Recent STAR results on high-pT probes Marco van Leeuwen, for the STAR collaboration

Baryon/meson ratios Anti-Baryon/meson ratio p/p ratio P. Fachini et al, QM08 Theory: X.-N. Wang, PRC 70, 031901 Large baryon/meson ratio at intermediate pT  Hadronisation by coalescence of quarks? New data extend pT-range in p+p Expect p mainly from gluons and DEg>DEq, no stronger suppression seen for p New STAR results on baryon fragmentation in p+p, see M. Heinz

Intermediate pT: the ridge 3 < pt,trig< 4 GeV/c Jet-like peak 4 < pt,trig < 6 GeV/c pt,assoc. > 2 GeV/c Au+Au 0-10% STAR preliminary Au+Au 0-10% STAR preliminary J. Putschke et al, QM06  trigger `Ridge’: associated yield at large , small Df associated Weak dependence of ridge yield on pT,trig  Relative contribution reduces with pT,trig Strong - asymmetry suggests coupling to longitudinal flow

Associated yields from coalescence Recombination of thermal (‘bulk’) partons ‘Shower-thermal’ recombination Baryon pT=3pT,parton Meson pT=2pT,parton Baryon pT=3pT,parton Meson pT=2pT,parton Hot matter Hot matter Hard parton (Hwa, Yang) No jet structure/associated yield Expect large baryon/meson ratio associated with high-pT trigger Expect reduced associated yield with baryon triggers 3<pT<4 GeV

Associated baryon/meson ratios Au+Au: 2 < pTtrig < 3 GeV/c Cu+Cu: 3 < pTtrig < 6 GeV/c C. Suarez et al, QM08 pTtrig > 4.0 GeV/c 2.0 < pTAssoc < pTtrig p+p / p++p- M. Lamont et al, QM05 J. Bielcikova et al, WWND07, C. Nattrass et al, QM08 Baryon/meson ratio in ridge close to inclusive, in jet close to p+p Different production mechanisms for ridge and jet?

Ridge event-by-event: 3-particle Dh-Dh 1 2 trigger No acceptance No acceptance Idea: Distinguish and 1 jet-like associated, 1 ridge-like: Bands along axes Ridge particles on either side: uniform Dh-Dh Both ridge particles on one side: diagonal structure

Dh-Dh results Dh-Dh structure: central peak + uniform P. Netrakanti et al, QM08 Radial distribution 3<pTTrig<10 GeV/c 1<pTAsso<3 GeV/c STAR Preliminary dAu dAu dAu Shaded : sys. error. Line : v2 uncer. STAR Preliminary STAR Preliminary AuAu 0-12% AuAu 0-12% STAR Preliminary AuAu 0-12% Note: 2x1 backgrounds subtracted STAR Preliminary Dh-Dh structure: central peak + uniform No substructure in ridge: no gluon fragments? Note absence of excess along axes: no jet+ridge events? Need to estimate expected signal strengths

Dependence on angle with reaction plane 0º 90º 3<pTtrig<4GeV/c in-plane fS=0 out-of-plane fS=90o A. Feng et al, QM08 STAR Preliminary 1 < pTasso < 1.5GeV/c 1.5 < pTasso < 2 GeV/c |Dh|>0.7 Ridge STAR Preliminary Jet

Reaction plane dependence II 3<pTtrig<4 Away side core/shoulder Near side: ridge/jet core shoulder A. Feng et al, QM08 A. Feng et al, QM08 20-60% jet part ridge part 1.5<pTassoc<2.0 GeV/c 1.0<pTasso<1.5GeV/c Yield in shoulder region independent of fs Jet yield approx no dependence on φs Yield in peak region decreases with fs Ridge yield decreases with φs Shape change due to core, not shoulder? Ridge larger for shorter path length?

System size dependence of di-hadron yields 6 < pT trig < 10 GeV/c pT assoc >3 GeV Near Side Away Side O. Catu et al, QM08 O. Catu et al, QM08 Modified frag: ZOWW, PRL 98, 212301 PQM: C. Loizides, Eur. Phys. J. C 49, 339-345 (2007) PQM: BDMPS-SW + rcoll scaling Mod Frag Model: ZOWW Higher twist + rpart scaling Need more model calculations to disentangle effects from energy loss model and density scaling/profile

Away-side IAA vs system size 6< pT trig < 10 GeV O. Catu et al, QM08 O. Catu et al, QM08 Theory - X.N.Wang private communication H.Zhang, J.F. Owens, E. Wang, X.N. Wang – PRL 98, 212301 zT=pT,assoc/pT,trig Away-side suppression constant for zT >0.3 Results agree with ZOWW modified fragmentation 2.8 ± 0.3 GeV2/fm for central Au+Au

Medium density from di-hadron measurement J. Nagle, WWND2008 associated  trigger d-Au IAA constraint DAA constraint DAA + scale uncertainty Au-Au Medium density from away-side suppression and single hadron suppression agree Theory: ZOWW, PRL98, 212301 e0=1.9 GeV/fm single hadrons However: Result depends on use of DAA vs IAA; theory curve does not match d+Au Current exp uncertainty driven by d+Au; more stats from run-8, BEMC p0 Data: STAR PRL 95, 152301

Fundamental quantity P(E) Radiation spectrum Radiation in realistic medium Salgado and Wiedemann, Phys. Rev. D68, 014008 ~15 GeV Renk, Eskola, PRC75, 054910 In realistic systems, energy loss is a broad distribution P(DE) Single-hadron and di-hadron observables fold production spectra with P(DE) Need more differential measurement to probe P(DE) Three techniques: g-jet, A. Hamed’s talk Jet-finding, J. Putschke, S. Salur, M. Heinz talks Multi-particle correlations, this talk

Di-hadron triggered analysis O. Barannikova, F. Wang, QM08 a.u. T1T2 pTa1 pTt1 pTt2 Signal + Background Background Signal A: pT>1.5 GeV/c T1: pT>5 GeV/c Au+Au 0-12% Df T2A1 -1 -2 1 2 3 4 5 1 _dN_ Ntrig d(Df ) STAR Preliminary T2A1_T1 T2A1 T1A1 2 4 Df (T1T2) T2: pT>4 GeV/c Idea: use back-to-back hadron pair to trigger on di-jet and study assoc yield Tune/control fragmentation bias and possibly geometry/energy loss bias Subtract two combinatorial terms: random T1, random T2

Di-hadron triggers in d+Au O. Barannikova, F. Wang, QM08 1 _dN_ Ntrig d(Df ) T2A1_T1 T2A1 d+Au 200 GeV Di-hadron trigger pTa1 pTt1 pTt2 2 1 T1: pT>5 GeV/c, T2: pT>4 GeV/c, A: pT>1.5 GeV/c STAR Preliminary -1 -2 1 2 3 4 5 Df (T2A1) Single hadron trigger Requiring away-side trigger increases yield  Selects higher underlying jet energies  Fragmentation bias changes pTa1 pTt1

Di-hadron triggers in Au+Au O. Barannikova, F. Wang, QM08 Di-hadron trigger 1 _dN_ Ntrig d(Df ) 4 pTa1 pTt1 pTt2 Au+Au 0-12% T2A1_T1 T2A1 2 Single hadron trigger pTa1 pTt1 STAR Preliminary -2 Df -1 -2 1 2 3 4 5 T1: pT>5 GeV/c, T2: pT>4 GeV/c A: pT>1.5 GeV/c Single trigger: broad away-side Di-hadron trigger: jet peaks on both near and away side Di-hadron trigger selects different events, has different bias

Au+Au vs d+Au comparison T1: pT>5 GeV/c, T2: pT>4 GeV/c, A: pT>1.5 GeV/c Au+Au d+Au Df -1 -2 1 2 3 4 5 1 _dN_ Ntrig d(Df ) STAR Preliminary 200 GeV Au+Au & d+Au T1 T.Renk,arXiv:0804.1204 2 density models O. Barannikova, F. Wang, QM08 T2 Au+Au similar to d+Au Model calculation: DE smallest when PTT1~PTT2 (still DE>0) Di-hadron trigger selects jet pairs with little or no energy loss in Au+Au To do: increase PTT1-PTT2

Multi-hadron cluster triggers Idea: Reduce fragmentation bias by clustering hadrons ‘proto-jet’ 0-12% Au+Au PT,sec seed > 3 GeV/c PT,sec seed > 4 GeV/c STAR Preliminary STAR Preliminary Multi-hadron trigger B. Haag WWND08 R S/B=0.7 Seed S/B=2.0 Secondary Seeds R=0.3 R=0.3 Associated track Combinatorial background in multi-hadron cluster can be tuned with pT,sec seed Use cluster energy for trigger: - R = 0.3 - pT,seed > 5 GeV/c - pT,sec seed > 3 GeV/c (not subtracted in present result)

Multi-hadron cluster trigger in Au+Au Away-side spectrum Au+Au Pythia 0-12% Au+Au STAR Preliminary B. Haag, WWND08 B. Haag, WWND08 PYTHIA Single-hadron and multi-hadron triggers give similar result Fragmentation bias does not change? PYTHIA also predicts similar results for multi-hadron and single-hadron triggers

Conclusion Intermediate pT (1 < pT < 5 GeV/c) Near side ridge and away-side double-peak Continue to explore with more differential studies: Identified particles: baryon/meson ratio larger in ridge than jet-like peak 3-particle Dh-Dh: no Dh substructure seen in ridge Angle wrt reaction plane: ridge larger in-plane, double-peak more prominent out-of-plane High-pT (pT > 6 GeV/c) Suppression due to E-loss – Theory well-controlled? Quantitative analyses being pursued - Single hadron and di-hadron suppression consistent - System size dependence indicates rpart scaling New techniques: multi-hadron analyses Back-to-back di-hadron trigger Multi-hadron cluster trigger Difficult to gain access to unbiased parton kinematics  Need g-jet, jet-finding, see talks by A. Hamed and J. Putschke

What’s to come: Run-8 d+Au Run-8 finished: d+Au + p+p Final total: 49 nb-1 + 660M FMS+fast-detector minbias events Large EMCal coverage New in Run-8 FMS: 2.5 < h < 4.0 + 45M minbias TPC events (~4x run-3 sample): improve reference for high-pT di- and multi-hadrons Use forward p0-p0 correlations to probe low-x in Au nucleus

Extra Slides

Extracting the transport coefficient Di-hadron suppression Di-hadrons Inclusive hadrons Zhang, H et al, nucl-th/0701045 Inclusive hadron suppression Di-hadrons provide stronger constrain on density 2-minimum narrower for di-hadrons Extracted transport coefficient from singles and di-hadrons consistent 2.8 ± 0.3 GeV2/fm