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Marc Vanderhaeghen

Publications and source records attributed to Marc Vanderhaeghen.

At least 73 records · Page 4Linked to original sources
arXiv2017

Theoretical analysis of the $γγ\to π^0 η$ process

We present a theoretical study of the $γγ\to πη$ process from the threshold up to 1.4 GeV in the $πη$ invariant mass. For the s-wave $a_0(980)$ resonance state we adopt a dispersive formalism using a coupled-channel Omnès representation, while the d-wave $a_2(1320)$ state is described as a Breit-Wigner resonance. An analytic continuation to the $a_0(980)$ pole position allows us to extract its two-photon decay width as $Γ_{a_0\toγγ}=0.27(4)$ keV.

hep-ph↗
arXiv2017

Light-by-light forward scattering sum rules for charmonium states

We apply three forward light-by-light scattering sum rules to charmonium states. We show that these sum rules imply a cancellation between charmonium bound state contributions, which are mostly known from the $γγ$ decay widths of these states, and continuum contributions above $D \bar D$ threshold, for which we provide a duality estimate. We also show that two of these sum rules allow to predict the yet unmeasured $γ^\ast γ$ coupling of the $χ_{c1}(1P)$ state, which can be tested at present high-luminosity $e^+ e^-$ colliders.

hep-ph↗
arXiv2017

Beam normal spin asymmetry for the $e p \to e Δ(1232)$ process

We calculate the single spin asymmetry for the $e p \to e Δ(1232)$ process, for an electron beam polarized normal to the scattering plane. Such single spin asymmetries vanish in the one-photon exchange approximation, and are directly proportional to the absorptive part of a two-photon exchange amplitude. As the intermediate state in such two-photon exchange process is on its mass shell, the asymmetry allows one to access for the first time the on-shell $Δ\to Δ$ as well as $N^\ast \to Δ$ electromagnetic transitions. We present the general formalism to describe the $e p \to e Δ$ beam normal spin asymmetry, and provide a numerical estimate of its value using the nucleon, $Δ(1232)$, $S_{11}(1535)$, and $D_{13}(1520)$ intermediate states. We compare our results with the first data from the Qweak@JLab experiment and give predictions for the A4@MAMI experiment.

hep-ph↗
arXiv2017

Analysis of Deeply Virtual Compton Scattering Data at Jefferson Lab and Proton Tomography

The CLAS and Hall A collaborations at Jefferson Laboratory have recently released new results for the ep->epγ reaction. We analyze these new data within the Generalized Parton Distribution formalism. Employing a fitter algorithm introduced and used in earlier works, we are able to extract from these data new constraints on the kinematical dependence of three Compton Form Factors. Based on experimental data, we subsequently extract the dependence of the proton charge radius on the quarks longitudinal momentum fraction.

hep-ph↗
arXiv2017

Spin-dependent sum rules connecting real and virtual Compton scattering verified

We present a detailed derivation of the two sum rules relating the spin polarizabilities measured in real, virtual, and doubly-virtual Compton scattering. For example, the polarizability $δ_{LT}$, accessed in inclusive electron scattering, is related to the spin polarizability $γ_{E1E1}$ and the slope of generalized polarizabilities $P^{(M1,M1)1}-P^{(L1,L1)1}$, measured in, respectively, the real and the virtual Compton scattering. We verify these sum rules in different variants of chiral perturbation theory, discuss their empirical verification for the proton, and prospect their use in studies of the nucleon spin structure.

hep-ph↗
arXiv2017

Light-by-light scattering sum rules in light of new data

We evaluate the light-quark meson contributions to three exact light-by-light scattering sum rules in light of new data by the Belle Collaboration, which recently has extracted the transition form factors of the tensor meson $f_2(1270)$ as well as of the scalar meson $f_0(980)$. We confirm a previous finding that the $η, η^\prime$ and helicity-2 $f_2(1270)$ contributions saturate one of these sum rules up to photon virtualities around 1 GeV$^2$. At larger virtualities, our sum rule analysis shows an important contribution of the $f_2(1565)$ meson and provides a first empirical extraction of its helicity-2 transition form factor. Two further sum rules allow us to predict the helicity-0 and helicity-1 transition form factors of the $f_2(1270)$ meson. Furthermore, our analysis also provides an update for the scalar and tensor meson hadronic light-by-light contributions to the muon's anomalous magnetic moment.

hep-ph↗
arXiv2017

Generalized polarizabilities of the nucleon in baryon chiral perturbation theory

The nucleon generalized polarizabilities (GPs), probed in virtual Compton scattering (VCS), describe the spatial distribution of the polarization density in a nucleon. They are accessed experimentally via the process of electron-proton bremsstrahlung ($ep\to epγ$) at electron-beam facilities, such as MIT-Bates, CEBAF (Jefferson Lab), and MAMI (Mainz). We present the calculation of the nucleon GPs and VCS observables at next-to-leading order in baryon chiral perturbation theory (B$χ$PT), and confront the results with the empirical information. At this order our results are predictions, in the sense that all the parameters are well-known from elsewhere. Within the relatively large uncertainties of our calculation we find good agreement with the experimental observations of VCS and the empirical extractions of the GPs. We find large discrepancies with previous chiral calculations---all done in heavy-baryon $χ$PT (HB$χ$PT)---and discuss the differences between B$χ$PT and HB$χ$PT responsible for these discrepancies.

hep-ph↗
arXiv2016

Two-photon exchange correction to $2S$-$2P$ splitting in muonic helium-3 ions

We calculate the two-photon exchange correction to the Lamb shift in muonic helium-3 ions within the dispersion relations framework. Part of the effort entailed making analytic fits to the electron-$^3$He quasielastic scattering data set, for purposes of doing the dispersion integrals. Our result is that the energy of the 2$S$ state is shifted downwards by two-photon exchange effects by 15.14(49) meV, in good accord with the result obtained from a potential model and effective field theory calculation.

nucl-th↗
arXiv2016

Accessing the real part of the forward $J/ψ$-p scattering amplitude from $J/ψ$ photo-production on protons around threshold

We provide an updated analysis of the forward $J/ψ$-p scattering amplitude, relating its imaginary part to $γp \to J/ψp$ and $γp \to c \bar c X$ cross section data, and calculating its real part through a once-subtracted dispersion relation. From a global fit to both differential and total cross section data, we extract a value for the spin-averaged $J/ψ$-p s-wave scattering length $a_{ψp} = 0.046 \pm 0.005$ fm, which can be translated into a $J/ψ$ binding energy in nuclear matter of $B_ψ= 2.7 \pm 0.3$ MeV. We estimate the forward-backward asymmetry to the $γp \to e^- e^+ p$ process around the $J/ψ$ resonance, which results from interchanging the leptons in the interference between the $J/ψ$ production and the Bethe-Heitler mechanisms. We show that to good approximation this asymmetry depends linearly on $a_{ψp}$, and can reach values around -25% for forthcoming $J/ψ$ threshold production experiments at Jefferson Lab. Its measurement can thus provide a very sensitive observable for a refined extraction of $a_{ψp}$.

hep-ph↗
arXiv2016

Tomographic image of the proton

We determine, based on the latest experimental Deep Virtual Compton Scattering experimental data, the dependence of the spatial size of the proton on the quark's longitudinal momentum. This results in a three-dimensional momentum-space image and tomography of the proton.

hep-ph↗
arXiv2015

Ballpark prediction for the hadronic light-by-light contribution to the muon (g-2)_μ

Using the momentum dependence of the dressed quark mass and the well-known formulae for the mass dependent quark-loop contribution to the light-by-light scattering insertions, we compute the hadronic light-by-light contribution to the muon anomalous magnetic moment. The relevant momentum running in the quark loop is calculated from the $π^0$ exchange contribution to the light- by-light scattering. Special emphasis on the reconstruction of the pseudoscalar transition form factor is made, and the $π^0$ contribution to the hadronic light-by-light is, as well, updated.

hep-ph↗
arXiv2015

Lepton universality test in the photoproduction of $e^- e^+$ versus $μ^- μ^+$ pairs on a proton target

In view of the significantly different proton charge radius extracted from muonic hydrogen Lamb shift measurements as compared to electronic hydrogen spectroscopy or electron scattering experiments, we study in this work the photoproduction of a lepton pair on a proton target in the limit of very small momentum transfer as a way to provide a test of the lepton universality when extracting the proton charge form factor. By detecting the recoiling proton in the $γp \to l^- l^+ p$ reaction, we show that a measurement of a ratio of $e^-e^+ + μ^-μ^+$ over $e^-e^+$ cross sections with a relative precision of around 2%, would allow for a test to distinguish between the two different proton charge radii currently extracted from muonic and electronic observables.

hep-ph↗
arXiv2015

Timelike Compton scattering off the neutron

We study the exclusive photoproduction of an electron-positron pair on a neutron target in the Jefferson Lab energy domain. The reaction consists of two processes: the Bethe-Heitler and the Timelike Compton Scattering. The latter process provides potentially access to the Generalized Parton Distributions (GPDs) of the nucleon. We calculate all the unpolarized, single- and double-spin observables of the reaction and study their sensitivities to GPDs.

hep-ph↗
arXiv2015

Single and double polarization observables in timelike Compton scattering off proton

We study the exclusive photoproduction of a lepton pair off the proton with the aim of studying the proton quark structure via the Generalized Parton Distributions (GPD) formalism. After deriving the amplitudes of the processes contributing to the $γP\to P' e^+e^-$, the Timelike Compton Scattering and the Bethe-Heitler process, we calculate all unpolarized, single- and double- beam-target spin observables in the valence region in terms of GPDs.

hep-ph↗
arXiv2014

Polarizability relations across real and virtual Compton scattering processes

We derive two relations involving spin polarizabilities of a spin-1/2 particle and consider their empirical implications for the proton. Using the empirical values of the proton anomalous magnetic moment, electric and magnetic charge radii, moments of the spin structure functions $g_1$, $g_2$, and of two spin polarizabilities, the present relations constrain the low-momentum behavior of generalized polarizabilities appearing in virtual Compton scattering. In the case of the proton, the dispersive model evaluations of the spin and generalized polarizabilities appear to be consistent with these relations. The ongoing measurements of different electromagnetic observables at the MAMI, Jefferson Lab, and HI$γ$S facilities may be able to put these relations to a test, or use them to unravel the low-energy spin structure of the nucleon.

nucl-th↗
arXiv2014

Two-loop massive scalar three-point function in a dispersive approach

We present a dispersion relation formalism to calculate a massive scalar two-loop vertex function. Such calculation is of direct relevance in the evaluation of the hadronic light-by-light contribution to the muon's anomalous magnetic moment due to meson poles. The discontinuity of the two-loop diagram is obtained by a sum of two- and three-particle cut contributions, which involve a phase space integration over the physical intermediate states. The real part of the vertex function is subsequently reconstructed through evaluation of a dispersion integral. We explicitly demonstrate that the dispersive formalism yields exactly the same result as the direct two-loop calculation.

hep-ph↗
arXiv2014

Anomalous magnetic moment of the muon in a dispersive approach

We present a new general dispersive formalism for evaluating the hadronic light-by-light scattering contribution to the anomalous magnetic moment of the muon. In the suggested approach, this correction is related to the imaginary part of the muon's electromagnetic vertex function. The latter may be directly related to measurable hadronic processes by means of unitarity and analyticity. As a test we apply the introduced formalism to the case of meson pole exchanges and find agreement with the direct two-loop calculation.

hep-ph↗