arXiv subjects

Ryan Plestid

Publications and source records attributed to Ryan Plestid.

At least 19 recordsLinked to original sources

arXiv2026

Explicitly on-shell currents in relativistic mean field models

Relativistic mean field models are a useful tool for modeling semi-leptonic scattering and photo production on nuclei. When using free-nucleon currents, it is often claimed that there exist so-called ``off-shell ambiguities''. Here we show that when the current is defined in terms of free-nucleon creation and annihilation operators, all ambiguities related to on-shell vs. off-shell Dirac algebra disappear. Genuine ambiguities persist because the current itself depends on the mean field responsible for nuclear binding; these would be fixed if a consistent background-field dependent current were used. As applications, we consider elastic scattering from a nucleus, and (very large) ambiguities that were previous reported in the literature in the context of coherent pion photoproduction. We explain how these ambiguities are removed by the procedure introduced herein.

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arXiv2026

The flavour of SU(15) composite quarks and leptons

We study the flavour structure of an $SU(15)_p$ confining chiral gauge theory in which the Standard Model (SM) quarks, leptons, and Higgs emerge as composite bound states. The couplings of two scalar fields in the conjugate antisymmetric ($\overline{\mathbf{105}}$) and conjugate symmetric ($\overline{\mathbf{120}}$) representations of $SU(15)_p$ provide two $SU(4)_F$ flavour-breaking spurions that generate both the SM Yukawa couplings and the flavour-changing processes. The up and down Yukawa matrices are tightly-correlated due to a "right-handed isospin" symmetry, which predicts a trivial CKM matrix in the absence of spontaneous symmetry breaking. The lepton Yukawas are correlated with the quarks due to a common source of flavour spurions. With a judicious Froggatt-Nielsen-like texture for the two flavour spurions we find that a benchmark fit with $\mathcal{O}(1)$ non-perturbative coefficients reproduces all six quark masses, three charged lepton masses, and the CKM matrix. The same spurions mediate charged lepton flavour violation, neutral meson mixing, rare kaon decays, and induce electric dipole moments. We compare the reach on the compositeness scale $\Lambda_{\rm pre}$ across these observables in the numerical benchmark and find that the electron EDM and $K^0$-$\bar{K}^0$ mixing provide the strongest sensitivity, reaching $\mathcal{O}(10^4)$ TeV, the lower end of the range probed by proton decay, while $\mu\to e\gamma$, $D^0$-$\bar{D}^0$ mixing, and $\mu$-$e$ conversion give complementary reach at $10^2$-$10^3$ TeV. The projected electron EDM sensitivity extends this to $\mathcal{O}(10^6)$ TeV, beyond the reach of planned proton decay searches.

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arXiv2026

Future directions in nuclear $\beta$ decay at FRIB and beyond

Motivated by the opportunities presented for studies relevant to nuclear structure, astrophysics, and fundamental symmetries with nuclear $\beta$ decay, the Facility for Rare Isotope Beams (FRIB) Theory Alliance topical program ``Future Directions in Nuclear $\beta$ Decays at FRIB'' was held in September of 2025. This white paper summarizes the main points of discussion over the two-week program, and it aims to provide a snapshot of the current status of the field while also highlighting important questions and opportunities for future work. We provide an overview of the experimental tools and techniques that enable modern $\beta$ decay studies, discuss the current state of nuclear many-body approaches used to study $\beta$ decays, and highlight the important science questions that can be addressed by weak decays.

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arXiv2026

Partial-wave unitarity and long-range interactions

Theories with massless particles contain $t$-channel (forward scattering) singularities that cause standard fixed order expressions for partial-wave amplitudes to be ill-defined. This presents an obstruction to systematically improvable partial-wave unitarity bounds. In this work, we study the construction of partial-wave amplitudes in a modified perturbation theory that incorporates long-range interactions focusing on the role of off-shell Coulomb modes. We find that there exists a universal description of the forward scattering region that renders the amplitudes renormalization scale independent. The resulting partial-wave amplitudes become well defined single-scale objects without spurious dependence on the infrared regulator, and we present a practical method for their computation order-by-order in perturbation theory.

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arXiv2026

Fluctuations in atom interferometers as a new tool for dark matter

We propose the use of the super-binomial variance in the count rate of an atom interferometer as a novel signature of dark matter. We show that the dark matter induced shift in this observable is enhanced by N, the number of atoms used per run of the interferometer, and therefore offers sensitivity that is enhanced by orders of magnitude relative to an independent-atom estimate. As an application, we consider dark matter that interacts with electrons, protons, and/or neutrons, via a long-range Yukawa interaction and new constraints on strongly interacting dark matter that thermalizes in the overburden of conventional direct detection experiments. We find that searches for super-binomial variance extend, and complement, existing atom interferometer observables; they are well suited to search for both short- and long-ranged forces.

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arXiv2025

$L_\mu-L_\tau$ gauge bosons in beam dumps and supernovae

We study the phenomenology of a sub-GeV $L_\mu-L_\tau$ gauge boson. We find discrepancies with existing literature in sensitivity projections for the upcoming SHiP experiment and in the treatment of supernovae cooling constraints. We present a quantitative analysis of different production modes in beam dumps and compare our results to previous work. In the context of supernovae, we re-evaluate the standard supernova cooling bounds from SN1987A and analyze additional supernova-based probes: diffusive cooling, constraints from the existence of low-energy supernovae, and the absence of a high-energy neutrino signal from SN1987A.

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arXiv2025

The $Z\alpha^2$ correction to superallowed beta decays in effective field theory and implications for $|V_{ud}|$

Superallowed ($0^+\rightarrow0^+$) beta decays currently provide the most precise extraction of quark mixing in the Standard Model. Their interpretation as a measurement of $|V_{ud}|$ relies on a reliable first-principles computation of QED radiative corrections expressed as a series in $Z\alpha$ and $\alpha$. In this work, we provide the first model-independent result for two-loop, $O(Z\alpha^2)$, long-distance radiative corrections where the nuclei are treated as heavy point-like particles. We use renormalization group analysis to obtain new results at $O(Z\alpha^3)$ for the coefficient of double-logarithms in the ratio of the maximal beta energy to the inverse nuclear size, $\Em/R^{-1}$. We use the Kinoshita-Lee-Nauenberg theorem to obtain new results at $O(Z^2\alpha^3)$ for the coefficient of logarithms in the ratio of maximal beta energy to the electron mass, $\log(2\Em/\me)$. We identify a structure-dependent, and therefore short-distance, contribution to the traditional $Z\alpha^2$ correction that should be revisited.. We provide the first comprehensive update to the long-distance corrections in almost forty years and comment on the impact of our findings for extractions of $|V_{ud}|$. We find that shifts in the long-distance corrections are $2.5\times$ larger than past estimates of their uncertainty, $1.5\times$ larger than the statistical uncertainty from the combined fit of superallowed decays, and about $1/2$ the size of estimated systematic error, which stems dominantly from nuclear structure effects.

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arXiv2025

Long-lived axionlike particles from electromagnetic cascades

We study axionlike particles (ALPs) in beam dump experiments, focusing on the Search for Hidden Particles (SHiP, at CERN) experiment and the Beam Dump eXperiment (BDX, at JLab). Many existing projections for sensitivity to ALPs in beam dump experiments have focused on production from either the primary proton/electron beam, or - in the case of SHiP - the secondary (high-energy) photons produced by neutral meson decays (e.g.,~$\pi^0\rightarrow\gamma \gamma$). In this work, we study the subsequent production of axions from the full electromagnetic shower in the target, finding order-of-magnitude enhancements in the visible decay yields across a wide range of axion masses. We update SHiP's sensitivity curve and provide new projections for BDX. Both experiments will be able to reach currently unexplored regions of ALP parameter space.

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arXiv2025

Coherently enhanced decoherence and cloud substructure of atom interferometers

We study how coherent scattering of a background gas off an atom (or other matter) interferometer can lead to enhanced signals from phase shifts and contrast loss. We focus on the inclusion of realistic features of atom interferometers such as finite temperature, cloud substructure, and time-dependent cloud radii. The inclusion of these effects, extending beyond the previously considered point-like cloud approximation, naturally allow us to study the smooth transition between the coherent and incoherent scattering regimes. We discuss how the formalism presented herein can be tested in the lab (with near-infrared photons or an eV-scale electron gun), and discuss an application for the detection of dark matter interacting via long-range forces.

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arXiv2025

Vertex corrections and wavefunction renormalization for atoms, nuclei, and other heavy composite particles

We study QED corrections to operator matrix elements involving heavy composite particles (e.g., heavy-mesons, nuclei, and atoms). We define a new notion of reducible and irreducible graphs which is useful for systems with many discrete excited states. The equivalence of the LSZ reduction formula and old fashioned perturbation theory is explicitly demonstrated. The self energy and vertex corrections are defined (to all orders), and the one-loop corrections are reduced to operator matrix elements which may be evaluated by hadronic, nuclear, or atomic theorists. The gauge dependence of the various pieces are studied in detail at one loop, and cancellation of spurious contributions are demonstrated in a class of covariant gauges; Coulomb gauge is also discussed. The formalism is applied to superallowed beta decay where the one-loop structure is connected to existing literature based on current algebra techniques. We further identify the well known $O(Z^2\alpha^2)$ isospin breaking correction from the intranuclear Coulomb field as arising from two-loop diagrams. We comment on future applications of our results to the radiative corrections necessary in extractions of $|V_{ud}|$, in particular for corrections that required beyond one-loop order.

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arXiv2025

Factorization and resummation of QED radiative corrections for neutron beta decay

Details of the two-loop analysis of long-distance QED radiative corrections to neutron beta decay are presented. Explicit expressions are given for hard, jet, and soft functions appearing in the factorization formula that describes the small mass/large energy limit. Power corrections, cancellation of singularities in the small mass expansion, renormalization scheme dependence, and bound state effects are discussed. The results impact the determination of $|V_{ud}|$ from the measured neutron lifetime.

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arXiv2025

Manifest Gauge Invariance for Structure Dependent Radiative Corrections to Processes Involving Atoms and Nuclei

Radiative corrections to reactions involving atoms or nuclei can become sensitive to the structure of the bound state. Generically, one encounters correlation functions of multiple currents which must satisfy Ward identities. At intermediate steps, however the Ward identities are obscured, and often violated by physically motivated approximation schemes. In this paper we outline a method to construct a representation of the aforementioned correlators that manifests gauge invariance in the limit of a heavy target (i.e., when recoil energy can be neglected). This representation then enables manifestly gauge invariant approximation schemes. Furthermore, the proposed representation naturally separates the largest contributions that dominate scattering amplitudes in the limit of a heavy constituent (e.g., proton) mass. We analyze elastic electron scattering from nuclei in detail, and also discuss radiative corrections to processes mediated by the weak interaction.

nucl-th↗

arXiv2025

The Fermi function and the neutron's lifetime

The traditional Fermi function ansatz for nuclear beta decay describes enhanced perturbative effects in the limit of large nuclear charge $Z$ and/or small electron velocity $\beta$. We define and compute the quantum field theory object that replaces this ansatz for neutron beta decay, where neither of these limits hold. We present a new factorization formula that applies in the limit of small electron mass, analyze the components of this formula through two loop order, and resum perturbative corrections that are enhanced by large logarithms. We apply our results to the neutron lifetime, supplying the first two-loop input to the long-distance corrections. Our result can be summarized as \begin{equation*} \tau_n \times |V_{ud}|^2\big[1+3\lambda^2\big]\big[1+\Delta_R\big] = \frac{5263.284(17)\,{\rm s}} {1 + 27.04(7)\times 10^{-3} }~, \end{equation*} with $|V_{ud}|$ the up-down quark mixing parameter, $\tau_n$ the neutron's lifetime, $\lambda$ the ratio of axial to vector charge, and $\Delta_R$ the short-distance matching correction. We find a shift in the long-distance radiative corrections compared to previous work, and discuss implications for extractions of $|V_{ud}|$ and tests of the Standard Model.

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arXiv2024

Off-shell vertices in heavy particle effective theories and $B\rightarrow D\pi \ell \nu$

We study the modifications to decay amplitudes in heavy to heavy semileptonic decays with multiple hadrons in the final state due to intermediate heavy hadrons being off-shell or having a finite width. Combining Heavy Hadron Chiral Perturbation Theory (HH$\chi$PT) with a BCFW on-shell factorization formula, we show that these effects induce $O(1/M)$ corrections to the standard results computed in the narrow-width approximation and therefore are important in extracting form factors from data. A combination of perturbative unitarity, analyticity, and reparameterization invariance fully determine these corrections in terms of known Isgur-Wise functions without the need to introduce new form factors. In doing so, we develop a novel technique to compute the boundary term at complex infinity in the BCFW formula for theories with derivatively coupled scalars. While we have used the $\bar B\rightarrow D\pi \ell\nu$ decay as an example, these techniques can generally be applied to effective field theories with (multiple) distinct reference vectors.

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arXiv2024

Probing Millicharged Particles at an Electron Beam Dump with Ultralow-Threshold Sensors

We propose to search for millicharged particles produced in high-intensity electron beam dumps using small ultralow-threshold sensors. As a concrete example, we consider a Skipper-CCD placed behind the beam dump in Hall A at Jefferson Lab. We compute the millicharged particle flux, including both electromagnetic cascade and meson productions emanating from an aluminum target. We find that the sensitivity of a modest 2x14 array of Skipper-CCDs can exceed the sensitivity of all existing searches for millicharged particle masses below 1.5 GeV, and is either competitive or world leading when compared to other proposed experiments. Our results demonstrate that small-scale ultralow threshold silicon devices can enhance the reach of accelerator-based experiments, while fitting comfortably within existing experimental halls.

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arXiv2024

Long-lived vectors from electromagnetic cascades at SHiP

We simulate dark-vector, $V$, production from electromagnetic cascades at the recently approved SHiP experiment. The cascades (initiated by photons from $\pi^0\rightarrow \gamma \gamma$) can lead to 3-4 orders of magnitude increase of the event rate relative to using primary production alone. We provide new SHiP sensitivity projections for dark photons and electrophilic gauge bosons, which are significantly improved compared to previous literature. The main gain in sensitivity occurs for long-lived dark vectors with masses below $\sim 50-300~{\rm MeV}$. The dominant production mode in this parameter space is low-energy annihilation $e^+ e^- \rightarrow V(\gamma)$. This motivates a detailed study of backgrounds and efficiencies in the SHiP experiment for sub-GeV signals.

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arXiv2024

The cosmology of ultralight scalar dark matter coupled to right-handed neutrinos

We consider ultralight scalar dark matter that couples to right-handed neutrinos. Due to the high density of neutrinos in the early universe, the background neutrino density dominates the dynamics of the scalar field, and qualitatively alters the field's cosmological evolution. This effect has not been included in previous literature, and changes the interpretation of cosmological data and its interplay with laboratory experiments. To illustrate these points a simplified model of a $1+1$ setup with a single scalar field is analyzed. We find that: {\it i}) The scalar field experiences an asymmetric potential and its energy density redshifts differently than ordinary matter. {\it ii}) Neutrino mass measurements at the CMB and oscillation experiments performed today complement one another (i.e., they constrain different regions of parameter space). {\it iii}) There exists potentially interesting cosmologies with either $O(1)$ variations in the dark matter density between the CMB and today, or $O(1)$ oscillations of neutrino mass.

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arXiv2024

Final state interactions for high energy scattering off atomic electrons

We consider the scattering of high energy leptons off bound atomic electrons focusing primarily on final state interactions i.e., the exchange of virtual photons between the outgoing energetic electron, and the heavy residual charged "debris" in the final state. These effects are inherently absent from calculations for a free electron at rest. Coulomb exchanges are enhanced by the large number of electrons in the atomic debris, and are unsuppressed by non-relativistic velocities in the debris. We find that these exchanges can be resummed using operator methods, and cancel at the level of the cross section until at least $O(\alpha^3)$. Furthermore, we argue that both final {\it and} initial state Coulomb exchanges (enhanced by the number of electrons in the atom) do not affect the cross section until at least $O(\alpha^3)$. Transverse photon couplings to non relativistic electrons are proportional to their small velocities, and rotational invariance suppresses their contribution to $O(\alpha^3)$. Our results are relevant for precision experiments involving neutrinos, electrons, positrons, and muons scattering off of atomic electrons in a fixed target.

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