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Rene Bellwied Wayne State University 19 th Winter Workshop on

Strange particle production at the intersection of soft and hard processes at RHIC. Rene Bellwied Wayne State University 19 th Winter Workshop on Nuclear Dynamics, Breckenridge, Feb 8 th -15 th. The Premise.

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Rene Bellwied Wayne State University 19 th Winter Workshop on

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  1. Strange particle production at the intersection of soft and hard processes at RHIC Rene Bellwied Wayne State University 19th Winter Workshop on Nuclear Dynamics, Breckenridge, Feb 8th-15th

  2. The Premise • We see high pt charged particle suppression at pt > 2 GeV/c compared to results in pp (CERN-UA experiments) at comparable incident energies • The effect can be measured by comparing central AA to pp (RAA factor) or by comparing central AA to peripheral AA (RCP factor) • This effect is particle species dependent as shown by the latest STAR results for neutral Kaon and Lambda particles, and by PHENIX for pions and protons.

  3. Preliminary Suppression of inclusive charged hadrons Centrality dependence normalized to NN (130 GeV, nucl-ex/0206011) Central/peripheral (200 GeV) • Clear evidence for high pT hadron suppression in central collisions • Suppression factor ~constant for 6<pT<12 GeV/c

  4. We have no clue, but we’re hip !! First particle identified spectra (PHENIX) Do baryons and mesons have different origin ? Is there a parton flavor dependence ? Science Magazine, 298 (2002) 718 4

  5. In STAR were PID of non decaying particles is limited in pt range by dE/dx, we use short lived particles decaying via a V0 topology, because the topology cuts allow us to reconstruct particles without a pt-cutoff 5

  6. preliminary preliminary One sees the mass dependence of the transverse expansion, which is well described by thermal and hydrodynamics models High pT strange particles in Au+Au

  7. preliminary preliminary The particle identified data For pt from 1.8-3.2 GeV/c in central collisions Λ production approximately follows Nbin scaling. At higher pthowever,a suppression with respect to Nbin scaling is seen for both KS and Λ. A significant difference is seen between the pt dependence of KS and Λ RAA. 7

  8. preliminary preliminary Comparison to (h++h-)/2 8

  9. preliminary preliminary Minimum bias v2 for 200 GeV Au+Au Fit can be blast wave or hydro See Raimond’s talk tomorrow

  10. Species dependence of v2 and RAA vs. pt. The v2 saturation and the the decrease in RAA appear to be loosely correlated for both KS and Λ. What physics is behind the pt scales of the saturation in v2 and the suppression in RAA. How does the particle species influence the pt dependences? 10

  11. The collision phases for high pt particles Initial parton parton collision (depends on parton structure functions) Parton traveling through medium – recombination / coalescence ? Parton fragmentation into jets (depends on fragmentation functions) Jets (hadrons) traveling through medium – scattering / absorption Which part is affected ? What causes the suppression ? • In pp: a.) Initial parton momentum fraction probabilities from parton distribution functions (PDF) (LO Glueck-Reya-Vogt 98) • b.)Momentum fraction carried away by leading hadron from fragmentation functions (FF) (LO Binnewies-Kniehl-Kramer) • c.) Constant K-factor and parton kT broadening function to account for NLO corrections 11

  12. Are there ‘normal’ nuclear effects ? • Nuclear Shadowing • Nuclear modifications to the parton distribution function in cold nuclear matter, measured and parametrized in the ‘shadowing function’ (EKS98) • Cronin effect • Multiple initial state scatterings of partons in cold nuclei lead to high pt particle enhancement compared to pp reactions and to kT broadening. Parametrized through kT broadening functions, which also includes ‘random walk’ elastic scattering corrections.

  13. Old pA measurements P.B. Straub et al., PRL 68 (1992) Fermilab experiments measuring R (W / Be) for identified particles at Ös of 27.4 and 51.3 GeV. Interpretation: Cronin effect drives all R(AA) above unity. Then at high pt all R(AA) approach unity which is the QCD limit. Interesting observation: Cronin effect seems to be mass and / or quark content dependent. (e.g. increased probability of gluon rescattering in nuclear matter relative to quarks is attributed to stronger K- effect)

  14. Initial state effect: gluon saturation Formation of new state of matter: Color Glass Condensate because of gluon saturation in Lorentz contracted nucleus at low x. Depends on parton packing factor. Structure functions will be modified. But does not describe back to back jet disappearance Final state effect: jet quenching in medium Energy loss in medium due to gluon radiation (QGP signature). Fragmentation functions will be modified. But gluon radiation should lead to additional soft particles, i.e. the charged particle multiplicity should go up. Two ideas: initial or final state effect ? 14

  15. At RHIC: Qs2 = 2 GeV2, pt=4 GeV/c Saturation scale The high density QCD regime (gluon saturation) See Dima’s talk tomorrow morning

  16. Gluon bremsstrahlung In hot nuclear matter the gluon interaction via gluon bremsstrahlung is enhanced even further (estimate by X.N.Wang by comparing RHIC to HERA data: factor 15 from 0.5GeV/fm to 7.3 GeV/fm) Does the gluon filter survive ? (L vs. L) DEµ gluon density Gluon interaction in cold and hot matter • Gluon interactions occur already in cold nuclear matter but the effects are different (A. Krzywicki et al., PLB 85,1979) In cold nuclear matter the triple-gluon coupling favors multiple gluon scattering, the fraction of gluon jets is enhanced at large pt, i.e.softening of gluon fragmentation function. Measurable by comparing K- (more gluon contribution) to K+ (more quark contribution) leading particles -> Gluon Filter 16

  17. Gyulassy-Vitev-Levin predictions Includes Cronin, Shadowing, and Quenching. Free parameter: initial gluon density. At SPS Cronin dominates but is still reduced by a factor 2 due to moderate jet quenching. Initial gluon density ~ 200 At RHIC jet quenching dominates, but as a f(pt) Cronin and quenching yield an almost constant RAA. Initial gluon density ~ 1000 At LHC hadronic fragments from energetic jets compensate the increasing jet quenching. Initial gluon density ~ 3000 17

  18. Who can distinguish between baryons and mesons ? If it’s an initial state effect It’s a generic effect acting on particle multiplicities and it should not be dependent on particle species If it’s a final state effect Energy loss of final state partons or hadrons will be species dependent and thus could lead to a different effect for baryons and mesons. Also, one has to consider the onset of soft particle production at lower pt and its effect on the soft/hard transition region Conclusion: let’s try soft/hard hybrid models 18

  19. Two ideas: soft/hard models Hydro with CGC saturation The soft part of the spectrum is dominated by hydrodynamics which affects the shape of the momentum spectrum differently for different species, i.e. the cross over point from soft to hard will be different from species to species. Baryon junctions with GLV bremsstrahlung The soft part of the spectrum is dominated by baryon junctions which affects the shape of the momentum spectrum differently for different species, i.e. the cross over point from soft to hard will be different from species to species. Conclusion: the ‘hard’ model is insensitive to the shape ? 19

  20. Interplay of soft and hard processes in GLV theory (v2 measurements) Hydrodynamics plus jet quenching describes general form of v2 pt dependence If pion at moderate pt is dominated by quenched pQCD, but baryon production is dominated by non pQCD effects such as Baryon junctions or hydro, then we expect qualitative different v2 behavior for different particle species

  21. preliminary Interplay of soft and hard processes in GLV theory (Vitev et al., hep-ph/0208108)

  22. If it’s an initial state effect Then the effect will still be there in dA collisions because the A still has gluon saturation. No QGP and thus no jet quenching, though. Perturbative QCD is the wrong approach for such a medium ! If it’s a final state effect Then the effect will not be there anymore in dA because no high energy loss medium (QGP) was formed. In this case the CGC model would be wrong. Compromise: one should see final state effects in addition to initial state effects, but not vice versa. The prominent role of dA collisions ? 22

  23. Summary and Outlook RAA, v2 and particle ratios for strange particles show a strong species dependence as a function of the measured pt. Are we seeing non-pQCD effects on baryons ? Is this a mass dependence or a flavor dependence? Are all these effects (v2, RAA, ratios as a function of pt) due to the same physics ? Is it initial state or final state effects. Is the difference due to soft interactions or hard interactions ? Charged particle measurements in dA will allow us to distinguish between initial and final state effects. Identified particle measurements in dA and AA will allow us to distinguish between different final state effects 23

  24. Recombination and fragmentation of partons (Mueller et al., nucl-th/0301087) dominated by fragmentation dominated by recombination (quark coalescence)

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