The observation of a resonance structure in the opening angle of the electron-positron pairs in the Li-7(p,e(+)e(-)) 8Be reaction was claimed and interpreted as the production and subsequent decay of a hypothetical particle (X17). Similar excesses, consistent with this particle, were later observed in processes involving He-4 and C-12 nuclei with the same experimental technique. The MEG II apparatus at PSI, designed to search for the mu (+) -> e(+)gamma decay, can be exploited to investigate the existence of this particle and study its nature. Protons from a Cockroft-Walton accelerator, with an energy up to 1.1 MeV, were delivered on a dedicated Li-based target. The gamma and the e(+)e(-)pair emerging from the Be-8(& lowast;) transitions were studied with calorimeters and a spectrometer, featuring a broader angular acceptance than previous experiments. We present in this paper the analysis of a four-week data-taking in 2023 with a beam energy of 1080 keV, resulting in the excitation of two different resonances with Q-value 17.6 and 18.1 MeV. No significant signal was found, and limits at 90% C.L. on the branching ratios (relative to the gamma emission) of the two resonances to X17 were set, R-17.6 < 1.8 x 10(-6 )and R-18.1 < 1.2 x 10(-5) in the mass range between 16.5 MeV/c(2) and 17.1 MeV/c(2).

The MEG II experiment, based at the Paul Scherrer Institut in Switzerland, reports the result of a search for the decay ^+ →e^+ from data taken in the first physics run in 2021. No excess of events over the expected background is observed, yielding an upper limit on the branching ratio of ℬ ( ^+ →e^+ ) < 7.5 × 10^-13 (90 ℬ ( ^+ →e^+ ) < 3.1 × 10^-13 (90

Straw tubes with resistive cathode and cathode readout were successfully manufactured and tested. The straw tubes were manufactured by ultrasonic welding technique. The resistive cathode electrode was made of diamond like carbon (DLC). The possibility of cathode signal readout from external strip electrodes was successfully demonstrated. The event coordinate along the straw can be accurately determined by applying center of gravity method to the individual strip signals.

The MEG II experiment, based at the Paul Scherrer Institut in Switzerland, reports the result of a search for the decay $\mu^+\to e^+\gamma$ from data taken in the first physics run in 2021. No excess of events over the expected background is observed, yielding an upper limit on the branching ratio of B($\mu^+\to e^+\gamma$)<$7.5 \times 10^{-13}$ (90% C.L.). The combination of this result and the limit obtained by MEG gives B($\mu^+\to e^+\gamma$)<$3.1 \times 10^{-13}$ (90% C.L.), which is the most stringent limit to date. A ten-fold larger sample of data is being collected during the years 2022-2023, and data-taking will continue in the coming years.

Abstract NarrowΞπ andΞ + π ± resonances produced by quasi-real photons have been search ed for by the COMPASS experiment at CERN. The study was stimulat ed by the recent observation of an exotic baryonic state decaying into Ξπ−, at a mass of 1862 MeV, interpreted as a pentaquark. While the ordinary hyperon states Ξ(1530)0 andΞ(1530)0 are clearly seen, no exotic baryon is observed in the data taken in 2002 an d 003.

The international PEN collaboration aims to obtain the branching ratio for the pion electronic decay π → eνe(γ), aka πe2, to a relative precision of 5 × 10−4 or better. The PEN apparatus comprises a number of detection systems, all contributing vital information to the PEN event reconstruction. This paper discusses the design, performance, and Monte Carlo simulation of the mini time projection chamber (mTPC) used for pion, muon, and positron beam particle tracking. We also review the use of the extracted trajectory coordinates in the analysis, in particular in constructing observables critical for discriminating background processes, and in maximizing the fiducial volume of the target in which decay event vertices can be accepted for branching ratio extraction without introducing bias.

Abstract We present the first direct search for lepton flavour violating muon decay mediated by a new light particle X, $$\upmu ^+ \rightarrow \mathrm {e}^+\mathrm {X}, \mathrm {X} \rightarrow \upgamma \upgamma \ $$ μ + → e + X , X → γ γ . This search uses a dataset resulting from $$7.5\times 10^{14}$$ 7.5 × 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 $$^2$$ 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 $$^2$$ 2 , down to $${\mathcal {O}}(10^{-11})$$ O ( 10 - 11 ) at 90% confidence level.

We report the results of first studies of the small resistive Well radiation detector (https://indico.inp.nsk.su/event/20/session/9/contribution/205/material/poster/0.pdf) based on the Well Electron Multiplier with the DLC (Diamond-Like Carbon) anode. The DLC film covers the printed circuit mesh surrounding a group of holes. The prototype built with this technology provides fast, 200 ns, evacuation of electrons from the resistive layer with surface resistivity of 25 MOhm/square. As shown with 55Fe X-ray source, the detector with the mesh surrounding each hole withstands 0.5 millions discharges at the gas gain of 32,000 without damage to materials and functionality.

We study signals(2) induced in the resistiveWell MPGD constructed with the single DLC layer evaporated on top of a printed circuit mesh which was designed with various configurations surrounding either single hole or group of holes. As shown, signals are induced on the output readout elements (strip/pad/pixel) by both electrons and positive ions drifting in the blind well-like hole. An additional electron component is clearly seen in signals and related to the charge dispersed along the resistive layer. We present for the first time, that the ion tail induced on the readout elements can be suppressed to the level below 1% by proper designing the detector.

Experimental results on pion decays obtained with the PIBETA spectrometer at the Paul Scherrer Institute (PSI) are reviewed. For pion beta decay π+ → π0е+ν (πβ), a precision measurement of relative probability yields Г(πβ) = [1.036 ± 0.004(stat) ± 0.004(syst) ± 0.003(π+→е+ν)] × 10–8, which implies Vud = 0.9728(30) for the corresponding element of the Cabibbo–Kobayashi–Maskawa mixing matrix. Using a sample of 65 × 103 events, relative probability of the π+→е+νγ radiative pion decay (RPD) in the kinematic region of Eγ > 10 MeV and θeγ > 40° is measured as Bexp = 73.86(54) × 10–8. A statistical analysis of measured Ee+ and Eγ distributions for this decay yield the values FV = 0.0258(17) and FA = 0.0117(17) for the pion weak formfactors. Assuming that FV linearly depends on the е+ν invariant mass q2 as FV(q2) = FV(0)(1 + aq2), the slope parameter is extracted as а = 0.10(6). The pion polarizability and neutral-pion lifetime are estimated as αE = 2.78(10) × 10–4 fm3 and τ(π0) = (8.5 ± 1.1) × 10–17 s, respectively. The data for decays π+→ е+ ν and \({\mu ^ + } \to {e^ + }v\bar v\gamma \) have been collected and are being processed. The follow-up PEN experiment aims at reducing the uncertainty on the π+ → е+ ν relative probability by almost an order of magnitude (to 5 × 10–4).

The MEG experiment, designed to search for the mu+->e+ gamma decay at a 10^-13 sensitivity level, completed data taking in 2013. In order to increase the sensitivity reach of the experiment by an order of magnitude to the level of 6 x 10-14 for the branching ratio, a total upgrade, involving substantial changes to the experiment, has been undertaken, known as MEG II. We present both the motivation for the upgrade and a detailed overview of the design of the experiment and of the expected detector performance.

A prototype of a 2D detector based on specially designed straw tubes with cathode data readout has been developed and tested. This detector exhibits comparable accuracies in measuring radial and longitudinal coordinates. Its rate capability is similar to the capabilities of traditional detectors whose tubes are smaller by half in diameter.

With few open channels and uncomplicated theoretical description, charged pion decays are uniquely sensitive to certain standard model (SM) symmetries, the universality of weak fermion couplings, and to aspects of pion structure and chiral dynamics. We review the current knowledge of the pion electronic decay π^+ → e^+ ν_e(γ), or π_e2(γ), and the resulting limits on non-SM processes. Focusing on the PEN experiment at the Paul Scherrer Institute (PSI), Switzerland, we examine the prospects for further improvement in the near term.

Allowed charged $\pi$ meson decays are characterized by simple dynamics, few available decay channels, mainly into leptons, and extremely well controlled radiative and loop corrections. In that sense, pion decays represent a veritable triumph of the standard model (SM) of elementary particles and interactions. This relative theoretical simplicity makes charged pion decays a sensitive means for testing the underlying symmetries and the universality of weak fermion couplings, as well as for studying pion structure and chiral dynamics. Even after considerable recent improvements, experimental precision is lagging far behind that of the theoretical description for pion decays. We review the current state of experimental study of the pion electronic decay $\pi^+ \to e^+\nu_e(\gamma)$, or $\pi_{e2(\gamma)}$, where the $(\gamma)$ indicates inclusion and explicit treatment of radiative decay events. We briefly review the limits on non-SM processes arising from the present level of experimental precision in $\pi_{e2(\gamma)}$ decays. Focusing on the PEN experiment at the Paul Scherrer Institute (PSI), Switzerland, we examine the prospects for further improvement in the near term.

Precise measurement of straw axial coordinate (along the anode wire) with accuracy compatible with straw radial coordinate determination by drift time measurement and increase of straw detector rate capability by using straw cathode readout instead of anode readout are presented.

We studied the radiative muon decay \({\upmu }^{+} \rightarrow \mathrm {e}^{+} {\upnu }\bar{{\upnu }}{\upgamma }\) by using for the first time an almost fully polarized muon source. We identified a large sample (\(\sim \)13,000) of these decays in a total sample of \(1.8\times 10^{14}\) positive muon decays collected in the MEG experiment in the years 2009–2010 and measured the branching ratio \(\mathcal {B}({\upmu } \rightarrow \mathrm {e} {\upnu }\bar{{\upnu }}{\upgamma }) = (6.03\pm 0.14\mathrm {(stat.)}\pm 0.53\mathrm {(sys.)})\times 10^{-8}\) for \(E_\mathrm {e}>45~\mathrm {MeV}\) and \(E_{{\upgamma }}>40~\mathrm {MeV}\), consistent with the Standard Model prediction. The precise measurement of this decay mode provides a basic tool for the timing calibration, a normalization channel, and a strong quality check of the complete MEG experiment in the search for \({\upmu }^{+} \rightarrow \mathrm {e}^{+} {\upgamma }\) process.

The MEG experiment makes use of one of the world’s most intense low energy muon beams, in order to search for the lepton flavour violating process \(\mu ^{+} \rightarrow \mathrm{e}^{+} \gamma \). We determined the residual beam polarization at the thin stopping target, by measuring the asymmetry of the angular distribution of Michel decay positrons as a function of energy. The initial muon beam polarization at the production is predicted to be \(P_{\mu } = -1\) by the Standard Model (SM) with massless neutrinos. We estimated our residual muon polarization to be \(P_{\mu } = -0.86 \pm 0.02 ~ \mathrm{(stat)} ~ { }^{+ 0.05}_{-0.06} ~ \mathrm{(syst)}\) at the stopping target, which is consistent with the SM predictions when the depolarizing effects occurring during the muon production, propagation and moderation in the target are taken into account. The knowledge of beam polarization is of fundamental importance in order to model the background of our \({\mu ^+ \rightarrow e^+ \gamma }\) search induced by the muon radiative decay: \(\mu ^{+} \rightarrow \mathrm{e}^{+} \bar{\nu }_{\mu } \nu _\mathrm{e} \gamma \).