The final results of the search for the lepton flavour violating decay \(\mathrm {\mu }^+ \rightarrow \mathrm {e}^+ \mathrm {\gamma }\) based on the full dataset collected by the MEG experiment at the Paul Scherrer Institut in the period 2009–2013 and totalling \(7.5\times 10^{14}\) stopped muons on target are presented. No significant excess of events is observed in the dataset with respect to the expected background and a new upper limit on the branching ratio of this decay of \( \mathcal{B} (\mu ^+ \rightarrow \mathrm{e}^+ \gamma ) < 4.2 \times 10^{-13}\) (90 % confidence level) is established, which represents the most stringent limit on the existence of this decay to date.

Background: The rate lambda(pp mu) characterizes the formation of pp mu molecules in collisions of muonic p mu atoms with hydrogen. In measurements of the basic weak muon capture reaction on the proton to determine the pseudoscalar coupling g(P), capture occurs from both atomic and molecular states. Thus knowledge of lambda(pp mu) is required for a correct interpretation of these experiments.Purpose: Recently the MuCap experiment has measured the capture rate Lambda(S) from the singlet p mu atom, employing a low-density active target to suppress pp mu formation [V. Andreev et al. (MuCap Collaboration), Phys. Rev. Lett. 110, 012504 (2013)]. Nevertheless, given the unprecedented precision of this experiment, the existing experimental knowledge in lambda(pp mu) had to be improved.Method: The MuCap experiment derived the weak capture rate from the muon disappearance rate in ultrapure hydrogen. By doping the hydrogen with 20 ppm of argon, a competing process to pp mu formation was introduced, which allowed the extraction of pp mu from the observed time distribution of decay electrons.Results: The pp mu formation rate was measured as lambda(pp mu) = (2.01 +/- 0.06(stat) +/- 0.03(sys)) x 10(6) s(-1). This result updates the pp mu value used in the abovementioned MuCap publication.Conclusions: The 2.5x higher precision compared to earlier experiments, and the fact that the measurement was performed under nearly identical conditions as the main data taking, reduces the uncertainty induced by lambda(pp mu) to a minor contribution to the overall uncertainty of Lambda(S) and g(P), as determined in the MuCap experiment. Our final value for lambda(pp mu) shifts Lambda(S) and g(P) by less than one-tenth of their respective uncertainties compared to our results published earlier.

The Michel parameter xi" has been determined from a measurement of the longitudinal polarization of positrons emitted in the decay of polarized and depolarized muons. The result, xi" = 0.981 +- 0.045stat +- 0.003syst, is consistent with the Standard Model prediction of unity, and provides an order of magnitude improvement in the relative precision of this parameter. This value sets new constraints on exotic couplings beyond the dominant V-A description of the leptonic weak interaction.

. The MuCap experiment at the Paul Scherrer Institute performed a high-precision measurement of the rate of the basic electroweak process of nuclear muon capture by the proton, μ^-+p→ n+ν_μ . The experimental approach was based on the use of a time projection chamber (TPC) that operated in pure hydrogen gas at a pressure of 10bar and functioned as an active muon stopping target. The TPC detected the tracks of individual muon arrivals in three dimensions, while the trajectories of outgoing decay (Michel) electrons were measured by two surrounding wire chambers and a plastic scintillation hodoscope. The muon and electron detectors together enabled a precise measurement of the μ p atom’s lifetime, from which the nuclear muon capture rate was deduced. The TPC was also used to monitor the purity of the hydrogen gas by detecting the nuclear recoils that follow muon capture by elemental impurities. This paper describes the TPC design and performance in detail.

A muon decay accompanied by a photon through the inner Bremmstrahlung process (μ→eνν¯γ, radiative muon decay) produces a time-correlated pair of positron and photon which becomes one of the main backgrounds in the search for μ→eγ decay. This channel is also an important probe of timing calibration and cross-check of whole the experiment. We identified a large sample (∼ 13000) of radiative muon decays in MEG data sample. The measured branching ratio in a region of interest in the μ→eγ search is consistent with the standard model prediction. It is also the first measurement of the decay from polarized muons. The precision measurement of this mode enables us to use it as one of the normalization channels of μ→eγ decay successfully reducing its uncertainty to less than 5%.

We studied the radiative muon decay $\mu^+ \to e^+\nu\nu\gamma$ by using for the first time an almost fully polarized muon beam. We identified a large sample (~13000) of these decays in a total sample of 1.8x10^14 positive muon decays collected in the MEG experiment and measured the branching ratio B($\mu^+ \to e^+\nu\nu\gamma$) = (6.03+-0.14(stat.)+-0.53(sys.))x10^-8 for E_e > 45 MeV and E_{\gamma} > 40 MeV, consistent with the Standard Model prediction. Moreover, the precise measurement of this decay mode provides the basic tool for the timing calibration and a strong quality check of the complete MEG experiment in the search for $\mu^+ \to e^+\gamma$ process.

The analysis of a combined data set, totaling 3.6 × 10(14) 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 yield a new upper limit on the branching ratio of this decay of 5.7 × 10(-13) (90% confidence level). This represents a four times more stringent limit than the previous world best limit set by MEG.

The MuCap experiment at the Paul Scherrer Institute has measured the rate Λ(S) of muon capture from the singlet state of the muonic hydrogen atom to a precision of 1%. A muon beam was stopped in a time projection chamber filled with 10-bar, ultrapure hydrogen gas. Cylindrical wire chambers and a segmented scintillator barrel detected electrons from muon decay. Λ(S) is determined from the difference between the μ(-) disappearance rate in hydrogen and the free muon decay rate. The result is based on the analysis of 1.2 × 10(10) μ(-) decays, from which we extract the capture rate Λ(S) = (714.9 ± 5.4(stat) ± 5.1(syst)) s(-1) and derive the proton's pseudoscalar coupling g(P)(q(0)(2) = -0.88 m(μ)(2)) = 8.06 ± 0.55.

The MEG (Mu to Electron Gamma) experiment has been running at the Paul Scherrer Institut (PSI), Switzerland since 2008 to search for the decay μ + →e + γ by using one of the most intense continuous μ + beams in the world. This paper presents the MEG components: the positron spectrometer, including a thin target, a superconducting magnet, a set of drift chambers for measuring the muon decay vertex and the positron momentum, a timing counter for measuring the positron time, and a liquid xenon detector for measuring the photon energy, position and time. The trigger system, the read-out electronics and the data acquisition system are also presented in detail. The paper is completed with a description of the equipment and techniques developed for the calibration in time and energy and the simulation of the whole apparatus.

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-flavor-violating decay þ ! e þ .The likelihood analysis of the combined data sample, which corresponds to a total of 1:8 Â 10 14 muon decays, gives a 90% C.L. upper limit of 2:4 Â 10 À12 on the branching ratio of the þ ! e þ decay, constituting the most stringent limit on the existence of this decay to date.

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 decay mu(+) -> e(+)gamma, 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(mu(+) -> e(+)gamma) <= 2.8 x 10(-11) (90% C.L.). This corresponds to the measurement of positrons and photons from similar to 10(14) stopped mu(+)-decays by means of a superconducting positron spectrometer and a 900 litre liquid xenon photon detector. (C) 2010 Elsevier B.V. All rights reserved.

This work was carried out as part of a project aiming at a greatly improved measurement of the muon capture rate from the singlet state of the μp atom. The experiment will be performed at the intense muon beam of PSI using a new experimental method allowing high precision measurements of the lifetime of muons stopped in ultra-pure deuterium-depleted hydrogen (protium). The basic element of the detector is a time projection chamber operating in hydrogen gas at 10bar pressure. The arrival times and trajectories of the incoming muons and the outgoing decay electrons are measured with this device providing effective suppression of background. The system of chambers and electronics is designed for the large muon stop rates required for attaining high statistical accuracy. During four beam periods at PSI, data were taken. Also, various studies of the MWPC performance in hydrogen were made including ageing studies of the chambers under irradiation with stopped muons and with alpha and beta sources. It was demonstrated that the MWPCs can operate in pure hydrogen under 10 bar pressure with gas gains up to 5000, which is sufficient for the detection of relativistic electrons.

A new experiment is under preparation with the aim to improve considerably the present knowledge of the rate Λ s , which should be measured on a level of 1% or better, for the basic electroweak capture reaction of a negative muon on the free proton μ p 1 s → n + ν μ . The capture rate will be determined by measuring the lifetime of μ − stopped in ultra pure hydrogen at 10 bar pressure and comparing it with the lifetime of the unbound μ + . A new experimental method was developed for this project which should allow measuring the μ − lifetime with at least 10 ppm precision. The basic element of the detector is operating in the hydrogen gas time projection chamber (TPC) surrounded by multi-wire proportional chambers (MWPCs) and scintillator counters. The arrival times and trajectories of the incoming muons and the outgoing decay electrons are measured with this device providing effective suppression of background. Using the TPC as an active target, we can monitor on-line the protium contamination by impurities with a sensitivity better than 10 −8 . This can be done by detecting the charged products of the muon capture reaction on these impurities. It was demonstrated that the TPC and MWPCs can operate in pure hydrogen under 10 bar pressure providing gas gain up to 10 000.

The Mucap experiment at the Paul Scherrer Institute is a high-precision measurement of the singlet rate of the basic electroweak process µ - + p → n + νµ (muon capture on the proton). The experimental approach is based on the use of a spec ially developed Time Projection Chamber (TPC) operating with ultra-pure hydrogen gas at 10 bar an d acting as an active muon stop target. Each muon stop in the gas is identified by the TPC and its de cay electron is detected by surrounding wire chambers and a plastic scintillation hodoscope. The TPC allows in connection with the electron detector to measure the μ μ μ μ-p lifetime to highest precision from which the capt ure rate is deduced. The design of the TPC and its experimental performance is described in detail.