A limit for the μeγ decay from the MEG experiment

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Abstract

A search for the decay μ+e+γ, performed at PSI and based on data from the initial three months of operation of the MEG experiment, yields an upper limit on the branching ratio of BR(μ+e+γ)2.8×1011 (90% C.L.). This corresponds to the measurement of positrons and photons from 1014 stopped μ+-decays by means of a superconducting positron spectrometer and a 900 litre liquid xenon photon detector.

Introduction

We report here on the results of a search for the lepton flavour violating decay μ+e+γ, based on data collected during the first three months period of the MEG experiment. This operates at the 590 MeV proton ring cyclotron facility of the Paul Scherrer Institut (PSI), in Switzerland.

Lepton flavour conservation in the Standard Model (SM) is associated with neutrinos being massless. Recent observations of neutrino oscillations [1] imply a non-zero mass and hence the mixing of lepton flavours. However, in minimal extensions to the SM, with finite but tiny masses, charged lepton flavour violating processes are strongly suppressed and beyond experimental reach.

Additional sources of lepton flavour violation (LFV) [2], [3], [4] appear in theories of supersymmetry, grand unification or in extra dimensions, giving predictions that have now become accessible experimentally. Hence, the present lack of observation of a signature of charged LFV may change with improved searches and reveal new physics beyond the SM or significantly constrain the parameter space of such extensions.

The strongest bounds on charged LFV come from the muon system, with the current limit for the branching ratio BR(μ+e+γ)1.2×1011 (90% C.L.), set by the MEGA experiment [5].

Section snippets

Experimental principle

The μ+e+γ process is characterized by a simple two-body final state, with the positron and photon being coincident in time and emitted back-to-back in the rest frame of the muon, each with an energy equal to half that of the muon mass.

There are two major sources of background, one from radiative muon decay (RMD) μ+e+νeν¯μγ and the other from accidental coincidences between a high energy positron from the normal muon decay μ+e+νeν¯μ (Michel decay) and a high energy photon from sources such as

Experimental layout and the MEG detector

A schematic of the experiment is shown in Fig. 1. Surface muons of 28MeV/c from one of the world's most intense sources, the πE5 channel at PSI, are stopped in a thin, partially depolarizing polyethylene target, placed at the centre of the positron spectrometer. To facilitate a stopping rate of 3×107μ+s1 in the 18 mg/cm2 thick target, with minimum beam-related background, a Wien filter and a superconducting transport solenoid (BTS) with a central degrader system are employed. The MEG beam

Monitoring and calibrations

The long term stability of the MEG experiment is an essential ingredient in obtaining high quality data over extended measurement periods. Continuous monitoring and frequent calibrations are a prerequisite. Apart from such items as the liquid xenon temperature and pressure, the drift chambers gas composition and pressure and the electronics temperature, a number of additional measurements must be performed to keep the subdetectors calibrated and synchronized. The three most important are

Event selection and resolutions

The data sample analyzed here was collected between September and December 2008 and corresponds to 9.5×1013 muons stopping in the target. At the first stage of the data processing, a data reduction is performed by selecting events with conservative criteria that require the time of the photon detector signal to be close to that of a timing counter hit, and at least one track to be detected by the drift chamber system. This reduces the data size to 16% of the recorded events. The pre-selected

Data analysis

The analysis algorithms are calibrated and optimized by means of a large data sample in the side-bands outside of the blinding-box. The background level in the signal region can also be studied with the event distribution in the side-bands since the primary source of background in this experiment is accidental.

The blinding-box is opened after completing the optimization of the analysis algorithms and the background study. The number of μ+e+γ events is determined by means of a maximum

Conclusion and prospects

A search for the lepton flavour violating decay μ+e+γ was performed with a branching ratio sensitivity of 1.3×1011, using data taken during the first three months period of the MEG experiment in 2008. With this sensitivity, which is comparable with the current branching ratio limit set by the MEGA experiment, a blind likelihood analysis yields an upper limit on the branching ratio of BR(μ+e+γ)2.8×1011 (90% C.L.).

The problem of the reduced performance of the drift chambers, due to

Acknowledgements

We are grateful for the support and co-operation provided by PSI as the host laboratory and to the technical and engineering staff of our institutes. This work is supported by DOE DEFG02-91ER40679 (USA), INFN (Italy) and MEXT KAKENHI 16081205 (Japan).

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Present address: DECTRIS Ltd., Neuenhoferstrasse 107, CH-5400 Baden, Switzerland.

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