Very soon the LHC will provide beams for heavy ion collisions at 5.52
TeV/nucleon. This center-of-mass energy results in a large cross-section for
producing high-$E_T$ ($>$ 50 GeV) jets that are distinct from the soft,
underlying event. This brings with it the possibility of performing full jet
reconstruction to directly study jet energy loss in the medium produced in
heavy ion collisions. In this note, we present the current state of jet
reconstruction performance studies in heavy ion events using the ATLAS
detector. We also discuss the possibilities of energy loss measurements
available with full jet reconstruction: single jet $R_{AA}$ and di-jet and
$\gamma$-jet correlations.Comment: 3 pages, 4 figures, write up of poster presented at ICHEP 2008,
  Philadelphia, PA, July 29 - Aug. 5, 200

Grau, N.

English

arXiv

Very soon the LHC will provide beams for heavy ion collisions at 5.52 TeV/nucleon. This center-of-mass energy results in a large cross-section for producing high-$E_T$ ($&gt;$ 50 GeV) jets that are distinct from the soft, underlying event. This brings with it the possibility of performing full jet reconstruction to directly study their energy loss in the medium produced in heavy ion collisions. In this note, we present the current state of jet reconstruction performance studies in heavy ion events using the ATLAS detector. We also discuss the possibilities of energy loss measurements available with full jet reconstruction: single jet $R_{AA}$ and di-jet and $gamma$-jet correlations

Grau, N

CERN Document Server

ATL-PHYS-PROC-2008-05505November200834th International Conference on High Energy Physics, Philadelphia, 2008Elucidating Jet Energy Loss in Heavy Ion CollisionsN. Grau for the ATLAS CollaborationColumbia University, Nevis Laboratories, Irvington, NY 10533, USAVery soon the LHC will provide beams for heavy ion collisions at 5.52 TeV/nucleon. This center-of-mass energy resultsin a large cross-section for producing high-ET (> 50 GeV) jets that are distinct from the soft, underlying event. Thisbrings with it the possibility of performing full jet reconstruction to directly study jet energy loss in the mediumproduced in heavy ion collisions. In this note, we present the current state of jet reconstruction performance studiesin heavy ion events using the ATLAS detector. We also discuss the possibilities of energy loss measurements availablewith full jet reconstruction: single jet RAA and di-jet and γ-jet correlations.1. INTRODUCTIONOver the last eight years, the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory (BNL) hasbeen providing heavy ion collisions in the attempt to create the Quark-Gluon Plasma (QGP) within the laboratory.Some of the most striking data from the first few years at RHIC were results from the apparent interaction ofhard scattered partons in the medium produced at RHIC. Single high-pT particles are suppressed relative to binarycollision-scaled p + p rates[1]; di-jets, measured from pairs of high-pT hadrons, are suppressed relative to p + p andhave a strongly modified angular correlation[2]; and heavy quarks, both charm and bottom, are suppressed as well[3].These data have been interpreted as the loss of energy of a colored parton traversing a colored medium, analogousto energy loss in QED. However, many of the details underlying QCD energy loss are not well understood. On theone hand, energy loss models are able to reproduce the single particle suppression with very different assumptionson the rate of energy loss[4]. On the other, multiple high-pT particle correlations may suffer from a bias for the jetto lose little if any energy[5, 6]. This is observed in data where the measured jet properties are quite similar betweenp+ p and A+A, and they have only slightly more constraining power on models.To overcome such biases, it is necessary to move beyond multi-particle correlations and measure jets directlyin heavy ion collisions. This note highlights the current status of jet reconstruction and energy loss measurementsexpected to be made with the ATLAS detector at the Large Hadron Collider (LHC). The large acceptance and nearlyhermetic electromagnetic and hadronic calorimeters were designed for jet measurements and are uniquely suited toperform these measurements in heavy ion collisions. Such measurments will result in an increased understanding ofhow colored partons lose energy in a colored environment, a direct test of QCD.2. JET RECONSTRUCTIONUnlike e+e− annihilation or p + p(p¯) collisions, heavy ion collisions result in a large, soft, underlying event. Thecharge particle multiplicity in central (head-on) Pb+ Pb collisions is expected to be between dN/dη=1500-3000. Toput this in perspective, an average of 70-150 GeV would exist in an R =√∆φ2 +∆η2 = 0.4 radius cone from theunderlying event alone. Therefore, jet clustering algorithms must be modified to handle such large backgrounds.Such a modification is not unlike the pile-up event subtraction[7] necessary at full LHC luminosity.Both a seeded cone algorithm and the kT algorithm have been explored for use in heavy ion collisions withinATLAS[8]. For the rest of this note we will focus on seeded cone jet results. The underlying event energy is removedby the following steps. First, regions of high energy in the calorimeter are identified as possible regions with a jet.Next, the average ET from the cells are calculated as a function of η and for each longitudinal calorimeter segment,excluding the regions in the first step. This average is then subtracted from all calorimeter cells. Finally, 0.1×0.1towers in ∆η ×∆φ built from a sum of subtracted cells are used as input to an R=0.4 cone algorithm with a towerthreshold of ET > 5 GeV.134th International Conference on High Energy Physics, Philadelphia, 2008 (GeV)TE0 50 100 150 200 250 300!d TN/dE2) d TE" 1/(2evt1/N-1210-1110-1010-910-810-710-610-510-410Input SpectrumReconstructed SpectrumFake SpectrumMaximal Fake Spectrum=2700!dN/dATLAS Preliminary (GeV)TE0 50 100 150 200 250reco/input00.511.52 = 2700!dN/d = 1700!dN/d = 460!dN/dATLAS PreliminaryFigure 1: Left: Binary-scaled PYTHIA jet spectrum (blue circles) compared to the raw reconstructed spectrum (open di-amonds), the maximal fake rate (dashed line), and the residual fake rate (red squares). The raw reconstructed spectrumis uncorrected for efficiency and ET resolution.Right: Ratio of raw reconstructed spectrum to the binary scaled expectedspectrum for three different underlying event multiplicities. The 20% difference at ET > 70 GeV before corrections indicatesthe level of sensitivity expected for jet RAA measurements. The non-statistical fluctuations in the data result from combiningdata sets from different ET ranges.The left panel of Fig. 1 shows the comparison of the input, reconstructed, maximal, and final fake rate.severalspectra. The input and reconstructed distributions are from di-jet PYTHIA events embedded in unquenched HIJINGwith dN/dη=2700. The raw reconstructed spectrum is not corrected for efficiency and ET resolutions. The raw fakespectrum is evaluated by running the jet algorithm with background subtraction directly on unquenched HIJINGwith a requirement that no jets with ET >10 GeV are produced. The final fake rate is determined after a cut on theshape of the energy distribution within the jet has been made. Such a cut is also applied to the raw spectrum. For70 GeV jets, the efficiency is 70%, the energy resolution is 25%, and the fake fraction is 3%.3. MEASURING JET ENERGY LOSSOnce jets can be reliably measured, their modification due to interactions with the medium will be explored. Inthis section we discuss modification of single jet rates[9] and the increased acoplanarity of di-jets because of theincoherent multiple scattering in the medium[10].The right panel of Fig. 1 shows the ratio of raw reconstructed to input spectra. Since the raw spectrum isuncorrected for efficiency and ET resolution, this represents a worst case scenario in the measurement of the jet RAAdefined asRAA =YieldA+A〈Ncoll〉Yieldp+p (1)Such a variable will be sensitive to details of energy loss. For example, if jets were perfectly reconstructed, RAA=1.However, because collisional energy loss will impart energy to the medium, all lost energy will not be recovered andRAA <1. Further, energy loss due to radiation outside of the cone size or the jet area will cause RAA < 1 of jets.A non-perturbative energy loss scenario based on AdS/CFT arguments would result in an RAA 1[11]. Therefore,studying the jet RAA as a function of cone size, seed ET , etc. should be sensitive to the details of jet energy loss[9].Fig. 2 shows two examples of jet azimuthal decorrelations to be measured in ATLAS. The left panel shows thereconstructed di-jet and the right panel shows the reconstructed γ-jet |∆φ| distributions. In a 2 → 2 process,the produced hard scattering products are directly back-to-back, i.e. at |∆φ| = pi. Intial and final state raditionresults in a natural broadening to this distributions. Further, radiation from in-medium energy loss will also broaden234th International Conference on High Energy Physics, Philadelphia, 2008|!"Dijet |0 1 2 3|!"/d|B dNA1/N01234 > 70 GeVBT > 100 GeV, EATE Cut2without HIJING Q Cut2with HIJING Q = 2700#dN/dATLAS Preliminary|!" - jet |#0 1 2 3!"/dpair dN#1/N02468<80 GeVT 60<E#<80 GeVTJet 60<EATLAS PreliminaryFigure 2: Left: Correlations between pairs of reconstructed jets for “trigger jet” A > 100 GeV and “associated jet” B > 70GeV. Right: Correlations between reconstructed isolated photons and reconstructed jets for both the γ and jet from 60-80GeV. The clear peaks at ∆φ = pi are indicative of 2 → 2 hard scattering. Multiple scattering and incoherent energy loss isexpected to broaden these disributions[10].this distribution[10]. However, this broadening has not been measured at RHIC[12]. This could indicate that thebroadening is small or it is a consequence of the bias to little energy loss of the two-particle correlation measurements.4. CONCLUSIONATLAS expects to perform ground breaking measurements in QCD energy loss studies using fully reconstructedjets. This note highlighted the current status of cone jet reconstruction and measurements of jet rates and jetcorrelations as sensitive tests of QCD energy loss.References[1] K. Adcox et al. (PHENIX Collaboration), Phys. Rev. Lett. 88 (2002) 022301[2] A. Adare et al. (PHENIX Collaboration), Phys. Rev. C78 (2008) 014901[3] A. Adare et al. (PHENIX Collaboration), Phys. Rev. Lett. 98 (2007) 172301[4] T. Renk, arXiv:hep-ph/0608333[5] T. Renk, Phys. Rev. C74 (2006) 024903[6] A. Dainese, C. Loizides, and G. Paic Eur. Phys. J. C38 (2005) 461[7] M. Cacciari and G. P. Salam, Phys. Lett. B659 (2008) 119[8] N. Grau (ATLAS Collaboration), J. Phys. G35 (2008) 104040[9] I. Vitev, arXiv:0806.0003 [hep-ph][10] R. Baier et al., Nucl. Phys. B484 (1997) 265[11] D. E. Kharzeev, arXiv:0806.0358 [hep-ph][12] J. Adams et al. (STAR Collaboration), Phys. Rev. Lett. 97 (2006) 1623013

Elucidating Jet Energy Loss in Heavy Ion Collisions

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