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

在大爆炸后的最初10微秒内，宇宙的温度非常高，以至于质子和中子等普通强子无法形成。相反，物质的主要形式是处于夸克-胶子等离子体状态的未结合夸克和胶子。大爆炸后时代的异常高温，大约2 × 10**12 K （200 MeV能量单位），今天只能通过在布鲁克海文国家实验室的相对论重离子对撞机（RHIC）上以极高能量碰撞重核的加速器实验来实现。对现有实验数据的分析表明，所达到的能量密度大于10**30 J/cm**3，比普通核物质的能量密度高出两个数量级，比夸克胶子等离子体形成的估计阈值高出10倍以上。然而，目前还没有关于等离子体达到的温度的任何实验信息。测量温度需要探测等离子体发出的热辐射。这种辐射由光子、电子对和来自虚光子衰变的正电子组成。不幸的是，在碰撞中形成的数千个其他粒子的衰变产生的巨大背景妨碍了迄今为止对热辐射的可靠测量。该提案旨在通过使用新技术获取探测器来直接测量和拒绝来自这些衰变的电子和正电子的压倒性背景，从而解决这个问题。剩余电子-正电子对的能量分布反映了在RHIC产生的夸克胶子等离子体的温度。此外，良好的背景抑制将有助于寻找手性对称恢复的证据。当宇宙从夸克胶子等离子体状态冷却下来时，夸克的凝聚自发地打破了高温下存在的手性对称性。这一步骤产生了正常物质中粒子的质量，与粒子内轻夸克的质量相比，它们的质量很大。对手性对称性的广泛理论研究预测，在RHIC达到的高温下，对称性应该恢复，粒子的性质可能会有显著的不同。我们将测量，和，粒子衰变成电子-正电子对并寻找它们的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.