Proton-antiproton suppression in200AGeV Au-Au collisions

Abstract

We discuss the measured nuclear suppression of p + p production in 200 AGeV Au-Au collisions at RHIC within radiative energy loss. For the AKK set of fragmentation functions, proton production is dominated by gluons, giving rise to the expectation that the nuclear suppression for p + p should be stronger than for pions due to the stronger coupling of gluons to the quenching medium. Using a hydrodynamical description for the soft matter evolution, we show that this is indeed seen in the calculation. However, the expected suppression factors for pions and protons are sufficiently similar that a discrimination with present data is not possible. In the high pT region above 6 GeV where the contributions of hydrodynamics and recombination to hadron production are negligible, the model calculation is in good agreement with the data on p + p suppression. The STAR collaboration has measured the nuclear suppression ratio R CP (yield for 0 − 12% central collisions divied by yield for 60 − 80% peripheral collisions) for π + + π − and p + p in 200 AGeV Au-Au collisions [1], finding that the suppression pattern of both pions and protons is very similar in the 6+ GeV momentum region. This is to some degree surprising, as the AKK fragmentation functions [2] (which roughly describe the STAR data on proton production in p-p collisions [3]) indicate that ∼ 80 % of p+p production comes from gluon jets whereas only ∼ 40 % of pion production is gluon-driven. This is not so in the older KKP set of fragmentation functions [4] which consequently underpredict proton production at STAR. However, gluons as colour octet states are expected to undergo stronger energy loss in medium than quarks, thus the dominance of gluon jets in hard proton production should map into a stronger suppression of p + p production as compared to π + + π − production. It becomes thus a quantitative question in the energy loss model to check how large this difference in suppression should be and if present data are able to resolve it. A framework to study energy loss within full expansion dynamics of soft matter has been presented in [5] and extended to a 3-d hydrodynamical model [6] in [7]. We utilize this framework to address the above question. We stress that given the scenario describing pionic R AA , the change to study the suppression of protons is only the trivial change of the fragmentation function, no further parameters are adjusted. Furthermore, since the AKK fragmentation function overpredicts p + p production in the high p T region in p-p collisions (and the fragmentation function of q(q) → p(p) is well constrained), thus overemphasizing the role of gluons, we stress that our results are an upper limit for the expected difference between proton and pion suppression. Let us now discuss the treatment of partons propagating * Electronic address: trenk@phys.jyu.fi † Electronic address: kari.eskola@phys.jyu.fi through the medium: our calculation follows the BDMPS formalism for radiative energy loss [8] using quenching weights as introduced by Salgado and W...

Jet quenching from QCD evolution

“…They are obtained from the vacuum and medium evolution equations, as given in Eqs. (19)(20) and (37)(38), respectively. For the vacuum input, we take KKP fragmentation functions [76], and choose the factorization, fragmentation, and renormalization scales Q = p Tc in the following numerical calculations.…”

“…The related attenuation of particle flux, commonly referred to as jet quenching [23,27] has attracted tremendous attention from both the theoretical and experimental communities. There has been great progress in studying jet quenching phenomena based on the parton energy loss picture, ranging from inclusive light hadron and heavy meson production [19][20][21][28][29][30][31][32][33][34][35][36] to di-hadron correlations [37][38][39][40] and γ+hadron correlations [41][42][43][44].…”

Systematic comparison of jet energy-loss schemes in a realistic hydrodynamic medium

“…The parameter K in Eq. (18) is regarded as a parameter to account for the uncertainty in the selection of α s and possible non-perturbative effects increasing the quenching power of the medium (see discussion in [69]). …”

Hard Probes at RHIC

“…Thus they ultimately sample a wide range of parton energies. In addition, the leading hadrons once measured are more likely to come from the surface of the expanding medium [31]. Fully reconstructed jets overcome to large extent these limitations, however their reconstruction in nucleus-nucleus collisions is difficult because of their relatively small production rate and large fluctuating background.…”