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

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 (R AA factor) or by comparing central AA to peripheral AA (R CP 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.

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

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

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

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

For p t from GeV/c in central collisions Λ production approximately follows N bin scaling. At higher p t however, a suppression with respect to N bin scaling is seen for both K S and Λ. A significant difference is seen between the p t dependence of K S and Λ R AA. 7 The particle identified data preliminary

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

Minimum bias v 2 for 200 GeV Au+Au Fit can be blast wave or hydro See Raimond’s talk tomorrow preliminary

The v 2 saturation and the the decrease in R AA appear to be loosely correlated for both K S and Λ. What physics is behind the p t scales of the saturation in v 2 and the suppression in R AA. How does the particle species influence the p t dependences? 10 Species dependence of v 2 and R AA vs. p t.

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 ? 11 The collision phases for high pt particles 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

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 k T broadening. Parametrized through k T broadening functions, which also includes ‘random walk’ elastic scattering corrections.

Old pA measurements P.B. Straub et al., PRL 68 (1992) Fermilab experiments measuring R (W / Be) for identified particles at  s 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)

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. 14 Two ideas: initial or final state effect ?

The high density QCD regime (gluon saturation) Saturation scale At RHIC: Q s 2 = 2 GeV 2, p t =4 GeV/c See Dima’s talk tomorrow morning

16 Gluon interaction in cold and hot matter 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 Gluon interactions occur already in cold nuclear matter but the effects are different (A. Krzywicki et al., PLB 85,1979) Gluon bremsstrahlung   gluon density 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 0.5 GeV/fm to 7.3 GeV/fm) Does the gluon filter survive ? (  vs.  )

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 R AA. Initial gluon density ~ 1000 At LHC hadronic fragments from energetic jets compensate the increasing jet quenching. Initial gluon density ~ Gyulassy-Vitev-Levin predictions

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 Who can distinguish between baryons and mesons ?

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 Two ideas: soft/hard models

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

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

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. 22 The prominent role of dA collisions ?

Summary and Outlook R AA, v2 and particle ratios for strange particles show a strong species dependence as a function of the measured p t. Are we seeing non-pQCD effects on baryons ? Is this a mass dependence or a flavor dependence? Are all these effects (v 2, R AA, ratios as a function of p t ) 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

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