Abstract The experimental ratio of

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ቤተ መጻሕፍቲ ባይዱ
High-energy heavy-ion collisions have become the focus of intense research because of the possibility of producing a decon ned quark-gluon plasma during such collisions 1,2]. The J= suppression has been suggested as a way to probe the screening between a charm quark with its antiquark partner in the plasma 3]. While the J= suppression has been observed 4,5] as predicted, the phenomenon can be explained by the absorption model 6{8], which was actually introduced earlier 9] to measure the total -N cross section using J= suppression. In the model of Gerschel and Hufner 6], the collision of a J= particle with baryons of the colliding nuclei may lead to the breakup of the J= into an open-charm pair. An e ective J= -baryon absorption cross section abs( N ) of 5-6 mb can explain the systematics of the suppression phenomenon 6,2]. One can alternatively describe J= suppression in terms of the interaction of the J= particle with produced hadrons (comovers) 7,8]. A comparison of the production of 0 with J= has been suggested to distinguish between decon nement and absorption 10]. The NA38 experimental measurements using protons and heavy ions at 200A GeV and 450A GeV reveal three features 11,12]: 1) 0= is approximately a constant in pA collisions, 0 independent of energies, 2) 0= decreases as the transverse energy ET increases in SU collisions, and 3) 0= for SU collisions is about 0.5 of that for pA collisions. The rst feature, further supported by other pA experiments 13], implies that in pA collisions 0 is suppressed in the same way as . We would like to describe an absorption model with abs( 0N ) = abs( N ) and with additional soft-particle disruption to explain all three features of the phenomenon. The production of J= or 0 occurs by the interaction of the partons of one baryon with the partons of the other baryon. The incipient cc pair is created with a radial dimension of the order of 0:06 fm at tcc. It is necessary for the incipient cc system to evolve to the bound state rms radius of 0.24 fm for at t and 0.47 fm for 0 at t 14,15]. Because J= is produced predominantly in the central rapidity region 16], the incipient cc pair is produced predominantly in the central rapidity region. In soft particle production in a baryon-baryon collision, we envisage Bjorken's inside0
2
outside cascade picture 17] or Webber's picture of gluon branching 18] as a q and a q (or a diquark) pull apart. After the collision, the diquark of one nucleon and the valence quark of the other nucleon pull apart and the gauge eld between them is polarized. Gluons are emitted at tg, and the system hadronizes at th. The characteristics of these produced gluons cannot yet be determined from nonperturbative QCD, but the shape of the rapidity distribution of produced gluons should be close to that of the produced hadrons. Thus, produced gluons are found predominantly in the central rapidity region. Because we shall use the J= (or 0) production rate in a nucleon-nucleon collision as a unit of reference, it is not necessary to include explicitly the interaction of the incipient cc system with gluons and their hadronized products in the same nucleon-nucleon collision, when we study pA and nucleus-nucleus (AB ) collisions. The space-time diagram for a typical pA collision is depicted schematically in Fig. 1a. The trajectory of an incipient cc pair, which is produced predominantly in the central rapidity region of the colliding baryons, does not cross the trajectories of soft particles produced in earlier or later collisions. Therefore, there is little interaction between the produced cc system and these soft particles. However, the cc system collides with baryons crossing its trajectory to lead to the breakup of the cc system into DDX . Such a reaction requires the production of at least one light-quark pair and is an inelastic process. In the collision at 200A GeV, the cc rapidities are separated from the baryon rapidities by about two units and the reaction cross section can be calculated in the Additive Quark Model (AQM) 19], whose approximate validity and connection with soft Pomeron exchanges have been re-assessed recently from the systematics of total hadron- and -hadron cross sections 20]. Using the Glauber theory and a Gaussian thickness function, the total cc-baryon inelastic cross section in the AQM is given by Eq. (12.27) of Ref. 2]:
ORNL-CTP-95-04 and HEP-PH 9506270
Suppression of
0
and
J=
in High-Energy Heavy-Ion Collisions
Cheuk-Yin Wong
Oak Ridge National Laboratory, Oak Ridge, TN 37831
(November 3, 1995)
Abstract
The experimental ratio of 0 to J= is approximately a constant in pA collisions, but decreases as the transverse energy increases in nucleus-nucleus collisions. These peculiar features can be explained as arising from approximately the same cc-baryon absorption cross section for 0 and J= but greater disruption probabilities for 0 than for J= due to the interaction of the cc system with soft particles produced in baryon-baryon collisions.