Search for non-Standard Model Physics with the TREK Detector at J-PARC

高精度电弱测量的研究长期以来一直被用作标准模型（SM）预测的严格测试，任何偏离这些预测的情况都将表明新的物理学。由于轻子普适性是标准模型的基本假设之一，因此半轻子K ~+衰变K ~+ →轻子+中微子（Kl_2）是进行这类检验的最佳途径之一。通过测量电（Ke 2）和μ子（Kμ2）衰变模式的分支比，Kl 2衰变中的强子形状因子被抵消，而Ke 2中的螺旋度抑制大大增强了对新物理的敏感性。我们停止的K+衰变实验旨在使系统和统计不确定性的综合改善达到两倍;这将使我们能够测试最近基于超对称中微子混合的预测。正电子谱中的μ子背景预计将大大低于CERN NA 62实验中的μ子背景。我们的新E36实验于2015年在J-PARC使用升级的TREK设备进行，使用来自K1.1BR光束线的停止K+光束。利用4层螺旋光纤跟踪器和位于12扇区环形谱仪每个间隙中的3个MWPC，对有源偏振光纤靶发射的Ke 2（P=247 MeV/c）和Kμ2（P=236 MeV/c）事例进行了动量分析。将应用228和215 MeV/c的动量切割，以分别从Ke 3和Kμ3中去除π 0背景。在这个实验中，仔细的电子和μ子粒子识别（PID）是必不可少的，因为分支比有很大的差异。这可以通过使用（1）围绕目标的阈值气凝胶切伦科夫计数器，（2）位于目标外部和磁扇区出口处的计数器之间的飞行时间的高分辨率（100 ps）测量，以及（3）放置在最后一个TOF计数器后面的铅玻璃EM簇射计数器来实现。除PID外，Ke 2和Kμ2事件的分析程序完全相同，以减少分析引起的系统不确定性。结构相关（SD）的背景事件，可以拒绝通过检测γ在768元素桶形CsI（Tl）量热计周围的目标区域。RK值的统计误差将由可接受的Ke 2事件的统计误差主导。在主环功率约为30 kW的情况下，我们在目标位置处观察到每次溢出（200 kHz）约106的K+强度。在约30天内累积的Ke 2事件数约为50，000，对应于δRK / RK的统计不确定性约为0.4%。详细的MC模拟表明，在正交中添加所有项目后，总体系统不确定性约为0.2%。** E36设备包括一个CsI量热计，因此我们也可以通过它们与正常光子的可能耦合来寻找暗光子。我们将研究π+ e+ e-和 μ+ e+ e-为暗光子耦合的证据。

The study of high-precision electroweak measurements has long been used as a stringent test of Standard Model (SM) predictions and any deviations from these predictions will indicate new physics. The semi-leptonic kaon decay, K+ --> lepton + neutrino (Kl2), is one of the best channels to perform such tests since lepton universality is one of the basic assumptions in the SM. By measuring the branching ratio of the electric (Ke2) and muonic (Kμ2) decay modes the hadronic form factor in Kl2 decay cancels out, and the helicity suppression in Ke2 greatly enhances the sensitivity to new physics. Our stopped K+ decay experiment aims to obtain a factor of two improvement in the combined systematic and statistical uncertainty; this will allow us to test a recent prediction based on SUSY neutrino mixing. The muon background in the positron spectrum is expected to be considerably lower than in the CERN NA62 experiment.******Our new E36 experiment was performed in 2015 with the upgraded TREK apparatus at J-PARC using a stopped K+ beam from the K1.1BR beamline. The Ke2 (P=247 MeV/c) and Kμ2 (P=236 MeV/c) events emitted from the active scintillating fibre target were momentum analyzed using a 4--layer spiral scintillating-fibre tracker and 3 MWPCs located in each gap of a 12-sector toroidal spectrometer. Momentum cuts at 228 and 215 MeV/c will be applied to remove the pi0 backgrounds from Ke3 and Kμ3, respectively. Careful electron and muon particle identification (PID) is essential in this experiment because of the large difference in branching ratios. This can be achieved using (1) threshold aerogel Cherenkov counters surrounding the target, (2) a high-resolution (100ps) measurement of the Time-of-Flight between counters located just outside the target and at the exits of the magnetic sectors, and (3) lead glass EM shower counters placed behind the last TOF counters. The analysis procedure will be exactly identical for both Ke2 and Kμ2 events except for the PID in order to reduce the systematic uncertainty due to the analysis. The structure dependent (SD) background events can be rejected by detecting the gammas in a 768 element barrel shaped CsI(Tl) calorimeter surrounding the target region. The statistical error on the RK value will be dominated by that of the accepted Ke2 events. With a main ring power of ~30 kW we observed a K+ intensity of ~ 106 per spill (200 kHz) at the target position. The number of accumulated Ke2 events in ~30 days was ~50,000, corresponding to a statistical uncertainty in δRK / RK ~ 0.4%. Detailed MC simulations indicate an overall systematic uncertainty of ~0.2% after adding all items in quadrature. ******The E36 apparatus includes a CsI calorimeter so we can also look for dark photons through their possible coupling to a normal photon. We will examine the final states of both π+ e+ e- and μ+ e+ e- for evidence of such dark photon couplings.