Rene Bellwied

Wayne State University

19th Winter Workshop on

Nuclear Dynamics, Breckenridge, Feb 8th-15th

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 (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.

Suppression of inclusive charged hadronsCentrality dependence normalized to NN

(130 GeV, nucl-ex/0206011)

• Clear evidence for high pT hadron suppression in central collisions• Suppression factor ~constant for 6<pT<12 GeV/c

Preliminary

Central/peripheral (200 GeV)

First particle identified spectra (PHENIX)

4Science Magazine, 298 (2002) 718

Do baryons and mesons have different origin ?

Is there a partonflavor 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

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High pT strange particles in Au+Au

One sees the mass dependence of the transverse expansion, which is well described by thermal and hydrodynamics models

preliminary preliminary

For pt from 1.8-3.2 GeV/c in central collisions Λ production approximately follows Nbin scaling.

At higher pt however, 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.

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The particle identified data

preliminary

preliminary

Comparison to (h++h-)/2

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preliminary

preliminary

Minimum bias v2 for 200 GeV Au+Au

Fit can beblast wave or hydro

See Raimond’stalk tomorrow

preliminary

preliminary

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?

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Species dependence of v2 and RAA vs. pt.

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 ?

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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 kT broadening. Parametrized through kT broadening functions, which also includes ‘random walk’ elastic scattering corrections.

Old pA measurementsP.B. Straub et al., PRL 68 (1992)Fermilab experiments measuring R (W / Be) for identified particlesat 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.

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Two ideas: initial or final state effect ?

The high density QCD regime (gluon saturation)

Saturationscale

At RHIC: Qs2 = 2 GeV2, pt=4 GeV/c

See Dima’s talktomorrow morning

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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 RAA.Initial gluon density ~ 1000

At LHC hadronic fragments from energetic jets compensate the increasing jet quenching.Initial gluon density ~ 3000

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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 models18

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 ?

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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/0208108)

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.

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The prominent role of dA collisions ?

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

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Recombination and fragmentation of partons (Mueller et al., nucl-th/0301087)

dominated by recombination(quark coalescence)

dominated by fragmentation