Collaborative Research: The MuLan Project-Development of instrumentation for a new high-precision determination of the Fermi coupling constant

“花木兰”计划将开发测量正介子寿命所需的仪器，其精度将达到前所未有的水平，即比目前世界平均水平高出20倍。μ子寿命决定了费米常数的值，费米常数决定了所有弱相互作用过程的强度，就像精细结构常数决定了所有电磁相互作用过程的强度一样。费米常数是粒子物理学标准模型的基本参数之一。瑞士Paul Scherrer研究所（PSI）的一种低能连续μ子束将被一种新的光束切割系统改造，以产生强烈的、短的μ子爆发，这些μ子将在薄目标上停止。正介子阻止了不合适的物质的衰变，就像在真空中一样。这些目标将被一套几乎密封的180个快速闪烁计时探测器所包围，每个探测器与一个最先进的波形数字化仪相耦合，并由一个高速数据采集系统读取所有数据。近100tb数据的在线分析将通过小型快速微处理器阵列进行。该仪器的第二次使用是与PSI正在进行的测量氢气中的负μ子寿命的工作相结合。负介子可以以与正介子完全相同的方式衰变，或者在氢中，它可以通过称为“捕获”的弱相互作用过程与质子相互作用。理论上可以非常准确地预测捕获率；然而，目前的实验有各种各样的解释问题，与理论有很大的分歧。如果这种分歧得到证实，这种差异将带来一种令人兴奋的可能性，即指向新的、未知的物理学。通过测量正负μ子寿命之间的差异，我们将可靠地确定μ子捕获率，并且精度比目前的测量高四倍以上。“花木兰”定时探测器、光束线和定制电子设备预计将在这一努力中发挥重要作用。

0079735Hertzog The MuLan Project will develop the instrumentation needed to measure the lifetime of the positive muon to unprecedented precision, namely, a factor of 20 better than the current world average. The muon lifetime fixes the value of the Fermi constant, which governs the strength of all weak-interaction processes, just as the fine structure constant governs the strength of allelectromagnetic-interaction processes. The Fermi constant is one of the fundamental parameters of the Standard Model of particlephysics. A low-energy, continuous-wave muon beam at the Paul Scherrer Institute (PSI) in Switzerland will be modified by a new beam chopper system in order to create intense, short bursts of muons, which will stop in thin targets. A positive muon stopped inappropriate materials decays as if it were in vacuum. Such targets will be surrounded by a nearly hermetic set of 180 fast-scintillator timing detectors, each coupled to a state-of-the-art waveform digitizer, and all readout by a high-speed data acquisition system. Online analysis of the nearly 100 terabytes of data will take place with a small array of fast microprocessors. A second use of the instrumentation is in conjunction with an ongoing effort at PSI to measure the negative muon lifetime inhydrogen gas. The negative muon can decay exactly in the same manner as the positive muon, or in hydrogen it can interact with aproton by the weak-interaction process known as "capture." The capture rate is predicted very accurately by theory; however,current experiments, which have various interpretation problems, are in disagreement with theory by a very significant amount. Ifthis disagreement is confirmed, this difference raises the exciting possibility of pointing to new and unaccounted for physics. Bymeasuring the difference between the positive and negative muon lifetimes, we will determine the muon capture rate reliably and at aprecision more than four times better than current measurements. The MuLan timing detectors, beamline and custom electronics areexpected to play a major role in this effort.