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Test of Lepton Flavour Universality with Kaon Decays

Final Report Summary - UNIVERSALEPTO (Test of Lepton Flavour Universality with Kaon Decays)

The rate at which a sub-nuclear particle, such as a K–meson (also known as kaon), decays is both a signature of its structure and a consequence of the forces which hold it together. The less rapid is its rate of decay, the less favourable are the configuration and properties of its structure for decay and the weaker is the force which causes its decay.
The constituents of a kaon are a “strange quark s“ and an “up anti-quark u ̅“. The kaon is the lightest, and therefore least energetic, of particles which have the property of strangeness and so the possibility of its decay to a lighter meson is suppressed to an extent that the dominant force for decay on the quark and the anti-quark is what physicists call “electroweak”.
The electroweak force is at the heart of physicists Standard Model (SM) of the physics of the Universe. It describes the interactions between quarks and leptons by virtue of electroweak fields which are attributable to the existence of electroweak bosons. Though around for decades, only recently with the discovery of the Higgs-boson have all the features of the SM been finally observed.
Measurements of the decay rates of kaons and the comparison of predictions calculated using the SM are therefore a unique window on the way the electroweak force works. When these decay rates are extremely small, and the electroweak force is therefore small, comparison of the expectation of the SM with measurements of them tests the validity of the SM extremely accurately. Furthermore, in such circumstances, if a SM expectation fails to predict successfully measurement, the discrepancy can be interpreted in terms of a new regime of physical law which underpins and explains understanding hitherto.
The UniversaLepto project, whose initiation is because of ERC funding, is concerned with measuring extremely rare kaon decays at a level which corresponds to 10 in a trillion. The aim is to obtain a hundred such decays so as to be able to compare meaningfully with SM expectation and to interpret any discrepancy in terms of the (small) distance scale, that is in terms of the (large) energy scale, where (may be?) is to be found the physics which explains and underpins the origin of the SM.
The UniversaLepto project is part of the international collaboration of physicists working on an experiment NA62 at CERN, Geneva. There, a beam of kaons is delivered to the NA62 apparatus. The ERC Advanced Grant award UniversaLepto has made possible the initiation of UK physicists in NA62. Specifically, it has funded the construction of a pivotal detector for fast “tagging” of individual kaons in the incident beam, and has already enabled UniversaLepto staff to lead work to publications in the name of the NA62 experiment. NA62 data-taking will continue until at the earliest the end of 2018, and throughout this period it is anticipated that the work of those in UniversaLepto on NA62 will continue to be supported by the UK agency STFC.