Hadron Blind Detector for Diagnostics of The Quark-Gluon Plasma at RHIC

在大爆炸后的最初10微秒内，宇宙的温度如此之高，以至于无法形成质子和中子等普通强子。相反，物质的主要形式是无束缚的夸克和胶子，这种状态被称为夸克-胶子等离子体。大爆炸后那个时代的极高温度，大约2×10**12K(以能量单位为200 MeV)，今天只有通过布鲁克海文国家实验室的相对论重离子对撞机(RHIC)以非常高的能量进行重核碰撞的加速器实验才能实现。对现有实验数据的分析表明，达到的能量密度超过10**30 J/cm*3，比普通核物质高出两个数量级，比估计的夸克胶子等离子体形成阈值高出10倍以上。然而，到目前为止，还没有关于等离子体达到的温度的任何实验信息。测量温度需要检测等离子体发出的热辐射。这种辐射由光子和虚光子衰变产生的电子和正电子组成。不幸的是，碰撞中形成的数以千计的其他粒子衰变的巨大背景阻碍了迄今为止对热辐射的可靠测量。这项提议旨在通过获得一个探测器来解决这个问题，该探测器使用新的技术来直接测量和拒绝那些衰变产生的压倒性的电子和正电子背景。剩余电子-正电子对的能量分布反映了RHIC产生的夸克胶子等离子体的温度。此外，良好的背景排斥将允许寻找手性对称性恢复的证据。随着宇宙从夸克胶子等离子体状态冷却下来，夸克的凝聚自发地打破了高温下存在的手征对称性。这一步骤产生了正常物质中的粒子质量，与粒子中轻夸克的质量相比，粒子的质量要大得多。对手征对称性的广泛理论研究预测，对称性应该在RHIC达到的高温下恢复，粒子的性质可能会有很大的不同。我们将测量Rho、omega和Phi粒子在电子-正电子对中的衰变，并寻找它们质量和宽度变化的证据。

During the first 10 microseconds following the Big Bang, the temperature of the universe was so high that ordinary hadrons such as protons and neutrons could not form. Instead, the dominantform of matter was unbound quarks and gluons in a state referred to as quark-gluon plasma. The extraordinarily high temperature of the epoch just after the Big Bang, approximately 2 x 10**12 K (200 MeV in energy units), is achievable today only via accelerator-based experiments colliding heavy nuclei at very high energy at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Lab. Analysis of available experimental data indicate that the energy density reached is more than 10**30 J/cm**3, exceeding that of ordinary nuclear matter by two orders of magnitude, and more than a factor of 10 above the threshold estimated for quark gluon plasma formation. However, there is as yet no experimental information whatsoever about the temperature reached by the plasma.Measuring the temperature requires detecting thermal radiation emitted by the plasma. This radiation consists of photons also pairs of electrons and positrons from decays of virtual photons.Unfortunately, enormous backgrounds from decays of the thousands of other particles formed in the collision have prevented reliable measurement of the thermal radiation to date. This proposal aims to solve this problem by acquisition of a detector using novel technology to directly measure and reject the otherwise overwhelming background of electrons and positrons from those decays. The energy distribution of the remaining electron-positron pairs reflects the temperature of the quark gluon plasma created at RHIC. Furthermore, good background rejection will allow a search for evidence of restoration of chiral symmetry. As the universe cooled down from the quark gluon plasma state, condensation of the quarks spontaneously broke the chiral symmetry present at high temperature. This step gave rise to the masses of the particles in normal matter, which are large compared to the masses of the light quarks inside the particles. Extensive theoretical study of chiral symmetry predicts that the symmetry should be restored at the high temperatures reached by RHIC and that the properties of particles can be significantly different. We will measure the decays of rho, omega and phi particles into electron-positron pairs and look for evidence of changes in their masses and widths.