arXiv subjects

Martin Hoferichter

Publications and source records attributed to Martin Hoferichter.

At least 55 records · Page 3Linked to original sources

arXiv2024

Prospects for PIONEER

Pion $β$ decay, $π^+ \toπ^0 e^+ ν_e$, offers a pristine way to measure the CKM matrix element $V_{ud}$ in a purely mesonic system, with excellent control over the hadronic matrix elements. We review the physics goals and current status of the PIONEER experiment, which aims at major improvements in the branching fractions for the $π^+\to e^+ ν_e$ decay in Phase I and for pion $β$ decay in Phases II and III of its experimental program, potentially leading to a measurement of $V_{ud}$ competitive with determinations from $β$ decays involving nucleons.

hep-ph↗

arXiv2023

Comparing phenomenological estimates of dilepton decays of pseudoscalar mesons with lattice QCD

Dilepton decays of pseudoscalar mesons have been drawing particular interest, thanks to their sensitivity to both the QCD dynamics at low energy and also signals beyond the Standard Model. In this context, we shortly review our recent study on an improved Standard-Model prediction for the rare decay $π^0\to e^+e^-$, and compare it with the first determination on the lattice that predicted also the $π^0\to γγ$ decay width as a byproduct. In addition, we discuss our recent work on $K_L\to\ell^+\ell^-$ decays and its connection to lattice QCD. We comment on the current uncertainty estimates and discuss how they could be improved profiting from future experiments and progress in lattice QCD.

hep-lat↗

arXiv2023

A phenomenological estimate of isospin breaking in hadronic vacuum polarization

Puzzles in the determination of the hadronic-vacuum-polarization contribution currently impede a conclusive interpretation of the precision measurement of the anomalous magnetic moment of the muon at the Fermilab experiment. One such puzzle concerns tensions between evaluations in lattice QCD and using $e^+e^-\to\text{hadrons}$ cross-section data. In lattice QCD, the dominant isospin-symmetric part and isospin-breaking (IB) corrections are calculated separately, with very different systematic effects. Identifying these two pieces in a data-driven approach provides an opportunity to compare them individually and trace back the source of the discrepancy. Here, we estimate the IB component of the lattice-QCD calculations from phenomenology, based on a comprehensive study of exclusive contributions that can be enhanced via infrared singularities, threshold effects, or hadronic resonances, including, for the first time, in the $e^+e^-\to3π$ channel. We observe sizable cancellations among different channels, with a sum that even suggests a slightly larger result for the QED correction than obtained in lattice QCD. We conclude that the tensions between lattice QCD and $e^+e^-$ data therefore cannot be explained by the IB contributions in the lattice-QCD calculations.

hep-ph↗

arXiv2023

Rescattering effects in nucleon-to-meson form factors and application to tau-lepton-induced proton decay

Nucleon decays put extremely stringent bounds on baryon-number-violating interactions. However, in case the corresponding operators involve only $τ$ leptons, the direct two-body decays, e.g., $p\toπ^0 τ^+$, are kinematically not allowed and nucleon decay can only proceed via an off-shell $τ$, leading to $p\to π^0\ell^+ν_\ell\barν_τ$. To calculate such processes, the momentum dependence of the form factors for nucleon-to-meson transitions, which describe the hadronization of the underlying process at the quark level, is needed. In this work, we point out new isospin and Fierz relations among such proton-kaon matrix elements and calculate the momentum dependence of the nucleon-to-meson form factors from the universal final-state interactions in terms of pion-nucleon or kaon-nucleon scattering phase shifts. We use these results to derive novel limits on the Wilson coefficients of the baryon-number-violating dimension-$6$ operators involving a $τ$ lepton, which were previously unconstrained.

hep-ph↗

arXiv2023

Isospin-breaking effects in the three-pion contribution to hadronic vacuum polarization

Isospin-breaking (IB) effects are required for an evaluation of hadronic vacuum polarization at subpercent precision. While the dominant contributions arise from the $e^+e^-\toπ^+π^-$ channel, also IB in the subleading channels can become relevant for a detailed understanding, e.g., of the comparison to lattice QCD. Here, we provide such an analysis for $e^+e^-\to 3π$ by extending our dispersive description of the process, including estimates of final-state radiation (FSR) and $ρ$-$ω$ mixing. In particular, we develop a formalism to capture the leading infrared-enhanced effects in terms of a correction factor $η_{3π}$ that generalizes the analog treatment of virtual and final-state photons in the $2π$ case. The global fit to the $e^+e^-\to 3π$ data base, subject to constraints from analyticity, unitarity, and the chiral anomaly, gives $a_μ^{3π}|_{\leq 1.8\,\text{GeV}}=45.91(53)\times 10^{-10}$ for the total $3π$ contribution to the anomalous magnetic moment of the muon, of which $a_μ^\text{FSR}[3π]=0.51(1)\times 10^{-10}$ and $a_μ^{ρ\text{-}ω}[3π]=-2.68(70)\times 10^{-10}$ can be ascribed to IB. We argue that the resulting cancellation with $ρ$-$ω$ mixing in $e^+e^-\to 2π$ can be understood from a narrow-resonance picture, and provide updated values for the vacuum-polarization-subtracted vector-meson parameters $M_ω=782.70(3)\,\text{MeV}$, $M_ϕ=1019.21(2)\,\text{MeV}$, $Γ_ω=8.71(3)\,\text{MeV}$, and $Γ_ϕ=4.27(1)\,\text{MeV}$.

hep-ph↗

arXiv2023

Axial-vector transition form factors and $e^+ e^- \to f_1 π^+ π^-$

We study the transition form factors (TFFs) of axial-vector mesons in the context of currently available experimental data, including new constraints from $e^+ e^- \to f_1(1285) π^+ π^-$ that imply stringent limits on the high-energy behavior and, for the first time, allow us to provide an unambiguous determination of the couplings corresponding to the two antisymmetric TFFs. We discuss how these constraints can be implemented in a vector-meson-dominance picture, and, in combination with contributions from the light-cone expansion, construct TFFs as input for the evaluation of axial-vector contributions to hadronic light-by-light scattering in the anomalous magnetic moment of the muon.

hep-ph↗

arXiv2023

Puzzles in the hadronic contributions to the muon anomalous magnetic moment

We summarize recent developments in the Standard-Model evaluation of the anomalous magnetic moment of the muon $a_μ$, both in the hadronic-light-by-light and hadronic-vacuum-polarization contributions. The current situation for the latter is puzzling as we are confronted with multiple discrepancies that are not yet understood. We present updated fits of a dispersive representation of the pion vector form factor to the new CMD-3 data set and quantify the tensions with the other high-statistics $e^+e^-\toπ^+π^-$ experiments in the contribution to $a_μ$ in the energy range up to 1 GeV, as well as in the corresponding contribution to the intermediate Euclidean window.

hep-ph↗

arXiv2023

Width effects of broad new resonances in loop observables and application to $(g-2)_μ$

In the phenomenology of strong interactions most physical states acquire a substantial width, and thus can only be defined in a model-independent way by pole positions and residues of the $S$-matrix. This information is incorporated in the Källén-Lehmann representation, whose spectral function characterizes the shape of the resonance and can be constrained by the dominant decay channels. Here, we argue that similar effects become important whenever beyond-the-Standard-Model particles possess a sizable decay width - as possible for instance in cases with a large branching fraction to a dark sector or strongly coupled scenarios - and show how their widths can be incorporated in the calculation of loop observables. As an application, we consider the anomalous magnetic moment of the muon, including both the direct effect of new physics and the possible indirect impact of a broad light $Z'$ on $e^+e^-\to\text{hadrons}$ cross sections. Throughout, we provide results for a general spectral function and its reconstruction from the one-loop imaginary part, where the latter captures the leading two-loop effects.

hep-ph↗

arXiv2023

On the role of isospin violation in the pion-nucleon $σ$-term

In recent years, a persistent tension between phenomenological and lattice QCD determinations of the pion-nucleon $σ$-term $σ_{πN}$ has developed. In particular, lattice-QCD calculations have matured to the point that isospin-violating effects need to be included. Here, we point out that the standard conventions adopted in both fields are incompatible, with the data-driven extraction based on the charged-pion mass, but lattice-QCD conventions relying on the mass of the neutral pion to define the isospin limit. The corresponding correction amounts to $Δσ_{πN}=3.1(5)\,\text{MeV}$ when evaluated in chiral perturbation theory with low-energy constants determined from a Roy-Steiner analysis of pion-nucleon scattering as well as $σ_{πN}$ itself. It reduces the tension with lattice QCD, and should be included in the comparison to phenomenological determinations. We also update the extraction from pionic atoms accounting for the latest measurement of the width of pionic hydrogen, $σ_{πN}=59.0(3.5)\,\text{MeV}$, and provide the corresponding set of scalar couplings of the nucleon.

hep-ph↗

arXiv2023

Improved limits on lepton-flavor-violating decays of light pseudoscalars via spin-dependent $μ\to e$ conversion in nuclei

Lepton-flavor-violating decays of light pseudoscalars, $P=π^0,η,η'\toμe$, are stringently suppressed in the Standard Model up to tiny contributions from neutrino oscillations, so that their observation would be a clear indication for physics beyond the Standard Model. However, in effective field theory such decays proceed via axial-vector, pseudoscalar, or gluonic operators, which are, at the same time, probed in spin-dependent $μ\to e$ conversion in nuclei. We derive master formulae that connect both processes in a model-independent way in terms of Wilson coefficients, and study the implications of current $μ\to e$ limits in titanium for the $P\toμe$ decays. We find that these indirect limits surpass direct ones by many orders of magnitude.

hep-ph↗

arXiv2023

Scrutinizing CKM unitarity with a new measurement of the $K_{μ3}/K_{μ2}$ branching fraction

Precision tests of first-row unitarity of the Cabibbo-Kobayashi-Maskawa matrix currently display two intriguing tensions, both at the $3σ$ level. First, combining determinations of $V_{ud}$ from superallowed $β$ decays with $V_{us}$ from kaon decays suggests a deficit in the unitarity relation. At the same time, a tension of similar significance has emerged between $K_{\ell 2}$ and $K_{\ell 3}$ decays. In this Letter, we point out that a measurement of the $K_{\mu3}/K_{μ2}$ branching fraction at the level of $0.2\%$ would have considerable impact on clarifying the experimental situation in the kaon sector, especially in view of tensions in the global fit to kaon data as well as the fact that the $K_{\mu2}$ channel is currently dominated by a single experiment. Such a measurement, as possible for example at NA62, would further provide important constraints on physics beyond the Standard Model, most notably on the role of right-handed vector currents.

hep-ph↗

arXiv2023

Nucleon form factors and the pion-nucleon sigma term

This talk summarizes the progress made since Lattice 2021 in understanding and controlling the contributions of towers of multihadron excited states with mass gaps starting lower than of radial excitations, and in increasing our confidence in the extraction of ground state nucleon matrix elements. The most clear evidence for multihadron excited state contributions (ESC) is in axial/pseudoscalar form factors that are required to satisfy the PCAC relation between them. The talk examines the broader question--which and how many of the theoretically allowed positive parity states $N(\textbf p)π(-\textbf p)$, $N(\textbf 0)π(\textbf 0)π(\textbf 0)$, $N(\textbf p)π(\textbf 0)$, $N(\textbf 0)π(\textbf p),\ \ldots$ make significant contributions to a given nucleon matrix element? New data for the axial, electric and magnetic form factors are presented. They continue to show trends observed in Ref[1]. The N${}^2$LO $χ$PT analysis of the ESC to the pion-nucleon sigma term, $σ_{πN}$, has been extended to include the $Δ$ as an explicit degree of freedom [2]. The conclusion reached in Ref [3] that $N π$ and $N ππ$ states each contribute about 10 MeV to $σ_{πN}$, and the consistency between the lattice result with $N π$ state included and the phenomenological estimate is not changed by this improvement.

hep-lat↗

arXiv2022

A dispersive analysis of $η'\toπ^+π^-γ$ and $η'\to \ell^+\ell^-γ$

We present a dispersive representation of the $η'$ transition form factor that allows one to account, in a consistent way, for the effects of $ρ$-$ω$ mixing in both the isoscalar and the isovector contributions. Using this formalism, we analyze recent data on $η'\to π^+π^-γ$ to constrain the isovector part of the form factor, individually and in combination with data for the pion vector form factor. As a first application, we use our results, in combination with the most recent input for the isoscalar part of the form factor, to predict the corresponding spectrum of $η'\to\ell^+\ell^-γ$, in particular we find the slope parameter $b_{η'}=1.431(23)\, \text{GeV}^{-2}$. With forthcoming data on the latter process, our results establish the necessary framework to improve the evaluation of the $η'$-pole contribution to the anomalous magnetic moment of the muon using experimental input from both $η'$ decay channels.

hep-ph↗

arXiv2022

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

If physics beyond the Standard Model (BSM) explains the $4.2σ$ difference between the Standard-Model and measured muon anomalous magnetic moment, $a_μ$, minimal flavor violation predicts a shift in the analog quantity for the $τ$ lepton, $a_τ$, 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 $τ$-pair production in $e^+e^-\rightarrow Υ(nS) \rightarrow$ $τ^+τ^-$, $n=1,2,3$. $\text{Re}\, F_2^\text{eff}$ can be measured from $e^-$-helicity-dependent transverse and longitudinal asymmetries in $τ$-pair events, which requires a longitudinally polarized $e^-$ beam. We discuss how Belle II asymmetry measurements could probe $a_τ^\text{BSM}$ 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 $a_τ$.

hep-ph↗

arXiv2022

Chiral extrapolation of hadronic vacuum polarization and isospin-breaking corrections

By far the biggest contribution to hadronic vacuum polarization (HVP) arises from the two-pion channel. Its quark-mass dependence can be evaluated by combining dispersion relations with chiral perturbation theory, providing guidance on the functional form of chiral extrapolations, or even interpolations around the physical point. In addition, the approach allows one to estimate in a controlled way the isospin-breaking (IB) corrections that arise from the pion mass difference. As an application, we present an updated estimate of phenomenological expectations for electromagnetic and strong IB corrections to the HVP contribution to the anomalous magnetic moment of the muon. In particular, we include IB effects in the $\bar K K$ channel, which are enhanced due to the proximity of the $\bar K K$ threshold and the $ϕ$ resonance. The resulting estimates make it unlikely that the current tension between lattice-QCD and data-driven evaluations of the HVP contribution is caused by IB corrections.

hep-ph↗

arXiv2022

Isospin-breaking effects in the two-pion contribution to hadronic vacuum polarization

Isospin-breaking (IB) effects in the two-pion contribution to hadronic vacuum polarization (HVP) can be resonantly enhanced, if related to the interference of the $ρ(770)$ and $ω(782)$ resonances. This particular IB contribution to the pion vector form factor and thus the line shape in $e^+e^-\to π^+π^-$ can be described by the residue at the $ω$ pole - the $ρ$-$ω$ mixing parameter $ε_ω$. Here, we argue that while in general analyticity requires this parameter to be real, the radiative channels $π^0γ$, $ππγ$, $ηγ$ can induce a small phase, whose size we estimate as $δ_ε=3.5(1.0)^\circ$ by using a narrow-width approximation for the intermediate-state vector mesons. We then perform fits to the $e^+e^-\to π^+π^-$ data base and study the consequences for the two-pion HVP contribution to the anomalous magnetic moment of the muon, its IB part due to $ρ$-$ω$ mixing, and the mass of the $ω$ resonance. We find that the global fit does prefer a non-vanishing value of $δ_ε=4.5(1.2)^\circ$, close to the narrow-resonance expectation, but with a large spread among the data sets, indicating systematic differences in the $ρ$-$ω$ region.

hep-ph↗

arXiv2022

Consequences of chirally enhanced explanations of $(g-2)_μ$ for $h\to μμ$ and $Z\to μμ$

With the long-standing tension between experiment and Standard-Model (SM) prediction in the anomalous magnetic moment of the muon $a_μ$ recently reaffirmed by the Fermilab experiment, the crucial question becomes which other observables could be sensitive to the underlying physics beyond the SM to which $a_μ$ may be pointing. While from the effective field theory (EFT) point of view no direct correlations exist, this changes in specific new physics models. In particular, in the case of explanations involving heavy new particles above the electroweak (EW) scale with chiral enhancement, which are preferred to evade exclusion limits from direct searches, correlations with other observables sensitive to EW symmetry breaking are expected. Such scenarios can be classified according to the $SU(2)_L$ representations and the hypercharges of the new particles. We match the resulting class of models with heavy new scalars and fermions onto SMEFT and study the resulting correlations with $h\toμμ$ and $Z\toμμ$ decays, where, via $SU(2)_L$ symmetry, the latter process is related to $Z\toνν$ and modified $W$-$μ$-$ν$ couplings.

hep-ph↗

arXiv2022

The Anomalous Magnetic Moment of the Muon: Beyond the Standard Model via Chiral Enhancement

The anomalous magnetic moment of the muon displays a $4.2σ$ tension with the Standard-Model prediction, if $e^+e^-\to \text{hadrons}$ data are used for hadronic vacuum polarization. In these proceedings we review possible explanations of this anomaly in terms of heavy new particles. As the necessary effect is of the order of the electroweak Standard-Model contribution, viable explanations with TeV-scale physics must involve an enhancement factor; in particular, one can obtain the chirality flip of the dipole operator via a sizable coupling to the Higgs doublet instead of the small muon Yukawa coupling. Such Standard-Model extensions then also predict effects in Higgs and $Z$-boson decays to muons, with details depending on the $SU(2)_L\times U(1)_Y$ representations of the new particles. We first review the general case of chirally enhanced new physics, before discussing in more detail the concrete example of leptoquark models.

hep-ph↗