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

2. 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. 3 Jets in nucleus-nucleus collisions Jets are the manifestation of high-p T 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-  E) + soft gluons (  E)

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

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

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

7. 7 Jets at RHIC 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. p+p @  s = 200 GeV STAR Au+Au @  s NN = 200 GeV

8. 8 RHIC: Jet studies with leading particles STAR Phys. Rev. Lett. 91, 072304 (2003). Pedestal&flow subtracted 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. Suppression of inclusive hadron yield Disappearance of away-side correlations

9. 9 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. Eskola et al., hep-ph/0406319 R AA ~0.2-0.3 for broad range of Surface emission bias limits sensitivity to

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

11. 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 E T Increased sensitivity to medium parameters

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

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

14. 14 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 10 4 jets needed to study fragmentation function in the z > 0.8 region. A. Accardi et al., hep-ph/0310274 CERN TH Yellow Report

15. 15 Jet physics at LHC: New challenges More than one jet E T > 20 GeV per event More than one particle p T > 7 GeV per event 1.9 TeV in cone of R =  2 +  2 < 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 j T Large distance from the jet axis Experiments need low- and high-p T capabilities for unbiased jet energy measurements and observation of low-p T hadrons from the gluon radiation. * For dN/dy = 5000. Unquenched Quenched (AliPythia) Quenched (Pyquen) p T < 2 GeV

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

17. 17 Optimal cone size Jets reconstructed from charged particles: Need reduced cone sizes and transverse momentum cut ! Energy contained in sub-cone R E ~ R 2 Jet Finders for AA do not work with the standard cone size used for pp (R = 0.7-1). R and p T cut have to be optimized according to the background conditions. Background reduced by 0.4 2 = 0.16 but 88% of signal preserved.

18. 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  plane ~R 2 Can be eliminated using impact parameter dependent background subtraction. Poissonian fluctuations of uncorrelated particles  E =  N  [ 2 +  p T 2 ] ~R Correlated particles from common source (low-E T jets) ~R

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

20. 20 CMS projected performance

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

22. 22 New challenges for ALICE Existing TPC+ITS+PID |  | < 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 central Pb–Pb pp

23. 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. 24 Measurement of the longitudinal jet structure dN/d   2 GeV 1GeV Ideal: No background Background estimated for Pb-Pb using HIJING 2 GeV 1GeV

25. 25 Measurement of the longitudinal jet structure Statistical error for E jet = 100 GeV, 10 4 events log(E/GeV) log(dN/dE) Background fluctuates up Background fluctuates down Bias towards higher Bg Systematics of Background Subtraction

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

27. 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-p T capabilities