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Jet-medium interaction in heavy-ion collisions

Jet-medium interaction in heavy-ion collisions. Rudolph C. Hwa University of Oregon. Hua-Zhong Normal University, Wuhan, China April, 2009. Outline. Introduction Ridges Dependence of ridge yield on trigger azimuth Hadron correlation in back-to-back jets Conclusion. 1. Introduction.

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Jet-medium interaction in heavy-ion collisions

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

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

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

  4. high pT High pT particles are suppressed.

  5. low intermediate high pT 2 6 hydro no rigorous theoretical framework pQCD 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. Parton distributions fragmentation medium effect At intermediate pT recombination model has been successful. pT distributions of  and p

  7. STAR 4 /K 3 2 1 0 Large Baryon/Meson ratio in the inclusive distributions Strong evidence in support of the recombination/coalescence model (Reco), since no other model can explain it in the intermediate pT region.

  8. P. Fachini, arXiv:0808.3110 B/M~1.7 up to pT~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. Correlation on the near side Primary correlation variables:,  Trigger Trigger   ,  are the variables of the associated particle relative to the trigger particle.

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

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

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

  14. Trigger: 3 < pT < 4 GeV/c Associated: 1.5 < pT < 2 GeV/c Not hard enough for pQCD to be reliable, too hard for hydrodynamics. • 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 We have no reliable theoretical framework in which to calculate all those subprocesses.

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

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

  17. out-of-plane in-plane 6 5 4 in-plane fS=0 out-of-plane fS=90o 3 STAR Preliminary 20-60% 2 1 STAR Preliminary top 5% • In 20-60%, away-side evolves from single-peak (φS =0) to double-peak (φS =90o). • In top 5%, double peak show up at a smaller φS. • At large φS, little difference between two centrality bins. Quark Matter 2008 -- A. Feng (STAR) Dependence on trigger azimuthal angle

  18. 6 5 4 in-plane fS=0 out-of-plane fS=90o 3 2 Study the area, which is the yield. 1 20-60% Out-of-plane assoc 3<pTtrig<4, 1.5<pTtrig<2.0 GeV/c Ridge In-plane STAR Preliminary Jet Ridge and Jet components are separated. Ridge shapes in  are similar.

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

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

  21. Chiu-Hwa -- PRC 79, 034901 (2009) Correlated emission model (CEM) Strong ridge is developed when the trigger direction is aligned with the flow direction. =0.09 ~20o Data: Feng QM08 3<pTtrig<4, 1.5<pTassoc<2.0 GeV/c

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

  23. 4. Hadron correlation in back-to-back jets What we have discussed is about RIDGE --- the effect of jet on the medium. Now we discuss the effect of medium on jets --- correlation of hadrons in di-jets. Hwa-Yang - 0812.2205 [PRC (09)]

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

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

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

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

  28. Back-to-back jets Yield is insensitive to pt

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

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

  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 Npart2/3.

  32. Barannikova (STAR) QM08 200 GeV Au+Au, 12% central 1 _dN_ Ntrigd(Df ) STAR Preliminary 200 GeV Au+Au & d+Au T1A1_T2 T2A1_T1 1 _dN_ Ntrigd(Df ) STAR Preliminary Au+Au d+Au 2 3 2 0 1 0 -2 -2 -1 -1 2 2 0 0 1 1 3 3 4 4 5 5 Df Df Au+Au vs d+Au comparison T1: pT>5 GeV/c, T2: pT>4 GeV/c, A: pT>1.5 GeV/c • 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).

  33. x10-3 Ntrig__ NevtNpart2/3 T1= 5 GeV/c 0.4 d+Au STAR Preliminary 0 100 0 Npart 200 300 0.015 #T1T2 pairs / #Single triggers #Di-Jets / #Single triggers 0.01 0.05 STAR Preliminary Npart 100 200 0 300 Surface effect • If the triggers have tangential bias: expect a term related to the surface: ~ R2 ~ Npart2/3 T1: pT>5GeV/c T2: pT>4GeV/c Barannikova (STAR) QM08

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

  36. Thank you.

  37. backup

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

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

  40. Jet + Ridge 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. Jet STAR preliminary STAR preliminary SS TS ForpT,trig as low as 3 GeV/c, the semihard parton is created not far from the surface because of absorption by the medium. 1. Centrality dependence TT Enhanced thermal partons are strongly dependent on medium

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

  42. grad u(x,y) => normal to the ellipse x0, y0 s t = distance from creation point to surface alongs t t’ x1, y1  Survivability function: Density: D(x, y) depends on TA,B(s) -- a la Glauber Fluctuation of ridge hadron at  from local flow direction  Fluctuation: Geometry Ellipse: y h x w

  43. 0 1 Observable ridge distribution per trigger II II I I III III IV IV Ridge particle distribution

  44. t0 a constant  t1 ~ 0.1 t0 Yield per trigger 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. Adjust N to fit overall normalization for top 5%; relative normalization for 20-60% not adjustable.

  45. CEM  s

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