Nuclear Theory·nucl-th

Friday·November 7, 2025

8 papers—4 primary·4 cross-listed

05
Light new physics and the $τ$ lepton dipole moments
Martin Hoferichter🇨🇭 · Gabriele Levati🇨🇭
Testing New-Physics (NP) scenarios that couple predominantly to the third generation is notoriously difficult experimentally, as exemplified by comparing limits for the $τ$ lepton dipole moments to those of electrons and muons. In this case, extracting limits from processes such as $e^{+} e^{-} \to τ^{+} τ^{-}$ often relies on effective-field-theory (EFT) arguments, which allow for model-independent statements, but only apply if the NP scale is sufficiently large compared to the center-of-mass energy. In this work we offer a comprehensive analysis of light NP contributions to the $τ$ dipole moments, providing a detailed account of the interpretation of asymmetry measurements in $e^{+} e^{-} \to τ^{+} τ^{-}$ that are tailored towards the extraction of dipole moments, for the test cases of new light spin- $0$ and spin- $1$ bosons. Moreover, we study the decoupling to the EFT limit in these scenarios and discuss the complementarity to constraints from other related processes, such as production in $e^{+} e^{-}$ reactions. While covering a wide range of light NP scenarios, as specific case study we present a detailed discussion of a tauphilic gauge vector boson at Belle II.
↳ hep-phhep-exnucl-thPRD(2026)·12 citations
06
Characterizing the initial state of hydrodynamics in pp and pA collisions
Gabriel Rabelo-Soares🇧🇷 · Gojko Vujanovic🇨🇦 · Giorgio Torrieri🇧🇷
The observation of seeming hydrodynamic-like behavior in proton-proton and proton-nucleus collisions presents us with the conceptual problem of how the initial state of such a hydrodynamic evolution should be characterized. This is an issue because, while nuclei can reasonably be approximated as ``large'' systems w.r.t. the characteristic Fermi momentum of their constituents, this is no longer true for nucleons. Hence, one would need to match a ``quantum'' theory, whose observables are described via highly non-commuting operators, to a ``classical'' hydrodynamics. Operationally assuming a ``fast'' thermalization, we survey what kind of object is best suited to such a matching condition. We show that it cannot be any of the objects usually associated with ``the 3D structure of the nucleon'' but rather a measure associated with entanglement entropy.
↳ hep-phnucl-th3 citations
07
Exclusive photoproduction of a di-meson pair with large invariant mass
Saad Nabeebaccus🇬🇧 · David Perez🇫🇷 · Lech Szymanowski🇵🇱 · Samuel Wallon🇫🇷
We consider the exclusive photoproduction of a di-meson pair with large invariant mass, $γ N \to N^{'} M_{1} M_{2}$ , in the framework of collinear factorisation. The mesons considered $M_{1}$ and $M_{2}$ are either pions or rho mesons, charged or neutral. We consider the kinematic regime characterised by a large invariant mass of the two-meson system, and a small deflection of the nucleon in the centre-of-mass frame. In this kinematic domain, the amplitude factorises into a perturbative hard part and non-perturbative parts described by Generalised Parton Distributions (GPDs) and Distribution Amplitudes (DAs). We automate the calculation of the fully differential cross section at leading twist and leading order, and we present some numerical results at JLab 12 GeV kinematics. This class of processes provides yet more exclusive $2 \to 3$ channels that can be used to extract GPDs.
↳ hep-phhep-exnucl-exnucl-th1 citation
08
Emergence of kaonium as a sharp resonance in photon-photon to meson-meson cross-sections
Alireza Beygi🇩🇪 · S. P. Klevansky🇩🇪 · R. H. Lemmer🇿🇦
We calculate the binding energies of the hypothetical mesonic atom, $K^{+} K^{-}$ (kaonium), using the $K^{+} K^{-} \to K^{+} K^{-}$ elastic scattering amplitude. Our findings are in line with previously reported results, which involve solving an eigenvalue equation of the Kudryavtsev-Popov type. Using chiral perturbation theory, we show that kaonium manifests itself as a sharp resonance around 992 MeV accompanying $f_{0} (980)$ or $a_{0} (980)$ in cross-sections for processes $γ γ \to π^{0} π^{0}$ or $γ γ \to π^{0} η$ . The latter process is particularly striking: the peak at the kaonium resonance energy is highly pronounced, with the ratio of the cross-sections $σ (γ γ \to π^{0} η) / σ (γ γ \to π^{0} π^{0}) \approx 9$ . Due to the short lifetime of kaonium ( $\sim 1 0^{- 18}$ s) and its small decay width ( $\sim 0.4$ keV), direct detection of this exotic atom poses a significant challenge and requires high experimental resolution. However, we show that once the formation of kaonium is considered in the cross-section, a better fit to the available experimental data is obtained.
↳ hep-phnucl-thPRD(2026)·0 citations

Affiliations

first authorsco-authorsvia INSPIRE

Light new physics and the τ lepton dipole moments

Characterizing the initial state of hydrodynamics in pp and pA collisions

Exclusive photoproduction of a di-meson pair with large invariant mass

Emergence of kaonium as a sharp resonance in photon-photon to meson-meson cross-sections

Affiliations

Light new physics and the $τ$ lepton dipole moments