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Hengameh Bagherian

Publications and source records attributed to Hengameh Bagherian.

8 recordsLinked to original sources
arXiv2026

Lepton Flavor Violation: From Muon Decays to Muon Colliders

We investigate the unique potential of a high-energy muon collider to probe lepton-flavor-violating signals arising from physics beyond the Standard Model (SM). Low-energy, precision searches for charged lepton-flavor violation (LFV) are projected to dramatically improve their sensitivity in the coming years, and could provide the first evidence of new physics. We interpret the sensitivity of these searches in terms of a set of LFV operators in the SM Effective Field Theory. The same operators are then probed at the TeV scale by evaluating the projected reach of a muon collider while accounting for SM backgrounds and using a parameterized detector simulation. We find that for most operators, a muon collider could confirm signals if they are seen at future low energy experiments, whereas for certain flavor combinations it extends the reach to scales well beyond those accessible at lower energies. We also project the sensitivity of a muon collider to lepton flavor-violating decays of the SM Higgs boson and demonstrate improved sensitivity to $h \to eτ$ and $h \to μτ$ by an order of magnitude compared to the High-Luminosity LHC. The importance of having multiple, complementary probes is illustrated by considering both various combinations of operators and relative sizes of flavor-violating transitions between generations under various assumptions for the flavor structure of new physics.

hep-ph↗
arXiv2025

The Bearable Inhomogeneity of the Baryon Asymmetry

We study the implications of precision measurements of light-element abundances, in combination with the Cosmic Microwave Background, for scenarios of physics beyond the Standard Model that generate large inhomogeneities in the baryon-to-photon ratio. We show that precision Big Bang Nucleosynthesis (BBN) places strong constraints on any mechanism that produces large-scale inhomogeneities at temperatures around or below the TeV scale. In particular, we find that fluctuations of order $25\%$ on comoving length scales larger than the horizon at $T \simeq 3~\mathrm{TeV}$ are incompatible with the observed light-element abundances. This sensitivity to early-universe physics arises because baryon-number inhomogeneities homogenize primarily through diffusion, a slow process. As a result, BBN serves as a novel probe of baryogenesis below the TeV scale, readily ruling out some proposed scenarios in the literature. We discuss the implications for electroweak baryogenesis, and further show that precision BBN provides a new probe of first-order phase transitions that generate gravitational waves in the pHz-mHz frequency range. This yields constraints on the electroweak phase transition, as well as first-order phase transitions that have been suggested as an explanation of the pulsar timing array signal. Finally, we comment on the future prospects for improving this probe.

hep-ph↗
arXiv2024

Evaporating Axion Dark Matter and the Hubble Constant

Axion-like particles are a well-motivated dark matter candidate that can form a condensate with low momentum and high occupation number. In the presence of dark radiation, this condensate loses energy, naturally increasing the energy density of the universe around matter-radiation equality without requiring additional inputs. This general mechanism may offer a solution to the Hubble tension.

hep-ph↗
arXiv2024

Stepping into the Forest: Confronting Interacting Radiation Models for the Hubble Tension with Lyman-$α$ Data

Models of interacting dark radiation have been shown to alleviate the Hubble tension. Extensions incorporating a coupling between dark matter and dark radiation (DM-DR) have been proposed as combined solutions to both the Hubble and $S_8$ tensions. A key feature of these extended models is a break in the matter power spectrum (MPS), suppressing power for modes that enter the horizon before the DM-DR interactions turn off. In scenarios with a massless mediator, modes that enter before matter-radiation equality get suppressed, whereas for massive mediators, the break is determined by the mediator mass, a free parameter. In this work, we test these models against probes of LSS: weak lensing, CMB lensing, full-shape galaxy clustering, and eBOSS measurements of the 1D Ly$α\,$ forest flux power spectrum. The latter are the most constraining since they probe small scales where many models predict the largest deviations. In fact, already within $Λ{\rm CDM}\,$, the eBOSS Ly$α\,$ data are in significant tension with Planck CMB data, with the Ly$α\,$ data preferring a steeper slope of the MPS at $k \sim h \mathrm{Mpc}^{-1}$. We find that the simplest dark radiation models, which improve the Hubble tension, worsen the fit to the Ly$α\,$ data. However, models with DM-DR interactions can simultaneously address both tensions.

astro-ph.CO↗
arXiv2024

Zero Modes of Massive Fermions Delocalize from Axion Strings

Massless chiral excitations can arise from the interactions between a fermion and an axion string, propagating along the string and allowing it to superconduct. The properties of these excitations, or zero modes, dictate how the string interacts with light and can thus have important phenomenological consequences. In this paper, we add a nowhere-vanishing Dirac mass for the fermion in the usual model of axion electrodynamics. We find that the zero modes exhibit an interesting phase structure in which they delocalize from the string's core as the mass increases, up until a critical value past which they disappear. We study this structure from an analytic perspective, with explicit numerical solutions, and via anomaly inflow arguments. Finally, we derive the two-dimensional effective theory of the zero mode and its interactions with the four-dimensional gauge field and show how this effective theory breaks down as the zero modes delocalize.

hep-th↗
arXiv2022

The Inflated Chern-Simons Number in Spectator Chromo-Natural Inflation

The chromo-natural inflation (CNI) scenario predicts a potentially detectable chiral gravitational wave signal, generated by a Chern-Simons coupling between a rolling scalar axion field and an SU(2) gauge field with an isotropy-preserving classical background during inflation. However, the generation of this signal requires a very large integer Chern-Simons level, which can be challenging to explain or embed in a UV-complete model. We show that this challenge persists in the phenomenologically viable spectator field CNI (S-CNI) model. Furthermore, we show that a clockwork scenario giving rise to a large integer as a product of small integers can never produce a Chern-Simons level large enough to have successful S-CNI phenomenology. We briefly discuss other constraints on the model, both in effective field theory based on partial-wave unitarity bounds and in quantum gravity based on the Weak Gravity Conjecture, which may be relevant for further explorations of alternative UV completions.

hep-th↗
arXiv2018

On-Chip Optical Convolutional Neural Networks

Convolutional Neural Networks (CNNs) are a class of Artificial Neural Networks(ANNs) that employ the method of convolving input images with filter-kernels for object recognition and classification purposes. In this paper, we propose a photonics circuit architecture which could consume a fraction of energy per inference compared with state of the art electronics.

cs.ET↗