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1. Precision Measurement of the Beam-Normal Single-Spin Asymmetry in Forward-Angle Elastic Electron-Proton Scattering

   https://doi.org/10.1103/PhysRevLett.125.112502  

   Androic, D.; Armstrong, D.S.; Asaturyan, A; Bartlett, K.; Beaufait, J.; Beminiwattha, R.S.; Benesch, J.; Benmokhtar, F.; Birchall, J.; Carlini, R.D.; et al (September 2020, Physical review letters)

   A beam-normal single-spin asymmetry generated in the scattering of transversely polarized electrons from unpolarized nucleons is an observable related to the imaginary part of the two-photon exchange process. We report a 2% precision measurement of the beam-normal single-spin asymmetry in elastic electron-proton scattering with a mean scattering angle of θlab=7.9° and a mean energy of 1.149 GeV. The asymmetry result is Bn=−5.194±0.067(stat)±0.082 (syst) ppm. This is the most precise measurement of this quantity available to date and therefore provides a stringent test of two-photon exchange models at far-forward scattering angles (θlab→0) where they should be most reliable. 

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2. Determining the leading-order contact term in neutrinoless double β decay

   https://doi.org/10.1007/JHEP05(2021)289  

   Cirigliano, Vincenzo; Dekens, Wouter; de Vries, Jordy; Hoferichter, Martin; Mereghetti, Emanuele (May 2021, Journal of High Energy Physics)

   null (Ed.)

   A bstract We present a method to determine the leading-order (LO) contact term contributing to the nn → ppe − e − amplitude through the exchange of light Majorana neutrinos. Our approach is based on the representation of the amplitude as the momentum integral of a known kernel (proportional to the neutrino propagator) times the generalized forward Compton scattering amplitude n ( p 1 ) n ( p 2 ) W + ( k ) → $$ p\left({p}_1^{\prime}\right)p\left({p}_2^{\prime}\right){W}^{-}(k) $$ p p 1 ′ p p 2 ′ W − k , in analogy to the Cottingham formula for the electromagnetic contribution to hadron masses. We construct model-independent representations of the integrand in the low- and high-momentum regions, through chiral EFT and the operator product expansion, respectively. We then construct a model for the full amplitude by interpolating between these two regions, using appropriate nucleon factors for the weak currents and information on nucleon-nucleon ( NN ) scattering in the 1 S 0 channel away from threshold. By matching the amplitude obtained in this way to the LO chiral EFT amplitude we obtain the relevant LO contact term and discuss various sources of uncertainty. We validate the approach by computing the analog I = 2 NN contact term and by reproducing, within uncertainties, the charge-independence-breaking contribution to the 1 S 0 NN scattering lengths. While our analysis is performed in the $$ \overline{\mathrm{MS}} $$ MS ¯ scheme, we express our final result in terms of the scheme-independent renormalized amplitude $$ {\mathcal{A}}_{\nu}\left(\left|\mathbf{p}\right|,\left|\mathbf{p}^{\prime}\right|\right) $$ A ν p p ′ at a set of kinematic points near threshold. We illustrate for two cutoff schemes how, using our synthetic data for $$ {\mathcal{A}}_{\nu } $$ A ν , one can determine the contact-term contribution in any regularization scheme, in particular the ones employed in nuclear-structure calculations for isotopes of experimental interest. 

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3. I=3/2 Nπ scattering and the ∆(1232) resonance on $$N_f=2+1$$ CLS ensembles using the stochastic LapH method

   https://doi.org/10.22323/1.363.0039  

   Andersen, Christian Walther; Bulava, John; Hörz, Ben; Morningstar, Colin (January 2020, Proceedings of Science)

   Calculations of the elastic I=3/2 nucleon-pion scattering phase shifts on two lattice QCD ensembles with mπ=200MeV and 280MeV are presented. The ensembles both employ Nf=2+1 Wilson clover fermions. We determine the Δ(1232) resonance parameters from a finite volume scattering analysis. In one study the single partial wave simplification is employed to compute the p-wave amplitude while in the other we treat the partial wave mixing between s- and p-wave contributions. Fitting our data to a Breit-Wigner resonance model we find mΔ/mπ=7.13(9) and 4.75(5) on the two ensembles respectively, showing that for a lighter quark mass the resonance mass moves from near the Nπ threshold to near the Nππ threshold, in agreement with experiment. 

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4. QCD angular momentum in N → Δ transitions

   https://doi.org/10.1016/j.physletb.2023.138083  

   Kim, June-Young; Won, Ho-Yeon; Goity, Jose L; Weiss, Christian (September 2023, Physics Letters B)

   N → transitions offer new possibilities for exploring the isovector component of the QCD quark angular momentum (AM) operator causing the J u−d flavor asymmetry in the nucleon. We extend the concept of QCD AM to transitions between baryon states, using light-front densities of the energy-momentum tensor in transversely localized states. We calculate the N → transition AM in the 1/Nc expansion, connect it with the J u−d flavor asymmetry in the nucleon, and estimate the values using lattice QCD results. In the same setup, we connect the transition AM to the transition GPDs sampled in hard exclusive electroproduction processes with N → transitions, enabling experimental study of the transition AM. 

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5. Dipolar-octupolar correlations and hierarchy of exchange interactions in ${\mathrm{Ce}}_2{\mathrm{Hf}}_2{\mathrm{O}}_7$

   https://doi.org/10.1103/j451-ztvr  

   Porée, Victor; Bhardwaj, Anish; Lhotel, Elsa; Petit, Sylvain; Gauthier, Nicolas; Yan, Han; Pomjakushin, Vladimir; Ollivier, Jacques; Quilliam, Jeffrey A; Nevidomskyy, Andriy H; et al (November 2025, Physical Review B)

   High-resolution neutron spectroscopy on ${\mathrm{Ce}}_2{\mathrm{Hf}}_2{\mathrm{O}}_7$reveals a correlated state characterized by distinct dipolar scattering signals—quasielastic and inelastic contributions consistent with ‘photon’ and ‘spinon’ excitations in quantum spin ice. These signals coexist with weak octupolar scattering. Fits of thermodynamic data using numerical methods indicate a dominant octupolar exchange, $J_x$or $J_y$, with substantial dipolar $J_z$and minute dipole-octupole $J_{xz}$couplings. The $J_{xz}$value is corroborated by an independent fit of the neutron scattering amplitude balance between dipolar and octupolar ‘photon’ contributions, highlighting its importance to understand neutron scattering results in this family. ${\mathrm{Ce}}_2{\mathrm{Hf}}_2{\mathrm{O}}_7$enriches the landscape of dipole-octupole pyrochlore physics, and reveals a ‘quantum multipolar liquid’ where hybrid correlations involve multiple terms in the moment series expansion, opening questions regarding their intertwining and hierarchy in quantum phases. 

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