Baryon Production in Heavy Ion Collisions

1. Baryon Production in Heavy Ion Collisions manifestations of multi-parton dynamics Paul Sorensen

2. Nucleus-nucleus collisions may probe the physics of this quark-hadron transition Today t = 51017 sec T=1 meV 197Au 197Au √sNN=200 GeV Quark-hadron transition Hadrons form - protons & neutrons t = 10-6 sec T=1 GeV The Planck epoch UrQMD Group – Frankfurt heavy-ion collisions San Diego — March 2006

3. pT indicates the length scale the hadron can probe 0.6 < pT/n (GeV/c) < 2.0 0.33 >  (fm) > 0.1 centrality dependence RCP is the number of particles produced in head-on collisions divided by the number produced in grazing collisions(expected increase with system size scaled out) For intermediate hadron probes, RCP depends on the number of quarks in the hadron. Production increases more quickly for hadrons with three instead oftwo quarks Central: head-on higher density fireball Peripheral: grazing less dense San Diego — March 2006

4. identified particle RAA/RCP e-Print Archive: nucl-ex/0510052 • A large suppression of hadrons is observed • Measurements interpreted in terms of energy-loss for quarks and gluons in medium • The suppression patterns depend on particle-type • Baryons exhibit less suppression • Measurements interpreted several ways: hydro-like flow, protons scaling with N-binary, coalescence/recombination, gluon junctions etc. San Diego — March 2006

5. baryon to meson ratio the faster increase of baryon production is common to many systems: it can’t be q vs. g fragmentation multi-parton dynamics (power corrections, higher twist) before being applied in Au+Au factorization should be established experimentally San Diego — March 2006

6. / expectations ratios may depend on flavor Oregon Reco. the expectation for Ω/ is probably not correct given the power-law shape in p+p San Diego — March 2006

7. lower √sNN is the baryon enhancement just a lack of baryon suppression? at lower √sNN the RCP of baryons is above unity San Diego — March 2006

8. anti-baryon to baryon ratios gluon vs quark hadronization predictions don’t work in A+A or p+p collisions “soft” physics extends quite high in pT San Diego — March 2006

9. implications for “charm” RAA charmelectron branching ratios depend on the hadron type: c decays yield electrons less frequently than D0 decays. when using non-photonic electrons to look for charm, charm can hide in the c A baryon enhancement in the charm sector can result in a non-photonic electron suppression San Diego — March 2006

10. an example • consider a charm spectrum that follows a power law • for Au+Au modify the default c/D ratio to match the /KS (while holding the total number charm hadrons fixed) • compare the decay electron spectra in p+p and Au+Au decay electron RAA The ratio shows that when c/D has the same form as /KS, even if the charm yield follows Nbin scaling, the electron spectrum will be suppressed San Diego — March 2006

11. example 2 Consider c/Dsimilar to/KS and charm RAA similar to light hadron RAA decay electron RAA Data are above the curve we cannot say charm quarks are suppressed as much as light quarks • The resulting electron RAA is below preliminary measurements! • Implies energy loss for charm quarks may be smaller than for light quarks San Diego — March 2006

12. preferred charm RAA A 2analysis shows that for this scenario, preliminary PHENIX data prefer charm hadron RAA = 1.35  light hadron RAA decay electron RAA San Diego — March 2006

13. higher pT decay electron RAA A charm baryon enhancement as parameterized by /KSdoes not change the non-photonic electron RAA above 5 GeV/c but what pT would a c/D enhancement persist to? We should measure c. bottom contribution is highly uncertain and not considered here? San Diego — March 2006

14. what about Ds/D0 decay electron RAA enhance the Ds yield by 50% (for this plot the Ds/D0 ratio is taken to be pT independent, not really a good assumption) Depending on the poorly known branching ratio, a Ds enhancement can also contribute to an electron suppression San Diego — March 2006

15. x z Non-central Collisions azimuthal dependence: v2 azimuthal momentum-space anisotropy: a self quenching probe of early interactions out-of-plane y Au nucleus in-plane Au nucleus sensitivity to the system geometry can arise through secondary rescattering, soft-gluon radiation, etc. San Diego — March 2006

16. in-plane baryon enhancement • At intermediate pT, v2 of hadrons display quark number dependence • Baryon yields have more in-plane enhancement than meson yields San Diego — March 2006

17. PRL 92 (2004) 052302; PRL 91 (2003) 182301 quark-number scaling • Since hadronization is a soft process, it can be modified by the presence of a QGP. • Simple models of hadronization by coalescence relate quark and hadron v2: • v2q = v2h(pT/n)/n, • n is the number of quarks in the hadron • Models imply • v2 is developed before hadrons form • deconfinement: long range interactions amongst quarks not hadrons San Diego — March 2006

18. run 4 v2(pT/n)/n update With higher precision, fine structure is observed: the scaling doesn’t follow NCQ (3 vs 2) exactly Is this related to a jet component, a breakdown of simple approximations, or something new? Q:Are coalescence models invalidated? A:Not necessarily. Imperfect v2/n was also anticipated within ReCo models. STAR Preliminary San Diego — March 2006

19. Inclusion of gluons in recombination was predicted to lead to a larger meson v2/n than baryon v2/n: B.Müller, et al. nucl-th/0503003 To 1st order: the constituents look like massive valence quarks with no sub-structure  g Probing more deeply reveals substructure in the form of sea quarks or gluons g qq Further daughter partons are revealed as the scale Q2 is increased Including contributions from sea quarks and gluons should cause deviations from v2/n a closer look STAR Preliminary San Diego — March 2006

20. comparisons to predictions Inclusion of gluons in recombination was predicted to lead to a larger meson v2/n than baryon v2/n: B.Müller, et al. nucl-th/0503003 Tantalizing indication for the fate of gluons and the nature of the constituents Systematic errors on the data and calculations need to be carefully addressed San Diego — March 2006

21. comparisons to predictions Effect of the quark momentum distribution inside the hadron: V.Greco, et al. Phys.Rev.C70:024901,2004 A momentum distribution within hadrons leads to v2/n breaking similar to data. Fragmentation component may be necessary at higher pT San Diego — March 2006

22. comparisons to predictions MPC + Coalescence/Jetset: jet fragmentation contribution + spatial correlations also can distort the scaling of v2/n: D. Molnar nucl-th/0406066 pT/n The original parton v2 is not recovered from scaling so the opacity puzzle remains. Calculation fails for p/ and RCP but it does predict the gradual drop in v2 at high pT. San Diego — March 2006

23. from hadronic x-sections? • / ratio should distinguish hadronic cross-sections from coalescence. • Is there a duality between the pictures? • interacting co-moving quarks form a hadron • interacting hadrons described by the sum of their constituent quarks • Both yield ~NCQ scaling. Y. Lu, et. al. here, larger proton v2 is from the additive quark model for hadronic cross-sections San Diego — March 2006

24. Corona Demonstrates that the corona effect can yield interesting pT and particle type dependences What about a “corona” in e+e- will the simple parameterization of the core get the K* or  v2 and RCP? San Diego — March 2006

25. another B/M enhancement away-side: comoving q/g enhance the probability for baryon production near-side like p+p • The large away-side B/M ratio also points to multi-gluon/quark dynamics: • wherever the density of comoving constituents is larger • in-plane: v2 • central vs. peripheral: RCP • and in the “wake” of quenched jets, it becomes easier to produce baryons. San Diego — March 2006

26. Conclusions: • the baryon enhancement in e+e-ggg seems to rule out a quark vs gluon jet fragmentation explanation: pbar/p ratios also confirm this • a mass hypothesis is strongly disfavored by , , , and  RCP and v2 (corona?) • Baryon junctions may be disfavored by the enhancements at √sNN=17 GeV (Nbin) • The baryon enhancement is generic to many systems (e+e-, d+Au): does v2/n point to interactions within a deconfined phase!? • Some open questions: • Do diquarks play a role in our observations? • Why does the [MPC + Coalescence/Jetset + spatial correlations] model fail to describe the data? Which part is wrong? • How much do hadronic cross sections contribute to the species dependence? Definitely shouldn’t be overlooked! • Outlook: • changes in hadronization induced by high energy densities may be a valuable tool to study -symmetry and confinement San Diego — March 2006