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High Q 2 probes of the Quark-Gluon Plasma

High Q 2 probes of the Quark-Gluon Plasma. Peter Jacobs Lawrence Berkeley National Laboratory. QCD: running of a S. Asymptotic Freedom. Confinement. High momentum. Low momentum. QCD jets in reality…. Final State Radiation (FSR). Detector. Initial State Radiation (ISR).

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High Q 2 probes of the Quark-Gluon Plasma

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  1. High Q2 probes of the Quark-Gluon Plasma Peter Jacobs Lawrence Berkeley National Laboratory ICNFP 2014

  2. QCD: running of aS Asymptotic Freedom Confinement High momentum Low momentum ICNFP 2014

  3. QCD jets in reality… Final State Radiation (FSR) Detector Initial State Radiation (ISR) Hadronization {p,K,p,n,…} Jet Beam Remnants p = (uud) Beam Remnants p = (uud) ICNFP 2014

  4. Modern jet reconstruction algorithms KT jet Cone jet • Cone algorithms • Mid Point Cone (merging + splitting) • SISCone (seedless, infra-red safe) • Sequential recombination algorithms • kT • anti-kT • Cambridge/ Aachen Algorithms differ in recombination metric: different ordering of recombination different event background sensitivities Jet Fragmentation Hard scatter What everyone uses now: FastJet (M. Cacciari, G. Salam, G. SoyezJHEP 0804:005 (2008)) ICNFP 2014

  5. Inclusive jet production in proton-proton collisions Good agreement with pQCD @ (N)NLO over a broad kinematic range Phys.Rev. D86 (2012) 014022 Phys.Lett. B722 (2013) 262 ALICE ICNFP 2014

  6. Now consider “matter” Phase structure of water Phase structure of QCD matter heat Limited exploration to date Explored in detail compress heat compress ICNFP 2014

  7. Hot QCD and high energy collisions of heavy nuclei Model calculation Real LHC Pb+Pb collision ICNFP 2014

  8. Hot QCD in the laboratory: colliders Au+Au √sNN=200 GeV Pb+Pb √sNN=2.76 TeV LHC @ CERN RHIC @ BNL STAR PHENIX CMS ALICE ATLAS ICNFP 2014

  9. Jets in heavy ion collisions • Controlled “beams” with well-calibrated intensity • Final-state interactions with colored matter are calculable • “Jet quenching”: quasi-tomographic probe of the Quark-Gluon Plasma ICNFP 2014

  10. Jet quenching in QCD collisional and radiative energy loss Jet quenching involves processes over wide range of Q2 Complex interplay of perturbative and non-perturbative dynamics Calculable on the lattice…? L Probability distribution of = expectation value of Wilson loop of spatial extent L (incorporates effects of the medium) Second moment: Total medium-induced jet energy loss (multiple soft scattering): ICNFP 2014

  11. High Q2processes in nuclear collisions (A+A): • no nuclear effects Glauber scaling of inclusive cross sections Quantifying nuclear effects for (inclusive) hard processes: ICNFP 2014

  12. Jet quenching via high pT hadrons Photons (color-neutral) Jet quenching Jet fragments (color-charged) ICNFP 2014

  13. Hadron suppression: RHIC vs LHC RHIC/LHC charged hadrons RHIC p0, h, direct g • RHIC/LHC: Qualitatively similar, quantitatively different • interplay between energy loss (~matter density) and spectrum shape ICNFP 2014

  14. Measurement of : data + modeling JET Collaboration, arXiv:1312.5003 Fit pQCD-based models to single-hadron suppression data at RHIC and LHC Compared to 5 years ago: significant improvement in precision due to LHC data For a 10 GeV light quark at time 0.6 fm/c: ICNFP 2014

  15. So why bother with fully reconstructed jets? Single hadrons Jets STAR • High pThadron suppression is a disappearance measurement: • We mainly observe the energetic remnants of those jets that have not interacted • We want a complete dynamical understanding of jet quenching: • Jet energy is not “lost”: where does it go? • Jets and jet quenching are partonic in nature • Hadrons are an annoyance that may screen the essential physics • Jet quenching at the partonic level fully reconstructed jets ICNFP 2014

  16. Jets in real heavy ion collisions LHC/CMS RHIC/Star • Different experiments take very different approaches to heavy ion jets • Experimental sensitivities • Treatment of backgrounds • Jet biases • Overall: work in progress • Diversity of approaches is crucial • Visual identification of energetic jets above background is fairly easy • Much harder: accurate measurement of jet energy within finite cone • Pb+Pb at LHC: over 100 GeV of uncorrelated background energy in cone R=0.4 • Uncorrelated background has complex structure, including multiple overlapping jets at multiple energy scales • Very challenging…but not intractable ICNFP 2014

  17. Dijet Asymmetry AJ • CMS version: • |hJet|<2 • Lead jet pT,1 > 120 GeV/c • Subleading jet pT,2 > 50 GeV • Df1,2 >2p/3 Quantify dijet energy imbalance by asymmetry ratio: Ratio reduces uncertainty in Jet Energy Scale ICNFP 2014

  18. LHC Pb+Pb: Dijet energy imbalance Enhanced energy asymmetry in central Pb+Pb collisions Qualitative indication of jet quenching QCD jets in QCD matter

  19. Inclusive Jet RAA: Pb+Pb @ LHC Compare PbPb to pp data Anti-kT jets with R = 0.2, 0.3, 0.4 x2 If PbPb = superposition of pp CMS PAS HIN-12-004 • Central (“head-on”) collisions: factor ~2 yield suppression • Full jet energy not recovered: still a disappearance measurement !? • Radiation scattered to large angle? softened below experimental sensitivity? Hot QCD Matter

  20. Inclusive jets: Au+Au at RHIC J. Rusnak, Hard Probes 2013 Lower jet energies, larger S/B than LHC: more difficult measurement Background suppression: require hard leading hadron in jet pTthresh=7 GeV pTthresh=5 GeV Trigger bias First systematically well-controlled heavy ion jet spectrum at RHIC Data on tape: pTjet > 50 GeV/c ICNFP 2014

  21. Jet suppression: RHIC vs LHC RHIC: central Au+Au LHC: central Pb+Pb Markedly larger jet suppression observed at LHC than at RHIC Different jet quenching dynamics? Larger “out of cone” radiation at LHC? ICNFP 2014

  22. Hadron vs jet suppression at RHIC Hadrons Jets • Jets are markedly less suppressed than hadrons at RHIC • Contrast LHC, where jet and hadron suppression are similar • Less out-of-cone radiation at RHIC? • Instructive to compare and contrast similar jet measurements at RHIC and LHC • Data-driven guidance on the nature of jet quenching • Strong constraints on theory/modelling ICNFP 2014

  23. Infrared+collinear safe jets in heavy ion collisions: hadron+jet correlations Semi-inclusive yield of jets recoiling from a high pT hadron trigger Measured Calculable in fixed-order pQCD Recoil jet Recoil jet spectrum in p+pcollisions • Consider two different trigger pT intervals • Higher pTtrigger • biases towards higher Q2 processes • harder recoil jet spectrum ICNFP 2014

  24. IRC-safe jet quenching: h+jet Central Pb+Pb (ALICE data) noise jets 8<pTtrig<9 GeV/c 20<pTtrig<50 GeV/c p+p Recoil jets Trigger-correlated jets Bkgd-corrected jet pT: r: event-wise bkgd density estimate Ai = jet area (Fastjet) But: no jet-by-jet rejection; retain complete jet population (signal +bkgd) S/B discrimination done entirely at the event-ensemble level ICNFP 2014

  25. IRC-safe jets: intra-jet broadening due to quenching? • Ratio of differential recoil yields • R=0.2/R=0.5 • Compare ratios for central • Pb+Pb and p+p No evidence of intra-jet broadening due to quenching within jet radius R~0.5 ICNFP 2014

  26. Large-angle scattering off the QGP d’Eramo et al, arXiv:1211.1922 Discrete scattering centers or effectively continuous medium? • Look at the rate of large-angle deflections • (DIS-like scattering off the QGP) • what are the quasi-particles? • Weak coupling: pQCD calculation • Strong coupling: AdS/CFT calculation Strong coupling: Gaussian distribution • Weak coupling: • hard tail ICNFP 2014

  27. h+jet @ LHC: medium-induced acoplanarity? Df “DIS off the QGP”: look at rate of large-angle scattering…? Compare to p+p No evidence of medium-induced acoplanarity ICNFP 2014 d’Eramo et al, arXiv:1211.1922

  28. CMS : photon-jet angular correlation “QGP Rutherfold experiment” Anti-kT jet R = 0.3 PbPb Photon Jet pp pp pp Photon “Backscattering?” Jet Azimuthal angle difference between photon and jet PLB 718 (2013) 773 ICNFP 2014

  29. Compare CMS g+jet/ALICE h+jet • Many differences: trigger, jet kinematics, jet selection bias, parton flavor bias,… • But still: distributions are similar • Difference in tails….? ICNFP 2014

  30. h+jet@ RHIC (STAR) Au+Au peripheral Au+Au central • Similar analysis as ALICE@LHC: • background correction done entirely at event-ensemble level • Systematically well-controlled measurement of small jet signal in large combinatorial background ICNFP 2014

  31. h+jet recoil yield suppression: RHIC vs LHC STAR central Au+Au ALICE central Pb+Pb Are these consistent? • Convert vertical suppression into horizontal shift: energy loss out of jet cone • RHIC: DE ~ 5 GeV • LHC: DE ~ 7 GeV R=0.5 Direct, IRC-safe measurement of partonic energy loss ICNFP 2014

  32. h+jet azimuthal distributions: RHIC vs LHC Df Au+Au 0-10% Au+Au 60-80% Au+Au 0-10% Pb+Pb 2.76 TeV 0-10% 40<pTcorr<60 GeV • RHIC/LHC: No evidence yet of large-angle scattering • (N)NLO pQCD calculations underway for baseline expectation • LHC Run 2: significant improvement in statistical precision ICNFP 2014

  33. Outlook • Jet quenching is a deep problem in QCD • Field-theoretical basis, several theory/modeling approaches • Experiment: challenging measurements, promising new ideas • Techniques in place for precise measurements of IRC-safe jets (not an approximation) in nuclear collisions at both RHIC and LHC, over the full kinematic range available • Needed for statistical precision: • LHC Run 2 • future RHIC running + upgrades • We are approaching the long-standing goal of applying jets as precisely controlled tomographic probes of hot QCD matter ICNFP 2014

  34. Extra slides ICNFP 2014

  35. Finite Temperature QCD on the lattice (mB=0) Slow convergence to non-interacting Steffan-Boltzmann limit What carries energy - complex bound states of q+g? “strongly-coupled” plasma? Energy density S. Borsanyi et al., JHEP 1011, 077 (2010) Temperature [MeV] Cross-over, not sharp phase transition (like ionization of atomic plasma) ICNFP 2014

  36. Inclusive jet spectrum: isolation of hard jet component G. De Barros et al., arXiv:1208.1518 • Require leading hadron of each jet candidate to be above pT threshold • Imposition of momentum scale discriminating hard from non-hard jets • Infrared-safe: large fraction of jet energy can still be carried by very soft radiation (down to ~200 MeV) • Collinear-unsafe: minimize pT cut and vary it to assess its effect unbiased biased ICNFP 2014

  37. Inclusive jets at RHIC: compare Au+Au and p+p Ratio of heavy ion jet yield to p+p jet cross section p+p spectrum with leading hadron bias biased Au+Au/unbiased p+p Bias persists to ~few times hadron pT threshold • Bias in Au+Au not markedly different than in p+p • Vacuum-like jets? ICNFP 2014

  38. Background density estimate • For each event: • Run jet finder, collect all jet candidates • Tabulate jet energy pT,ijetand area Ai jet • Event-wide median energy density: STAR Preliminary Jet candidate pT corrected event-wise for median background density: • ~half the jet population has pT<corr> < 0 • Not interpretable as physical jets • But we do not reject this component explicitly by a cut in pT<corr>: • Contains crucial information about background or “combinatorial” jets • Rejected at later step by imposition of a specific (transparent) bias on candidates STAR Preliminary ICNFP 2014

  39. True and measured jet spectra • ATLAS/CMS algorithm: • reject jet candidates based on pT<corr> • Correct for missing yield by simulation simulation STAR Preliminary • STAR/ALICE: • keep entire pT<corr> distribution • Reject background based on other observables Analysis steps: Isolate the real hard jet component and suppress combinatorial component “Unfold” the effects of energy smearing on the hard jet component ICNFP 2014

  40. Vacuum reference: NLO vs MC shower NLO: D. de Florian arXiv:0904.4402 MC shower and NLO differ Compare ALICE p+p@7 TeV (not shown): MC shower strongly favored Important lesson for jet quenching via pQCD@NLO p+p √s=2.76 TeV ICNFP 2014

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