Susan Gardner

We employ the joint QED and QCD factorization of deeply inelastic, electron-proton scattering with generic initial state radiation to probe the possibility of exotic particle emission—i.e., of weakly coupled particles originating from a dark or hidden sector—through anomalous energy loss. We leverage this possibility through the consideration of phase-space-limited kinematic regions, for which the emission of an additional, undetected particle can particularly impact the associated cross section. In this first paper, as a proof of principle, we focus on radiation from the incoming electron, considering the modification of the lepton distribution function from the emission of particles, that could have spin of up to 2 and various, well-motivated electron couplings. We illustrate the sensitivity of our approach through the computation of the modified cross sections for the emission of MeV-GeV mass-scale, spin 0 particles in kinematics chosen for their sensitivity to initial state electron radiation and suitable to the forward-backward detection sensitivity of the ePIC detector at the Electron Ion Collider.

The neutron lifetime anomaly speaks to the possibility of exotic decay channels of the neutron. The very existence of neutron stars constrains the strength of such effects, and in this talk I develop how precisely determined energy-loss constraints, particularly anomalous binary-pulsar period lengthening, limit not only the total baryon loss rate across the star but also the parameters of the particle physics models that can produce such loss. To do this, we compute the new processes in the dense nuclear medium found at the core of a neutron star, employing the techniques of relativistic mean-field theory. Focusing on scenarios in which the dark-sector particles do not accumulate in the star, we extract limits on in-vacuum exotic neutron decays, and we determine them for various equations of state, noting their implications.

Processes that violate baryon number, most notably proton decay and n n ¯ transitions, are promising probes of physics beyond the Standard Model (BSM) needed to understand the lack of antimatter in the Universe. To interpret current and forthcoming experimental limits, theory input from nuclear matrix elements to UV complete models enters. Thus, an interplay of experiment, effective field theory, lattice QCD, and BSM model building is required to develop strategies to accurately extract information from current and future data and maximize the impact and sensitivity of next-generation experiments. Here, we briefly summarize the main results and discussions from the workshop ‘INT-25-91W: Baryon Number Violation: From Nuclear Matrix Elements to BSM Physics,’ held at the Institute for Nuclear Theory, University of Washington, Seattle, WA, 13–17 January 2025.

The Sun is not quite a perfect sphere, and its oblateness, thought to be induced through its rotation, has been measured using optical observations of its radius. Its gravitational quadrupole moment can then be deduced using solar models, or through helioseismology, and it can also be determined from measurements of its gravitational effects on Mercury’s orbit. The various assessments do not appear to agree, with the most complete and precise orbital assessments being in slight excess of other determinations. This may speak to the existence of a nonluminous disk or ring, where we also note evidence for a circumsolar dust ring within Mercury’s orbit from the Solar TErrestrial RElations Observatory (STEREO) mission. Historically, too, a protoplanetary disk may have been key to reconciling the Sun’s metallicity with its neutrino yield. The distribution of the nonluminous mass within Mercury’s orbit can modify the relative size of the optical and orbital quadrupole moments in different ways. We develop how we can use these findings to limit the mass of a dark disk, ring, or halo in the immediate vicinity of the Sun, and we note how future observational studies of the inner Solar System can not only refine these constraints but can also help to identify and to assess the mass of its dark-matter component. Published by the American Physical Society 2025

Tensions in the measurements of neutron and kaon weak decays, such as of the neutron lifetime, may speak to the existence of new particles and dynamics not present in the Standard Model (SM). In scenarios with dark sectors, particles that couple feebly to those of the SM appear. We offer a focused overview of such possibilities and describe how the observations of neutron stars, which probe either their structure or dynamics, limit them. In realizing these constraints, we highlight how the assessment of particle processes within dense baryonic matter impacts the emerging picture—and we emphasize both the flavor structure of the constraints and their broader connections to cogenesis models of dark matter and baryogenesis.

We investigate how our baryon-loss limits from anomalous binary-pulsar period lengthening can be interpreted microscopically to yield specific constraints on the particle physics of baryon number violation within a neutron star. We focus on the possibility of anomalous baryon disappearance via dark baryon processes and on scenarios in which the produced dark-sector particles do not survive to influence the response of the star to baryon-number-violating effects. We flesh out the conditions for which this may occur, as well as other key assumptions. We then turn to the analysis of particle processes in the dense nuclear medium found at the core of a neutron star, employing the techniques of relativistic mean-field theory. Using our study of in-medium effects and limits on macroscopic baryon number violation we extract limits on in-vacuum baryon-number-violating processes, and we determine them for various equations of state. We conclude by noting the implications of our results for models of dark-sector-enabled baryogenesis.

We investigate how our baryon-loss limits from anomalous binary-pulsar period lengthening can be interpreted microscopically to yield specific constraints on the particle physics of baryon number violation within a neutron star. We focus on the possibility of anomalous baryon disappearance via dark baryon processes and on scenarios in which the produced dark-sector particles do not survive to influence the response of the star to baryon-number-violating effects. We flesh out the conditions for which this may occur, as well as other key assumptions. We then turn to the analysis of particle processes in the dense nuclear medium found at the core of a neutron star, employing the techniques of relativistic mean-field theory. Using our study of in-medium effects and limits on macroscopic baryon number violation, we extract limits on in-vacuum baryon-number-violating processes, and we determine them for various equations of state. We conclude by noting the implications of our results for models of dark-sector-enabled baryogenesis. Published by the American Physical Society 2024

We construct the effective Hamiltonian for hadronic parity violation in strangeness-nonchanging ($ΔS=0$) processes in next-to-leading order (NLO) in QCD, for all isosectors, and at a renormalization scale of 2 GeV, thus extending our earlier leading-order (LO) analysis. Hadronic parity violation, studied in the context of the low-energy interactions of nucleons and nuclei, exposes the complex interplay of weak and strong interactions in these systems, and thus supports our extension to NLO. Here we exploit the flavor-blind nature of QCD interactions to construct the needed anomalous dimension matrices from those computed in flavor physics, which we then use to refine our effective Hamiltonian and finally our predicted parity-violating meson-nucleon coupling constants, to find improved agreement with few-body experiments.