Muon Decay Research Articles

Muon decay spin asymmetry

We compute the spin asymmetry of the muon decay through O(alpha^2) in perturbative QED. These two-loop corrections are about a factor five (twenty) smaller than the current statistical (systematic) uncertainty of the most precise measurement, performed by the TWIST collaboration. We point out that at O(alpha^2) the asymmetry requires a careful definition due to multi-lepton final states and suggest to use familiar QCD techniques to define it in an infra-red safe way. We find that the TWIST measurement of the asymmetry is in excellent agreement with the Standard Model.

Heavy-flavour elliptic flow measured in Pb–Pb collisions at sNN=2.76 TeV with ALICE

Abstract The ALICE Collaboration has measured the production of open heavy-flavour hadrons relative to the reaction plane in Pb–Pb collisions at s NN = 2.76 TeV . The anisotropy is quantified in terms of the second harmonic, the elliptic flow v 2 , in the Fourier expansion of the particle azimuthal distribution. The v 2 measurements are presented for prompt charm mesons, i.e. D 0 , D + , D ⁎+ , and heavy-flavour decay electrons at mid-rapidity, as well as for heavy-flavour decay muons at forward rapidity for various centrality intervals. The results are compared with the ones for charged particles and with model calculations of charm and beauty quark transport and energy loss in high-density strongly-interacting matter at high temperature.

Phenomenology in supersymmetric neutrinophilic Higgs model with sneutrino dark matter

We study a supersymmetric neutrinophilic Higgs model with large neutrino Yukawa couplings where neutrinos are Dirac particles and the lightest right-handed (RH) sneutrino is the lightest supersymmetric particle (LSP) as a dark matter candidate. Neutrinophilic Higgs bosons need to be rather heavy by the precise determination of the muon decay width and dark radiation constraints for large Yukawa couplings. From the Large Hadron Collider constraints, neutrinophilic Higgsino mass need to be heavier than several hundred GeV or close to the RH sneutrino LSP mass. The latter case is interesting because the muon anomalous magnetic dipole moment can be explained with a relatively large lightest neutrino mass, if RH sneutrino mixings are appropriately fine tuned in order to avoid stringent lepton flavor violation constraints. Dark matter is explained by asymmetric RH sneutrino dark matter in the favoured region by the muon anomalous magnetic dipole moment. In other regions, RH sneutrino could be an usual WIMP dark matter.

Nonperturbative QED: Muon Structure and Decay

A nonperturbative QED is presented which is free of the divergences of perturbative QED. The theory comprises two equations of motion (EOM’s) for a relativistic electron which are mutually coupled by gauge-invariant self-electromagnetic interaction terms calculated from Maxwell’s equations. The first is Dirac’s equation itself, which Dirac required to be compatible with the material equation of continuity and which accounts for the electron’s rest-mass energy and spin-1/2 nature. The second is an EOM with Dirac form for a mass-0, spin-1/2 particle which I require to be compatible with the electromagnetic equation of continuity and which accounts for the electron’s charge and electromagnetic self energy such that the combined equations account for the electron’s rest-mass energy, spin, charge, and self energy and may therefore be regarded as a complete relativistic-electron theory. Due to the transverse nature of Maxwell’s equations the second EOM actually comprises two EOM’s: an EOM with magnetic-field interaction (MFEOM) accounting for radiative contributions to atomic structure and an EOM with electric-field interaction (EFEOM) investigated in this paper. The use of all three EOM’s together may be considered to describe radiation-dressed states of matter as opposed to radiation-bare states of matter as described by Dirac’s or Schroedinger’s equation alone. I argue that the EFEOM is physically appropriate for a neutrino for large separations of the two particles and is possibly associated with the electroweak force. On the other hand I show that for small interparticle separations the EFEOM shows binding on a GeV energy and Fermi-unit length scale such that one bare electron and two neutrinos with net spin-1 are possibly related to the muon or “heavy” electron. A model is constructed for muon structure and decay.

Measurements of the heavy-flavour nuclear modification factor in p–Pb collisions at sNN=5.02 TeV with ALICE at the LHC

The heavy-flavour nuclear modification factor R pPb has been measured with the ALICE detector in p–Pb collisions at the nucleon–nucleon center of mass energy s NN = 5.02 TeV at the CERN LHC in a wide rapidity and transverse momentum range, as well as in several decay channels. R pPb is consistent with unity within uncertainties at mid-rapidity and forward rapidity. In the backward region a slight enhancement of the yield of heavy-flavour decay muons is found in the region 2 < p T < 4 GeV / c . The results are described within uncertainties by theoretical calculations that include initial-state effects. The measurements add experimental evidence that the suppression of heavy-flavour production observed at high p T in central Pb–Pb collisions with respect to pp collisions is due to a medium effect induced by the interaction of heavy quarks with the partonic matter.

Heavy-flavour production and nuclear modification factor in Pb–Pb collisions at sNN=2.76 TeV with ALICE

The measurement of heavy-flavour production and nuclear modification factor in heavy-ion collisions provides insights into the mechanisms of parton energy loss in the hot and dense medium formed in these collisions. ALICE results on heavy-flavour decay electrons and muons and on D mesons are presented.

Z and W boson production in pPb collisions with CMS

Abstract The electroweak boson production is an important benchmark measurement in ultra-relativistic heavy-ion collisions which can provide constraints on the nuclear parton distribution functions. In this paper the first results from the proton–lead collision data taken in early 2013 are presented. The Z boson production cross section is measured in the muon decay channel in bins of transverse momentum and rapidity together with the forward–backward ratio. The W production is studied in the muon and electron decay channels and the differential cross sections, lepton-charge and forward–backward asymmetries are computed as a function of the lepton pseudorapidity. All results are compared with theory predictions with and without nuclear modification of the parton distribution functions showing hints of nuclear effects.

The ultra-lightweight support structure and gaseous helium cooling for the Mu3e silicon pixel tracker

The Mu3e experiment searches for charged lepton flavor violation in the rare decay μ→eee. In order to reach a sensitivity of better than 10−16, more than 109 muon decays per second have to be observed over a running time of one year. Precise determination of particle momentum, vertex position and time are necessary for background suppression. These requirements can be met by combining an ultra-lightweight tracker based on High-Voltage Monolithic Active Pixel Sensors (HV-MAPS) with a timing system which consists of a scintillating fiber detector and a tile hodoscope. As the momentum of particles from muon decay at rest is below 53 MeV/c, the silicon pixel tracker resolution is dominated by multiple Coulomb scattering. This leads to extreme requirements for the material budget of the tracking detector of below 0.1% of a radiation length per layer. Even though the target power consumption of the HV-MAPS detector is as low as 150 mW/cm2, the detector cooling must be very efficient and at the same time avoid adding material inside the active tracking volume.

Search for mu LFV from MEG

The MEG experiment searches for a lepton flavor violating muon decay, μ→eγ, discovery of which gives a clear evidence of the new physics beyond the Standard Model of the elementary particle physics. By analyzing the data taken in 2009 and 2010, which corresponds to 1.8 × 1014 muon decays in the stopping target, the result was consistent with null signal. The most stringent upper limit on the branching ratio of 2.4 × 10−12 is set.

Right-handed chirality of muon neutrinos in polarized muon decay at rest

In this paper, we analyze the polarized muon decay at rest admitting the non-standard \(V+A\) interaction in addition to the standard \(V-A\) interaction. We show that the angle–energy distribution of Dirac muon neutrinos (\(\nu _{\mu }\)’) is sensitive to the interference terms between the standard \(V-A\) coupling of left-chirality \(\nu _{\mu }\) and exotic \(V+A\) coupling of right-chirality one, which are proportional to the transverse components of \(\nu _{\mu }\) spin polarization. The interferences do not vanish in the limit of massless \(\nu _{\mu }\) and include the relative phases to test the CP violation. It allows one to calculate a neutrino flux for an assumed configuration of detector in the case of standard model prediction, and the upper limits on the neutrino flux for the left–right \(\nu _{\mu }\) mixture. It is also demonstrated that the effects of neutrino mass and mixing are tiny corrections and can be omitted. The most important conclusion is that even in the massless \(\nu _{\mu }\) limit, there are physical effects connected with the mixture of left- and right-handed helicity components in a spin \(1/2\) quantum state, when the exotic interaction \(V+A\) is admitted, contrary to the electron observables, where all the possible interferences vanish.

Particle decays in the presence of a neutrino background

Several non-threshold reactions which may be used to detect the cosmic neutrino background are presented. The corresponding cross-sections are calculated analytically within the Standard Model. These reactions are sensitive not only to the electron neutrinos but also to the muon and tau neutrinos. It is also pointed out that they mimic lepton number violation as well as angular-momentum non-conservation in muon and meson decays. Some possibilities of experimental observation of the neutrino background are indicated.

Acceleration-induced scalar field transitions ofn-particle multiplicity

In this paper we calculate the effect of acceleration on the decay and excitation rates of scalar fields into a final state of arbitrary multiplicity. The analysis is carried out using standard field operators as well as an Unruh-DeWitt detector. Using the equivalence of the two methods, we show how to correctly setup the computation and interpret the results in terms of the particle content of the initial and final state Rindler and Minkowski spacetimes. We find the dominant transition pathway, and thus final state multiplicity, is acceleration dependent. The formalisms developed are then used to analyze the electron and muon system. We compute the transition rates and lifetimes for accelerated electrons and muons as well as the branching fractions for muon decay.

Electroweak boson production in heavy-ion collisions with CMS

The Compact Muon Solenoid (CMS) is fully equipped to measure leptonic decays of electroweak probes in the high multiplicity environment of nucleus-nucleus collisions. Electroweak boson production is an important benchmark process at hadron colliders. Precise measurements of W and Z production in heavy-ion collisions can help to constrain nuclear PDFs as well as serve as a standard candle of the initial state in PbPb collisions at the LHC energies. The inclusive and differential measurements of the Z boson yield in the muon and electron decay channel will be presented, establishing that no modification is observed with respect to pp collisions, scaled by the number of incoherent nucleon-nucleon collisions.

Measurement of the intrinsic electron neutrino component in the T2K neutrino beam with the ND280 detector

The T2K experiment has reported the first observation of the appearance of electron neutrinos in a muon neutrino beam. The main and irreducible background to the appearance signal comes from the presence in the neutrino beam of a small intrinsic component of electron neutrinos originating from muon and kaon decays. In T2K, this component is expected to represent 1.2% of the total neutrino flux. A measurement of this component using the near detector (ND280), located 280 m from the target, is presented. The charged current interactions of electron neutrinos are selected by combining the particle identification capabilities of both the time projection chambers and electromagnetic calorimeters of ND280. The measured ratio between the observed electron neutrino beam component and the prediction is 1.01+-0.10 providing a direct confirmation of the neutrino fluxes and neutrino cross section modeling used for T2K neutrino oscillation analyses. Electron neutrinos coming from muons and kaons decay are also separately measured, resulting in a ratio with respect to the prediction of 0.68+-0.30 and 1.10+-0.14, respectively.

Variations of muon flux in the atmosphere during thunderstorms

The variations of muon flux that are occasionally registered by ground-based scintillation detectors during thunderstorms are investigated. The variations are mainly negative (deficit) and originate due to the electric field of thunderclouds. They last from several minutes up to several hours and demonstrate minor (up to a few percent) deviations against the background. We develop an appropriate theoretical model to describe this phenomenon by investigating the two processes responsible for it: (i) the transformation of an energetic spectrum of muons in the electric field due to their acceleration or deceleration, and (ii) the decay of muons conditioned by a short life span. The change of muon flux near the ground is derived at a given altitudinal distribution (profile) of the electric field. Two possible opposite cases of an altitudinal profile of a two-layered electric field are considered: (i) upward directed in the lower layer of the cloud and downward directed in the upper layer and (ii) vice versa. We find that in case (i), the deficit of muon fluxes is observed by the detector at any threshold energy. It emerges due to a decrease of the flux of positive muons decelerated in the lower layer of a thundercloud. The corresponding increase of the flux of accelerated muons turns out to be smaller because of their partial decaying. As a result, an uncompensated decrease of total flux of muons is formed near the ground. However, in case (ii), the deficit is smaller, and it may become positive when it is observed by detectors with a threshold energy larger than $\ensuremath{\sim}300\text{ }\mathrm{MeV}$. The results may help estimate the height and electric field of thunderclouds by analyzing the records of muon detectors with different energy thresholds.

Charged lepton spectrum approximation in a three body nucleon decay

Only phase space is typically used to obtain final state particle spectra in rare decay searches, which is a crude approximation in the case of three body processes. We will demonstrate how both dynamics and phase space can be approximately accounted for, in processes such as nucleon decays $p \rightarrow e^+ \bar{\nu} \nu$ or $p \rightarrow \mu^+ \bar{\nu} \nu$ originating from Grand Unification models, using general effective Fermi theory formalism of electroweak muon decay $\mu \rightarrow e^+ \bar{\nu} \nu$. This approach allows for a more precise and only weakly model dependent approximation of final particle spectra for these and similar decays, which may improve rare process searches in current and near-future experiments.

Electroweak bosons in heavy ion collisions in CMS

Abstract The Compact Muon Solenoid (CMS) is fully equipped to measure leptonic decays of electroweak probes in the high multiplicity environment of nucleus–nucleus collisions. The inclusive and differential Z boson yields in the muon and electron decay channels are presented, together with measurements of the yield of W bosons decaying into a muon and an (anti)neutrino as a function of centrality, and the W charge asymmetry as a function of rapidity. The results confirm the binary scaling hypothesis, and show that possible modifications due to nuclear PDFs with respect to pp collisions, scaled by the number of elementary nucleon–nucleon collisions, are within the statistical and systematic uncertainties of the current measurements.

Bs,d→ℓ+ℓ−in the Standard Model with Reduced Theoretical Uncertainty

We combine our new results for the O(αem) and O(αs2) corrections to Bs,d→ℓ+ℓ-, and present updated branching ratio predictions for these decays in the standard model. Inclusion of the new corrections removes major theoretical uncertainties of perturbative origin that have just begun to dominate over the parametric ones. For the recently observed muonic decay of the Bs meson, our calculation gives B¯(Bs→μ+μ-)=(3.65±0.23)×10-9.

Measurement of Inner Bremsstrahlung in Polarized Muon Decay with MEG

A muon decay accompanied by a photon through the inner Bremmstrahlung process (μ→eνν¯γ, radiative muon decay) produces a time-correlated pair of positron and photon which becomes one of the main backgrounds in the search for μ→eγ decay. This channel is also an important probe of timing calibration and cross-check of whole the experiment. We identified a large sample (∼ 13000) of radiative muon decays in MEG data sample. The measured branching ratio in a region of interest in the μ→eγ search is consistent with the standard model prediction. It is also the first measurement of the decay from polarized muons. The precision measurement of this mode enables us to use it as one of the normalization channels of μ→eγ decay successfully reducing its uncertainty to less than 5%.

Feasibility study of an active target for the MEG experiment

We consider the possibility to have an active target for the upgrade of the MEG experiment (MEG II). The active target should work as (1) a beam monitoring, to continuously measure the muon stopping rate and therefore provide a direct evaluation of the detector acceptance (or an absolute normalization of the stopped muon); and as (2) an auxiliary device for the spectrometer, to improve the determination of the muon decay vertex and consequently to achieve a better positron momentum and angular resolutions, detecting the positron from the muon decay. In this work we studied the feasibility of detecting minimum ionizing particle with a single layer of 250 μm fiber and the capability to discriminate between the signal induced by either a muon or a positron.