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Jet Structure of Baryons and Mesons in Nuclear Collisions

Jet Structure of Baryons and Mesons in Nuclear Collisions. Why jets in nuclear collisions? Initial state What happens in the nuclear medium? Medium modification of jet fragmentation. PHENIX Collaboration Barbara Jacak Stony Brook University Aug. 16, 2004. why jets in nuclear collisions?.

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Jet Structure of Baryons and Mesons in Nuclear Collisions

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  1. Jet Structure of Baryons and Mesons in Nuclear Collisions • Why jets in nuclear collisions? • Initial state • What happens in the nuclear medium? • Medium modification of jet fragmentation PHENIX Collaboration Barbara Jacak Stony Brook University Aug. 16, 2004

  2. why jets in nuclear collisions? System expands & cools e, pressure builds up Hard scattering, heavy quark production. Rate calculable from QCD + nuclear geometry. p, K, p, n, f, L, D, X, W, d, Reflect (thermal) properties when collisions cease Quark gluon plasma? use short wavelength probes. Fast q, g nearly ideal

  3. Expected interaction with the medium Hard scattering happens early affected by initial state nucleus Hard partons traverse the interesting stuff Energy loss by induced gluon radiation Modification of fragmentation outside the medium??  recombination with medium partons  radiated gluons nearby!

  4. p-p PRL 91 (2003) 241803 Good agreement with NLO pQCD Parton distribution functions Fragmentation functions To generalize for nuclei: fa/N(xa,Q2,r)  fa/N(xa,Q2) . Sa/A(xa,r) . tA(r) Nuclear modification to structure function (shadowing, saturation, etc.) Nuclear thickness function Initial state: p+p collisions p0 well described by pQCD and usual fragmentation functions p0

  5. ( pQCD x Ncoll) / background Vogelsang/CTEQ6 ( pQCD x Ncoll) / (background x Ncoll) [w/ the real suppression] [if there were no suppression] pQCD in Au+Au? direct photons Probe calculation works! Au+Au 200 GeV/A: 10% most central collisions Preliminary pT (GeV/c) []measured / []background = measured/background

  6. d+Au: Cronin Effect (RdA>1): Multiple Collisions broaden pT spectrum Nuclear medium modifies initial state • Probe response of coldnuclear matter with increased number of collisions. • Initial state multiple soft or semi-hard scattering • kT broadening Cronin effect for baryons > mesons But shouldn’t initial state scattering and fragmentation factorize?!

  7. Turn to nuclear collisions: single particles h/p0 ratio shows baryons enhanced for pT < 5 GeV/c

  8. Greco, Ko, Levai: PRC 68 (2003)034904 But observed enhancement can be explained by recombination of thermal quarks from an expanding quark gluon plasma. NOT Jet-like! The baryons scale with Ncoll ! Not suppressed!!? Jet-like

  9. trigger 2 particle correlations near side Df < 90° Partner from same jet away side Df > 90° opposing jet So, do jet analysis in Au+Au • Trigger: • hadron with pT > 2.5 GeV/c • Identify as baryon or meson • Biased, low energy, high z jets! • Plot Df of associated partners • Count associated lower pT particles for each trigger • “conditional yield” Near side yield: number of jet associated particles from same jet in specified pT bin Away side yield: jet fragments from opposing jet

  10. Underlying event is big! Collective flow causes another correlation in them: associated particles with non-flow angular correlations -> jets! Treat as 2 Gaussians B(1+2v2(pTtrig)v2(pTassoc)cos(2)) Subtract the underlying event includes ALL triggers (even those with no associated particles in the event) combinatorial background large in Au+Au! CARTOON 1 dN flow+jet Ntrig d flow jet

  11. 2 particle correlations Select particles with pT= 2.5-4.0GeV/c Identify them as mesons or baryons via Time-of-flight Find second particle with pT = 1.7-2.5GeV/c Plot distribution of the pair opening angles

  12. intermediate pT baryons ARE from jets • jet partner equally likely for trigger baryons & mesons • Same side: slight decrease with centrality for baryons • Larger partner probability than pp, dAu • Away side: partner rate as in p+p confirms jet source of baryons! • “disappearance” of away-side jet for both baryons and mesons

  13. baryon Meson trigger Fries, Bass & Mueller nucl-th/0407102 What’s going on? Radiated gluons are collinear (inside jet cone) Increases partner yield Thermal quark recombination Dilutes jet partner yield

  14. Jet partner distribution on trigger side Corrected to full jet yield Partner spectrum flatter, indicates jet source Partners soften in most central collisions Fragmentation fn. modified! Jet partners Inclusive

  15. (4-6 GeV/c) Partner spectrum Hwa & Yang nucl-th/0407081 Soft-hard recomb. can also explain baryon Cronin effect! So, could expect hard-soft recombination (C.M. Ko) Baryon formation is not outside medium • formation time from hadron size, Rh and mass, mh • In laboratory frame: tf ~ Rh (Eh /mh) • consider 2.5 GeV pT hadrons • tf,p~ 9-18 fm/c (Rh~0.5-1 fm); tf,p~ 2.7 fm/c (Rh~1 fm)

  16. Conclusions • Baryon excess has a significant jet component • Dilution becoming visible in most central collisions • Jet fragmentation is modified by the medium • Au+Au jets richer in soft hadrons than p+p or d+Au • effect of induced radiation? • Au+Au jets baryon yield increases with medium volume • effect of prevalent soft quarks? • See some evidence that jet fragments are beginning to thermalize in the medium, even on near side

  17. Jets in PHENIX • Large multiplicity of charged particles • --solution: find jets in a statistical manner • using angular correlations of particles • mixed events give combinatorial background • 2 x 90 degree acceptance in phi and ||<0.35 --solution: correct for azimuthal acceptance, but not for  acceptance • Elliptic flow correlations • --solutions: • use published strength values • and subtract • (could integrate over 90° • to integrate all even • harmonics to zero) PHENIX PRL 91 (2003) 182301

  18. d+Au Compare p+p and d+Au to PYTHIA

  19. R. Fries, et al pQCD spectrum shifted by 2.2 GeV Teff = 350 MeV Are extras from the (soft) underlying event? Hydro. expansion at low pT + jet quenching at high pT. Coalesce (recombine) boosted quarks  hadrons enhances mid pT hadrons baryons especially

  20. Exponential: Power law: Phase space filled with partons:coalesce into hadrons Use lowest Fock state, i.e. valence quarks • ReCo of hadrons: convolution of Wigner functions • Where does ReCo win? Wab(1;2) = wa(1)wb(2) fragmenting parton: ph = z p, z<1 recombining partons: p1+p2=ph R. Fries

  21. Coalescence Model results Fries et al: Phys.Rev. C68 (2003) 044902 Greco, Ko, Levai: PRC 68 (2003)034904 • particle ratios and spectra OK • intermediate pT hadrons from coalescence of flowing partons NOT from jets, so no jet-like associated particles

  22. kT, jTat RHIC from p+p Data Statistical Errors Only di-hadron J. Rak, Wed. J. Rak, DNP03 PHENIX preliminary Df near-side away-side

  23. Pions in 3 detectors in PHENIX • Charged pions from TOF • Neutral pions from EMCAL • Charged pions from RICH+EMCAL Cronin effect gone at pT ~ 8 GeV/c

  24. Does Cronin enhancement saturate? • A different approach: • Intrinsic momentum broadening in the excited projectile proton: • hpA: average number of collisions: X.N.Wang, Phys.Rev.C 61 (2000): no upper limit. Zhang, Fai, Papp, Barnafoldi & Levai, Phys.Rev.C 65 (2002): n=4 due to proton d dissociation.

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