NIKHEF June 10, 2005 Jets in Nuclear Collisions1 Jets in Nuclear Collisions at RHIC and LHC Peter Jacobs CERN and Lawrence Berkeley National Laboratory

NIKHEF June 10, 2005 Jets in Nuclear Collisions2

NIKHEF June 10, 2005 Jets in Nuclear Collisions3

NIKHEF June 10, 2005 Jets in Nuclear Collisions4 Jets at RHIC p+p  dijet+X Au+Au  ??? nucleon parton jet Find this……….in this

NIKHEF June 10, 2005 Jets in Nuclear Collisions5 Partonic energy loss in a colored medium Bjorken’s collisional energy loss generates only small effects But medium-induced bremsstrahlung is more effective: Bjorken, Gyulassy, Pluemer, Wang, Baier, Dokshitzer, Mueller, Pegne, Schiff, Levai, Vitev, Zhakarov, Wang, Salgado, Wiedemann, Armesto… Essential physics: virtual gluons multiple scatter off color charges in medium  decohere  E sensitive to color-charge density of the medium  E not directly sensitive to color correlations (i.e. deconfinement) Unique non-abelian feature: system size dependence  E ~ L 2

NIKHEF June 10, 2005 Jets in Nuclear Collisions6 Partonic energy loss in cold nuclear matter: Hermes semi-inclusive DIS off nuclei P. DiNezza JPhysG 30, S783 Charged hadron yields in N and Kr relative to deuterium z= fraction of   energy carried by hadron strong nuclear-dependent suppression for hard fragments Theory: partonic energy loss  L 2 dependence E Wang and XN Wang, PRL 89, hadronic absorption + rescaled fragmentation A Accardi et al NuclPhys A Data consistent with partonic energy loss but not decisive

NIKHEF June 10, 2005 Jets in Nuclear Collisions7 Partonic energy loss in hot matter Multiple soft interactions: ~linear dependence of energy loss on gluon density  glue  measure   color charge density at early hot, dense phase Gluon bremsstrahlung Opacity expansion (few hard scatters): (without expansion) (with expansion)

NIKHEF June 10, 2005 Jets in Nuclear Collisions8 Partonic energy loss via leading hadrons - Energy loss  softening of fragmentation  suppression of leading hadron yield

NIKHEF June 10, 2005 Jets in Nuclear Collisions9 Binary collision scalingp+p reference Inclusive production: quantifying nuclear effects Glauber theory for p+A collisions: high Q 2 processes are incoherent  d  pA =A d  pp AA A+A collisions:

NIKHEF June 10, 2005 Jets in Nuclear Collisions10 p+p: inclusive hadrons vs NLO pQCD NLO: W. Vogelsang 00 charged hadrons NLO calculations OK; p+p reference under control

NIKHEF June 10, 2005 Jets in Nuclear Collisions11 p+p: direct photons Another check of NLO pQCD at RHIC: direct photons

NIKHEF June 10, 2005 Jets in Nuclear Collisions12 Origin of high p T hadrons at  s=200 GeV p+p   0 at mid-rapidity DqDq DgDg p T  (GEV/c) pTpT initial statefinal state S. Kretzer, hep-ph/ fraction of total p T <~9 GeV/c: mainly gluon fragments pT>~9 GeV/c: mainly quark fragments Initial state (pdf): ~ Final state (fragm.): ~ pTpT Trigger bias

NIKHEF June 10, 2005 Jets in Nuclear Collisions13 Inclusive hadron yields in 200 GeV Au+Au p T (GeV) PHENIX PHOBOS

NIKHEF June 10, 2005 Jets in Nuclear Collisions14 Inclusive hadrons yields in central Au+Au collisions are suppressed Qualitatively different than conventional nuclear effects: initial state multiple scattering (“Cronin enhancement”) shadowing Binary Collision scaling Factor 5 suppression: large effect  charged hadrons R AA

NIKHEF June 10, 2005 Jets in Nuclear Collisions15 Initial or final state effect? Initial state? is p+p the correct reference? Final state? partonic energy loss How to discriminate? Turn off final state  d+Au collisions

NIKHEF June 10, 2005 Jets in Nuclear Collisions16 Inclusive yields not suppressed in d+Au PHENIX PHOBOS BRAHMS STAR PRL 91, /3/4/5 Hadron suppression in central Au+Au is a final state effect

NIKHEF June 10, 2005 Jets in Nuclear Collisions17 Cross check: direct photon production Direct  dominant channel for p T >10 GeV is Compton process q+g  jet+  Photon has no color charge  production should not be suppressed by energy loss in colored medium

NIKHEF June 10, 2005 Jets in Nuclear Collisions18 Direct photons are not suppressed Photons scale as binary collisions while   are suppressed:  consistent with partonic energy loss

NIKHEF June 10, 2005 Jets in Nuclear Collisions19 Caveat: fragmentation photons Energy loss of fragmenting quark?  modification of photon rates LHC: fragmentation  dominates to p T ~50 GeV (Arleo et al hep-ph/ ) direct Bremsstrahlung pQCD calculation: W. Vogelsang Compton Annihilation Bremsstrahlung  E?

NIKHEF June 10, 2005 Jets in Nuclear Collisions20 Another cross check:  production  suppression ~   suppression  partonic energy loss followed by fragmentation in vacuum PHENIX preliminary   world average   ~invariant with system

NIKHEF June 10, 2005 Jets in Nuclear Collisions21 What do we learn from inclusive hadron suppression? Partonic energy loss calculations: observed suppression requires initial density >~30 times cold nuclear matter density Eskola et al., hep-ph/ Suppression only supplies lower bound on ~ density

NIKHEF June 10, 2005 Jets in Nuclear Collisions22 Surface emission (“trigger bias”) ? Inclusive measurements  insensitive to opacity of bulk  need coincidence measurements to probe deeper Eskola et al., hep-ph/ R AA ~ for broad range of Large energy loss  opaque core

NIKHEF June 10, 2005 Jets in Nuclear Collisions23 “Jets” via dihadron azimuthal distributions p+p  dijet trigger: highest p T track, p T >4 GeV/c  distribution: 2 GeV/c<p T <p T trigger normalize to number of triggers trigger Phys Rev Lett 90,

NIKHEF June 10, 2005 Jets in Nuclear Collisions24 Dihadrons in Au+Au vs p+p Au+Au peripheral Au+Au central Near-side: peripheral and central Au+Au similar to p+p  trigger bias: recoil heads towards core Strong suppression of back-to-back correlations in central Au+Au Phys Rev Lett 90,

NIKHEF June 10, 2005 Jets in Nuclear Collisions25 Initial or final state effect? Initial state?Final state? partonic energy loss How to discriminate? Turn off final state  d+Au collisions

NIKHEF June 10, 2005 Jets in Nuclear Collisions26 Final state suppression? d+Au dihadrons Near-side: p+p, d+Au, Au+Au similar Back-to-back: Au+Au strongly suppressed relative to p+p and d+Au Suppression of the back-to-back high p T correlation in central Au+Au is a final-state effect Phys Rev Lett 91,

NIKHEF June 10, 2005 Jets in Nuclear Collisions27 Away-side suppression: non-central collisions Back-to-back suppression strength correlated with reaction plane orientation  suppression is sensitive to propagation length in medium STAR PhysRevLett 93, Non-central (20-60%) trigger in-plane trigger out-of-plane

NIKHEF June 10, 2005 Jets in Nuclear Collisions28 Quarks mass effects? In vacuum, gluon radiation suppressed at  < m Q /E Q “dead cone” effect: heavy quarks fragment hard into heavy mesons Dead cone also implies lower energy loss (Dokshitzer-Kharzeev, 2001) : –energy distribution  d  /d  of radiated gluons suppressed by angle-dependent factor –suppress high-  tail Q Dokshitzer, Khoze, Troyan, JPG 17 (1991) Dokshitzer and Kharzeev, PLB 519 (2001) 199. Dokshitzer

NIKHEF June 10, 2005 Jets in Nuclear Collisions29 Ratio of heavy/light meson yields at RHIC More detailed considerations: multiple scattering fills dead cone; fragmentation; q vs g color charge End result: at RHIC only modest effects expected relative to light quark suppression Armesto, Dainese, Salgado, Wiedemann, PRD 71 (2005) D/light meson Au+Au 200 GeV

NIKHEF June 10, 2005 Jets in Nuclear Collisions30 R AA p T (GeV) Inclusive electron R AA p T <5 GeV/c: electrons dominated by charm significant electron suppression seen similar to light quark suppression consistent with theory Limited dynamic range at RHIC to study dead cone effect

NIKHEF June 10, 2005 Jets in Nuclear Collisions31 Jet quenching at RHIC Consistent picture: core of reaction volume is opaque to jets  surface-biased trigger  observed jets fragment in vacuum High p T measurements: inclusive hadrons suppressed direct photons unsuppressed (no color charge) near-side dihadron correlations ~unchanged back-to-back dihadron correlations suppressed azimuthal modulation of correlations vis a vis reaction plane

NIKHEF June 10, 2005 Jets in Nuclear Collisions32 Where do jet energy and momentum go? Look at lower momentum correlated hadrons 4< p T,trig < 6 GeV p T,assoc > 2 GeV STAR nucl-ex/ p T,assoc > 0.15 GeV Suppression of high momentum  enhancement of low momentum pairs Recoil distribution soft and broad ~ cos (  ) (momentum conservation) But: s/b~1/200 (difficult background subtraction), no neutral energy  suggestive but not “jet reconstruction”

NIKHEF June 10, 2005 Jets in Nuclear Collisions33 Soft  hard physics: where does jet-like behavior emerge? p T ~ few GeV/c: hadronization time is ~few fm/c ~ size of medium  hadronization in medium?

NIKHEF June 10, 2005 Jets in Nuclear Collisions34 Recall indications of fragmentation in vacuum   world average   ~invariant with system near-side correlations invariant PHENIX prelim. (“factorization”)

NIKHEF June 10, 2005 Jets in Nuclear Collisions35  /K 0 s But simple jet phenomenology is absent at intermediate p T ~2-5 GeV/c in central Au+Au Inclusive yields: Mesons are suppressed baryons are not Limited to 2<p T <5 GeV R CP ~ R AA

NIKHEF June 10, 2005 Jets in Nuclear Collisions36 Intermediate p T II: constituent quark scaling of elliptic flow Scale by n=3 for baryons, n=2 for mesons

NIKHEF June 10, 2005 Jets in Nuclear Collisions37 Intermediate p T III:Meson vs baryon-led dihadrons Associated yields similar for meson and baryon triggers PHENIX nucl-ex/ |η|<0.35 meson – π, K baryon – p, p |η|<0.7 STAR preliminary Intermediate p T : 2.5<p T trig <4.0 GeV/c; 1.7<p T assoc <2.5 GeV/c

NIKHEF June 10, 2005 Jets in Nuclear Collisions38 fragmenting parton: p h = z p, z<1 recombining partons: p 1 +p 2 =p h Intermediate p T IV: hadronization via quark coalescence? Correlation data require recombination of soft and hard partons: interplay between hard scattering and medium recombination from thermal + hard scattering sources provides natural explanation of baryon enhancement, elliptic flow scaling

NIKHEF June 10, 2005 Jets in Nuclear Collisions39 RHIC: summary to date jet structure is strongly modified in dense matter signals are large and statistically robust, testable multiple ways  consistent with partonic energy loss via induced gluon radiation  medium is very dense: >~ 30 times cold nuclear matter intermediate p T : complex phenomena, interplay between bulk medium and hard processes  window into partonic equilibration? Open issues: no differential measurement of  E (only lower bound) no direct observation of induced radiation how does the medium respond to energy loss? coupling of radiation to medium? no accurate accounting of full jet energy dependence on color charge (q/g) and quark mass of probe ….

NIKHEF June 10, 2005 Jets in Nuclear Collisions40 All results shown are from here Analysis in progress

NIKHEF June 10, 2005 Jets in Nuclear Collisions41 Jets in nuclear collisions at the LHC ATLAS CMS ALICE 2007: p+p 14 TeV 2008: Pb+Pb 5.5 TeV

NIKHEF June 10, 2005 Jets in Nuclear Collisions42 Jet rates and kinematic reach at LHC are huge relative to RHIC: E T jet >100 GeV ~ 10 6 /year 1 per minbias event Example: ALICE EMCal acceptance (Rates in CMS and ATLAS acceptances are factor ~10 larger) Hard process rates at the LHC

NIKHEF June 10, 2005 Jets in Nuclear Collisions43 Jets in nuclear collisions at the LHC (in one slide) LHC is a new physics regime  surprises higher density  stronger medium effects? Jet cross sections are huge: robust statistics enable precise, microscopic studies Detailed probes of energy loss mechanisms: Kinematic reach in jet E T is huge: from RHIC (large quenching effects) to asymptotia (small quenching effects?) Robust tests of quark mass dependence, color charge coupling  +jet  fragmentation function Hadronization of high energy jets (> ~100 GeV): many fragments still have modest p T <10 GeV/c intermediate p T : breakdown of factorization? coupling of radiation to medium?  new phenomena?

NIKHEF June 10, 2005 Jets in Nuclear Collisions44 Jets in heavy ion collisions ALICE preliminary 100 GeV jet in central Pb+Pb Energy (GeV) Large backgrounds  optimal resolution using small jet cones R~0.3? Complex underlying event fluctuations in heavy ion events: full jet reconstruction is difficult jet trigger is tricky (large background fluctuations) real jet capabilities will only be known with first data

NIKHEF June 10, 2005 Jets in Nuclear Collisions45 Observables: jet broadening and softening Longitudinal momentum fraction z along the thrust axis of a jet: p T relative to thrust axis: Medium Modification of Jet Shapes and Jet Multiplicities Salgado and Wiedemann hep-ph/

NIKHEF June 10, 2005 Jets in Nuclear Collisions46 Dead-cone revisited: heavy/light LHC charm/light beauty/light p T ~10-20 GeV/c: light mesons from gluons, charm effectively massless  well-controlled discimination of color-charge and mass effects Armesto, Dainese, Salgado, Wiedemann, PRD 71 (2005)

NIKHEF June 10, 2005 Jets in Nuclear Collisions47 Z+jet Z has no color charge  no in-medium interaction Background negligible Precise calibration of recoil jet: in principle gives detailed measurement of modified fragmentation ZZ

NIKHEF June 10, 2005 Jets in Nuclear Collisions48 Z+jet: cross sections Z     Pb+Pb counts/year F. Maltoni, CERN Important but statistically challenging measurement Even LHC has its limitations!

NIKHEF June 10, 2005 Jets in Nuclear Collisions49 Z+ multiple jets? Z+1 jet dominates over accessible range

NIKHEF June 10, 2005 Jets in Nuclear Collisions50 Summary Jet structure is strongly modified in dense matter Signals are large and statistically robust, testable multiple ways  very high parton density early in collision evolution Intermediate p T : complex phenomena, interplay between bulk medium and hard processes  window into partonic equilibration? Conclusions to date from jet quenching studies at RHIC are largely qualitatively Many promising future directions for detailed, precision jet studies at RHIC and LHC jet as probe of the medium medium as probe of the jet

NIKHEF June 10, 2005 Jets in Nuclear Collisions51 Phys Rev Lett 91, /3/4/5