arXiv subjects

Ahmad Alachkar

Publications and source records attributed to Ahmad Alachkar.

3 recordsLinked to original sources

arXiv2026

Axion-Induced Casimir Interaction Between Graphene Plates

Axion dark matter may induce observable electromagnetic effects in resonant cavity systems and potentially lead to modifications of the Casimir interaction. In this context, graphene represents an attractive platform owing to its tunable electromagnetic properties, and the fact that its electromagnetic response can be modelled microscopically from first principles within quantum field theory. The electromagnetic response induced by axion dark matter is investigated in a planar cavity consisting of parallel graphene interfaces in the presence of a homogeneous external magnetic field, incorporating finite temperature, chemical potential and dissipation through the graphene conductivity. Closed analytical expressions are obtained for the induced electric field and the resulting pressure. The pressure exhibits resonant enhancement at a series of plate separations satisfying $d_n=(2\pi n-\phi(r))/m_a$, where $m_a$ is the axion mass and the phase $\phi(r)$ is determined by the reflection coefficient $r$, which depends on the graphene conductivity evaluated at $\omega=m_a$. The resonant structure is strongly influenced by the graphene chemical potential and damping parameter. In particular, increased doping, for example via a gate voltage, sharpens the resonances and amplifies the axion-induced signal. By comparing the resonantly enhanced signal with the conventional Casimir background, the parametric regimes in which the effect could become experimentally relevant are identified, with the strongest sensitivity obtained for highly doped low-dissipation graphene configurations operated near resonance. These results demonstrate that graphene-based Casimir-type configurations may provide a sensitive framework for probing axion-induced electromagnetic phenomena and highlight the interplay between axion electrodynamics, cavity resonances, and material properties in low-dimensional systems.

hep-ph↗

arXiv2024

Dilatonic Couplings and the Relic Abundance of Ultralight Dark Matter

Models of scalar field dark matter where the scalar is a dilaton have a special behaviour, since non-trivial couplings, $d$, to matter result in a contribution to the potential for the field which is proportional to the trace of the stress-energy tensor. We look in more detail at the dilaton mass, $m_ϕ$, and initial conditions required to yield the correct relic abundance for couplings that are not already excluded by terrestrial experiments. In minimal models with only couplings accessible to terrestrial searches, we find that dilaton dark matter with $m_ϕ \gtrsim 10^{-10}$ eV requires couplings suppressed compared to constraints from equivalence principle (EP) tests and fifth force searches in order to not produce too much dark matter, improving on the strongest current experimental constraints by up to $\sim {\cal O}(10)$, with consequences for the proposed mechanical resonator dilaton DM searches. In non-minimal or universally coupled models, the unconstrained couplings of the dilaton to e.g. the top quark can strongly influence the relic abundance at all masses. In particular, this implies that atom interferometry searches at masses $m_ϕ\approx 10^{-19}\text{ eV}$ are unable to constrain the early Universe behaviour or UV physics of the dilaton. We also find that dilatonic couplings allow for compatibility of $m_ϕ\gtrsim 10^{-7}\text{ eV}$ with an observably large tensor-to-scalar ratio in the cosmic microwave background, which is not possible for a decoupled scalar of the same mass.

hep-ph↗

arXiv2023

Dark Matter Constraints from the Eccentric Supermassive Black Hole Binary OJ 287

OJ 287 is a blazar thought to be a binary system containing a ~ 18 billion solar mass primary black hole accompanied by a ~ 150 million solar mass secondary black hole in an eccentric orbit, which triggers electromagnetic flares twice in every ~ 12 year orbital period when it traverses the accretion disk of the primary. The times of these emissions are consistent with the predictions of general relativity calculated to the 4.5th post-Newtonian order. The orbit of the secondary black hole samples the gravitational field at distances between O(10) and O(50) Schwarzschild radii around the primary, and hence is sensitive to the possible presence of a dark matter spike around it. We find that the agreement of general-relativistic calculations with the measured timings of flares from OJ 287 constrains the mass of such a spike to < 3% of the primary mass.

hep-ph↗