Time structure of anomalous J/ψ and ψ′ suppression in nuclear collisions

Abstract

The data for the mean squared transverse momentum p 2 t (E t ) as function of transverse energy E t of J/ψ and ψ ′ produced in Pb-Pb collisions at the CERN-SPS are analyzed and it is claimed that they contain information about the time structure of anomalous suppression. A transport equation which describes transverse motion of J/ψ and ψ ′ in the absorptive medium is proposed and solved for a QGP and a comover scenario of suppression. While the comover approach accounts for the data fairly well without adjusting any parameter, the fit to the data within the QGP scenario requires to assume anomalous suppression to become effective rather late, 3−4 fm/c after the nuclear overlap.The discovery in 1996 of anomalous J/ψ suppression in Pb-Pb collisions at the SPS has been one of the highlights of the research with ultrarelativistic heavy ions at CERN [1]. Does it point to the discovery of the predicted quark gluon plasma (QGP)? Six years later the situation is still confused, since several models -with and without the assumption of a QGP -describe the observed suppression, after at least one parameter is adjusted. The data on the mean squared transverse momentum p 2 t (E t ) [2] for the ψ (this symbol stands for J/ψ and ψ ′ ) in the regime of anomalous suppression and as a function of transverse energy E t have received less attention -for no good reason. We claim: p 2 t (E t ) contains additional information about the nature of anomalous suppression and may help to distinguish between different scenarios. In this paper we investigate how the time structure of anomalous suppression influences the values p 2 t (E t ). This idea has already been considered more than 10 years ago [3] (c.f. also more recent works [4,5]) and is based on the following observation ( Fig.1): Anomalous suppression is not an instantaneous process, but takes a certain time depending on the mechanism. During this time ψ's produced with high transverse momenta may leak out of the parton/hadron plasma and escape suppression. As a consequence, low p t ψ's are absorbed preferentially and the p 2 t of the surviving (observed) ψ's show an increase δ p 2 t , which grows monotonically with the mean time t A , when anomalous absorption acts [5]. In this letter we propose a general formalism of how to incorporate the effect of leakage into the various models, which have been proposed to describe 1

J/ψ transport in QGP and pt distribution at SPS and RHIC

“…Since the matter produced in central collisions is denser and hotter than that in peripheral collisions, the J/ψ with large p t has more chance to be absorbed by the QGP; On the other hand, the J/ψ with large p t has more chance to escape the anomalous suppression region, when its duration time in the QGP is shorter than the anomalous suppression time. This is the leakage effect initially indicated by Matsui and Satz [1] and studied in detail in the transport approach in [18]. The former mechanism suppresses the p 2 t , while the later enhances the p 2 t .…”

“…(12), has been shown in Ref. [18] to be important in the consideration of charmonium p t distribution. When we know the time evolution of the produced medium and the Ψ dissociation cross section in the medium, we can calculate the number of Ψ and its p t distribution at freeze-out time τ f with Eq.…”

“…where f N is the Ψ transverse distribution function at impact parameter b after the normal suppression [18],…”

“…The QGP evolution is calculated with a 2 + 1 dimensional boost invariant relativistic hydrodynamics [19]. Since J/ψ is heavy enough, a classical Boltzmann-type transport equation in the transverse phase space [18] is used to describe the evolution of its transverse distribution function. We assume that the local equilibrium is reached at a proper time τ 0 when the normal suppression has ceased and the anomalous suppression starts.…”

J/ψProduction in Quark-Gluon Plasma

“…Since the Ψ (=J/ψ, χ c ) is heavy, we use a classical Boltzmann-type transport equation to describe its evolution. The distribution function f Ψ (p t , x t , τ |b) in the central rapidity region and in the transverse phase space (p t , x t ) at fixed impact parameter b is controlled by the equationThe second term on the left-hand side arises from free-streaming of Ψ with transverse velocity v Ψ = p t / p 2 t + m 2 Ψ , which leads to the "leakage" effect and is needed to explain the averaged transverse momentum squared at the SPS [15]. The anomalous suppression and regeneration mechanisms are reflected in the loss term α Ψ and gain term β Ψ , respectively.…”