Proposal to Instrument an Endcap Electromagnetic Calorimeter for Spin Physics with STAR at RHIC

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9971947Vigdor These funds will be used to construct a detector that will enable a qualitative advance in understanding of the internal structure ofprotons. Protons are known to be composed of smaller particles called quarks, antiquarks and gluons, particles that have never been observed in isolation. The contributions of the gluons and the antiquarks to the proton's properties are important, but elusive. For example, it has been inferred that the gluons dominate the mass of the proton, and hence the mass of the universe. One goal of research with this new detector is to measure the contributions of gluons and antiquarks to the proton's overall spin, a fundamental property associated with its intrinsic magnetism. These measurements are critical because other experiments performed during the past decade have suggested that the quarks and antiquarks combined account for less than a third of the knownproton spin. The detector proposed is an Endcap Electromagnetic Calorimeter (EEMC) to be added to the major STAR (Solenoidal Tracker at RHIC) detector system currently being commissioned at the new Relativistic Heavy Ion Collider facility. RHIC and STAR were originally designed to search for new phases of strongly interacting matter in collisions of very energetic heavy nuclei. The EEMC wouldcritically enhance their capabilities as a platform for a complementaryclass of experiments, utilizing colliding beams of spin-polarizedprotons at unprecedentedly high energies. It would do so by providing detection of high-energy gamma rays, electrons, positrons and neutral pi-mesons in a kinematic regime of optimal sensitivity to unmeasured contributions to the proton's intrinsic spin. The enhanced STAR detector would allow the world's best measurement of the integral gluon spin contribution, by detecting gamma rays from hard polarized proton collisions in coincidence with "jets" of hadrons -- products when a polarized quark in one proton beam strikes a polarized gluon in the other. In addition, the EEMC will facilitate important tests of the Standard Model of elementary particle properties, including the fundamental theory (quantum chromodynamics) of the interactions among quarks, antiquarks and gluons.