Search for Physics Beyond the Standard Model through ultra-rare Kaon decays with the NA62 Experiment at CERN

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The Standard Model of particle physics (SM) fully describes high energy interactions in normal matter, as witness the discovery of the Higgs-like boson at LHC at CERN, without explaining dark matter or dark energy. Two approaches pursued in particle physics to search beyond the SM are the direct search for new particles at high energy, and the study of rare decays precisely predicted by the SM. The NA62 experiment at CERN is designed to pursue this second approach by precisely measuring the decay of the charged kaon into a pion and two neutrinos that is predicted very precisely by the SM to occur only once for every ten billion times that a kaon decays into everything else possible. The hope is to find something different from the prediction to help us to explain why the SM works so well and to understand how to improve it.The entire system of detectors for NA62 takes up 270 metres and is located at CERN in an underground cavern. Kaon mesons do not exist in ordinary matter and about fifty million of them are produced per second, together with another billion pions and muons, by more than one thousand billion protons per second extracted from the SPS CERN accelerator and interacting with a beryllium target. The NA62 detectors are designed to recognize the kaons and to measure all the different types of particles that can be produced in their decay. The key ingredients are: i) kaon identification by measuring the Cherenkov radiation specific to their mass and speed, ii) precise reconstruction of the decay of the kaon, iii) the separation between pions, muons and electrons by measuring their masses and energies, and iv) the identification of photons with an accuracy of better than one part in a hundred thousand. It is vital that no particles escape detection and to this end an instrumented volume of liquid krypton measures the energy and position of the high-energy electrons and photons travelling forwards, while a system of 12 lead-glass detectors around the beamline detects all remaining photons. In order to perturb as little as possible the momentum and angles of the charged particles, lightweight straw-tubes have been built into two pairs of chambers, separated by a bending magnet producing a large magnetic field, to measure their position and momentum. This enables us to use the conservation of energy and momentum to reject much of the background to the wanted decay. A 17-metre-long Cherenkov detector filled with gaseous Neon has been installed to improve the distinction between charged pions and muons, provided principally by a calorimeter built from iron plates separated by scintillators that measure the energy released when the particles come to rest. The experiment is designed to keep all of the rare decays we wish to study and to sample all other kaon decays in an unbiased way with very low probability. The key point in reconstructing the final state particles is not just that they are consistent with the very rare decay we are seeking, but that they are not consistent with any other type of decay. To do this in a reliable way we must measure all backgrounds from data rather than relying on simulation. The experiment is scheduled to run up to 2018. The 2014 and 2015 data should allow us to observe a few of the very rare kaon decays into a pion and two neutrinos at the level of the Standard Model sensitivity, while the precision measurement of the decay probability will be achievable only at the end of the data taking. This measurement, together with the results from the many future direct and indirect searches produced concurrently at LHC, will help to distinguish the mechanisms beyond the Standard Model that may produce a coherent picture of the observed universe.