Jet-Medium Interactions with Identified Particles

Identified particles have long been of great interest at RHIC in large part because of the baryon/meson differences observed at intermediate pT and the implications for hadronization via quark coalescence. With recent high statistics data identified particles are also now central to understanding the details of the jet-medium interactions and energy loss and hadron formation at intermediate and high pT . In particular, high pT identified particle spectra along with two-particle correlations triggered with direct photons, neutral pions or electrons from heavy flavor decay with hadrons can provide information about how medium modifications to jet fragmentation depend on parton type. I will review recent results with identified particles both in heavy ion systems and the reference measurements in p+p collisions. 1. Hard Scattering at RHIC Hard probes of relativistic heavy ion collisions have long been a valuable probe of the hot matter created in relativistic heavy ion collisions. Their main value comes from the fact that their production is both calculable in perturbative QCD and measurable in proton-proton collisions, providing quantified expectations for heavy ion collisions. Deviations from proton-proton expectations provide measurements of the effects of the hot dense matter on the propagation of fast partons. In high energy experiments hard probes are often measured through directly reconstructing full jets. Such measurements are not naturally suited to the large soft background of heavy ion collisions, especially when one wants to study the modification of jets from baseline proton-proton collisions measurements (for a discussion of jet reconstruction in heavy ion collisions see [1, 2]). More experimentally straightforward are single particle spectra and two particle correlations which provide complementary observables with relatively large rates. Particle identification provides a valuable experimental handle for changing the probe partons (e.g. light quarks, gluons, heavy quarks, photons) and for studying the effect of the matter on hadron formation [3, 4, 5, 6]. Current results from RHIC allow detailed quantitative studies of how jet modifications depend on parton type and provide insights on the interactions of the matter with fast partons and the hadron formation process. 2. Single Particle Production The most stringent constraint on the opacity of the hot matter, within a particular theoretical model, comes from π0 RAA [7]. The RAA of other particles can provide additional information Email address: [email protected] (Anne M. Sickles) Preprint submitted to Nuclear Physics A September 15, 2009 ar X iv :0 90 7. 49 21 v2 [ nu cl -e x] 1 4 Se p 20 09 about parton-medium interactions and hadron formation. Expectations for the RAA of protons and anti-protons were that they would be more suppressed than π0 since gluon jets were thought to produce more baryons than quark jets do (however, recent fragmentation functions incorporating the STAR p, p̄ spectra cast some doubt on this [8]). Since gluons have a larger QCD color factor, they should lose more energy. However, recent measurements from STAR show that RAA(p, p̄) > RAA(π), Fig. 1. At intermediate pT , 2 < pT < 6 GeV/c, this difference is widely thought to be from recombination (see Ref [9] and references therein), though realistic calculations taking into account spatial correlations, energy and momentum conservation and gluons have not yet been done. For pT > 6GeV/c, the difference between RAA(p, p̄) and RAA(π) is smaller but still significant and is not understood. One possible idea is that as the jet partons traverse the matter they change flavor after scattering on matter partons [10, 11], for example a fast quark could scatter off a medium gluon and emerge as a fast gluon. Discovery of these conversions would be extremely interesting as it would provide a means to study the mean free path of partons in the matter. However, this mechanism could make RAA(p, p̄) ≈ RAA(π), but cannot make RAA(p, p̄) > RAA(π) so it cannot, in itself, explain the current data. Alternatively, it has been proposed that baryons and anti-baryons observed at RHIC at high pT are produced in higher twist QCD processes in the initial state [5]. Processes such as uu→ pd̄ can occur in QCD, but they are typically rare compared to production via jet fragmentation. However, since the protons produced directly in the hard scattering are small enough to propagate through the matter without interacting, similar to a direct photon. Such p and p̄ would also be produced in proton-proton collisions, however the dense matter in the heavy ion collision selects out these protons because the partons that would lead to fragmentation p and p̄ production lose energy. This mechanism naturally leads to RAA(p, p̄) > RAA(π). The decreasing v2 of p and p̄ [12] observed by PHENIX is also easily explained if an increasing fraction of the p and p̄ is insensitive to the path length through the matter and this also explains the centrality dependence of p and p̄ triggered correlations at intermediate pT [13]. 3. Two Particle Correlations Two particle correlations have been used extensively to investigate jet production at RHIC. They are complementary to single particle observables, especially in that they are expected to have a different sensitivity to geometry. Single particle observables are thought to be strongly biased toward particles toward partons which have lost less than the average amount of energy which is dominated by those with a short path length through the matter. Requiring a correlated particle changes the surface bias depending on the pT and ∆φ between the trigger and associated particle. At high pT , two-particle back-to-back correlations can help constrain the path length dependence of energy loss. Previous measurements with charged hadrons have shown a strong suppression of the away side hadrons when a high pT trigger is required [15, 16]. At intermediate pT with hadron-hadron correlations a third peak at |∆φ − π| ≈1 radian, termed the shoulder is observed [16, 17, 18]. Both the trigger particle and pT dependence of this structure are of great interest in determining its origin and sensitivity to properties of the matter. New measurements with the 2007 RHIC run allow further exploration of this with identified triggers and smaller pT bins. Identified trigger particles are important because of the dependence of RAA on the hadron species out to the highest measured pT .