arXiv subjects

Michael Shamma

Publications and source records attributed to Michael Shamma.

10 recordsLinked to original sources

arXiv2026

On the Origins of Varying Gauge Couplings

Variations in the gauge couplings of the Standard Model have been searched for experimentally and proposed as solutions to open questions in fundamental physics. Varying gauge couplings can arise from a dynamical scalar field coupled to the gauge kinetic term. This mechanism has been invoked extensively, assuming a minimal linear coupling of the scalar to the gauge bosons. In this work, we investigate how this operator is generated from the ultraviolet perspective. We argue that in weakly coupled, renormalizable completions in four spacetime dimensions, gauge invariance forces the leading dependence of the effective gauge coupling on the scalar to be logarithmic rather than linear. The gauge coupling evolution in these scenarios can be entirely described by the renormalization-group running with dynamical mass thresholds. Beyond leading order or four dimensions, we provide examples showing that more general behavior is possible. Finally, we discuss the phenomenological implications of dynamically evolving gauge couplings, particularly in early-universe settings.

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arXiv2025

Genesis of baryon and dark matter asymmetries through ultraviolet scattering freeze-in

We introduce a new mechanism for the simultaneous generation of baryon and dark matter asymmetries through ultraviolet-dominated freeze-in scatterings. The mechanism relies on heavy Majorana neutrinos that connect the visible Standard Model sector to a dark sector through the neutrino portal. Following reheating of the visible sector to a temperature well below the heavy neutrino masses, we show that 2-to-2 scattering processes can populate the dark sector and generate both baryon and dark matter asymmetries. In some parameter regions, the dominant source of baryon asymmetry can be charge transfer from the dark sector, a process we call dark wash-in. We also demonstrate that annihilation of the dark matter to massless states within the dark sector can deplete the symmetric population without destroying the net baryon charge to leave only an asymmetric dark matter abundance today. Depending on the specific model parameters, the observed baryon and dark matter abundances can be attained with heavy neutrino masses $M_N \gtrsim 10^{10}$ GeV, and dark matter masses in the range 0.1 GeV $\lesssim m_\chi \lesssim 10^3$ GeV if the dark matter relic abundance is mainly asymmetric and even lower masses if it is symmetric.

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arXiv2024

Probing Lepton Number Violation at Same-Sign Lepton Colliders

Same-sign lepton colliders offer a promising environment to probe lepton number violation. We study processes that change lepton number by two units in the context of Majorana heavy neutral leptons and neutrinophilic scalars at $\mu$TRISTAN, a proposed same-sign muon collider. Our work shows that such colliders, with modest energy and luminosity requirements, can either reveal direct evidence of lepton number violation or significantly constrain unexplored regions of parameter space, especially in the case of a neutrinophilic scalar.

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arXiv2024

Dark Matter from Dark Glueball Dominance

New gauge forces can play an important role in the evolution of the early universe. In this work we investigate the cosmological implications of a pure Yang-Mills dark sector that is dominantly populated after primordial inflation. Such a dark sector takes the form of a bath of dark gluons at high temperatures, but confines at lower temperatures to produce a spectrum of dark glueballs. These glueballs then undergo a freezeout process such that the remnant population is nearly completely dominated by the lightest state. To reproduce the observed cosmology, this lightest glueball species must decay to the Standard Model to repopulate and reheat it. At leading order, this can occur through a connector operator of dimension-6. In contrast, other glueballs can be parametrically long-lived or stable, and remain as contributors to dark matter or modify the observed cosmology through their later decays. In this work we study the evolution of such dark sectors in detail. We demonstrate that stable remnant glueballs can produce the measured dark matter abundance. We also derive broad constraints on non-Abelian dark sectors from overproduction of remnant glueballs when they are stable or from their destructive impact when they are able to decay.

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arXiv2024

Signatures of Bulk Neutrinos in the Early Universe

Neutrino masses and quantum gravity are strong reasons to extend the standard model of particle physics. A large extra dimension can be motivated by quantum gravity and can explain the small neutrino masses with new singlet states that propagate in the bulk. In such a case, a Kaluza-Klein tower of sterile neutrinos emerges. We revisit constraints on towers of sterile neutrinos that come from cosmological observables such as the effective number of noninteracting relativistic species and the dark matter density. These limits generically rule out micron-sized extra dimensions. We explore the weakening of these constraints to accommodate an extra dimension close to the micron size by assuming that the universe reheated after inflation to a low temperature. We discuss how such a possibility can be distinguished in the event of a positive signal in a cosmological observable.

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arXiv2022

Early-Universe Model Building

Theoretical investigations into the evolution of the early universe are an essential part of particle physics that allow us to identify viable extensions to the Standard Model as well as motivated parameter space that can be probed by various experiments and observations. In this white paper, we review particle physics models of the early universe. First, we outline various models that explain two essential ingredients of the early universe (dark matter and baryon asymmetry) and those that seek to address current observational anomalies. We then discuss dynamics of the early universe in models of neutrino masses, axions, and several solutions to the electroweak hierarchy problem. Finally, we review solutions to naturalness problems of the Standard Model that employ cosmological dynamics.

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arXiv2022

New Ideas in Baryogenesis: A Snowmass White Paper

The Standard Model of Particle Physics cannot explain the observed baryon asymmetry of the Universe. This observation is a clear sign of new physics beyond the Standard Model. There have been many recent theoretical developments to address this question. Critically, many new physics models that generate the baryon asymmetry have a wide range of repercussions for many areas of theoretical and experimental particle physics. This white paper provides an overview of such recent theoretical developments with an emphasis on experimental testability.

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arXiv2021

Dark Freeze-out Cogenesis

We propose a new mechanism where a multi-component dark sector generates the observed dark matter abundance and baryon asymmetry and thus addresses the coincidence between the two. The thermal freeze-out of dark matter annihilating into meta-stable dark partners sets the dark matter relic abundance while providing the out-of-equilibrium condition for baryogenesis. The meta-stable state triggers baryon asymmetry production by its decay well after the freeze-out and potentially induces a period of early matter domination before its decay. The dark matter and baryon abundances are related through number conservation within the dark sector (cogenesis). The "coincidence" is a natural outcome with GeV- to TeV-scale symmetric dark matter and the dark sector's interactions with the Standard Model quarks. We present a UV-complete model and explore its phenomenological predictions, including dark matter direct detection signals, LHC signatures of new massive particles with color charges and long-lived particles with displaced vertices, dark matter-induced nucleon conversions, (exotic) dark matter indirect detection signals, and effects on the cosmological matter power spectrum. As a side result, we provide a novel analytical treatment for dark sector freeze-out, which may prove useful in the study of related scenarios.

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arXiv2020

Unraveling the Dirac Neutrino with Cosmological and Terrestrial Detectors

We point out a correlation between the effective number of relativistic degrees of species $\Delta N_\text{eff}$, the cosmologically measured $m_{\nu,\text{sterile}}^{\text{eff}}$, and the terrestrially measured neutrino mass sum and effective electron neutrino mass, $\Sigma m_\nu$ and $m_{\nu_e}$, which arises in the Dirac neutrino hypothesis. If the neutrinos are Dirac particles, and if the active neutrinos' sterile partners were once thermalized in the early universe, then this new cosmological relic would simultaneously contribute to the effective number of relativistic species, $\Delta N_\text{eff}$, and also lead to a correlation between the cosmologically-measured effective sterile neutrino mass $m_{\nu,\text{sterile}}^\text{eff}$ and the terrestrially-measured active neutrino mass sum $\Sigma m_\nu$. We emphasize that specifically correlated deviations in $\Delta N_\text{eff}\gtrsim 3$, $m_{\nu,\text{sterile}}^\text{eff}$ and $\Sigma m_\nu$ above their standard predictions could be the harbinger revealing the Dirac nature of neutrinos. We provide several benchmark examples, including Dirac leptogenesis, that predict a thermal relic population of the sterile partners, and we discuss the relevant observational prospects with current and near-future experiments. If the correlation highlighted in this work is observed in future surveys, it could be interpreted as supporting evidence of Dirac neutrino masses.

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arXiv2020

WIMP Cogenesis for Asymmetric Dark Matter and the Baryon Asymmetry

We propose a new mechanism where asymmetric dark matter (ADM) and the baryon asymmetry are both generated in the same decay chain of a metastable weakly interacting massive particle (WIMP) after its thermal freeze-out. Dark matter and baryons are connected by a generalized baryon number that is conserved, while the DM asymmetry and baryon asymmetry compensate each other. This unified framework addresses the DM-baryon coincidence while inheriting the merit of the conventional WIMP miracle in predicting relic abundances of matter. Examples of renormalizable models realizing this scenario are presented. These models generically predict ADM with sub-GeV to GeV-scale mass that interacts with Standard Model quarks or leptons, thus rendering potential signatures at direct detection experiments sensitive to low mass DM. Other interesting phenomenological predictions are also discussed, including: LHC signatures of new intermediate particles with color or electroweak charge and DM induced nucleon decay; the long-lived WIMP may be within reach of future high energy collider experiments.

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