Description
The search for charged lepton flavour violation (CLFV) has an enormous discovery potential in probing new physics Beyond the Standard Model (BSM). The observation of a CLFV transition would be an undeniable sign of the presence of BSM physics which goes beyond non-zero masses for neutrinos. Furthermore, CLFV measurements can provide a way to distinguish between different BSM models, which may not be possible through other means. So far muonic CLFV processes have the best experimental sensitivity because of the huge number of muons which can be produced at several facilities world-wide. In coming years new muon beam-lines will be built, leading to several orders of magnitude increase in beam intensity. Among muonic CLFV processes, $\mu\to e$ conversion is one of the most important processes, having several advantages compared to other such processes.
We describe the COMET experiment, which is searching for $\mu\to e$ conversion in a muonic atom at the J-PARC proton accelerator laboratory in Japan. The COMET experiment has taken a staged approach; the first stage, COMET Phase-I, is currently under construction at J-PARC, and is aiming at a factor 100 improvement over the current limit. The second stage, COMET Phase-II is seeking another 100 improvement (a total of 10,000), allowing a single event sensitivity (SES) of $2.6\times 10^{-17}$ with $2\times 10^{7}$ seconds of data-taking. Further improvements by one order of magnitude, from refinements to the experimental design and operation are being considered within the beam power and the beam time as originally assumed. Such a sensitivity could be translated into probing many new physics constructions up to $\mathcal{O}(10^{4})$ TeV energy scales, which would go far beyond the level that can be reached directly by collider experiments. The search for CLFV $\mu\to e$ conversion is thus highly complementary to BSM searches at the LHC.
