Cronin Effect in Hadron Production Off Nuclei

Abstract

Recent data from RHIC for high-pT hadrons in gold-gold collisions raised again the long standing problem of quantitatively understanding the Cronin effect, i.e. nuclear enhancement of high-pT hadrons due to multiple interactions in nuclear matter. In nucleus-nucleus collisions this effect has to be reliably calculated as baseline for a signal of new physics in high-pT hadron production. The only possibility to test models is to compare with available data for pA collisions, however, all existing models for the Cronin effect rely on a fit to the data to be explained. We develop a phenomenological description based on the light-cone QCD-dipole approach which allows to explain available data without fitting to them and to provide predictions for pA collisions at RHIC and LHC. We point out that the mechanism causing Cronin effect drastically changes between the energies of fixed target experiments and RHIC-LHC. High-pT hadrons are produced incoherently on different nucleons at low energies, whereas the production amplitudes interfere if the energy is sufficiently high. PACS: 24.85.+p, 13.85.Ni, 25.40.Qa It was first observed back in 1975 [1] that high-p T hadrons are not suppressed in proton-nucleus collisions, but produced copiously. This effect named after James Cronin demonstrates that bound nucleons cooperate producing high-p T particles. Indeed, it has been soon realized that multiple interactions which have a steeper than linear A-dependence lead to the observed enhancement. An adequate interpretation of the Cronin effect has become especially important recently in connection with data from RHIC for high-p T hadron production in heavy ion collisions [2,3]. The observed suppression factor can be understood as a product of two terms. One is due to multiple interactions within the colliding nuclei, analogous to the Cronin effect. The second factor arises from final state interaction with the produced medium, the properties of which are thus probed. This second factor, the main goal of the experiment, can be extracted from data only provided that the Cronin effect for nuclear collisions can be reliably predicted. However, in spite of the qualitative understanding of the underlying dynamics of this effect, no satisfactory quantitative explanation of existing pA data has been suggested so far. Available models contain parameters fitted to the data to be explained (e.g. see [4,5,6]) and miss important physics. In this paper we suggest a comprehensive description of the dynamics behind the Cronin effect resulting in parameterfree predictions which agree with available data.First of all, the mechanism of multiple interactions significantly changes with energy. At low energies a highk T parton is produced off different nucleons incoherently, while at high energies it becomes a coherent process. This is controlled by the coherence lengthwhere k T is the transverse momentum of the parton produced at mid rapidity and then hadronizing into the detected hadron with transverse momentum p T . For a coherence length which is...

Jet quenching by (pre-)hadronic final state interactions at RHIC

“…This 'Cronin effect' is probably related to an increase of the average transverse momentum squared k 2 ⊥ of the partons in the nuclear medium. One may speculate that such an enhancement of k 2 ⊥ is due to induced initial semi-hard gluon radiation in the medium, which is not present in the vacuum due to color neutrality [25,37]. Since the microscopic mechanisms are beyond the scope of our present analysis, we simulate this effect in the transport approach by increasing the average k 2 ⊥ in the string fragmentation with the number of previous collisions N prev as [35,53,59]…”

Measurement of D-meson production versus multiplicity in p-Pb collisions at s N N = 5.02 $$ \sqrt{{\mathrm{s}}_{\mathrm{NN}}}=5.02 $$ TeV

“…The leading twist nuclear shadowing calculation [60] assumes the GlauberGribov approach of the collision geometry and predicts the dependence of the nPDF on the collision impact parameter. The estimates of the initial-state k T broadening due to multiple soft collisions also consider a dependence on the collision impact parameter [46,47]. Initial-state parton energy loss is also expected to evolve with the collision geometry as a consequence of the different nuclear density, though detailed calculations including this effect are not yet available.…”

“…Alternatively, when the production process is dominated by gluons at low Bjorken-x, the nucleus can be described by the Colour-Glass Condensate (CGC) effective theory as a coherent and saturated gluonic system [41][42][43][44]. The kinematics of the partons in the initial state can be affected by multiple scatterings (transverse momentum broadening, or k T broadening) [45][46][47] or by gluon radiation (energy loss) [48] before the heavy-quark pair is produced. Gluon radiation may also occur after the heavy-quark pair…”

Measurement of the D0 Meson Production in Pb–Pb and p–Pb Collisions

“…If the coherence length is longer than the nuclear radius all amplitudes interfere coherently and result in a collective parton distribution of the nucleus. In this regime hard partons originate mainly from radiated gluons and the gluon shadowing is responsible of a reduction of the Cronin effect [82][83][84].…”