arXiv subjects

Martin Hoferichter

Publications and source records attributed to Martin Hoferichter.

At least 19 recordsLinked to original sources

arXiv2026

Ab initio calculations of parity-violating electron scattering off $^{48}$Ca and $^{208}$Pb

Parity-violating electron scattering off nuclei both serves as a low-energy precision probe to test electroweak interactions and allows one to access neutron distributions inside nuclei. It has implications for strong interactions in dense neutron-rich environments, also providing constraints for the properties of matter in neutron stars. Precision measurements are available for $^{48}$Ca and $^{208}$Pb by the CREX and PREX collaborations, respectively, and their interpretation requires advanced nuclear-structure calculations to draw firm conclusions. We perform the first ab initio calculations of the parity-violating asymmetry $A_\text{PV}$ based on nuclear forces from chiral effective field theory, fully including corrections due to Coulomb distortion effects. Based on these results, we critically reexamine correlation analyses employed to infer weak radii and quantify the resulting tensions between ab initio and experimental results. We find that ab initio calculations prefer values of $A_\text{PV}$ slightly smaller and larger than observed for $^{48}$Ca and $^{208}$Pb, respectively, with a global significance of $1.9\sigma$. Using theoretically consistent inputs for charge and weak densities, we infer from the experimental $A_\text{PV}$ a neutron skin of $^{208}$Pb of $R_n-R_p = 0.187(25)(18)$ fm, substantially smaller than that reported by PREX II.

nucl-th↗

arXiv2026

Four-fermion operators, $Z$-boson exchange, and $\tau$ lepton dipole moments

Asymmetry measurements in $e^+e^-\to\tau^+\tau^-$ constitute a promising avenue to obtain competitive constraints on the $\tau$ dipole moments, the anomalous magnetic moment $a_\tau$ and the electric dipole moment $d_\tau$, especially, once a polarized electron beam becomes available, as possible at a future polarization upgrade of the SuperKEKB collider. While the main challenges concern the measurement of these asymmetries and the calculation of radiative corrections at the relevant level of precision, at subleading orders also electroweak effects and the potential impact of four-fermion operators parameterizing other beyond-the-Standard-Model scenarios besides those described by dipole operators need to be taken into consideration. Here, we show that $Z$-boson contributions arise at the level of $\simeq 3\times 10^{-6}$, while we estimate the largest possible effect from four-fermion operators as $\simeq 10^{-5} C \, v^2/\Lambda^2$. In addition, we observe that four-fermion-operator insertions at the loop level can probe Wilson coefficients that are otherwise not constrained directly, and that the imaginary part generated by insertions of the dipole operator at loop level opens another potential avenue towards a determination of $a_\tau$ without the need for a polarized electron beam. Despite the inherent loop suppression, a measurement of the required normal asymmetry $A_N^\pm$ with a precision of $\lesssim 10^{-5}$ would allow one to probe the Schwinger term, which could define an intermediate goal to be realized in the current setting at Belle II.

hep-ph↗

arXiv2026

Reconciling hadronic and partonic analyticity in $b\to s\ell\ell$ transitions

Rare $B$-meson decays mediated by $b\to s\ell\ell$ transitions constitute sensitive probes of physics beyond the Standard Model, and have triggered considerable interest due to hints for deviations from the Standard-Model prediction. To establish a discrepancy beyond a reasonable doubt, control over the nonlocal matrix elements involving charm loops is essential, which, for large spacelike virtualities, can be constrained by an operator product expansion with coefficients known at two-loop order. We observe that the analytic structure of this partonic calculation, whose understanding is important to put forward rigorous parameterizations, follows from simple triangle topologies and demonstrate explicitly how dispersion relations are fulfilled even in the case of anomalous thresholds. Crucially, these anomalous contributions match onto the ones expected when considering hadronic degrees of freedom, proving that the partonic calculation does not miss anomalous effects and justifying its use in regions of parameter space in which a perturbative description applies.

hep-ph↗

arXiv2026

Pion $\beta$ decay and $\tau\to\pi\pi\nu_\tau$ beyond leading logarithms

The consistent matching of short-distance contributions and hadronic matrix elements is crucial for precise predictions of weak processes involving hadrons. In this Letter, we address this point for charged-current processes involving two pions -- pion $\beta$ decay $\pi^\pm\to\pi^0 e^\pm\nu_e$ and hadronic $\tau$ decays $\tau^\pm\to\pi^\pm\pi^0\nu_\tau$ -- whose decay rates depend on the so-called $\gamma W$ box correction. Using recent results from lattice QCD, we show how to formulate the matching beyond leading-logarithmic accuracy, in particular, how to cancel the dependence on the scheme choice for evanescent operators. As main results, we obtain a prediction for the decay rate of pion $\beta$ decay with theory uncertainties improved by a factor of three, which renders theory uncertainties negligible for future determinations of $V_{ud}$ even beyond the reach of the PIONEER experiment, and an evaluation of isospin-breaking corrections to $\tau\to\pi\pi\nu_\tau$ with negligible uncertainty from the short-distance matching, as necessary for a future $\tau$-based determination of the hadronic-vacuum-polarization contribution to the anomalous magnetic moment of the muon.

hep-ph↗

arXiv2025

The anomalous magnetic moment of the muon: status and perspectives

We review the status of the anomalous magnetic moment of the muon as a precision probe of physics beyond the Standard Model (SM) after the release of the final results from the Fermi National Accelerator Laboratory (FNAL) Muon $g-2$ experiment and the second White Paper of the Muon $g-2$ Theory Initiative. While the SM prediction requires further improvements by a factor of four to fully leverage the sensitivity achieved in experiment, the FNAL measurement will set the standard for many years to come, and we discuss a variety of features of the experimental campaign that made this achievement possible. In going forward, we discuss current efforts to improve the SM prediction, and imagine how an experiment would have to be devised to surpass 124 ppb in precision.

hep-ph↗

arXiv2025

Improved Standard-Model predictions for $\eta^{(\prime)}\to \ell^+ \ell^-$

The rare decays $\eta^{(\prime)}\to\ell^+\ell^-$, $\ell\in\{e,\mu\}$, are highly suppressed in the Standard Model, both by their chirality structure and the required loop attaching the lepton line to the $\eta^{(\prime)}\to\gamma^*\gamma^*$ matrix element. The latter is described by a single scalar function, the transition form factor, which has recently been studied in great detail for $\eta^{(\prime)}$ in the context of the pseudoscalar-pole contributions to hadronic light-by-light scattering in the anomalous magnetic moment of the muon. Based on these results, we evaluate the corresponding prediction for the $\eta^{(\prime)}$ dilepton decays, supplemented by an improved evaluation of the asymptotic contributions including pseudoscalar mass effects. In particular, the dispersive representation for the $\eta^{(\prime)}$ transition form factors allows us, for the first time, to perform a robust evaluation of the imaginary parts due to subleading channels besides the dominant two-photon cut. Our final results are $\text{Br}[\eta\to e^+e^-]=5.37(4)(2)[4]\times 10^{-9}$, $\text{Br}[\eta\to \mu^+\mu^-]=4.54(4)(2)[4]\times 10^{-6}$, $\text{Br}[\eta'\to e^+e^-]=1.80(2)(3)[3]\times 10^{-10}$, and $\text{Br}[\eta'\to \mu^+\mu^-]=1.22(2)(2)[3]\times 10^{-7}$, where the errors refer to the uncertainty in the normalized branching fraction, the one propagated from $\text{Br}[\eta^{(\prime)}\to\gamma\gamma]$, and the total uncertainty, respectively. The branching fraction for $\eta\to\mu^+\mu^-$ exhibits a mild $1.6\sigma$ tension with experiment, and we explore the bounds that can be derived on physics beyond the Standard Model.

hep-ph↗

arXiv2025

Radiative corrections to $\tau\to\pi\pi\nu_\tau$

Hadronic $\tau$ decays present an opportunity to determine the isovector part of the hadronic-vacuum-polarization contribution to the anomalous magnetic moment of the muon in a way complementary to $e^+e^-\to\text{hadrons}$ cross sections. However, the required isospin rotation is only exact in the isospin limit, and corrections need to be under control to draw robust conclusions, most notably for $\tau\to\pi\pi\nu_\tau$ decays to determine the two-pion contribution, $a_\mu^\text{HVP, LO}[\pi\pi,\tau]$. In this work, we present a novel analysis of the required radiative corrections using dispersion relations, thereby extending in a model-independent way the previous analysis in chiral perturbation theory (ChPT) beyond the threshold region. In particular, we include the dominant structure-dependent virtual corrections from pion-pole diagrams, leading to sizable changes in the vicinity of the $\rho(770)$ resonance. Moreover, we work out the matching to ChPT and devise a strategy for a stable numerical evaluation of real-emission contributions near the two-pion threshold, which proves important to capture isospin-breaking corrections enhanced by the threshold singularity. For the numerical analysis, we use a dispersive representation of the pion form factor including the $\rho'$, $\rho''$ resonances, perform fits to the available data sets for the $\tau\to\pi\pi\nu_\tau$ spectral function, and calculate the corresponding radiative correction factor $G_\text{EM}(s)$ in a self-consistent manner. Based on these results, we evaluate the $\tau$-specific isospin-breaking corrections to $a_\mu^\text{HVP, LO}[\pi\pi,\tau]$.

hep-ph↗

arXiv2025

Light new physics and the $\tau$ lepton dipole moments

Testing New-Physics (NP) scenarios that couple predominantly to the third generation is notoriously difficult experimentally, as exemplified by comparing limits for the $\tau$ lepton dipole moments to those of electrons and muons. In this case, extracting limits from processes such as $e^+e^-\to\tau^+\tau^-$ often relies on effective-field-theory (EFT) arguments, which allow for model-independent statements, but only apply if the NP scale is sufficiently large compared to the center-of-mass energy. In this work we offer a comprehensive analysis of light NP contributions to the $\tau$ dipole moments, providing a detailed account of the interpretation of asymmetry measurements in $e^+e^-\to\tau^+\tau^-$ that are tailored towards the extraction of dipole moments, for the test cases of new light spin-$0$ and spin-$1$ bosons. Moreover, we study the decoupling to the EFT limit in these scenarios and discuss the complementarity to constraints from other related processes, such as production in $e^+e^-$ reactions. While covering a wide range of light NP scenarios, as specific case study we present a detailed discussion of a tauphilic gauge vector boson at Belle II.

hep-ph↗

arXiv2025

Improved calculation of radiative corrections to $\tau\to\pi\pi\nu_\tau$ decays

A reliable calculation of radiative corrections to $\tau\to\pi\pi\nu_\tau$ decays is an important prerequisite for using hadronic $\tau$ decays for a data-driven evaluation of the hadronic-vacuum-polarization contribution to the anomalous magnetic moment of the muon, $a_\mu^\text{HVP, LO}[\pi\pi,\tau]$. In this Letter, we present an improved model-independent analysis of these radiative corrections, including, for the first time, effects beyond point-like pions in the evaluation of the loop diagrams. These structure-dependent corrections, implemented via a dispersive representation of the pion form factor, lead to significant changes compared to previous calculations due to enhancements near the $\rho(770)$ resonance. We also devise strategies for the matching to chiral perturbation theory and a stable implementation of the real corrections down to the two-pion threshold, which shows that some higher-order isospin-breaking corrections need to be kept due to a strong threshold enhancement. Finally, we perform dispersive fits to the currently available $\tau\to\pi\pi\nu_\tau$ spectra and discuss the consequences for isospin-breaking corrections in the evaluation of $a_\mu^\text{HVP, LO}[\pi\pi,\tau]$.

hep-ph↗

arXiv2025

Light new physics and the $\tau$ lepton dipole moments: prospects at Belle II

While electron and muon dipole moments are well-established precision probes of physics beyond the standard model, it is notoriously challenging to test realistic new-physics (NP) scenarios for the $\tau$ lepton. Constructing suitable asymmetries in $e^+e^-\to\tau^+\tau^-$ has emerged as a promising such avenue, providing access to the electric and magnetic dipole moment once a polarized electron beam is available, e.g., with the proposed polarization upgrade of the SuperKEKB $e^+e^-$ collider. However, this interpretation relies on an effective-field-theory (EFT) argument that only applies if the NP scale is large compared to the center-of-mass energy. In this Letter we address the consequences of the asymmetry measurements in the case of light NP, using light spin-0 and spin-1 bosons as test cases, to show how results can again be interpreted as constraints on dipole moments, albeit in a model-dependent manner, and how the decoupling to the EFT limit proceeds in these cases. In particular, we observe that the imaginary parts generated by light new particles can yield nonvanishing asymmetries even without electron polarization, which can again be interpreted as constraints on the $\tau$ anomalous magnetic moment. This proposed measurement, thus, presents a novel opportunity for NP searches that can be realized already with present data at Belle II.

hep-ph↗

arXiv2025

Nucleon mass: trace anomaly and $\sigma$-terms

We give a pedagogical introduction to the origin of the mass of the nucleon. We first review the trace anomaly of the energy-momentum tensor, which generates most of the nucleon mass via the gluon fields and thus contributes even in the case of vanishing quark masses. We then discuss the contributions to the nucleon mass that do originate from the Higgs mechanism via the quark masses, reviewing the current status of nucleon $\sigma$-terms that encode the corresponding matrix elements.

hep-ph↗

arXiv2025

Towards testing $(g-2)_\tau$ in $e^+e^-\to\tau^+\tau^-$: radiative corrections and projections for Belle II

The arguably most promising avenue towards testing physics beyond the Standard Model in the anomalous magnetic moment of the $\tau$ proceeds via suitably constructed asymmetries in $e^+e^-\to\tau^+\tau^-$ in the presence of a polarized electron beam. Such a program, as could be realized at Belle II assuming a polarization upgrade of the SuperKEKB $e^+e^-$ collider, crucially relies on a careful consideration of radiative corrections. In this work, we present the complete one-loop result for the fully polarized $e^+e^-\to\tau^+\tau^-$ process and its implementation in the Monte-Carlo integrator McMule. As an application, we discuss projections relevant for measurements at Belle II, both with and without electron polarization, and outline the necessary steps for a generalization to next-to-next-to-leading order.

hep-ph↗

arXiv2025

Baryon Number Violation: From Nuclear Matrix Elements to BSM Physics

Processes that violate baryon number, most notably proton decay and $n\bar n$ transitions, are promising probes of physics beyond the Standard Model (BSM) needed to understand the lack of antimatter in the Universe. To interpret current and forthcoming experimental limits, theory input from nuclear matrix elements to UV complete models enters. Thus, an interplay of experiment, effective field theory, lattice QCD, and BSM model building is required to develop strategies to accurately extract information from current and future data and maximize the impact and sensitivity of next-generation experiments. Here, we briefly summarize the main results and discussions from the workshop "INT-25-91W: Baryon Number Violation: From Nuclear Matrix Elements to BSM Physics," held at the Institute for Nuclear Theory, University of Washington, Seattle, WA, January 13-17, 2025.

hep-ph↗

arXiv2025

Extracting the chiral anomaly from $e^+e^-\to 3\pi$

The strength of the interaction of three pions and a photon, $F_{3\pi}$ is predicted by the axial anomaly in terms of the pion decay constant, a relation that is frequently used to constrain low-energy radiative processes involving pions, but only tested experimentally at the $10\%$ level. Here, we present a new avenue to test this prediction, via a fit of a dispersive description of the $\gamma^*\to3\pi$ amplitude to data for $e^+e^-\to 3\pi$. From the global fit to SND, CMD-2, and BaBar data we obtain $F_{3\pi}=33.1(1.7)\,\text{GeV}^{-3}$, in agreement with the chiral prediction at the level of $5\%$. We also consider dispersive fits to the recent data by Belle II, in which case we observe tensions with the dispersive constraints, the width parameters of $\omega$ and $\phi$, and the chiral anomaly.

hep-ph↗

arXiv2025

Hadronic light-by-light scattering in the anomalous magnetic moments of electron and $\tau$

In Refs. [1,2] we provided a complete dispersive evaluation of the hadronic light-by-light (HLbL) contribution to the anomalous magnetic moment of the muon. While the evaluation strategy was developed for the kinematic situation determined by the muon mass, a similar approach also applies to the HLbL corrections to the anomalous magnetic moments of the electron and $\tau$ lepton, shifting the sensitivity in the loop integrals to smaller and larger momenta, respectively. In this Letter, we propagate the corresponding uncertainties of the various contributions, obtaining $a_e^\text{HLbL}= 3.51(23)\times 10^{-14}$ and $a_\tau^\text{HLbL}= 3.77(29)\times 10^{-8}$.

hep-ph↗

arXiv2025

Kaon Physics: A Cornerstone for Future Discoveries

The kaon physics programme, long heralded as a cutting-edge frontier by the European Strategy for Particle Physics, continues to stand at the intersection of discovery and innovation in high-energy physics (HEP). With its unparalleled capacity to explore new physics at the multi-TeV scale, kaon research is poised to unveil phenomena that could reshape our understanding of the Universe. This document highlights the compelling physics case, with emphasis on exciting new opportunities for advancing kaon physics not only in Europe but also on a global stage. As an important player in the future of HEP, the kaon programme promises to drive transformative breakthroughs, inviting exploration at the forefront of scientific discovery.

hep-ph↗

arXiv2025

Signs of Non-Monotonic Finite-Volume Corrections to $g_A$

We study finite-volume (FV) corrections to determinations of $g_A$ via lattice quantum chromodynamics (QCD) using analytic results and numerical analysis. We observe that $SU(2)$ Heavy Baryon Chiral Perturbation Theory does not provide an unambiguous prediction for the sign of the FV correction, which is not surprising when one also considers large-$N_c$ constraints on the axial couplings. We further show that non-monotonic FV corrections are naturally allowed when one considers either including explicit $\Delta$-resonance degrees of freedom or one works to higher orders in the chiral expansion. We investigate the potential impact of these FV corrections with a precision study of $g_A$ using models of FV corrections that are monotonic and non-monotonic. Using lattice QCD data that is approximately at the 1% level of precision, we do not see significant evidence of non-monotonic corrections. Looking forward to the next phase of lattice QCD calculations, we estimate that calculations that are between the 0.1%-1%-level of precision may be sensitive to these FV artifacts. Finally, we present an update of the CalLat prediction of $g_A$ in the isospin limit with sub-percent precision, $g_A^{\rm QCD} = 1.2674(96)$.

hep-lat↗

arXiv2025

Improved evaluation of the electroweak contribution to muon $g-2$

A precise evaluation of the electroweak contribution to the anomalous magnetic moment of the muon requires control over all aspects of the Standard Model, ranging from Higgs physics, over multi-loop computations for bosonic and (heavy-)fermion diagrams, to non-perturbative effects in the presence of light quarks. Currently, the dominant uncertainties arise from such hadronic effects in the vector-vector-axial-vector three-point function, an improved understanding of which has recently emerged in the context of hadronic light-by-light scattering. Profiting from these developments as well as new perturbative and non-perturbative input for the charm contribution, we obtain $a_\mu^\text{EW}=154.4(4)\times 10^{-11}$.

hep-ph↗