We have built and we will upgrade electromagnetic calorimeters for the very forward region of the STAR detector at Brookhaven National Lab. It is well known that when energetic particles are produced at large angles to the incident beam axis (at high transverse momentum), the underlying process involves scattering between pairs of individual quarks or gluons. However, scattering that produces energetic particles at forward angles, relative to the beam axis, is likely to involve the interaction of a single high energy quark with one or more low energy gluons. Measurements of these forward processes are important to understand how a single fast quark interacts with a soft multi-gluon field. From general arguments, saturation of these gluon fields is expected to occur when the number of participating gluons is large. Theoretical models predict such effects. Comparisons will be made between pp scattering and dAu scattering for production of forward hadrons.
A similar pp measurement, with one proton polarized in a direction transverse to the beam (up), leads to a left-right asymmetry in the forward cross section. We plan new measurements of this asymmetry, with detailed characterization of the correlated particle production that will distinguish between models that require initial state quark orbital angular momentum from models that involve the fragmentation of highly polarized quarks into final state jet fragments. The resolution of this question is an important step in understanding the roles of spin and angular momentum for proton constituents.
