Search for New Physics in a Kaon Decay

The present theory of the electroweak interaction (referred to as the Standard Model has been very successful in explaining all presently observed experimental facts but we are not satisfied with the present state of this theory: it has too many arbitrary parameters; several of its predictions have not yet been confirmed experimentally. Nobody believes that this Standard Model is the final theory of the electroweak interaction. Also we would like to see a better unity in particle physics and the next step should be the confirmation of a unified theory of the electroweak and strong interactions. It is hoped that the expected start of the Large Hadron Collider at CERN will soon be able to test several predictions of the electroweak theory; the first one would be the observation of the Higgs boson. Many other predictions will also be submitted to experimental tests. This approach consists in going to higher energies and is referred as to an exploration of the energy frontier. Another approach consists in using more modest accelerators, delivering particles at lower energies, but looking for observables which do not exist in the Standard Model. Here the experiments are aimed at the observation of small (sometimes very small) effects at much lower energies. This approach is referred to as the precision frontier. The aim of this approach is not to observe new particles but to observe small effects which will be used to put upper limits on theoretical predictions and possibly reject some of these predictions. A long time ago, Sakurai, a Japanese physicist, proposed an experiment based on the measurement of the transverse polarization of the muon emitted in the following weak decay: positive kaon going to a neutral pion plus a positive muon plus a neutrino (K+ p μ + n μ). In such an experiment it is impossible to detect the neutrino. The energy and direction of the neutral pion can be detected by observing the two gamma rays produced in its decay. The positive muon can be detected, its kinetic energy measured, and its (very small) transverse polarization measured by using a special polarimeter. In such an experiment the main problem is the contribution of spurious effects due to the inevitable imperfections of the detector. The elimination of these spurious effects is possible by exploiting symmetries in the conception of the detector and the data analysis. A previous experiment was run at the KEK laboratory, located in Tsukuba, Japon, by using the existing synchrocyclotron and a specially devised detector.