Recent data from PHENIX presented at QM2002 conference included convincing evidence that the hadrons measured at high transverse momentum are indeed the leading particles of jets, coming from the fragmentation of the hard-scattered partons. In Au+Au, the evolution of the strength of the azimuthal correlations with increasing transverse momentum of particles in the jet associated with the triggering particle indicates a mean jet fragmentation transverse momentum 400 MeV/c, in agreement with measurements from p+p collisions. STAR also had a very interesting result on correlations to high particles which indicated that the jet opposite in azimuth to the trigger high particle was observed in peripheral Au+Au collisions but tended to vanish with increasing centrality. Taken together the data from two experiments are consistent with idea that the `unsuppressed' jets coming from the surface of collision zone were really observed and that the degree of high suppression is directly related to the distance hard scattered quark is traveling in the matter. This first experimental evidence that hot and dense nuclear matter was actually produced in heavy-ion collisions and is responsible for observed modifications to parton evolution in general immediately raises a number of interesting questions about mass and flavor dependence of the evolution regime (related discussion can be found in almost any theory paper on the subject). Answering those questions requires knowing the nature of scattered parton (tagging) and large acceptance (rate for correlation studies).
The very first natural candidates for tagging are gluon jets. The flavor neutral hard scattered fragmenting gluons are natural source of the high mesons which do not share any valence quarks with the incoming nucleons. The analysis similar to what was first attempted by the ABCDHW Collaboration in 1985 [Z.Phys.C - Particles and Fields 27, 205 (1985)]. will be based upon short-range quantum number correlations within trigger jet. And the goal of the analysis is of direct measurement of the evolution of a gluon rescattering cross section, as a function of time from a point-like configuration at t=0 to the freezout time at which final state particles are formed.
On the other end of the mass scale are heavy quarkonia and heavy-quark jets, the latter fragment into charmed and beauty particles further decaying into final states which could be tagged by the presence of strangeness. In a few years PHENIX will build and install high-resolution silicon vertex detector and new powerfull central tracking. In the events triggered by high electrons new central detector will be used to associate electron trajectories to decay vertexes and validate the hypothesis of the heavy quark parent of the trigger particle. To make further conclusions about charm or beauty nature of the decaying particle and thus about mass of a hard scattered parton, additional information would be required, one of the sources of this data being the sign of the kaon if present among decay products. Same sign kaon is as good an indicator of the beauty quark as a same sign second lepton. The job looks even more challenging whenever we consider the prospects of studying associated production of heavy flavors. Kaon identification presents the only viable solution to this problem: heavy flavors have nearly 50 % probability to produce charged kaon in the final state, we can think of triggering on kaons at a second level to identify and record golden events. We should understand that the physics potentials of the flavor tagging with identified kaons can be realized only in case if PHENIX construct the central detector and implement high PID over large acceptance (provided central tracking has momentum measuring capabilities over the whole azimuth). If we accept this idea - it would be natural to consider two-stage PID program, when at first - we construct and install the detector based upon already approved detector technology (BELLE-type) in the west PHENIX arm and continue R&D program aimed to develop a new type of high granularity aerogel Cerenkov counters with readout employing hybrid phototubes or large area solid state photocathodes. This new detector will match TPC acceptance and (together with HBD) provide for hadron PID outside of the acceptance of the PHENIX central arms.