The heavy-ion program of the CMS experiment

Abstract We present the capabilities of the CMS experiment to explore the heavy-ion physics program of the CERN Large Hadron Collider (LHC). Collisions of lead nuclei at energies up to s N N = 5.5 TeV will probe quark and gluon matter at unprecedented values of energy density. The prime goal of this research is to study the fundamental theory of the strong interaction (QCD) in extreme conditions of temperature, density and parton momentum fraction. We give an overview of the potential of CMS to carry out a full set of representative Pb-Pb measurements both in “soft” and “hard” regimes. Measurements include “bulk” observables — charged hadron multiplicity, low- p T inclusive identified hadron spectra and elliptic flow — which provide information on the collective properties of the system; as well as perturbative processes — such as quarkonia, heavy quarks, jets, γ–jet and high p T hadrons — which yield “tomographic” information on the hottest and densest phases of the reaction. In addition, reference measurements that have been performed on early p+p collision data will be reviewed.


Introduction
The study of the strong interaction in extreme temperature and density conditions has been the driving force for experiments from the Bevalac to the the Large Hadron Collider. The RHIC experiments have produced the evidence that at the energy in c.m.s. √ s = 63-200 GeV per nucleon pair a strongly interacting quark gluon liquid is produced [1]. The scaling of the elliptic flow with quark number, the suppression of fast quarks in the medium are clear signals of this. At both SPS and RHIC energies the suppression of the J/ψ resonance suggests that a very high temperatures system was created [2,3,4]. In addition there is evidence that at small parton momentum fraction the initial state of the nuclei may be a sheet of gluons, the color glass condensate [5,6].
The start of the heavy-ion program at the LHC with lead-lead collisions at the energy in c.m.s. √ s = 2.76 TeV per nucleon pair is planned for the end of 2010. It seems that this is the last publication about the CMS heavyions program without the real data. In 2013-14 LHC plans to collide Pb nuclei at √ s = 5.5 TeV which is 28 times higher than the highest energy available at the RHIC. We expect the initial state to be dominated by saturated parton distribution with relevant range of parton momentum fraction x as low as 10 −5 and a characteristic saturation momentum, Q 2 s ≃ 5-10 GeV 2 /c 2 [7]. The collisions should produce the hard probes such as jets, high-p T hadrons, heavy-quarks, quarkonia and large yields of the weakly interacting perturbative probes (direct photons, dileptons, Z 0 and W ± bosons) [8].

The CMS detector
The central feature of the CMS apparatus [9] is a 3.8 T superconducting solenoid, of 6 m internal diameter. Within the field volume there are the silicon pixel and strip tracker, the crystal electromagnetic calorimeter (ECAL) and the brass-scintillator hadronic calorimeter (HCAL). Muons are measured in gas chambers embedded in the iron return yoke. CMS has extensive forward calorimetry, HF (3 < |η| < 5.2), CASTOR (5.3 < η < 6.6) and Zero Degree (|η| > 8.3) calorimeters.
Near mid-rapidity (|η| < 2.4) charged particles are tracked by three layers of silicon pixel detectors, made of 66 million 100 × 150 µm 2 pixels, followed by ten microstrip layers, with strips of pitch between 80 and 180 µm. The silicon tracker provides the vertex position with ∼ 15 µm accuracy. The good momentum resolution of the tracker allows us to clearly resolve the Υ-family. The ECAL has an energy resolution of better than 0.5 %. The HCAL combined with the ECAL measures jets with jet energy resolution ∆E/E ≈ 100 %/ √ E ⊕ 5 %. The calorimeter cells are grouped in projective towers, of granularity ∆η × ∆φ = 0.087 × 0.087 at central rapidities and 0.175 × 0.175 at forward rapidities. Muons are measured in the pseudorapidity window |η| < 2.4, with detection planes made of three technologies: drift tubes, cathode strip chambers, and resistive plate chambers. Matching the muons to the tracks measured in the tracker results in a transverse momentum p T resolution 1-5 % for up to 1 TeV/c.

Bulk ("hydro") measurements in A-A collisions
The charged particle multiplicity per unit of rapidity at mid-rapidity is related to the entropy density in the collisions and fixes the global properties of the produced medium. The unexpectedly low multiplicities seen at RHIC have lent support to the color glass picture and it will be interesting to see if this model works at LHC energies. CMS is planning to make a first day measurement of the charged particle multiplicities by two methods: 1) hit counting in the pixels using a energy loss dE/dx cut and 2) tracklets with a vertex constraint. Using the CMS detector we can accurately reconstruct particle pseudorapidity distribution dN/dη using the hit counting technique.
Measurements of hadron momentum spectra and ratios at low p T are an important tool to determine the amount of collective radial flow and the thermal and chemical conditions of the system at freeze-out. CMS has developed a special low p T tracking algorithm based on the pixels. This allows us to identify particles by comparing dE/dx and the momentum of track. Inclusive hadron spectra can be measured from p ≃100 MeV/c up to p ≃1 GeV/c for pions and kaons and up to p ≃ 2 GeV/c for protons.
Unless the two lead nuclei collide head on the overlap region will have an elliptical shape. For a liquid, this initial space anisotropy is translated into a final elliptical asymmetry in momentum space. However for a gas any anisotropy should be much weaker. The elliptic flow parameter, v 2 is the strength of the second harmonic of the the azimuthal distribution of hadrons with respect to the reaction plane. Comparing the experimental v 2 with hydrodynamical calculations will show us how close the matter is fully thermalized perfect fluid close. There is a number of predictions for elliptic flow in heavy-ion collisions at the LHC energy [10,11,12]; compared with the RHIC energies the decrease, increase or saturation will be possible. CMS will measure v 2 using both tracker and calorimeters [13]. The differential p T and η dependencies of the elliptic flow calculated using method with event plane angle determination in Pb-Pb collisions for impact parameter b = 9 fm are shown in Fig. 1 and Fig. 2, respectively. HYDJET event generator [14] was used with full GEANT simulation of the CMS detector responses.

Hard ("tomographic") probes of dense QCD matter
Hard probes, i.e. objects with large transverse momentum and/or high mass are of crucial importance for several reasons: (i) they originate from parton scattering with large momentum transfer Q 2 and are directly coupled to the fundamental QCD degrees of freedom; (ii) their production timescale is short, allowing them to propagate and potentially be affected by the medium; (iii) their cross-sections can be theoretically predicted with pQCD.
One of the major discoveries at the RHIC is the suppression of high p T hadrons compared to what would be expected the corresponding number of binary pp collisions. This effect is known as jet quenching. The nuclear modification factor, R AA (p T ) is defined by the ratio of particle yield in heavy-ion collisions to the binary collisions scaled yield in pp collisions. It provides a convenient first measure of the strength of jet quenching. In a typical run without the high level trigger we expect to be able to measure R AA out to 150 GeV/c, see Fig. 3. Using the HLT we can double our p T reach, see Fig. 4.
New hard probes are available at the LHC, such as boson-tagged (γ, Z 0 ) jet production. We have developed algorithms to reconstruct jets, high-p T tracks and photons in the heavy ion environment. Photon-jet events are a convenient way to study jet fragmentation since the photon does not loose energy as it propagates through the partonic medium. The ratio of the reconstructed quenched fragmentation function to the unquenched one can be measured accurately for tracks with p T values between 1 and 7 × 10 −3 times the momentum of the photon.
The suppression of heavy-quark bound states in high energy A-A collisions was one of the first proposed signatures for a deconfined medium of quarks and gluons to be actually observed in experiment [2,4]. At the LHC the Υ family will be available with large statistics for the first time. Unlike the J/ψ family the botomonium one will be less affected by the recombination process. Our simulated dimuon spectra for the J/ψ and Υ families are shown in Figs

Summary
CMS is a superb detector for measuring muons, photons, jets and charged tracks from lead-lead collisions at high rate over a very large rapidity range. In this paper we have been able to highlight some of our capabilities to use both soft and hard probes such as multiplicity, low and high p T spectra of charged particles, elliptic flow, photons, jets and quarkonia to study QCD at very high temperatures, high energy densities and also very low x. Lack of space prevents us from discussing other probes such as dijet correlations, ultra-peripheral collisions etc.