STAR results on Medium Properties and Response of the Medium to Energetic Partons

1. STAR results on Medium Properties and Response of the Medium to Energetic Partons Bedanga Mohanty (For the STAR Collaboration) Variable Energy Cyclotron Centre, Kolkata Outline • Motivation • Parton Energy Loss • Medium Response to Energetic Partons • Summary

2. Motivation Correlations play a significant role in understanding medium properties

3. Leading/trigger particle Near side Associated particles Dj Absence of medium STAR Preliminary Away side New STAR high pT p+p results away near Basic Approach Calibrated probe Look for modification Medium formed in heavy-ion collisions Higher the trigger particle pT more probable it is from a jet and more well defined is the jet axis Jet and high pT particle production in pp understood in pQCD framework STAR : PRL 97 (2006) 252001 STAR : PLB 637 (2006) 161 Is there any modification in heavy ion collisions ?

4. Di-hadron Single y (fm) y (fm) x (fm) x (fm) Less surface bias Limited sensitivity of RAA to P(E,E) T. Renk, PRC 74 (2006) 034906 T. Renk and Eskola,hep-ph/0610059 2IAA 2RAA Di-hadron correlations more robust probes of initial density ~ H. Zhong et al., PRL 97 (2006) 252001 Advantage of Di-hadron Correlations

5. Enhanced correlated yield at large  on near side Away side shape modification 2.5 < pTtrig< 4 GeV/c 1< pTassoc < 2.5 GeV/c d+Au Au+Au Medium response STAR: PRL 95 (2005) 152301 J.G. Ulery, QM 2005 STAR : J. Putschke, QM2006 STAR : M. J. Horner, QM2006 pTtrig=3-6 GeV/c, 2 GeV/c <pTassoc< pTtrig Reappearance of di-jets STAR : PRL 97 (2006) 162301 Current Observations in STAR High pT suppression Away side yield modification Parton Eloss pTlp : 4 - 6 GeV/c STAR : PLB 655 (2007) 104 STAR : PRL 97 (2006) 152301 STAR : PRL 91 (2003) 072304 pTasoc : 2 GeV/c - pTlp What does these features reveal about the medium ?

6. Mechanism of energy loss in medium - What is the Path length dependence of energy loss ? - L2 or L Do we see a Color charge dependence of energy loss ? What is the probability distribution of parton energy loss ? Do partons loose energy continuously or discretely? Do they give answers to … Few hard interactions or multiple soft interactions ? Where does the energy from the absorbed jets go or how are they distributed in the medium? - Shock waves in recoil direction - Coupling of radiation to collective flow

7. Results on Partonic Eloss Parton Eloss Medium response Reappearance of di-jets Results to be discussed in this talk : Is there a difference in quark and gluon Eloss ? Patricia Fachini (Parallel Talk on 5th Feb),Zhangbu Xu (Plenary Talk on 5th Feb) Path length effects of parton Eloss : Dihadron Fragmentation functions Oana Catu (Parallel talk on 8th Feb) Di-hadron correlations with respect to reaction plane Aoqi Feng (Parallel Talk on 5th Feb) Probability distribution of parton Eloss : -hadron correlations A. Hamed (Parallel Talk on 8th Feb)

8. Anti-particle to particle ratio Baryon & meson NMF STAR Preliminary STAR Preliminary Eg ~ 9/4 X.-N. Wang, PRC 70 (2004) 031901 Eq 2 D E <q> ^ L  a C s Anti-Baryon to meson ratio STAR Preliminary Is there a difference in q, g Eloss Color factor effects of the type 9/4 not observed in pion, (anti-)proton ratios and Rcp upto pT ~ 12 GeV/c Patricia Fachini, Parallel Talk, 5th February STAR : PLB 637 (2006) 161 STAR : PRL 97 (2006) 152301 STAR : PLB 655 (2007) 104

9. 6< pT trig < 10 GeV IAA STAR Preliminary Npart • Inconsistent with PQM calculations • Modified fragmentation model better C. Loizides, Eur. Phys. J. C 49, 339-345 (2007) H. Zhong et al., PRL 97 (2006) 252001 Away-side Di-hadron Fragmentation Function zT=pTassoc/pTtrig 1/Ntrig dN/dzT STAR Preliminary STAR Preliminary IAA zT • Denser medium in central Au+Au collisions compared to central Cu+Cu • zT distributions similar for Au+Au and Cu+Cu for similar Npart Oana Catu, Parallel Talk, 8th February

10. trigger in-plane trigger out-of-plane Observations : • 20-60% : away-side : from single-peak (φS =0) to double-peak (φS =90o) • Top 5% : double peak show up at a smaller φS • At large φS, little difference between two centrality bins Di-hadron Correlations w.r.t Reaction Plane in-plane fS=0 3< pTtrig < 4 GeV/c, pTasso : 1.0- 1.5 GeV/c out-of-plane fS=90o 20-60% STAR Preliminary Au+Au 200 GeV top 5% STAR Preliminary d+Au Aoqi Feng, Parallel Talk, 5th February

11. i ( i - )2 yi RMS = i yi RMS Path Length Effects STAR Preliminary v2 sys. error v2{RP} in-plane: similar to dAu in 20-60%. broader than dAu in top 5%. Out-of-plane: not much difference between the two centrality bins. v2{4} Au+Au 200 GeV 3< pTtrig < 4 GeV/c 1.0 < pTasso < 1.5 GeV/c Away-side features reveal path length effects Aoqi Feng, Parallel Talk, 5th February

12. Jet X.-N. Wang et al PRL 77(1996)231 • 1st measurement of away-side -h correlations • Suppression similar level as inclusives in central collisions Note : Ejet = E Promptg A. Hamed (Parallel Talk on 8th Feb) Eloss probability distribution : -hadron correlation • Provides constraints on Eloss probability distributions • Possibly gives full accounting of jet energy loss T. Renk : PRC 74 (2006) 034906

13. Results on Medium Response Parton Eloss Reappearance of di-jets Medium response Results to be discussed in this talk : Conical Emission : Final 3-particle results with higher pT trigger, PID correlations Jason Ulery (Plenary Talk on 8th Feb), Gang Wang (Parallel Talk on 8th Feb), Guoliang Ma (Poster), Jiaxu Zuo (Poster), Quan Wang (Poster) Ridge in heavy ion collisions : Identified particle correlations, 3-particle correlations Correlations w.r.t RP Pawan Netrakanti (Plenary Talk on 5th Feb), Aoqi Feng, Parallel Talk, 5th Feb, Christine Nattrass (Parallel Talk on 8th Feb), Jiaxu Zuo (Poster), Cristina Suarez (Poster), Betty Abelev (Poster) , Navneet Kumar(Poster)

14. STAR Preliminary near near dAu e-h correlation STAR Preliminary STAR Preliminary Medium Medium away away Conical emission or deflected jets ? (1-2)/2 deflected jets (1-2)/2 Central Au+Au 0-12% Conical Emission Conical Emission Experimental evidence of Conical emission 3 <pT-trig < 4 GeV/c 1 < pT-assoc < 2 GeV/c • Two component approach • Correlated to trigger (jets..) • Uncorrelated to trigger • (except via anisotropic flow) • Bkg normalization 3-particle ZYAM See also Jason Ulery - Plenary Talk on 8th February, Gang Wang (Parallel Talk on 8th Feb

15. C3  STAR Preliminary  Subtraction of v2v2v4 terms using on v2 = 0.06 Subtraction of v2v2v4 term using v2 = 0.12 Strength and shape of away side structures observed depends on assumed magnitude of flow coefficients In cumulant approach: no conclusive evidence for conical emission so far Claude Pruneau : STAR : QM2008(Poster), PRC 74 (2006) 064910 Mach Cone or Cerenkov Gluons Cone angle (radians) • Mach-cone: Angle independent of associated pT • Cerenkov gluon radiation: Decreasing angle with associated pT STAR Preliminary pT(GeV/c) Naively the observed cone angle ~ 1.36 radians leads to very small (time averaged) velocity of sound in the medium

16. d+Au, 40-100% Au+Au, 0-5% 3 < pT(trig) < 6 GeV2 < pT(assoc) < pT(trig) Ridge in Heavy Ion Collisions Au+Au d+Au What does these features reveal about the medium ? Perhaps tells us how the energy lost by partons are distributed in the medium

17. STAR Preliminary Ridge persists up to high pT-trig TRidge ~ Tinclusive < Tjet Indication of two contributions Jet contribution + contribution arising due to jet Propagating in the medium STAR Preliminary What could be the different physics possibilities ? Features of the Ridge (at QM2006) STAR Preliminary Yield at large  independent on  STAR : J. Putschke, QM2006

18. Different Physics Possibilities QCD bremsstrahlung radiation boosted by transverse flow S.A.Voloshin, Phys.Lett.B. 632(2007)490 E.Shuryak, hep-ph:0706.3531 In medium radiation and longitudinal flow push N.Armesto et.al Phys.Rev.Lett. 93(2007) 242301 Broadening of quenched jets in turbulent color fields A.Majumder et.al Phys. Rev. Lett.99(2004)042301 Recombination between thermal and shower partons at intermediate pT R.C. Hwa & C.B. Chiu Phys. Rev. C 72 (2005) 034903 Momentum Kick Model C.Y. Wong hep-ph:0712.3282 Qualitatively consistent with the features of ridge • Following are the new approaches attempted to disentangle • different physics possibilities • 3-particle correlation • Identified particle correlation • Di-hadron correlation with respect to reaction plane • System size dependence

19. Jet and Ridge : Observations • Near-side jet yield independent of colliding system, Npart and trigger particle type • High pT-trigleads to higher jet yields • Supports : Parton fragmentation after parton Eloss in the medium • Ridge yield increases with Npart Oana Catu, Parallel Talk, 8th February Christine Nattrass, Parallel Talk, 8th February

20. Jet ridge v2 modulated background + ridge + jet • inferred v2 of events with ridge pair ~ inclusive • inferred v2 of events with jet pair < inclusive Particle Ratios and v2 : Jet & Ridge Jet Cone vs. Inclusive Ridge vs. Inclusive STAR Preliminary STAR Preliminary Jet : /K0s ~ 0.5 < inclusive Ridge : /K0s ~ 1 ~ inclusive • Ratios in cone smaller than inclusive • Ratios in ridge similar to inclusive Cristina Suarez - Poster, Paul Sorenson - Talk 9th Feb, Navneet K. Pruthi - Poster Jiaxu Zuo - Poster

21. jet ridge Ridge : Di-hadron Correlation w.r.t RP STAR Preliminary Au+Au 200 GeV 20-60% Observations : Ridge: decreases with φS. Little ridge at larger φS. Jet: slightly increases with φS. General agreement with d+Au 3< pTtrig < 4 GeV/c, pTasso : 1.0- 1.5 GeV/c Interpretation : Strong near-side jet-medium interaction in reaction plane, generating sizable ridge Minimal near-side jet-medium interaction perpendicular to reaction plane Aoqi Feng, Parallel Talk, 5th February

22. dAu : Jets AuAu : 200 GeV STAR Preliminary 3<pTTrig<10 1<pTAsso<3 ||<0.7 Uniform overall excess of associated particles not due to correlated emission = + Ridge : 3-particle Correlation Jets In-medium radiated gluons diffused in In-medium radiated gluons still collimated Jet fragmentation and diffused gluons Pawan K. Netrakanti, Plenary Talk, 5th February

23. Outlook : Di-Jets and Jet Reconstruction Parton Eloss Reappearance of di-jets Medium response Results to be discussed in this talk : Di-jets triggered correlations - Olga Barannikova, Parallel Talk on 8th February Multi-hadron cluster triggered correlations Brooke Haag - Poster

24. T1: pT>5GeV/c T2: pT>4GeV/c A1:pT>1.5GeV/c What happens to these features if we trigger on di-jets ? A1 T1 T2 Di-jet trigger Di-Jets Triggered Correlations Observation of di-jets

25. 200 GeV Au+Au, 12% central 1 _dN_ Ntrigd(Df ) 4 T2A1_T1 T2A1 T1: pT>5GeV/c T2: pT>4GeV/c A: pT>1.5GeV/c 200 GeV Au+Au & d+Au 2 1 _dN_ Ntrigd(Df ) STAR Preliminary Au+Au d+Au 3 0 STAR Preliminary 2 -2 1 1 _dN_ Ntrigd(Dh ) 1 _dN_ Ntrigd(Df ) T2A1_T1 T2A1_T1 T1A1_T2 Au+Au 12% central |Df |<0.7 12% Central 40-60% MB 60-80% MB 0 2 -2 -2 -1 -1 2 0 0 1 1 3 3 4 4 5 5 0 STAR Preliminary STAR Preliminary Df -1 -1.5 0.5 0 -0.5 1.5 1 Df Df Dh Di-Jets Triggered Correlations No Away-side suppression, No Shape modification, no ridge Olga Barannikova, Parallel Talk, 8th February

26. R Seed Associated track Secondary Seeds Multi-hadron Cluster Triggered Correlations Motivation: Explore jet-biases in di-hadron correlations Start developing jet reconstruction Away-side spectrum 0-12% Au+Au Add 12-15 GeV trigger Multi-hadron trigger STAR Preliminary Use cluster energy for trigger: - R = 0.3 - pT,seed > 5 GeV - pT,sec seed > 3 GeV Single-hadron and multi-hadron triggers give similar result Re-confirms single high pT triggered correlation results probes jet-like correlations Brooke Haag - Poster

27. Differences (due to color factor) in energy loss between quarks and gluons not observed in the measured pT range • Dense medium formed in Au+Au collisions compared to central Cu+Cu collisions • Cu+Cu and Au+Au : zT distributions similar for collisions with similar Npart- feature not consistent with PQM calculations • Path length effects observed : Broader RMS for away-side distribution in di-hadron correlations from in-plane to out-of-plane • 1st measurement of away-side suppression in -h correlation Summary : Parton Eloss STAR Preliminary STAR Preliminary

28. Summary : Medium Response • Strong jet-medium interaction observed. • Signals of conical emission observed in central Au+Au Collisions at 200 GeV in 2-component approach • Medium responds through ridge formation. • New observations should provide significant constrains on the mechanism of ridge formation • Particle ratios in ridge similar to inclusive measurements • Di-hadron correlations with respect to reaction plane • indicates - ridge is dominated in-plane, consistent with • medium density effect • o 3-particle correlations : jet fragmentation + an overall uniform • excess of associated particles not due to correlated emission STAR Preliminary Ridge vs. Bulk STAR Preliminary Jet Cone vs. Bulk STAR Preliminary AuAu STAR Preliminary dAu STAR Preliminary STAR Preliminary

29. Thanks Thanks to STAR Collaboration Argonne National Laboratory Moscow Engineering Physics Institute Institute of High Energy Physics - Beijing City College of New York University of Birmingham NIKHEF and Utrecht University Brookhaven National Laboratory University of California, Berkeley Ohio State University Panjab University University of California - Davis Pennsylvania State University University of California - Los Angeles Institute of High Energy Physics - Protvino Universidade Estadual de Campinas Purdue University Carnegie Mellon University University of Illinois at Chicago Pusan National University University of Rajasthan Creighton University Rice University Nuclear Physics Inst., Academy of Sciences Instituto de Fisica da Universidade de Sao Paulo Laboratory of High Energy Physics - Dubna University of Science and Technology of China Particle Physics Laboratory - Dubna Shanghai Institue of Applied Physics Institute of Physics. Bhubaneswar SUBATECH Indian Institute of Technology. Mumbai Texas A&M University Indiana University Cyclotron Facility University of Texas - Austin Institut Pluridisciplinaire Hubert Curien Tsinghua University University of Jammu Valparaiso University Variable Energy Cyclotron Centre. Kolkata Kent State University Wayne State University University of Kentucky Institute of Modern Physics, Lanzhou Warsaw University of Technology Lawrence Berkeley National Laboratory University of Washington Massachusetts Institute of Technology Institute of Particle Physics Yale University Max-Planck-Institut fuer Physics Michigan State University University of Zagreb

30. Eg ~ 9/4 Eq 2 D E <q> ^ L  a C s Color Factors and q,g Eloss QCD : For SU(3) : Nc = 3 CA = 3 (gluons), CF = 4/3 (quarks) S = 0.119 ALEPH : ZPC 76 (1997) 1 OPAL :EJPC 20 (2001) 601 SU(3) is the gauge group for QCD i,j represent fermion field indices and a,b gauge field indices An opportunity to relate experimental observable (of Eloss) to basic ingredient of QCD - Gauge Group through Color Factors