
0 10 20 30 40 50 60 70 80 90
100 MomTheta_antiprotons
2−10
1−10
1
MomTheta_antiprotons
FTS Full Reco Antiprotons, 4.6 GeV
(f)
Momentum [GeV] 0 0.5 1 1.5 2 2.5 3 3.5
T he
ta [D
0 10 20 30 40 50 60 70 80 90
100 MomMCTheta_pi_minus
1−10
FTS Full MC Tracks Pi minus, GeV
(g)
Momentum [GeV] 0 0.5 1 1.5 2 2.5 3 3.5
T he
ta [D
0 10 20 30 40 50 60 70 80 90
100
MomTheta_pi_minus
1−10
1
10
MomTheta_pi_minus
FTS Full Reco Pi minus, 4.6 GeV
(h)
Momentum [GeV] 0 0.5 1 1.5 2 2.5 3 3.5
T he
ta [D
0 10 20 30 40 50 60 70 80 90
100 MomMCTheta_pi_plus
1−10
1
10
MomMCTheta_pi_plus
FTS Full MC Tracks Pi plus, 4.6 GeV
(i)
Momentum [GeV] 0 0.5 1 1.5 2 2.5 3 3.5
T he
ta [D
0 10 20 30 40 50 60 70 80 90
100 MomTheta_pi_plus Entries 3020 Mean x 0.3879 Mean y 13.39 Std Dev x 0.214 Std Dev y 9.329
1−10
1
FTS Full Reco Pi plus, 4.6 GeV
(j)
28
If we now look at the plots presented in Figure 25, which corresponds to the same reaction but beam momentum 7 GeV, (a) shows the MonteCarlo simulation and (b) the reconstructed tracks. In this case, the total momentum ranges from 0 to 6 GeV and the angles from 0 to 100 in the former and from 0 to 6 GeV, with an empty zone in the middle, for total momenta and angles up to ∼ 60 for the latter. Once again there are few protons and antiprotons tracks reconstructed for momenta around 0.5 GeV and a larger number in the region from 3 to 6 GeV and with angles lower than 20 . On the other hand, the reconstructed pions with momenta ranging from 0 to 1.5 GeV and with corresponding angles up to 60 . The explanation for the tracks at high angle could be the same as stated in the case of beam momentum 4.6 GeV, however further investigations would have to be done if we want a deeper explanation.
The behavior of the plots is very similar between configurations in both cases of beam momentum, so the same discussion applies. They are shown in appendix B.
Momentum [GeV] 0 1 2 3 4 5 6
T he
ta [D
100 MomMCTheta
1−10
1
10
MomMCTheta Entries 60000 Mean x 1.254 Mean y 24.2 Std Dev x 1.247 Std Dev y 14.85
Final state particles MCTracks FTS Full, 7 GeV
(a)
T he
ta [D
eg ]
0
10
20
30
40
50
60
70
80
90
100
MomTheta Entries 7305 Mean x 1.433 Mean y 9.738 Std Dev x 1.611 Std Dev y 7.41
1−10
10 MomTheta
Final state particles Reco tracks FTS Full, 7 GeV
(b)
T he
ta [D
100 MomMCTheta_protons
1−10
1
10MomMCTheta_protons
FTS Full MC Tracks Protons, 7 GeV
(c)
T he
ta [D
eg ]
0
10
20
30
40
50
60
70
80
90
100 MomTheta_protons Entries 829 Mean x 4.343 Mean y 5.994 Std Dev x 0.6353 Std Dev y 2.695
2−10
1−10
1
MomTheta_protons Entries 829 Mean x 4.343 Mean y 5.994 Std Dev x 0.6353 Std Dev y 2.695
FTS Full Reco Protons, 7 GeV
(d)
29
T he
ta [D
1−10
FTS Full MC Tracks Antiprotons, 7 GeV
(e)
T he
ta [D
100 MomTheta_antiprotons
2−10
1−10
1
MomTheta_antiprotons
FTS Full Reco Antiprotons, 7 GeV
(f)
T he
ta [D
100 MomMCTheta_pi_plus
1−10
1
10
MomMCTheta_pi_plus
FTS Full MC Tracks Pi plus, 7 GeV
(g)
T he
ta [D
eg ]
0
10
20
30
40
50
60
70
80
90
100
MomTheta_pi_plus Entries 2881 Mean x 0.6098 Mean y 10.81 Std Dev x 0.3858 Std Dev y 7.856
1−10
1
FTS Full Reco Pi plus, 7 GeV
(h)
T he
ta [D
1−10
1
10
MomMCTheta_pi_minus
FTS Full MC Tracks Pi minus, 7 GeV
(i)
T he
ta [D
100 MomTheta_pi_minus
1−10
1
10MomTheta_pi_minus
FTS Full Reco Pi minus, 7 GeV
(j)
30
2.7 Efficiency of angle of reconstructed tracks
On the following plots, efficiency studies were performed. Efficiency is taken as the ratio between the number of MonteCarlo tracks and the number of reconstructed tracks, in this case in terms of emmision angle. In Figure 26 the efficiency for all particles is shown for the three possible set ups in different colors: Magenta for FTSFull, blue for FTS1234 and red for FTS1256. Here we can see that the efficiency for angles up to 10 is good, and from there it starts to drop until it goes to zero. This is expected because the FTS is designed to have good acceptance for particles emmited at low angles in the forward direction. In Figure 27 from (a) to (d), the efficiency is shown for each particle and for the three set ups as well. In the case of protons and antiprotons, Figure 27 (a) and (b), the efficiency for angles up to 10 is close to one and then it decreases. In the case of pions, shown in Figure 27 (c) and (d), the efficiency at low angles is close to 0.8 but it decreases slower than the case of protons and antiprotons. From these two plots, we can refer again to the fact that in section 2.6, tracks of pions emmited at high angles were reconstructed.
For beam momentum 7 GeV, the plots look very similar as the ones corresponding to beam momentum 4.6 GeV in all cases. The efficiency for all final state particles is shown in Figure 28, while at Figure 29 for each one individually. Same colors were applied.
Angle [Degrees] 0 10 20 30 40 50 60 70 80
E ffi
ci en
Reconstruction efficiency FTS, final state particles, 4.6 GeV
Figure 26: Efficiency of reconstructed tracks as function of the angle for all final state particles, beam momentum 4.6 GeV
31
Angle [Degrees] 0 10 20 30 40 50 60 70 80
E ffi
ci en
(a)
Angle [Degrees] 0 10 20 30 40 50 60 70 80
E ffi
ci en
(b)
Angle [Degrees] 0 10 20 30 40 50 60 70 80
E ffi
ci en
(c)
Angle [Degrees] 0 10 20 30 40 50 60 70 80
E ffi
ci en
(d)
Figure 27: Reconstruction efficiency as function of angle for beam momentum 4.6 GeV: (a) Protons (b)Antiprotons (c)Pi Plus (d)Pi Minus
32
Angle [Degrees] 0 10 20 30 40 50 60 70 80
E ffi
ci en
Reconstruction efficiency FTS, final state particles, 7 GeV
Figure 28: Efficiency of reconstructed tracks as function of the angle for all final state particles, beam momentum 7 GeV.
33
Angle [Degrees] 0 10 20 30 40 50 60 70 80
E ffi
ci en
(a)
Angle [Degrees] 0 10 20 30 40 50 60 70 80
E ffi
ci en
(b)
Angle [Degrees] 0 10 20 30 40 50 60 70 80
E ffi
ci en
(c)
Angle [Degrees] 0 10 20 30 40 50 60 70 80
E ffi
ci en
(d)
Figure 29: Reconstruction efficiency as function of angle for beam momentum 7 GeV: (a) Protons (b)Antiprotons (c)Pi Plus (d)Pi Minus
34
2.8 Central + Forward detection efficiency
In this section, efficiencies as defined in section 2.7 for the central tracker and the different FTS set ups are compared. Figure 30 shows the comparisson for (a)FTS1234, (b)FTS1256 and (c)FTSFull. In these plots, the full tracking system is represented in blue (forward + central), while the forward tracker alone is represented in red and the central tracker in green. For small angles the efficiency of the forward tracker is good but as the angle increases, it goes to zero. On the other hand, it is the central tracker which shows good efficiency for large angles and bad efficiency for low angles. From comparing we can see that in the case of FTS1234 and FTS1256, the complete system (blue line) has a significant drop in the zone corresponding to the end of the central tracker and the beggining of the forward tracker. This is however not seen in the case of the FTSFull. The latter can have different explanations, but the most reasonable thing is that as it was mentioned before, both the central tracker (green line) and the forward tracker (red line) individually are different in the three cases of set up used. This means that not only the forward tracker influences the behaviour in this zone, but there are some components missing. The lack of pixels in the Mvd which area supposed to cover the area surrounding the beam line could affect the reconstruction of low angle particles and thus the efficiency drops in the in between zone. The latter is an hypothesis and needs further investigation to be accepted.
Angle [Degrees] 0 20 40 60 80 100 120 140 160 180
E ffi
ci en
(a)
Angle [Degrees] 0 20 40 60 80 100 120 140 160 180
E ffi
ci en
(b)
Angle [Degrees] 0 20 40 60 80 100 120 140 160 180
E ffi
ci en
(c)
Figure 30: Efficiency of STT + FTS for all final state particles.
35
Angle [Degrees] 0 20 40 60 80 100 120 140 160 180
E ffi
ci en
(a)
Angle [Degrees] 0 20 40 60 80 100 120 140 160 180
E ffi
ci en
(b)
Angle [Degrees] 0 20 40 60 80 100 120 140 160 180
E ffi
ci en
(c)
Figure 31: Efficiency of STT + FTS for all final state particles. Beam momentum 7 GeV.
36
3 Conclusions
A more complete comparisson between partial and full set ups would require to take into account more than one reaction, however this study was good to give an idea of how the forward tracker works and how the reaction products interact with the forward stations.
We can conclude that in case of the studied reaction involving Ξ−-Ξ+ hyperons, the number of reconstructed tracks is independent on the beam momentum. In both cases, half of the total number of events don’t have reconstructed tracks and for the rest at least one track is reconstructed. For both beam momenta the number of events for which more than one track is reconstructed decreases, and there are no events for which all 6 final state particles tracks are reconstructed. At first sight, there seems to be no big difference in changing configurations from FTS1234 to F1256 or FTSFull (Figure 10 and 11). However if one looks at the number of reconstructed tracks and standard deviations in Tables 4 and 5, we can distinguish two things: the first one is that for protons and antiprotons the numbers are almost the same for the three cases of set ups, the second is that for pions there are less tracks reconstructed in the case of FTSFull. This can be attributed to the fact that we are only taking into account the reconstruction before any fit or cut which means that eventhough there are more reconstructed tracks in the partial set ups, these migh have less quality.
As the plots showed at Figure 2.2, it was found that there is an error in the position of station 6 in the FTS1256 set up, as it appears to be almost one meter away from Station 5. The correct distance between the last two stations is of the order of a few centimeters as it is shown in FTSFull. There number of hits in the statios decreases as we move away from the interaction point, but there is still a big number of hits in the last stations, which means that they are still very important. Moving on into the number of hits in the stations corresponding to each of the set ups, it was concluded that the zone surrounding the beam line has higher hit densities and it is lower at the edges in the case of all stations. Another important result from section 2.2 is that most of the reconstructed tracks have hits in all the stations corresponding to the used configuration. This means that for the partial set ups there are mostly tracks that have 32 hits, whereas in the full set up most of the tracks have 48 hits. The main difference between the partial set ups is that there is a bigger number of tracks reconstructed with 32 tracks in the case of FTS1256 than in the case of FTS1234, which means that not a lot of tracks manage to reach Station 5. For each FTS set up, there are some differences in the devices corresponding to the central tracker which also influences in the behaviour of the tracks.
We also looked at the number of hits in the GEM planes that are connected with tracks in the forward spectrometer. This is important because maybe these hits are the only information that at some point could relate the barrel with the forward part. However, this is not straightforward. It was found that in both cases of beam momenta the higher number of hits in the GEM planes corresponds to the full set up, while the difference between the reduced set ups is not very distinguishable. This result shows once again that there are effects on the FTS information due to the changes in other devices of the tracking system, in this case the extra GEM plane in the full set up increases the possibility of having a track with hits in the GEM and in the stations of the forward spectrometer. It was also seen that most of the reconstructed tracks in the forward spectrometer have at least one hit in the GEM planes.
This work was also focused in the comparisson between the MonteCarlo and the reconstructed tracks. This was done firstly by looking at histograms showing the momentum in Z direction vs radial momentum and secondly using histograms showing total momenta vs angle of emission. For the former it was found that the higher radial momentum simulated tracks are not reconstructed. It was possible to distinguish between particles in the reconstructed tracks by learning that the protons and antiprotons have higher radial and longitudinal momenta than the pions. What this tells us is that in general the FTS is important on the reconstruction of lower momenta particles.
On the other hand, what we see at the plots corresponding to total momenta vs emission angle for both beam momenta, is a considerable reduction of the number or reconstructed tracks when it comes to high angle tracks. There are some pions reconstructed tracks at angles higher than 40. However most of the reconstructed tracks (for all particles) have angles no larger than 20. This is expected because the FTS is designed to detect those particles that are emitted at low angles, nevertheless some particles that are produced far from the interaction point and at higher angles could by some other effect be able to hit the FTS stations. This has to be further studied taking a
37
closer look to these cases.
The efficiency of the reconstructed tracks at the FTS was plotted by taking the ratio between the number of simulated and the reconstructed tracks in terms of angle. It was found that if the efficiency is very high (∼1) at angles lower than 10, decreasing when the angle is increasing: it is very low at 20 and it goes close to zero for angles higher than 40. If we look at the particles separately, pions in angles below 10 are less efficiently reconstructed than protons and antiprotons for these angles. This is another confirmation of the fact that there are reconstructed pion tracks at higher angles. Once again, a comparisson between set ups was done, where it is seen that there are not significative differences between set ups. This was done for both beam momenta and the behavior is very similar in both cases. The calculated efficiencies were also used to check the full PANDA set up composed by central and barrel spectrometers. With this, it was possible to cross check the good efficiency regions corresponding with the purpose of the components, meaning that high efficiency is expected for low angles in the case of all configurations of FTS and high efficiency for big angles is expected for the central tracker. The result in this case is what expected, however the difference between set ups is more evident. There is a region where the efficiency drops very fast and goes up again, corresponding with the border between the spectrometers in the case of the partial set ups. This drop is not present in the case of the full set up. Investigations follow to see if this drop is due to the missing pixels and GEM plane in the partial set ups.
What we can in general conclude, is that there are a lot of factors that have to be taken into account when comparing between set ups. One of them is that there are not only differences between the set ups in the forward spectrometer but also in the central part. If we want to be more precise on finding these differences, a deeper study has to be done, however with these results we have valuable information such as the regions of momenta where pions and (anti)protons are reconstructed, the range of emmission angle of the particles whose tracks can be reconstructed in the FTS, the behaviour of the efficiency of all configurations of FTS and also the number of tracks reconstructed in each case. The Forward tracking system is essential for PANDA to reconstruct the information belonging particles emmited for at low angles. This type of study is important to the development of reconstruction algorithms.
References
[1] Grape Sophie, Studies of PWO Crystals and Simulations of the pp → ΛΛ, ΛΣ0 Reactions for the PANDA Experiment Uppsala University, 2009
[2] Wikipedia, https: // commons. wikimedia. org/ wiki/ File: Standard_ Model_ of_ Elementary_
Particles. svg. File:Standard Model of Elementary Particles.svg. January 10, 2018
[3] PANDA Website https: // panda. gsi. de/ oldwww/ framework/ det_ iframe. php? section= Tracking The PANDA Detector - Tracking January 10, 2018
[4] K. A. Olive et al. Particle Data Group, Chin. Phys. C 38 (2014) 090001. doi:10.1088/1674-1137/38/9/090001
[5] Spataro Stefano, Simulation and event reconstruction inside the PandaRoot framework Journal of Physics: Conference Series, (Vol. 119) 2008, IOP Publishing
[6] Stockmanns, T. (2017). Introduction PANDA Computing School 2017 - SUT.
[7] Schumann J., Entwicklung eines schnellen algorithmus zur suche von teilchenspuren im straw tube tracker des panda-detektors. Bsc. Thesis, Aachen University of Applied Sciences, 2013.
[8] Andersson Bjorn, Nordstrom Johan Prototype of an online track and event reconstruction scheme for thePANDA experiment at FAIR, January, 2017
[9] Strandlie, Are and Fruhwirth, Rudolf, Track and vertex reconstruction: From classical to adaptive methods Rev. Mod. Phys. Vol 82. Issue 2, pages 1419–1458, American Physical Society May, 2010 https://link.aps.org/
doi/10.1103/RevModPhys.82.1419
38
[11] Jerzy Smyrski Geometry of the Forward Tracking Stations - Internal Document 2013
39
A.1 FTS1234 4.6 GeV
Z momentum [GeV] 0.5− 0 0.5 1 1.5 2 2.5 3 3.5
R ad
1.2 hMCMom_r_z
1−10
1
10
MC tracks FTS1234, 4.6 GeV
(a)
Z momentum [GeV] 0.5− 0 0.5 1 1.5 2 2.5 3 3.5
R ad
1.2 hMom_r_z
1−10
1
Reconstructed tracks FTS1234, 4.6 GeV
(b)
Z momentum [GeV] 0.5− 0 0.5 1 1.5 2 2.5 3 3.5
R ad
1.2 hMCMom_z_r_protons
2−10
1−10
1
hMCMom_z_r_protons
Protons MC tracks FTS1234, 4.6 GeV
(c)
Z momentum [GeV] 0.5− 0 0.5 1 1.5 2 2.5 3 3.5
R ad
0
0.2
0.4
0.6
0.8
1
1.2 hMom_z_r_protons Entries 732 Mean x 2.57 Mean y 0.2736 Std Dev x 0.5827 Std Dev y 0.119
2−10
1−10
Protons Reconstructed tracks FTS1234, 4.6 GeV
(d)
40
Z momentum [GeV] 0.5− 0 0.5 1 1.5 2 2.5 3 3.5
R ad
2−10
1−10
1
Anti-Protons MC tracks FTS1234, 4.6 GeV
(e)
Z momentum [GeV] 0.5− 0 0.5 1 1.5 2 2.5 3 3.5
R ad
2−10
1−10
1
Anti-Protons Reconstructed tracks FTS1234, 4.6 GeV
(f)
Z momentum [GeV] 0.5− 0 0.5 1 1.5 2 2.5 3 3.5
R ad
0
0.2
0.4
0.6
0.8
1
1.2
hMCMom_z_r_pi_plus
1−10
Pi Plus MC tracks FTS1234, 4.6 GeV
(g)
Z momentum [GeV] 0.5− 0 0.5 1 1.5 2 2.5 3 3.5
R ad
2−10
1−10
1
10
hMom_z_r_pi_minus
Pi minus Reconstructed tracks FTS1234, 4.6 GeV
(h)
Z momentum [GeV] 0.5− 0 0.5 1 1.5 2 2.5 3 3.5
R ad
0
0.2
0.4
0.6
0.8
1
1.2
hMCMom_z_r_pi_plus
1−10
(i)
Z momentum [GeV] 0.5− 0 0.5 1 1.5 2 2.5 3 3.5
R ad
2−10
1−10
1
10
hMom_z_r_pi_minus
Pi minus Reconstructed tracks FTS1234, 4.6 GeV
(j)
41
A.2 FTS1256 4.6 GeV
Z momentum [GeV] 0.5− 0 0.5 1 1.5 2 2.5 3 3.5
R ad
1.2 hMCMom_r_z
1−10
1
10
MC tracks FTS1256, 4.6 GeV
(a)
Z momentum [GeV] 0.5− 0 0.5 1 1.5 2 2.5 3 3.5
R ad
1.2 hMom_r_z
1−10
1
Reconstructed tracks FTS1256, 4.6 GeV
(b)
Z momentum [GeV] 0.5− 0 0.5 1 1.5 2 2.5 3 3.5
R ad
0
0.2
0.4
0.6
0.8
1
1.2
hMCMom_z_r_protons
2−10
1−10
1
hMCMom_z_r_protons
Protons MC tracks FTS1256, 4.6 GeV
(c)
Z momentum [GeV] 0.5− 0 0.5 1 1.5 2 2.5 3 3.5
R ad
0
0.2
0.4
0.6
0.8
1
1.2
2−10
1−10
Protons Reconstructed tracks FTS1256, 4.6 GeV
(d)
42
Z momentum [GeV] 0.5− 0 0.5 1 1.5 2 2.5 3 3.5
R ad
2−10
1−10
1
Anti-Protons MC tracks FTS1256, 4.6 GeV
(e)
Z momentum [GeV] 0.5− 0 0.5 1 1.5 2 2.5 3 3.5
R ad
2−10
1−10
1 hMom_z_r_antiprotons
Anti-Protons Reconstructed tracks FTS1256, 4.6 GeV
(f)
Z momentum [GeV] 0.5− 0 0.5 1 1.5 2 2.5 3 3.5
R ad
1.2 hMCMom_z_r_pi_minus
1−10
10 hMCMom_z_r_pi_minus
Pi minus MC tracks FTS1256, 4.6 GeV
(g)
Z momentum [GeV] 0.5− 0 0.5 1 1.5 2 2.5 3 3.5
R ad
1−10
1
10
hMom_z_r_pi_minus
Pi Minus Reconstructed tracks FTS1256, 4.6 GeV
(h)
Z momentum [GeV] 0.5− 0 0.5 1 1.5 2 2.5 3 3.5
R ad
0
0.2
0.4
0.6
0.8
1
1.2
hMCMom_z_r_pi_plus
1−10
(i)
Z momentum [GeV] 0.5− 0 0.5 1 1.5 2 2.5 3 3.5
R ad
(j)
43
R ad
1.8 2
1−10
1
10
MC tracks FTS1234, 7 GeV
(a)
R ad
1.8 2
hMom_r_z Entries 7500 Mean x 1.382 Mean y 0.1614 Std Dev x 1.586 Std Dev y 0.1718
1−10
1
Reconstructed tracks FTS1234, 7 GeV
(b)
R ad
hMCMom_z_r_protons
2−10
1−10
1
hMCMom_z_r_protons
Protons MC tracks FTS1234, 7 GeV
(c)
R ad
1.8 2
2−10
1−10
1
Protons Reconstructed tracks FTS1234, 7 GeV
(d)
44
R ad
2−10
1−10
1
Anti-Protons MC tracks FTS1234, 7 GeV
(e)
R ad
2−10
1−10
1 hMom_z_r_antiprotons
Anti-Protons Reconstructed tracks FTS1234, 7

detector signatures of - hyperons in the forward

Documents