Phantom Jets: the  puzzle and v 2 without hydrodynamics Rudolph C. Hwa University of Oregon Early Time Dynamics in Heavy Ion Collisions Montreal, July 2007

2 Jets Conventional jet structure Phantom jet ?

3  puzzle  distribution of associated particles shows what seems like jet structure. p T distribution is exponential; thus no contribution from jets Bielcikova (STAR) Blyth (STAR) SQM 06

4 The specific problem Phantom jet How phantom jets solve the  puzzle The more general problem v 2 without hydrodynamics Do not assume fast thermalization. There is no need to conjecture sQGP; no basis for concluding perfect liquid.

5 Jet structure Putschke, QM06     J+RJ+R R J

6 Ridge Putschke, QM06 Jet Dependence on p T (trig) and p T (assoc) 3 - 4

7 Bielcikova, QM06 Dependence on particle species    

8 Thus we have a ridge without any significant peak on top. The ridge would not be there without a hard scattering, but it does not appear as a usual jet.  is formed by the s quarks in the ridge, since s quark in the shower is suppressed. But phantom jets of intermediate p T are there with or without  trigger. Triggering on  is the experimental way to select events to exhibit the properties of phantom jets. Phantom Jet

9 In the case of usual jets, a hard- scattered parton near the surface loses energy to the medium. Recombination of enhanced thermal partons gives rise to the ridge, elongated along  The peak is due to thermal-shower recombination in both  and  Chiu & Hwa, PRC 72, (2005) ridge bg R J pTpT Power-law behavior is a sign of TS recombination peak It generates shower partons outside.

10    and  spectra Exponential  thermal Phantom jets  ridges   has associated particles above background.  puzzle can thus be resolved.

11 Hadronization by recombination Hwa&Yang,PRC(2004)

12 Hwa & Yang, PRC(2007) nucl-th/

13  dist. yield H 0 our only free parameter. 6.9  trigger (thermal s quarks in ridge) T’=0.33 GeV Associated particles (thermal q quarks in ridge)

14 STAR data nucl-ex/ (a) H 0 =0.795 (b) H 0 = <p T trig < <p T assoc <p T trig <1% variation Chiu & Hwa, It leads to 20% change in dN/d  and yield. All ridge ! The  problem is not a puzzle any more.

15 Predictions Ridge height: dN R /d  ~0.06 p/  ratio >1 in the ridge for p T >2 GeV/c similar behavior for  -triggered events Implications Even for p T up to 6 GeV/c, one should not think of  as a product of fragmentation of hard partons. Phantom jets and ridges are present, irrespective of  trigger, so long as there are semi-hard partons near the surface to generate enhanced thermal partons.

16 hydrodynamical results It is an assumption. What if it is regarded as unacceptable? Azimuthal Anisotropy Conventional approach: hydrodynamical flow high pressure gradient requires fast thermalization.  0 =0.6 fm/c sQGP perfect liquid leads to momentum space asymmetry:  v 2 >0

17 Relevant physics must be sensitive to the initial configuration (a) Soft physics --- hydrodynamics No commonly accepted mechanism for fast development of pressure gradient (b) Hard physics --- high-p T jet quenching Process too rare at high p T (c) Medium hard physics --- semi-hard scattering Soft enough to have frequent occurrences, hard enough to create intermediate-p T jets at early times.

18 Phantom jets --- ridges |  | <  = cos -1 (b/2R)   At any given  Each scattering sends semi-hard partons in random directions on average, jet direction is normal to the surface. If the phantom jets are soft enough, there are many of them, all restricted to |  | < .

19 Bulk partons pions Bulk+Ridge partons pions Hwa&Yang,PRC(2004)

20 v2v2 Small p T region

21 ridge spectrum harder than inclusive h +,- (~ MeV in slope parameter) “Jet”/ridge yield vs. p t,assoc. in central Au+Au preliminary Au+Au 0-10% preliminarySTAR preliminary Ridge Jet Ridge/Jet yield STAR preliminary “jet” slope ridge slope inclusive slope Putschke HP06 TT

22 Use  T=45 MeV Get T”=2.12 GeV Max of sin2  (b) at  =  /4 b=√2 R=10 fm centrality 50% At small p T The first time that a connection is made between ridge and v 2. PHENIX 40-50% T=0.28 GeV

% 30-40% 20-30% 10-20% 5-10% STAR

% 30-40% 20-30% 10-20% 5-10% STAR

25 Centrality dependence v2v2 b v2v2 N part v2v2 % centrality p T =0.5 GeV/c sin2  (b)

26 Proton 40-50% at small p T

27 In peripheral collisions there are some complications. It is harder to produce protons in the bulk because of lower density of soft partons. (remember pp collisions) Thermal parton distributions in F uud are not factorizable. T in B(p T ) is lower. Thus phantom jets are relatively more effective in enhancing the thermal partons. So B(p T )/R(p T ) for proton is smaller than in pion Hence, v 2 (p T,b) continues to increase for  (b) smaller than  /4.

28 For p T >1.5 GeV/c, shower partons must be considered for both  and p spectra. Jet dominance (>3GeV/c) will saturate v 2. For p T <1.5 GeV/c, the analysis is simple, and the result can be expressed in analytic form. No part of it suggests that the medium behaves like a perfect fluid.

29 Conclusion Phantom jets produced by semi-hard parton scattering create ridges that are important in low and intermediate p T physics.  up to 6 GeV/c is produced by thermal partons in the ridge and can have associated particles. Azimuthal anisotropy is mainly a ridge effect. No fast thermalization or hydrodynamical flow are needed. Calling v 2 “elliptic flow” may be misleading.