1 Parton Recombination at all p T Rudolph C. Hwa University of Oregon Hard Probes 2004 Ericeira, Portugal, November 2004 2 Outline How parton recombination resolves a number of puzzles. Shower partons Inclusive distributions at all p T in Au+Au and d+Au collisions (Cronin effect) Dihadron correlations Forward production in d+Au collisions Why fragmentation is inadequate at intermediate p T in heavy-ion collisions. 3 Conventional approach to hadron production at high p T D(z) h q AA There are numerous evidences against this framework: hard scattering fragmentation in heavy-ion collisions. 4 Exhibit #1 R p/ 1 Not possible in fragmentation model: R p/ u 5 k T broadening by multiple scattering in the initial state. Unchallenged for ~30 years. If the medium effect is before fragmentation, then should be independent of h= or p Exhibit #2 in pA or dA collisions Cronin Effect Cronin et al, Phys.Rev.D (1975) p q h A p > 6 7 Exhibit #3 Jet structure Hard parton jet { (p 1 ) + (p 2 ) + (p 3 ) + } trigger particleassociated particles The distribution of the associated particles should be independent of the medium if fragmentation takes place in vacuum. 8 Exhibit #3 Jet structure for Au+Au collisions is different from that for p+p collisions pp Fuqiang Wang (STAR) Quark Matter 2004 9 Azimuthal anisotropy Exhibit #4 v 2 (p) > v 2 ( ) at p T > 2.5 GeV/c v 2 : coeff. of 2nd harmonic in distribution 10 Exhibit #5 Backward-forward asymmetry at intermed. p T in d+Au collisions 11 The problem: Fragmentation of hard partons that works so well in leptonic and hadronic processes is only one of many components when the hard parton is in the environment of many soft partons, as in heavy-ion collisions. 12 How can recombination solve the puzzles? Parton distribution p p 1 +p 2 pq (recombine)(fragment) hadron momentum higher yieldheavy penalty 13 D(z) h q AA Conventional approach h Thermal-shower recombination thermal shower 14 Shower partons in fragmentation process known from data (e+e-, p, ) known from recombination model can be determined hard parton meson fragmentation shower partons recombination 15 Shower parton distributions u g s s d duvalence sea L L D Sea K NS L D V G G D G L L s D K Sea G G s D K G RR RKRK 5 SPDs are determined from 5 FFs. 16 Hwa & CB Yang, PRC (2004). Shower Parton Distributions u -> u valence g -> u u -> d u -> s g -> s 17 BKK fragmentation functions 18 hard parton (u quark) An example 19 Inclusive distribution of pions in any direction thermal shower Pion formation: Proton formation: uud distribution Determine T by fitting dN /dp T at low p T ( v 2 ( ) at p T > 2.5 GeV/c v 2 : coeff. of 2nd harmonic of distribution PHENIX, PRL 91 (2003) 38 Molnar and Voloshin, PRL 91, (2003). Parton coalescence implies that v 2 (p T ) scales with the number of constituents STAR data 39 Exhibit #5 Forward-backward asymmetry at intermed. p T in d+Au collisions STAR preliminary data 40 Rapidity dependence of R CP in d+Au collisions BRAHMS nucl-ex/ R CP < 1 at =3.2 Central more suppressed than peripheral collisions This can be understood in terms of parton recombination. 41 Forward production in d+Au collisions Notation: Centrality 0-20% = % 0.7 for all and Soft component: Pion distribution C( , ) Same hadronization process as at =0. 42 Centrality (Impact Parameter) Dependence d for d + Au Collisions at 200 GeV Preliminary can be calculated. 43 Hwa, Yang, and Fries, nucl-th/ Our prediction on the + spectra for 0 3.2, 0.1 0.7 44 T=constant, no free parameter No new physics, e.g. gluon saturation, explicitly. 45 Include and dependences of T (one free parameter) and momentum degradation Hwa, Yang, Fries, nucl-th/ 46 Without any more free parameters 47 Summary Traditional picture pTpT hardsoft pQCD + FF More realistic picture pTpT hardsoftsemi-hard (low)(intermediate) thermal-thermalthermal-shower (high) shower-shower 48 All anomalies at intermediate p T can be understood in terms of recombination of thermal and shower partons Recombination is the hadronization process. At p T > 9 GeV/c fragmentation dominates, but it can still be expressed as shower-shower recombination. Hence, parton recombination at all p T. 49 Conclusion in one sentence Due to the soft parton environment in heavy-ion collisions at RHIC, hadronization by thermal-shower recombination dominates all phenomena on particle production at intermediate p T for all centralities and all rapidities. 50 All of the work presented was done in collaboration with Chunbin Yang Hua-zhong Normal University Wuhan, China