An experimental perspective on first jet measurements at LHC: Lessons from RHIC Dan Magestro, The Ohio State University ALICE-USA Collaboration Meeting October 15-16, 2005

ALICE-USA 10/ Dan Magestro, Ohio State University Jets in hadronic collisions High-energy hadronic collisions: collisions of constituent partons High-energy hadronic collisions: collisions of constituent partons Each parton carries fraction (x) of hadron’s momentum Each parton carries fraction (x) of hadron’s momentum “Hard-scattered” outgoing partons back-to-back in azimuth (  ), “Hard-scattered” outgoing partons back-to-back in azimuth (  ), not back-to-back in pseudorapidity (  ) due to different x not back-to-back in pseudorapidity (  ) due to different x p p Transverse planeSide view of collision DM, Hard Probes 2004

ALICE-USA 10/ Dan Magestro, Ohio State University Jets as probes of nuclear collisions Partons in vacuum fragment characteristically → calibrated probe Can it be calibrated at LHC? parton  Particle Data Group, PLB 592 (2004) = p T (hadron) / p T (parton)

ALICE-USA 10/ Dan Magestro, Ohio State University 1 fm/c5 fm/c 10 fm/c 50 fm/c hadronization initial state pre-equilibrium QGP and hydrodynamic expansion hadronic phase Jets as probes of nuclear collisions - 2 , K, p spectra Momentum-space anisotropy Particle ratios Resonance production HBT interferometry Nuclear coalescence Strange baryon spectra QCD hard scattering, jets Heavy particles (charm) Nuclear modification Direct photons, leptops QCD hard scattering, jets 2. Hard parton scatterings early in collision  direct probe ALICE is the only LHC detector that will measure all HI observables ALICE is the only LHC detector that will measure all HI observables

ALICE-USA 10/ Dan Magestro, Ohio State University Observable #1: Inclusive spectra RHIC: Initial + final state effects  R AB ~ constant RHIC: Initial + final state effects  R AB ~ constant STAR, Phys. Rev. Lett. 91 (2003) B. Cole, QM2005 baryons RHIC measurements

ALICE-USA 10/ Dan Magestro, Ohio State University Observable #1: Inclusive spectra Significant hardening of spectra Significant hardening of spectra LHC expectation 1.Reduced sensitivity to initial-state kinematic effects 2.Larger variation of energy-loss with p T

ALICE-USA 10/ Dan Magestro, Ohio State University Observable #1: Inclusive spectra R AA exhibits strong p T dependence R AA exhibits strong p T dependence I. Vitev LHC expectation 5x change! Note: collisional energy loss not considered in this model  even MORE suppression?

ALICE-USA 10/ Dan Magestro, Ohio State University Jet and dijet measurements at LHC Calorimetry, jet-cone algorithms Try to measure full jet energy Try to measure full jet energy Difficult for lower energy jets due to background Difficult for lower energy jets due to background Tracking, 2-particle correlations Correlate high-p T trigger particles with associated particles Correlate high-p T trigger particles with associated particles Sample fragmentation function, neutral energy (~1/3) lost Sample fragmentation function, neutral energy (~1/3) lost  Trigger 0  /2   (radians) 0 Near side Away side E T < 20 GeV E T >(>) 20 GeV Background contains jets at LHC!

ALICE-USA 10/ Dan Magestro, Ohio State University Jet correlations and the underlying event Spatial anisotropy in collision induces 2 nd -order harmonic Spatial anisotropy in collision induces 2 nd -order harmonic PRL 90 (2003) p+p Au+Au, 20-40%

ALICE-USA 10/ Dan Magestro, Ohio State University Observable #2: Back-to-back dijets RHIC results (3) Higher p T → Away-side suppression Pedestal&flow subtracted STAR, PRL 91 (2003) Lower p T → Away-side enhancement p T (assoc) > 2 GeV/cp T (assoc) > 0.15 GeV/c 4 < p T (trig) < 6 GeV/c   STAR, PRL 95 (2005)

ALICE-USA 10/ Dan Magestro, Ohio State University Observable #2: Back-to-back dijets RHIC results (3) Higher p T → Away-side peak emerges: Dijets  8 < p T (trig) < 15 GeV/c DM (STAR), nucl-ex/

ALICE-USA 10/ Dan Magestro, Ohio State University Aside: At RHIC we’ve now reached pQCD regime O. Barannikova (STAR), QM2005 At RHIC, pQCD fragmentation dominates hadron production above ~6 GeV/c At RHIC, pQCD fragmentation dominates hadron production above ~6 GeV/c R. Fries At LHC, pQCD dominance predicted to set in a bit higher At LHC, pQCD dominance predicted to set in a bit higher

ALICE-USA 10/ Dan Magestro, Ohio State University Observable #2: Back-to-back dijets LHC expectation Strong p T dependence of energy loss at LHC requires dijet analyses over full p T range!! Low-E T jets: Two-particle correlations Low-E T jets: Two-particle correlations High-E T jets: Jet reconstuction above background High-E T jets: Jet reconstuction above background

ALICE-USA 10/ Dan Magestro, Ohio State University Observable #2: Back-to-back dijets LHC expectation – Low-E T Two-particle correlations necessary Two-particle correlations necessary Difficult: large suppression + poor acceptance for away-side jet at low-E T Difficult: large suppression + poor acceptance for away-side jet at low-E T

ALICE-USA 10/ Dan Magestro, Ohio State University Observable #2: Back-to-back dijets LHC expectation – High-E T Multiple jets + background reduces purity at “intermediate” E T Multiple jets + background reduces purity at “intermediate” E T Trigger capabilities NEEDED to access high E T Trigger capabilities NEEDED to access high E T Taken from B. Wyslouch, Hard Probe 2004

ALICE-USA 10/ Dan Magestro, Ohio State University LHC operations plan Main objectives: Main objectives: Terminate installation in Febr Terminate installation in Febr First pp collisions in summer 2007, √s = 14 TeV First pp collisions in summer 2007, √s = 14 TeV First long heavy ion (Pb+Pb) run end of 2008, √s = 5.5 TeV First long heavy ion (Pb+Pb) run end of 2008, √s = 5.5 TeV Higher jet rates: first jet-quenching questions can be answered with ~ events, e.g. prediction for strong p T dependence of energy loss Higher jet rates: first jet-quenching questions can be answered with ~ events, e.g. prediction for strong p T dependence of energy loss Parallel efforts statistical & reconstructed jets needed to explore full p T range Parallel efforts statistical & reconstructed jets needed to explore full p T range

ALICE-USA 10/ Dan Magestro, Ohio State University An opportunity for early Pb beam? 2005 LHC Project Workshop (Chamonix XIV) 2005 LHC Project Workshop (Chamonix XIV) Pb ions may be available at very early stage of LHC operation Pb ions may be available at very early stage of LHC operation Jowett (CERN): Likely short Pb “Pilot Run” end of p+p Pilot Run in late 2007 Jowett (CERN): Likely short Pb “Pilot Run” end of p+p Pilot Run in late 2007 Depends on ability to commission ion cycle in SPS in 2006 Depends on ability to commission ion cycle in SPS in 2006 Luminosity = (few) x cm -2 s -1  ~10 5 events in < 1 day Luminosity = (few) x cm -2 s -1  ~10 5 events in < 1 day This could be a great opportunity to answer first (zeroth) energy loss questions with a one-day pilot run