arXiv subjects

Aqeel Ahmed

Publications and source records attributed to Aqeel Ahmed.

At least 19 recordsLinked to original sources

arXiv2026

Primordial Dirac Leptogenesis

We present a novel realization of Dirac leptogenesis based on the post-inflationary reheating phase of the early universe. An asymmetry generated within the scalar sector via CP-violating and out-of-equilibrium inflaton decays is transferred to chiral neutrinos through Yukawa interactions and then to baryons via electroweak sphalerons. We describe in detail a minimal realization of this mechanism that naturally accommodates small neutrino Yukawa couplings and results in contributions to the effective number of relativistic species, $N_{\text{eff}}$, testable in upcoming cosmological observations.

hep-ph↗

arXiv2025

Long-Lived-Particle Signals of a Composite Hidden Sector through the Neutrino Portal

We explore the signals of a scenario in which the composite states of a strongly coupled hidden sector couple to the Standard Model through the neutrino portal, giving rise to the neutrino masses. We consider a framework in which the hidden sector is conformal in the ultraviolet and the compositeness scale lies below the weak scale. If the lightest composite state in the hidden sector is a scalar, its decay rate back to the Standard Model is suppressed by angular momentum considerations and can naturally be small, giving rise to long-lived particle signals. We determine the current constraints on this class of models and explore the reach of future collider and beam dump searches. We find that FASER, SHiP, and Belle II can potentially probe a significant part of the unexplored parameter space.

hep-ph↗

arXiv2025

Radiative Symmetry Breaking with a Scale Invariant Seesaw

We study a scale invariant inverse seesaw model that radiatively generates the electroweak scale, the Standard Model (SM) neutrino masses, and stabilizes the electroweak vacuum. Previous studies have noted that the SM Higgs potential and the electroweak scale can be radiatively generated via the minimal seesaw mechanism. This scenario, called the ``Neutrino Option'', was UV-completed by a scale invariant framework. However, these models predict singlet neutrino and scalar masses at $10^7-10^9$ GeV, beyond any experimental reach and leave the electroweak vacuum meta/un-stable at high energies. In this work, we propose modifications to this framework that lower the singlet neutrino masses to experimentally accessible scales through an inverse seesaw mechanism, while fully stabilizing the electroweak vacuum with an additional singlet scalar. The possibility of generating the observed baryon asymmetry of the universe via leptogenesis is also explored.

hep-ph↗

arXiv2025

Composite Dark Matter and Neutrino Masses from a Light Hidden Sector

We study a class of models in which the particle that constitutes dark matter arises as a composite state of a strongly coupled hidden sector. The hidden sector interacts with the Standard Model through the neutrino portal, allowing the relic abundance of dark matter to be set by annihilation into final states containing neutrinos. The coupling to the hidden sector also leads to the generation of neutrino masses through the inverse seesaw mechanism, with composite hidden sector states playing the role of the singlet neutrinos. We focus on the scenario in which the hidden sector is conformal in the ultraviolet, and the compositeness scale lies at or below the weak scale. We construct a holographic realization of this framework based on the Randall-Sundrum setup and explore the implications for experiments. We determine the current constraints on this scenario from direct and indirect detection, lepton flavor violation and collider experiments and explore the reach of future searches. We show that in the near future, direct detection experiments and searches for $μ\rightarrow e$ conversion will be able to probe new parameter space. At colliders, dark matter can be produced in association with composite singlet neutrinos via Drell Yan processes or in weak decays of hadrons. We show that current searches at the Large Hadron Collider have only limited sensitivity to this new production channel and we comment on how the reconstruction of the singlet neutrinos can potentially expand the reach.

hep-ph↗

arXiv2025

General form of effective operators from hidden sectors

We perform a model-independent analysis of the dimension-six terms that are generated in the low energy effective theory when a hidden sector that communicates with the Standard Model (SM) through a specific portal operator is integrated out. We work within the Standard Model Effective Field Theory (SMEFT) framework and consider the Higgs, neutrino and hypercharge portals. We find that, for each portal, the forms of the leading dimension-six terms in the low-energy effective theory are fixed and independent of the dynamics in the hidden sector. For the Higgs portal, we find that two independent dimension-six terms are generated, one of which has a sign that, under certain conditions, is fixed by the requirement that the dynamics in the hidden sector be causal and unitary. In the case of the neutrino portal, for a single generation of SM fermions and assuming that the hidden sector does not violate lepton number, a unique dimension-six term is generated, which corresponds to a specific linear combination of operators in the Warsaw basis. For the hypercharge portal, a unique dimension-six term is generated, which again corresponds to a specific linear combination of operators in the Warsaw basis. For both the neutrino and hypercharge portals, under certain conditions, the signs of these terms are fixed by the requirement that the hidden sector be causal and unitary. We perform a global fit of these dimension-six terms to electroweak precision observables, Higgs measurements and diboson production data and determine the current bounds on their coefficients.

hep-ph↗

arXiv2024

Conformal little Higgs models

Little Higgs models address the hierarchy problem by identifying the SM Higgs doublet as pseudo-Nambu--Goldstone bosons (pNGB) arising from global symmetries with collective breakings. These models are designed to address the little hierarchy problem up to a scale of $Λ\!\sim\! {\cal O}(10)$ TeV. Consequently, these models necessitate an ultraviolet (UV) completion above this scale. On the other hand, conformal extensions of the Standard Model are intriguing because scales emerge as a consequence of dimensional transmutation. In this study, we present a unified framework in which the electroweak hierarchy problem is tackled through a conformal symmetry collectively broken around the TeV scale, offering an appealing UV completion for little Higgs models. Notably, this framework automatically ensures the presence of the required UV fixed points, eliminating the need for careful adjustments to the particle content of the theory. Moreover, this framework naturally addresses the flavor puzzles associated with composite or little Higgs models. Furthermore, we suggest that in this framework all known little Higgs models can be UV-completed through conformal dynamics above the scale $Λ$ up to arbitrary high scales.

hep-ph↗

arXiv2023

Transport in strained graphene: Interplay of Abelian and axial magnetic fields

Immersed in external magnetic fields ($B$), buckled graphene constitutes an ideal tabletop setup, manifesting a confluence of time-reversal symmetry (${\mathcal T}$) breaking Abelian ($B$) and ${\mathcal T}$-preserving strain-induced internal axial ($b$) magnetic fields. In such a system, here we numerically compute two-terminal conductance ($G$), and four- as well as six-terminal Hall conductivity ($σ_{xy}$) for spinless fermions. On a flat graphene ($b=0$), the $B$ field produces quantized plateaus at $G=\pm |σ_{xy}|=(2n+1) e^2/h$, where $n=0,1,2, \cdots$. The strain induced $b$ field lifts the two-fold valley degeneracy of higher Landau levels and leads to the formation of additional even-integer plateaus at $G=\pm |σ_{xy}|= (2,4,\cdots)e^2/h$, when $B>b$. While the same sequence of plateaus is observed for $G$ when $b>B$, the numerical computation of $σ_{xy}$ in Hall bar geometries in this regime becomes unstable. A plateau at $G=σ_{xy}=0$ always appears with the onset of a charge-density-wave order, causing a staggered pattern of fermionic density between two sublattices of the honeycomb lattice.

cond-mat.mes-hall↗

arXiv2023

Impact of 700keV Ni++ ion irradiation on structural and optical properties of GaN

In this paper, we present the effects of high-energy Ni++ ion irradiation on the structural and optical properties of GaN films. Three different irradiation doses of 1013, 1014, and 1015 ions/cm2 were used while keeping the ion energy at 700keV. The irradiation induced structural and optical changes in GaN films were measured using X-ray diffraction (XRD) and UV-Vis spectroscopy. The XRD measurements on irradiated films discovered several extra peaks in the XRD spectrum compared with the as-grown GaN film indicating the formation of new phases or defects in the GaN film due to the ion irradiation. The intensity of these extra peaks increases with increasing ion dose, suggesting that the density of defects in the GaN film also increases with increasing ion dosage. The UV-Vis measurements revealed a decrease in the bandgap of the irradiated GaN films from 3.40 eV for the pristine GaN film to 3.26 eV for the film irradiated with highest dose of 1015 ions/cm2. The decrease in bandgap can be attributed to the creation of defects and/or the formation of new phases in the GaN film due to the ion irradiation.

cond-mat.mtrl-sci↗

arXiv2023

Ultraviolet freeze-in dark matter through the dilaton portal

We study a class of models in which the Standard Model (SM) and dark matter (DM) belong to a conformal/scale-invariant theory at high energies. Scale invariance is spontaneously broken at scale $f$, giving rise to a dilaton as the corresponding Goldstone boson. In the low energy theory, we assume that DM interacts with the SM solely through the dilaton portal, which is suppressed by the conformal breaking scale $f$. For $f\gg{\rm TeV}$, the portal interactions are extremely weak, resulting in DM not being in thermal equilibrium with the SM. Thus, ultraviolet freeze-in production of DM occurs through the dilaton portal, being most effective at the maximum temperature of the SM bath. The temperature evolution is greatly impacted by the reheating dynamics, which we parametrize using a general equation of state $w$ and temperature at the end of reheating $T_{\rm rh}$. We analyze the implications of the reheating dynamics for DM production in this framework and identify regions of parameter space that result in the observed DM relic abundance for a wide range of DM masses and reheating temperatures for scalar, vector, or fermion DM.

hep-ph↗

arXiv2023

Higgs boson induced reheating and ultraviolet frozen-in dark matter

A reheating phase in the early universe is an essential part of all inflationary models during which not only the Standard Model (SM) quanta are produced but it can also shed light on the production of dark matter. In this work, we explore a class of reheating models where the reheating is induced by a cubic interaction of the inflaton $ϕ$ to the SM Higgs boson ${\textbf h}$ of the form $g_{hϕ} M_{\rm Pl} ϕ|{\textbf h}|^2$ adopting the $α$-attractor T-model of inflation. Assuming inflaton as a background field such interaction implies a $ϕ$-dependent mass term of the Higgs boson and a non-trivial phase-space suppression of the reheating efficiency. As a consequence, the reheating is prolonged and the maximal temperature of the SM thermal bath is reduced. In particular, due to oscillations of the inflaton field the $ϕ$-dependent Higgs boson mass results in periodic transitions between phases of broken and unbroken electroweak gauge symmetry. The consequences of these rapid phase transitions have been studied in detail. A purely gravitational reheating mechanism in the presence of the inflaton background, i.e., for $g_{hϕ}=0$, has also been investigated. It turned out that even though it may account for the total production of SM radiation in the absence of $g_{hϕ}$, its contribution to the reheating is subdominant for the range of $g_{hϕ}$ considered in this work. As a dark matter candidate, a massive Abelian vector boson, $X_μ$, has been considered. Various production mechanisms of $X_μ$ have been discussed including (i) purely gravitational production from the inflaton background, (ii) gravitational freeze-in from the SM quanta, (iii) inflaton decay through a dim-5 effective operator, and (iv) Higgs portal freeze-in and Higgs decay through a dim-6 effective operator. Parameters that properly describe the observed relic abundance have been determined.

hep-ph↗

arXiv2022

Implications of time-dependent inflaton decay on reheating and dark matter production

We discuss the production of radiation and dark matter assuming a time-dependent inflaton decay rate during the reheating period. It is shown that the time dependence of the inflaton decay rate can substantially modify the reheating dynamics. As an illustration, a leading interaction between the inflaton $ϕ$ and the Higgs doublet ${\boldsymbol h}$ of the form $ϕ|{\boldsymbol h}|^2$ was adopted. In the presence of such interaction, the Higgs doublet acquires a $ϕ$-dependent mass which generates vacuum-expectation-value due to inflaton oscillations and breaks the Standard Model gauge symmetry. This leads to a time-dependent inflaton decay rate during the reheating period, and consequently, the production of radiation and dark matter during this period is modified. Regions of the parameter space that describe the observed value of the dark matter abundance were found and compared with the standard case when inflaton the decay rate is constant.

hep-ph↗

arXiv2021

Structural Order of the Molecular Adlayer Impacts the Stability of Nanoparticle-on-Mirror Plasmonic Cavities

Immense field enhancement and nanoscale confinement of light are possible within nanoparticle-on-mirror (NPoM) plasmonic resonators, which enable novel optically-activated physical and chemical phenomena, and render these nanocavities greatly sensitive to minute structural changes, down to the atomic scale. Although a few of these structural parameters, primarily linked to the nanoparticle and the mirror morphology, have been identified, the impact of molecular assembly and organization of the spacer layer between them has often been left uncharacterized. Here, we experimentally investigate how the complex and reconfigurable nature of a thiol-based self-assembled monolayer (SAM) adsorbed on the mirror surface impacts the optical properties of the NPoMs. We fabricate NPoMs with distinct molecular organizations by controlling the incubation time of the mirror in the thiol solution. Afterwards, we investigate the structural changes that occur under laser irradiation by tracking the bonding dipole plasmon mode, while also monitoring Stokes and anti-Stokes Raman scattering from the molecules as a probe of their integrity. First, we find an effective decrease in the SAM height as the laser power increases, compatible with an irreversible change of molecule orientation caused by heating. Second, we observe that the nanocavities prepared with a densely packed and more ordered monolayer of molecules are more prone to changes in their resonance compared to samples with sparser and more disordered SAMs. Our measurements indicate that molecular orientation and packing on the mirror surface play a key role in determining the stability of NPoM structures and hence highlight the under-recognized significance of SAM characterization in the development of NPoM-based applications.

physics.optics↗

arXiv2021

Intrinsic Luminescence Blinking from Plasmonic Nanojunctions

Plasmonic nanojunctions, consisting of adjacent metal structures with nanometre gaps, can support localised plasmon resonances that boost light matter interactions and concentrate electromagnetic fields at the nanoscale. In this regime, the optical response of the system is governed by poorly understood dynamical phenomena at the frontier between the bulk, molecular and atomic scales. Here, we report ubiquitous spectral fluctuations in the intrinsic light emission from photo-excited gold nanojunctions, which we attribute to the light-induced formation of domain boundaries and quantum-confined emitters inside the noble metal. Our data suggest that photoexcited carriers and gold adatom - molecule interactions play key roles in triggering luminescence blinking. Surprisingly, this internal restructuring of the metal has no measurable impact on the Raman signal and scattering spectrum of the plasmonic cavity. Our findings demonstrate that metal luminescence offers a valuable proxy to investigate atomic fluctuations in plasmonic cavities, complementary to other optical and electrical techniques.

physics.optics↗

arXiv2020

Gravitational production of vector dark matter

A model of vector dark matter that communicates with the Standard Model only through gravitational interactions has been investigated. It has been shown in detail how does the canonical quantization of the vector field in varying FLRW geometry implies a tachyonic enhancement of some of its momentum modes. Approximate solutions of the mode equation have been found and verified against exact numerical ones. De Sitter geometry has been assumed during inflation while after inflation a non-standard cosmological era of reheating with a generic equation of state has been adopted which is followed by the radiation-dominated universe. It has been shown that the spectrum of dark vectors produced gravitationally is centered around a characteristic comoving momentum $k_\star$ that is determined in terms of the mass of the vector $m_X$, the Hubble parameter during inflation $H_{\rm I}$, the equation of state parameter $w$ and the efficiency of reheating $γ$. Regions in the parameter space consistent with the observed dark matter relic abundance have been determined, justifying the gravitational production as a viable mechanism for vector dark matter. The results obtained in this paper are applicable within various possible models of inflation/reheating with non-standard cosmology parametrized effectively by the corresponding equation of state and efficiency of reheating.

hep-ph↗

arXiv2020

A Minimal Model for Neutral Naturalness and pseudo-Nambu-Goldstone Dark Matter

We outline a scenario where both the Higgs and a complex scalar dark matter candidate arise as the pseudo-Nambu-Goldstone bosons of breaking a global $SO(7)$ symmetry to $SO(6)$. The novelty of our construction is that the symmetry partners of the Standard Model top-quark are charged under a hidden color group and not the usual $SU(3)_c$. Consequently, the scale of spontaneous symmetry breaking and the masses of the top partners can be significantly lower than those with colored top partners. Taking these scales to be lower at once makes the model more natural and also reduces the induced non-derivative coupling between the Higgs and the dark matter. Indeed, natural realizations of this construction describe simple thermal WIMP dark matter which is stable under a global $U(1)_D$ symmetry. We show how the Large Hadron Collider along with current and next generation dark matter experiments will explore the most natural manifestations of this framework.

hep-ph↗

arXiv2020

A light dilaton at the LHC

In this paper, we explore the possibility that a light dilaton can be the first sign of new physics at the LHC. The dilaton could emerge in approximate scale invariant UV completions of the SM as the Goldstone boson associated with the spontaneous breaking of the scale invariance. We study in detail the phenomenology of the dilaton at the LHC in the mass range of [$10-300$] GeV including the case where the dilaton can mix with the SM Higgs boson, leading to an interesting interplay between direct and indirect constraints. A possibility that the dilaton acts as a portal to a dark sector is also considered. As a minimal realization, the dark sector includes a dark photon lighter than the dilaton implying sizeable missing energy signatures. Several simplified benchmark models that can encode different UV completions are discussed, for which we scrutinize the current and future LHC reach.

hep-ph↗

arXiv2019

Dilaton portal in strongly interacting twin Higgs models

We consider a strongly interacting twin Higgs (SITH) model where an ultraviolet completion of twin Higgs mechanism is realized by a strongly coupled approximately scale invariant theory. Besides the Standard Model (SM) and twin sectors, the low energy effective theory contains a relatively light scalar called a dilaton --- the pseudo Goldstone boson of spontaneously broken scale invariance. The dilaton provides a unique portal between the SM and twin sectors whose phenomenology could provide an important probe of the twin Higgs mechanism. As a concrete example, we consider a holographic twin Higgs model where the role of the dilaton is played by the radion. The phenomenology of this model is fully determined by a few parameters and our analysis concludes that at the HL-LHC (14 TeV) and HE-LHC (27 TeV) with 3000/fb most of the natural parameter space can be probed.

hep-ph↗

arXiv2019

Dynamical origin of flavor hierarchies in a warped extra dimension

Extensions of the Standard Model featuring a warped extra dimension compactified on an $S^1/\mathbb{Z}_2$ orbifold, in which the fermions and gauge bosons live in the bulk of the fifth dimension, offer one of the most compelling mechanisms for addressing both the hierarchy problem and the flavor puzzle of the Standard Model. However, the five-dimensional mass terms of bulk fermions must be odd functions on the orbifold, and as such they should be described by a field depending on the coordinate of the extra dimension. We demonstrate the feasibility of dynamically generating these fermion bulk masses with a bulk scalar field in warped extra dimensions. The bulk scalar acquires a vacuum expectation value, which is odd under the orbifold symmetry and gives rise to the fermion bulk masses through non-universal Yukawa-like interactions. Like in the conventional Randall-Sundrum setup, the localization of the different fermion zero modes along the extra dimension naturally explains the observed flavor structure and four-dimensional mass hierarchy of the SM fermions. We study the phenomenological implications of the backreaction on the metric and the modified fermion profiles due to the bulk scalar field on electroweak precision and flavor observables. Using up-to-date data, we show that the contributions to the $S$, $T$, and $ε_K$ parameters require the mass of the first Kaluza-Klein gluon resonance to be of order 14 and 10 TeV in the minimal and the custodial model, respectively, regardless of the effect of the backreaction. Furthermore, effective flavor-changing interactions among the SM fermions induced by the bulk scalar are discussed. We also comment on the potential impact of the Higgs portal interaction of the bulk scalar on the couplings of the Higgs boson.

hep-ph↗