Lepton Flavour Violation (LFV)
The Standard Model of particle physics is a theory which describes the interaction of all fundamental particles, under the mediation of three of the four known forces, namely the Weak, Strong and Electromagnetic. Particles which are mediated by only the Weak and Electromagnetic force are known as leptons, and have a lepton number of +1 (antiparticles having a corresponding lepton number of -1). For interactions within the Standard Model lepton number is conserved.In this Standard Model, lepton flavour conservation is built in by assuming vanishingly small neutrino masses. However, neutrino mixing has been experimentally confirmed by the discovery of neutrino oscillations, and lepton flavour conservation is therefore known to be violated. However, LFV of charged leptons has yet to be observed experimentally.
It is known that the contribution of neutrino mixing to LFV is extremely small, since it is proportional to (m&nu/mW)4, with a branching ratio of the order 10-54. Therefore, the discovery of LFV would imply new physics beyond neutrino oscillations, and vice-versa. This new physics includes supersymmetric models, extra dimensional models, models with new gauge bosons, models with new heavy leptons, lepto-quark models and so on. The branching ratios for these extensions are model dependent, with the branching ratio for SUSY models being anywhere from 10-14 to 10-25 . However, any beyond-SM results from the LHC will likely constrain such models significantly, leading to rather robust predictions of lepton flavour violation (LFV).
Supersymmetric Extension
| It is known that LFV has significant contributions from SUSY, with the contribution being particularly large if SUSY particles exist in the LHC energy range. In minimum SUSY extension (MSSM), LFV of charged leptons would occur through mixing of their corresponding sleptons. Slepton mixing is difficult to study in high energy collider experiments such as the LHC. The consistency of LHC results with LFV will, therefore, be one of the definitive and necessary checks in establishing new physics beyond the SM. |
Experimental LFV
Since the first search by Hincks and Pontecorvo in 1947, experimental searches for LFV have been continuously carried out with various elementary particles, such as muons, taus and kaons. The upper limits have been improved at an approximate rate of two orders of magnitude per decade with the present, and possible near future, upper limits of various LFV decays listed in the adjacent table. It can be seen that the sensitivity of the muon system to LFV is very high, primarily due to the large number of muons available for current experimental searches (approx. 1014-1015 &mu/year).|
The processes which utilise this high intensity muon source to the greatest effect are those of rows 3 and 4, namely the interactions of incident muons with atomic targets. Measurements that call for coincidence requirements in detection of daughter particles (i.e. the processes in rows 1 and 2) would suffer from huge accidental backgrounds of electrons, therefore making it extremely difficult to distinguish between signal and background. For this reason, &mu--e- conversion is sought after, employing a dense target such as Aluminium or Titanium to increase the relative branching ratio. There are two major experiments, mu2e and COMET, both in the research and design stage, with the intention of probing this variety of lepton violation. UCL is a member of the Coherent Muon to Electron Transition (COMET) collaboration, spearheaded by Osaka University, which aims to probe for LFV with a sensitivity of 10-16. The collaboration is also working on a subsequent experiment (PRISM) which aims to increase the sensitivity to LFV by a factor of 100, to 10-18. | 
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Last Modified : 14:35:51 10 Jun 2009