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Precision Lepton Flavour Conservation Tests in Kaon Decays

Final Report Summary - KAONLEPTON (Precision Lepton Flavour Conservation Tests in Kaon Decays)

Particle physics studies the sub-atomic constituents of matter and the laws of their mutual interactions. Our current knowledge is embodied in a mathematical description called the Standard Model (SM). It has been spectacularly successful, having withstood a generation of extremely precise experimental tests, including the recent Higgs boson discovery. However, the SM cannot provide the full description of the subatomic physics as it does not incorporate gravity, offers no explanation for the fermion mass hierarchy, or for a range of other phenomena. Furthermore, evidence for the existence of unknown types of matter comes from cosmology: for example, there is amply evidence from astrophysical observations that the Universe is dominated by the "dark matter", rather than the "normal" luminous matter. Models proposed to address these problems, collectively termed "new physics", predict the existence of as yet undiscovered particles. The quest for new physics which underpins the SM is the primary challenge of modern particle physics.

New phenomena beyond the SM description can be searched for in the laboratory using two methods. The "energy frontier" experimental methodology addresses this challenge directly. It is based on measurements of particle interactions in collisions of highest possible energy, aiming to be above the threshold for associated production of new quantum numbers thereby leading to discovery. This approach is the "raison d’être" for the experimental challenge of colliding beams of the highest possible energy and intensity, culminating in TeV-energy (Terascale) proton-proton interactions at the Large Hadron Collider (LHC) at the European Laboratory for Particle Physics (CERN).

A complementary "precision frontier" approach is to study carefully very rare lower-energy processes, which can be very accurately described by the SM, in high beam intensity experiments. New physics at high-mass scale can manifest itself in dynamical effects at lower, accessible energy as higher order corrections to the basic process. Therefore the differences from expectations in such processes would prove the existence, and give information on the form of, new physics. This methodology has superior energy reach with respect to the "energy frontier" approach. Historically, it has been successful in discovering new phenomena. For instance, the existence of the charm quark was predicted in 1970 on the basis of the suppression of flavour-changing neutral currents in precision low-energy experiments (the "GIM mechanism") before its direct observation in 1974.

One of the major research directions at the precision frontier is the study of the origin of Lepton Flavour (LF) conservation. This cornerstone principle of the SM, postulating that lepton coupling to gauge bosons is independent of lepton type ("flavour"), is an accidental rather than a fundamental symmetry. As a result, it is violated in many extensions of the SM. The recent discovery of neutrino oscillations has led to a conclusion that the LF symmetry is approximate rather than exact. This also implies that the neutrinos have non-zero masses, which (in addition to constituting a non-SM phenomenon by itself) opens the questions about the possible Majorana nature of the neutrino, as well as the origin of the small (sub-eV) observed neutrino mass scale. This has triggered increased interest in LF violating (LFV) phenomena, including experimental searches for the neutrinoless nuclear double beta decays and the LFV decays of hadrons and leptons.

The KaonLepton project has been dedicated to searches for LFV phenomena at the highest precision of down to 10^{-12} at the NA62 experiment at the European Laboratory for Particle Physics (CERN) by searcing for the "forbidden" decays of the charged K+ meson. The project relied on the unique very intense kaon beam in the world provided by the CERN accelerator complex. Complementing the LFV searches in other sectors and surpassing many of them in precision, and building on the state-of-the-art technology and unprecedented sensitivity of the existing NA62 experiment, it filled a major research gap by obtaining new limits on several lepton number violating decay modes of the K+ meson at the 10^{-11} level, and providing a unique data set for related measurements.