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19/02/2009

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH

The readiness of CMS for exotic physics at LHC

CERN-PH-EP/2009-004

19 February 2009

by Archana Sharma, CMS-CERN

Abstract

The Large Hadron Collider (LHC) at CERN is a 14 TeV proton-proton collider to be operational in 2009,

permitting experiments to take data at a design luminosity of 1034

cm-2

sec-1

. With an enormous physics potential, the

Compact Muon Solenoid (CMS) is one of the two general purpose detectors at LHC that will independently

distinguish some of natures most interesting phenomena. This requires an efficient and precise detection of

particles in the inner detectors of the experiment, namely the tracker and calorimeters contained in a solenoid

magnet of 4T. The muon system is housed within the elements of the return yoke. At design luminosity, anunprecedented particle rate places stringent demands and technological challenges on the complete detector design

and concept. The lead tungstate crystal electromagnetic calorimeter (ECAL,) hermetic and homogeneous is

expected to reach excellent performances in order to guarantee the full CMS discovery potential of the Higgs boson

(H) for which a good energy resolution is crucial. The Hadron Calorimeter is designed to measure energy of

hadron jets and single hadrons. Muons provide a clear signature for many of the interesting processes which will be

studied at the LHC. The goal of the muon system is to provide identification, track reconstruction and trigger. To do

this, the CMS muon system employs several detectors: Drift Tubes (DT), Cathode Strip Chambers (CSC) and

Resistive Plate Chambers (RPC). In this paper, a short description and status of each subsystem of the CMS

experiment will be presented.

Invited Plenary Paper at the DAE BNRS Symposium on High Energy Physics,

Banaras Hindu UniversityVaranasi 221 005, India

Dec 14-18 2008

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1. IntroductionThe Standard Model in Particle Physics describes the present understanding of nature and the

fundamental forces, and is corroborated well by experiment in most aspects. Nevertheless a fewunanswered questions remain, for example, why is the top quark much more heavy than the other

quarks. The mass of the top is equivalent to that of the gold nucleus! What is the origin of mass?Astrophysics/cosmological measurements show that most matter in the universe is not explained

by the standard model and what is dark energy and dark matter?

Explanation of the origin of mass has been offered by Higgs, Brout and Englert [1] by

introducing a new field and particle: A scalar Higgs field and contend that at least one new scalar

particle should exist: Higgs the last missing particle in the Standard Model.

One of the main missions of the large hadron collider (LHC), which will operate at the Terascale,

is to discover the Higgs. The two multipurpose experiments ATLAS [2] and CMS [3] will

discover the Higgs, if it exists, after 2-3 years of operation and if the Higgs does not exist, LHCshould see spectacular new effects. Higgs boson searches in various decay channels [4] make

CMS a discovery experiment. Beyond the Higgs Particle, there could be supersymmetry: a new

symmetry in Nature which may provide us with candidate particles for Dark Matter which couldbe produced for the first time in the laboratory when the LHC will be operational.

Supersymmetric particles decay and produce a cascade of jets, leptons and missing (transverse)

energy and very clear signatures are expected in CMS and ATLAS. If extra space dimensionsexist, gravity force becomes strong and main detection modes at the experiments are a large

missing (transverse) energy and resonance production [5]. The LHC can detect extra dimensions

for scales up to 5 to 9 TeV [6].

Black Holes are a direct prediction of Einsteins general theory on relativity. If the Planck scale

is in ~TeV region we can expect Quantum Black Hole production opens the exciting perspective

to study Quantum Gravity in the laboratory [7]. Quantum Black Holes are harmless for theenvironment as they decay within less than 10-27seconds [7]. Other exotica includes Little Higgs,

ZZ/WW resonances, Technicolor, Split Susy, New Gauge Bosons and Hidden Valleys [7]. There

will definitely be surprises and investigations into the unknown at LHC.

The physics program at LHC will discover or exclude the Higgs in the mass region up to 1 TeV,measure its properties, discover supersymmetric particles, if they exist, up to 2-3 TeV and

discover extra space dimensions, if these are on the TeV scale, and black holes. CP violation in

the B sector and B physics will be studied in the experiment called LHCb [8]. Precision

measurements of the top mass, W masss, anomalous couplings. heavy ion collisions, search forquark gluon plasma QCD and diffractive (forward) physics in a new regime are some of the

other items on the menu of LHC physics.

Nevertheless, the physicist gold is the Higgs which is also the main goal for LHC, see Figure [1].

On the other hand, from extensive simulations one learns that a glimpse of discovery physics

namely, supersymmetry at LHC, Squark and gluino masses up to about 2 TeVwith 10 fb-1, 2.5

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Fig. 2 Detecting supersymmetry in the CMS experiment, one can see the two jets with energy

reconstructed in the electromagnetic calorimeter.

All signatures require an excellent understanding of our detector and build on the measurements

of the Standard Model processes and signatures of leptons, jets and missing energy.

2. The Large Hadron ColliderLimits on circular machines are given by dipole magnet strength and synchrotron radiation, forproton and electron colliders and have been tackled by using superconducting magnets and RF

power respectively. In table [1] are given details of high energy collider machines.

Machine Location Year Beams Energy

(GeV)

Luminosity

(cm-2s-1)

SPPS CERN 1981 Pp 630-900 6.10

30

Tevatron

FNAL 1987 Pp 1800-2000 1031

-1032

SLC SLAC 1989 e+e- 90 1030LEP CERN 1989 e+e- 90-200 10

31-10

32

HERA DESY 1992 Ep 300 1031

-1032

RHIC BNL 2000 pp (AA) 200-500 1032

LHC CERN 2009 pp (AA) 10-14

TeV 1033

-1034

Table 1. Recent High Energy Colliders

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The LHC surpasses existing accelerators and colliders in several aspects, the major ones beingthat the energy of the beam, namely 7 TeV, is achieved within all the constraints of the existing

26.7 km LEP [9] tunnel. Comparing HERA and the Tevatron with LHC there is a factor of 2 in

field and a factor of 4 in size, while a factor of 100 in luminosity. The combination of very highfield magnets and very high beam intensities required to attain luminosity is a great challenge.

3. The CMS ExperimentCMS is the heaviest LHC experiment; it is built on the surface, in 15 vertical slices with layers

like that of an onion and has about 100 million electronic channels in total. Each channel is

activated 40 million times per second as the LHC collision rate is 40 MHz. An on-line triggerselects events and reduces the rate from 40MHz to 100 Hz. With a length of CMS is about 25 m

and the radius approximately 10 m it weighs ~ 12500 tons. Fig [3] shows the schematic of the

experiment. At the centre of the detector is a solenoid [10] which operates at a magnetic field of4 T and houses the inner tracking detectors namely finely segmented pixels and silicon strips

which track the particle trajectories and measure their momentum. The inner detectors and

electronics are exposed to high doses of radiation and they are designed to withstand it. Tominimize radiation damage in silicon the whole detector is kept at -20oC.

Fig. 3. Schematic of the CMS Experiment

The Electromagnetic Calorimeter (ECAL) measures the energy of electrons, positrons & photonsfor which it employs crystals of PbWO4 (lead tungstate) which give off scintillation light

proportionately when high energy particles shower. A Russian factory in a former military

complex produced most of the crystals, while the remainder was produced in China. It tookabout ten years to grow all 78,000 crystals to stringent specifications, taking around two days to

artificially grow each one. Installed in the CMS ECAL, they measure the energy of electrons and

photons (e/). In the Hadron Calorimeter (HCAL) are installed over a million World War II brass

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shell casements from the Russian Navy in making some of its detector components. Comprising36 wedges, each of which is made of dense brass layers interspersed with plastic scintilators it

will identify muons and measure their momenta. CMS uses three types of muon detectors: drift

tubes (DT), cathode strip chambers (CSC) and resistive plate chambers (RPC). The totalsensitive area of detectors is about 6000m2.

4. CMS Detector CommissioningIn preparation for the Sept 10th 2008 Startup of LHC, the CMS detector was commissioned and

ready to take data. The beam-pipe was installed in spring 2008 and baked out, the pixel and

Silicon strip tracker were installed, commissioned and operated. The calorimeters, both ECALand HCAL have been installed and are operational. The solenoid magnet has been tested in 2008

up to 3.8 Tesla which is the nominal field. Earlier during 2006, the magnet was tested

extensively in a test program of three months, called Magnet test and Cosmic Challenge (MTCC)[11]. The Muon systems, DT/RPC in barrel, CSC in end caps are fully operational while for the

forward RPCs commissioning is underway. The level-1 Trigger and DAQ is functional, software

and computing infrastructure is ready and final releases for data taking are organized. Insummary, CMS is prepared for physics and ready for imminent beam late in 2009.

Starting from May 2007, intermittent exercises of 3-10 days have been devoted to global

commissioning exercises with installed detectors and electronics underground, using final powerand cooling infrastructure in the underground experimental cavern (UXC55) and the service

cavern (USC55). The incremental goals from one exerice to the next focus on increased

complexity and larger scale. The frequency of the runs increased towards LHC start-up, whereCMS eventually became a 24/7 running experiment ready for beam.

During August last year, the 4th exercise named CRUZET (Cosmics Run at Zero Tesla) took

place. This was the first Global run with the final CMS configuration during which data 38

million cosmic triggers were logged. In total of ~ 300 million cosmic triggers were recorded at0T. The next exercise was that called CRAFT (Cosmic Run at Four Tesla), which was a global

run with the magnet operational at 3.8 Tesla, during which another 300 million cosmic triggers

were recorded.

During these exercises, most detectors were operational and muon signals were tracked through

the muon system, strip tracker (and pixels when cosmic muon close to beam pipe), ECAL and

HCAL. This required synchronization of all electronic signals and global track fitting used foralignment and detector performance studies. For the Silicon tracker, good Landau shapes were

observed in all sub-detectors and signal to noise 20-30 was measured [12]. For the pixel detector

both the barrel and forward were operational with a good efficiency and noise performance [13].

When the LHC beam was switched on, the first splash events with a closed Collimator and

~2.109protons on collimator, 150 m upstream of CMS were observed by CMS as shown in Fig.

[4].

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Fig.4 First beam splash events recorded in the CMS detector at LHC start up Sept. 10, 2008

5. Conclusions and OutlookThere have been long deliberations to plan what to extract from the first few pb

-1at LHC. The

standard model will be rediscovered and precision measurements will be done as the event rates

for SM processes are large. The rates for production of Ws and Zs are also large and will be

used for precision calibrations, understand backgrounds in search for new physics, and precision

measurements and concentrate on data driven methods for determining backgrounds andunderstanding the detector.

The first particle bunches injected in the LHC accelerator and the signals have been recorded bythe CMS detector demonstrating that this experiment is ready for data taking at LHC start uplater this year.

6. References1. P.W. Higgs, Phys. Lett. 12 (1964) 132; F. Englert and R. Brout, Phys. Rev. Lett. 13

(1964) 3212. ATLAS Collaboration Technical Proposal 1996 CERN/LHCC/96-413. CMS Collaboration Technical Proposal 1994 CERN/LHCC 94-384. S. Basu, These proceedings5. CMS Collaboration, G.L. Bayatian et al, J.Phys.G34:995-1579,2007.6. L. Vacavant, I. Hinchliffe J.Phys.G27:1839-1850,2001.7. Steven B. Giddings, Scott D. Thomas, Phys.Rev.D65:056010,2002, e-Print: hep-

ph/0106219, Savas Dimopoulos, Greg L. Landsberg, Phys.Rev.Lett.87:161602,2001,e-

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8. J.-J. Blaising, A. De Roeck, J. Ellis, F. Gianotti, P. Janot, G. Rolandi, and D. Schlatter,Potential LHC Contributions to Europe's Future Strategy at the High Energy Frontier; J.

Ellis, arXiv:hep-ph/0611237.9. The LEP Accelerator at CERN10.

Compact Muon Solenoid, IEEE Transactions onApplied Superconductivity, June 2006Volume: 16, Issue: 2, 517-520

11.The CMS Magnet Test and Cosmic Challengehttp://cerncourier.com/cws/article/cern/29836

12.CRAFT exercise during October-November 200813.Beam splash events at the CMS experiment at LHC start up Sept. 10, 2008.

http://ieeexplore.ieee.org/xpl/RecentIssue.jsp?punumber=77http://ieeexplore.ieee.org/xpl/tocresult.jsp?isnumber=34433&isYear=2006http://cerncourier.com/cws/article/cern/29836http://cerncourier.com/cws/article/cern/29836http://ieeexplore.ieee.org/xpl/tocresult.jsp?isnumber=34433&isYear=2006http://ieeexplore.ieee.org/xpl/RecentIssue.jsp?punumber=77