Forward Meson Production and Spin Asymmetries at RHIC

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It is now a well-established fact that nucleons, protons and neutrons, are made up of more elementary particles called quarks. The main physics program supported by this grant is directed at the study of the quark structure of nucleons and nuclei. Research will be carried out at the Brookhaven National Laboratory as part of a collaboration called STAR. This collaboration is focused on the scattering of high energy polarized protons from nuclei with the goal of understanding the structure of the proton. The proposed research will use the forward electromagnetic calorimeter, called FMS, which is part of the STAR detector system to measure neutral particles that are produced at small scattering angles. The measured rates of such particle production have been found to depend strongly on the direction of polarization of the proton beam, an effect that has confounded attempts at a theoretical understanding based on the fundamental theory of the strong force, Quantum Chromodynamics (QCD). Further studies of this particles detected using the FMS will guide future theoretical develpments and help us to understand more about the quark and gluon structure of nucleons.This proposal supports the Penn State group to operate the FMS detector in the STAR experiment for polarized proton RHIC runs and to analyze data from those runs. The STAR FMS is an electromagnetic calorimeter that detects collision fragments produced in the forward pseudo-rapidity region. Particles observed in this region are associated with the interactions of the most energetic and most polarized quarks. For transversely polarized protons, the large dependence of the cross section on the sign of polarization is a long-standing mystery. While the QCD Lagrangian is known, our ability to use it to predict observables is still limited. In contrast to the electromagnetic force, where the role of spin constituents of an atom is fairly well understood, the role of spin among the partons (constituents of a bound state proton) remains near our knowledge frontier. The most useful model for scattering by the QCD force is perturbative QCD, or pQCD, which is applicable when the collisions between protons constituents exchange large momentum. The goal of this research is to determine experimentally where pQCD applies and how to modify models to extend applicability.