Muon Research Articles

Radiative seesaw mechanism for charged leptons

We discuss a mechanism where charged lepton masses are derived from one-loop diagrams mediated by particles in a dark sector including a dark matter candidate. We focus on a scenario where the muon and electron masses are generated at one loop with new $\mathcal{O}(1)$ Yukawa couplings. The measured muon anomalous magnetic dipole moment, $(g\ensuremath{-}2{)}_{\ensuremath{\mu}}$, can be explained in this framework. As an important prediction, the muon and electron Yukawa couplings can deviate significantly from their standard model predictions, and such deviations can be tested at High-Luminosity LHC and future ${e}^{+}{e}^{\ensuremath{-}}$ colliders.

Muon spinning its way to new physics

Muon spinning its way to new physics

Elastic positron–proton scattering at low Q$$^2$$

Systematic differences in the the proton's charge radius, as determined by ordinary atoms and muonic atoms, have caused a resurgence of interest in elastic lepton scattering measurements. The proton's charge radius, defined as the slope of the charge form factor at Q$^2$=0, does not depend on the probe. Any difference in the apparent size of the proton, when determined from ordinary versus muonic hydrogen, could point to new physics or need for the higher order corrections. While recent measurements seem to now be in agreement, there is to date no high precision elastic scattering data with both electrons and positrons. A high precision proton radius measurement could be performed in Hall B at Jefferson Lab with a positron beam and the calorimeter based setup of the PRad experiment. This measurement could also be extended to deuterons where a similar discrepancy has been observed between the muonic and electronic determination of deuteron charge radius. A new, high precision measurement with positrons, when viewed alongside electron scattering measurements and the forthcoming MUSE muon scattering measurement, could help provide new insights into the origins of the proton radius puzzle, and also provide new experimental constraints on radiative correction calculations.

Forbidden dark matter annihilations into Standard Model particles

We present kinematically forbidden dark matter annihilations into Standard Model leptons. This mechanism precisely selects the dark matter mass that gives the observed relic abundance. This is qualitatively different from existing models of thermal dark matter, where fixing the relic density typically leaves open orders of magnitude of viable dark matter masses. Forbidden annihilations require the dark matter to be close in mass to the particles that dominate its annihilation rate. We show examples where the dark matter mass is close to the muon mass, the tau mass, or the average of the tau and muon masses. We find that most of the relevant parameter space can be covered by the next generation of proposed beam-dump experiments and future high-luminosity electron positron colliders. Forbidden dark matter predicts large couplings to the Standard Model that can explain the observed value of (g − 2)μ.

Reaching the Planck scale with muon lifetime measurements

Planck scale modified dispersion relations are one way how to capture the influence of quantum gravity on the propagation of fundamental point particles effectively. We derive the time dilation between an observer's or particle's proper time, given by a Finslerian length measure induced from a modified dispersion relation, and a reference laboratory time. To do so, the Finsler length measure for general first order perturbations of the general relativistic dispersion relation is constructed explicitly. From this we then derive the time dilation formula for the $\kappa$-Poincar\'e dispersion relation in several momentum space bases, as well as for modified dispersion relations considered in the context of string theory and loop quantum gravity. Most interestingly we find that the momentum Lorentz factor in the present and future colliders can, in principle, become large enough to constrain the Finsler realization of the $\kappa$-Poincar\'e dispersion relation in the bicrossproduct basis as well as a string theory inspired modified dispersion relation, at Planck scale sensitivity with the help of the muon's lifetime.

Implementation of Local Chiral Interactions in the Hyperspherical Harmonics Formalism

With the goal of using chiral interactions at various orders to explore the properties of the few-body nuclear systems, we write the recently developed local chiral interactions as spherical irreducible tensors and implement them in the hyperspherical harmonics expansion method. We devote particular attention to three-body forces at next-to-next-to leading order, which play an important role in reproducing experimental data. We check our implementation by benchmarking the ground-state properties of 3H, 3He, and 4He against the available Monte Carlo calculations. We then confirm their order-by-order truncation error estimates and further investigate uncertainties in the charge radii obtained by using the precise muonic atom data for single-nucleon radii. Having local chiral Hamiltonians at various orders implemented in our hyperspherical harmonics suites of codes opens up the possibility to test such interactions on other light-nuclei properties, such as electromagnetic reactions.

Muon-Induced Single-Event Upsets in 20-nm SRAMs: Comparative Characterization With Neutrons and Alpha Particles

Negative and positive muon-induced single-event upsets (SEUs) are studied in 20-nm bulk planar SRAMs. Muon irradiation is performed using a mono-energetic source with varying the muon energy. The energy dependence of the cross sections (CSs) of SEUs and multiple-cell upsets (MCUs) shows the significant contribution of muon capture reactions for the negative muon, as reported in previous studies. Interestingly, MCU events are found for the positive muon, in contrast to the previous studies. The CSs for the negative and positive muons are compared with that for the other terrestrial radiations: high-energy neutrons, thermal neutrons, and alpha particles. The voltage dependence of the SEU CS, together with the empirical model for charge collection, demonstrates the difference in the contributing secondary ions among the negative muon, the high-energy neutron, and the thermal neutron. The MCU events are thoroughly analyzed in terms of their ratio to the total events and their fail bit patterns. The results reveal that the MCU characteristics for the negative muon are different from that for the other terrestrial radiations due to the muon capture reactions, where parasitic bipolar effects and the isotropic emission of secondary ions are important factors.

Time evolution calculation of muon catalysed fusion: Emission of recycling muons from a two-layer hydrogen film

Recycling muon having approximately 10 keV kinetic energy produced in a muon catalysed fusion (μCF) cycle is expected to be a source of negative muon beam with high-spatial/time coherence. We report a time evolution calculation of μCF cycle with a particular focus on the emission of recycling muons from a hydrogen film target. Rate equations for the μCF reactions are solved using the 4th-order Runge-Kutta method. Considering a two-layer solid hydrogen film target composed of 1 mm hydrogen-deuterium layer and 2 µm deuterium-tritium layer, we have numerically simulated the time evolution of the muonic states and their populations in these layers. The muon atomic and molecular processes in the layers are solved simultaneously by introducing a probability of muonic deuterium atoms traveling between the layers via the Ramsauer-Townsend effect. The use of T2 amplifies the recycling muon emission. A weak dependence of the emission fraction of the recycling muons on the T2 concentration is predicted, which can be utilized to minimize the usage of tritium.

Muonium hydride: The lowest density crystal

A muonium hydride molecule is a bound state of muonium and hydrogen atoms. It has half the mass of a parahydrogen molecule and very similar electronic properties in its ground state. The phase diagram of an assembly of such particles is investigated by first principle quantum simulations. In the bulk limit, the low-temperature equilibrium phase is a crystal of extraordinarily low density, lower than that of any other known atomic or molecular crystal. Despite the low density and particle mass, the melting temperature is surprisingly high (close to 9 K). No (metastable) supersolid phase is observed. We investigated the physical properties of nanoscale clusters (up to 200 particles) of muonium hydride and found the superfluid response to be greatly enhanced compared to that of parahydrogen clusters. The possible experimental realization of these systems is discussed.

Radiative muon mass models and (g \u2212 2)\u03bc

Recent measurements of the Higgs-muon coupling are directly probing muon mass generation for the first time. We classify minimal models with a one-loop radiative mass mechanism and show that benchmark models are consistent with current experimental results. We find that these models are best probed by measurements of (g − 2)μ, even when taking into account the precision of Higgs measurements expected at future colliders. The current (g − 2)μ anomaly, if confirmed, could therefore be a first hint that the muon mass has a radiative origin.

Testing Quantum Electrodynamics with Exotic Atoms.

Precision study of few-electron, high-Z ions is a privileged field for probing high-field, bound-state quantum electrodynamics (BSQED). However, the accuracy of such tests is plagued by nuclear uncertainties, which are often larger than the BSQED effects under investigation. We propose an alternative method with exotic atoms and show that transitions may be found between circular Rydberg states where nuclear contributions are vanishing while BSQED effects remain large. When probed with newly available quantum sensing detectors, these systems offer gains in sensitivity of 1 to 2 orders of magnitude, while the mean electric field largely exceeds the Schwinger limit.

Search for heavy neutral leptons in W+→μ+μ±jet decays

A search is performed for heavy neutrinos in the decay of a W boson into two muons and a jet. The data set corresponds to an integrated luminosity of approximately 3.0fb-1 of proton–proton collision data at centre-of-mass energies of 7 and 8TeV collected with the LHCb experiment. Both same-sign and opposite-sign muons in the final state are considered. Data are found to be consistent with the expected background. Upper limits on the coupling of a heavy neutrino with the Standard Model neutrino are set at 95 % confidence level in the heavy-neutrino mass range from 5 to 50GeV/c2. These are of the order of 10 - 3 for lepton-number-conserving decays and of the order of 10 - 4 for lepton-number-violating heavy-neutrino decays.

Particle mystery deepens, as physicists confirm that the muon is more magnetic than predicted

Particle mystery deepens, as physicists confirm that the muon is more magnetic than predicted

Measuring the Magnet that Measures the Muon

Measuring the Magnet that Measures the Muon

Confined muonic hydrogen-like atoms

A muonic hydrogen-like atom is formed when replacing the electron in it by a negative muon $$\mu ^-$$ . The Schrodinger equation corresponds to the one of the atoms with a binding particle $$(\mu ^-)$$ with mass $$M_{\mu } \approx 207m_e$$ , where $$m_e$$ is the electron mass. The muon, in comparison with the electron, moves along an orbit which remains much closer to the nucleus and the associated binding energy is considerably increased. In this report, we have studied how the ground state energy changes for H, $$He^{+}$$ and $$Li^{+2}$$ muonic atoms when subjected to very high compression. In order to simulate such a compression regime for the atoms, we have considered two confinement models: In the first, we assume for each system a nuclear point mass, whereas in the second the nucleus consists of a finite volume with a uniformly distributed charge. The energy correction due to the nucleus finite size is carried out at first-order perturbation theory. We found that the maximum of the energy correction becomes more important as the confinement and mass number of the atom nucleus grow.

Cosmic antihelium-3 nuclei sensitivity of the GAPS experiment

The General Antiparticle Spectrometer (GAPS) is an Antarctic balloon experiment designed for low-energy (0.1–0.3 GeV/n) cosmic antinuclei as signatures of dark matter annihilation or decay. GAPS is optimized to detect low-energy antideuterons, as well as to provide unprecedented sensitivity to low-energy antiprotons and antihelium nuclei. The novel GAPS antiparticle detection technique, based on the formation, decay, and annihilation of exotic atoms, provides greater identification power for these low-energy antinuclei than previous magnetic spectrometer experiments. This work reports the sensitivity of GAPS to detect antihelium-3 nuclei, based on full instrument simulation, event reconstruction, and realistic atmospheric influence simulations. The report of antihelium nuclei candidate events by AMS-02 has generated considerable interest in antihelium nuclei as probes of dark matter and other beyond the Standard Model theories. GAPS is in a unique position to detect or set upper limits on the cosmic antihelium nuclei flux in an energy range that is essentially free of astrophysical background. In three 35-day long-duration balloon flights, GAPS will be sensitive to an antihelium flux on the level of 1.3−1.2+4.5·10−6 m-2sr-1s-1(GeV/n)-1 (95% confidence level) in the energy range of 0.11–0.3 GeV/n, opening a new window on rare cosmic physics.

Transition-Edge Sensor Optimization for Hard X-ray Applications

Arrays of superconducting transition-edge sensor (TES) microcalorimeters are attractive for some X-ray experiments where the combination of higher collection efficiency than crystal spectrometers and much better energy resolution than semiconductor detectors is enabling. For hard X-ray (>2 keV) applications, a TES is often integrated with an extra absorber to extend its dynamic range, quantum efficiency and collecting area. For >100 keV applications, millimeter size absorbers have been manually attached to TES pixels after fabrication. At lower energies, the absorber size needs to be much smaller, because a large heat capacity degrades energy resolution. In that case, the absorber is directly electroplated or evaporated on the TES during the fabrication process. In this work, we present TES microcalorimeters optimized for the intermediate hard X-ray energy region (20 keV ~ 100 keV) that have thick absorbers directly deposited during the TES fabrication process. This microcalorimeter is designed to measure high-resolution muonic atom transition X-ray spectra as a method to study quantum electrodynamics (QED) effects. The absorber is comprised of evaporated gold and electroplated bismuth layers and is designed to optimize dynamic range, collecting efficiency, and energy resolution in the 20 keV to 45 keV energy range. A prototype with pure gold absorbers has been fabricated and characterized, in order to obtain a first experimental reference in the detector development process.

Chemical effect on muonic atom formation through muon transfer reaction in benzene and cyclohexane samples

Abstract To investigate the chemical effect on the muon capture process through a muon transfer reaction from a muonic hydrogen atom, the formation rate of muonic carbon atoms is measured for benzene and cyclohexane molecules in liquid samples. The muon transfer rate to carbon atoms of the benzene molecule is higher than that to the carbon atoms of the cyclohexane molecule. Such a deviation has never been observed among those molecules for gas samples. This may be because the transfers occur from the excited states of muonic hydrogen atoms in the liquid system, whereas in the gas system, all the transfers occur from the 1s (ground) state of muon hydrogen atoms. The muonic hydrogen atoms in the excited states have a larger radius than those in the 1s state and are therefore considered to be affected by the steric hindrance of the molecular structure. This indicates that the excited states of muonic hydrogen atoms contribute significantly to the chemical effects on the muon transfer reaction.

Dispersive evaluation of the Lamb shift in muonic deuterium from chiral effective field theory

We merge the dispersive relation approach and the ab initio method to compute nuclear structure corrections to the Lamb shift in muonic deuterium. We calculate the deuteron response functions and corresponding uncertainties up to next-to-next-to-next-to-leading order in chiral effective field theory and compare our results to selected electromagnetic data to test the validity of the theory. We then feed response functions calculated over a wide range of kinematics to the dispersion-theory formalism and show that an improved accuracy is obtained compared to that with the use of available experimental data in the dispersive analysis. This opens up the possibility of applying this hybrid method to other light muonic atoms and supplementing experimental data with ab initio theory for kinematics where data are scarce or difficult to measure with the goal of reducing uncertainties in estimates of nuclear structure effects in atomic spectroscopy.

Redetermination of the strong-interaction width in pionic hydrogen

The hadronic width of the ground state of pionic hydrogen has been redetermined by X-ray spectroscopy to be varGamma ^{pi mathrm {H}}_{1s}=(856,pm ,16_mathrm{stat},pm ,22_mathrm{sys}) meV. The experiment was performed at the high-intensity low-energy pion beam of the Paul Scherrer Institute by using the cyclotron trap and a high-resolution Bragg spectrometer with spherically bent crystals. Coulomb de-excitation was studied in detail by comparing its influence on the line shape by measuring the three different transitions Kalpha , Kbeta , and Kgamma at various hydrogen densities. The pion-nucleon scattering lengths and other physical quantities extracted from pionic-atom data are in good agreement with the results obtained from pion-nucleon and nucleon-nucleon scattering experiments and confirm that a consistent picture is achieved for the low-energy pion-nucleon sector with respect to the expectations of chiral perturbation theory.