arXiv subjects

Rakshanda Goswami

Publications and source records attributed to Rakshanda Goswami.

4 recordsLinked to original sources

arXiv2026

Elastic lepton-proton two-photon exchange scattering: An exact HB$χ$PT analysis including hadronic effects at NNLO

We present an exact analytical evaluation of the two-photon exchange (TPE) correction to the elastic lepton-proton differential scattering cross section at low-energies within the framework of heavy-baryon chiral perturbation theory. Our analysis focuses on the kinematic regime relevant to the ongoing MUSE experiment, and we therefore restrict the intermediate states to the dominant elastic channel. All loop integrals are evaluated analytically without approximations. Radiative and chiral recoil contributions of the proton are included, retaining kinematical and dynamical TPE corrections to the cross section through next-to-next-to-leading order [i.e., ${\mathcal O}(α/M^2)$] accuracy in the recoil expansion where $M$ is the proton mass. At this chiral order, pion-loop contributions demonstrate that structure-dependent TPE effects arise through the proton form factors. Our analytical results for the scattering cross section reveal non-vanishing residual proton structure effects of ${\mathcal O}(α/M^2)$, despite substantial cancellations between TPE box and crossed-box contributions. Such effects were entirely absent at this accuracy in our earlier analysis based on the soft-photon approximation. Although the next-to-leading-order contributions are numerically sizable, the next-to-next-to-leading-order TPE corrections are found to be small, thereby indicating that the chiral expansion exhibits reasonably good perturbative convergence.

hep-ph↗

arXiv2025

Chiral enhancement in soft-photon bremsstrahlung and charge asymmetry in low-energy lepton-proton scattering

We carry out a Lorentz gauge evaluation of the single-soft photon bremsstrahlung radiative corrections to the unpolarized elastic lepton-proton scattering cross section at low energies using the framework of heavy baryon chiral effective field theory (HB$χ$PT). We systematically incorporate all next-to-leading order [i.e., ${\mathcal O}(α^3/M)$] contributions to the cross section arising from radiative and proton's low-energy recoil effects. An important component of the soft-photon bremsstrahlung corrections arises from the interference between lepton and proton bremsstrahlung processes, which are characterized as charge-odd. We identify a class of Feynman diagrams involving the final-state radiating proton that induces a chiral enhancement, thereby modifying the naive amplitude hierarchy based on the standard chiral power-counting scheme. The recently derived HB$χ$PT result for the analytically computed exact two-photon exchange (TPE) counterpart at NLO is combined with our charge-odd soft-photon bremsstrahlung contribution to yield the complete charge-odd radiative correction. By incorporating the leading hadronic correction to the leading order Born one-photon exchange (OPE) process within the charge-even radiative correction, we obtain a prediction for the charge asymmetry observable, which can be compared with forthcoming MUSE data on elastic lepton-anti-lepton scattering off the proton. In all our calculations, we use finite lepton masses.

hep-ph↗

arXiv2025

Lepton-proton two-photon exchange with improved soft-photon approximation including proton's structure effects in HB$χ$PT

We present an improved evaluation of the two-photon exchange correction to the unpolarized lepton-proton elastic scattering process at very low-energies relevant to the MUSE experiment, where only the dominant intermediate elastic proton is considered. We employ the framework of heavy baryon chiral perturbation theory and invoke the soft-photon approximation in order to reduce the intricate 4-point loop-integrals into simpler 3-point loop-integrals. In the present work, we adopt a more robust methodology compared to an earlier work along the same lines for analytically evaluating the loop-integrations, and incorporate important corrections at next-to-next-to-leading order. These include the proton's structure effects which renormalize the proton-photon interaction vertices and the proton's propagator. Finally, our results allow a model-independent estimation of the charge asymmetry for the scattering of unpolarized massive leptons and anti-leptons.

hep-ph↗

arXiv2024

Scale-(in)dependence in quantum 4-body scattering

We investigate the multi-channel 4-body scattering system using regularized 2- and 3-body contact interactions. The analysis determines the sensitivity of bound-state energies, scattering phase shifts and cross sections on the cutoff parameter ($λ$), and the energy gaps between scattering thresholds. The latter dependency is obtained with a 2-body scale fixed to an unnaturally large value and a floating 3-body parameter. Specifically, we calculate the binding energies of the shallow 3- and 4-body states, dimer-dimer and trimer-atom scattering lengths, and the trimer-atom to dimer-dimer reaction rates. Employing a potential renormalized by a large 2-body scattering length and a 3-body scale, we find all calculated observables to remain practically constant over the range $6\textrm{fm}^{-2}<λ<10\textrm{fm}^{-2}$. Divergences in scattering lengths emerge for critical 3-body parameters at which thresholds are degenerate. Such threshold effects are found to be independent of the regulator cutoff. Furthermore, at those critical points where the dimer-dimer and trimer-atom thresholds overlap, we predict an enhancement of the inelastic over the elastic scattering event. Such an inversion between elastic- and rearrangement-collision probabilities indicates a strong sensitivity of the 4-body reaction dynamics on the 3-body parameter at finite 2-body scale. This phenomenon is absent in earlier studies which differ in the renormalization scheme. As this discrepancy arises for all considered cutoffs, a more comprehensive parametrization of short-distance structure is necessary: sole cutoff variation does not reveal non-perturbative change in reaction rates conjectured to be due to a combined effect of the finite 2-body range and the specific choice for the 3-body parameter.

nucl-th↗