arXiv subjects

Karim Ghorbani

Publications and source records attributed to Karim Ghorbani.

At least 19 recordsLinked to original sources

arXiv2026

Multi Component Dark Matter in a Minimal Model

We study a minimal $\mathbb{Z}_2$-symmetric extension of the Standard Model containing two singlet fermions and a singlet scalar that interact with the SM particles through the Higgs-portal. We identify regions of parameter space in which all three new particles are kinematically stable, giving rise to a multi component dark matter (DM) scenario. The parameter space consistent with the observed dark matter relic abundance are determined, and the contribution of each component to the total relic density is evaluated. While the DM-nucleon elastic scattering cross sections of the two fermionic dark matter components are loop-suppressed, the corresponding cross section of the scalar dark matter particle arises at tree level and is therefore expected to dominate. We find a viable region of the parameter space in which the scalar dark matter candidate with mass range of approximately $125-400$ GeV, evades current direct detection (DD) bounds while contributing only a small fraction of the observed relic density. In contrast, the fermionic dark matter possesses a loop-suppressed DD cross section that lies below the neutrino floor and can constitute a substantial fraction of the total relic density.

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arXiv2025

Fermion Dark Matter Effect on Electroweak Phase Transition

The addition of extra scalars to the Standard Model (SM) of particle physics enriches the vacuum structure and consequently gives rise to strong first-order phase transitions (EWPT) in the early universe. We raise the question that how the EWPT is affected by the addition of fermions in models beyond the SM, and address this question by studying the EWPT in a dark matter model comprising a singlet scalar and two Dirac fermions. The singlet scalar develops a nonzero vacuum expectation value (VEV), and the lighter fermion plays the role of the dark matter. The model evades the stringent direct detection bounds due to the presence of two fermions. We first show that applying the high-temperature approximation, no first-order phase transition is found. Then we demonstrate that when including the full finite temperature corrections to the effective potential, the first-order phase transition becomes possible, nevertheless, all the phase transitions will be weak. We therefore deduce that the addition of fermions reduces the strength of the EWPT.

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arXiv2024

Loop quantum effects on direct detection prediction in two-scalar dark matter scenario

We investigate the effect of quantum corrections on the elastic scattering cross section of dark matter off nucleus in two-scalar dark matter model. Among two extra singlet scalars in the two-scalar model, the lighter one is stable and plays the role of dark matter candidate and the heavier one contributes in dark matter co-annihilation processes in thermal history of the early universe. It is already known that the two-scalar model at tree level, unlike the single-scalar dark matter model, can easily evade the bounds from direct detection (DD) experiments. The claim here is that taking into account the loop effects, in some regions of the parameter space, the DM-nucleon cross section becomes larger than the tree level contribution. Therefore, loop effects move the regions which were below the neutrino floor at tree level, up to the regions which are detectable by future DD experiments.

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arXiv2023

Secluded Scalar Dark Matter and the Muon Anomalous Magnetic Moment

We consider a dark matter model with a singlet scalar, $χ$, as our dark matter (DM) candidate which is secluded from the Standard Model (SM) and annihilates to the singlet scalar, $ϕ$, via a contact interaction. The singlet scalar, $ϕ$, has a leptophilic interaction with the SM leptons and may decay leptonically at tree level, and decays into a pair of photons at loop level. The focus in this work is to consider DM masses below 10 GeV. It is found a viable secluded region in the parameter space after imposing the observed relic density. There is a one-loop interaction between scalar dark matter and the atomic electron in this model. We then apply the available direct detection bounds from Xenon10, Xenon1T, and DarkSide on the DM-electron elastic scattering cross section. While the model can explain the muon anomalous magnetic moment, we put bounds from current and future lepton collider experiments.

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arXiv2023

(sub)GeV Dark Matter in the $U(1)_X$ Higgs Portal Model

In this research we consider a $U(1)_X$ gauge boson acting as a dark matter candidate. The vector dark matter (DM) gets mass when a complex singlet scalar breaks the gauge symmetry spontaneously, adding a second Higgs boson to the spectra. The dark matter candidates communicate with the SM particles via a scalar-Higgs portal. In this work, we concentrate on the masses of the vector dark matter and the scalar mediator below 10 GeV, aka light dark matter. Although we assume thermal freeze-out for the vector DM using the zero-moment of the full Boltzmann equation to calculate the relic abundance, we explore the effects of the second-moment when the vector DM annihilates resonantly. As typically light DM is highly sensitive to CMB bounds, we focus on two thermal mechanisms which alleviate this bound: dark matter annihilation via forbidden channels and near a pole. Other bounds from colliders, thermalization conditions, beam-dump experiments, and astrophysical observations are imposed. Taking into account all the bounds including the direct detection upper limits, the viable space is achieved.

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arXiv2023

Loop enhancement of direct detection cross section in a fermionic dark matter model

We investigate the effect of one loop quantum corrections on the elastic scattering of dark matter off the nucleon in a fermionic dark matter model. The model introduces two new singlet fermions and a singlet scalar. The fermions communicate with the SM particles through a Higgs portal. It is found that some viable regions in the parameter space respecting the bounds from the observed relic density, the Higgs invisible decay width, and direct detection experiment, will be shrunk significantly when one loop effects are taken into account. The regions already resided below the neutrino floor, partly may come into regions which are testable by the current or future direct detection experiments. In addition, some regions being viable at tree level, may be excluded when quantum corrections are included.

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arXiv2023

$Z_3$ Scalar Dark Matter with Strong Positron Fluxes

We explore a class of simplified extensions to the Standard Model containing a complex singlet scalar as a dark matter candidate accompanied by a vector-like lepton as a mediator, both charged under a new $Z_3$ symmetry. In its simplest form, the new physics couples only to right-handed electrons, and the model is able to accommodate the correct dark matter relic abundance around the electroweak scale up to several TeV evading the strongest constraints from perturbativity, collider and dark matter searches. Furthermore, the model is capable to enhance naturally positron fluxes by several orders of magnitude presenting a box-shape spectra. This framework opens up a lot of phenomenological possibilities depending on the quantum charge assignments of the new fields.

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arXiv2022

$W$-Boson Mass Anomaly from Scale Invariant 2HDM

The recently reported measurement of the $W$-boson mass by CDF-II collaboration is significantly heavier than that of the Standard Model prediction. We study this anomaly in the scale invariant Two-Higgs-Doublet-Model (SI-2HDM) with a $\mathbb{Z}_2$ symmetry to avoid the flavor-changing-neutral-current (FCNC). In this scenario the Higgs particle is the classically massless scalon in the SI-2HDM gaining its mass by radiative corrections, hence being naturally light. Moreover, because of the scale symmetry the model is more predictive respect to the generic 2HDM. We show that the oblique parameters depending on the masses of the charged and CP-even (CP-odd) neutral scalar components of the SI-2HDM denoted respectively by $M_{H^\pm}$ and $M_h$ ($M_A$), are large enough to accommodate the $W$-boson mass anomaly in the model. There are as well viable regions in the mass spectrum of the SI-2HDM that evade the experimental bounds from colliders on the charged Higgs and neutral scalars.

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arXiv2022

The hierarchy problem and the vacuum stability in two-scalar dark matter model

We consider an extension to the Standard Model (SM) with two extra real singlet scalars which interact with the SM Higgs particle. The lighter scalar is taken as the dark matter (DM) candidate. We show that the model successfully explains the relic abundance of the DM in the universe and evades the strong bounds from direct detection experiments while respecting the perturbativity and the vacuum stability conditions. In addition, we study the hierarchy problem within the Veltman approach by solving the renormalization group equations at one-loop. We demonstrate that the addition of the real singlet scalars contributes to the Veltman parameters which in turn results in satisfying the Veltman conditions much lower than the Planck scale $ Λ_\text{Pl}$ down to the electroweak scale. Therefore, the presence of the extra scalars solves the fine-tuning problem of the Higgs mass. For the case of the two-scalar DM model we find two representative points in the viable parameter space which satisfy also the Veltman conditions at $Λ= 300$ GeV and $Λ= 1$ TeV.

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arXiv2021

Light vector dark matter with scalar mediator and muon g-2 anomaly

We study a model with a vector dark matter (DM) candidate interacting with the SM charged leptons through a scalar portal. The dark matter candidate acquires mass when the complex scalar breaks an abelian gauge symmetry spontaneously. The scalar interacts with the SM charged leptons through a dimension-6 operator. The scalar mediator induces elastic scattering of dark matter with electrons at tree level and also DM-nucleon interaction when the effects from scalar-Higgs mixing are also taken into account. Given the recent results from Xenon1T upper bounds on DM-electron elastic scattering cross section where the strongest sensitivity lies in the range $\sim {\cal O}$(1) GeV, we find the viable space in the parameter space respecting constraints from the observed relic density, direct detection, muon $(g_μ-2)$ anomaly, $e^+ e^-$ colliders, electron beam-dump experiments and astrophysical observables. It is shown that the current upper bounds of Xenon1T on DM-electron interaction is partially sensitive to the regions in the viable parameter space which is already excluded by the electron beam-dump experiment, Orsay. We also find that there are viable DM particles with masses $\sim {\cal O}(1)$ GeV evading the direct detection but stand well above the neutrino floor. Almost the same viable regions are found when we apply the direct detection upper limits on the DM-proton spin-independent cross section.

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arXiv2021

Singlet Scalars as Dark Matter and the Muon g-2 Anomaly

We explore a simple model containing two singlet scalars as dark matter supplemented with a vector-like lepton. Evading collider and dark matter constraints, the model is able to accommodate the correct dark matter relic abundance and the muon g-2 anomaly when the masses of the three new fields are around 100 GeV and the values of the minimum amount of required couplings are order one.

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arXiv2019

Strongly First-Order Phase Transition in Real Singlet Scalar Dark Matter Model

The extension of the standard model by a real gauge singlet scalar is the simplest but the most studied model with sometimes controversial ideas on the ability of the model to address the dark matter and the electroweak phase transition issues simultaneously. For this model, we obtain analytically slightly different conditions for strongly first-order electroweak phase transition and apply that in computation of the dark matter relic density where the real scalar plays the role of the dark matter particle. We show that the scalar in this model before imposing the invisible Higgs decay constraint, can be responsible for all or part of the dark matter abundance, while at the same time gives rise to a strongly first-order electroweak phase transition required for the baryogenesis. When the constraints from the direct detection experiments such as XENON100 or LUX/XENON1t are considered, the model is excluded completely.

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arXiv2019

A Simultaneous Study of Dark Matter and Phase Transition: Two-Scalar Scenario

The simplest extension of the Standard Model by only one real singlet scalar can explain the observed dark matter relic density while giving simultaneously a strongly first-order electroweak phase transition in the early universe. However, after imposing the invisible Higgs decay constraint from the LHC, the parameter space of the single scalar model shrinks to regions with only a few percent of the DM relic abundance and when adding the direct detection bound, e.g. from XENON100, it gets excluded completely. In this paper, we extend the Standard Model with two real guage singlet scalars, here $s$ and $s'$, and show that the electroweak symmetry breaking may occur via different channels. Despite very restrictive first-order phase transition conditions for the two-scalar model in comparison to the single scalar model, there is a viable space of parameters in different phase transition channels that simultaneously explains a fraction or the whole dark matter relic density, a strongly first-order electroweak phase transition and still evading the direct detection bounds from the latest LUX/XENON experiments while respecting the invisible Higgs decay width constraint from the LHC.

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arXiv2019

Leading Loop Effects in Pseudoscalar-Higgs Portal Dark Matter

We examine a model with a fermionic dark matter candidate having pseudoscalar interaction with the standard model particles where its direct detection elastic scattering cross section at tree level is highly suppressed. We then calculate analytically the leading loop contribution to the spin independent scattering cross section. It turns out that these loop effects are sizable over a large region of the parameter space. Taking constraints from direct detection experiments, the invisible Higgs decay measurements, observed DM relic density, we find viable regions which are within reach in the future direct detection experiments such as XENONnT.

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arXiv2018

Split fermionic WIMPs evade direct detection

We consider a model with two gauge singlet fermionic WIMPs communicating with the SM particles by a singlet scalar mediator via a Higgs portal. While the light WIMP is stable and plays the role of the dark matter (DM) candidate, the heavy partner is a short-lived WIMP without contribution to the current DM relic density. Along with the coannihilation effects the heavy WIMP, acting as a mediator in $t$- and $u$-channel DM annihilation cross sections, has a significant effect in finding the viable parameter space against the direct detection constraints provided by XENON1t and LUX experiments. This is an extension to the minimal singlet fermionic DM model whose entire parameter space (except a resonance region) excluded by the latest direct detection experiments. It is found out that there are viable regions in the parameter space which evade direct detection upper bounds and respect the observed DM relic density by WMAP/Planck. We also found that the Fermi-LAT upper limits on the DM annihilation cross section into $b\bar b$ can exclude small regions of the viable parameter space which elude direct detection experiments. This model exemplifies a case within the WIMP paradigm whose DM candidate can escape direct detection experiments nontrivially. Such models are interesting to be studied in collider experiments like the LHC.

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arXiv2018

DAMPE Electron-Positron Excess in Leptophilic $Z'$ model

Recently the DArk Matter Particle Explorer (DAMPE) has reported an excess in the electron-positron flux of the cosmic rays which is interpreted as a dark matter particle with the mass about $1.5$ TeV. We come up with a leptophilic $Z'$ scenario including a Dirac fermion dark matter candidate which beside explaining the observed DAMPE excess, is able to pass various experimental/observational constraints including the relic density value from the WMAP/Planck, the invisible Higgs decay bound at the LHC, the LEP bounds in electron-positron scattering, the muon anomalous magnetic moment constraint, Fermi-LAT data, and finally the direct detection experiment limits from the XENON1t/LUX. By computing the electron-positron flux produced from a dark matter with the mass about $1.5$ TeV we show that the model predicts the peak observed by the DAMPE.

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arXiv2017

Renormalization group equation analysis of a pseudoscalar portal dark matter model

We investigate the vacuum stability and perturbativity of a pseudoscalar portal dark matter model with a Dirac dark matter (DM) candidate, through the renormalization group equation analysis at one-loop order. The model has a particular feature which can evade the direct detection upper bounds measured by XENON100 and even that from planned experiment XENON1T. We first find the viable regions in the parameter space which will give rise to correct DM relic density and comply with the constraints from Higgs physics. We show that for a given mass of the pseudoscalar, the mixing angle plays no significant role in the running of the couplings. Then we study the running of the couplings for various pseudoscalar masses at mixing angle $θ= 6^\circ$, and find the scale of validity in terms of the dark coupling, $λ_{d}$. Depending on our choice of the cutoff scale, the resulting viable parameter space will be determined.

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arXiv2017

Mono-Higgs signature in fermionic dark matter model

In light of the Higgs boson discovery, we explore mono-Higgs signature in association with dark matter pair production at the LHC in a renormalizable model with a fermionic dark matter candidate. For two channels with $γγ+\text{MET}$ and $b \bar b+\text{MET}$ in the final state we simulate the SM backgrounds and signal events at $\sqrt{s} = 14$ TeV. We then estimate the LHC sensitivities for various benchmark points for two integrated luminosities ${\cal L} = 300~\text{fb}^{-1}$ and ${\cal L} = 3~ \text{ab}^{-1}$. We constrain the Yukawa coupling of the dark matter-SM interaction, taking into account bounds from mono-Higgs signature, observed dark matter relic density, Higgs physics, perturbativity requirement and electroweak measurements. Concerning the mono-Higgs search, it turns out that the channel with the largest branching ratio, $b \bar b$ channel, provides better sensitivity. There are found regions in the parameter space of the model compatible with all the bounds mentioned above which can be reached in future LHC studies.

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