The CMS heavy-ion program will probe QCD matter under extreme conditions. Its
capabilities for the study of global observables and soft probes are described.Comment: 6 pages, 10 figures. Presented at the International Conference on
  Strangeness in Quark Matter 2007, Levoca, Slovakia, 28 June 200

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Ferenc Siklér

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Soft probes of high density QCD physics with CMS

The CMS heavy-ion program will probe QCD matter under extreme conditions. Its capabilities for the study of global observables and soft probes are described.The CMS heavy-ion program will probe QCD matter under extreme conditions. Its capabilities for the study of global observables and soft probes are described

Sikler, Ferenc

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Available on CMS information server CMS CR 2007/054CMS Conference Report4 October 2007Soft probes of high density QCD physicswith CMSFerenc Sikle´rKFKI Research Institute for Particle and Nuclear Physics, 1121 Budapest, HungaryCMS collaborationAbstractThe CMS heavy-ion program will probe QCD matter under extreme conditions. Its capabilities forthe study of global observables and soft probes are described.Presented at the International Conference on Strangeness in Quark Matter, Levocˇa, Slovakia, 28 June 2007Total Number of Particles0 500 1000 1500Counts0200400600800Tower Threshold:E>1.4GeVallnTowers>1nTowers>2nTowers>3η-10 0 10η/dchdN01000200030004000no cutsE > 1 GeV TE   > 1 GeVFigure 1: Left: Estimated loss of low multiplicity events due to triggering requirements on the number of towersfor cuts on E in minimum bias p-p collisions at 14 TeV. Right: Pseudorapidity distribution of charged hadrons incentral Pb-Pb collisions at 5.5 TeV per nucleon from the HYDJET generator. Particle selection to mimic the level-1trigger is applied for total 〈E〉 and transverse 〈ET 〉 energy.1 IntroductionThe CMS experiment at the LHC is a general purpose detector designed to explore physics at the TeV energy scale.It has a large acceptance and hermetic coverage. The various subdetectors are: a silicon tracker with pixels andstrips (|η| < 2.4); electromagnetic (|η| < 3) and hadronic (|η| < 5) calorimeters; and muon chambers (|η| < 2.4).The acceptance is further extended with forward detectors (|η| < 6.8). CMS detects leptons and both charged andneutral hadrons. In the following the CMS soft physics capabilities are described. For a recent extensive reviewsee Ref. 1.2 Soft physics2.1 Minimum bias triggerTo maximize efficiency, it is desirable to trigger on regions with a large number of produced particles. The level-1 trigger read-out will utilize hadronic cells, towers. In order to reject non-collision events, such as those due tobeam-gas collisions, the interaction trigger could require a coincidence of signals from both hadronic forward (HF)calorimeters. The rejection of such spurious events would not be achievable using only the central calorimeter asa trigger. At the same time, this choice would reduce the efficiency for single diffractive processes.The p-p trigger will be based on counting towers with energy above the 1.4 GeV threshold, in both forward hadroniccalorimeters (3 < |η| < 5). A minimal number of hits (1, 2 or 3) will be required on one or on both sides. Thetrigger efficiency for different total number of particles using several symmetric settings is shown in Fig. 1-left. Incase of higher luminosity, events with random trigger could also be taken, because overlapping events can also bereconstructed.The Pb-Pb trigger will be similar since there are many particles produced in the region of the forward calorimeters(Fig. 1-right). Although triggering will be possible for the most central collisions with transverse energy cuts(ET ), more peripheral collisions with lower multiplicity will suffer from inefficiency. For total energy cuts (E),only events with b > 12 fm suffer from inefficiency, relative to the p-p baseline.2.2 Centrality determinationIn CMS, a simple method of determining the impact parameter on an event-by-event basis is to use the transverseenergy deposited in the calorimeters, which decreases strongly from central to peripheral collisions. Due to itsrelatively low initial parton density, the very forward rapidity region covered by the HF and CASTOR calorimeters,|η| > 3, is expected to be nearly free of final-state rescattering compared to the central rapidity region. Thereforethe energy deposition is determined mainly by the initial nuclear geometry of the collision rather than by final-state2b, fm0 2 4 6 8 10 12 14, GeVTE020040060080010001200η-2 0 2η/dchdN010002000300040005000Simulated Primary TracksReconstructedSystematic Uncertainty (15%)Figure 2: Left: Correlation between the impact parameter, b, and the transverse energy, ET, deposited in theforward rapidities, 5.3 < |η| < 6.7, covered by the CASTOR calorimeter, as simulated with HIJING for 3000minimum bias Pb-Pb collisions. Right: Comparison of the original distribution of primary simulated tracks (largepoints) to the estimate obtained from the reconstructed hits in the innermost pixel layer (smaller points with statis-tical error bars).dynamics (Fig. 2-left). Using the forward spectator neutron energy measured in both zero degree calorimeters canimprove the experimental impact parameter resolution of a few tenths of a fermi.2.3 Charged particle multiplicityIt is possible to measure the rapidity density and multiplicity of produced charged particles event-by-event. Thetechnique is based on the relation between the pseudorapidity distribution of reconstructed hit clusters in the inner-most layer of the pixel tracker and that of charged particle tracks originating from the primary vertex. Correctionsare needed for hits from non-primary sources: looping particles in magnetic field, secondaries and noise. Becausethe amount of energy deposited is proportional to the length traversed in the silicon, charged particle tracks withlonger path lengths will deposit more energy. This observation is a powerful method for eliminating most non-primary hits. A cut in cosh η selects only hits where the energy deposition is consistent with particles emanatingfrom the true collision vertex. Based on previous measurements and recent studies, an accuracy of about 2% isexpected with systematic errors below 10% (Fig. 2-right).2.4 Low pT trackingIn CMS, the measurement of charged particle trajectories is achieved primarily using the silicon tracker with bothpixels and strips, embedded in a 4 T magnetic field. The high granularity silicon pixel tracker consists of threebarrel layers (at about 4, 7 and 11 cm radius) and two endcap disks. There are about 66 million pixels with anarea of 100 × 150 µm2. The strip part is a combination of single- and double-sided layers with ten barrel and nineforward layers on each side (9.3 million channels). The reconstruction capabilities at lower pT are limited by thehigh magnetic field and effects of the detector material. In addition, in central Pb-Pb collisions the high occupancyof the silicon strips makes the inclusion of these strips in charged particle tracking difficult.The track finding procedure, modified for low pT , starts by pairing two hits from different layers. During thesearch for the third hit, the following requirements must be fulfilled: the track must come from the cylinder oforigin (given by its radius, half-length and position along the beam-line); the pT of the track must be above theminimal value pT,min; and the track must be able to reach the layer where the third hit may be located. In thesmall volume of the pixel detector the magnetic field is practically constant and the charged particles propagate onhelices. The projection of a helix or a cylinder onto the transverse plane is a circle. Each requirement defines aregion of allowed track trajectories. They are enclosed by a pair of limiting circles which can be constructed usingsimple geometrical transformations. A third hit candidate is accepted if its position is within a region which takesinto account the expected multiple scattering.3 0 0.2 0.4 0.6 0.8 1 0  0.2  0.4  0.6  0.8  1AcceptancepT [GeV/c]pionkaonprot 0 0.05 0.1 0.15 0.2 0.25 0  0.2  0.4  0.6  0.8  1Fake ratepT [GeV/c]single min. bias p+plow-lumi min. bias p+phigh-lumi min. bias p+psingle medium Pb+Pbsingle central Pb+PbFigure 3: Left: Acceptance of the track reconstruction algorithm as a function of pT for tracks with |η| < 1. Valuesare given separately for pions (circles), kaons (triangles) and (anti)protons (squares). Right: Reconstruction rate offake tracks as a function of pT , for tracks with |η| < 1, for single, low luminosity and high luminosity minimumbias p-p events and for central and mid-central Pb-Pb collisions. 0 50 100 150 200 250 300 350 400 450 500 0.2  0.3  0.4  0.5  0.6  0.7  0.8Mpi+ pi− [GeV/c2]M = 0.496 GeV/c2σ = 0.016 GeV/c2KS0 0 5 10 15 20 25 30 35 40 45 1  1.05  1.1  1.15  1.2  1.25  1.3  1.35  1.4Mppi [GeV/c2]M = 1.114 GeV/c2σ = 0.006 GeV/c2Λ−ΛFigure 4: Left: Invariant mass distribution of reconstructed K0S → pi+pi− in single minimum bias p-p collisions.The mass distribution of the background is indicated with a dashed histogram. Right: Invariant mass distributionof reconstructed Λ → p + pi+ and Λ → p + pi− in single minimum bias p-p collisions with dE/dx selection forthe secondary proton or antiproton.While high pT tracks are relatively clean, uncorrelated hit clusters can often be combined to form fake low pTtracks. A hit cluster contains more information than just its position. Its geometrical shape depends on the angleof incidence of the particle: large angles will result in longer clusters. We can thus check whether the measuredshape of the cluster is compatible with the predicted angle of incidence of the track: if any of the hits in the tripletis not compatible, the triplet is removed from the list of track candidates.Using this modified pixel hit triplet finding algorithm, charged particles down to very low pT can be reconstructed(Fig. 3-left). The acceptance and efficiency is at 80–90%, while the pT resolution is about 6%. At the same timea low fake track rate is achieved thanks to the geometrical shape of the hit cluster. It is below 10% even in centralPb-Pb collisions for pT > 0.4 GeV/c (Fig. 3-right). It is possible to study identified particle spectra (down to pTof 0.1− 0.3 GeV/c) and yields, multiplicity distributions and correlations.4E_HE_beEntries    2.25267e+07Mean   0.001429RMS     1.823, radϕ-3 -2 -1 0 1 2 3, GeVϕdE/d020406080100120140160180200barrel+endcapendcapbarrelEnergy HCAL+ECAL (b+e)b, fm2 4 6 8 10 12), rad0Ψ(σ00.10.20.30.40.50.6Figure 5: Left: Azimuthal dependence of the total energy deposition in the CMS calorimeters in Pb-Pb col-lisions at b = 9 fm (solid histogram). The barrel and endcap regions are shown separately. Right: Eventplane resolution, σ(Ψ0), as a function of impact parameter in Pb-Pb collisions with total particle multiplicitiesN0 (b = 0 fm) = 58 000 (solid histogram) and 84 000 (dashed histogram).2.5 Identified particlesParticles can be identified by decay topology and by energy loss measurements in the silicon (in the range p < 1−2GeV/c, benefitting from analogue readout). Weakly-decaying neutral particles have a sizeable probability to decayfar from the primary event vertex. Likewise, the silicon detectors can be used to reconstruct photons throughtheir conversion to e+e− pairs in the material of the beam-pipe, silicon pixels and supports. The search for V0candidates is reduced to determining the closest distance between two track helices. The K0S is reconstructed witha resolution of 16 MeV/c2 and an average mass of 0.496 GeV/c2 (Fig. 4-left). The Λ and Λ peaks are located at1.114 GeV/c2 with a 6 MeV/c2 resolution (Fig. 4-right). The proton signal was strongly enhanced by a cut onthe truncated mean of their dE/dx, removing almost all the background. In the case of single collisions or lowluminosity p-p running, the resonances can be exclusively identified. For high luminosity p-p running or Pb-Pbcollisions, the inclusive yield can still be extracted. Multi-strange baryons (Ξ−, Ω−), open charm (D0, D∗+) andopen beauty (B→ J/ψ +K) can also be studied.2.6 Elliptic flowThe reaction plane in Pb-Pb collisions can be reconstructed using electromagnetic and hadronic calorimeters, byextracting harmonic angular coefficients of the energy deposition distribution dE/dϕ (Fig. 5-left). The calculatedevent plane resolution as a function of impact parameter is shown in Fig. 5-right. The interplay of multiplicity andanisotropic flow in opposite centrality directions gives the best resolution in semi-central collisions. The resultswill improve by adding tracker information and using forward detectors such as the zero degree calorimeter. Theazimuthal anisotropy coefficient can also be determined using the two-particle azimuthal correlator, without theevent plane angle. The advantage of this method is that it automatically corrects for the detector anisotropies.3 SummaryThe CMS detector has a good capability for global event characterization and physics with soft probes. Theperformance in minimum bias triggering, centrality determination, measurement of charged particle multiplicity,low pT tracking, particle identification and measurement of elliptic flow has been shown.AcknowledgmentThe author wishes to thank to the Hungarian Scientific Research Fund (T 048898).5References[1] CMS Collaboration, “CMS Physics: Technical Design Report vol. 2. Addendum on High Density QCD withHeavy Ions,” CERN-LHCC-2007-009. To appear in J Phys G.6

Soft probes of high density QCD physics with CMS

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