arXiv subjects

Ahmad Mohamadnejad

Publications and source records attributed to Ahmad Mohamadnejad.

At least 19 recordsLinked to original sources

arXiv2025

Study of gravitational waves from phase transitions in three-component dark matter

This paper studies gravitational waves in a dark matter model composed of three types of particles with distinct spins, along with a scalar field $ϕ$ that mediates interactions between Standard Model particles and dark matter. It discusses the electroweak phase transition following the Big Bang, during which all particles are initially massless due to the inactive Higgs mechanism. As temperature decreases, the effective potential reaches zero at two points, leading to two minima at the critical temperature ($T_c$), and eventually to a true vacuum state. The formation of new vacuum bubbles, where electroweak symmetry is broken and particles acquire mass, generates gravitational waves as these bubbles interact with the fabric of space-time. The paper derives the gravitational wave frequency and detection range based on the model's parameters, aligning with observational data from the Planck satellite and detection thresholds from PandaX-4T and XENONnT for some parameter points. It concludes by comparing the predicted background gravitational wave density with the sensitivities of LISA, BBO and $μ$-Ares detectors.

hep-ph↗

arXiv2025

Gravitational waves from high-temperature vacuum decay in scale-invariant models: nanohertz vs. millihertz regimes

We present a comprehensive analysis of high-temperature vacuum decay and the resulting stochastic gravitational wave (GW) background within the framework of general scale-invariant models. The effective potential is constructed to include tree-level contributions, the Coleman-Weinberg correction, finite-temperature effects, and the Daisy resummation technique, culminating in a high-temperature form. We investigate the dynamics of the first-order phase transition, calculating the critical and nucleation temperatures, the supercooling parameter, and the key transition parameters $α$ (transition strength) and $β$ (inverse duration). The vacuum decay is found to be dominated by sphaleron transitions rather than quantum tunneling. We compute the full GW spectrum arising from bubble collisions, sound waves, and turbulence. An extensive numerical scan reveals two distinct phenomenological regimes: one produces nanohertz-frequency GW signals potentially detectable by Pulsar Timing Arrays (PTA), while the other yields millihertz-frequency signals that are prime targets for future space-based interferometers like the Laser Interferometer Space Antenna (LISA).

hep-ph↗

arXiv2025

Spin Trio: a dark matter scenario

We investigate a beyond Standard Model (SM) featuring five new fields. Four fields encompassing three distinct spin states - scalar ($ S $), spinor ($ ψ^{1,2} $), and vector ($ V_μ $) - together form the multi-component dark matter (DM), while the fifth (scalar) field ($ ϕ$) carries a unit charge under a dark $ U_{D}(1) $ gauge symmetry, enabling SM-DM interactions via the Higgs portal. Although the model maintains classical scale invariance, loop effects break electroweak symmetry. The parameter space is constrained by scale invariance, DM relic density, and direct detection results. Our study aims to identify feasible model regions and evaluate detectability in future experiments. We analyze processes like DM annihilations, semi-annihilations, and conversions, integrating them into Boltzmann equations to calculate DM abundances. Random parameter scans reveal regions compatible with current data, including constraints from direct detection experiments like XENONnT and PandaX-4T . Our results show the model's viability across a broad range of DM masses.

hep-ph↗

arXiv2024

Gravitational wave in a filtered vector dark matter model

We consider a first order phase transition (FOPT) for a Vector Dark Matter (VDM) in the early universe in which its mass may partially arise from such mechanism in the hidden sector. We calculate the ratio of VDM that may enter the bubble for various bubble wall velocities as well as various nucleation temperatures that produce the measured dark matter relic abundance via bubble filtering. In the following, we focus on gravitational wave (GW) signals which produced by FOPT and show that it can be detectable at the DECIGO, TianQin, LISA and Ultimate-DECIGO (UDECIGO) experiments.

hep-ph↗

arXiv2024

Gravitational wave effects and phenomenology of a two-component dark matter model

We study an extension of the Standard Model (SM) which could have two candidates for dark matter (DM) including a Dirac fermion and a vector dark matter (VDM) under a new $U(1)$ gauge group in the hidden sector. The model is classically scale-invariant and the electroweak symmetry breaks because of loop effects. We investigate the parameter space allowed by current experimental constraints and phenomenological bounds. We probe the parameter space of the model in the mass range $1< M_V<5000$ GeV and $1<M_ψ<5000$ GeV. It has been shown that there are many points in this mass range that are in agreement with all phenomenological constraints. The electroweak phase transition has been discussed and it has been shown that there is region in the parameter space of the model consistent with DM relic density and direct detection constraints that, at the same time, can lead to first order electroweak phase transition. The gravitational waves produced during the phase transition could be probed by future space-based interferometers such as LISA and BBO.

hep-ph↗

arXiv2023

Thermal Leptophilic Light Vector Dark Matter with Spinor Mediator and Muon (g-2) Anomaly

Inspired by the recently new measurement of $(g-2)_μ$ at FermiLab and reported upper bound for electron-dark matter (DM) recoil by the XENON1T collaboration, we revisited phenomenology of a light MeV scale vector dark matter in a leptophilic extension of standard model while a new spinor field plays the role of mediator. A viable parameter space is considered to discuss the possibility of light dark matter relic density as well as anomalous magnetic moment of the muon. We study DM-electron direct detection and cosmological bounds on the parameters space of the model. It is shown that although new bound of $(g-2)_μ$ anomaly greatly confines the parametric space of the model, the thermal light dark matter can exist for $\rm M_{DM}\sim 10^{-1}-10^{1}~\rm GeV$.

hep-ph↗

arXiv2022

Electroweak phase transition and gravitational waves in a two-component dark matter model

We investigate an extension of the Standard Model (SM) with two candidates for dark matter (DM). One of them is a real scalar field and the other is an Abelian gauge field. Except for these two, there is another beyond SM field which has unit charge under a dark $ U_{D}(1) $ gauge symmetry. The model is classically scale invariant and the electroweak symmetry breaks because of the loop effects. Although SM is extended with a new dark symmetry and three fields, because of scale invariance, the parameter space is strictly restricted compared to other two-component DM models. We study both DM phenomenology and electroweak phase transition and show that there are some points in the parameter space of the model consistent with DM relic density and direct detection constraints, while at the same time can lead to first order electroweak phase transition. The gravitational waves produced during the phase transition could be probed by future space-based interferometers such as Laser Interferometer Space Antenna (LISA) and Big Bang Observer (BBO).

hep-ph↗

arXiv2020

Vacuum and Symmetry Breaking: New Approach

We propose a new mechanism for symmetry breaking in which, apart from particle degrees of freedom, topological degrees of freedom also emerge. In this method, a decomposition for the fields of the Yang-Mills-Higgs theory is introduced and Lagrangian is written based on new variables. This new Lagrangian does not change the dynamics of the theory, at least at the classical level. We study the spontaneous symmetry breaking for this new Lagrangian and show that how it works in Abelian and non-Abelian gauge theories. In the case of Abelian gauge theory our method adds nothing new to the so-called Higgs mechanism. However, in the non-Abelian case topological degrees of freedom, as classical fields, arise. Finally, we reacquire our results considering a new definition for the vacuum.

hep-th↗

arXiv2020

Gravitational waves from scale-invariant vector dark matter model: Probing below the neutrino-floor

We study the gravitational waves (GWs) spectrum produced during the electroweak phase transition in a scale-invariant extension of the Standard Model (SM), enlarged by a dark $ U(1)_{D} $ gauge symmetry. This symmetry incorporates a vector dark matter (DM) candidate and a scalar field (scalon). Because of scale invariance, the model has only two independent parameters and for the parameter space constrained by DM relic density, strongly first-order electroweak phase transition can take place. In this model, for a narrow part of the parameter space, DM-nucleon cross section is below the neutrino-floor limit, and therefore, it cannot be probed by the future direct detection experiments. However, for a benchmark point form this narrow region, we show the amplitude and frequency of phase transition GW spectrum fall within the observational window of space-based GW detectors such as eLISA.

hep-ph↗

arXiv2020

DAMPE Excess from Leptophilic Vector Dark Matter: Model Independent Approach

We study all extensions of the Standard Model (SM) with a vector dark matter (VDM) candidate which can explain the peak structure observed by recent DAMPE experiment in electron-positron cosmic ray spectrum. In this regard, we consider all leptophilic renormalizable VDM-SM interactions through scalar, spinor, and vector mediators. We show that only two out of six possible models could produce DAMPE signal by annihilation of VDM with the mass about 1.5 TeV in a nearby subhalo whilst simultaneously satisfying constraints from DM relic density, direct/indirect detection, and the collider bounds. These two models are the ones with scalar/pseudoscalar mediator $ ϕ$ with $ M_ϕ \in [1500,3000] $ GeV.

hep-ph↗

arXiv2019

Accidental scale-invariant Majorana dark matter in leptoquark-Higgs portals

We study a classically accidental scale-invariant extension of the Standard Model (SM) containing three additional fields, a vector leptoquark ($ V_μ $), a real scalar ($ ϕ$), and a neutral Majorana fermion ($ χ$) as a dark matter (DM) candidate. The scalar $ ϕ$ (scalon) and Majorana fermion $ χ$ are both singlets under the SM gauge group, while $ V_μ $ has (\textbf{3}, \textbf{1}, 2/3) quantum numbers under the $ SU(3)_{c} \times SU(2)_{L} \times U(1)_{Y} $. The Majorana DM couples to the SM sector via both Higgs and leptoquark portals. We perform a scan over the independent parameters to determine the viable parameter space consistent with the Planck data for DM relic density, and with the PandaX-II and LUX direct detection limits for the spin-independent (SI) and spin-dependent (SD) DM-nucleon cross section. The model generally evades indirect detection constraints while being consistent with collider data.

hep-ph↗

arXiv2019

Cosmological evolution of the proton-to-electron mass ratio in an extended Brans-Dicke theory

We study variation of the proton-to-electron mass ratio $ μ=m_{p}/m_{e} $ by incorporating standard model (SM) of particle physics into an extended Brans-Dicke theory. We show that the evolution of the Higgs vacuum expectation value (VEV), with expansion of the Universe, leads to the variation of the proton-to-electron mass ratio. This is because the electron mass is proportional to the Higgs VEV, while the proton mass is mainly dependent on the quantum chromodynamics (QCD) energy scale, i.e., $ Λ_{QCD} $. Therefore, using the experimental and cosmological constraints on the variation of the $ μ$ we can constrain the variation of the Higgs VEV. This study is important in understanding the recent claims of the detection of a variation of the proton-to-electron mass ratio in quasar absorption spectra.

gr-qc↗

arXiv2019

Conformal vector dark matter and strongly first-order electroweak phase transition

We study a conformal version of the Standard Model (SM), which apart from SM sector, containing a $ U_{D}(1) $ dark sector with a vector dark matter candidate and a scalar field (scalon). In this model the dark sector couples to the SM sector via a Higgs portal. The theory is scale-invariant in lowest order, therefore the spontaneous symmetry breaking of scale invariance entails the existence of a scalar particle, scalon, with vanishing zeroth-order mass. However, one-loop corrections break scale invariance, so they give mass to the scalon. Because of the scale invariance, our model is subjected to constraints which remove many of the free parameters. We put constraints to the two remaining parameters from the Higgs searches at the LHC, dark matter relic density and dark matter direct detection limits by PandaX-II. The viable mass region for dark matter is about 1-2 TeV. We also obtain the finite temperature one-loop effective potential of the model and demonstrate that finite temperature effects, for the parameter space constrained by dark matter relic density, induce a strongly first-order electroweak phase transition.

hep-ph↗

arXiv2018

Skyrme-Faddeev Lagrangian from reformulated Georgi-Glashow model

Recently we proposed a decomposition for fields of the Georgi-Glashow model and interpreted Cho's decomposition as a result of some constraints on Georgi-Glashow's fields. Now, using the decomposition form that Faddeev and Niemi proposed, we introduce a simple and novel method to derive the Skyrme-Faddeev Lagrangian from the reformulation of the Georgi-Glashow model with an extra constraint. As we showed before, this extra constraint leads to appearance of both vortices and monopoles.

hep-th↗

arXiv2018

Scale-Invariant Two Component Dark Matter

We study a scale invariant extension of the standard model which can explain simultaneously dark matter and the hierarchy problem. In our set-up, we introduce a scalar and a spinor as two-component dark matter in addition to scalon field as a mediator. Interesting point about our model is that due to scale invariant conditions, compared to other two-component dark matter models, it has lower independent parameters. Possible astrophysical and laboratory signatures of two-component dark matter candidate are explored and it is shown that the most contribution of observed relic density of dark matter can be determined by spinor dark matter. Detectability of these dark matter particles is studied and the direct and invisible Higgs decay experiments are used to rule out part of the parameter space of the model. In addition, the dark matter self-interactions are considered and shown that their contribution saturate this constraint in the resonant regions.

hep-ph↗

arXiv2018

Searching for Vector-Like Quarks in a Fermionic Dark Matter Model with Pseudoscalar: A Resonance Case

We propose a minimal model of a fermionic dark matter with a pseudoscalar mediator and N generation of vector-like quarks. We calculate the relic density and obtain new constraint on the generation of the aforementioned quarks. Concerning phenomenological aspects, we probe the presenting model via direct and indirect approaches. Finally, as an illustrative example, we evaluate a resonance case which has been (would be) the subject of major experiments aiming to detect new particles. Performing this analysis results in significant constraints on the coupling strength of the vector-like quarks.

hep-ph↗

arXiv2017

Vacuum of the Standard Model and its Cosmological Aspects

Recently, we have proposed a definition for the vacuum and suggest a mechanism for symmetry breaking. In this mechanism extra massless fields, vacuum fields, arise. We apply our method to the standard model of particle physics and obtain vacuum sector of this model. Vacuum field of the standard model is a four-vector with electromagnetic nature. This field is invariant under $ U(1)_{em} $ gauge transformation. We speculate that it could be responsible for many cosmological phenomena like inflation, dark energy and cosmic magnetic fields.

hep-ph↗

arXiv2016

Reformulation of the Georgi-Glashow model and some constraints on its classical fields

We study the SU(2) Georgi-Glashow model and suggest a decomposition for its fields and obtain a Lagrangian based on new variables. We use Cho's restricted decomposition as a result of a vacuum condition of the Georgi-Glashow model. This model with no external sources leads us to the Cho extended decomposition. We interpret the puzzling field, $ \textbf{n} $, in Cho's decomposition as the color direction of the scalar field in the Georgi-Glashow model. We also study another constraint, condensate phase, and generalize Cho's extended decomposition. Finally, we argue about a decomposition form that Faddeev and Niemi proposed in this constrained Georgi-Glashow model.

hep-th↗