Forward Meson Production and Spin Asymmetries at RHIC

It is a well-established fact that protons and neutrons are made up of smaller constituents called quarks and gluons. Protons are collections of quarks bound by the strong nuclear force much as atoms are collections of electrons bound to the nucleus by the electric force. By studying the proton collisions at high energy we learn how these quarks are bound inside the proton. The proton also has an intrinsic property called spin, which can be visualized by thinking of the proton as a sphere spinning on its axis, much like the Earth rotates on its axis. In particular, when high energy beams of spin-aligned protons collide at Brookhaven National Laboratory's Relativistic Heavy Ion Collider (RHIC) the resulting particles can reveal how much of the proton's spin is carried by its gluons or by different types of quarks. The goal of this project is search for the gluon contribution to the spin of the proton, which is identified as an important question by the Nuclear Science Advisory Committee. These collisions are observed as photons and electrons in the STAR detector and its component part, called the FMS, which will be maintained as part of this project. This award will fund the Penn State group to prepare, calibrate and operate the FMS for upcoming RHIC runs and to analyze data recently collected. The FMS is an electromagnetic calorimeter covering the pseudo-rapidity region from 2.6 to 4. It detects collision fragments from neutral pions, eta mesons, jets, single photons and electrons. The upcoming polarized RHIC run will be emphasizing the transverse proton spin dependence for production of electron-positron Drell-Yan pairs. This will allow measurement of the transverse single spin asymmetry, defined as the difference in cross section for proton spin up or down divided by the sum of those cross sections. The measured asymmetry is sensitive to the strong nuclear forces in a regime where current models of Perturbative Quantum Chromodynamics are evolving.