Mu to electron conversion with the COMET experiment

Abstract

The COMET experiment at J-PARC is under construction and will search for mu to electron conversion in aluminium at the 3.10 level, four orders of magnitude better than current limits. The experiment will take place in two phases with a first phase aiming at a two orders of magnitude improvement. 1 Charged lepton flavour violation with COMET The COMET experiment is currently in preparation at JPARC. The aim is to investigate charged lepton violation by searching for mu to e transitions from muons stopped in aluminium. The collaboration currently has 177 participants from 33 institutes from Belarus, Canada, China, Czech Republic, France, Georgia, Germany, India, Japan, Malaysia, Russia, Saudi Arabia, South Korea, United Kingdom and Vietnam. First data taking is expected in 2018. Lepton flavour violation does not feature in the standard model which only describes flavour violations in the charged current interactions of quarks. However neutrino oscillation shows that neutral lepton flavour violation does occur in nature and this, so far, is the one unambiguous observation in particle physics of an interaction beyond the basic standard model. The actual mechanism of lepton flavour violation in the neutrino sector remains to be discovered and so establishing if there is measurable lepton flavour violation in the charged sector is crucial in the search for a wider theory or at least the necessary extension of the standard model to encompass neutrino oscillation. Flavour is a crucial part of particle physics and whilst it is becoming increasingly well parameterised there is little real appreciation of its role. Consequently it is necessary to acquire as much information as possible if a valid theory is to emerge. An important consequence of flavour changing processes with three families is the mixing matrix with the crucial phase which can yield CP violating processes. Observation of charged lepton flavour processes will almost certainly lead to additional sources of CP violation which could contribute to the unexplained baryon – antibaryon asymmetry. 1.1 Mu to e transitions A search for a muon changing to an electron is the transition most accessible for experimental investigation of charged lepton violation. Of course it could be that the main transition is from the tau but the lower production rate and the very short lifetime make it much harder to achieve very low rates. Three processes for to e transitions are currently being investigated Present limits are currently: Br( → e < 5.7.10, Br( → eee) < 1.10 and to e conversion on gold lower than 6.10. Currently →e and →eee are being investigated at PSI. Two experiments are in construction to investigate to e conversion, COMET at J-PARC and Mu2e at FNAL. As a result of neutrino oscillation charged lepton violation must take place at some level This is shown in Figure 1, however the rate from this process is very small, ~(m /mW) and so effectively unobservable. Figure 1. Mu to e transition as a result of neutrino oscillation. Possible processes for charged lepton violation can be classified as ‘Photonic’, Figure 2, and Non-Photonic, Figure 3. Figure 2. Photonic processes for to e Transitions Figure 3. Non-photonic processes for to e Transitions These can be summarised by a Lagrangian[1] , 01010 (2016) EPJ Web of Conferences DOI: 10.1051/ conf/201611801010 epj 118 © The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0 (http://creativecommons.org/licenses/by/4.0/). 201 FCCP 5