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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. Jets in hadronic collisions. DM, Hard Probes 2004. High-energy hadronic collisions: collisions of constituent partons
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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
Jets in hadronic collisions DM, Hard Probes 2004 • High-energy hadronic collisions: collisions of constituent partons • Each parton carries fraction (x) of hadron’s momentum • “Hard-scattered” outgoing partons back-to-back in azimuth (), • not back-to-back in pseudorapidity () due to different x Side view of collision Transverse plane p p Dan Magestro, Ohio State University
Jets as probes of nuclear collisions - 1 • Partons in vacuum fragment characteristically →calibrated probe Can it be calibrated at LHC? Particle Data Group, PLB 592 (2004) parton = pT(hadron) / pT(parton) Dan Magestro, Ohio State University
Jets as probes of nuclear collisions - 2 QCD hard scattering, jets QGP and hydrodynamic expansion hadronic phase initial state pre-equilibrium hadronization 1 fm/c 5 fm/c 50 fm/c 10 fm/c • Hard parton scatterings early in collision direct probe • ALICE is the only LHC detector that will measure all HI observables Momentum-space anisotropy QCD hard scattering, jets Heavy particles (charm) Strange baryon spectra Direct photons, leptops Resonance production Nuclear coalescence Nuclear modification HBT interferometry , K, p spectra Particle ratios Dan Magestro, Ohio State University
Observable #1: Inclusive spectra baryons RHIC measurements • RHIC: Initial + final state effects RAB ~ constant STAR, Phys. Rev. Lett. 91 (2003) 072304 B. Cole, QM2005 Dan Magestro, Ohio State University
Observable #1: Inclusive spectra LHC expectation • Significant hardening of spectra • Reduced sensitivity to initial-state kinematic effects • Larger variation of energy-loss with pT Dan Magestro, Ohio State University
Observable #1: Inclusive spectra LHC expectation • RAA exhibits strong pT dependence I. Vitev 5x change! Note: collisional energy loss not considered in this model even MORE suppression? Dan Magestro, Ohio State University
Jet and dijet measurements at LHC Trigger Near side Away side 0 /2 0 (radians) ET < 20 GeV ET >(>) 20 GeV Calorimetry, jet-cone algorithms • Try to measure full jet energy • Difficult for lower energy jets dueto background Tracking, 2-particle correlations • Correlate high-pTtrigger particles with associated particles • Sample fragmentation function, neutral energy (~1/3) lost Background contains jets at LHC! Dan Magestro, Ohio State University
Jet correlations and the underlying event PRL 90 (2003) 082302 • Spatial anisotropy in collision induces 2nd-order harmonic p+p Au+Au, 20-40% Dan Magestro, Ohio State University
Observable #2: Back-to-back dijets Pedestal&flow subtracted RHIC results (3) Higher pT→ Away-side suppression Lower pT→ Away-side enhancement pT(assoc) > 2 GeV/c pT(assoc) > 0.15 GeV/c STAR, PRL 95 (2005) 152301 STAR, PRL 91 (2003) 072304 4 < pT(trig) < 6 GeV/c Dan Magestro, Ohio State University
Observable #2: Back-to-back dijets RHIC results (3) Higher pT→ Away-side peak emerges: Dijets 8 < pT(trig) < 15 GeV/c DM (STAR), nucl-ex/0510002 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 R. Fries • At LHC, pQCD dominance predicted to set in a bit higher Dan Magestro, Ohio State University
Observable #2: Back-to-back dijets LHC expectation • Low-ET jets: Two-particle correlations • High-ET jets: Jet reconstuction above background Strong pT dependence of energy loss at LHC requires dijet analyses over full pT range!! Dan Magestro, Ohio State University
Observable #2: Back-to-back dijets LHC expectation – Low-ET • Two-particle correlations necessary • Difficult: large suppression + poor acceptance for away-side jet at low-ET Dan Magestro, Ohio State University
Observable #2: Back-to-back dijets Taken from B. Wyslouch, Hard Probe 2004 LHC expectation – High-ET • Multiple jets + background reduces purity at “intermediate” ET • Trigger capabilities NEEDED to access high ET Dan Magestro, Ohio State University
LHC operations plan • Main objectives: • Terminate installation in Febr. 2007 • First pp collisions in summer 2007, √s = 14 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 ~104-105 events, e.g. prediction for strong pT dependence of energy loss • Parallel efforts statistical & reconstructed jets needed to explore full pT range Dan Magestro, Ohio State University
An opportunity for early Pb beam? • 2005 LHC Project Workshop (Chamonix XIV) • 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 • Depends on ability to commission ion cycle in SPS in 2006 • Luminosity = (few) x 1024 cm-2s-1 ~105 events in < 1 day This could be a great opportunity to answer first (zeroth) energy loss questions with a one-day pilot run Dan Magestro, Ohio State University