Physics at the high-mass scale can also be manifest in dynamical effects at lower, accessible energy. Thus, the existence of new high-mass-scale physics implies new mechanisms by which new interactions between constituents, quarks and gluons with leptons, must also exist. This proposal is for a precision measurement at the NA62 experiment at the European Laboratory for Particle Physics (CERN) of the ratio R(K) of the branching ratios (BR) of two rare, leptonic, decays of the K+, namely K+ to e+ neutrino and K+ to mu+ neutrino, with a precision of 0.2%. When compared with the prediction of the Standard Model (uncertainty 4E-4), the measurement will be sensitive to physics at the Terascale (TeV energy). The experiment is based on a sample of about 10E13 in-flight K+ decays, for which selection and background rejection exploit precision kinematic reconstruction and particle identification. The team will contribute a Cerenkov detector, which will time-stamp individual K +decays in the decay volume of the experiment at the 50 MHz K+ beam rate. When operating in the NA62 experiment, the sensitivity to K decay BRs will then be better than 10E-11. Such a measurement of R(K) will certainly constrain in a unique manner the nature of Terascale physics. The reality of measurements at this level of precision, which may point to lepton flavour violation, will make possible a number of definitive tests concerned with the violation of lepton flavour universality. The importance of the measurements is therefore compelling, and especially so when taken in the context of imminent measurements using long baseline neutrino beams.
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