PHENIX experiment at RHIC

Abstract

The purpose of my talk is to present an overview of the PHENIX experiment (Pioneering High Energy Nuclear Interaction experiment) at RHIC. Physics issues related to the quark-gluon plasma, in particular, in relation to this experiment, are listed in Table 1. Two important aspects exist for the quark-gluon plasma. One is “deconfinement”, which is closely related to the Debye screening of the QCD potential. The basic mechanism of J/$ suppression is that, if the J/1c, ra d ius is longer than the screening length, then cz would not be able to find a bound state due to a Debye suppression of the long-range term of the CC potential and, thus, J/+ production is suppressed’). The degree of suppression depends strongly on the relative size difference between the meson radius and the screening length. Since r($‘) = 0.51 fm > r(J/$) = 0.25 fm > r(T) = 0.13 fm, we expect that 4’ must melt first, then J/$, and finally T. A stronger suppression of $J’ than Jill, reported by NA38 at this conference2) is, therefore, very impressive, and this result compels us to study these vector mesons systematically at RHIC. We plan to measure J/?c, in the mid-rapidity region by dielectrons, and J/+, +’ and ‘I at forward angles by dimuons. The other important element is “chiral symmetry restoration”. Because the mass of the d-meson is close to twice the kaon mass, and because both 4 and K could be distorted in the quark-gluon plasma, it was predicted3) that a change would occur in: (a) the branching ratio between leptonic and hadronic channels, (b) the mass of the 4, and (c) the width of the 4. Here, high-resolution &spectroscopy is required to study these points. We plan to measure &mesons by both electron and hadron channels. Thermal radiation of a hot gas has been a very topical subject for many years4). There was a confusing period, at least to me, in which it was debated whether or not the radiation was enhanced or suppressed when the phase transition occurred. Recently, a consensus among theorists seems to be that the gluon content is high at an early stage of the quark-gluon plasma and an enhancement must be expected in the region of mr (or, pT for photons) greater th an 2-3 GeV’). We plan to investigate this topic using the photon measurement. The nature of the phase transition is a very interesting point to study. If the phase transition is first-order, then, an entropy jump would be expected at Tc, because internal degrees of freedom of the constituents increase by a factor of about 12 from the pionic gas to the quark-gluon gas. If one plots T as a function of the energy density (E) of the system, the value of T increases with & in the phase of a pionic gas until it reaches Tc. The system stays at this temperature, even as E increases, until a sufficient energy density