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Jets in Heavy Ion Collisions at the LHC

Jets in Heavy Ion Collisions at the LHC. Andreas Morsch CERN. Outline. What are the new opportunities but also experimental challenges of jet physics on Heavy Ion Collisions ? How can jets be reconstructed in the high multiplicity heavy ion events ?

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Jets in Heavy Ion Collisions at the LHC

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  1. Jets in Heavy Ion Collisions at the LHC Andreas Morsch CERN

  2. Outline • What are the new opportunities but also experimental challenges of jet physics on Heavy Ion Collisions ? • How can jets be reconstructed in the high multiplicity heavy ion events ? • How can we observe modifications of the jet structure and use them as a tool to test the medium ?

  3. Jets in nucleus-nucleus collisions • Jets are the manifestation of high-pT partons produced in a hard collisions in the initial state of the nucleus-nucleus collision. • These partons undergo multiple interaction inside the collision region prior to fragmentation and hadronisation. • In particular they loose energy through medium induced gluon radiation and this so called “jet quenching” has been suggested to behave very differently in cold nuclear matter and in QGP. Simplistically:Jet(E)→Jet(E-DE)+ soft gluons (DE)

  4. Medium induced parton energy loss Example: BDMPS Baier, Dokshitzer, Mueller, Peigne, Schiff (1996); Zakharov (1997); Wiedemann (2000); Gyulassy, Levai, Vitev (2000); Wang ... Coherent sum over scatterings with free path length l and mean qT transfer m Medium characterized by transport coefficient: Expect large effects ! Needs large range of E to measure DE(E)

  5. Consequences for the jet structure Decrease of leading particle pT Increased mult. of low-pT Particles from radiation. Increase of pT rel. to jet-axis Energy outside jet cone Dijet energy imbalance and acoplanarity AA pp

  6. But also background from underlying event … • … and this has important consequences for • Jet identification • Jet energy reconstruction • Resolution • Bias • Low-pT background for the jet structure observables

  7. Jets at RHIC p+p @ s = 200 GeV STAR Au+Au @ sNN = 200 GeV In central Au-Au collisions standard jet reconstruction algorithms fail due to the large energy from the underlying event (125 GeV in R< 0.7) and the relatively low accessible jet energies (< 20 GeV). Use leading particles very successfully as a probe.

  8. STAR Phys. Rev. Lett. 91, 072304 (2003). Pedestal&flow subtracted RHIC: Jet studies with leading particles Suppression of inclusive hadron yield Disappearance of away-side correlations • In central Au+Au • Strong suppression of inclusive hadron yield in Au-Au collisions • Disappearance of away-side jet • No suppression in d+Au • Hence suppression is final state effect.

  9. Eskola et al., hep-ph/0406319 RAA~0.2-0.3 for broad range of Sensitivity to transport coefficient • RHIC measurements are consistent with pQCD-based energy loss simulations. However, they provide only a lower bound to the initial color charge density. Surface emission bias limits sensitivity to

  10. Bias from the production spectrum 100 GeV Jet Mean value shifts to pLeading/Eparton =0.6 • Strong bias on fragmentation function • … which we want to measure • But also low efficiency since only tail is relevant. pLeading [GeV]

  11. Advantages of reconstructed jets • Since more of the original parton energy is collected: • Reduced Surface bias • Reduced bias on parton energy • Makes measurement of the fragmentation function possible • Possibility to observe directly the quenched jet and the particles from gluon radiation. • Increases statistics at high ET • Increased sensitivity to medium parameters

  12. Jet structure observables Longitudinal Structure Transverse Structure Borghini,Wiedemann, hep-ph/0506218 Salgado, Wiedemann, Phys. Rev. Lett. 93: 042301 (2004) Sensitive to out-of-cone radiation. I. Lokhtin

  13. g Direct measurement of J. Casalderrey-Solana and XNW, arXiv:0705.1352 [hep-ph].

  14. A. Accardi et al., hep-ph/0310274 CERN TH Yellow Report Jet physics at LHC: Rates • Jet rates are high at energies at which they can be reconstructed over the large background from the underlying event. • Reach to about 200 GeV • Provides lever arm to measure the energy dependence of the medium induced energy loss • 104 jets needed to study fragmentation function in the z > 0.8 region.

  15. Jet physics at LHC: New challenges • More than one jet ET> 20 GeV per event • More than one particle pT > 7 GeV per event • 1.9 TeV in cone of R = Dh2+Df2 < 1 ! (*) • We want to measure modification of leading hadron and the hadrons from the radiated energy. Small S/B where the effect of the radiated energy should be visible: • Low z • Low jT • Large distance from the jet axis • Experiments need low- and high-pT capabilities for unbiased jet energy measurements and observation of low-pT hadrons from the gluon radiation. Unquenched Quenched (AliPythia) Quenched (Pyquen) pT < 2 GeV * For dN/dy = 5000.

  16. Jet reconstruction in Heavy Ion Collisions • How to reconstructs jets above a large fluctuation background (DEBg) ? • Restrict identification and reconstruction to domain in which Emeas >> DEBg • Cone size R < 1 • pT-cut • Also in this case there is a bias due to the input spectrum • Identified jets are on average more collimated.

  17. Jets reconstructed from charged particles: Energy contained in sub-cone R Need reduced cone sizes and transverse momentum cut ! Optimal cone size Jet Finders for AA do not work with the standard cone size used for pp (R = 0.7-1). R and pT cut have to be optimized according to the background conditions. E ~ R2 Background reduced by 0.42 = 0.16 but 88% of signal preserved.

  18. Background fluctuations • Background fluctuations limit the energy resolution. • Fluctuations caused by event-by-event variations of the impact parameter for a given centrality class. • Strong correlation between different regions in h-f plane • ~R2 • Can be eliminated using impact parameter dependent background subtraction. • Poissonian fluctuations of uncorrelated particles • DE = N[<pT>2 +DpT2] • ~R • Correlated particles from common source (low-ET jets) • ~R

  19. Jet finder in HI environment: Principle • Other algorithms have been tested successfully • FASTJET kT-algorithm (M. Cacciari, G. Salam) • Deterministic annealing (D. Perrino) • Important because they show different systematics for the background subtraction) Rc Loop1: Background estimation from cells outside jet cones Loop2: UA1 cone algorithm to find centroid using cells after background subtraction

  20. CMS projected performance

  21. h ATLAS projected performance Standard ATLAS solution -cone algorithm (R = 0.4) - is intensively studied with different samples Jet position resolution Jet energy resolution Jet finding & energy measurement work for ET > 40 GeV (15 GeV in pp)

  22. central Pb–Pb pp New challenges for ALICE • Existing TPC+ITS+PID • |h| < 0.9 • Excellent momentum resolution up to 100 GeV • Tracking down to 100 MeV • Excellent Particle ID • New: EMCAL • Pb-scintillator • Energy resolution ~15%/√E • Energy from neutral particles • Trigger capabilities

  23. Expected resolution including EMCAL Jet reconstruction using charged particles measured by TPC + ITS And neutral energy from EMCAL. Attention: ALICE quotes fluctuations relative to ideal jet with R = 1.0

  24. Measurement of the longitudinal jet structure 2 GeV 1GeV 2 GeV 1GeV dN/dx x Background estimated for Pb-Pb using HIJING Ideal: No background

  25. log(dN/dE) Background fluctuates down Background fluctuates up Bias towards higher Bg log(E/GeV) Measurement of the longitudinal jet structure Statistical error for Ejet = 100 GeV, 104 events Systematics of Background Subtraction

  26. g Measurement of the longitudinal jet structure g-jet correlation • Eg = Ejet • Opposite direction • Direct photons are not perturbed by the medium • Parton in-medium-modification through the fragmentation function • Caveats • Statistics • Systematics from fragmentation photons Robust signal but underestimation of jet energy biases x to lower values.

  27. Summary • We can look forward to very interesting physics with reconstructed jets in Heavy Ion collisions at the LHC • High rates providing sufficient energy lever-arm to map out the energy dependence of jet quenching. • Large effects: Jet structure changes due to energy loss and the additional radiated gluons. • Experiments suited for jet measurements in Heavy Ion Collisions • ATLAS and CMS: larger acceptance, more statistics. • ALICE: excellent PID and low-pT capabilities

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