Forward physics issues for RHIC II: The current state of physics at RHIC I is primarily based on the physics at 90 degrees. In this region, which is dominated by moderate-to-low x partons, one expects final state interactions to drive the system to a Quark-Gluon Plasma. This region has the largest total particle density, and thus presumably the highest parton density. Pushing into kinematic regimes away from mid-rapidity offers substantial insight into the initial stages of the collision, as well as the collision dynamics. Of course, to take full advantage of physics in the forward region, one must be able to perform momentum measurements and particle ID up to pT~2-3 GeV, which is challenging with longitudinal momenta of 20-30 GeV at large rapidities. Furthermore, the beam energy itself is a stringent limitation on the possible Q^2 reached in a reaction, simply due to kinematic limits (pT max ~ sqrt(s)/(2*sinh(y)) since most of the available energy for high-rapidity particles is longitudinal. One approach to forward physics at RHIC II is to focus less on high-rates and rare probes, but to also push for substantial detector acceptance to allow unique global measurements that will be completely unavailble at the LHC. - measurement of high-pT particles vs. y out to the kinematic limit - extensive particle ID, e.g. with a large acceptance RICH. - large enough acceptance to measure total energy flow event-by-event. (a fantasy would be to estimate the net-baryon distribution per event) These measurements can be associated with several physics topics: 1. Saturation 2. Soft physics in 4-pi (Hydro, universality, etc) 3. Heavy flavor 4. Jet physics at forward angles Saturation physics ------------------ One of the dominant physics paradigms of recent years is the Color Glass Condensate. At very low-x, gluons can be coherent over large longitudinal distances. This means that a probe encountering a substantial transverse density of soft partons will see them all as a single parton at a much higher-Q^2, the "saturation scale". According to this picture, this single scale encodes both the number and spectrum of gluons produced out of the initial-state wave functions. The coherence tends to reduce the entropy of the final state, and thus leads to a hardening of particle spectra, perhaps to the point where pertubrative QCD methods can be used safely. To reach the saturation regime, where gluons from different parton cascades interact with each other, one must push down to the lowest x available. This can be achieved either by increasing the beam energy, since the typical x-values scale as 1/s, or by increasing the rapidity of the detected particles. In a simple picture where a pion results from the scattering of two gluons, particle production at large rapidity is the sampling of the nucleon structure function (which typically goes as (1-x)^4) by a parton at very low x relative to the oncoming particle. In this picture, the saturation scale runs as Q^2 ~ exp(-lambda*y), and so it is feasible that much of the particle production in the forward region is dominated by saturation physics. Signatures of the CGC in the forward region are a systematic hardening of the spectrum due to gluon recombination, associated with a suppression relative to p+p physics. Hints of this may already be visible in the recent BRAHMS data at forward rapidities, although it is not clear at present whether or not this is a kinematic or saturation effect. In either case, the presence of non-trivial effects away from mid-rapidity, and the enormous suppression relative to the assumption of perfect factorization of hard processes at y=0, clearly point to the need to understanding the physics in this regime. Soft / universal / hydrodynamic physics ---------------------------------------- An emerging problematic in our understanding of the collision dynamics is the nature of the longitudinal distributions. As noted by Landau years ago, while the entropy per unit rapidity peaks at y=0, the energy released in hadronic reactions continues predominantly in the forward region. The nature of this energy flow has many effects on bulk particle production - Collective flow has become an incredibly rich experimental subject, with extensive information available at mid-rapidity, for a wide range of identified particles up to moderately-high pT. Extending similar studies in the forward direction, already begun by STAR and PHOBOS will be crucial to decode the information available in this range of measurements. - the amount of energy liberated seems to be directly related to the particle multiplicity, as predicted by Fermi & Landau in the 1950's to be approximately s^1/4 (a formula only approximately true relative to the data). This multiplicity appears to be universal in a variety of collision systems, and studying this in detail in p+p and p+A may be very illuminating. This will require high resolution particle tracking or calorimetry at least one unit away from beam rapidity. - While Bjorken predicted that particle production near y=0 should be boost invariant (i.e. independent of y), all data to date shows a dramatic violation of this invariance. Rather, distributions appear to be gaussian, as might have been predicted by Landau hydrodynamics. It remains to be seen how charge, flavor and baryon number are distributed over 4-pi. These measurements have fundamental interest since they address the very concept of baryon number conservation, which is still not fully understood at the level of QCD. Heavy Flavor in the forward direction: -------------------------------------- Heavy flavor is an excellent probe of the gluon distributions, via quarkonium states (J/Psi, Upsilon, etc.) Thus, the same tools which will be used for deconfinement studies can also be used to probe gluon distributions Jet physics at forward angles ----------------------------- In the light of the back-to-back disappearance measurements, it is important to determine that the jet is actually recoiling and being quenched, and thus is not simply being shifted a unit (or more) in rapidity. It also allows direct tests of factorization in heavy ion collisions (i.e. the hard processes should be more-or-less independent of the soft, above a reasonable cutoff pT). Detector ideas: --------------- - tracking in the forward region dipole magnets silicon tracking? - calorimetry: projective geometry, bjorken's FAD/FELIX/RHIC" ideas - multiplicity measurements PHOBOS-style rings - particle ID - how much acceptance is feasible? RICH Aerogel - heavy flavor muon or electron acceptance