Publications

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MEG: Muon to Electron and Gamma

A.M. Baldini, T. Mori

SciPost Phys. Proc. 5, 019 (2021) · published 6 September 2021 | · pdf

The possible existence of the muon to electron and gamma decay predicted by many new physics scenarios is investigated by stopping positive muons in a very thin target and measuring emitted photons and positrons with the best possible resolutions. Photons are measured by a 2.7 ton ultra pure liquid xenon detector while positron trajectories are measured in a specially designed gradient magnetic field by low-mass drift chambers and precisely timed by scintillation counters. A first phase of the experiment (MEG) ended in 2016, and excluded the existence of the decay with branching ratios larger than 4.2x10**(-13) (90% C.L.). This provides approximately 30 times stronger constraints on a variety of new physics models than previous experiments. In the second phase (MEG II), most of the detectors have been upgraded by adopting up-to-date technologies to improve the search sensitivity by another order of magnitude down to O(10**(-19) ). MEG II will pursue new physics beyond the Standard Model complementary to high energy collider experiments with a compatible or even higher sensitivity.

2 citations
MuSun - Muon Capture on the Deuteron

P. Kammel on behalf of the MuSun collaboration

SciPost Phys. Proc. 5, 018 (2021) · published 6 September 2021 | · pdf

The MuSun experiment is a precision measurement of the rate for nuclear muon capture on the deuteron, designed to resolve a long-standing disagreement between experiment and theory, and to determine an important low-energy constant relevant for a variety of weak and strong dynamics. The experiment is based on a novel active target method employing a pure deuterium cryogenic time-projection chamber. The data taking was completed in two main campaigns and the analysis is well advanced. The unique chal- lenges and corresponding strategy of the experiment as well as the status of the analysis are presented.

2 citations
MuCap: Muon Capture on the Proton

Malte Hildebrandt, Claude Petitjean

SciPost Phys. Proc. 5, 017 (2021) · published 6 September 2021 | · pdf

The singlet muon capture rate $\Lambda_S$ on the proton $\mu^-\ p \to \nu_\mu n$ is determined in a high precision lifetime measurement. The main apparatus consists of a new hydrogen time projection chamber as muon detector, developed by PSI, surrounded by cylindrical wire chambers and a plastic scintillator hodoscope as electron detectors. The parameter $\Lambda_S$ is evaluated as the difference between the inverse $\mu\,p$ lifetime and that of the free $\mu^+$. The result $\Lambda_S^\text{MuCap} = (715.6 \pm 5.4^\text{stat} \pm 5.1^\text{sys})\,\text{s}^{-1}$ is in excellent agreement with the prediction of chiral perturbation theory $\Lambda_S^{\chi\text{PT}} = (715.4 \pm 6.9)\,\text{s}^{-1}$. From $\Lambda_S^\text{MuCap}$ a recent analysis derives for the induced pseudoscalar coupling $g^\text{MuCap}_p = 8.23 \pm 0.83$ whereas $\bar{g}^{\chi\text{PT}}_p = 8.25 \pm 0.25$.}

1 citation
Mulan: a part-per-million measurement of the muon lifetime and determination of the Fermi constant

R. Carey, T. Gorringe, D.W. Hertzog

SciPost Phys. Proc. 5, 016 (2021) · published 6 September 2021 | · pdf

The part-per-million measurement of the positive muon lifetime and determination of the Fermi constant by the MuLan experiment at the Paul Scherrer Institute is reviewed. The experiment used an innovative, time-structured, surface muon beam and a near-4π, finely-segmented, plastic scintillator positron detector. Two in-vacuum muon stopping targets were used: a ferromagnetic foil with a large internal magnetic field, and a quartz crystal in a moderate external magnetic field. The experiment acquired a dataset of 1.6e12 positive muon decays and obtained a muon lifetime τ_µ = 2 196 980.3(2.2) ps (1.0 ppm) and Fermi constant G_F = 1.166 378 7(6) × 10−5 GeV^−2 (0.5 ppm). The thirty fold improvement in τ_µ has proven valuable for precision measurements in nuclear muon capture and the commensurate improvement in G_F has proven valuable for precision tests of the standard model.

2 citations
Measurement of the transverse polarization of electrons emitted in neutron decay – nTRV experiment

K. Bodek, A. Kozela

SciPost Phys. Proc. 5, 015 (2021) · published 6 September 2021 | · pdf

This paper recalls the main achievements of the nTRV experiment which measured two components of the transverse polarization ($\sigma_{T_{1}}$, $\sigma_{T_{2}}$) of electrons emitted in the $\beta$-decay of polarized, free neutrons and deduced two correlation coefficients, $R$ and $N$, that are sensitive to physics beyond the Standard Model. The value of time-reversal odd coefficient $R$, 0.004$\pm$0.012$\pm$0.005, significantly improved limits on the relative strength of imaginary scalar coupling constant in the weak interaction. The value obtained for the time-reversal even correlation coefficient $N$, 0.067$\pm$0.011$\pm$0.004, agrees with the Standard Model expectation, providing an important sensitivity test of the electron polarimeter. One of the conclusions of this pioneering experiment was that the transverse electron polarization in the neutron $\beta$-decay is worth more systematic exploring by measurements of yet experimentally not attempted correlation coefficients such as $H$, $L$, $S$, $U$ and $V$. This article presents a brief outlook on that questions.
Pionic hydrogen and deuterium

D. Gotta, L. M. Simons

SciPost Phys. Proc. 5, 014 (2021) · published 6 September 2021 | · pdf

The measurement of strong-interaction shift and broadening in pionic hydrogen and deuterium yields pion-nucleon scattering lengths as well as the threshold pion-production strength on isoscalar NN pairs. Results from recent high-resolution experiments at PSI using crystal spectrometers allow important comparisons with the outcome of the modern low-energy description of QCD within the framework of effective field theories.
Cyclotron trap

D. Gotta, L. M. Simons

SciPost Phys. Proc. 5, 013 (2021) · published 6 September 2021 | · pdf

The cyclotron trap was developed at SIN/PSI to increase the stopping density of nega- tively charged particle beams for the formation of exotic atoms in low pressure gases. A weak focusing magnetic field, produced by superconducting solenoids, is used. Particles are injected radially through the fringe field to a moderator, which decelerates them into orbits bound by the field. Further deceleration by moderators and/or low-pressure gases leads the particles to the center of the device, where they can be stopped or eventually extracted. Experiments became feasible with this technique, such as those dealing with pionic hydrogen/deuteriumat SIN/PSI.Muonic hydrogen laser experiments also became possible with the extraction of muons from the cyclotron trap. The formation of antipro- tonic hydrogen in low pressure targets led to successful experiments at LEAR/CERN.

1 citation
The $\pi^0$ mass and the first experimental verification of Coulomb de-excitation in pionic hydrogen

M. Daum, P.-R. Kettle

SciPost Phys. Proc. 5, 012 (2021) · published 6 September 2021 | · pdf

The most precise value for the $\pi^0$ mass was obtained from the measurement of the mass difference $m_{\pi^-}-m_{\pi^0} = 4.593\,64(48)$\,MeV/c$^2$ in the charge exchange reaction $\pi^-$p $\rightarrow \pi^0$n at PSI. With the most precise charged pion mass value, $m_{\pi^+} = 139.570\,21(14)$\,MeV/c$^2$ and the validity of the CPT theorem ($m_{\pi^-} = m_{\pi^+}$), a value $m_{\pi^0} = 134.976\,57(50)$\,MeV/c$^2$ is obtained. The measurements also revealed, for the first time, evidence of an unexpectedly large contribution from Coulomb de-excitation states during the pionic atom cascade.
The mass of the $\pi^+$

M. Daum , P.-R. Kettle

SciPost Phys. Proc. 5, 011 (2021) · published 6 September 2021 | · pdf

At the Paul Scherrer Institute, the most precise pion mass value was obtained from a precision determination of the muon momentum, $p_{\mu^+}$, in pion decay at rest, $\pi^+ \rightarrow \mu^+ + \nu_{\mu}$. We find $p_{\mu^+} = (29.792\,00 \pm 0.000\,11$ MeV/c and $m_{\pi^+}$ = (139.570\,21 $\pm$ 0.000\,14) MeV/c$^2$. This value is more precise, however, in agreement with the recent compilation of the Particle Data Group for $m_{\pi^-}$. The agreement of $m_{\pi^+}$ with the most precise and recent single measurement, $m_{\pi^-}$ = (139.570\,77 $\pm$ 0.000\,18) MeV/c$^2$ is only fair (2.43 $\sigma$). From our $p_{\mu^+}$ value and using the above mentioned $m_{\pi^-}$ mass and the mass of the muon, $m_{\mu^+} = (105.658\,374\,5 \pm 0.000\,002\,4)$ MeV/c$^2$ we obtain an upper limit for the muon neutrino mass with 90 \%confidence, $m_{\mu_{\nu}} \leq 230$ keV/c$^2$. One also obtains a new quantitative measure of CPT invariance in the pion sector: $(m_{\pi^+} - m_{\pi^-})/m_{\pi}(\mbox{av}) = (-2.9 \pm 2.0)\cdot 10^{-6}$, an improvement by two orders of magnitude.
The mass of the $\pi^-$

M. Daum, D. Gotta

SciPost Phys. Proc. 5, 010 (2021) · published 6 September 2021 | · pdf

The most precise values of the mass of the negatively charged pion have been determined from several measurements of X-ray wavelengths for transitions in pionic atoms at PSI. The Particle Data Group gives the average $m_{\pi^-}= (139.570 61 \pm 0.000 24) MeV/c^2$.

4 citations

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