aa r X i v : . [ nu c l - t h ] S e p High- P T Physics with Identified Particles
Rainer J. Fries a , b and Wei Liu a a Cyclotron Institute, Texas A & M University, College Station, TX 77843, USA b RIKEN / BNL Research Center, Brookhaven National Laboratory, Upton, NY 11973, USA
Abstract
The suppression of high- P T particles in heavy ion collisions was one of the key discoveries at theRelativistic Heavy Ion Collider. This is usually parameterized by the average rate of momentum-transfer squared to this particle, ˆ q . Here we argue that measurements of identified particles athigh P T can lead to complementary information about the medium. The leading particle of a jetcan change its identity through interactions with the medium. Tracing such flavor conversionscould allow us to constrain the mean free path. Here we review the basic concepts of flavorconversions and discuss applications to particle ratios and elliptic flow. We make a predictionthat strangeness is enhanced at high P T at RHIC energies while its elliptic flow is suppressed.For the past decade high momentum particles and jets have been used to probe the quarkgluon plasma (QGP) phase created at the Relativistic Heavy Ion Collider (RHIC). The energyloss of a fast parton su ff ered in the medium carries information about the typical momentumtransfer µ along the path, more precisely about the transport coe ffi cient ˆ q = µ /λ [1, 2, 3, 4, 5, 6].We have recently argued that the mean free path λ of a fast parton could be determined separatelyby measuring the change in hadro-chemistry induced by the medium [7, 8].Here we discuss a model based on conversions of the leading particle of a jet. Just as partonscan lose energy through collisions and induced radiation, they can scatter through channels inwhich the identity of the fastest parton in the initial and final state are not the same. Examplesare binary collisions like q + ¯ q ↔ g + g or q + g ↔ g + q which can lead to conversions ofquarks into gluons and vice versa. Here the first parton on each side has a large momentum (theleading jet parton) and the second parton in the initial state is a thermal parton from the quarkgluon plasma. The rate of flavor conversions depends on the mean free path λ of fast partons.Conversions between quarks and gluons should obscure their di ff erent color factors couplingthem to the medium. Instead of a relative factor 9 / q only an average color factor should beobservable in a long enough medium. It was pointed out that a larger quenching for gluons couldbe reflected in more suppression of protons compared to pions given the preference of gluonto proton fragmentation in modern fragmentation functions [9]. Jet conversions should softenthis e ff ect and increase the proton to pion ration in central collisions [10, 11]. Fig. 1 shows theratio of nuclear modification factors R AA for protons and pions with and without conversionsbetween quarks and gluons taken into account [7]. Clearly, flavor conversions lead to less protonsuppression.Conversions had also been discussed before for photons and dileptons. Fast quarks andgluons can create real or virtual photons through Compton and annihilation processes with themedium, q + g → γ + q , q + ¯ q → γ + g . It has been realized over the years that this process canmake a large contribution to the overall yield of photons or dileptons [12, 13, 14, 15, 16, 17]. Fig. Preprint submitted to Nuclear Physics A July 12, 2018 with conv w/o conv PHENIX prelim R g AA p T (GeV/c) Au+Aus = 200 GeV with conv w/o conv R p AA / R p AA p T (GeV/c) Au+Aus = 200 GeV
Figure 1: Left panel: The nuclear modification factor R AA for direct photons with and without conversions switchedon, calculated in the model introduced in [7] (preliminary PHENIX data from [18]). Right panel: The ratio of nuclearmodification factors for protons and pions is approaching one if conversions are allowed. with conv w/o conv R S AA p T (GeV/c) Au+Aus = 200 GeV
Figure 2: R AA for neutral kaons with and without conversion processes allowed. The strangeness in the jet sample isdriven towards equilibrium by coupling it chemically to the quark gluon plasma. < g + g → s + ¯ s and in particular kick-out reac-tions like g + s → s + g can lead to an enrichment of strangeness in the jet sample. Obviously, fora su ffi ciently long medium the jet sample would be driven toward chemical equilibrium throughinteractions with the chemically equilibrated medium. Fig. 2 shows the expected nuclear modi-fication factor for neutral kaons at RHIC. For the perturbative elastic rates chosen in [7] with a K factor of 4 we see a clear enhancement extending above 10 GeV / c .We have also checked the e ff ect of conversions on heavy quark production [19]. We do notfind any significant yields of charm or bottom quarks at high P T from these processes. The same2
10 15 20 25 300.000.050.100.150.20 20--30%Au+Au @ 200 GeV g u,d sv p T (GeV) g u,d sv p T (GeV) Figure 3: Left panel: The azimuthal asymmetry v for light quarks, strange quarks and gluons without conversions.Right panel: the same with conversions. The v for light quarks and gluons is not similar, while strange quarks exhibit asuppression. is true even at LHC energies. The reason is that thresholds and low center of mass energiessuppress pair production in interactions of jets with the medium, and kick-out reactions su ff erfrom small heavy quark densities in the medium to begin with. Even for charm at LHC we donot expect chemical equilibration and there is no large chemical gradient between heavy quarksin jets and the medium.We have also studied the e ff ect of flavor conversions on the azimuthal asymmetry v . It wasfirst pointed out in Ref. [20] that photons from jet-medium interactions should be more abun-dantly produced in the direction where the medium is thicker, leading to a negative contributionto v . See [21] for comprehensive calculations of photon elliptic flow. It is important to realizethat this mechanism is generally true for all particles produced in jet-medium interactions [22].In particular, we expect it to hold for additional strange hadrons produced at RHIC. Fig. 3 showsthe e ff ect of flavor conversions on the azimuthal asymmetry v of up, down and strange quarks aswell as gluons. As predicted, conversions make light quarks and gluons behave similarly whilethere is a significant decrease in the v of strange quarks due to the additional large yield witha negative contribution. Note that the total v is determined from summing up all sources ofparticles both with positive and negative v . Fig. 4 shows the resulting azimuthal asymmetry v that we expect for kaons at RHIC [22].In summary, we have shown that jet conversions can lead to measurable signatures in hadronproduction at high P T . We predict an enhancement of strange hadrons at high P T at RHIC and asuppression of their azimuthal asymmetry coe ffi cient v . Acknowledgments
We acknowledge support by the RIKEN / BNL Research Center and DOE grant DE-AC02-98CH10886.
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