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Jet-medium interactions in heavy ion collisions

Jet-medium interactions in heavy ion collisions. Olga Evdokimov for the STAR Collaboration. Outline. STAR detector Detector components Specifics of Heavy Ion jet studies Jet quenching in heavy ion collisions: Angular correlations for jet studies Full jet reconstruction for HI collisions

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Jet-medium interactions in heavy ion collisions

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  1. Jet-medium interactions in heavy ion collisions Olga Evdokimov for the STAR Collaboration

  2. Outline • STAR detector • Detector components • Specifics of Heavy Ion jet studies • Jet quenching in heavy ion collisions: • Angular correlations for jet studies • Full jet reconstruction for HI collisions • Summary & Outlook Olga Evdokimov University of Illinois at Chicago

  3. The STAR detector ToF EMC FPD TPC PMD FMS • Uniform 2p azimuthal acceptance • Extended rapidity coverage • Excellent identification capabilities Olga Evdokimov University of Illinois at Chicago

  4. HI collisions: the environment Data: High multiplicities → background levels → new techniques for jet studies Physics: Strongly-interacting partonic medium (ask me how we know) → modified jets Olga Evdokimov University of Illinois at Chicago

  5. Hard parton scattering leading particle hadrons q leading particle hadrons q q hadrons q leading particle hadrons leading particle Jet-medium interactions • Why? • How to spot? • Correlations and hadron distributions High pT spectra and RAA Triggered and inclusive correlations: pp vs. AA • Reconstructed jets Cross-sections and RAAJet Shape modifications (broadening) Jet-jet, hadron-jet correlations Olga Evdokimov University of Illinois at Chicago

  6. Jet quenching: the discovery PRL 91 (2003) 072303 • High pT hadron suppression: • Final state effect in Au+Au collisions • Observation extends to all accessible pT range 4<pTtrig<6 GeV/c 2<pTassoc<pTtrig • Signature two-particle correlation result: • “Disappearance” of the away-side jet in central Au+Au collisions • (for associated hadrons pTassoc>2) • Effect vanishes in peripheral/d+Au collisions PRL 91 (2003) 072304 Olga Evdokimov University of Illinois at Chicago

  7. Angular pair-correlations proton-proton 84-93% 74-84% 64-74% 55-64% 46-55% φΔ ηΔ STAR Preliminary 18-28% 9-18% 28-38% 5-9% 0-5% φΔ ηΔ Evolution of several correlation structures is observed Shown are 200 GeV minimum bias Au+Au data Similar analysis performed for 200 GeV Cu+Cu, and 62 GeV Au+Au and Cu+Cu data Olga Evdokimov University of Illinois at Chicago

  8. Transverse momentum scan Zoom in on jets: follow pT evolution pT>0.3 GeV/c pT>0.5 GeV/c pT>1.1 GeV/c pT>1.5 GeV/c STAR Preliminary diminishing e+e- Unlike-charge-sign pairs from 10% most central 200 GeV Cu+Cu data Fit same-side with asymmetric 2D Gaussian + symmetric 2D Gaussian f width amplitude h width amplitude width 0 0.6 1.2 pT(GeV/c) 0 0.6 1.2 pT(GeV/c) 0 0.6 1.2 pT(GeV/c) 0 0.6 1.2 pT(GeV/c) 0 0.6 1.2 pT(GeV/c) Olga Evdokimov University of Illinois at Chicago

  9. “Triggered” correlations 3 < pT,trigger< 4 GeV/c P RC 80 (2009) 064912 pt,assoc. > 2 GeV/c • Near-side correlation structure: • Central Au+Au: cone-like + ridge-like • Ridge correlated with jet direction • Approximately independent of Dhand trigger pT Olga Evdokimov University of Illinois at Chicago

  10. 200 GeV 62 GeV What is same-side ridge? 3.0 < pTtrigger 6.0 GeV/c; 1.5 < pTassoc < pTtrigger • Medium modified jet ? • Correlated with jet and persists to highest trigger pT • Jet yield decreases with beam energy, consistent with pQCD • Ridge/jet ratio consistent between 200 and 62 GeV data Olga Evdokimov University of Illinois at Chicago

  11. 90º 0º What is same-side ridge? • Jet modified medium? • Ridge pt-spectra are ‘bulk-like’ and approx. independent of pTtrig • Ridge decreases from in-plane to out-of-plane Olga Evdokimov University of Illinois at Chicago

  12. assoc. trig2 trig1 Di-jets through correlations Trig1 is highest pT particle in event with 5-10 GeV/c Trig2 (pT > 4 GeV/c) is back-to-back with Trig1 Associated particles pT > 1.5 GeV/c 200 GeV Au+Au and d+Au same-side away-side associated particle pT spectra STAR Preliminary • No evidence of medium modifications Di-jets observed - all tangential? Olga Evdokimov University of Illinois at Chicago

  13. Jet-energy calibration Direct g - hadron correlations arXiv:0912.1871 STAR Preliminary • Similar medium effect on the away-side of 0 and dir •  is this due to tangential emission? • rich sample has lower near-side • yields compared to those of the • 0 different bias Olga Evdokimov University of Illinois at Chicago

  14. Full jet reconstruction • Why? • High pT hadrons bias towards non-interacting jets • Full jet reconstruction reduces the bias • Hadronization • Why just now? • Very complex due to underlying event • Algorithmic biases • Data driven correction schemes Pythia smeared Unfolding unfolded Olga Evdokimov University of Illinois at Chicago

  15. Inclusive jet measurements Central Au+Au: background correction leads to a factor of 2 uncertainty in cross-section Olga Evdokimov University of Illinois at Chicago

  16. Inclusive jet ratios R=0.4 RAAJet Yields: R=0.2/R=0.4 RAA of pions ~ 0.2 RAAJet> RAAp: recover larger fraction of cross-section, but not all Significant ratio suppression wrt. pp → medium induced ET broadening Olga Evdokimov University of Illinois at Chicago

  17. Jet-hadron correlations 1.0<pTassoc<2.5 GeV/c STAR Preliminary • 0-20% Au+Au • pp STAR Preliminary flat BG subtraction STAR Preliminary • Au+Au vs. pp: • Softer momentum distribution for recoil hadrons • Significant away-side broadening Olga Evdokimov University of Illinois at Chicago

  18. Summary &Outlook: • Jet quenching discovered at RHIC has been extensively studied for years: • Modification of angular shapes, production rates, transverse momentum and energy distributions • Evolution of jet studies in HI: • Inclusive hadron measurements → di-hadron correlations → multi-particle correlations → full jets • Outlook: • Towards “theory-ready” (partonic) energy loss measurements • Disentangling experimental biases Olga Evdokimov University of Illinois at Chicago

  19. Back-Up Olga Evdokimov University of Illinois at Chicago

  20. peak η width peak amplitude STAR Preliminary STAR Preliminary 200 GeV 62 GeV 83-94% 55-65% 0-5% 46-55% STAR Preliminary STAR Preliminary STAR Preliminary STAR Preliminary Transverse particle density ηΔ width Low pT ridge Same-side evolution • Sharp transition in both amplitude and width at ρ ~ 2.5 • Similar ρ-trends at 62 and 200GeV Olga Evdokimov University of Illinois at Chicago

  21. 90º 0º in-plane fS=0 out-of-plane fS=90o Path-length effects 3<pTtrig<4GeV/c • Same-side yield • Jet: d+Au ~ Au+Au • Ridge decreases from in-plane to out-of-plane • Elliptic flow effects? STAR Preliminary Olga Evdokimov University of Illinois at Chicago

  22. KT jet Cone jet Jet reconstruction algorithms • Cone algorithms • Mid Point Cone (merging + splitting) • SISCone (seedless, infra-red safe) • Sequential recombination algorithms • kT • anti-kT • Cambridge/ Aachen Jet by P. Jacobs Fragmentation Hard scatter Olga Evdokimov University of Illinois at Chicago

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