Grants and Contributions:
Grant or Award spanning more than one fiscal year. (2017-2018 to 2019-2020)
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 (K l2 ), 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 (K e2 ) and muonic (K μ2 ) decay modes the hadronic form factor in K l2 decay cancels out, and the helicity suppression in K e2 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 K e2 (P=247 MeV/c) and K μ2 (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 pi 0 backgrounds from K e3 and K μ3 , 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 K e2 and K μ2 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 R K value will be dominated by that of the accepted K e2 events. With a main ring power of ~30 kW we observed a K + intensity of ~ 10 6 per spill (200 kHz) at the target position. The number of accumulated K e2 events in ~30 days was ~50,000, corresponding to a statistical uncertainty in δR K / R K ~ 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.