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