Jet-medium interaction in heavy-ion collisions Rudolph C. Hwa University of Oregon Hua-Zhong Normal University, Wuhan, China April, 2009

2 Outline 1.Introduction 2.Ridges 3.Dependence of ridge yield on trigger azimuth 4.Hadron correlation in back-to-back jets 5.Conclusion

3 1. Introduction Jet-medium interaction has one well-known consequence: Jet Quenching --- studied in pQCD at high p T. One way to learn about the dense, hot medium created in heavy-ion collision is to probe it with hard partons. There are other ways of studying the jet-medium interaction that reveal a broad variety of its nature.

4 High p T particles are suppressed. high p T

5 pTpT 26 lowintermediatehigh pQCDhydrono rigorous theoretical framework But that is where abundant experimental data exist, especially on hadronic correlations that characterize the interaction between jets and medium. What can we learn from the abundant data?

6 p T distributions of  and p At intermediate p T recombination model has been successful. Parton distributions fragmentation medium effect

7  /K STAR Strong evidence in support of the recombination/coalescence model (Reco), since no other model can explain it in the intermediate pT region. Large Baryon/Meson ratio in the inclusive distributions

8 P. Fachini, arXiv: B/M~1.7 up to p T ~11 GeV/c! How is it to be explained by fragmentation?

9 2. Ridges Single-particles inclusive distribution can reveal only limited information about the nature of jet-medium interaction. For more information we need to consider two-particle correlation. Ridges are the response of the medium to the passage of semihard partons, detected in di-hadron correlation.

10 Primary correlation variables: ,    Trigger Correlation on the near side ,  are the variables of the associated particle relative to the trigger particle.

11 Putschke, Quark Matter 2006 STAR Ridge R J     J+RJ+R ridge R Jet J Jet: medium effect on hard parton Ridge: effect of hard parton on medium Structure of particles associated with a trigger

12 R yield increases with N part  medium effect 1. Centrality dependence Jet+Ridge (  ) Jet (  ) Jet  ) Putschke, QM06 p t,assoc. > 2 GeV STAR preliminary 2. p T,trig dependence Strongly correlated to jet production, even for trigger momentum < 4 GeV/c. Four features about Ridges

13 3. Dependence on p T,assoc Putschke, QM06 Ridge is exponential in p T,assoc slope independent of p T,trig 4. Baryon/meson ratio Suarez QM08 B/M in ridge even higher than in inclusive distr.

14 Trigger: 3 < p T < 4 GeV/c Associated: 1.5 < p T < 2 GeV/c Not hard enough for pQCD to be reliable, too hard for hydrodynamics. We have no reliable theoretical framework in which to calculate all those subprocesses. Physical processes involve: semihard parton propagating through dense medium energy loss due to soft emission induced by medium enhancement of thermal partons hydro flow and hadronization ridge formation above background

15  associated particles  These wings are useful to identify the Ridge SS trigger TT ridge (R) STST peak (J) Partonic basis for ridge formation Mesons: Baryons: TTT in the ridge Suarez QM08 B/M in ridge even higher than in inclusive distr. It can only be explained by Recombination.

16 3. Dependence of ridge yield on the trigger azimuthal angle  Trigger restrict |  |<0.7 What is the direction of the trigger  T ? irrelevantvery relevant

17 Quark Matter A. Feng (STAR) Dependence on trigger azimuthal angle in- plane out-of-plane top 5% 20-60% in-plane  S =0out-of-plane  S =90 o In 20-60%, away-side evolves from single-peak (φ S =0) to double-peak (φ S =90 o ). In top 5%, double peak show up at a smaller φ S. At large φ S, little difference between two centrality bins. STAR Preliminary

18 STAR Preliminary in-plane  S =0out-of-plane  S =90 o Ridge Jet 3<p T trig <4, 1.5<p T trig <2.0 GeV/c 20-60% assoc Ridge and Jet components are separated. In-plane Out-of- plane Ridge shapes in  are similar. Study the area, which is the yield.

19 Jet and Ridge Yield 20-60%top 5% jet part, near-side ridge part, near-side jet part, near-side ridge part, near-side Ridge: seem to decrease with φ s. More significant in 20-60% than top 5%. Jet: seem to slightly increase with φ s. Strong near-side jet-medium interaction in reaction plane, generating sizable ridge? Minimal near-side jet-medium interaction perpendicular to reaction plane? STAR Preliminary 3<p T trig <4, 1.5<p T assoc <2.0 GeV/c

20 The medium expands during the successive soft emission process, and carries the enhanced thermal partons along the flow. If not, then the effect of soft emission is spread out over a range of surface area, thus the ridge formation is weakened. Correlation between  s and  Semihard parton directed at  s, loses energy along the way, and enhances thermal partons in the vicinity of the path. ss But parton direction  s and flow direction  are not necessarily the same. ss  Reinforcement of emission effect leads to a cone that forms the ridge around the flow direction .  Flow direction  normal to the surface

21 3<p T trig <4, 1.5<p T assoc <2.0 GeV/c Data: Feng QM08 =0.09 ~20 o Chiu-Hwa -- PRC 79, (2009) Correlated emission model (CEM) Strong ridge is developed when the trigger direction is aligned with the flow direction.

22  s >0 In CEM we found an asymmetry in the  distribution trigger pt=3-4 GeV/c Jet Ridge  s | CEM model STAR Preliminary Ridge: assoc pt=1-1.5 GeV/c Ridge: assoc pt=1.5-2 GeV/c Jet: assoc pt=1.5-2 GeV/c Netrakanti QM09 R only  s <0

23 What we have discussed is about RIDGE --- the effect of jet on the medium. 4. Hadron correlation in back-to-back jets Now we discuss the effect of medium on jets --- correlation of hadrons in di-jets. Hwa-Yang [PRC (09)]

24 c=0 (0%) most centralc=0.5 (50%) mid-central Near-side jet p TTTSSS L: path length in medium In reality, L cannot be fixed. Experiment can only specify centrality c. Single-particle distribution kq

25 c=0.05 c=0.86 Inclusive spectra fitted by one parameter for each centrality Fit by the average of 2 parameters:  0,  ; data >100 pts.

26 Associated particle on near-side jet [TS+SS] nearly independent of c

27 Suppression factor Fraction of energy loss ~ 15% Near-side jets originate from the rim to minimize energy loss Trigger bias Insensitive to centrality

28 Back-to-back jets Yield is insensitive to p t

29 Suppression factor Fraction of energy loss ~ 0.7 much larger than on near side ~ 0.15 Away-side hard parton travels a longer distance in the medium, losing more momentum. much larger than Anti-trigger bias

30 Symmetric dijets Let ptpt ptpt pbpb pbpb away Same degree of quenching on both sides. near knows nothing about the away side.

31 The only way that can be true is that all symmetric dijets are tangential jets at any c. Suppressions on both sides are similar, independent of c. Surface-to-volume ratio is N part 2/3.

32 Au+Au vs d+Au comparison T1A1_T2 T2A1_T1  _dN_ N trig d  ) STAR Preliminary 200 GeV Au+Au, 12% central Di-jets are suppressed. Once select di-jets, away-side associated particles NOT suppressed. Shapes of near- and away-sides similar. Central Au+Au ~ d+Au. No energy loss for triggered di-jets! Tangential di-jets (or punch-through without interactions). T1: p T >5 GeV/c, T2: p T >4 GeV/c, A: p T >1.5 GeV/c Au+Au d+Au  _dN_ N trig d  ) STAR Preliminary GeV Au+Au & d+Au Barannikova (STAR) QM08

33 Surface effect T1: p T >5GeV/c T2: p T >4GeV/c If the triggers have tangential bias: expect a term related to the surface: ~ R 2 ~ N part 2/3 STAR Preliminary T1= 5 GeV/c N part 0 N trig __ N evt N part 2/3 0.4 d+Au x STAR Preliminary #T1T2 pairs / #Single triggers #Di-Jets / #Single triggers N part Barannikova (STAR) QM08

34 Conclusion We have discussed jet-medium interaction at intermediate p T. Effect of jets on medium: Semi-hard parton -> energy loss to medium -> Ridge. Our interpretation is that the ridge is formed by the recombination of thermal partons enhanced by jet. The prediction on asymmetry has been verified by data. Effect of medium on dijets: Energy loss to medium -> strong correlation between jets. It is hard to probe the medium interior by dijets because of dominance by tangential jets --- also verified by data on 2jet+1 correlation.

35 Will the problem be clarified at LHC? Physics at LHC is not likely to be simply the extrapolation from RHIC. Di-hadron correlation will be far more complicated. Many people predict that p/  ratio ~0.5 for 10<p T <20 GeV/c in single particle distribution (by fragmentation). We (RH & CBYang) predicted 5< p/  <20 due to jet-jet recombination. I doubt it.

36 Thank you.

37 backup

38 There is severe damping on the away side, but no damping on the near side. to detector undamped absorbed

39 A more revealing way to see the properties of jet-medium interaction is to examine the azimuthal dependence of jet production trigger associated particle Dihadron correlations

40 1. Centrality dependence STAR preliminary Jet SSTS STAR preliminary Jet + Ridge TT For p T,trig as low as 3 GeV/c, the semihard parton is created not far from the surface because of absorption by the medium. Enhanced thermal partons are strongly dependent on medium Ridge is formed by recombination of enhanced thermal partons due to energy loss of a semihard parton created near the surface as it traverses the medium.

41 What partons? Putschke, QM06 Ridge is exponential in p T,assoc 3. Dependence on p T,assoc Thermal partons correlated to jets Inverse slope: T’ (for R) > T (for inc.)  T~40-50 MeV/c quark ~ exp(-q T /T’) hadron ~ exp(-p T /T’) RF ~  (p T -  i q iT )  T’ same for quarks and hadrons Ridge is formed by enhanced thermal partons p t,assoc. > 2 GeV STAR preliminary 2. p T,trig dependence

42 Geometry x y h w Ellipse: grad u(x,y) => normal to the ellipse  x 0, y 0 x 1, y 1 t ss t = distance from creation point to surface along  s Survivability function: Density: D(x, y) depends on T A,B (s) -- a la Glauber t’  Fluctuation: Fluctuation of ridge hadron at  from local flow direction 

43  01 Ridge particle distribution Observable ridge distribution per trigger III IIIIV III IIIIV

44 Yield per trigger a constant t0t0 t 1 ~ 0.1 t 0 Adjust N to fit overall normalization for top 5%; relative normalization for 20-60% not adjustable. N encapsules all uncalculable effects of the soft processes involved in the ridge formation, and is not essential to the study of the  s dependence.

45 CEM ss 