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

长期以来，高精度电弱测量的研究一直被用作标准模型（SM）预测的严格检验，与这些预测的任何偏差都将表明新的物理学。半轻子衰变K+—>轻子+中微子（Kl2）是进行此类测试的最佳通道之一，因为轻子普适性是SM中的基本假设之一。通过测量电（Ke2）和介子（Kμ2）衰变模式的分支比，发现Kl2衰变中的强子形式因子可以抵消，而Ke2中的螺旋抑制极大地提高了对新物理的灵敏度。我们的停止K+衰变实验旨在获得系统不确定性和统计不确定性组合的两个改进因子；这将使我们能够测试最近基于超对称性中微子混合的预测。正电子谱中的介子背景预计比CERN NA62实验中的要低得多。******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波束线的K+束流。利用4层螺旋闪烁光纤跟踪器和位于12扇形环形光谱仪每个间隙的3个MWPCs，对主动闪烁光纤靶发射的Ke2 （P=247 MeV/c）和Kμ2 （P=236 MeV/c）事件进行动量分析。将分别应用228和215 MeV/c的动量削减来去除Ke3和Kμ3的pi0背景。由于分支比的巨大差异，在本实验中，仔细的电子和介子粒子识别（PID）是必不可少的。这可以通过以下方法实现：(1)目标周围的阈值气凝胶切伦科夫计数器，(2)高分辨率（100ps）测量目标外部和磁扇区出口计数器之间的飞行时间，以及(3)放置在最后一个TOF计数器后面的铅玻璃EM shower计数器。除了PID之外，Ke2和Kμ2事件的分析过程将完全相同，以减少由于分析引起的系统不确定性。通过在目标区域周围的768元素桶形CsI（Tl）量热计中检测伽马，可以排除结构相关（SD）背景事件。RK值的统计误差将由可接受的Ke2事件的统计误差决定。在主环功率为~30 kW的情况下，我们观察到目标位置的K+强度为~ 106 /泄漏（200 kHz）。~30 d内Ke2事件的累计次数为~5万次，δRK / RK ~ 0.4%的统计不确定性。详细的MC模拟表明，在加上正交的所有项目后，总体系统不确定性约为0.2%。****** E36装置包括一个CsI量热计，所以我们也可以通过它们可能与正常光子的耦合来寻找暗光子。我们将研究π+ e+ e-和μ+ e+ e-的最终态，以寻找这种暗光子耦合的证据。

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.