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

[9971947 . vigdor]这些资金将用于建造一个探测器，这将使人们对质子的内部结构的理解有质的提高。众所周知，质子是由更小的粒子组成的，这些粒子被称为夸克、反夸克和胶子，这些粒子从未被单独观察到。胶子和反夸克对质子性质的贡献很重要，但难以捉摸。例如，据推测胶子支配着质子的质量，因而也支配着宇宙的质量。这种新型探测器的研究目标之一是测量胶子和反夸克对质子整体自旋的贡献，这是与其内在磁性相关的基本特性。这些测量是至关重要的，因为在过去十年中进行的其他实验表明，夸克和反夸克加起来只占已知质子自旋的不到三分之一。该探测器是一个终端电磁量热计（EEMC），将被添加到目前在新的相对论重离子对撞机设施中服役的主要STAR （RHIC电磁跟踪器）探测器系统中。RHIC和STAR最初是为了在高能重核碰撞中寻找强相互作用物质的新相而设计的。电子电磁兼容将极大地增强他们的能力，作为一个补充实验类的平台，利用前所未有的高能自旋极化质子束的碰撞。它将提供高能伽马射线、电子、正电子和中性介子的探测，在一个对质子固有自旋的未测量贡献具有最佳灵敏度的运动体系中。增强的STAR探测器将允许世界上最好的测量整体胶子自旋贡献，通过检测硬极化质子碰撞产生的伽马射线与强子“喷流”的巧合-当一个质子束中的极化夸克撞击另一个质子束中的极化胶子时产生的产物。此外，电磁兼容将促进基本粒子特性标准模型的重要测试，包括夸克、反夸克和胶子之间相互作用的基本理论（量子色动力学）。

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.