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KaonLepton Report Summary

Project ID: 336581
Funded under: FP7-IDEAS-ERC
Country: United Kingdom

Mid-Term 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, the studies of the galactic motion suggest the Universe is dominated by a yet-to-be-identified “dark matter”, rather than the “normal” luminous matter observed by telescopes. 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). The accessible new physics mass scale probed by this method is technologically limited by the beam energy.

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 (currently up to ~100 TeV). Historically, it has been tremendously 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; both the interpretation of precision data and the ultimate discovery at high energy experiments were acknowledged by Nobel Prizes.

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 an upsurge of 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 is dedicated to searching for LFV phenomena at the highest precision of ~10−12 at the NA62 experiment at the European Laboratory for Particle Physics (CERN) via a search for the “forbidden” decays of kaons (K+) and pions (π0). The project relies on the unique most 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 provides a cost-effective way of filling a major research gap. Any observation of a LFV process would be an unambiguous signal of non-SM physics at subatomic scale, and thus a major scientific discovery. The research provides unique sensitivity to the Majorana neutrino mass term and the supersymmetric (SUSY) scenarios (including SUSY with the R-parity breaking, currently gaining interest given the non-observation of the “standard” SUSY at the LHC). As a result of the project, entire classes of new physics models (postulated to explain such phenomena as the dark matter) will be confirmed, rigorously constrained or eliminated.

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United Kingdom
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