Grant: Subatomic Physics Envelope - Project

<p> 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 (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 proposed 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.</p> <p>Our new E36 experiment will be performed with the upgraded TREK apparatus at J-PARC using a stopped K+ beam. The Ke2 (P=247 MeV/c) and Kµ2 (P=236 MeV/c) events emitted from the active scintillating fibre target will be 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 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 will 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 TOF2 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 will 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 Ke2 events. Assuming a main ring power of 30 kW we expect a K+ intensity of ~200 kHz at the target position. Hence the number of accumulated Ke2 events after 50 days will be ~ 250,000, corresponding to a statistical uncertainty in deltaR_K / R_K ~ 0.2%. Detailed MC simulations indicate an overall systematic uncertainty of 0.15% after adding all items in quadrature. The statistical uncertainty can be reduced even further with more additional beam time.</p>