Muon Anomalous Magnetic Moment Research Articles

Neutrino masses, flavor anomalies, and muon g−2 from dark loops

The lepton sector of the Standard Model is at present haunted by several intriguing anomalies, including an emerging pattern of deviations in $b \to s \ell \ell$ processes, with hints of lepton flavor universality violation, and a discrepancy in the muon anomalous magnetic moment. More importantly, it cannot explain neutrino oscillation data, which necessarily imply the existence of non-zero neutrino masses and lepton mixings. We propose a model that accommodates all the aforecited anomalies, induces neutrino masses and provides a testable dark matter candidate. This is achieved by introducing a dark sector contributing to the observables of interest at the 1-loop level. Our setup provides a very economical explanation to all these open questions in particle physics and is compatible with the current experimental constraints.

A large muon EDM from dark matter

We explore a model of dark matter (DM) that can explain the reported discrepancy in the muon anomalous magnetic moment and predict a large electric dipole moment (EDM) of the muon. The model contains a DM fermion and new scalars whose exclusive interactions with the muon radiatively generate the observed muon mass. Constraints from DM direct and indirect detection experiments as well as collider searches are safely evaded. The model parameter space that gives the observed DM abundance and explains the muon g – 2 anomaly leads to the muon EDM of dμ ≃ (4-5) × 10−22e cm that can be probed by the projected PSI muEDM experiment. Another viable parameter space even achieves {d}_{mu }=mathcal{O}left({10}^{-21}right) e cm reachable by the ongoing Fermilab Muon g − 2 experiment and the future J-PARC Muon g − 2/EDM experiment.

Collider signatures of a scalar leptoquark and vectorlike lepton in light of muon anomaly

Despite the immense success of the Standard Model (SM), the hunt for physics beyond the Standard Model (BSM) has continued. Extension of the SM gauge group or the particle content of the SM remains a viable solution to many observed anomalies, such as the anomalous magnetic moment of muon, flavor violation, dark matter, etc. In this work, we consider a BSM model which includes a leptoquark, a vectorlike lepton, and a real scalar singlet. The model accounts for a dark matter candidate through a ${\mathbb{Z}}_{2}$ symmetry in addition to predicting the muon ($g\ensuremath{-}2$) in agreement with the experimental measurement. We also show that the experimental measurements of the lepton flavor violation are satisfied in a wide range of parameter space. The viable parameter space of the model is used to study the collider signatures arising from the pair production of the leptoquark in the $2\ensuremath{\mu}+2b+{\overline{)E}}_{T}$ channel at the 14 TeV LHC run.

Constraints on the hadronic light-by-light in the Melnikov-Vainshtein regime

The muon anomalous magnetic moment continues to attract attention due to the possible tension between the experimentally measured value and the theoretical Standard Model prediction. With the aim to reduce the uncertainty on the hadronic light-by-light contribution to the magnetic moment, we derive short-distance constraints in the Melnikov-Vainshtein regime which are useful for data-driven determinations. In this kinematical region, two of the four electromagnetic currents are close in the four-point function defining the hadronic light-by-light tensor. To obtain the constraints, we develop a systematic operator product expansion of the tensor in question to next-to-leading order in the expansion in operators. We evaluate the leading in αs contributions and derive constraints for the next-to-leading operators that are also valid nonperturbatively.

Measurement of e+e− → π+π−π0 at BABAR and calculation of (g−2)μ

We present a study of the process e+e−→π+π−π0 at BABAR using the initial-state radiation technique. The analysis is based on the full BABAR data set, 469fb−1, recorded at and near the ϒ(4S) resonance. From the fit to the measured three pion mass spectrum we determine the products Γ(V→e+e−)B (V→π+π−π0) for the ω and ϕ resonances, and the branching ratio B (ρ→π+π−π0). The latter isospin-breaking decay is observed with 6σ significance. The e+e−→π+π−π0 cross section is measured from 0.62 GeV to 3.5 GeV. The measured cross section is used to calculate the leading-order hadronic contribution to the muon anomalous magnetic moment from this exclusive final state with improved accuracy. A short status is given on the current discrepancy between the predicted value and the measured value of the muon anomalous magnetic moment.

A Flexible Divergence Elimination Method for Calculating Lepton Magnetic Moments in Quantum Electrodynamics

A precise calculation of the lepton anomalous magnetic moments (AMM) requires an evaluation of the quantum electrodynamics (QED) Feynman diagrams up to five independent loops. The complicated structure of ultraviolet (UV), infrared (IR) and mixed divergences in the corresponding integrals makes it difficult to calculate these high-order contributions in reasonable computer time frame.We demonstrate a method that eliminates all divergences point by point in Feynman parametric space (before integration) and possesses a flexibility that can be used for improving the precision of the numerical integration. This flexibility is especially actual for calculating the contributions of the Feynman diagrams with electron loops to the muon AMM. 3-loop and 4-loop numerical test results are provided.The subtraction procedure is based on a forest formula with linear operators applied to the Feynman amplitudes of UV divergent subdiagrams. It is similar to BPHZ; the difference is in the linear operators used and in the way of combining them. It is equivalent to the on-shell renormalization after summation over diagrams: no residual renormalization is required.

Muon (g − 2) in U(1) L μ − L τ scotogenic model extended with vector like fermion

The latest results of anomalous muon magnetic moment at Fermilab show a discrepancy of 4.2 σ between the Standard Model (SM) prediction and experimental value. In this work, we revisit symmetry with in the paradigm of scotogenic model which explains muon (g − 2) and neutrino mass generation, simultaneously. The mass of new gauge boson generated after the spontaneous symmetry breaking of is constrained, solely, in light of the current neutrino oscillation data to explain muon (g − 2). In particular, we have obtained two regions I and II, around 150 MeV and 500 MeV, respectively, in plane which explain the neutrino phenomenology. Region I is found to be consistent with muon neutrino trident (MNT) bound (g μ τ ≤ 10−3) to explain muon (g − 2), however, region II violates it for mass range . We, then, extend the minimal gauged scotogenic model by a vector like lepton (VLL) triplet ψ T . The mixing of ψ T with inert scalar doublet η leads to chirally enhanced positive contribution to muon anomalous magnetic moment independent of Z μ τ mass. Furthermore, we have, also, investigated the implication of the model for 0ν β β decay and CP violation. The non-observation of 0ν β β decay down to the sensitivity of 0.01 eV shall refute the model. The model, in general, is found to be consistent with both CP conserving and CP violating solutions.

Flavor anomalies meet the LHC

Currently there are several discrepancies between the standard model (SM) prediction and experimental value in the flavor observables, e.g. RD* , BR(B̄ → D+K−), RK(*) and muon anomalous magnetic moment. If we interpret the deviation as a hint for the physics beyond the SM, $ \mathcal {O} $(1) TeV new particles are often implied. Therefore it is natural to search those new particles at the large hadron colliders. In this note we interplay the flavor anomalies and collider physics.

Preparations for a search of the muon EDM at PSI

Electric dipole moments (EDMs) of fundamental particles violate time invariance and the combined symmetry of charge and parity (CP). The existence of a large muon EDM (muEDM) is made plausible by tensions with Standard Model predictions for semileptonic decays of heavy meson measured at LHCb, BaBar, and Belle, as well as the muon’s anomalous magnetic moment (AMM). A discovery of the muEDM would manifest CP and lepton flavor universality (LFU) violation, revealing physics beyond the SM (BSM). The most sensitive muEDM search to date provides an upper limit of 1.8 × 10−19 e cm (CL 95%), extracted from high-precision data collected to measure the muon AMM. At the Paul Scherrer Institute, we are setting up a dedicated search for the muEDM using, for the first time, the frozen-spin technique to target an ultimate sensitivity better than 6 × 10−23 e cm. This novel technique increases the sensitivity to EDM-induced spin precession by cancelling the AMM-induced precession with the application of a precisely tuned electric field perpendicular to the muon momentum and the magnetic field. In this configuration, the dominant source of precession is the EDM coupling to the large relativistic electric field in the muon rest frame, generated by its motion in a strong 3 T uniform magnetic field. In a precursor experiment, we will apply the frozen-spin technique in a compact solenoid demonstrating a sensitivity of better than 3 × 10−21 e cm, probing uncharted and otherwise inaccessible territory in BSM theories.

Reappraisal of the minimal flavoured Z′ scenario

Recent results from the intensity frontier indicate the tantalizing possibility of violation in lepton flavour universality. In light of this we revisit the minimal phenomenological Z′ model taking in account both vectorial and axial-vectorial flavour violating couplings to the charged leptons. We make a systematic study to identify the minimal framework that can simultaneously explain the recent results on anomalous magnetic moment of muon and electron while remaining in consonance with RK(⁎), Bs0−Bs0¯ mixing and angular observables in the B+→K+⁎μ+μ− channel reported by the LHCb collaboration. We demonstrate that the neutrino trident data imply a further SU(2)L violation in the leptonic couplings of the exotic Z′.

Addendum: Timelike and spacelike kernel functions for the hadronic vacuum polarization contribution to the muon anomalous magnetic moment (2022 J. Phys. G: Nucl. Part. Phys. 49 055001)

This addendum provides results complementary to those obtained in [J. Phys. G 49, 055001 (2022)]. Specifically, an equivalent form of the relation, which binds together the ‘spacelike’ kernel functions for the hadronic vacuum polarization contribution to the muon anomalous magnetic moment , is obtained. It is shown that the infrared limiting value of the ‘spacelike’ and ‘timelike’ kernel functions, which enter the representations for involving the Adler function and the R-ratio, is identical to the corresponding QED contribution to the muon anomalous magnetic moment of the preceding order in the electromagnetic coupling. The next-to-leading order contributions (which includes the leptonic and hadronic insertions) and (which includes the double hadronic insertion), are studied. The three kernel functions appearing in the representations for , which involve the hadronic vacuum polarization function, Adler function, and the R-ratio, are presented for the cases of the electron and τ-lepton loop insertions.

The neutrinoless double beta decay in the colored Zee-Babu model

We study the neutrinoless double beta decay in the colored Zee-Babu model. We consider three cases of the colored Zee-Babu model with a leptoquark and a diquark introduced. The neutrino masses are generated at two-loop level, and the constraints given by tree-level flavor violation processes and muon anomalous magnetic moment (g−2)μ have been considered. In our numerical analysis, we find that the standard light neutrino exchange contribution can be canceled by new physics contribution under certain assumption and condition, leading to a hidden neutrinoless double beta decay. The condition can be examined comprehensively by future complementary searches with different isotopes.

The Anomalous Magnetic Moment of Muon

Known as one of the most hopeful fields to find new physics beyond the standard model, the anomalous magnetic moment of muon has gained much attention for a long time and become even more important after the Fermi National Accelerator Laboratory’s result came out in 2018. This paper shows the general works and achievements in this exciting field. Those include experiments operated by the Brookhaven National Laboratory and Fermi National Accelerator Laboratory to measure the g-2 factor, the calculation based on the standard model, and a possible extension of the standard model that can explain the experimental results. This paper is an introduction for anyone interested in this field.

A comprehensive study of the vector leptoquark with U(1)B3-L2 on the B-meson and muon g − 2 anomalies

Abstract Recently reported anomalies in various B-meson decays and also in the anomalous magnetic moment of muon (g − 2)μ motivate us to consider a particular extension of the standard model incorporating new interactions in the lepton and quark sectors simultaneously. Our minimal choice would be the leptoquark. In particular, we take the vector leptoquark (U1) and comprehensively study all related observables including ${(g-2)_{\mu }},\ R_{K^{(*)}},\ R_{D^{(*)}}$, B → (K)ℓℓ′, where ℓℓ′ are various combinations of μ and τ, and also lepton flavor violation in the τ decays. We find that a hybrid scenario with an additional $U(1)_{B_3-L_2}$ gauge boson provides a common explanation for all these anomalies.

M_W, dark matter and a_mu in the NMSSM

We study regions in the parameter space of the NMSSM which are able to simultaneously explain the current measured values for the W mass M_W and the muon anomalous magnetic moment a_mu , and provide a dark matter relic density consistent with the observations as well as constraints from detection experiments. The corresponding regions feature light charginos and sleptons in the 100 GeV–1 TeV range, at least some of them with masses below 175 GeV such such that the electroweakly-interacting SUSY particles generate sufficiently large contributions to M_W. The LSP is always singlino-like with a mass below 150 GeV, and could possibly remain invisible even at future detection experiments. Decays of electroweak sparticles proceed through cascades via staus and/or sleptons which makes their detection challenging. We propose benchmark points for future searches of such sparticles. The lightest CP-even scalar may have a mass in the 95–98 GeV range with, however, modest signal rates in view of the mild excesses reported in this range at LEP and by CMS at the LHC.

Toward testing the magnetic moment of the tau at one part per million

If physics beyond the Standard Model (BSM) explains the $4.2\sigma$ difference between the Standard-Model and measured muon anomalous magnetic moment, $a_{\mu}$, minimal flavor violation predicts a shift in the analog quantity for the $\tau$ lepton, $a_{\tau}$, at the $10^{-6}$ level, and even larger effects are possible in generic BSM scenarios such as leptoquarks. We show that this produces equivalent BSM deviations in the Pauli form factor, $F_2(s)$, at $s=(10\,\text{GeV})^2$, and report the first complete two-loop prediction of $\text{Re}\, F_2^\text{eff}(100\,\text{GeV}^2)=-268.77(50)\times 10^{-6}$ for resonant $\tau$-pair production in $e^+e^-\rightarrow \Upsilon(nS) \rightarrow$ $\tau^+\tau^-$, $n=1,2,3$. $\text{Re}\, F_2^\text{eff}$ can be measured from $e^-$-helicity-dependent transverse and longitudinal asymmetries in $\tau$-pair events, which requires a longitudinally polarized $e^-$ beam. We discuss how Belle II asymmetry measurements could probe $a_{\tau}^\text{BSM}$ at $10^{-6}$, assuming such a polarization upgrade of the SuperKEKB $e^+e^-$ collider, and conclude by outlining the next steps to be taken in theory and experiment along this new avenue for exploring realistic BSM effects in $a_\tau$.

Fermion masses and mixings, dark matter, leptogenesis and $$g-2$$ muon anomaly in an extended 2HDM with inverse seesaw

We propose a predictive \(Q_4\) flavored 2HDM model, where the scalar sector is enlarged by the inclusion of several gauge singlet scalars and the fermion sector by the inclusion of right-handed Majorana neutrinos. In our model, the \(Q_4\) family symmetry is supplemented by several auxiliary cyclic symmetries, whose spontaneous breaking produces the observed pattern of SM charged fermion masses and quark mixing angles. The light active neutrino masses are generated from an inverse seesaw mechanism at one loop level thanks to a remnant preserved \(Z_2\) symmetry. Our model successfully reproduces the measured dark matter relic abundance and is consistent with direct detection constraints for masses of the DM candidate around \(\sim\) 6.3 TeV. Furthermore, our model is also consistent with the lepton and baryon asymmetries of the Universe as well as with the muon anomalous magnetic moment.

High-power test of an interdigital H -mode drift tube linac for the J-PARC muon g−2 and electric dipole moment experiment

We conducted a high-power test of a prototype cavity of a 324-MHz interdigital H-mode drift tube linac (IH-DTL) for the precise measurement of the muon anomalous magnetic moment ($g\ensuremath{-}2$) and electric dipole moment (EDM). This prototype cavity (short IH) was developed to verify the fabrication methodology for the IH-DTL cavity with a monolithic drift tube structure. The electromagnetic field distribution was measured and compared with the finite element method simulation results, and the fabrication accuracy of the monolithic drift tube was confirmed to satisfy the requirements. After 40 h of conditioning, the short IH was stably operated with an rf power of 88 kW, which corresponds to a 10% higher accelerating field than the design field (${E}_{0}$) of $3.0\text{ }\text{ }\mathrm{MV}/\mathrm{m}$. In addition, the thermal characteristics and frequency response were measured, verifying that the experimental data were consistent with the three-dimensional model. In this paper, the design, fabrication, and low-power and high-power tests of this IH-DTL for muon acceleration are described.

Muon (g − 2) and the W-boson mass anomaly in a model based on Z4 symmetry with a vector-like fermion

Abstract The latest results of the CDF-II Collaboration show a discrepancy of 7σ with standard model expectations. There is also a 4.2σ discrepancy in the measurement of the muon magnetic moment reported by Fermilab. We study the connection between neutrino masses, dark matter, the Muon (g − 2) experiment, and the W-boson mass anomaly within a single coherent framework based on a Z4 extension of the scotogenic model with a vector-like lepton (VLL). Neutrino masses are generated at the one-loop level. The inert doublet also provides a solution to the W-boson mass anomaly through correction in oblique parameters S, T, and U. The coupling of the VLL triplet ψT to the inert doublet η provides a positive contribution to the muon anomalous magnetic moment. In the model, the VLL triplet provides a lepton portal to dark matter, $\eta _R^0$. The model predicts a lower bound mee > 0.025 eV at 3σ, which is well within the sensitivity reach of the 0νββ decay experiments. The model explains the muon anomalous magnetic moment Δaμ for 1.3 < yψ < 2.8 and a DM candidate mass in the range $152\text{ GeV}\lt M_{\eta _{R}^{0}}\lt 195\text{ GeV}$. The explanation of the W-boson mass anomaly further constrains the mass of the DM candidate, $M_{\eta _{R}^{0}}$, in the range $154\text{ GeV}\lt M_{\eta _{R}^{0}}\lt 174\text{ GeV}$.

Contribution of exclusive (π0π0,π0η,ηη)γ channels to the leading order HVP of the muon g−2

We evaluate the contributions of $(\pi^0\pi^0, \pi^0\eta, \eta\eta)\gamma$ exclusive channels to the dispersion integral of the leading order HVP of the muon anomalous magnetic moment. These channels are included in some way in previous evaluations of the $\pi^0\omega, \eta\omega$ and $\eta\phi$ contributions to $a_{\mu}^{\rm had, LO}$, where the vector resonances (decaying into $\pi^0/\eta+ \gamma$) are assumed to be on-shell. Since the separation of resonance and background contributions in a given observable is, in general, a model-dependent procedure, here we use pseudoscalar mesons and the photon as the $in$ and $out$ states of the $e^+e^- \to (\pi^0\pi^0, \pi^0\eta, \eta\eta)\gamma$ $S$-matrix, such that the cross section contains the interferences among different contributing to the amplitudes. We find $a^{\rm had, LO}_{\mu}(P^0_1P^0_2\gamma)=(1.13\pm 0.13 ) \times 10^{-10}$, where uncertainties stem mainly from vector meson dominance model parameters. Improved experimental studies of these exclusive channels in the whole range below 2 GeV would reduce model-dependency.