arXiv subjects

Mohammad Aghaie

Publications and source records attributed to Mohammad Aghaie.

8 recordsLinked to original sources

arXiv2026

Neutron disappearance and the LZ nuclear recoil event

The LZ experiment reported a nuclear recoil with anomalous energy $E_R=248$ keV, around the typical neutron motion in xenon. This motivates interpreting the event as the disappearance of a bound neutron, either through spontaneous decay or a dark-matter-induced reaction. If the energy released is large, the predicted recoil spectrum is broad around the observed value; only $\sim10\%$ of xenon events avoid extra nuclear activity; the disappearance rate is target-independent and mildly constrained by oxygen experiments. In the special case of near-threshold DM-induced neutron disappearance, it can be open in xenon and closed in oxygen, the quiet fraction rises to $\sim30\%$, the recoil spectrum turns into lines.

hep-ph↗

arXiv2026

Anomaly-free axion-like particle in Nelson-Barr models

We study Nelson--Barr models with a discrete $Z_N$ symmetry that solve the strong CP problem through spontaneous CP violation, and show that they naturally predict a light axion-like particle (ALP) without introducing any additional ingredients. Unlike the QCD axion, this ALP is anomaly-free: its couplings to photons and gluons are highly suppressed, rendering it naturally long-lived. Instead, it couples to quarks through flavor-violating interactions whose structure is dictated by the CKM matrix. These interactions induce rare meson decays, providing a unique probe of the Nelson--Barr mechanism. We study the cosmological production of the ALP through both freeze-in and misalignment mechanisms. We show that the parameter space in which the observed relic abundance is explained by the freeze-in mechanism is subject to stringent constraints from precision flavor experiments and stellar cooling bounds from SN1987A, leaving only a small viable region that will be comprehensively tested by future structure-formation observations such as the Vera Rubin Observatory and next-generation X-ray missions like Athena, GECCO and THESEUS. In contrast, misalignment production remains a robust and viable mechanism for explaining the observed dark matter abundance over a broad region of parameter space. Our results demonstrate that precision flavor measurements, cosmological observations, and X-ray searches provide complementary probes of this anomaly-free ALP and, consequently, of the Nelson--Barr solution to the strong CP problem.

hep-ph↗

arXiv2026

A particle on a ring or: how I learned to stop worrying and love $θ$-vacua

Recently, Ai, Cruz, Garbrecht, and Tamarit (arXiv:2001.07152, arXiv:2404.16026, arXiv:2511.04216) claimed that the strong CP problem can be avoided by adopting a particular order of limits in the Euclidean path integral, in which the spacetime volume is taken to infinity before summing over all topological sectors. We critically examine this proposal using exactly solvable examples of one-dimensional quantum mechanics on a ring, namely the quantum rotor and the quantum pendulum. These systems provide fully controlled settings with known $θ$-dependent spectra. We find that the ACGT procedure fails to reproduce the correct energy spectrum. Since the spectrum is a direct physical observable, this result demonstrates that the proposed order of limits cannot be justified and conclusions about CP conservation in QCD cannot be based on this prescription alone.

hep-ph↗

arXiv2026

Self-Interaction Bounds on Ultralight Dark Matter Couplings to Matter

Ultralight dark matter (ULDM) couplings to matter fields and ULDM self-interactions are typically treated as independent probes. However, since the ULDM-matter couplings unavoidably induce self-interactions through quantum loop corrections, bounds on self-interacting ULDM from astrophysical and cosmological observations will also limit the coupling strength to matter. Applying this argument, we find that self-interaction bounds can impose strong constraints on the linear ULDM couplings to neutrinos, excluding a large portion of parameter space that is widely considered for probing ULDM via neutrino oscillation experiments. In addition, the self-interaction bounds also limit the quadratic ULDM couplings to electrons and light quarks, which can become stronger than from the stringent test of equivalence-principle violation. Our results demonstrate that the extreme observational sensitivity of cosmic microwave background and structure formations to repulsive self-interactions can robustly translate into powerful constraints on the ULDM interactions with fundamental particles.

hep-ph↗

arXiv2026

(H)ALPing the 511 keV line: A thermal DM interpretation of the 511 keV emission

We propose a novel framework where MeV-scale Dirac Dark Matter annihilates into axion-like particles, providing a natural explanation for the 511 keV gamma-ray line observed in the Galactic Center. The relic abundance is determined by p-wave annihilation into two axion-like particles, while s-wave annihilation into three axion-like particles, decaying into $e^+ e^-$ pairs, accounts for the line intensity. Remarkably, this model, assuming a standard Navarro-Frenk-White profile, reproduces the observed emission morphology, satisfies in-flight annihilation and cosmological bounds, and achieves the correct relic density, offering a compelling resolution to this longstanding anomaly.

hep-ph↗

arXiv2025

Minimal Dark Matter in the sky: updated Indirect Detection probes

Minimal Dark Matter is among the simplest and most predictive Dark Matter frameworks, with the Majorana SU(2) 5-plet as its smallest accidentally stable real representation. We present a comprehensive reassessment of its indirect-detection signals. The $γ$-ray flux from both Sommerfeld-enhanced annihilations and bound-state formation is calculated, incorporating next-to-leading-order corrections and next-to-leading-log resummation of the relevant electroweak effects. In the Milky Way halo, bound-state formation dominates the flux near 100 GeV. The corresponding low-energy spectrum is used to place constraints based on Fermi-LAT observations of Galactic diffuse emission, while the high-energy part of the spectrum is employed to forecast the required observation time for several of the Milky Way's dwarf spheroidal galaxies using the Cherenkov Telescope Array Observatory (CTAO). Fermi-LAT data strongly disfavor the lower edge of the thermal mass window, even under conservative assumptions about the inner Galaxy density profile. Furthermore, several hundred hours of forthcoming CTAO observations of northern dwarfs should be sufficient to probe the central mass value.

hep-ph↗

arXiv2024

Bounds on Ultralight Dark Matter from NANOGrav

The compelling evidence for the detection of the stochastic gravitational wave background by NANOGrav imposes constraints on the mass of compact cores of ultralight dark matter, also known as "solitons", surrounding supermassive black holes found at the centers of large galaxies. The strong dynamical friction between the rotating black holes and the solitons competes with gravitational emission, resulting in a suppression of the characteristic strain in the nHz frequency range. Our findings robustly rule out ultralight dark matter particles with masses ranging from $1.3\times 10^{-21}$ eV to $1.4\times 10^{-20}$ eV condensing into solitons around supermassive black holes.

astro-ph.CO↗

arXiv2024

Axion Dark Matter from Heavy Quarks

We propose simple scenarios where the observed dark matter abundance arises from decays and scatterings of heavy quarks through freeze-in of an axion-like particle with mass in the $10 {\rm \, keV} - 1 {\rm \, MeV}$ range. These models can be tested by future X-ray telescopes, and in some cases will be almost entirely probed by searches for two-body decays $K \to π+ {\rm invis.}$ at NA62. As a byproduct, we discuss the cancellation of IR divergencies in flavor-violating scattering processes relevant for thermal axion production, and derive the general contribution to axion-photon couplings from all three light quarks.

hep-ph↗