Jet Physics in ALICE Mercedes López Noriega - CERN for the ALICE Collaboration Hot Quarks 2006 Villasimius, Sardinia - Italy

Mercedes López Noriega - Hot Quarks gluon radiation Physics motivation High energy partons, resulting from an initial hard scattering, will create a high energy collimated spray of particles → jets Partons traveling through a dense color medium are expected to lose energy via medium induced gluon radiation, “jet quenching”, and the magnitude of the energy loss depends on the gluon density of the medium Total jet energy is conserved, but “quenching” changes the jet structure and fragmentation function Measurement of the parton fragmentation products reveals information about the QCD medium

Mercedes López Noriega - Hot Quarks Outline Partonic energy loss at RHIC Jet Rates at the LHC Full reconstruction of jets –Reconstructing jets in ALICE –Jet observables Conclusion

Mercedes López Noriega - Hot Quarks nucl-ex/ x5 ● PHENIX (π 0 ) High-p T suppression in central AuAu collisions High-p T hadrons of recoiling jet suppressed in AuAu but not in dAu gluon radiation Evidence for partonic energy loss in heavy ion collisions 1/N trigger dN/d(  ) PRL91, (2003) Results from RHIC

Mercedes López Noriega - Hot Quarks Full jet reconstruction Eskola et al., hep-ph/ Leading Particle Reconstructed Jet Ideally, the analysis of reconstructed jets will allow us to measure the original parton 4-momentum and the jet structure. → Study the properties of the medium through modifications of the jet structure: – Decrease of particles with high z, increase of particles with low z – Broadening of the momentum distribution perpendicular to jet axis Leading particle becomes fragile as a probe Surface emission: –Small sensitivity of R AA to medium properties. For increasing in medium path length L, the momentum of the leading particle is less and less correlated with the original parton 4-momentum.

Mercedes López Noriega - Hot Quarks Jet rates at the LHC Huge jet statistics from E T ~10 GeV to E T ~200 GeV Jets with E T > 50 GeV will allow full reconstruction of hadronic jets, even in the underlying heavy- ion environment. Multijet production per event extents to ~ 20 GeV 100

Mercedes López Noriega - Hot Quarks ALICE A Large Ion Collider Experiment Central barrel: |  |< 0.9 High-p T capabilities –High p T charged hadrons identification –Momentum resolution better than 10% up to 100 GeV/c –Photons Proposed electromagnetic calorimeter –  /3 <  <  –|  | < 0.7 –Energy resolution ~15% Central PbPb Collisions at √s = 5.5 TeV dN ch /dy = dE T /d  ~ TeV

Mercedes López Noriega - Hot Quarks Jet reconstruction in ALICE Collimation: ~ 80% energy around jet axis in R < 0.3 Background energy in cone of size R is ~R 2 and background fluctuations ~R. In pp-collisions jets: excess of transverse energy within a typical cone of R = 1. In heavy-ion collisions – jets reconstructed using smaller cone sizes – subtract energy from underlying event Main limitations: Background energy. Reduced by: – reducing the cone size (R = ) – transverse momentum cut (p T = 1-2 GeV/c) Background energy fluctuations: – event-by-event fluctuations – Poissonian fluctuations of uncorrelated particles – fluctuations of correlated particles

Mercedes López Noriega - Hot Quarks Intrinsic performance background — no p T cut — p T >1 GeV/c — p T >2 GeV/c E T jet 100 GeV 150 GeV 50 GeV 30 GeV R E(R) [GeV] Background energy contained in a subcone of radius R reduced by: reducing the cone size cutting on p T Limited cone size leads to a low energy tail Charged reconstruction (TPC) dominated by charged to neutral fluctuations (TPC+EMCal) (TPC) (TPC – “RHIC like”) E T = 100 GeV, R = 0.4

Mercedes López Noriega - Hot Quarks Reconstructed E T -spectrum Study properties of the medium through the modifications on the transverse jet structure Jet shape (dE/dr) and jet particle momentum perpendicular to jet axis (j t ) vs. reconstructed energy Study hard processes with low p T observables by measuring the fragmentation function to low p T. Energy loss and radiated energy Decrease of hadrons in the high-z part and increase of hadrons in the low-z region of fragmentation function (z = p T /E T jet ) 10 7 central events R = 0.4 Charged jets

Mercedes López Noriega - Hot Quarks Jet shape and j T Fraction of energy in a subcone of radius R. Salgado, Wiedemann, Phys.Rev.Lett.93, (2004) --- vacuum QCD medium Fraction of energy in a subcone of radius R. Lowering p parallel to the axis + increasing p perpendicular to axis → increase of the jet size Differences are difficult to measure but → increase on mean j T

Mercedes López Noriega - Hot Quarks N. Borghini, U. Wiedemann Increase on # of particles with low z Decrease on # of particles with high z Statistical error 10,000 jets E T cone > 70 GeV Hump-backed Plateau Representing the fragmentation function: Hump-backed Plateau. Charged jets. Particles from medium induced gluon radiation in ξ ~ 4-6 For E T ~ 100 GeV, S/B ~ p T ~ 1.8 GeV/c Leading Particles S/B > 0.1

Mercedes López Noriega - Hot Quarks “RHIC like” analysis pp events at √s NN = 14 TeV. Fully reconstructed events. Trigger: 4 < p T < 8 GeV/c – Assoc.: p T < 4 GeV/ in steps of 0.5 GeV/c 0 < p T assoc < < p T assoc < < p T assoc < < p T assoc < < p T assoc < < p T assoc < < p T assoc < < p T assoc < 4.0

Mercedes López Noriega - Hot Quarks Conclusion Copious production of jets in PbPb collisions at the LHC ALICE will be able to identify jets –in reduced cones R = to reduce contributions from the underlying event –the proposed EMCal will improved jet reconstruction ALICE will study the properties of the medium through the jet structure observables (radiated energy is observed in low-p T particles): –jet shape: increase on the jet size –momentum perpendicular to the jet axis, j T : increase of mean value, broadening of the distribution –fragmentation function: increase on the number of particles with low z and decrease on the number of particles with high z ALICE will also: –study the dependence of the energy loss in particle species –study gamma-jet correlations

Mercedes López Noriega - Hot Quarks Backup Slides

Mercedes López Noriega - Hot Quarks Out-of-cone fluctuations E T = 100 GeV

Mercedes López Noriega - Hot Quarks Production rate weighted resolution function Intrinsic resolution limited to  E/E ~ (15-20)% Production rate changes factor of 3 within  E Production rate weighted resolution function has to be studied. Input spectrum for different cone energy of charged jets.

Mercedes López Noriega - Hot Quarks Jet-structure observables Jet particle momentum perpendicular to jet axis (j t )  jTjT Salgado, Wiedemann, hep-ph/

Mercedes López Noriega - Hot Quarks Photon-tagged jets Dominant processes: g + q → γ + q (QCD Compton) q + q → γ + g (Annihilation) p T > 10 GeV/c   -jet correlation – E  = E jet – Opposite direction Direct photons are not perturbed by the medium Parton in-medium-modification through the fragmentation function

Mercedes López Noriega - Hot Quarks Prompt  -hadron correlations Prompt  - hadron   - hadron Azimuthal Correlation in pp collisions at LHC energies (PYTHIA) Azimuthal Correlation in pp collisions at LHC energies (PYTHIA). Trigger particle: Highest p T g. Associated particle: Charged particles with p T > 1 GeV/c. Useful to estimate the fraction of decay photons present in the experiment

Mercedes López Noriega - Hot Quarks “RHIC like” analysis? Trigger: 5 < p T < 8 GeV/c – Assoc.: p T < 5 GeV/ in steps of 0.5 GeV/c Pythia events.