The CMS detector is an excellent tool for measuring high mass and low-x phenomena in heavy-ion collisions. Its exceptional acceptance and resolution combined with a fast and sophisticated trigger offer the potential for unexpected discoveries

Veres, Gabor I

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Available on CMS information server CMS CR 2006/032CMS Conference Report11 June 2006Heavy Ion Physics at the LHC with CMSGa´bor I. Veresa)Massachussetts Institute of Technology, Cambridge, MA USACMS collaborationAbstractThe CMS detector is an excellent tool for measuring high mass and low-x phenomena in heavy-ioncollisions. Its exceptional acceptance and resolution combined with a fast and sophisticated triggeroffer the potential for unexpected discoveries.Presented at Strangeness in Quark Matter 2006, UCLA Los Angeles, March 26, 2006a) Present address: Eo¨tvo¨s Lora´nd University, Deptartment of Atomic Physics, Budapest H-1117, HungaryFigure 1: Schematic transverse slice of the detector with n, γ, e, µ+ and pi+ tracks.1 IntroductionAt LHC it will be possible to explore strongly interacting matter at higher densities, higher temperatures and longerlifetimes than ever before. The cross sections of many hard probes, such as flavorless mesons with hidden charmand bottom (J/Ψ and Υ) as well as those with open flavor (D and B mesons), energetic photons, electroweak gaugebosons (W± and Z0), and high-pT jets are too small to be easily accessible experimentally at the √sNN = 0.2 TeVcollision energy at RHIC. However, at the LHC Pb+Pb energy of 5.5 TeV [1] these cross sections will be orders ofmagnitude larger. Partons with high transverse momentum are predicted to suffer radiative and collisional energyloss in the plasma [2], suppressing the yield of jets found with high transverse energy in a heavy-ion collisioncompared to a p+p collision. The measured quarkonia yields depend on the properties of the high energy densitypartonic matter created in the collision. The abundance and variety of these hard probes at the LHC makes thestudy of partonic energy loss possible. The increase of collision energy also extends the kinematical region studiedto low Bjorken-x at small scattering angles, even at y = 0. At y = 6 CMS can sample 400 times smaller x valuesthan RHIC. The initial gluon density (and also the measured hadron abundances) will be governed by saturationeffects in this low-x region, described in the framework of the Color Glass Condensate [3], a new region to beexplored in the QCD phase diagram.Analysis of the simulated data show that the CMS detector is a powerful tool for studying the following aspectsof heavy-ion physics: charged particle multiplicity (event-by-event); azimuthal asymmetry of particle production[4]; quarkonia and heavy quark production [5]; jets and individual particles at high pT ; jet fragmentation and jetshapes; correlations between jets and other jets, photons, and Z0 bosons [6]; neutral and charged energy fluctuationsto search for Centauro, DCC and strangelet states [7]; ultraperipheral collisions [4] and comparisons between p+p,p+A and A+A collisions.2 The CMS detectorThe CMS detector system was designed primarily to study new physics in p+p interactions at collision rates ofabout 1 GHz (corresponding to 1034 cm−2s−1 luminosity). The main components of CMS [8] are the trackingsystem, made of silicon pixel and strip layers; finely segmented electromagnetic and hadronic calorimeters; a largesolenoid magnet with a 4T field; and, outside the magnet coil, muon detectors and absorbers serving also as amagnet yoke, as shown in Figure 1. The readout of all the systems is fast enough to allow not only lower leveldecisions to be made, but detailed analysis of each Pb+Pb event by the high level trigger computer farm at themaximum expected Pb+Pb collision rate of about 8 kHz. The CMS detector features a unique, large geometricalacceptance of tracking and calorimetry in pseudorapidity, η, and full 2pi coverage in azimuthal angle.The CMS tracker is a cylinder of 5.5 m length and 1.1 m radius with endcaps, featuring three silicon pixel detectorlayers in the center (R ≤ 10 cm) with less than 15 µm spatial resolution and (in the barrel region) 10 silicon strip2layers surrounding the pixels with 10-60 µm resolution in Rφ. The electromagnetic calorimeter is assembled fromlead-tungstate crystals. The barrel covers |η| < 1.48 and the endcaps extend to |η| < 3. The granularity of thebarrel is uniform in the η − φ plane, corresponding to a segmentation of 1 × 1 degrees at η ≈ 0. The hadroniccalorimeter is made of brass-scintillator sandwiches, arranged in a barrel and two endcap segments. The θ × φgranularity (at η ≈ 0) is 5×5 degrees. The forward calorimeters, made of iron and quartz fibers, cover 3 < |η| < 5.Muons are identified by the muon system covering the |η| < 2.4 region, while their momentum is measured by theSi tracker.3 Heavy-ion collisions with the CMS detectorAlthough designed for p+p collisions, various capabilities of CMS makes it a very powerful heavy-ion detectorsystem. The high readout rate allows inspection of all minimum bias Pb+Pb events by the high level triggerfarm. The 4T magnetic field in a large volume and the high granularity of the silicon tracker results in excellentmomentum resolution, ∆pT /pT < 1.5% (for pT < 100 GeV/c). The resolution of the track impact parameterat the event vertex is less than 50 µm, and improves to 20 µm at high pT (above 10 GeV/c). The CMS heavy-ion group is building two Zero Degree Calorimeters for event characterization and ultra-peripheral measurements[9]. In addition, the group has proposed building the CASTOR calorimeter to cover the 5 < |η| < 7 region.The performance of the CMS detector was extensively studied in simulations of Pb+Pb events with reasonableassumptions of the charged multiplicity, azimuthal anisotropy, jet, muon and hadron spectra [10].Besides the momentum and impact parameter resolution, the algorithmic tracking efficiency in the high-multiplicityheavy-ion environment is about 80% with less than a few percent fake track rate for pT > 1 GeV/c, assuming acharged particle density of about dN/dy = 3000. Even at these high multiplicities, the pixel layers operate at lessthan a few percent occupancy. That also makes it possible to evaluate the charged particle pseudorapidity distri-bution dNch/dη in the |η| < 2.5 range using the single innermost silicon pixel layer, even in one central Pb+Pbevent.The quarkonium states are reconstructed from decays to muon pairs. The muons are efficiently detected andidentified in the muon system above pT > 3.5 GeV/c. Muon background sources from K, pi and heavy quarkdecays are included. After one month Pb+Pb run, the estimated number of reconstructed J/Ψ, Υ and Υ′ states isin the 104 range, allowing measurements as a function of rapidity and pT . With the excellent momentum resolutionof the CMS tracker, an overall Υ mass resolution of 90 MeV/c2 can be achieved in Pb+Pb collisions in the fullmuon detector acceptance, separating the members of the Υ family, as shown on the left panel of Figure 2.The high granularity and good resolution of the CMS calorimeters help to isolate individual jets over the back-ground of the heavy-ion collision. Charged particles with pT < 0.8 GeV/c will not reach the calorimeters in the4T axial magnetic field. Early results from RHIC showed the importance of jet measurements for hadron yieldsuppression at pT > 3 GeV/c and the disappearance of back-to-back correlations of high-pT particles. These re-sults suggest significant in-medium energy loss of fast partons, absent in d+Au collisions. Due to the much largerjet production cross section, this energy loss will be accessible experimentally by observing fully formed jets. Atthe LHC collision energy, about five million dijets with ET > 100 GeV are expected to hit the calorimeter barrelover a one month Pb+Pb run [6]. Jets are reconstructed using an iterative cone algorithm modified to subtract theunderlying soft background. The jet-finding efficiency and purity is shown in the right panel of Figure 2. The lowerlimit of transverse energy needed for efficient and clean reconstruction is about 50 GeV. The energy resolution forjets with 100 GeV transverse energy at η ≈ 0 is about 16%. With the CMS tracker, the longitudinal and transversemomentum distributions of the charged component of the jets identified in the calorimeter can be studied with highprecision. These distributions are expected to be influenced by the produced medium. Thus, comparisons to p+pcollisions and the centrality dependence of jet structure measurements are highly important for the CMS heavy-ionprogram. The ability to study the balance between jets and γ or Z0 bosons emitted back-to-back is especiallyexciting, since the bosons are not affected by the strongly interacting environment.4 SummaryThe CMS detector studies the hot and dense medium created in heavy-ion collisions by measuring quarkoniumproduction, jet energy loss and structure, charged particle spectra, charged particle multiplicity event-by-eventand particle flow. These studies are possible due to the high resolution and highly segmented tracker and largeacceptance calorimetry, making the CMS detector a unique and powerful tool to study heavy-ion collisions at theLHC.3)2 (GeV/c-µ+µM8.5 9 9.5 10 10.5 112NeV/0.025 GeV/c01000200030004000--N++N ×-2 +-NFigure 2: Unlike sign muon invariant mass spectra for the Υ family (left panel), and jet reconstruction efficiencyand purity in the calorimeters as a function of transverse energy in Pb+Pb collisions (right panel).5 AcknowledgmentsParticipation in the Conference was possible due to the generous support from the organizers; from the HungarianScientific Research Fund under contracts T048898 and F049823; and from the Ja´nos Bolyai Research Grant.References[1] Brandt D 2000 LHC Project Report 450[2] Salgado C A and Wiedemann U A 2003 Phys. Rev. D 68 014008[3] McLerran L D and Venugopalan R 1994 Phys. Rev. D 49 2233; 1999 Phys. Rev. D 59 094002; Iancu E,Leonidov A, McLerran L D 2001 Nucl. Phys. A 692 583[4] Baur G et al 2000 CMS Note/2000-060[5] Bedjidian M et al 2004 CERN-2004-009 247, arXiv:hep-ph/0311048[6] Accardi A et al 2004 CERN-2004-009 123, arXiv:hep-ph/0310274[7] Angelis A L S and Panagiotou A D 1997 J. Phys. G 23 2069; http://zdc.web.cern.ch/ZDC/[8] CMS Collaboration Technical Proposal CERN/LHCC 94-38[9] Adler C et al 2001 Nucl. Instrum. Meth. A 470 488[10] Lokhtin I P and Snigirev A M 2006 Eur. Phys. J. C 45 211

Heavy Ion Physics at the LHC with CMS

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