Towards a New μ→eγ Search with the MEG II Experiment: From Design to Commissioning

The MEG experiment represents the state of the art in the search for the Charged Lepton Flavour Violating μ+→e+γ decay. With its first phase of operations at the Paul Scherrer Institut (PSI), MEG set the most stringent upper limit on the BR (μ+→e+γ)≤4.2×10−13 at 90% confidence level, imposing one of the tightest constraints on models predicting LFV-enhancements through new physics beyond the Standard Model. An upgrade of the MEG experiment, MEG II, was designed and it is presently in the commissioning phase, aiming at a sensitivity level of 6×10−14. The MEG II experiment relies on a series of upgrades, which include an improvement of the photon detector resolutions, brand new detectors on the positron side with better acceptance, efficiency and performances and new and optimized trigger and DAQ electronics to exploit a muon beam intensity twice as high as that of MEG (7×107 μ+/s). This paper presents a complete overview of the MEG II experimental apparatus and the current status of the detector commissioning in view of the physics data taking in the upcoming three years.

muCool: muon cooling for high-brightness μ+ beams

A number of experiments with muons are limited by the poor phase space quality of the muon beams currently available. The muCool project aims at developing a phase-space cooling method to transform a surface \mu^+μ+ beam with 4 MeV energy and 1 cm size into a slow muon beam with eV energy and 1 mm size. In this process the phase space is reduced by a factor of 10^{9}-10^{10}109−1010 with efficiencies of 2\cdot 10^{-5}-2\cdot 10^{-4}2⋅10−5−2⋅10−4. The beam is then re-accelerated to keV-MeV energies. Such a beam opens up new avenues for research in fundamental particle physics with muons and muonium atoms as well as in the field of \muμSR spectroscopy.

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

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-4\piπ, 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 1.6 \times 10^{12}1.6×1012 positive muon decays and obtained a muon lifetime \tau_{\mu} = 2\, 196\, 980.3(2.2)τμ=2196980.3(2.2)~ps (1.0~ppm) and Fermi constant G_F = 1.166\, 378\, 7(6) \times 10^{-5}F=1.1663787(6)×10−5 GeV^{-2}−2 (0.5~ppm). The thirty-fold improvement in \tau_{\mu}τμ has proven valuable for precision measurements in nuclear muon capture and the commensurate improvement in G_FF has proven valuable for precision tests of the standard model.

Looking forward to lepton-flavor-violating ALPs

We assess the status of past and future experiments on lepton flavor violating (LFV) muon and tau decays into a light, invisible, axion-like particle (ALP), a. We propose a new experimental setup for MEG II, the MEGII-fwd, with a forward calorimeter placed downstream from the muon stopping target. Searching for μ → ea decays MEGII-fwd is maximally sensitive to LFV ALPs, if these have nonzero couplings to right-handed leptons. The experimental set-up suppresses the (left-handed) Standard Model background in the forward direction by controlling the polarization purity of the muon beam. The reach of MEGII-fwd is compared with the present constraints, the reach of Mu3e and the Belle-II reach from τ → ℓa decays. We show that a dedicated experimental campaign for LFV muon decays into ALPs at MEG II and Mu3e will be able to probe the ALP parameter space in an unexplored region well beyond the existing astrophysical constraints. We study the implications of these searches for representative LFV ALP models, where the presence of a light ALP is motivated by neutrino masses, the strong CP problem and/or the SM flavor puzzle. To this extent we discuss the majoron in low-scale seesaw setups and introduce the LFV QCD axion, the LFV axiflavon and the leptonic familon, paying particular attention to the cases where the LFV ALPs constitute cold dark matter.

The Search for μ+ → e+γ with 10–14 Sensitivity: The Upgrade of the MEG Experiment

The MEG experiment took data at the Paul Scherrer Institute in the years 2009–2013 to test the violation of the lepton flavor conservation law, which originates from an accidental symmetry that the Standard Model of elementary particle physics has, and published the most stringent limit on the charged lepton flavor violating decay μ+→e+γ: BR(μ+→e+γ) <4.2×10−13 at 90% confidence level. The MEG detector has been upgraded in order to reach a sensitivity of 6×10−14. The basic principle of MEG II is to achieve the highest possible sensitivity using the full muon beam intensity at the Paul Scherrer Institute (7×107 muons/s) with an upgraded detector. The main improvements are better rate capability of all sub-detectors and improved resolutions while keeping the same detector concept. In this paper, we present the current status of the preparation, integration and commissioning of the MEG II detector in the recent engineering runs.

The Physics Case for an Electron-Muon Collider

An electron-muon collider with an asymmetric collision profile targeting multi-ab-1 integrated luminosity is proposed. This novel collider, operating at collision energies of, e.g., 20–200 GeV, 50–1000 GeV, and 100–3000 GeV, would be able to probe charged lepton flavor violation and measure Higgs boson properties precisely. The collision of an electron and muon beam leads to less physics background compared with either an electron-electron or a muon-muon collider, since electron-muon interactions proceed mostly through higher-order vector boson fusion and vector boson scattering processes. The asymmetric collision profile results in collision products that are boosted towards the electron beam side, which can be exploited to reduce beam-induced background from the muon beam to a large extent. With this in mind, one can imagine a lepton collider complex, starting from colliding order 10 GeV electron and muon beams for the first time in history and to probe charged lepton flavor violation, then to be upgraded to a collider with 50-100 GeV electron and 1-3 TeV muon beams to measure Higgs properties and search for new physics and finally to be transformed to a TeV-scale muon-muon collider. The cost should vary from order 100 million to a few billion dollars, corresponding to different stages, which make the funding situation more practical.