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Imtiaz Khan

Publications and source records attributed to Imtiaz Khan.

At least 37 records · Page 2Linked to original sources

Beyond Classical Instability Limits of Anisotropic Self-gravitating Fluid Configurations in Hu-Sawicki Inspired f(R) Gravity

In this draft, we investigate the dynamical instability of a restricted class of non-static, axially symmetric, self-gravitating fluid configurations within a Hu-Sawicki inspired f(R) gravity model. The matter source is described by an anisotropic energy-momentum tensor containing three principal stresses and an off-diagonal stress component. For the adopted vorticity free geometry, conservation equations are formulated, and a linear perturbation scheme is applied to separate the equilibrium and time-dependent sectors. This procedure yields a collapse equation that governs the evolution of the perturbed compact configuration. The associated instability conditions are then derived in terms of the adiabatic index Γ under the Newtonian and post-Newtonian approximations, whereas the resulting bounds show that the onset of instability depends not only on the stiffness of the fluid, but also on the background energy density, directional pressure anisotropies, metric perturbations, and higher-curvature contributions generated by the Hu-Sawicki model. The general relativistic limit is recovered by suppressing the modified gravity parameters, while the isotropic limit reproduces the classical Chandrasekhar threshold. These results demonstrate that curvature corrections and anisotropic stresses can appreciably modify the conventional instability conditions of axially symmetric compact systems.

gr-qc↗

Dilaton-Induced Resonant Production of Ultralight Vector Dark Matter

A dilatonic half-mass resonance can produce ultralight vector dark matter only if the Floquet instability becomes efficient before the oscillating spectator scalar dominates the cosmic expansion. We formulate this requirement in terms of the microscopic modulation parameter $ε_i=Φ_i/M$ and the gravitational onset fraction $r_i=Φ_i^2/(6\Mpl^2)$. For a background with constant equation-of-state parameter $w_b$, the narrow-band Floquet exponent obeys $μ/H\propto a^{3w_b/2}$; during radiation domination this ratio grows as $a^{1/2}$, while it remains constant for matter-like expansion. Imposing that the delayed instability occurs before spectator domination yields the amplitude-independent bound $M/\Mpl\lesssim\sqrt6,c_1\simeq0.31$, with $c_1\simeq1/8$ determined by the linear half-mass branch. An explicit expanding-background analysis confirms that $a_\star<a_{\rm dom}$ for sub-Planckian $M$, whereas $M\simeq\Mpl$ postpones efficient growth until after domination. Combining this embedding condition with the efficient-transfer normalization gives $m_{γ'}\propto r_i^{-2}$, implying that the ultralight range $m_{γ'}\sim10^{-20}$--$10^{-18},{\rm eV}$ corresponds to $r_i\sim10^{-5}$--$10^{-4}$ rather than to early spectator domination. The polarization-resolved canonical analysis shows that longitudinal production is more strongly concentrated in the infrared than transverse production, while derivative terms from canonical normalization modify the leading Floquet exponents at order unity. Stückelberg and Higgsed completions impose distinct ultraviolet consistency conditions, including radial decoupling and symmetry-restoration constraints. The viable branch is therefore radiation-era, perturbative, infrared-dominated, and associated with a sub-Planckian kinetic scale.

hep-ph↗

Disformal Kerr Imprints on BHL Accretion: Shock Morphology, PSD Signatures, and Observational QPO Counterparts

We reveal the effect of the spacetime parameters on the accretion morphology formed through the BHL mechanism around a slowly rotating disformal Kerr black hole. Thus, we investigate the measurable signatures of these parameters on the hydrodynamical morphology and the timing behavior of the accreting flow. It is shown that even weak disformal deviations from the Kerr solution modify the shock-cone structure, enhance the density in the post-shock region, and produce coherent oscillations in the accretion rate. The Kerr model produces coherent peaks at 42.99 Hz and 68.13 Hz, and these frequencies are consistent with the high-frequency QPOs observed from the source GRS 1915+105. In the models where the deviations from the Kerr solution are weak, low-frequency QPOs are produced and found to be coherent. These frequencies also fall within the frequency range observed in Galactic black-hole binaries. On the other hand, the models with large deviations from Kerr can be used to explain observational results that are more irregular, broad-band, and contain multiple peaks. In addition, by using inverse-mass scaling in this work, the numerically calculated frequencies are also compared with observations of intermediate-mass and supermassive black holes. In particular, the disformal black-hole models are found to be consistent with the observational results obtained from the sources M82 X-1, NGC 5408 X-1, and RE J1034+396. This comparison also allows the possible black-hole mass range of observed sources to be inferred from the relation between simulated and observed frequencies. This makes BHL accretion in disformal Kerr geometry a powerful framework for connecting modified-gravity black-hole spacetimes with observable QPO phenomenology.

astro-ph.HE↗

Long Inflation Screens Euclidean-Wormhole Initial States

Euclidean wormholes can prepare inflation in non--Bunch--Davies initial states, but long Lorentzian expansion screens this memory from the CMB. We derive a visibility bound for Euclidean-matched Bogoliubov data: the pivot excitation satisfies $|β_*| \lesssim e^{-2N_{\rm pre}}$, and smooth Euclidean filters confine residual signatures to a comoving edge $k_w=a_iM$. Only near-minimal inflation, or an edge inside the observable window, leaves detectable scalar, tensor, and higher-point imprints. For longer inflation, wormhole-prepared perturbations are driven to the Bunch--Davies prediction. Euclidean memory therefore, becomes a quantitative bound on inflationary duration, with direct targets in CMB polarization and large-scale structure: the longer inflation lasts, the less of the wormhole remains on the sky.

hep-ph↗

Dilaton-Flattened Axion Inflation

We present a solvable same-sector effective theory for anomaly-inspired axion inflation, in which a heavy trace-anomaly mode dynamically backreacts on the axion potential. The tree-level elimination of the radial field resums the backreaction into a closed-form Lambert-$W$ potential, naturally flattening the hilltop potential without external plateau operators. By deriving the exact trough metric, we evaluate all the observables on the fully reduced one-field action, bypassing uncontrolled kinetic approximations. Calibrated at $N_\star=56$, reheating-compatible branches yield $r\simeq0.033$--$0.036$ and $α_s\simeq-(4.6$--$4.7)\times10^{-4}$, comfortably satisfying the current ACT/SPT/BICEP constraints. The evolution remains strictly adiabatic ($m_\perp^2/H^2\gtrsim6.1$, $Ω/H\lesssim7.6\times10^{-4}$) with negligible sound-speed and metric corrections. We provide analytic control over the constant-$w_{\rm eff}$ reheating map, the $N_{\rm re}=0$ boundary, and robustness against vacuum-offset deformations. This Lambert-$W$ backbone establishes a precise, deformable benchmark for confining axion inflation, with microscopic matching and reheating microphysics accessible as systematic EFT refinements.

hep-ph↗

Topological magnetotransport in modified-Haldane systems

We present a theoretical study of quantum magneto-transport and magneto-optical (M-O) properties in modified-Haldane model; which is applicable to diverse classes of two-dimensional (2D) quantum materials such as buckled Xene monolayers and transition metal dichalcogenide (TMDC) monolayers. By varying the staggered sublattice potential and intrinsic spin-orbit coupling, we identify distinct topological regimes and analyze their manifestations in the emergence of Landau levels, the evolution of the density of states, and the characteristics of M-O absorption spectra. Using the Kubo formalism, we compute the longitudinal and Hall M-O conductivities and show that inter-Landau-level (inter-LL) transitions produce characteristic resonance features that provide optical signatures of the underlying topological phases. Within this framework, we demonstrate electrically tunable topological phase transitions in buckled silicene. Extending our study to monolayer TMDCs, we show that inspite of large band gap, the spin-valley coupling provides a powerful tool for tailoring M-O absorption features across wide range of 2D materials. Collectively, these results underscore modified-Haldane-model materials as an ideal testbed for engineering quantum transport, with promising applications in topological photonics, valleytronic devices, and next-generation optoelectronics.

cond-mat.mes-hall↗

Floquet-driven thermal transport in topological Haldane lattice systems

In this paper, we employ a modified Haldane lattice model to investigate the light-driven, spin- and valley-dependent anomalous Nernst effect in two-dimensional hexagonal topological systems. We demonstrate that two-dimensional buckled materials exhibit a hierarchy of electrically and optically tunable topological phases when subjected to off-resonant circularly polarized light in the presence of intrinsic spin-orbit coupling and a staggered sublattice potential. Within a Berry-curvature-driven transport framework, we systematically analyze charge-, spin-, and valley-resolved anomalous Nernst responses and identify their correspondence with distinct topological regimes. A finite charge Nernst conductivity arises under optical driving combined with spin-orbit coupling, whereas the generation of a pure valley Nernst current requires the simultaneous presence of sublattice asymmetry and off-resonant light. Substrate-induced inversion asymmetry further enables thermally driven valley currents with tunable magnitude and sign. We find that single-spin and single-valley Nernst responses occur in selected insulating and metallic phases, while the valley Nernst signal is suppressed in spin-polarized and anomalous quantum Hall phases. Extending our analysis to monolayer MoS$_2$, we show that strong spin-orbit coupling and broken inversion symmetry allow fully spin- and valley-polarized Nernst currents over a broad energy window. The temperature dependence of the Nernst response exhibits characteristic signatures of topological phase transitions, establishing the anomalous Nernst effect as a sensitive probe of field-engineered band topology in two-dimensional Dirac materials.

cond-mat.other↗

LHC Run-3, Dark Matter and Supersymmetric Spectra in the Supersymmetric Pati-Salam Model

Driven by the growing agreement between the experimentally measured muon anomalous magnetic moment and its SM prediction, we reexamine phenomenological consequences of the MSSM, which is embedded in the supersymmetric $SU(4)_C \times SU(2)_L \times SU(2)_R$ Pati-Salam model. In contrast to earlier studies that predominantly favored a specific sign for the Higgsino mass parameter, our analysis systematically explores both $μ> 0$, and $μ< 0$ scenarios in light of current collider, cosmological, and DM constraints. Within this framework, we identify viable parameter space regions where the observed DM relic density is reproduced through multiple mechanisms: co-annihilations involving sbottom-neutralino, gluino-neutralino, stop-neutralino, stau-neutralino, and chargino-neutralino coannihilation, as well as resonant s-annihilation channel via the pseudoscalar Higgs boson. We demonstrate that all such scenarios are consistent with present bounds from LHC supersymmetry searches, the Planck~2018 DM relic density bound, and current limits from DD DM searches. Our results reveal characteristic mass spectra associated with these mechanisms. In particular, sbottom-neutralino coannihilation typically requires sbottom masses near $2.8~\text{TeV}$, while gluino-neutralino and stop-neutralino coannihilation scenarios allow gluino masses in the range $1$--$3~\text{TeV}$ and stop masses between $1$ and $3.5~\text{TeV}$. In coannihilation-dominated regions, the stau and chargino masses may reach values as high as $3.8~\text{TeV}$, whereas viable $A$ resonance solutions are realized for pseudoscalar Higgs masses spanning approximately $1.6$--$3.8~\text{TeV}$. We anticipate that a portion of the parameter space will be accessible to supersymmetry searches in LHC Run-3 and future runs.

hep-ph↗

Interlayer Coupling and Floquet-Driven Topological Phases in Bilayer Haldane Lattices

We investigate Floquet-driven topological phase transitions in an AB-stacked bilayer Haldane lattice with tunable intralayer hopping anisotropy. By combining interlayer hybridization, Haldane flux, and off-resonant circularly polarized light, we obtain controlled transitions among Dirac, semi-Dirac, and higher-Chern insulating phases. As the hopping anisotropy increases, the two inequivalent Dirac points move toward each other and merge at the Brillouin-zone $\mathbf{M}$ point, where a semi-Dirac dispersion emerges with linear and quadratic momentum dependence along orthogonal directions. In this regime, competition between the intrinsic Haldane mass and the Floquet-induced mass drives a sequence of sharp topological transitions with Chern numbers $C=0,\pm1,\pm2$. We further show that interlayer coupling qualitatively reshapes the Floquet band topology by inducing helicity-dependent and valley-selective band inversions at the K and K$'$ points, thereby stabilizing higher-Chern phases in the valence bands. These changes are accompanied by redistribution of the Berry curvature, bulk gap closings, and the collapse or sign reversal of quantized anomalous Hall plateaus. As the system approaches the semi-Dirac limit, the topological phase space narrows and disappears at the critical merger point, beyond which the system becomes topologically trivial even when it remains gapped. Overall, the bilayer geometry broadens the scope of Floquet topological control by enabling dynamically tunable higher-Chern phases and valley-dependent Hall responses governed by interlayer coupling and light helicity.

cond-mat.mes-hall↗

The Light Neutralino Dark Matter at Future Colliders in the MSSM with the Generalized Minimal Supergravity (GmSUGRA)

We perform a detailed investigation of light right-handed slepton bulk regions, together with the Higgs- and $Z$-resonance regimes, in the Minimal Supersymmetric Standard Model (MSSM) with Generalized Minimal Supergravity (GmSUGRA), focusing on the higgsino mass parameter scenario $μ< 0$, given that the anomalous magnetic moment of the muon may now be consistent with the Standard Model (SM) prediction. A systematic numerical exploration of the parameter space is carried out, where the bulk region is conservatively defined by the mass-splitting ratio \[ \mathcal{R}_{\tildeϕ} = \frac{m_{\tildeϕ} - m_{\tildeχ_1^0}}{m_{\tildeχ_1^0}} \gtrsim 10\%. \] In particular, we analyze the case in which the right-handed stau ($\tildeτ_R$) emerges as the Next-to-Lightest Supersymmetric Particle (NLSP). We uncover a sizeable parameter space consistent with current experimental bounds, including limits from LHC supersymmetry searches, the Planck 2018 relic density measurement, and direct detection constraints on neutralino-nucleon scattering. This region naturally accommodates bulk annihilation channels mediated by right-handed sleptons. We obtain robust upper bounds on the masses of the lightest neutralino, $m_{\tildeχ_1^0} \lesssim 143~\text{GeV}$, and the right-handed stau, $m_{\tildeτ_R} \lesssim 158~\text{GeV}$. The identified bulk region lies within the prospective reach of forthcoming dark matter direct detection facilities such as LUX-ZEPLIN, as well as future high-energy $e^+e^-$ colliders including FCC$_{\text{ee}}$ and CEPC. Conversely, scenarios with the right-handed selectron as the NLSP have already been excluded by current LHC data. Furthermore, the parameter space consistent with our findings yields contributions to the anomalous magnetic moment of the muon, $a_μ= (g_μ- 2)/2$.

hep-ph↗

Revisiting the Electroweak Supersymmetry from the Generalized Minimal Supergravity

We explore the Electroweak Supersymmetry (EWSUSY) scenario within the Minimal Supersymmetric Standard Model (MSSM) under the Generalized Minimal Supergravity (GmSUGRA) framework, given that the anomalous magnetic moment of the muon may now be consistent with the Standard Model (SM) prediction, we consider both signs of the Higgsino mass parameter, $μ< 0$ and $μ> 0$. A comprehensive scan of the parameter space is performed, subject to the experimental constraints from the LHC SUSY searches, Planck 2018 relic density, and LUX-ZEPLIN (LZ) direct detection limits. We identify the viable regions featuring neutralino dark matter production via coannihilation with stau, chargino, stop, sbottom, and gluino, as well as through $A$-funnel, Higgs-resonance, and $Z$-resonance mechanisms. Notably, the $μ< 0$ scenario yields a broader allowed parameter space, including for the first time sbottom-neutralino coannihilation solutions in GmSUGRA, which are absent for $μ> 0$. While the Higgs-pole and $Z$-pole regions for $μ> 0$ are largely excluded by the current LZ bounds, substantial viable regions remain for $μ< 0$. Gluino coannihilation scenarios are strongly constrained by the current LHC data. The characteristic mass ranges of interest include sbottoms (0.7-1.3~TeV), stops (up to 1.0~TeV for $μ> 0$ and 1.3~TeV for $μ< 0$), staus and charginos (up to 1.5~TeV), and pseudoscalar Higgs bosons in the $A$-funnel (0.4-1.4~TeV). Moreover, the supersymmetric contributions to the muon anomalous magnetic moment remain within a $2σ$ deviation from the SM prediction. And our findings suggest that significant portions of the parameter space can be probed at the future LHC SUSY searches and upcoming dark matter direct detection experiments.

hep-ph↗

Revisiting the Realistic Intersecting D6-Brane Model with positive and negative μ Terms

In light of current constraints from supersymmetry (SUSY) searches within the LHC, as well as findings from direct dark matter detection experiments such as LUX-ZEPLIN (LZ), we revisit the three-family Pati-Salam model derived from intersecting D6-branes in Type IIA string theory compactified on the $T^6/(\mathbb{Z}_2 \times \mathbb{Z}_2)$ orientifold, known for its realistic low-energy phenomenology. Since the muon anomalous magnetic moment might be in accordance with the Standard Model prediction, we conduct a comprehensive scan over the model's parameter space for each sign of the Higgsino mass parameter, $μ< 0$ and $μ> 0$. We find that the gravitino mass is typically greater than 1.5 TeV in both scenarios while simultaneously satisfying the LHC SUSY bounds, B-physics observables, and the Higgs mass constraint. Within the experimentally viable region of the parameter space, the sparticle mass spectra fall within the following ranges: gluinos lie in the range 2-18 TeV; first- and second-generation squarks and sleptons span 3-16 TeV and 1-6 TeV, respectively. For third-generation sfermions, the lightest stop, which can satisfy the dark matter relic density via neutralino-stop coannihilation consistent with the \textit{Planck} 5$σ$ bounds, has a mass in the range 0.5-1.2 TeV. The lightest neutralino can be as heavy as 2.9 TeV. Additionally, the lightest stau can be as light as 200 GeV or as heavy as 5.2 TeV. We identify several viable mechanisms, including multiple coannihilation channels and resonance mechanisms, by which the observed dark matter relic abundance is successfully realized.

hep-ph↗

Quantum magnetotransport in monolayer $\mathrm{Pt_{2}HgSe_{3}}$

We present a theoretical framework to investigate quantum magnetotransport in monolayer jacutingaite, focusing on its response to external electric fields and off-resonant circularly polarized laser irradiation. Our analysis reveals a sequence of topological phase transitions triggered by tuning these external parameters. We find that the zeroth LL exhibits spin- and valley-polarized splitting, leading to four distinct peaks in the DOSs for the $K$ and $K'$ valleys. Using the Kubo formalism, we calculate both longitudinal and Hall magneto-optical conductivities based on the Kane-Mele model. Our results demonstrate that external electric, magnetic, and off-resonant optical fields can control these conductivities. These findings highlight monolayer jacutingaite as a highly tunable platform with strong potential for future applications in photonics, optoelectronics, and topological quantum devices.

cond-mat.mes-hall↗

The Light Neutralino Dark Matter in the Generalized Minimal Supergravity (GmSUGRA)

We investigate both the $Z$ and $H$ poles solutions for the Higgsino mass parameter $μ>0$ and $μ<0$ for the neutralino dark matter in light of the LHC supersymmetry searches and the direct detection dark matter experiments, LUX-ZEPLIN (LZ), in the Generalized Minimal Supergravity (GmSUGRA). Our study indicates that the latest experimental constraints from the LHC and LZ Collaborations exclude the light Higgsinos in the $Z$ and $H$ pole regions for the $μ>0$ case. Interestingly, for the $μ< 0$ case, a very light Higgsinos can still be consistent with the current constraints from the electroweakino searches and LZ experiment in the $Z$ and $H$ poles. Consequently, the $μ< 0$ case appears more promising and thus requires the dedicated efforts to make definitive conclusions about their current status from the experimental Collaborations. In this framework, our findings indicate a deviation of up to $2σ$ from the central value of \( a_μ\equiv (g-2)_μ/2 \), resonating with the experimental results reported by CMD and BDM.

hep-ph↗

The Right-Handed Slepton Bulk Regions for Dark Matter in the Generalized Minimal Supergravity (GmSUGRA)

We study the light right-handed slepton bulk regions for dark matter from the Generalized Minimal Supergravity (GmSUGRA) in the Minimal Supersymmetric Standard Model (MSSM). In our comprehensive numerical studies, we show that $\mathcal{R_{\tildeϕ}}\gtrsim10\%$ is a conservative criteria to formulate bulk region, where $\mathcal{R_{\tildeϕ}}\equiv({m_{\tildeϕ}-m_{\tildeχ_1^0}})/{m_{\tildeχ_1^0}}$. For right-handed stau as the Next to the Lightest Supersymmetric Partcile (NLSP), we find a large viable parameter space, consistent with the current LHC constraints, Planck2018 dark matter relic density bounds, and direct bounds on neutralino-nucleons scattering cross-section that naturally supports the right-handed stau bulk regions for dark matter. In particular, the upper bounds on the masses of the Lightest Supersymmetric Particle (LSP) neutralino and right-handed stau are about 120.4 GeV and 138 GeV, respectively. This bulk region may be beyond the current LHC reach and could be probed at LUX-ZEPLIN, a next-generation dark matter direct detection experiment, the Future Circular Collider (FCC-ee) at CERN, and the Circular Electron Positron Collider (CEPC). However, the scenario with the right-handed selectron as the NLSP is excluded by the LHC supersymmetry searches.

hep-ph↗

Revisit a realistic intersecting D6-brane with modified soft SUSY terms

Because there are a few typos in the supersymmetry breaking sfermion masses and trilinear soft term, regarding the current Large Hadron Collider (LHC) and dark matter searches, we revisit a three-family Pati-Salam model based on intersecting D6-branes in Type IIA string theory on a $\mathbf{T^6/(\Z_2\times \Z_2)}$ orientifold with a realistic phenomenology. We study the viable parameter space and discuss the spectrum consistent with the current LHC Supersymmetry searches along with the dark matter relic density bounds from the Planck 2018 data. For the gluinos and first two generations of sfermions, we observe that the gluino mass is in the range [2, 14] TeV, the squarks mass range is [2, 13] TeV and the sleptons mass is in the range [1, 5] TeV. We achieve the cold dark matter relic density consistent with 5$σ$ Planck 2018 bounds via A-funnel and coannihilation channels such as stop-neutralino, stau-neutralino, and chargino-neutralino. Except for a few chargino-neutralino coannihilation solutions, these solutions also satisfy current nucleon-neutralino spin-independent and spin-dependent scattering cross-sections and may be probed by future dark matter searches.

hep-ph↗

Investigating the GmSUGRA in the MSSM through the long-lived bino NLSP at the HL-LHC

The axino, the supersymmetric partner of axion, is a well-motivated warm/hot dark matter candidate, and provides a natural solution to the relic density problem for the bino-like neutralino if it is the lightest supersymmetric particle (LSP). With the Generalized Minimal Supergravity, we study such kind of the viable parameter space where the bino-like neutralino is the next-to-LSP (NLSP) and the axino is the LSP. In addition, we consider a scenario where the bino is a long-lived NLSP with the lifetime varying from $10^{-6}$s to $10^{-4}$s, and then propose a new signal searching scheme involving one displaced photon together with the large missing transverse momentum at the HL-LHC. The bino-like lightest neutralino lies under or around 100 GeV and is produced as a decay product of the right-handed sleptons.The relevant axion coupling $f_a$ can be probed up to $\mathcal{O}(10^9)$ GeV at 2$σ$ level for the right-handed slepton mass under 300 GeV and the lightest neutralino mass under 100 GeV.

hep-ph↗

Probing Relatively Heavier Right-Handed Selectron at the CEPC, $\rm\bf {FCC_{ee}}$ and ILC

We employ the low energy Minimal Supersymmetric Standard Model (MSSM) to explore the parameter space associated with $Z$-pole and Higgs-pole solutions. Such parameter spaces can not only saturate the cold dark matter relic density bound within 5$σ$ set by the Planck 2018, but also satisfy the other standard collider mass bounds and B-physics bounds. In particular, we show that the right-handed selectron can be light. Thus, we propose a search for the relatively heavier right-handed selectron at the future lepton colliders with the center-of-mass energy $\sqrt{s}=240$ GeV and integrated luminosity 3000 $\rm{fb^{-1}}$ via mono-photon channel: $e^{+}_{R} e^{-}_{R}\rightarrow {\tilde χ_{1}^{0}(bino)}+{\tilde χ_{1}^{0}(bino)}+γ$. We show that for the $Z$-pole case the right-handed selectron will be excluded up to 180 GeV and 210 GeV respectively at 3$σ$ and 2$σ$, while the right-handed selectron will be excluded up to 140 GeV and 180 GeV respectively at 3$σ$ and 2$σ$ in case of Higgs-pole.

hep-ph↗