Muon Decay Research Articles

Measurement of W- and Z-boson production in p—Pb collisions at √sNN = 5.02 TeV with ALICE at the LHC

In hadronic collisions, electroweak bosons (W and Z) are produced in initial hard scattering processes and they are not affected by the strong interaction. In proton-proton collisions, they have been suggested as standard candles for luminosity monitoring and their measurement can improve the evaluation of detector performances. In nucleus-nucleus and proton-nucleus collisions, electroweak bosons provide a check at first order of the validity of binary collision scaling, while small deviations allow studying the nuclear modifications of parton distribution functions. In ALICE, the production of W and Z bosons is measured via the contribution of W-boson decays to the inclusive pT-differential muon yield and the invariant mass of unlike-sign muon pairs from Z-boson decays, respectively, at large rapidities. This paper reports on the production cross sections of muons from W muonic decay and of Z boson in p- Pb collisions at √sNN = 5.02 TeV, which are compared to the theoretical calculations, and on the W-boson yields normalized to the average number of binary nucleon-nucleon collisions as a function of the event activity.

A cosmic Ray Muon Experiment: a Way to Teach Standard Model of Particles at Community Colleges

This experiment is an example of research for early undergraduate students and of its benefits and challenges as an accessible strategy for community colleges, in the spirit of the report on improving undergraduate STEM education from the US President's Council of Advisors on Science and Technology. The goals of this project include measuring average low- energy muon flux, day/night flux difference, time dilation, energy spectra of electrons and muons in arbitrary units, muon decay curve, average lifetime of muons. From the lifetime data we calculate the weak coupling constant gw, electric charge e and the Higgs energy density.

Non-standard interactions and the CP phase measurements in neutrino oscillations at low energies

We study the effects of non-standard interactions (NSI) and the genuine CP phase $\delta_D$ in neutrino oscillations at low, $E_\nu \lesssim 1\,\mbox{GeV}$, and very low, $E_\nu \lesssim 0.1\,\mbox{GeV}$, energies. For experimental setup with baseline and neutrino energy tuned to the first 1-3 oscillation maximum, we develop a simple analytic formalism to show the effects of NSI. The vacuum mimicking and its violation as well as the use of the separation basis play a central role in our formalism. The NSI corrections that affect the CP phase measurement mainly come from the violation of vacuum mimicking as well as from the corrections to the 1-3 mixing angle and mass-squared difference. We find that the total NSI correction to the $\nu_\mu - \nu_e$ probability $P_{\mu e}$ can reach $20\% - 30\%$ ($1 \sigma$) at T2(H)K. Correspondingly, the correction to the CP phase can be as large as $50^\circ$ and hence significantly deteriorates the CP sensitivity at T2(H)K. The proposed TNT2K experiment, a combination of T2(H)K and the short baseline experiment $\mu$Kam that uses the Super-K/Hyper-K detector at Kamioka to measure the oscillation of the antineutrinos from muon decay at rest ($\mu$DAR), can substantially reduce the degeneracy between NSI and the genuine CP phase $\delta_D$ to provide high CP sensitivity. The NSI correction to $P_{\mu e}$ is only $2\%$ ($1 \sigma$) for $\mu$DAR neutrinos.

A Multigap Resistive Plate Chambers array for the Extreme Energy Events Project

The Extreme Energy Events (EEE) Project is a Centro Fermi - CERN - INFN - MIUR Collaboration Project, for the study of extremely high-energy cosmic rays, which exploits the Multigap Resistive Plate Chamber (MRPC) technology. The excellent time resolution and good tracking capability of this detector allows us to study Extensive Air Showers (EAS) with an array of telescopes distributed all over the Italian territory. Each telescope is installed in a High School, with the additional goal to introduce students to particle and astroparticle Physics. The EEE array is composed, so far, of 47 telescopes, each made of three MRPC planes, spanning more than 10 degrees in latitude and 11 in longitude, organized in clusters and single telescope stations. The status of the experiment and the results, obtained during two recent coordinated data taking periods, will be reported. The observation of Forbush decreases, coincidence events among different telescopes and the muon decay, using more than 5 billion tracks collected in the last few months, are of particular interest.

Leptons from decay of mesons in the laser-induced particle pulse from ultra-dense protium p(0)

Kaons and pions are observed by their characteristic decay times of 12, 52 and 26 ns after impact of relatively weak ns-long laser pulses on ultra-dense hydrogen H(0), as reported previously. The signal using an ultra-dense protium p(0) generator with natural hydrogen is now studied. Deflection in a weak magnetic field or penetration through metal foils cannot distinguish between the types of decaying mesons. The signals observed are thus not caused by the decaying mesons themselves, but by the fast particles often at [Formula: see text][Formula: see text]MeV u[Formula: see text] formed in their decay. The fast particles are concluded to be mainly muons from their relatively small magnetic deflection and strong penetration. This is further supported by published studies on the direct observation of the beta decay of muons in scintillators and solid converters using the same type of p(0) generator.

Hyperfine splitting in a true muonium toO(mμα6): Two-photon-annihilation contribution from other flavors

The two-photon-annihilation contribution to the true muonium hyperfine splitting arising from $e$ and $\tau$ loops is obtained analytically at order $m_\mu\alpha^6$. The contribution to the hyperfine splitting is $-2.031092873 m_\mu\alpha^6/n^3\pi^2=-793.926988/n^3$ MHz. The contribution to the triplet true muonium decay rate has also been obtained and was found to be $9.825708266 m_\mu\alpha^6/n^3\pi^2=3840.737698/n^3$ MHz. Additional results have been computed for other purely leptonic bound states.

Bound muon decay spectrum in the leading logarithmic accuracy

We compute the dominant, logarithmically enhanced radiative corrections to the electron spectrum in the bound muon decay in the whole experimentally interesting range. The corrected spectrum fits well the TWIST results. The remaining theoretical error, dominated by the nuclear charge distribution, can be reduced in the muon-electron conversion searches by measuring the spectrum slightly below the New Physics signal window.

Finite-temperature modification of heavy particle decay and dark matter annihilation

We apply the operator product expansion (OPE) technique to the decay and annihilation of heavy particles in a thermal medium with temperature below the heavy particle mass, m_chi. This allows us to explain two interesting observations made before: a) that the leading thermal correction to the decay width of a charged particle is the same multiplicative factor of the zero-temperature width for a two-body decay and muon decay, and b) that the leading thermal correction to fermionic dark matter annihilation arises only at order T^4/m_chi^4. The OPE further considerably simplifies the computation and factorizes it into model-independent matrix elements in the thermal background, and short-distance coefficients to be computed in zero-temperature field theory.

Standard Model in multiscale theories and observational constraints

We construct and analyze the Standard Model of electroweak and strong interactions in multiscale spacetimes with (i) weighted derivatives and (ii) $q$-derivatives. Both theories can be formulated in two different frames, called fractional and integer picture. By definition, the fractional picture is where physical predictions should be made. (i) In the theory with weighted derivatives, it is shown that gauge invariance and the requirement of having constant masses in all reference frames make the Standard Model in the integer picture indistinguishable from the ordinary one. Experiments involving only weak and strong forces are insensitive to a change of spacetime dimensionality also in the fractional picture, and only the electromagnetic and gravitational sectors can break the degeneracy. For the simplest multiscale measures with only one characteristic time, length and energy scale $t_*$, $\ell_*$ and $E_*$, we compute the Lamb shift in the hydrogen atom and constrain the multiscale correction to the ordinary result, getting the absolute upper bound $t_*<10^{-23}\,{\rm s}$. For the natural choice $\alpha_0=1/2$ of the fractional exponent in the measure, this bound is strengthened to $t_*<10^{-29}\,{\rm s}$, corresponding to $\ell_*<10^{-20}\,{\rm m}$ and $E_*>28\,{\rm TeV}$. Stronger bounds are obtained from the measurement of the fine-structure constant. (ii) In the theory with $q$-derivatives, considering the muon decay rate and the Lamb shift in light atoms, we obtain the independent absolute upper bounds $t_* < 10^{-13}{\rm s}$ and $E_*>35\,\text{MeV}$. For $\alpha_0=1/2$, the Lamb shift alone yields $t_*<10^{-27}\,{\rm s}$, $\ell_*<10^{-19}\,{\rm m}$ and $E_*>450\,\text{GeV}$.

Measurement of event-shape observables in [Formula: see text] events in pp collisions at [Formula: see text][Formula: see text] with the ATLAS detector at the LHC.

Event-shape observables measured using charged particles in inclusive Z-boson events are presented, using the electron and muon decay modes of the Z bosons. The measurements are based on an integrated luminosity of 1.1~mathrm{fb}^{-1} of proton–proton collisions recorded by the ATLAS detector at the LHC at a centre-of-mass energy sqrt{s}=7 {mathrm{TeV}}. Charged-particle distributions, excluding the lepton–antilepton pair from the Z-boson decay, are measured in different ranges of transverse momentum of the Z boson. Distributions include multiplicity, scalar sum of transverse momenta, beam thrust, transverse thrust, spherocity, and {mathcal {F}}-parameter, which are in particular sensitive to properties of the underlying event at small values of the Z-boson transverse momentum. The measured observables are compared with predictions from Pythia 8, Sherpa, and Herwig 7. Typically, all three Monte Carlo generators provide predictions that are in better agreement with the data at high Z-boson transverse momenta than at low Z-boson transverse momenta, and for the observables that are less sensitive to the number of charged particles in the event.

Light sterile neutrinos, lepton number violating interactions, and the LSND neutrino anomaly

We develop the consequences of introducing a purely leptonic, lepton number violating non-standard interaction (NSI) and standard model neutrino mixing with a fourth, sterile neutrino in the analysis of short-baseline, neutrino experiments. We focus on the muon decay at rest (DAR) result from the Liquid Scintillation Neutrino Experiment (LSND) and the Karlsruhe and Rutherford Medium Energy Neutrino Experiment (KARMEN). We make a comprehensive analysis of lepton number violating, NSI effective operators and find nine that affect muon decay relevant to LSND results. Two of these preserve the standard model (SM) value 3/4 for the Michel rho and delta parameters and, overall, show favorable agreement with precision data and the electron anti-neutrino signal from LSND data. We display theoretical models that lead to these two effective operators. In the model we choose to apply to DAR data, both electron anti-neutrino appearance from muon anti-neutrino oscillation and electron anti-neutrino survival after production from NSI decay of the positive muon contribute to the expected signal. This is a unique feature of our scheme. We find a range of parameters where both experiments can be accommodated consistently with recent global, sterile neutrino fits to short baseline data. We comment on implications of the models for new physics searches at colliders and comment on further implications of the lepton number violating interactions plus sterile neutrino-standard model neutrino mixing.

Liquid xenon calorimeter for MEG II experiment with VUV-sensitive MPPCs

The MEG II experiment is an upgrade of the MEG experiment to search for the charged lepton flavor violating decay of muon, μ+→e+γ. The MEG II experiment is expected to reach a branching ratio sensitivity of 4×10−14, which is one order of magnitude better than the sensitivity of the current MEG experiment. The performance of the liquid xenon (LXe) γ-ray detector will be greatly improved with a highly granular scintillation readout realized by replacing 216 photomultiplier tubes (PMTs) on the γ-ray entrance face with 4092 Multi-Pixel Photon Counters (MPPCs). For this purpose, we have developed a new type of MPPC which is sensitive to the LXe scintillation light in vacuum ultraviolet (VUV) range, in collaboration with Hamamatsu Photonics K.K. We have measured the performance of the MPPC in LXe, and an excellent performance has been confirmed including high photon detection efficiency (>15%) for LXe scintillation light. An excellent performance of the LXe detector has been confirmed by Monte Carlo simulations based on the measured properties of the MPPC. The construction of the detector is in progress, aiming to start physics data taking in 2017.

Gamma rays from muons from WIMPs: Implementation of radiative muon decays for dark matter analyses

Dark matter searches in gamma ray final states often make use of the fact that photons can be produced from final state muons. Modern Monte Carlo generators and DM codes include the effects of final state radiation from muons produced in the dark matter annihilation process itself, but neglect the O(1%) radiative correction that arises from the subsequent muon decay. After implementing this correction we demonstrate the effect that it can have on dark matter phenomenology by considering the case of dark matter annihilation to four muons via scalar mediator production. We first show that the AMS-02 positron excess can no longer easily be made consistent with this final state once the Fermi-LAT dwarf limits are calculated with the inclusion of radiative muon decays, and we next show that the Fermi-LAT galactic centre gamma excess can be improved with this final state after inclusion of the same effect. We provide code and tables for the implementation of this effect in the popular dark matter code micrOMEGAs, providing a solution for any model producing final state muons.

CMS results on electroweak probes from LHC Run 1

Electroweak boson production is an important benchmark process in high-energy heavy-ion collisions at the LHC. The Z and W bosons do not participate in the strong interaction and their leptonic decays provide medium-blind probes of the initial state of the collisions. Final results on Z boson production in PbPb collisions compared to pp collisions are presented. The centrality dependence confirms the binary scaling of hard probes in heavy-ion collisions and the differential cross sections show that initial state effects are small compared to the statistical precision of the available data. Measurements of the W and Z boson production in pPb collisions, combining both the muon and electron decay channels are also presented. The data are compared to theory predictions for nuclear modifications of the parton distributions and show a clear sensitivity to these effects.

Question of Lorentz violation in muon decay

Possibilities to test the Lorentz invariance of the weak interaction in muon decay are considered. We derive the direction-dependent muon-decay rate with a general Lorentz-violating addition to the W-boson propagator. We discuss measurements of the directional and boost dependence of the Michel parameters and of the muon lifetime as a function of absolute velocity. The total muon-decay rate in the Lorentz-violating Standard Model Extension is addressed. Suggestions are made for dedicated (re)analyses of the pertinent data and for future experiments.

Study of Z boson production in pPb collisions at sNN=5.02TeV

The production of Z bosons in pPb collisions at sqrt(s[NN]) = 5.02 TeV is studied by the CMS experiment via the electron and muon decay channels. The inclusive cross section is compared to pp collision predictions, and found to scale with the number of elementary nucleon-nucleon collisions. The differential cross sections as a function of the Z boson rapidity and transverse momentum are measured. Though they are found to be consistent within uncertainty with theoretical predictions both with and without nuclear effects, the forward-backward asymmetry suggests the presence of nuclear effects at large rapidities. These results provide new data for constraining nuclear parton distribution functions.

A new muon-pion collection and transport system design using superconducting solenoids based on CSNS**Supported by National Natural Science Foundation of China (11527811)

A new muon and pion capture system is proposed for the China Spallation Neutron Source (CSNS), currently under construction. Using about 4% of the pulsed proton beam (1.6 GeV, 4 kW and 1 Hz) of CSNS to bombard a cylindrical graphite target inside a superconducting solenoid, both surface muons and pions can be acquired. The acceptance of this novel capture system - a graphite target wrapped up by a superconducting solenoid - is larger than the normal muon beam lines using quadrupoles at one side of the separated muon target. The muon and pion production at different capture magnetic fields was calculated using Geant4. The bending angle of the capture solenoid with respect to the proton beam was also optimized in simulation to achieve more muons and pions. Based on the layout of the muon experimental area reserved at the CSNS project, a preliminary muon beam line was designed with multi-purpose muon spin rotation areas (surface, decay and low-energy muons). Finally, high-flux surface muons (108/s) and decay muons (109/s) simulated by G4beamline will be available at the end of the decay solenoid based on the first phase of CSNS. This collection and transport system will be a very effective beam line at a proton current of 2.5 μA.

Azimuthal asymmetry of recoil electrons in neutrino–electron elastic scattering as signature of neutrino nature

In this paper, we show how a presence of the exotic scalar, tensor weak interactions in addition to the standard vector-axial (V-A) one may help to distinguish the Dirac from Majorana neutrinos in the elastic scattering of (anti)neutrino beam off the unpolarized electrons in the limit of vanishing (anti)neutrino mass. We assume that the incoming (anti)neutrino beam comes from the polarized muon decay at rest and is the left-right chiral mixture with assigned direction of the transversal spin polarization with respect to the production plane. We display that the azimuthal asymmetry in the angular distribution of recoil electrons is generated by the interference terms between the standard and exotic couplings, which are proportional to the transversal (anti)neutrino spin polarization and independent of the neutrino mass. This asymmetry for the Majorana neutrinos is distinct from the one for the Dirac neutrinos through the absence of interference between the standard and tensor couplings. Additionally, the interference term between the standard and scalar coupling of the only left chiral neutrinos, absent in the Dirac case, appears. We also indicate the possibility of utilizing the azimuthal asymmetry measurements to search for the new CP-violating phases. Our analysis is model-independent and consistent with the current upper limits on the non-standard couplings.

Muon decay: an old, yet alive experiment in the university physics curriculum

The classic experiment on muon decay is proposed in the undergraduate or master physics curriculum, making use of a single scintillation detector and observing in coincidence the delayed signal due to the electron following muon decay. Two experimental setups, with different geometries and scintillation materials, were considered, making use of standard equipment available in most undergraduate labs. The results obtained in the two cases were compared and discussed. For a better understanding of the results, and to also evaluate the possible effect of material surrounding the detector, detailed GEANT simulations were carried out for both experimental setups. Such simulations are also intended as an additional student activity to virtually exploit in a short time the effect of modifying detector and material configuration, which cannot be achieved during a single student session. The results of this experiment and its interpretation also proved to be a useful way to discuss with students many topics of interest in modern physics, such as—among others—the time dilation, the dependence of muon lifetime in matter, the Fermi weak interaction coupling constant and the parity violation in weak interactions.

Neutrinos from STORed Muons, nuSTORM

Neutrino beams produced from the decay of muons in a racetrack-like decay ring (the so called Neutrino Factory) provide a powerful way to study neutrino oscillation physics and, in addition, provide unique beams for neutrino interaction studies.The Neutrinos from STORed Muons (nuSTORM) facility is a neutrino factory-like facility designed for short baseline neutrino oscillation and neutrino interaction studies. However, due to the particular nature of nuSTORM, it can also provide an intense, very pure, muon neutrino beam from pion decay, which is possibly suitable for long-baseline neutrino oscillation searches as well. This so-called “Neo-conventional” muon neutrino beam from nuSTORM makes nuSTORM a hybrid neutrino factory.In this paper, the facility is described and a description of the neutrino beams and the precision to which they can be determined is given. Sensitivity plots are presented that indicate how well the facility can perform for short-baseline oscillation searches and show its potential for a neutrino interaction physics program. Finally, the concept of the “Neo-conventional” muon neutrino beam available at the nuSTORM facility is briefly discussed.