Hadronic Contribution Research Articles

Leading hadronic contribution to the muon g − 2 from lattice QCD

We compute the leading order hadronic vacuum polarization contribution to the anomalous magnetic moment of the muon. The calculations are performed using four flavors of stout smeared staggered quarks, with quark masses at their physical values. The continuum limit is taken using six different lattice spacings ranging from 0.132 fm down to 0.064 fm. All strong isospin breaking and electromagnetic effects are accounted for to leading order. A controlled infinite volume limit is taken thanks to dedicated simulations performed in box sizes up to 11 fm. Putting all these ingredients together, we find [(gµ − 2)/2]LO−HVP = 707.5[5.5] 10−10, which has a total uncertainty of 0.8%. Compared to determinations based on the e+e− → hadrons cross section, our result significantly reduces the tension between the standard model prediction for the muon g − 2 and its experimental value.

Hadronic contributions to the muon g − 2 in holographic QCD

We discuss the recent progress made in using bottom-up holographic QCD models in calculating hadronic contributions to the anomalous magnetic moment of the muon, in particular the hadronic light-by-light scattering contribution, where holographic QCD naturally satisfies the Melnikov-Vainshtein constraint by an infinite series of axial vector meson contributions.

MUonE, muon g−2 and electroweak precision constraints within 2HDMs

Two Higgs doublet models are attractive scenarios for physics beyond the Standard Model. In particular, lepton-specific manifestations remain contenders to explain the observed discrepancy between the anomalous magnetic moment of the muon $a_\mu$ predicted within the Standard Model and recent observations at Fermilab and BNL. Dominant uncertainties that affect $a_\mu$ have motivated the MUonE experiment to access the hadronic vacuum polarisation contribution that impacts $a_\mu$ via elastic muon-electron scattering. In this work, we contrast the high precision that is achievable within the MUonE context with constraints from flavour physics, precision electroweak constraints and LHC searches as well as their extrapolations for a range of two Higgs doublet models with a softly broken $\mathbb{Z}_2$ symmetry. We find that the sensitivity of MUonE does not extend beyond the parameter regions that are already excluded by other constraints. MUonE will therefore provide a detailed measurement of the hadronic vacuum polarisation contribution which then transparently informs $a_\mu$ interpretations in 2HDMs without modifications of correlations from beyond the Standard Model interactions. In passing we extend earlier results of LHC and flavour projections to lepton-specific 2HDM (Types X and Y) scenarios, and comment on the possibility of modifying the value of the W-boson mass; we briefly discuss the implications for a strong first-order electroweak phase transition for these models.

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.

Window observable for the hadronic vacuum polarization contribution to the muon g−2 from lattice QCD

Euclidean time windows in the integral representation of the hadronic vacuum polarization contribution to the muon $g-2$ serve to test the consistency of lattice calculations and may help in tracing the origins of a potential tension between lattice and data-driven evaluations. In this paper, we present results for the intermediate time window observable computed using O($a$) improved Wilson fermions at six values of the lattice spacings below 0.1\,fm and pion masses down to the physical value. Using two different sets of improvement coefficients in the definitions of the local and conserved vector currents, we perform a detailed scaling study which results in a fully controlled extrapolation to the continuum limit without any additional treatment of the data, except for the inclusion of finite-volume corrections. To determine the latter, we use a combination of the method of Hansen and Patella and the Meyer-Lellouch-L\"uscher procedure employing the Gounaris-Sakurai parameterization for the pion form factor. We correct our results for isospin-breaking effects via the perturbative expansion of QCD+QED around the isosymmetric theory. Our result at the physical point is $a_\mu^{\mathrm{win}}=(237.30\pm0.79_{\rm stat}\pm1.22_{\rm syst})\times10^{-10}$, where the systematic error includes an estimate of the uncertainty due to the quenched charm quark in our calculation. Our result displays a tension of 3.9$\sigma$ with a recent evaluation of $a_\mu^{\mathrm{win}}$ based on the data-driven method.

Muon g − 2 from the lattice

We briefly report on the status of lattice calculations for the hadronic contributions to the anomalous magnetic moment of the muon. After the analysis of the first run of the experiment, the Fermilab's Muon g−2 experiment published a new measurement leading to a 4.2σ tension with the theory estimate published by the Muong−2Theory Initiative. Both the theory estimate and the experimental value have comparable uncertainties and the theory error is dominated by hadronic contributions. In this report, we describe the main challenges of such lattice calculations and discuss recent results not yet taken into account in the previous theory estimate.

Hadronic vacuum polarization contributions to the muon g-2 in the space-like region

We present simple analytic expressions to compute the hadronic vacuum polarization contribution to the muon g-2 in the space-like region up to next-to-next-to-leading order. These results can be employed in lattice QCD calculations of this contribution as well as in space-like determinations based on scattering data, like that expected from the proposed MUonE experiment at CERN.

Exploring the Hadronic Origin of LHAASO J1908+0621

Recent observations by the Large High Altitude Air Shower Observatory (LHAASO) have paved the way for the observational detection of PeVatrons in the Milky Way, thus revolutionizing the field of γ-ray astrophysics. In this paper, we study one such detected source, LHAASO J1908+0621, and explore the origin of multi-TeV γ-ray emission from this source. A middle-aged radio supernova remnant SNR G40.5–0.5 and a GeV pulsar PSR J1907+0602 are cospatial with LHAASO J1908+0621. Dense molecular clouds are also found to be associated with SNR G40.5–0.5. We explain the multi-TeV γ-ray emission observed from the direction of LHAASO J1908+0621, by the hadronic interaction between accelerated protons that escaped from the SNR shock front and cold protons present inside the dense molecular clouds, and the leptonic emission from the pulsar wind nebula (PWN) associated with the pulsar J1907+0602. Moreover, we explain lower energy γ-ray emission by considering the radiative cooling of the electrons that escaped from SNR G40.5–0.5. Finally, the combined lepto-hadronic scenario was used to explain the multiwavelength spectral energy distribution of LHAASO J1908+0621. Although not yet significant, an IceCube hotspot of neutrino emission is spatially associated with LHAASO J1908+0621, indicating a possible hadronic contribution. In this paper, we show that if a hadronic component is present in LHAASO J1908+0621, then the second-generation IceCube observatory will detect neutrinos from this source.

Probing light mediators at the MUonE experiment

The MUonE experiment, that aims to provide a precise measurement of the hadronic vacuum polarization contribution to the muon $g-2$ via elastic muon-electron scattering, has also the potential to explore the parameter space of light new physics. Exploiting the process $\mu^- N \to \mu^- N X$, where $N$ is the target nucleus and X is a new physics light mediator, we demonstrate that MUonE can be sensitive to new regions of parameter space for sub-GeV dark photons. In particular, thanks to its muon beam, MUonE will be able to explore uncharted parameter space regions for the $L_\mu-L_\tau$ model. Finally, we also find that MUonE can probe the parameter space of axion-like particles for different assumptions of the couplings to electrons, muons and photons.

Hadronic vacuum polarization contribution to the muon g−2 in holographic QCD

We evaluate the leading-order hadronic vacuum polarization contribution to the anomalous magnetic moment of the muon with two light flavors in minimal hard-wall and soft-wall holographic QCD models, as well as in simple generalizations thereof, and compare with the rather precise results available from dispersive and lattice approaches. While holographic QCD cannot be expected to shed light on the existing small discrepancies between the latter, this comparison in turn provides useful information on the holographic models, which have been used to evaluate hadronic light-by-light contributions where errors in data-driven and lattice approaches are more sizable. In particular, in the hard-wall model that has recently been used to implement the Melnikov-Vainshtein short-distance constraint on hadronic light-by-light contributions, a matching of the hadronic vacuum polarization to the data-driven approach points to the same correction of parameters that has been proposed recently in order to account for next-to-leading order effects.

Single π0 production in μe scattering at MUonE

The recently proposed MUonE experiment at CERN aims at providing a novel determination of the leading order hadronic contribution to the muon anomalous magnetic moment through the study of elastic muon-electron scattering at relatively small momentum transfer. The anticipated accuracy of the order of 10ppm demands for high-precision prediction in radiative corrections to the μe scattering as well as for robust quantitative estimates of all possible background processes. In this letter, the contribution due to the emission of a neutral pion through the process μe→μeπ0 is studied and its numerical impact is discussed in different phase space configurations by means of the upgraded Monte Carlo event generator MESMER. In fact, single π0 production could be a source of reducible background for the measurement of the QED running coupling constant at MUonE and it could be also an important background for possible New Physics searches at MUonE involving 2→3 processes, in phase space regions complementary to the ones characteristic of the elastic μe scattering.

Status of the MUonE experiment

The MUonE experiment has been proposed to measure the differential cross section of μe elastic scattering, by colliding the 160 GeV muons of the CERN M2 beam with atomic electrons of thin target plates. From a very precise measurement of the shape one can achieve a competitive determination of the leading hadronic contribution to the muon magnetic moment, independent from the other existing ones. In preparation for the Test Run with a reduced setup the detector geometry has been optimised. Expected yields for a first physics run with limited statistics are discussed, together with prospects for the assessment of the main systematic uncertainties.

Timelike and spacelike kernel functions for the hadronic vacuum polarization contribution to the muon anomalous magnetic moment

Abstract The complete set of relations, which mutually express the spacelike and timelike kernel functions for the hadronic vacuum polarization contribution to the muon anomalous magnetic moment a μ HVP in terms of each other, is obtained. By making use of the derived relations the explicit expression for the next-to-leading order spacelike kernel function, which enters the representation for a μ HVP involving the hadronic vacuum polarization function, is obtained. The corresponding next-to-leading order spacelike kernel function, which appears in the representation for a μ HVP involving the Adler function, is calculated numerically. The obtained results can be employed in the assessments of the hadronic vacuum polarization contribution to the muon anomalous magnetic moment in the framework of the spacelike methods, such as lattice studies, MUonE project, and others.

ChPT estimate of the strong-isospin-breaking contribution to the anomalous magnetic moment of the muon

First-principles lattice determinations of the Standard Model expectation for the leading order hadronic vacuum polarization contribution to the anomalous magnetic moment of the muon have become sufficiently precise that further improvement requires including strong and electromagnetic isospin-breaking effects. We provide a continuum estimate of the strong-isospin-breaking contribution, ${a}_{\ensuremath{\mu}}^{\mathrm{SIB}}$, using $SU(3)$ chiral perturbation theory. The result is shown to be dominated by resonance-region contributions encoded in a single low-energy constant whose value is known from flavor-breaking hadronic $\ensuremath{\tau}$ decay sum rules. Implications of the form of the result for lattice determinations of ${a}_{\ensuremath{\mu}}^{\mathrm{SIB}}$ are also discussed.

New physics in s → d semileptonic transitions: rare hyperon vs. kaon decays

We investigate the potential of rare hyperon decays to probe the short distance structure in the sto dvoverline{v} and s → dℓ+ℓ− transitions. Hyperon decays into neutrinos left({B}_1to {B}_2voverline{v}right) can be reliably predicted by using form factors determined in baryon chiral perturbation theory. Their decay rates are sensitive to different short-distance operators, as compared to their kaon counterparts, and the corresponding branching fractions are in the range of 10−14 ∼ 10−13 in the standard model. In the context of the low-energy effective theory, we find that the anticipated BESIII measurements of the {B}_1to {B}_2voverline{v} decays would lead to constraints on new physics in the purely axial vector overline{d}{gamma}_{mu }{gamma}_5s current that are stronger than the present limits from their kaon siblings Kto pi pi voverline{v} . On the other hand, although hyperon decays into charged leptons are dominated by long-distance hadronic contributions, angular observable such as the leptonic forward-backward asymmetry is sensitive to the interference between long- and short-distance contributions. We discuss the sensitivity to new physics of a potential measurement of this observable in comparison with observables in the kaon decays KL→ μ+μ− and K+→ π+μ+μ−. We conclude that the current kaon bounds are a few orders of magnitude better than those that could be obtained from Σ+→ pμ+μ− except for two scenarios with new physics in the left(overline{d}{gamma}^{mu }sright)left(overline{mathrm{ell}}{gamma}_{mu }{gamma}_5mathrm{ell}right) and left(overline{d}{gamma}^{mu }{gamma}_5sright)left(overline{mathrm{ell}}{gamma}_{mu}mathrm{ell}right) currents. Finally, we point out that the loop effects from renormalization group evolution are important in this context, when relating the low-energy effective field theory to new physics models in the UV.

Two-Loop Four-Fermion Scattering Amplitude in QED.

We present the first fully analytic evaluation of the transition amplitude for the scattering of a massless into a massive pair of fermions at the two-loop level in quantum electrodynamics. Our result is an essential ingredient for the determination of the electromagnetic coupling within scattering reactions, beyond the currently known accuracy, which has a crucial impact on the evaluation of the anomalous magnetic moment of the muon. It will allow, in particular, for a precise determination of the leading hadronic contribution to the (g-2)_{μ} in the MUonE experiment at CERN, and therefore can be used to shed light on the current discrepancy between the standard model prediction and the experimental measurement for this important physical observable.

Data-driven approaches to the evaluation of hadronic contributions to the (g − 2)μ

In this talk I reviewed the data-driven theoretical calculation of the hadronic contributions to the anomalous magnetic moment of the muon in the Standard Model mainly as it has been presented in the White Paper, but also including the most recent developments. All this is presented in the light of the new measurement of (g − 2)μ recently released by the Fermilab experiment, which led to an increase of the discrepancy with the Standard Model from 3.7 to 4.2σ.

Constraints on Hadronic Contributions to LHAASO Sources with Neutrino Observations

Abstract The Large High Altitude Air Shower Observatory (LHAASO) detected 12 gamma-ray sources above 100 TeV, which are the possible origins of Galactic cosmic-rays. We summarize the neutrino measurements by IceCube and ANTARES in the vicinity of LHAASO sources to constrain the contribution of hadronic gamma-rays in these sources. We find that the current observations constrain hadronic gamma-rays to contribute no more than ∼60% of the gamma-rays from the Crab Nebula. Gamma-rays from two LHAASO sources, LHAASO J1825−1326 and LHAASO J1907+0626, are dominated by leptonic components up to ∼200 TeV, under the hypotheses in the analysis by IceCube. The uncertainties of the constraint on the hadronic gamma-ray emission are discussed. We also constrain the total 100 TeV gamma-ray emission from TeV pulsar wind nebulae by relying on the remarkable sensitivity of LHAASO at that energy.

Novel evaluation of the hadronic contribution to the muon’s g−2 from QCD

We evaluate the hadronic contribution to the $g-2$ of the muon by deriving the low-energy limit of quantum chromodynamics (QCD) and computing in this way the hadronic vacuum polarization. The low-energy limit is a non-local Nambu--Jona-Lasinio (NJL) model that has all the parameters fixed from QCD, and the only experimental input used is the confinement scale that is known from measurements of hadronic physics. Our estimations provide a novel analytical alternative to the current lattice computations and we find that our result is close to the similar computation performed from experimental data. We also comment on how this analytical approach technique, in general, may provide prospective estimates for hadronic computations from dark sectors and its implication in BSM model-building in future.

Study of the process e+e−→π+π−π0 using initial state radiation with BABAR

The process e+e−→π+π−π0γ is studied at a center-of-mass energy near the ϒ(4S) resonance using a data sample of 469 fb−1 collected with the BABAR detector at the PEP-II collider. We have performed a precise measurement of the e+e−→π+π−π0 cross section in the center-of-mass energy range from 0.62 to 3.5 GeV. In the energy regions of the ω and ϕ resonances, the cross section is measured with a systematic uncertainty of 1.3%. The leading-order hadronic contribution to the muon magnetic anomaly calculated using the measured e+e−→π+π−π0 cross section from threshold to 2.0 GeV is (45.86±0.14±0.58)×10−10. From the fit to the measured 3π mass spectrum we have determined the resonance parameters Γ(ω→e+e−)B(ω→π+π−π0)=(0.5698±0.0031±0.0082) keV, Γ(ϕ→e+e−)B(ϕ→π+π−π0)=(0.1841±0.0021±0.0080) keV, and B(ρ→3π)=(0.88±0.23±0.30)×10−4. The significance of the ρ→3π signal is greater than 6σ. For the J/ψ resonance we have measured the product Γ(J/ψ→e+e−)B(J/ψ→3π)=(0.1248±0.0019±0.0026) keV.21 MoreReceived 4 October 2021Accepted 12 November 2021DOI:https://doi.org/10.1103/PhysRevD.104.112003Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasParticle interactionsPhysical SystemsLight mesonsQuarkoniaTechniquesLepton collidersParticle productionParticles & Fields