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Status of ATLAS and Preparation for the Pb-Pb Run

Jirı Dolejsı a for the ATLAS Collaboration

aCharles University, Faculty of Mathematics and Physics, IPNP,V Holesovickach 2, CZ-180 00 Praha 8, Czech Republic

Abstract

The ATLAS experiment took its first beam data in September 2008 and is actively preparing forthe planned start of LHC collision data-taking in 2009. This preparation includes hardware andsoftware commissioning, as well as calibration and cosmic data analysis. The status and perfor-mance of the ATLAS detector will be discussed, with a view towards the Pb+Pb run expected in2010.

1. Performance of ATLAS and experience from the 2008 run1

This contribution aims to illustrate of the ATLAS performance observed during the beam2

tests, cosmic runs and at the moment of first protons injected into LHC. For detailed information3

on ATLAS and its expected performance look at [2] and [3].4

Figure 1: Single beam splash event on September 10th, 2008: particles from a beam hitting the collimator 140 m fromATLAS are detected by the whole detector.

Preprint submitted to Nuclear Physics A July 22, 2009

Figure 2: Single beam splash event on September 10th, 2008: Event display with a more detailed look at muon detectors,calorimeters and the inner detector in the top row.

Inner detector5

Most data come from the cosmic run which started on September 14th, 2008:6

Figure 3: Left panel: The ATLAS inner detector. Right panel: Display of cosmic ray event going through the pixeldetector.

The whole inner detector performed remarkably well – the pixel detector reached efficiency7

of about 98.4%, 99% of SCT Barrel and 97% of SCT EndCaps were operational, 99% of all8

modules were operational and 99.8% strips alive. 97% of TRT channels were reading out, 99%9

of electronics working. 2.9 M tracks have been collected in TRT. Repairs and improvements,10

especially of the cooling, are scheduled for 2009.11

At the right panel of Fig. 3 you can see a cosmic ray event going through the SCT and pixel12

detector. Shown are the XY view (of SCT and pixels and of pixels alone) and an RZ view. The13

track has a hit in each of the layers in both the upper and the lower hemisphere (with two hits in14

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the innermost pixel layer due to a module overlap). Apart from the signal hits there is only one15

other hit in the pixel detector demonstrating the very low noise level in the detector.16

Alignment of the inner detector elements is illustrated by Fig. 4. Cosmic tracks crossing17

the entire ID leave hits in both the upper and lower halves of the ID. These tracks can be split18

near the interaction point and fitted separately, resulting in two collision-like tracks that can then19

be compared. The plots show the difference in the d0 track distance to the vertex. Tracks are20

selected to have pT > 2 GeV, |d0| < 50 mm, |z0| < 400 mm (in other words they are required to21

go through the innermost pixel layer). Tracks also are required to have a hit in the Pixel B layer,22

3 Pixel hits and in total 7 Silicon hits.23

d0 [mm]-0.8 -0.6 -0.4 -0.2 -0 0.2 0.4 0.6 0.8

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Aligned geometrym=49m, =-11

MC perfect geometrym=32m, =-1

Nominal geometry

ATLAS PreliminarySiUp-SiLow Tracks

Figure 4: The difference in the d0 track parameter between the two split tracks in the inner detector.

ATLAS calorimeters24

Figure 5: Left panel: The ATLAS calorimeters. Right panel: Energy summed over 100 splash events in EM Presampler.

The liquid argon calorimeter operated well in 2008: 97% of power supplies were OK, 99.92%25

channels of the calorimeter working. The tasks for 2009 include repairs of faulty power supplies26

and improvements of the monitoring. The performance of the barrel electromagnetic liquid argon27

calorimeter, more precisely of its first layer – presampler – is illustrated on the right panel of Fig.28

5. Clearly higher signal is recorded on the left side and on upper hemisphere, due to the origin29

of the splash particles and due to the structure of the tunnel and the detector. This feature of the30

recorded data from the EM calorimeter is illustrated further on the left panel of Fig. 6.31

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The tile calorimeter also performed very well with 99.2% power supplies and controls in32

perfect shape and 98.6% cells alive. The calibration is ready or being commissioned, cell equal-33

ization reached a level of few %. The tasks for 2009 are further tuning of the 137Cs cell inter-34

calibration, monitoring of PMT stability, data quality monitoring. The right panel of Fig. 635

illustrates the performance of the Tile calorimeter in splash events.36

Figure 6: The sum of 100 splash events from Sept. 10th. Left panel: The 8-fold φ structure of the signal from barrelEM calorimeter induced by the toroid endcap is clearly visible at large radius (layers S1, S2, S3). The 16-fold structureis due to additional matter and shielding at low radius (Presampler PS). Right panel: The signal from the tile hadroniccalorimeter, again with the same φ structure. The up-down asymmetry is also due to the material in front of the detector.

Muons37

The ATLAS muon system with its huge air core toroids and all subdetectors performed also38

well during 2008 with further improvement aimed for 2009 (e.g. improvement from 98.3% MDT39

channels already working to get 99.8% after the shutdown).40

Figure 7: Left panel: The ATLAS muon detectors: MDT and CSC for precision tracking; TGC and RPC for trigger.Right panel: Cosmic muon map reconstructed by off-line RPC standalone muon monitoring projected on surface (81 mup from the beam line).

The performance of muon detectors could be illustrated by the right panel of Fig. 7 – cosmic41

muon map of the ATLAS cavern and of the access shafts.42

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Forward detectors43

Preparation of the ATLAS forward detectors (see Fig. 8) started later then building the al-44

ready mentioned central subdetectors. LUCID (Luminosity Cerenkov Integrating Detector) is45

a running detector in advanced status of commissioning; nevertheless, some consolidation and46

repairs are ongoing during the shutdown period. ZDC (Zero Degree Calorimeter), for heavy ion47

physics, is at an advanced state of construction. ALFA (Absolute Luminosity for ATLAS) is48

expected to be ready in 2010.49

Figure 8: Left panel: ALFA (Absolute Luminosity for ATLAS) at 240 m. Middle panel: ZDC (Zero Degree Calorimeter) at 140 m. Right panel: LUCID (Luminosity Cerenkov Integrating Detector) at 17 m from the collision point.

Data acquisition50

The performance of the data acquisition chain is briefly illustrated by Fig. 9 with numbers of51

recorded cosmic events with different triggers and also by all the figures from the subdetectors52

already shown. The results presented here and many more are available at TWiki pages [1].53

Figure 9: Cosmic data since Sept 13, 2008. 216 M events. 400,000 files in 21 inclusive streams.

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2. Heavy Ion Physics with the ATLAS Detector at the LHC54

This short part will show only briefly some results from the forthcoming Physics Performance55

Report devoted to heavy ion physics. Although ATLAS is not a dedicated heavy-ion detector,56

its unprecedented acceptance (see Fig. 10) and other properties make it a very promising tool to57

study Pb+Pb collisions. More details are presented in parallel contributions to this conference:58

Jets by Nathan Grau, Quarkonia and Z0 by Marzia Rosati and Direct photons by Mark Baker.59

Figure 10: Acceptance of ATLAS.

The tracking performance is illustrated in Fig. 11. The efficiency of about 70% is well60

acceptable and fake rate above 1 GeV/c is negligible almost independently on η.61

Figure 11: Left panel: Tracking efficiency and fake rate in |η| < 1 extracted from a sample of central (b = 2 fm,dN/dη = 2700) HIJING [4] events produced with quenching effects turned off. Right panel: Top: Tracking efficiency asa function of pseudorapidity for tracks with 3 < pT < 8 GeV extracted from the same central sample of events. Bottom:Fake rate as function of pseudo-rapidity for the same tracks as above.

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Thanks to the complete azimuthal coverage we can well reproduce the simulated elliptic flow,62

different methods offer sensitivity to non-flow effects, see Fig. 12.63

Figure 12: Transverse momentum dependence of the reconstructed v2: from the event plane method (red squares), two-particle correlations (blue stars), the Lee-Yang Zeros method (green triangles) input flow as extrapolated from RHIC data(line)

Advanced calorimetry of ATLAS together with elaborated methods for the subtraction of the64

background from the underlaying event will enable the studies of jets and their energy losses.65

For a small illustration look at Fig. 13.66

Figure 13: Left panel: Reliable reconstruction of fragmentation function D(z): Reconstructed tracks with pT > 2 GeVmatching calorimeter jets. Right panel: The scale of possible modifications of fragmentation function by quenchingcomparison of PYTHIA [5] and PYQUEN [6]

Di-muon measurements with the invariant mass resolution good enough to separate the dif-67

ferent upsilon states (see Fig. 14) can elucidate the details of quarkonia suppression.68

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Figure 14: Di-muon invariant mass distribution as expected for one month of data, taking into account acceptance andefficiency, for decay muons in the barrel region only (|η| < 1).

3. Summary69

• ATLAS is fully operational, recorded several hundred million cosmic events.70

• Ongoing activities enable further detector improvements, calibration, refinement of moni-71

toring, software tools.72

• Extensive preparations for Pb+Pb program show a promising performance of ATLAS for73

heavy ion beams.74

• ATLAS heavy-ion group will participate in initial p+p data taking to get reference data for75

heavy ion program and to tune the analysis techniques.76

References77

[1] https://twiki.cern.ch/twiki/bin/view/Atlas/AtlasResults78

[2] The ATLAS Collaboration, G. Aad et al. JINST 3 (2008) S08003.79

[3] The ATLAS Collaboration, G. Aad et al.: arXiv:0901.0512 ; CERN-OPEN-2008-020.80

[4] M. Gyulassy and X.-N. Wang, Comput. Phys. Commun. 83 (1994) 307, nucl-th/9502021.81

[5] T. Sjostrand, Comp. Phys. Commun. 82,(1994) 74, T. Sjostrand, S. Mrenna and P. Skands, JHEP 0605 (2006) 026.82

[6] I. P. Lokhtin and A. M. Snigirev (2006), HEP-PH/0406038.83

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