aa r X i v : . [ nu c l - e x ] S e p Measurements of high- p T probes in heavy ion collisions at CMS G. I. Veres a on behalf of the CMS collaboration a CERN, Geneva, Switzerland
Abstract
The capabilities of the CMS detector at the LHC will be described for measuring high- p T hadrons, photons and jets in heavy ion collisions. Detailed simulations of various studies plannedwith the CMS apparatus, including charged particle tracking, jet reconstruction using calorime-try, dimuon and isolated photon detection and the measurement of in-medium fragmentationfunctions using high- p T photon-jet correlations will be discussed.High- p T processes have proven to be an important tool in investigating the hot, dense mattercreated in the collision of relativistic heavy ions. The large suppression observed for high- p T hadronic yields in central heavy ion collisions relative to the binary scaling of p + p collisions isevidence of large partonic energy loss in the medium. At RHIC energies, several complicationshave arisen in the interpretation of the high- p T phenomena. The high- p T hadrons (and jets) aresurface-biased if the created medium is opaque, masking the real amount of medium-inducedparton energy loss [1]. Fragility of some observables (like nuclear modification factors) hinderstheir discriminative power between models and model parameters [2].In the new energy frontier at the LHC a larger p T region, fully developed jets, and severalnew observables will be accessible. New ways that will be available to study energy loss include γ -jet, Z -jet, dijet correlations, and jet fragmentation function measurements. Since the photon(or Z ) escapes from the medium with little interaction, it gives a calibrated probe of the originalparton energy. The first heavy ion run at the LHC is scheduled for late 2010 at √ s NN ≈ p T phenomena in heavy ion collisions. The highly segmented electromagnetic andhadronic calorimeters have a large longitudinal coverage up to | η | < .
6, including the CASTORcalorimeter [4], and the Zero Degree Calorimeter will detect spectator neutrons at | η | > .
3. Themuon detectors will be used to reconstruct Z , J /ψ , Υ particles, with coverage of | η | < .
4. Thesilicon tracker system can reconstruct charged tracks with good e ffi ciency and purity. The siliconpixel layers have less than 2% hit occupancy even in heavy ion collisions. The high level triggerwill be able to inspect each event before the trigger decision, including complicated on-linereconstruction algorithms [5]. Some more details on these capabilities will be given below.The tracking performance of the CMS detector was studied under a conservative assumptionon the charged particle multiplicity for central Pb + Pb events, at dN ch / dy | y = = ffi ciency and less than 5% fake rate in the 1 < p T <
30 GeV / c range for charged particles. The p T resolution was found to be 1-3% depending on η and p T . The left panel of Fig. 1 shows the expected transverse impact parameter resolution ofcharged tracks that will become relevant for displaced vertex reconstruction [6]. The p T reachof the tracking in CMS was extended down to p T ≈
200 MeV / c, and particle identificationcapability based on the specific energy loss (dE / dx) in the silicon tracker was demonstrated [7]. Preprint submitted to Nuclear Physics A November 21, 2018 [GeV/c] T p [ c m ] t i p s <0.5 h -0.5 < < 2.5 h
2 <
Figure 1: Left: p T dependence of the transverse impact parameter resolution achieved in heavy ion events (with dN ch / dy | y = = p T distributions of the pions, kaons and protons produced in 125 central Pb + PbHYDJET events at 5.5 TeV / nucleon. The reconstructed and simulated p T spectra of identified hadrons are also shown in Fig. 1 (right).The CMS detector features excellent dimuon reconstruction capabilities, due to the high,4 Tesla magnetic field and accurate tracking and muon detectors. Figure 2 shows the recon-structed dimuon mass spectra in the J / Ψ (left) and Υ (right) dimuon mass regions for simulatedcentral Pb + Pb events with dN ch / d η | η = = . nb − integratedluminosity. The signal to background ratio is about 5 for the J / Ψ and 1 for the Υ where bothmuons have | η | < .
8. A mass resolution of 54 MeV / c can be achieved for the Υ states [7].High energy jets can be reconstructed in the CMS calorimeters using the iterative cone algo-rithm with event-by-event background subtraction. Although the determination of the absolutejet energy scale is di ffi cult, the jet finding e ffi ciency is more than 50% at E T =
50 GeV and closeto 100% above 70 GeV [8]. ) (GeV/c - m + m M E v en t s / . G e V / c +- N -- N ++ N · y J/ ’ y |<0.8 h both muons with | ) (GeV/c - m + m M E v en t s / . G e V / c +- N -- N ++ N · U ’ U ’’ U = 54 MeV/c s |<0.8 h both muons with | Figure 2: Reconstructed invariant mass spectra of opposite-sign and like-sign muon pairs from simulated central Pb + Pbevents with dN ch / d η | η = = J / Ψ (left) and Υ (right) mass regions, where both muons have | η | < . The E T resolution is about 16% at E T =
100 GeV with a background of dN ch / dy | y = = -3 -2 -1 0 1 2 3 f -3-2-10123 [ G e V ] T E Ecal+Hcal awayT )/E awayT -E g T (E -1 -0.5 0 0.5 1 E n t r i e s PYTHIAp+p (cid:10) 5.5 TeV > 70 GeV T p > 70 GeV T g E > 70 GeV
Taway
E > 3 ap fD Figure 3: Left: energy response on the η − φ plane for a simulated p + p event producing a γ -jet final state embedded ina Pb + Pb event at √ s NN = . φ . Right: the balance between the gammaand the away side parton E T using the PYTHIA event generator. The typical angular resolution of the jet axis obtained is σ φ = .
03 and σ η = .
02. By applyinga series of jet triggers in a nominal 0 . nb − run, the statistical p T reach of the jet and chargedhadron spectrum measurement is 500 and 300 GeV / c, respectively [5, 9]. The high statistics ofreconstructible jets opens the way to more detailed jet quenching studies.The high resolution electromagnetic calorimeter with large coverage and segmentation makesit possible to analyze γ -jet events, and use the measured γ energy to calibrate the jet energyscale, coincidentally measuring the properties of the quenched away side jet. Figure 3 shows asimulated γ -jet event as seen by the calorimeters, and the relatively tight correlation between the E T of the γ and that of the away side parton at the event generator level [10]. Isolated photons areselected for this analysis using an isolation cut based on a combination of various cluster shapevariables, suppressing π -s produced in jets usually associated with large hadronic activity. [GeV] T E
100 200 300 E n t r i e s pe r e v en t / G e V -5 -4 -3 -2 -1 Non-isolated particlesIsolated photonsIsolated hadrons
CMS Preliminary ) T /p T =ln(E x x d N / d j e t s / N -3 -2 -1 CMS Preliminary
Quenched Fragmentation FunctionUnderlying Event ContributionPb+Pb, 0-10% central > 70GeV
Clus.T > 1GeV/c, E T Track p
Figure 4: Left: the E T distribution for isolated photons, hadrons, and non-isolated particles that pass the photon isola-tion cut, according to simulation. Right: fragmentation function of quenched high energy jets (symbols) including thecontribution of the underlying event (histogram) at low p T . A signal to background ratio of 4.5 for isolated photons is achieved, as shown on the left3 /E T z=p d N / d z j e t s / N -3 -2 -1 CMS Preliminary > 70GeV
Clus.T > 1GeV/c, E T Track pUnderlying event subtractedQuenched Fragmentation FunctionMC Truth ) T /p T =ln(E x F r ag m en t a t i on F un c t i on R a t i o CMS Preliminary
Quenched / Unquenched > 70GeV g T EReconstructed Pb+Pb / MC p+pMC truth p+p
Figure 5: Left: reconstructed (symbols) and truth (histogram) fragmentation function of quenched jets with E γ T >
70 GeV in Pb + Pb collisions at √ s NN = . panel of Fig. 4. As a next step, the away-side jet was reconstructed, on the opposite side of the γ ,requiring E T >
30 GeV. The fragmentation function, dN / d ξ where ξ = ln ( E T / p T ), was obtainedfrom the charged particle tracks that were reconstructed within the jet cone of radius R = nb − heavy ion run at √ s NN = . E γ T >
70 GeV and 1200 events with E γ T >