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Large p/  Ratio without Jet Correlations at RHIC and LHC

Large p/  Ratio without Jet Correlations at RHIC and LHC. The Omega challenge. Rudolph C. Hwa University of Oregon. Quark Matter 06. November 14-20, 2006 Shanghai, China. In regions where fragmentation of jets is dominant. In regions where recombination is dominant. : qq.

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Large p/  Ratio without Jet Correlations at RHIC and LHC

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  1. Large p/ Ratio without Jet Correlations at RHIC and LHC The Omega challenge Rudolph C. Hwa University of Oregon Quark Matter 06 November 14-20, 2006 Shanghai, China

  2. In regions where fragmentation of jets is dominant • In regions where recombination is dominant : qq • In regions where is suppressed, compared to

  3. 1. At midrapidity in central collisions 2. Strange hadron production Subjects of this talk Where Forward production at RHIC: hard to find qbar at large x 4. pT<20 GeV/c at LHC: 2-jet recombination

  4. However, it is more urgent to answer the Omega challenge presented by the STAR data at this meeting. At a STAR collaboration meeting at BNL in Feb 2006, I showed a slide concluding the discussion on Omega production --

  5. Since shower partons make insignificant contribution to  production for pT<8 GeV/c, no jets are involved. Predict: no associated particles giving rise to peaks in , near-side or away-side. A prediction that can be checked now! Select events with  or  in the 3<pT<6 region, and treat them as trigger particles. Thermal partons are uncorrelated, so all associated particles are in the background. The details behind this prediction will be presented by C.B.Yang in a parallel talk in 3.1.

  6. STAR Ruan (Tuesday, plenary) Barranikova (Wed, plena.) Bielcikova (Sunday, 3.1) Phantom jet At face value the data falsify the prediction and discredits RM. I now explain why the prediction was wrong and how the data above can be understood. Yang’s talk tomorrow is still right. Recombination still works, but we need a new idea.

  7. (1)  spectrum is exponential up to 6 GeV/c. (2)  triggered events have associated particles. The core issue is the (seemingly) contradictory phenomena: (1) means that there is no contribution from hard scattering, which is power-law behaved; hence, there is no jet. (2) means that there is jet structure. The resolution is to recognize that it is a phantom jet.

  8. 3<pt,trigger<4 GeV pt,assoc.>2 GeV Au+Au 0-10% preliminary Jet+Ridge ()Jet () Jet) preliminary  yield,)    Npart Calderon showed on Tuesday But p/ ratio depends on centrality. A lot of action is going on in the ridge!

  9. J/R~10-15% Jet+ridge Jet only J. Bielcikova (HP06) at lower pt(assoc)  trigger even lower! Jet+Ridge on near side J. Putschke, QM-1.3 Unidentified charged hadron

  10. The ridge has been interpreted as the recombination of enhanced thermal partons due to the energy loss to the medium by the passage of hard parton. Radial expansion does not broaden the ridge under the peak in  Longitudinal expansion results in broad  ridge Chiu & Hwa, PRC 72, 034903 (2005) So the ridge is important.

  11. T = 15 MeV/c Pedestal (ridge) in  P1 2 < p2 < 4 GeV/c, P1 = 0.04 T ’ adjusted to fit pedestal That is for unidentified charged hadron trigger. Now, for  triggered events, there is not enough statistics to separate Jet from Ridge. I expect J/R<<1 for pT(assoc) as low as 1.5 GeV/c.

  12. Thus we have a ridge without any significant peak on top. The ridge would not be there without a hard scattering,but it is not a usual jet, because it contain no shower partons, only thermal partons. Phantom Jet When pT(trig) is low, and the trigger is , it is not in the jet, since s quark is suppressed in the shower partons. The s quarks in the ridge form the . One can see the usual peak when pT(assoc) is increased, and the ridge height will decrease.

  13. The  looks like a peak, but it is all ridge. Resolution of the  puzzle The ridge contains thermalized partons: u, d, s Hence, sss recombine to form the trigger . Other partons can form the associated particles. (1)The pT distribution of  is exponential. (2)There are associated particles. Our earlier prediction that there is no jet is still right, if ‘jet’ is meant to be the usual jet. But we were wrong to conclude that there would be no associated particles, because a phantom jet is associated with the  and it is the ridge that sits above the background.

  14. Since  is among the particles in the ridge and is formed by TTT recombination, everything calculated previously remains valid, as to be reported by C.B. Yang. See talks by J. Bielcikova, S. Blyth, and C.B. Yang in session 3.1 tomorrow.

  15. Predictions for  triggered events: • The ridge should be found in . • The ridge has abundant u, d, s. So the associated particles should have the characteristic feature of recombination, i.e., large p/ and /K ratios, ~O(1). • Since the ridge arises out of enhanced thermal partons, the associated particles should have exponential pT distribution. End of excursion to 

  16. BRAHMS has data at √s=62.4 GeV Forward Production

  17. BRAHMS has data at √s=62.4 GeV Forward Production

  18. There should be no partners associated with triggers at pT≈2.5 Gev/c. At large  hard scattering with large pT is suppressed at kinematical boundary. Since there are no hard partons to generate the usual jets or phantom jets, there is no jet structure, not even ridges. Only thermal partons contribute at large   pT distribution is exponential

  19. Hwa & Yang, nucl-th/0605037

  20. Hwa & Yang, nucl-th/0605037 (to be revised)

  21. 2 hard partons p  1 shower parton from each Two-jet recombination at LHC Hwa & Yang, PRL 97, 042301 (2006) New feature at LHC: density of hard partons is high. High pT jets may be so dense that neighboring jet cones may overlap. If so, then the shower partons in two nearby jets may recombine.

  22. single jet Proton-to-pion ratio at LHC  -- probability of overlap of 2 jet cones (pT)~pT-7

  23. But they are part of the background of an ocean of hadrons from other jets. GeV/c The particle detected has some associated partners. There should be no observable jet structure distinguishable from the background.

  24. For both (a) forward production at RHIC and (b) 10<pT<20 GeV/c at LHC, we expect large p/ ratio and no associated particles above background. Summary The  puzzle is resolved by recognizing the existence of ridge (without the usual jet) that constitutes the observed associated particles, while keeping the exponential pT dist of .

  25. Back-up slides

  26. PHOBOS, nucl-ex/0509034 Back et al, PRL 91, 052303 (2003) Forward production of hadrons Without knowing pT, it is not possible to determine xF But now BRAHMS has pT distribution

  27. BRAHMS has data at √s=62.4 GeV Forward Production What is significant about it?

  28. AuAu collisions BRAHMS, nucl-ex/0602018

  29. xF = 0.9 xF = 0.8 xF = 1.0 TFR TS ? TTT TT

  30. In pB collision the partons that recombine must satisfy p B A B But in AB collision the partons can come from different nucleons Theoretically, can hadrons be produced at xF > 1? (TFR) It seems to violate momentum conservation, pL > √s/2. In the recombination model the produced p and  can have smooth distributions across the xF = 1 boundary.

  31. FR TRF proton • : momentum degradation factor pion Hwa & Yang, PRC 73,044913 (2006) •  not constrained by data • no regeneration of soft partons • pT distribution not studied • p/ to be determined

  32. Many issues to consider about forward production 1. Trans-fragmentation region (TRF) xF>1 2. Momentum degradation of partons in traversing nuclear medium. (baryon stopping in pA collisions) 3. Regeneration of soft partons and gluon conversion. 4. More than one forward nucleons can contribute to the formation of a hadron at large x. (x>1 is possible) 5. Transverse momentum distribution near the kinematical boundary. (suppression of hard scattering) 6. Large p/ ratio. 7. Correlation at large .

  33. x 1 y y’ v Poissonian average over  A N Quark distribution after degradation Forward production at low pT is not a process that can be studied in pQCD. A model is needed to treat momentum degradation: valon model.

  34. Gluon conversion Momentum lost after  collisions:  momentum increase for sea quarks Regeneration of soft partons and gluon conversion Sum of parton momentum fractions: u,d sea quarks Valence quark

  35. is suppressed in forward region, so  production is also suppressed. Contribution from different forward nucleons

  36. Proton Pion Hwa & Yang, nucl-th/0605037

  37. Transverse momentum distribution Hard-scattered partons near kinematical boundary is suppressed. Thermal partons in central region: chemical equilibrium In forward region:

  38. overlap probability pion Given pT , k and k’ can be smaller,thus enhancing fi(k)fi’(k’). Effect is even more pronounced for proton formation.

  39. Does not approach limiting dist. for 1-jet Limiting distribution for 1-jet fragmentation Fragmentation of a parton to a proton has very low probability, but recombination of shower partons from two jets increases the yield.

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