Muon Decay Research Articles

Scintillating fibre detector for the Mu3e experiment

Mu3e is a dedicated experiment to search for the rare lepton flavour violating decay μ+ → e+e−e+. Its ultimate goal is to find or exclude this process with a sensitivity of one in 1016 muon decays. This constitutes four orders of magnitude improvement with respect to the current state-of-the art. A thin multi-layer scintillating fibre detector consisting of 250μm fibres read out on both sides with silicon photomultiplier arrays provides an excellent time measurement with σ<500ps in order to reject combinatorial background at a muon stopping rate around 108 muon/s, concurrently minimizing the material budget to X/X0<0.3%. The design, performance and readout concept, including the dedicated readout chip MuTRiG, is presented.

The Muon g − 2 experiment at Fermilab

The Muon g−2 experiment at Fermilab will measure the anomalous magnetic moment of the muon to a precision of 140 ppb, reducing the experimental uncertainty by a factor of 4 compared to the previous measurement at BNL (E821). The measurement technique adopts the storage ring concept used for E821, with magic-momentum muons stored in a highly uniform 1.45 T magnetic dipole field. The spin precession frequency is extracted from an analysis of the modulation of the rate of higher-energy positrons from muon decays, detected by 24 calorimeters and 3 straw tracking detectors. Compared to the E821 experiment, muon beam preparation, storage ring internal hardware, field measuring equipment, and detector and electronics systems are all new or significantly upgraded. Herein, I report on the status of the experiment as of Sept. 2016, presenting the magnetic field uniformity results after the completion of the first round of shimming and outlining the construction progress of the main detector systems.

Weak currents in nondiagonal leptonic processes

Nondiagonal leptonic processes are studied on the basis of neutral (NC) and charged (CC) weak current interactions. NC and CC descriptions of recoil electron spectra upon the elastic scattering of neutral (anti)leptons, including arbitrary vector and axial-vector current constants that can be real and complex or purely imaginary, are presented. Based on the NC description, the spectrum of muons originating upon the inelastic scattering of a muonic neutrino by a polarized target electron, the total cross section of this process and neutron conversion of an electron–muon pair, as well as the charged and neutral components of the electron spectrum emitted in muon decay are obtained. The possibility of forbidden nondiagonal neutral electron–muon currents emerging in these processes is analyzed.

The Mu3e Experiment

The Mu3e experiment will search for lepton flavor violation in the neutrinoless muon decay μ+→e+e−e+ with a sensitivity down to 10−16 (90% C.L.) using the world most intense muon beam at the Paul Scherrer Institute. The μ+→e+e−e+ decay is strongly suppressed in the Standard Model whilst several Beyond the SM models predict observable effects accessible to the new generation of LFV experiments. The search for the μ+→e+e−e+ decay requires a large acceptance detector capable of measuring up to 2 × 109 decays of stopped muons per second with excellent momentum, space, and time resolution to suppress backgrounds below the 1016 level. The required Mu3e detector performance is possible thanks to tracking detectors based on thin monolithic active silicon pixel sensors (HV-MAPS) and very precise timing measurements using scintillating fibers and tiles both coupled to silicon photo-multipliers. The first phase of the Mu3e experiment has been recently approved. Following an intense R&D program the construction of the detector is about to start. First data taking is expected in 2019.

A hybrid setup for fundamental unknowns in neutrino oscillations using T2HK (\u03bd) and \u03bc-DAR left(overline{nu}right)

Neutrino mass hierarchy, CP-violation, and octant of θ23 are the fundamental unknowns in neutrino oscillations. In order to address all these three unknowns, we study the physics reach of a setup, where we replace the antineutrino run of T2HK with antineutrinos from muon decay at rest (μ-DAR). This approach has the advantages of having higher statistics in both neutrino and antineutrino modes, and lower beam-on backgrounds for antineutrino run with reduced systematics. We find that a hybrid setup consisting of T2HK (ν) and μ-DAR left(overline{nu}right) in conjunction with full exposure from T2K and NOνA can resolve the issue of mass hierarchy at greater than 3σ C.L. irrespective of the choices of hierarchy, δCP, and θ23. This hybrid setup can also establish the CP-violation at 5σ C.L. for ∼ 55% choices of δCP, whereas the same for conventional T2HK left(nu + overline{nu}right) setup along with T2K and NOνA is around 30%. As far as the octant of θ23 is concerned, this hybrid setup can exclude the wrong octant at 5σ C.L. if θ23 is at least 3° away from maximal mixing for any δCP.

Renormalisation-group improved analysis of \u03bc \u2192 e processes in a systematic effective-field-theory approach

In this article, a complete analysis of the three muonic lepton-flavour violating processes μ → eγ, μ → 3e and coherent nuclear μ → e conversion is performed in the framework of an effective theory with dimension six operators defined below the electroweak symmetry breaking scale mW . The renormalisation-group evolution of the Wilson coefficients between mW and the experimental scale is fully taken into account at the leading order in QCD and QED, and explicit analytic and numerical evolution matrices are given. As a result, muonic decay and conversion rates are interpreted as functions of the Wilson coefficients at any scale up to mW . Taking the experimental limits on these processes as input, the phenomenology of the mixing effects is investigated. It is found that a considerable set of Wilson coefficients unbounded in the simplistic tree-level approach are instead severely constrained. In addition, correlations among operators are studied both in the light of current data and future experimental prospects.

Search for a light dark photon in muonium decay

About 0.85 × 105 events involving stopped μ + and muonium decay were observed in a nuclear photoemulsion with the aim of searches for a light dark photon (DPh) in the decay process DPh → e + e −. With a probability of about 10−5, no event of the decay μ+ → е + ν μ ν е accompanied by an electron–positron pair was observed. In the interval of 1.1 MeV ≤ m DPh ≤ 60 MeV, the mixing parameter e 2 was estimated at about 10−5 to 10−4.

Search for heavy gauge W′ bosons in events with an energetic lepton and large missing transverse momentum at [formula omitted

A search is presented for W′ bosons in events with an electron or muon and large missing transverse momentum, using proton–proton collision data at s=13 TeV collected with the CMS detector in 2015 and corresponding to an integrated luminosity of 2.3 fb−1. No evidence of an excess of events relative to the standard model expectations is observed. For a W′ boson described by the sequential standard model, upper limits at 95% confidence level are set on the product of the production cross section and branching fraction and lower limits are established on the new boson mass. Masses below 4.1 TeV are excluded combining electron and muon decay channels, significantly improving upon the results obtained with the 8 TeV data. Exclusion limits at 95% confidence level on the product of the W′ production cross section and branching fraction are also derived in combination with the 8 TeV data. Finally, exclusion limits are set for the production of generic W′ bosons decaying into this final state using a model-independent approach.

Muon anomalous magnetic moment through the leptonic Higgs portal

An extended Higgs sector may allow for new scalar particles well below the weak scale. In this work, we present a detailed study of a light scalar $S$ with enhanced coupling to leptons, which could be responsible for the existing discrepancy between experimental and theoretical determinations of the muon anomalous magnetic moment. We present an ultraviolet completion of this model in terms of the lepton-specific two-Higgs doublet model and an additional scalar singlet. We then analyze a plethora of experimental constraints on the universal low energy model, and this UV completion, along with the sensitivity reach at future experiments. The most relevant constraints originate from muon and kaon decays, electron beam dump experiments, electroweak precision observables, rare $B_d$ and $B_s$ decays and Higgs branching fractions. The properties of the leptonic Higgs portal imply an enhanced couplings to heavy leptons, and we identify the most promising search mode for the high-luminosity electron-positron colliders as $e^+{+}e^-\to\tau^+{+}\tau^-{+}S \to \tau^+{+}\tau^-{+}\ell{+}\bar \ell$, where $\ell =e,\mu$. Future analyses of existing data from BaBar and Belle, and from the upcoming Belle II experiment, will enable tests of this model as a putative solution to the muon $g-2$ problem for $m_S < 3.5$ GeV.

Open heavy-flavour and electroweak boson measurements via the (di-)muonic decay channel with ALICE at the LHC

Heavy flavours (charm and beauty) and electroweak bosons (W and Z) are produced in initial hard partonic scatterings. The former interact strongly with the medium formed in ultra-relativistic heavy-ion collisions throughout its evolution, thus making them well suited to investigate its properties. Furthermore, heavy-flavour measurements in proton-nucleus collisions can be used to investigate initial-state effects whereas in proton-proton (pp) collisions they are considered an important test for perturbative Quantum ChromoDynamics (pQCD) predictions. In addition, open heavy-flavour measurements in pp collisions are used as a reference for proton-lead (p–Pb) and lead-lead (Pb–Pb) collisions. On the other hand, electroweak bosons and their leptonic decay products only interact weakly with the QCD matter and thus are suitable probes to test the validity of binary-collision scaling of hard processes. Moreover, their measurements in p–Pb collisions could help to constrain nuclear parton distribution functions. The ALICE muon spectrometer allows the measurement of open heavy flavour, W- and Z-boson production at forward rapidity (−4.0 < η < −2.5) exploiting their (di)muonic decay channel. In this talk the results obtained with the LHC Run I data in pp, p–Pb and Pb–Pb collisions will be discussed and compared with theoretical predictions.

James Watson Cronin

James Watson Cronin

Background identification system in MEG II experiment based on high-rate scintillation detector with SiPM readout

The MEG experiment has been searching for the lepton flavor violating process, μ+ arrow e+γ, which is a clear evidence of new physics models beyond the Standard Model. The upgrade experiment (MEG II) is currently being prepared to obtain one order higher branching ratio sensitivity ℬ < 5.0 × 10−14 by using the world's most intense muon beam up to ∼108 μ+/s and upgraded detectors with considerably improved performance. One of the keys for the upgrade is to suppress the background rate which is significantly increased with the higher muon decay rate. In the MEG II experiment, the Radiative Decay Counter (RDC) will be newly introduced for active background identification. The RDC is able to identify the most dominant background due to photons from Radiative Muon Decay and improve the sensitivity by 22%. In this paper, the concept of the RDC and its development are described.

Measurement of the transverse momentum spectra of weak vector bosons produced in proton-proton collisions at sqrt{s}=8 TeV

The transverse momentum spectra of weak vector bosons are measured in the CMS experiment at the LHC. The measurement uses a sample of proton-proton collisions at sqrt{s}=8 TeV, collected during a special low-luminosity running that corresponds to an integrated luminosity of 18.4 ± 0.5 pb−1. The production of W bosons is studied in both electron and muon decay modes, while the production of Z bosons is studied using only the dimuon decay channel. The ratios of W− to W+ and Z to W differential cross sections are also measured. The measured differential cross sections and ratios are compared with theoretical predictions up to next-to-next leading order in QCD.

W and Z boson production in p-Pb collisions at sqrt{s_{mathrm{NN}}}=5.02 TeV

The W and Z boson production was measured via the muonic decay channel in proton-lead collisions at sqrt{s_{mathrm{NN}}}=5.02 TeV at the Large Hadron Collider with the ALICE detector. The measurement covers backward (−4.46 < ycms < −2.96) and forward (2.03 < ycms < 3.53) rapidity regions, corresponding to Pb-going and p-going directions, respectively. The Z-boson production cross section, with dimuon invariant mass of 60 < mμμ < 120 GeV/c2 and muon transverse momentum (pTμ) larger than 20 GeV/c, is measured. The production cross section and charge asymmetry of muons from W-boson decays with pTμ > 10 GeV/c are determined. The results are compared to theoretical calculations both with and without including the nuclear modification of the parton distribution functions. The W-boson production is also studied as a function of the collision centrality: the cross section of muons from W-boson decays is found to scale with the average number of binary nucleon-nucleon collisions within uncertainties.

A generalized description of the time dependent signals in extensive air shower detectors and its applications

The expected signal in extensive air shower (EAS) detectors can be predicted with a 10% accuracy by a parameterization that depends on a set of global shower parameters: the energy, the depth of the electromagnetic shower maximum (Xmax) and the overall muon content. By classifying shower particles in four components (muonic, purely electromagnetic, electromagnetic stemming from muon interactions and decay and electromagnetic-from-low-energy hadrons), shower-to-shower fluctuations are minimized. We follow this scheme to propose a model to describe the arrival time distributions of secondary particles as measured with surface detectors in an EAS experiment. This model is then used to reconstruct Xmax in Monte Carlo data sets. As an example, we show that for the case of the Pierre Auger Observatory Xmax can be reconstructed with an accuracy of about 45 g/cm2 at 1019 eV.

Excited Muon Searches at the FCC-Based Muon-Hadron Colliders

We study the excited muon production at the FCC-based muon-hadron colliders. We give the excited muon decay widths and production cross-sections. We deal with the μp→μ⋆q→μγq process and plot the transverse momentum and normalized pseudorapidity distributions of final state particles to define the kinematical cuts best suited for discovery. By using these cuts, we get the mass limits for excited muons. It is shown that the discovery limits obtained on the mass of μ⋆ are 2.2, 5.8, and 7.5 TeV for muon energies of 63, 750, and 1500 GeV, respectively.

The Partial Truncated Icosahedron Phoswich Detector Array: A Light Charged Particle Array for Pionic Fusion Measurements

The Partial Truncated Icosahedron (ParTI) phoswich detector array has been designed to detect charged pions and other light charged particles in pionic fusion reactions. The array has been constructed and characterized in a series of beam experiments. It is composed of 15 plastic/thalium-doped cesium iodide (CsI(Tl)) phoswich detector units arranged on the faces of a truncated icosahedron geometry which covers approximately 20% of the solid angle. The phoswich detectors have been shown to be capable of isotopic identification of Z = 1 and Z = 2 particles and elemental identification of at least up to Z = 3 using fast vs. slow pulse shape discrimination (PSD). Some advantages of employing digital electronics are discussed including identification of charged pions independent of PSD using their characteristic waveform response and selective event triggering using a muon decay trigger. A calibration method for the array is also described.

Next-to-leading order prediction for the decay mu to ekern0.22em left({e}^{+}{e}^{-}right);nu kern0.2em overline{nu}

We present the differential decay rates and the branching ratios of the muon decay with internal conversion, mu to ekern0.22em left({e}^{+}{e}^{-}right);nu kern0.2em overline{nu} , in the Standard Model at next-to-leading order (NLO) in the on-shell scheme. This rare decay mode of the muon is among the main sources of background to the search for μ → eee decay. We found that in the phase space region where the neutrino energies are small, and the three-electron momenta have a similar signature as in the μ → eee decay, the NLO corrections decrease the leading-order prediction by about 10 − 20% depending on the applied cut.

Fully differential NLO predictions for the rare muon decay

Using the automation program GoSam, fully differential NLO corrections were obtained for the rare decay of the muon $\mu\to e\nu\bar\nu ee$. This process is an important Standard Model background to searches of the Mu3e collaboration for lepton-flavour violation, as it becomes indistinguishable from the signal $\mu\to 3e$ if the neutrinos carry little energy. With our NLO program we are able to compute the branching ratio as well as custom-tailored observables for the experiment. With minor modifications, related decays of the tau can also be computed.

A generalized description of the signal size in extensive air shower detectors and its applications

The number as well as the energy and angular distributions of particles in extensive air showers (EAS) depend on the stage of the shower development and the distance to the shower axis. In this work we derive an analytic parameterization of the particle distributions at ground from air shower simulations convolved with the response of a surface detector array. Shower particles are classified into four components according to the shower component they belong to: the muonic component, the electromagnetic component stemming from muon interactions and muon decay, the purely electromagnetic component, and the newly introduced electromagnetic component from low-energy hadrons. Using this scheme, we will show that the total signal at ground level for different surface detectors can be described with minimal fluctuations with parameterizations depending on the primary energy, position of the shower maximum, and the overall number of muons in the shower. The simulation results for different combinations of primaries and hadronic interaction models are reproduced with an accuracy better than 5–10% in the range from 100 m to 2000 m from the shower core. This parameterization is then used as a Lateral Distribution ansatz to reconstruct showers in current EAS experiments. Since this ansatz depends on physical parameters, it opens the possibility to infer them from data.