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Register a new userLepton Flavour Violating Muon Decay at MEG
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Authors
Hajime Nishiguchi
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The MEG Experiment searches for a lepton flavour violating decay, $mu^+tomathrm{e}^+gamma $, with a branching-ratio sensitivity of $10^{-13}$ in order to explore the parameter region predicted by many theoretical models beyond the Standard Model. Detector construction and the Engineering Run were completed in 2007, and the first Physics Run will be carried out in 2008. In this paper, the prospects of MEG Physics Run in 2008 is described in addition to the experimental overview.
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We present the first direct search for lepton flavour violating muon decay mediated by a new light particle X, $mu^+ to mathrm{e}^+mathrm{X}, mathrm{X} to gammagamma$. This search uses a dataset resulting from $7.5times 10^{14}$ stopped muons collected by the MEG experiment at the Paul Scherrer Institut in the period 2009--2013. No significant excess is found in the mass region 20--45 MeV/c$^2$ for lifetimes below 40 ps, and we set the most stringent branching ratio upper limits in the mass region of 20--40 MeV/c$^2$, down to $mathcal{O}(10^{-11})$ at 90% confidence level.
We present a new result based on an analysis of the data collected by the MEG detector at the Paul Scherrer Institut in 2009 and 2010, in search of the lepton flavour violating decay mu->e gamma. The likelihood analysis of the combined data sample, which corresponds to a total of 1.8 x 10**14 muon decays, gives a 90% C.L. upper limit of 2.4 x 10**-12 on the branching ratio of the mu->e gamma decay, constituting the most stringent limit on the existence of this decay to date.
A search for the lepton flavour violating decay $tau^-to mu^-mu^+mu^-$ is performed with the LHCb experiment. The data sample corresponds to an integrated luminosity of $1.0mathrm{,fb}^{-1}$ of proton-proton collisions at a centre-of-mass energy of $7mathrm{,Tekern -0.1em V}$ and $2.0mathrm{,fb}^{-1}$ at $8mathrm{,Tekern -0.1em V}$. No evidence is found for a signal, and a limit is set at $90%$ confidence level on the branching fraction, $mathcal{B}(tau^-to mu^-mu^+mu^-) < 4.6 times 10^{-8}$.
A search for the lepton-flavour violating decay $D^0 to e^pm mu^mp$ is made with a dataset corresponding to an integrated luminosity of 3.0 fb$^{-1}$ of proton-proton collisions at centre-of-mass energies of $7$ TeV and $8$ TeV, collected by the LHCb experiment. Candidate $D^0$ mesons are selected using the decay $D^{*+} to D^0 pi^+$ and the $D^0 to e^pm mu^mp$ branching fraction is measured using the decay mode $D^0 to K^-pi^+$ as a normalisation channel. No significant excess of $D^0 to e^pm mu^mp$ candidates over the expected background is seen, and a limit is set on the branching fraction, $mathcal{B}(D^0 to e^pm mu^mp) < 1.3 times 10^{-8}$, at 90 % confidence level. This is an order of magnitude lower than the previous limit and it further constrains the parameter space in some leptoquark models and in supersymmetric models with R-parity violation.
Lepton-flavour violating tau-decays are predicted in many extensions of the Standard Model at a rate observable at future collider experiments. In this article we focus on the decay tau to mu mu antimu, which is a promising channel to observe lepton-flavour violation at the Large Hadron Collider LHC. We present analytic expressions for the differential decay width derived from a model-independent effective Lagrangian with general four-fermion operators, and estimate the experimental acceptance for detecting the decay tau to mu mu antimu at the LHC. Specific emphasis is given to decay angular distributions and how they can be used to discriminate new physics models. We provide specific predictions for various extensions of the Standard Model, including supersymmetric, little Higgs and technicolour models.
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