1 / 51

Jets in Nuclear Collisions: Experimental Aspects

Jets in Nuclear Collisions: Experimental Aspects. Peter Jacobs CERN and Lawrence Berkeley National Laboratory. Lecture 2. Jets in Nuclear Collisions. Introduction: jets in elementary collisions what is a jet? pdfs and fragmentation functions

eliana-pickett
Download Presentation

Jets in Nuclear Collisions: Experimental Aspects

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Jets in Nuclear Collisions: Experimental Aspects Peter Jacobs CERN and Lawrence Berkeley National Laboratory Lecture 2 Jets in Nuclear Collisions

  2. Jets in Nuclear Collisions • Introduction: jets in elementary collisions • what is a jet? • pdfs and fragmentation functions • characteristics of gluon, light quark and heavy quark jets • Hard processes in nuclear collisions • nuclear geometry and scaling rules • experimental issues: collider parameters, luminosity • Partonic energy loss and heavy ion collisions • leading hadrons • correlations • what have we learned? • Open questions and future prospects at RHIC and LHC Jets in Nuclear Collisions

  3. Jets in Nuclear Collisions

  4. Jets in Nuclear Collisions

  5. jet parton nucleon nucleon Jets at RHIC Find this……….in this p+p jet+jet (STAR@RHIC) Au+Au ??? (STAR@RHIC) Jets in Nuclear Collisions

  6. Partonic energy loss in a colored medium (discussed in detail by Nestor Armesto) Bjorken, Gyulassy, Pluemer, Wang, Baier, Dokshitzer, Mueller, Pegne, Schiff, Levai, Vitev, Zhakarov, Wang, Salgado, Wiedemann, Armesto… • Bjorken’s collisional energy loss generates only small effects • But medium-induced bremsstrahlung is more effective: • Essential physics: radiated gluon decoheres due to multiple interactions with medium • DE sensitive to color-charge density of the medium • Unique non-abelian feature: system size dependence DE ~ L2 Jets in Nuclear Collisions

  7. Partonic energy loss in cold nuclear matter? Hermes semi-inclusive DIS off nuclei 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  L2 dependenceE Wang and XN Wang, PRL 89, 162301 • hadronic absorption + rescaled fragmentationA Accardi et al NuclPhys A720 131 • Data consistent with partonic energy loss but not decisive P. DiNezza JPhysG 30, S783 Jets in Nuclear Collisions

  8. Partonic energy loss in hot matter Multiple soft interactions: (without expansion) Gluon bremsstrahlung Opacity expansion (few hard scatters): (with expansion) • ~linear dependence of energy loss on gluon density glue: • measure DE color charge density at early hot, dense phase Jets in Nuclear Collisions

  9. Binary collision scaling p+p reference Partonic energy loss via leading hadrons Energy loss  softening of fragmentation  suppression of leading hadron yield Jets in Nuclear Collisions

  10. p+p inclusive spectra vs NLO pQCD NLO: W. Vogelsang p0 charged hadrons NLO calculations OK; p+p reference under control Jets in Nuclear Collisions

  11. pT (GeV) Inclusive hadron yields in 200 GeV Au+Au PHENIX PHOBOS Jets in Nuclear Collisions

  12. Binary Collision scaling Inclusive hadrons yields in central Au+Au collisions are suppressed Factor 5 suppression: large effect • Qualitatively inconsistent with conventional nuclear effects: • initial state multiple scattering (“Cronin enhancement”) • shadowing Jets in Nuclear Collisions

  13. partonic energy loss Initial or final state effect? Initial state? Final state? e.g. gluon saturation How to discriminate? Turn off final state d+Au collisions Jets in Nuclear Collisions

  14. Inclusive yields not suppressed in d+Au STAR PHOBOS PRL 91, 072302/3/4/5 BRAHMS PHENIX Hadron suppression in central Au+Au is a final state effect Jets in Nuclear Collisions

  15. q+g  jet+g Cross check: direct photon production Direct g: dominant channel for pT>10 GeV is Compton process Photon does not carry color charge  production should not be suppressed by medium-induced radiation Jets in Nuclear Collisions

  16. Direct photons are not suppressed Photons scale as binary collisions while p0 are suppressed:  consistent with partonic energy loss Jets in Nuclear Collisions

  17. h/p0 world average Another test: h production PHENIX preliminary h/p0 ~invariant with system h suppression ~ p0 suppression  partonic energy loss followed by fragmentation in vacuum Jets in Nuclear Collisions

  18. Eskola et al., hep-ph/0406319 What do we learn from inclusive hadron suppression? see lectures by Nestor Armesto Partonic energy loss calculations: observed suppression requires initial density >~30 times cold nuclear matter density Suppression only supplies lower bound on ~ density Jets in Nuclear Collisions

  19. Eskola et al., hep-ph/0406319 RAA~0.2-0.3 for broad range of ? Surface emission (“trigger bias”) Large energy loss  opaque core Inclusive measurementsinsensitive to opacity of bulk  need coincidence measurements to probe deeper Jets in Nuclear Collisions

  20. trigger “Jets” via dihadron azimuthal distributions p+p  dijet • trigger: highest pT track, pT>4 GeV/c • Df distribution: 2 GeV/c<pT<pTtrigger • normalize to number of triggers Phys Rev Lett 90, 082302 Jets in Nuclear Collisions

  21. Dihadrons in Au+Au vs p+p Au+Au peripheral Au+Au central pedestal and flow subtracted Phys Rev Lett 90, 082302 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 Jets in Nuclear Collisions

  22. partonic energy loss Initial or final state effect? Initial state? Final state? e.g. gluon saturation How to discriminate? Turn off final state d+Au collisions Jets in Nuclear Collisions

  23. pedestal and flow subtracted Final state suppression? d+Au dihadrons Phys Rev Lett 91, 072304 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 pT correlation in central Au+Au is a final-state effect Jets in Nuclear Collisions

  24. PhysRevLett 93, 252301 trigger in-plane STAR Non-central (20-60%) trigger out-of-plane Away-side suppression: non-central collisions Back-to-back suppression strength correlated with reaction plane orientation  suppression is sensitive to propagation length in medium Jets in Nuclear Collisions

  25. Jet quenching at RHIC • High pT 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 • Consistent picture: core of reaction volume is opaque to jets •  surface-biased trigger • observed jetsfragment in vacuum Jets in Nuclear Collisions

  26. Where do jet energy and momentum go? Look at lower momentum correlated hadrons 4< pT,trig < 6 GeV pT,assoc > 2 GeV pT,assoc > 0.15 GeV STAR nucl-ex/0501016 • Suppression of high momentumenhancement of low momentum pairs • recoil distribution soft and broad ~ cos (Df) (momentum conservation) • but S/B~1/200: difficult background subtraction Jets in Nuclear Collisions

  27. minimum bias Low pT dihadron correlations: uncertainties Recall CDF dihadron analysis from lecture 1: • Ambiguities: • hadrons from jets vs underlying event • momentum conservation effects • resonances Jets in Nuclear Collisions

  28. cos(Df) STAR, nucl-ex/0501016 Dihadron correlations: uncertainties (cont’d) • low pTassoc: • signal/bkgd = 1/200 • large v2 corrections • normalization is ambiguous “Too much” energy in recoil peak: pickup from medium? My personal view: this analysis is interesting and provocative but not yet quantitative Jets in Nuclear Collisions

  29. shape is sensitive to v2 correction Evidence for shock waves? M. Horner (STAR); see also PHENIX lectures by Edward Shuryak, Nestor Armesto Broad recoil peak exhibits possible substructure Work in progress: look for news at Quark Matterfrom STAR and PHENIX Jets in Nuclear Collisions

  30. Where does jet-like behavior emerge? • Time scale for hadronization for pT~ few GeV/c is ~ few fm/c •  hadronization in medium? • Factorization in nuclear collisions? Jets in Nuclear Collisions

  31. h/p0 world average Recall indications of factorization h/p0 ~invariant with system PHENIX prelim. near-side peaks unchanged Jets in Nuclear Collisions

  32. L/K0s But simple jet phenomenology (factorization) breaks down at intermediate pT ~2-5 GeV/c Mesons are suppressed, baryons are not Limited to 2<pT<5 GeV Jets in Nuclear Collisions

  33. Intermediate pT II: constituent quark scaling of elliptic flow Scale by n=3 for baryons, n=2 for mesons Jets in Nuclear Collisions

  34. |η|<0.7 PHENIX nucl-ex/0408007 |η|<0.35 STAR preliminary meson – π, K baryon – p, p Intermediate pT III:Meson vs baryon-led dihadrons Intermediate pT: 2.5<pTtrig<4.0 GeV/c; 1.7<pTassoc<2.5 GeV/c Associated yields similar for meson and baryon triggers Jets in Nuclear Collisions

  35. fragmenting parton: ph = z p, z<1 recombining partons: p1+p2=ph Intermediate pT IV: hadronization via quark coalescence • recombination from thermal + hard scattering sources • provides natural explanation of baryon enhancement, elliptic flow scaling Correlation data require recombination of soft and hard partons: interplay between hard scattering and medium Jets in Nuclear Collisions

  36.    Near-side correlations at intermediate pT Dan Magestro, STAR d+Au, 40-100% Near side: small Df New puzzle: two distinct components in Dh 1. d+Au, Au+Au: short range, “jet-like” 2. Au+Au only: long range, flat STAR preliminary Au+Au, 0-5% 3 < pT(trig) < 6 GeV2 < pT(assoc) < pT(trig) Jets in Nuclear Collisions

  37. correlation width correlated yield Dh correlations (cont’d) • Recombination effects? Coupling of radiation to flow medium? • Long-range correlation: interplay of jet quenching and transverse radial flow? Voloshin, nucl-th/0312065 Armesto et al. Jets in Nuclear Collisions

  38. Jets @ 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 pT:complex phenomena, interplay between bulk medium and hard processes  window into partonic equilibration? • Open issues: • differential measurement of DE (not lower bound) • shock waves in recoil direction? • coupling of induced radiation to collective flow? • no direct observation of induced radiation • no accurate accounting of full jet energy • dependence on color charge (q/g) and quark mass of probe • …. Jets in Nuclear Collisions

  39. Jets in Nuclear Collisions • Introduction: jets in elementary collisions • what is a jet? • pdfs and fragmentation functions • characteristics of gluon, light quark and heavy quark jets • Hard processes in nuclear collisions • nuclear geometry and scaling rules • experimental issues: collider parameters, luminosity • Partonic energy loss and heavy ion collisions • leading hadrons • correlations • what have we learned? • Open questions and future prospects at RHIC and LHC Jets in Nuclear Collisions

  40. 84 days to QM05… Au+Au results to date are from here Jets in Nuclear Collisions

  41. pq,g > 10 GeV/c all h RHIC II R. Bellwied, RHIC II workshop Jets in Nuclear Collisions

  42. CMS Jets in nuclear collisions at the LHC ALICE ATLAS 2007: p+p collisions @ 14 TeV 2008: Pb+Pb collisions @ 5.5 TeV Jets in Nuclear Collisions

  43. 1 per minbias event Hard process rates at the LHC ALICE EMCal: convenient example I found on my laptop Rates in CMS and ATLAS acceptances are yet larger Jet rates and kinematic reach at LHC are large! Jets in Nuclear Collisions

  44. 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 ET is huge: from RHIC (large quenching effects) to asymptotia (small quenching effects?) • Robust tests of quark mass dependence, color charge coupling • g/Z+jet  fragmentation function • Hadronization of high energy jets (> ~100 GeV): • many fragments still have modest pT<10 GeV/c • intermediate pT: breakdown of factorization? • coupling of radiation to medium? •  new phenomena? Jets in Nuclear Collisions

  45. Some obvious comments on preparing for the LHC • It is crucial to continue developing new ideas and to anticipate where the most exciting physics lies in LHC heavy ion collisions • However, it is equally crucial to build flexible instruments that can respond to the surprises when they come •  especially important but difficult to maintain flexibility at trigger level for rare processes Jets in Nuclear Collisions

  46. 100 GeV jet in central Pb+Pb S.L. Blyth, QM04 Energy (GeV) Jets in ALICE 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 Jets in Nuclear Collisions

  47. Observables: jet broadening and softening Medium Modification of Jet Shapes and Jet Multiplicities C.A. Salgado, U. A. Wiedemann hep-ph/0310079 Longitudinal momentum fraction z along the thrust axis of a jet: pT relative to thrust axis: Cleanest measurements: g+jet, Z+jet (but low-ish cross sections even at LHC) Jets in Nuclear Collisions

  48. Smaller energy loss for heavy quarks ? Dokshitzer Q Dokshitzer, Khoze, Troyan, JPG 17 (1991) 1602. Dokshitzer and Kharzeev, PLB 519 (2001) 199. • In vacuum, gluon radiation suppressed at q < mQ/EQ  “dead cone” effect • Dead cone implies lower energy loss(Dokshitzer-Kharzeev, 2001): • energy distribution wdI/dw of radiated gluons suppressed by angle-dependent factor • suppress high-w tail Jets in Nuclear Collisions

  49. charm/light beauty/light Ratio of heavy/light meson yields Armesto, Dainese, Salgado, Wiedemann, PRD 71 (2005) 054027. More detailed considerations: multiple scattering fills dead cone; fragmentation; q vs g color charge pT~10-20 GeV/c: light mesons from glue, charm effectively massless  well-controlled discimination of color-charge and mass effects Jets in Nuclear Collisions

  50. Conclusions to date from jet quenching at RHIC are largely qualitatively Many promising future directions for detailed, precision jet studiesat RHIC and LHC medium as probe of the jet jet as probe of the medium Summary of Lecture 2 • 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 pT:complex phenomena, interplay between bulk medium and hard processes window into partonic equilibration? Jets in Nuclear Collisions

More Related