MEG Experiment Research Articles

μ→eγdecay in an MSSM extension

An analysis is given of the decay $\ensuremath{\mu}\ensuremath{\rightarrow}e+\ensuremath{\gamma}$ in an minimal supersymmetric standard model extension with a vectorlike generation. Here mixing with the mirrors allows the possibility of this decay. The analysis is done at the one-loop level with the exchange of charginos and neutralinos and of sleptons and mirror sleptons in the loops. A one-loop analysis with W- and Z-boson exchange and mirror leptons and neutrinos is also considered. The effects of $CP$-violating phases from the new sector on the decay $\ensuremath{\mu}\ensuremath{\rightarrow}e\ensuremath{\gamma}$ are analyzed in detail. The constraints arising from the current upper limit on the branching ratio $\mathcal{B}(\ensuremath{\mu}\ensuremath{\rightarrow}e\ensuremath{\gamma})$ from the MEG experiment of $2.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}$ (at 90% C.L.) on the parameter space of supersymmetry (SUSY) models and on vectorlike models are explored. Further, the MEG experiment is likely to improve the upper limit by an order of magnitude in the coming years. The improved limits will allow one to probe a much larger domain of the parameter space of the extended models.

New result from the MEG Experiment at PSI and the MEG upgrade

The analysis of a combined data set, totaling 3.6×1014 stopped muons on target, in the search for the lepton flavor violating decay μ+→e+γ is presented. The data collected by the MEG experiment at the Paul Scherrer Institut show no excess of events compared to background expectations and thus yield a new upper limit on the branching ratio of this decay of 5.7×10−13 @ 90% CL [J. Adam, et al., New constraint on the existence of the μ+→e+γ decay, Phys. Rev. Lett. 110 (2013) 201801. arXiv:1303.0754]. At this point of data taking the background became dominant and an improvement of the sub detectors resolution was needed in order to obtain in few years a significant improvement in the branching ratio upper limit. An upgrade of the MEG experiment was proposed [A. Baldini, F. Cei, C. Cerri, S. Dussoni, L. Galli, et al., MEG Upgrade Proposal, arXiv:1301.7225] and approved in January 2013.

Charged Lepton Flavor Violation: Latest Results and Future Plans of the MEG Experiment

Charged lepton flavor violation processes are ideal probes for new physics due to the suppression of Standard Model backgrounds. Currently, the MEG collaboration is searching for μ+ → e+γ decay with unprecedented precision and has recently published a new analysis based on data collected in the years 2009–2011. This resulted in an upper limit on the branching ratio of 5.7 · 10−13 at 90% confidence level, which represents a four times more stringent limit than the previous world best limit set by MEG. The details behind this result, as well as the current status and future plans of theMEG experiment are reported here.

Muon charged Lepton Flavor Violation search in Europe: the μ+ → e+γ and the μ+ → e+e−e+ decays

Lepton flavor violation (LFV) research is currently one of the most exciting branches of particle physics. Flavor violating processes, such as μ+ → e+γ and μ+ → e+e−e+, which are strongly suppressed in the Standard Model (SM), are very sensitive to new physics. The MEG experiment and the Mu3e experiment, which search for the μ+ → e+γ and the μ+ → e+e−e+ decay respectively, are two precision physics experiments at the forefront of field. They are housed at the Paul Scherrer Institut (PSI), in Switzerland, which provides the most intense continuous muon beam in the world. A summary of the status of the two experiments is given.

The LXe calorimeter and the pixelated timing counter in the MEG II experiment

The MEG experiment is to look for a lepton flavor violating μ+→e+γ decay with an unprecedented sensitivity, and we set an upper limit of the branching ratio for this decay, 5.7 × 10−13 at 90% C.L. in 2013 which is a twenty times more stringent limit than the previous experiment, MEGA. Since the sensitivity improvement was limited by the accidental background, we have considered the major detector upgrade. A proposal was submitted to PSI committee, and was approved in 2013, which aims at a sensitivity enhancement of one order of magnitude compared with the current MEG experiment. Here mainly two components of the MEG detector will be introduced, a γ-ray calorimeter with 900 L of liquid xenon (LXe), and a pixelated timing counter. The LXe detector will be improved by increasing the granularity at the incident face, by replacing the current PMTs with a larger number of smaller photosensors (MPPC) and optimizing the photosensor layout also on the lateral faces. A new highly segmented, fast timing counter array will replace the old system to allow improved timing resolution capabilities.

Upgrade of LXe gamma-ray detector in MEG experiment

We are searching for the μ→e+γ in the MEG experiment. In order to improve the search sensitivity down to about 5 × 10−14, which is one order higher than the sensitivity goal of the current stage of the experiment. We are planning a major upgrade of the experiment including the upgrade of the liquid xenon (LXe) γ-ray detector. The main item of the upgrade of the xenon detector is a replacement of the current PMTs located on the γ-ray incident face with smaller photosensors such as MPPC. The energy and position resolutions are expected to be significantly improved, especially for the event where γ-ray converts at a shallow point in the LXe. Because the MPPC operational in liquid xenon is not yet commercially available, we are developing special MPPC in collaboration with Hamamatsu Photonics. The detection efficiency for LXe scintillation light is measured with prototype sensors and found to be already at the level necessary for the upgrade of the experiment.

New Positron Spectrometer for MEG Experiment Upgrade

An upgrade of the MEG experiment, which searches for the lepton flavor violating decay, μ→eγ, at the highest sensitivity ever, is planned in order to improve the sensitivity down to ∼5×10−14. We plan to employ a stereo wire drift chamber with a unique volume for the tracking and a pixelated scintillation detector with silicon photomultiplier (SiPM) readout for the timing measurement with improved efficiency and resolutions. We will describe the expected performance and the R&D status of the new spectrometer especially focusing on the new timing counter, which is expected to contribute better resolution of the relative timing between positron and gamma-ray.

Search for mu LFV from MEG

The MEG experiment searches for a lepton flavor violating muon decay, μ→eγ, discovery of which gives a clear evidence of the new physics beyond the Standard Model of the elementary particle physics. By analyzing the data taken in 2009 and 2010, which corresponds to 1.8 × 1014 muon decays in the stopping target, the result was consistent with null signal. The most stringent upper limit on the branching ratio of 2.4 × 10−12 is set.

A simulation tool for scintillating fibers coupled to SiPM for MIP and heavy ionizing particle identification

The MEG experiment searches for the μ+→e+γ decay by stopping on a thin passive target the most intense continuous muon beam in the world. We are studying the possibility to have an active target, which should continuously monitor the muon beam and provide a precise measurement of the decay vertex, complementing the new spectrometer. The detector is based on thin scintillating fibers coupled to silicon photomultipliers (SiPMs), exploiting their insensitivity to the magnetic field, competitive PDE with respect to nominal PMT QE and low HV supply.This device can be used as a stand alone tool. In this paper we present a Monte Carlo simulation for a single fiber coupled to SiPM in order to study the propagation of the photons through the fiber and the response of the SiPM.

Operation and performance of the trigger system of the MEG experiment

The MEG experiment at PSI is searching for the μ+ → e+γ decay with a sensitivity B(μ+ → e+γ) ≈ 5 × 10−13 on its branching ratio.The world's most intense continuous muon beam (up to 108μ+/s) is stopped on a thin target, and e+γ pairs must becollected and measured. An effective on-line event selection system (trigger), able to suppressthe huge beam-related background, while preserving the signal efficiency, is mandatory.A digital approach is adopted in order to accomplish this objective.Detector signals are processed on-line to perform an adequate event reconstruction, within a 450 ns latency,by means of Field Programmable Gate Arrays (FPGA). This approachis flexible enough to allow the acquisition of calibration events in parallel with the main data stream so as to monitor thedetector behaviour throughout the run.In this paper we describe the implementation of the trigger algorithms on the FPGA, and their resolutions and efficiencies as well.Each observable used in the on-line event selection is described in details, together with the firmware implementation of the algorithms.We finally show how this arrangement provides the experiment with an effective trigger system fulfilling all the requirements andleading to an overall DAQ efficiency close to 100%.

Searching for charged lepton flavor violation: New results from the MEG experiment

Discovery of charged lepton flavor violation would be unambiguous evidence of new physics beyond the Standard Model. The MEG experiment focuses on μ+ → e+γ decay and has recently published a search result based on data collected in 2009 and 2010. No evidence of this decay mode is found in this data sample, resulting in an upper limit on the branching ratio of 2.4 · 10−12 at 90% confidence level, the best limit ever achieved for this process. This result as well as the current status and prospects of the MEG experiment are reported here.

Feasibility study of an active target for the MEG experiment

We consider the possibility to have an active target for the upgrade of the MEG experiment (MEG II). The active target should work as (1) a beam monitoring, to continuously measure the muon stopping rate and therefore provide a direct evaluation of the detector acceptance (or an absolute normalization of the stopped muon); and as (2) an auxiliary device for the spectrometer, to improve the determination of the muon decay vertex and consequently to achieve a better positron momentum and angular resolutions, detecting the positron from the muon decay. In this work we studied the feasibility of detecting minimum ionizing particle with a single layer of 250 μm fiber and the capability to discriminate between the signal induced by either a muon or a positron.

Research and Development activities at INFN Pisa on a new drift chamber for the MEG experiment upgrade

We present the research and development activities performed at the Istituto Nazione di Fisica Nucleare (INFN) sezione di Pisa about the new drift chamber for the MEG experiment upgrade.

Design and test of an extremely high resolution Timing Counter for the MEG II experiment: preliminary results

The design and tests of Timing Counter elements for the upgrade of the MEG experiment, MEG II, are presented. The detector is based on several small plates of scintillator with a Silicon PhotoMultipliers dual-side readout. The optimisation of the single counter elements (SiPMs, scintillators, geometry) is described. Moreover, the results obtained with a first prototype tested at the Beam Test Facility (BTF) of the INFN Laboratori Nazionali di Frascati (LNF) are presented.

Measuring temporal, spectral and spatial changes in electrophysiological brain network connectivity

The topic of functional connectivity in neuroimaging is expanding rapidly and many studies now focus on coupling between spatially separate brain regions. These studies show that a relatively small number of large scale networks exist within the brain, and that healthy function of these networks is disrupted in many clinical populations. To date, the vast majority of studies probing connectivity employ techniques that compute time averaged correlation over several minutes, and between specific pre-defined brain locations. However, increasing evidence suggests that functional connectivity is non-stationary in time. Further, electrophysiological measurements show that connectivity is dependent on the frequency band of neural oscillations. It is also conceivable that networks exhibit a degree of spatial inhomogeneity, i.e. the large scale networks that we observe may result from the time average of multiple transiently synchronised sub-networks, each with their own spatial signature. This means that the next generation of neuroimaging tools to compute functional connectivity must account for spatial inhomogeneity, spectral non-uniformity and temporal non-stationarity. Here, we present a means to achieve this via application of windowed canonical correlation analysis (CCA) to source space projected MEG data. We describe the generation of time–frequency connectivity plots, showing the temporal and spectral distribution of coupling between brain regions. Moreover, CCA over voxels provides a means to assess spatial non-uniformity within short time–frequency windows. The feasibility of this technique is demonstrated in simulation and in a resting state MEG experiment where we elucidate multiple distinct spatio-temporal-spectral modes of covariation between the left and right sensorimotor areas.

The MEG experiment at PSI

We present the latest results on the lepton flavor violating decay μ → e γ based on an analysis of the data collected by the MEG detector at the Paul Scherrer Institute during the years 2009–2011. The analysis of the combined data set, corresponding to 3.6 × 1014 muons stopped on target provides a 90% C.L. upper limit of 5.7 × 10−13 on the μ → e γ branching ratio, constituting the most stringent limit on the existence of this decay to date. After having successfully completed the 2012 and 2013 runs, an upgrade of the experiment, already approved by founding agencies and host laboratory, is foreseen. The status of the R&D, which has been started since 2012, will be also discussed.

A GLANCE BEYOND THE STANDARD MODEL: LATEST RESULTS FROM THE MEG EXPERIMENT

The MEG experiment started taking data in 2009 looking for the Standard Model suppressed decay μ → e + γ, which, if observed, can reveal Beyond Standard Model physics. It makes use of state-of-the art detectors optimized for operating in conditions of very high intensity, rejecting as much background as possible. The data taking ended August 2013 and an upgrade R&D is started to push the experimental sensitivity. The present upper limit on the decay Branching Ratio (BR) is presented, obtained with the subset of data from 2009 to 2011 run, together with a description of the key features of the upgraded detector.

Neutrino textures and charged lepton flavor violation in light ofθ13, MEG, and LHC data

In light of recent results from the LHC, MEG and neutrino experiments, we revisit the issue of charged lepton flavour violation (LFV) in supersymmetric theories with massive neutrinos, where flavour-violating soft supersymmetry-breaking masses for sleptons are induced naturally by radiative corrections. We link our results to the expectations for neutrino mixing angles in SU(5) enhanced by an abelian flavour symmetry, with particular focus on $\theta_{13}$. We focus on the radiative decays $\ell_i \rightarrow \ell_j \gamma$ and on detection prospects at the LHC and a linear collider (LC). We use supersymmetric parameters consistent with cosmological considerations and with LHC searches for supersymmetry and the Higgs mass. We find a class of scenarios where the LHC may be sensitive to LFV sparticle decays and LFV processes could be detectable at a LC with centre-of-mass energy above 1 TeV, whereas LFV lepton decays may be suppressed by cancellations in the decay amplitudes.

Development of pixelated scintillation detector for highly precise time measurement in MEG upgrade

The MEG experiment in search for the lepton flavor decay μ+→e+γ is planned to be upgraded to reach the branching ratio sensitivity down to 5×10−14, which is one order of magnitude higher than that of the current phase of the experiment. A pixelated scintillation detector with an ultimate time resolution is under development for the positron time measurement in the upgraded experiment. Single pixel time resolutions better than 50ps (rms) are achieved with prototype counters and an excellent overall time resolution at a level of 30–35ps (rms) is found to be achievable in the full-scale detector by averaging the positron impact times over multiple hit counters.

Supersymmetric Lepton Flavour Violation and Neutrinos mass textures: an update

We revise the prospects for the observation of charged lepton flavour violation (LFV), in the light of the recent results from the LHC, MEG and neutrino experiments. We work in the context of the Minimal Supersymmetric Standard Model (MSSM) extended with massive neutrinos arising from a see-saw mechanism. The connection between the observed neutrino oscillations with flavour oscillations in the charged sector is established using a model for the Yukawa couplings arising from a SU(5) grand unified theory (GUT) with Abelian flavour symmetries. We discuss in this scenario the possible observation on the radiative decays μ → eγ and τ → μγ and LFV processes that could be detectable at a linear collider (LC) with a centre-of-mass energy in the TeV range.