arXiv subjects

M. Vanderhaeghen

Publications and source records attributed to M. Vanderhaeghen.

At least 19 recordsLinked to original sources

arXiv2026

Subtracted Dispersion Relations for Virtual Compton Scattering off the Proton

We present a once-subtracted dispersion relation (DR) formalism for the virtual Compton scattering (VCS) process from threshold up to the $\Delta(1232)$ energy region. The formalism aims at extracting the nucleon's electric and magnetic generalized polarizabilities from the $e^- p \to e^- \gamma p$ process, in view of the precision goals of the present and near future experiments at Jefferson Lab. The present work improves upon the existing unsubtracted DR formalism in several ways. The required $s$- and $t$-channel discontinuities in the subtracted dispersion integrals are obtained in a largely data-driven manner in this energy region. The $s$-channel dispersive integrals are reconstructed from $\gamma^\ast p \to \pi N \to \gamma p$ using pion photo- and electro-production data, while the $t$-channel dispersion integrals are evaluated from $\gamma^\ast \gamma \to \pi \pi \to N \bar N$ using recent dispersive analyses of both the $\gamma^\ast \gamma \to \pi \pi$ and $\pi \pi \to N \bar N$ processes. We compare our results to VCS data and show the sensitivity of the observables to the nucleon's scalar generalized polarizabilities, which enter the present formalism as subtraction constants.

hep-ph↗

arXiv2023

Non-forward radiative corrections to electron-carbon scattering

Radiative corrections to elastic scattering represent an important part of the interpretation of electron-induced nuclear reactions at small energy transfers, where they make for a dominant part of background. Here we present and validate a new event generator for mimicking QED radiative processes in electron-carbon scattering that exactly calculates the coherent sum of the Bethe-Heitler amplitudes for the leading diagrams and can be reliably employed for a more robust determination of inelastic cross-sections.

hep-ph↗

arXiv2022

Theoretical analysis of the extraction of neutron skin thickness from coherent π0 photoproduction off nuclei

Background: Coherent π0 photoproduction on heavy nuclei has been suggested as a reliable tool to infer neutron skin thicknesses. To this aim, various experiments have been performed, especially on 208Pb. Purpose: We analyze the sensitivity of that reaction process to the nucleonic density, and especially to the neutron skin thickness, for 12C, 40Ca and 208Pb, for which reliable data exist, and on 116,124Sn, for which measurements have been performed in Mainz. We study also the role played by the π0-nucleus final-state interaction. Method: A model of the reaction is developed at the impulse approximation considering either plane waves or distorted waves to describe the π0-nucleus scattering in the outgoing channel. Results: Our calculations are in good agreement with existing data, especially for 208Pb. The sensitivity of the theoretical cross sections to the choice of the nucleonic density is small, and below the experimental resolution. Conclusions: Coherent π0 photoproduction is mostly an isoscalar observable that bares no practical sensitivity to the neutron skin thickness. To infer that structure observable it should be coupled to other reaction measurements, such as electron scattering, or by comparing experiments performed on isotopes of the same chemical element.

nucl-th↗

arXiv2021

Virtual Compton scattering at low energies with a positron beam

The Virtual Compton scattering (VCS) process at low energies explores the electromagnetic structure of the proton in terms of generalized polarizabilities (GPs). In the one-photon exchange approximation, VCS can be accessed with exclusive photon production reactions with electron or positron beams. The extraction of the GPs from VCS with electron beam has seen substantial progress over the past two decades. Nonetheless, a consistent picture of the GPs from low to higher scales demands further investigations. Complementary measurements with positron beams offer an unique possibility, and we present an impact study of such experimental program.

hep-ph↗

arXiv2020

The anomalous magnetic moment of the muon in the Standard Model

We review the present status of the Standard Model calculation of the anomalous magnetic moment of the muon. This is performed in a perturbative expansion in the fine-structure constant $α$ and is broken down into pure QED, electroweak, and hadronic contributions. The pure QED contribution is by far the largest and has been evaluated up to and including $\mathcal{O}(α^5)$ with negligible numerical uncertainty. The electroweak contribution is suppressed by $(m_μ/M_W)^2$ and only shows up at the level of the seventh significant digit. It has been evaluated up to two loops and is known to better than one percent. Hadronic contributions are the most difficult to calculate and are responsible for almost all of the theoretical uncertainty. The leading hadronic contribution appears at $\mathcal{O}(α^2)$ and is due to hadronic vacuum polarization, whereas at $\mathcal{O}(α^3)$ the hadronic light-by-light scattering contribution appears. Given the low characteristic scale of this observable, these contributions have to be calculated with nonperturbative methods, in particular, dispersion relations and the lattice approach to QCD. The largest part of this review is dedicated to a detailed account of recent efforts to improve the calculation of these two contributions with either a data-driven, dispersive approach, or a first-principle, lattice-QCD approach. The final result reads $a_μ^\text{SM}=116\,591\,810(43)\times 10^{-11}$ and is smaller than the Brookhaven measurement by 3.7$σ$. The experimental uncertainty will soon be reduced by up to a factor four by the new experiment currently running at Fermilab, and also by the future J-PARC experiment. This and the prospects to further reduce the theoretical uncertainty in the near future-which are also discussed here-make this quantity one of the most promising places to look for evidence of new physics.

hep-ph↗

arXiv2019

Neutron skins of atomic nuclei: per aspera ad astra

The complex nature of the nuclear forces generates a broad range and diversity of observational phenomena. Heavy nuclei, though orders of magnitude less massive than neutron stars, are governed by the same underlying physics, which is enshrined in the nuclear equation of state. Heavy nuclei are expected to develop a neutron-rich skin where many neutrons collect near the surface. Such a skin thickness is strongly sensitive to the poorly-known density dependence of the symmetry energy near saturation density. An accurate and model-independent determination of the neutron-skin thickness of heavy nuclei would provide a significant first constraint on the density dependence of the nuclear symmetry energy. The determination of the neutron-skin thickness of heavy nuclei has far reaching consequences in many areas of physics as diverse as heavy-ion collisions, polarized electron and proton scattering off nuclei, precision tests of the standard model using atomic parity violation, and nuclear astrophysics. While a systematic and concerted experimental effort has been made to measure the neutron-skin thickness of heavy nuclei, a precise and model-independent determination remains elusive. How to move forward at a time when many new facilities are being commissioned and how to strengthen the synergy with other areas of physics are primary goals of this review.

nucl-ex↗

arXiv2019

Nucleon resonance contributions to unpolarised inclusive electron scattering

The first CLAS12 experiments will provide high-precision data on inclusive electron scattering observables at a photon virtuality $Q^2$ ranging from 0.05 GeV$^2$ to 12 GeV$^2$ and center-of-mass energies $W$ up to 4 GeV. In view of this endeavour, we present the modeling of the resonant contributions to the inclusive electron scattering observables. As input, we use the existing CLAS electrocoupling results obtained from exclusive meson electroproduction data off protons, and evaluate for the first time the resonant contributions based on the experimental results on the nucleon resonance electroexcitation. The uncertainties are given by the data and duly propagated through a Monte Carlo approach. In this way, we obtain estimates for the resonant contributions, important for insight into the nucleon parton distributions in the resonance region and for the studies of quark-hadron duality.

hep-ph↗

arXiv2018

Dispersion Theory in Electromagnetic Interactions

We review various applications of dispersion relations (DRs) to the electromagnetic structure of hadrons. We discuss the way DRs allow one to extract information on hadron structure constants by connecting information from complementary scattering processes. We consider the real and virtual Compton scattering processes off the proton, and summarize recent advances in the DR analysis of experimental data to extract the proton polarizabilities, in comparison with alternative studies based on chiral effective field theories. We discuss a multipole analysis of real Compton scattering data, along with a DR fit of the energy-dependent dynamical polarizabilities. Furthermore, we review new sum rules for the double-virtual Compton scattering process off the proton, which allow for model independent relations between polarizabilities in real and virtual Compton scattering, and moments of nucleon structure functions. The information on the double-virtual Compton scattering is used to predict and constrain the polarizability corrections to muonic hydrogen spectroscopy.

hep-ph↗

arXiv2018

Nucleon and nuclear structure through dilepton production

Transverse momentum distributions and generalized parton distributions provide a comprehensive framework for the three-dimensional imaging of the nucleon and the nucleus experimentally using deeply virtual semi-exclusive and exclusive processes. The advent of combined high luminosity facilities and large acceptance detector capabilities enables experimental investigation of the partonic structure of hadrons with time-like virtual probes, in complement to the rich on-going space-like virtual probe program. The merits and benefits of the dilepton production channel for nuclear structure studies are discussed within the context of the International Workshop on Nucleon and Nuclear Structure through Dilepton Production taking place at the European Center for Theoretical Studies in Nuclear Physics and Related Areas (ECT$^{\star}$) of Trento. Particularly, the double deeply virtual Compton scattering, the time-like Compton scattering, the deeply virtual meson production, and the Drell-Yan processes are reviewed and a strategy for high impact experimental measurements is proposed.

nucl-ex↗

arXiv2018

$χ_{cJ}\rightarrow e^{+}e^{-}$ decays revisited

We present a calculation of the width for $χ_{cJ}\rightarrow e^{+}e^{-}$ decay. The amplitude of the process is computed within the NRQCD framework. The leading-order contribution is described by two terms associated with the two different integration domains in the electromagnetic loop describing two-photon annihilation of the heavy quark-antiquark pair. The corresponding operators are defined in the framework of NRQCD. The matrix element of one of these operators describes a configuration with an ultrasoft photon and can be associated with the higher Fock state of the heavy meson. In order to compute this contribution we use the heavy hadron chiral perturbation theory. We obtain that this contribution is numerically dominant. The obtained estimates for the decay widths of the $χ_{c1}$ and $χ_{c2}$ states are $0.09$ eV and $0.06$ eV, respectively.

hep-ph↗

arXiv2017

Production of a tensor glueball in the reaction $γγ\rightarrow G_2π^0$ at large momentum transfer

We study the production of a tensor glueball in the reaction $γγ\rightarrow G_2π^0$. We compute the cross section at higher momentum transfer using the collinear factorisation approach. We find that for a value of the tensor gluon coupling of $f^T_g\sim 100$~MeV, the cross section can be measured in the near future by the Belle~II experiment.

hep-ph↗

arXiv2017

Two-photon exchange contribution to elastic $e^-$-proton scattering: Full dispersive treatment of $πN$ states and comparison with data

We evaluate the two-photon exchange correction to the elastic electron-proton scattering cross section within a dispersive framework. Besides the elastic contribution, we account for all $πN$ intermediate state contributions using the phenomenological MAID fit as an input. We develop a novel method for the analytical continuation of the two-photon exchange amplitudes into the unphysical region and generalize our previous work to the momentum transfer region $ 0.064~\mathrm{GeV}^2 \lesssim Q^2 \lesssim 1~\mathrm{GeV}^2$. We compare our results with recent OLYMPUS, CLAS and VEPP-3 data as well as with empirical fits and estimates in the forward angular region.

hep-ph↗

arXiv2017

Radiative decays $χ_{cJ}\rightarrow Vγ$ within the QCD factorisation framework

We present a discussion of radiative decays $χ_{cJ}\rightarrowγρ(ω, ϕ)$. The decay amplitudes are computed within the QCD factorisation framework. NRQCD has been used in the heavy meson sector as well as a collinear expansion in order to describe the overlap with light mesons in the final state. The colour-singlet contributions to all helicity amplitudes have been computed using the light-cone distribution amplitudes of twist-2 and twist-3. All obtained expressions are well defined at least in the leading-order approximation. The colour-octet contributions have also been studied in the Coulomb limit in order to exhibit their scaling behaviour. In order to understand the relevance of the different contributions we perform numerical estimates using the colour-singlet contributions. For the $χ_{c1}\rightarrow γV_\perp$ decays, to vector mesons with transverse polarisation, we find that the colour-singlet contribution potentially allows for a reliable description. On the other hand, for the $χ_{c1}\rightarrow γV_\Vert$ decays, to vector mesons with longitudinal polarisation, our finding indicate that the colour-octet mechanism may be important for a good description. We expect that more accurate measurements of the decay $χ_{c2}\rightarrowγV_{\perp,\Vert} $ can help to better understand the mechanism of radiative decays.

hep-ph↗

arXiv2017

Two-photon exchange corrections to elastic $e^-$-proton scattering: Full dispersive treatment of $πN$ states at low momentum transfers

We evaluate the pion-nucleon intermediate-state contribution to the two-photon exchange (TPE) correction in the elastic electron-nucleon scattering within a dispersive framework. We calculate the contribution from all $πN$ partial waves using the MAID parametrization. We provide the corresponding TPE correction to the unpolarized $e p$ scattering cross section in the region of low momentum transfer $ Q^2 \lesssim 0.064~\mathrm{GeV}^2$, where no analytical continuation into the unphysical region of the TPE scattering amplitudes is required. We compare our result in the forward angular region with an alternative TPE calculation, in terms of structure functions, and find a good agreement, indicating a small contribution at low $Q^2$ due to discontinuities beyond $ πN$. We also compare our results with empirical fits.

hep-ph↗

arXiv2016

First measurement of proton's charge form factor at very low $Q^2$ with initial state radiation

We report on a new experimental method based on initial-state radiation (ISR) in e-p scattering, in which the radiative tail of the elastic e-p peak contains information on the proton charge form factor ($G_E^p$) at extremely small $Q^2$. The ISR technique was validated in a dedicated experiment using the spectrometers of the A1-Collaboration at the Mainz Microtron (MAMI). This provided first measurements of $G_E^p$ for $0.001\leq Q^2\leq 0.004 (GeV/c)^2$.

nucl-ex↗

arXiv2016

Two-photon exchange correction to muon-proton elastic scattering at low momentum transfer

We evaluate the two-photon exchange (TPE) correction to the muon-proton elastic scattering at small momentum transfer. Besides the elastic (nucleon) intermediate state contribution, which is calculated exactly, we account for the inelastic intermediate states by expressing the TPE process approximately through the forward doubly virtual Compton scattering. The input in our evaluation is given by the unpolarized proton structure functions and by one subtraction function. For the latter, we provide an explicit evaluation based on a Regge fit of high-energy proton structure function data. It is found that, for the kinematics of the forthcoming muon-proton elastic scattering data of the MUSE experiment, the elastic TPE contribution dominates, and the size of the inelastic TPE contributions is within the anticipated error of the forthcoming data.

hep-ph↗

arXiv2016

Two-photon exchange correction in elastic unpolarized electron-proton scattering at small momentum transfer

We evaluate the two-photon exchange (TPE) correction to the unpolarized elastic electron-proton scattering at small momentum transfer $ Q^2 $. We account for the inelastic intermediate states approximating the double virtual Compton scattering by the unpolarized forward virtual Compton scattering. The unpolarized proton structure functions are used as input for the numerical evaluation of the inelastic contribution. Our calculation reproduces the leading terms in the $ Q^2 $ expansion of the TPE correction and goes beyond this approximation by keeping the full $ Q^2 $ dependence of the proton structure functions. In the range of small momentum transfer, our result is in good agreement with the empirical TPE fit to existing data.

hep-ph↗

arXiv2016

Subtracted dispersion relation formalism for the two-photon exchange correction to elastic electron-proton scattering: comparison with data

We apply a subtracted dispersion relation formalism with the aim to improve predictions for the two-photon exchange corrections to elastic electron-proton scattering observables at finite momentum transfers. We study the formalism on the elastic contribution, and make a detailed comparison with existing data for unpolarized cross sections as well as polarization transfer observables.

hep-ph↗