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

长期以来，对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SM的基本假设之一，半轻子介子衰变，即K+-&gt；轻子+中微子(KL2)，是进行此类实验的最佳通道之一。通过测量电子(KE_2)和缪子(K_μ_2)衰变模式的分支比，KL_2衰变中的强子形状因子被抵消，KE_2中螺旋度的抑制大大提高了对新物理的敏感性。我们停止的K+衰变实验旨在获得系统和统计不确定度组合的两倍改善；这将使我们能够检验最近基于SUSY中微子混合的预测。正电子光谱中的Muon背景预计将比CERN NA62实验中的低得多。*我们新的E36实验是在2015年用J-Parc的升级后的Trek设备进行的，使用的是来自K1.1BR光束线的停止的K+束流。利用4层螺旋闪烁光纤跟踪器和位于12扇区环形光谱仪每个缝隙中的3个MWPC，对有源闪烁纤维靶发射的KE2(P=247MeV/c)和Kμ2(P=236MeV/c)事件进行了动量分析。228MeV/c和215MeV/c的动量削减将分别用于从KE3和Kμ3中去除Pi0背景。由于分支比的巨大差异，仔细的电子和Muon粒子识别(PID)在这个实验中是必不可少的。这可以使用(1)目标周围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高分辨率(100ps)飞行时间测量，以及(3)放置在最后的TOF计数器后面的铅玻璃EM簇射计数器来实现。KE2和Kμ2事件的分析程序将完全相同，但PID除外，以减少分析引起的系统不确定性。通过检测目标区周围的768元素桶形CSI(TL)量热计中的伽马，可以拒绝结构相关(SD)背景事件。RK值的统计误差将由接受的KE2事件的统计误差主导。在主环功率为~30kW的情况下，我们在目标位置观察到K+强度为每溢出(200 KHz)~106。~30天内累积的KE2事件数为~50,000次，相当于δRK/RK~0.4%的统计不确定度。详细的MC模拟表明，在将所有项以求积方式相加后，总体系统不确定度为~0.2%。*E36设备包括CSI量热计，因此我们还可以通过暗光子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.