arXiv subjects

Aqeel Ahmed

Publications and source records attributed to Aqeel Ahmed.

34 records · Page 2Linked to original sources

arXiv2018

Multi-Component Dark Matter: the vector and fermion case

Multi-component dark matter scenarios constitute natural extensions of standard single-component setups and offer attractive new dynamics that could be adopted to solve various puzzles of dark matter. In this work we present and illustrate properties of a minimal UV-complete vector-fermion dark matter model where two or three dark sector particles are stable. The model we consider is an extension of the Standard Model (SM) by spontaneously broken extra $U(1)_X$ gauge symmetry and a Dirac fermion. All terms in the Lagrangian which are consistent with the assumed symmetry are present, so the model is renormalizable and consistent. To generate mass for the dark-vector $X_μ$ the Higgs mechanism with a complex singlet $S$ is employed in the dark sector. Dark matter candidates are the massive vector boson $X_μ$ and two Majorana fermions $ψ_\pm$. All the dark sector fields are singlets under the SM gauge group. The set of three coupled Boltzmann equations has been solved numerically and discussed. We have performed scans over the parameter space of the model implementing the total relic abundance and direct detection constraints. The dynamics of the vector-fermion dark matter model is very rich and various interesting phenomena appear, in particular, when the standard annihilations of a given dark matter are suppressed then the semi-annihilations, conversions and decays within the dark sector are crucial for the evolution of relic abundance and its present value. Possibility of enhanced self-interaction has been also discussed.

hep-ph↗

arXiv2018

Heavy Higgs of the Twin Higgs Models

Twin Higgs models are the prime illustration of neutral naturalness, where the new particles of the twin sector, gauge singlets of the Standard Model (SM), ameliorate the little hierarchy problem. In this work, we analyse phenomenological implications of the heavy Higgs of the Mirror Twin Higgs and Fraternal Twin Higgs models, when electroweak symmetry breaking is linearly realized. The most general structure of twin Higgs symmetry breaking, including explicit soft and hard breaking terms in the scalar potential, is employed. The direct and indirect searches at the LHC are used to probe the parameter space of Twin Higgs models through mixing of the heavy Higgs with the SM Higgs and decays of the heavy Higgs to the SM states. Moreover, for the Fraternal Twin Higgs, we study the production and decays of twin glueball and bottomonium states to the SM light fermions, which have interesting signatures involving displaced vertices and are potentially observable at the colliders.

hep-ph↗

arXiv2017

Clockwork Goldstone Bosons

The clockwork mechanism has recently been proposed as a natural way to generate hierarchies among parameters in quantum field theories. The mechanism is characterized by a very specific pattern of spontaneous and explicit symmetry breaking, and the presence of new light states referred to as `gears'. In this paper we begin by investigating the self-interactions of these gears in a scalar clockwork model and find a parity-like selection rule at all orders in the fields. We then proceed to investigate how the clockwork mechanism can be realized in 5D linear dilaton models from the spontaneous symmetry breaking of a complex bulk scalar field. We also discuss how the clockwork mechanism is manifest in the scalar components of 5D gauge theories in the linear dilaton model, and build their 4D deconstructed analogue. Finally we discuss attempts at building both 4D and 5D realizations of a non-abelian scalar clockwork mechanism, where in the latter we consider scenarios in which the Goldstone bosons arise from 5D scalar and 5D gauge fields.

hep-ph↗

arXiv2017

Implications of the absence of high-mass radion signals

Given the disappearance of the 750 GeV di-photon LHC signal and the absence of signals at high mass in this and other channels, significant constraints on the mixed Higgs-radion of the five-dimensional Randall-Sundrum model arise. By combining all channels, these constraints place a significant radion-mass-dependent lower bound on the radion vacuum expectation value that is fairly independent of the amount of Higgs radion mixing.

hep-ph↗

arXiv2015

Higgs dark matter from a warped extra dimension

We present a 5D $\mathbb{Z}_2$-symmetric IR-UV-IR model with a {\it warped KK-parity} under which the bulk fields have towers of either even or odd KK-modes. We show that this $\mathbb{Z}_2$-symmetric geometry is equivalent to two times the UV-IR geometry (Randall-Sundrum model) provided each bulk field is subject to Neumann (or mixed) and Dirichlet boundary conditions at the UV-brane for even and odd fields, respectively. The 5D Standard Model (SM) bosonic sector is considered, such that in the 4D low-energy effective theory the $\mathbb{Z}_2$-even zero-modes correspond to the SM degrees of freedom, whereas the $\mathbb{Z}_2$-odd zero modes serve as a dark sector. In the zero-mode scalar sector, the even scalar mimics the SM Higgs boson, while the odd scalar (dark-Higgs) is stable and serves as a dark matter candidate. Implications for this dark matter are discussed; it is found that the dark-Higgs can provide only a small fraction of the observed dark matter abundance.

hep-ph↗

arXiv2015

Radius stabilization and dark matter with a bulk Higgs in warped extra dimension

We employ an $SU(2)$ bulk Higgs doublet as the stabilization field in Randall-Sundrum model with appropriate bulk and brane-localized potentials. The gauge hierarchy problem can be solved for an exponentially IR-localized Higgs background field with mild values of fundamental parameters of the 5D theory. We consider an IR-UV-IR background geometry with the 5D SM fields in the bulk such that all the fields have even and odd tower of KK-modes. The zero-mode 4D effective theory contains all the SM fields plus a stable scalar, which serves as a dark matter candidate.

hep-ph↗

arXiv2015

Higgs Dark Matter from a Warped Extra-Dimension -- the truncated-inert-doublet model

We construct a 5D $\mathbb{Z}_2$-symmetric model with three D3-branes: two IR ones with negative tension located at the ends of an extra-dimensional interval and a UV-brane with positive tension placed in the middle of the interval -- IR-UV-IR model. The background solutions for this geometric setup are found without and with taking into account the backreaction of the matter fields. A 5D $SU(2)$ Higgs doublet is employed as the Goldberger-Wise stabilizing field in this geometry and solutions of the 5D coupled scalar-gravity equations are found by using the superpotential method. Within this setup we investigate the low-energy (zero-mode) effective theory for the bulk Standard Model (SM) bosonic sector. The $\mathbb{Z}_2$-even zero-modes correspond to known standard degrees of freedom, whereas the $\mathbb{Z}_2$-odd zero modes might serve as a dark sector. The effective low-energy scalar sector contains a scalar which mimics the SM Higgs boson and a second stable scalar particle (dark-Higgs) is a dark matter candidate; the latter is a component of the zero-mode of the $\mathbb{Z}_2$-odd Higgs doublet. The model that results from the $\mathbb{Z}_2$-symmetric background geometry resembles the Inert Two Higgs Doublet Model. The effective theory turns out to have an extra residual $SU(2)\times U(1)$ global symmetry that is reminiscent of an underlying 5D gauge transformation for the odd degrees of freedom. At tree level the SM Higgs and the dark-Higgs have the same mass; however, when leading radiative corrections are taken into account the dark-Higgs turns out to be heavier than the SM Higgs. Implications for dark matter are discussed; it is found that the dark-Higgs can provide only a small fraction of the observed dark matter abundance.

hep-ph↗

arXiv2014

Thick-Brane Cosmology

We search for time-dependent solutions for the 5-dimensional system of a scalar field canonically coupled to gravity. Time-independent and time-dependent scalar field configurations with the most general homogeneous and isotropic 4D metric are considered. For the case of time-independent scalar field, the time evolution of the scale factor is obtained for different values of the spatial curvature $k=0,\pm 1$. In the case of time-dependent scalar field, two classes of solutions are discussed and an extension of the superpotential formalism is proposed.

hep-th↗

arXiv2014

Generalized Randall-Sundrum model with a single thick brane

A generalized version of the Randall-Sundrum model-2 with different cosmological constants on each side of a brane has been discussed. A possibility of replacing the singular brane by a configuration of a scalar field has been also considered, the Einstein equations for this setup were solved and stability of the solution discussed. It has been shown that under mild assumptions the relation between cosmological constants and the brane tension obtained in the brane limit does not depend on the particular choice of the regularizing profile of the scalar field.

hep-th↗

arXiv2013

On Fuzzy semihyperrings

In this article we introduce the study of fuzzy semihyperrings and fuzzy R-semihypermodules, where R is a semihyperrings and R-semihypermodules are represntations of R. In particular, semihyperrings all of whose hyperideals are idempotent, called fully idempotent semihyperrings, are investigated in a fuzzy context. It is proved, among other results, that a semihyperring R is fully idempotent if and only if the lattics of fuzzy hyperideals of R is distributive under the sum and product of fuzzy hyperideals. It is also shown that the set of proper fuzzy prime hyperideals of a fully idempotent semihyperring R admits the structure of a topological space, called the fuzzy prime spectrum of R.

math.GM↗

arXiv2013

Brane modeling in warped extra-dimension

Five-dimensional scenarios with infinitesimally thin branes replaced by appropriate configurations of a scalar field were considered. A possibility of periodic extra dimension was discussed in the presence on non-minimal scalar-gravity coupling and a generalized Gibbons-Kallosh-Linde sum rule was found. In order to avoid constraints imposed by periodicity, a non-compact spacial extra dimension was introduced. A five dimensional model with warped geometry and two thin branes mimicked by a scalar profile was constructed and discussed. In the thin brane limit the model corresponds to a set-up with two positive-tension branes. The presence of two branes allows to address the issue of the hierarchy problem which could be solved by the standard warping of the four dimensional metric provided the Higgs field is properly localized. Stability of the background solution was discussed and verified in the presence of the most general perturbations of the metric and the scalar field.

hep-th↗

arXiv2011

Comparative Study of B_{c} --> D_{s}^{*}l^{+}l^{-} Decays in Standard Model and Supersymmetric Models

A comparative study of the exclusive rare $B_{c}\rightarrow D_{s}^{\ast}\ell^{+}\ell^{-}$ ($\ell=μ, τ$) decays has been made in the minimal supersymmetric models (MSSM) and the SUSY SO(10) GUT models. In this context, various physical observables such as branching ratios $(\mathcal{BR})$, forward-backward asymmetries $(\mathcal{A}_{FB})$, lepton polarization asymmetries $(P_{L,N,T})$ and helicity fractions ($f_{L,T}$) of $D_{s}^{\ast}$ meson by using the the QCD sum rules form factors have been investigated. It is found that the SUSY effects are characteristically prominent to that of the SM values for these observables. For instance, in SUSY I and SUSY II, the forward-backward asymmetry does not cross zero which is mainly due to the same sign of the $C_{7}^{eff}$ and $C_{9}^{eff}$ Wilson coefficients. Similarly in SUSY SO(10) GUT models due to the complex nature of the new Wilson coefficients -- corresponding to the new operators arising due to the contribution of neutral Higgs bosons (NHBs) -- the above mentioned observables are sizably affected. Therefore the analysis of said observables in charmed semileptonic $B$ meson decays can put some stringent constraints on the parameter space of SUSY variants and can serve as a windowpane to look beyond the SM.

hep-ph↗

arXiv2011

Fourth-generation SM imprints in B -> K^l^+l^- decays with polarized K^

The implication of the fourth-generation quarks in the B -> K^*l^+l^- (l=mu,tau) decays, when K^* meson is longitudinally or transversely polarized, is presented. In this context, the dependence of the branching ratio with polarized K^* and the helicity fractions (f_{L,T}) of K^* meson are studied. It is observed that the polarized branching ratios as well as helicity fractions are sensitive to the NP parameters, especially when the final state leptons are tauons. Hence the measurements of these observables at LHC can serve as a good tool to investigate the indirect searches of new physics beyond the Standard Model.

hep-ph↗

arXiv2011

K_1(1270)-K_1(1400) mixing and the fourth generation SM effects in B-->K_1l^+l^- decays

The implications of the fourth generation quarks in the B-->K_{1}(1270,1400)l^{+}l^{-} with l=mu, tau decays are studied, where the mass eigenstates K_{1}(1270) and K_{1}(1400) are the mixture of ^{1}{P}_{1} and ^{3}{P}_{1} states with the mixing angle θ_{K}. In this context, we have studied various observables like branching ratio (BR), forward backward asymmetry (A_{FB}) and longitudinal and transverse helicity fractions (f_{L,T}) of K_{1} meson in B-->K_{1}l^{+}l^{-} decays. To study these observables, we have used the Light Cone QCD sum rules form factors and set the mixing angle θ_{K}=-34^{o}. It is noticed that the BR is suppressed for K_{1}(1400) as a final state meson compared to that of K_{1}(1270). Same is the case when the final state leptons are tauons rather than muons. In both the situations all the above mentioned observables are quite sensitive to the fourth generation effects. Hence the measurements of these observables at LHC, for the above mentioned processes can serve as a good tool to investigate the indirect searches for the existence of fourth generation quarks.

hep-ph↗

arXiv2011

Analysis of B_{C} --> D_{s}^{*}l^{+}l^{-} in the Standard Model Beyond Third Generation

The weak decays of B_{c} --> D_{s}^{*}l^{+}l^{-} (l=mu,tau) are investigated in the Standard Model with fourth generation (SM4). Taking fourth generation quark mass m_{t'} of about 300 to 600 GeV with the CKM matrix elements V_{t'b}^{*}V_{t's} in the range (0.03-1.2)*10^{-2} and using the new CP odd phase (phi_{sb}) to be 90^{o} the analysis of decay rates, forward-backward asymmetries, lepton polarization asymmetries and the helicity fractions of D_{s}^{*} meson in B_{c} --> D_{s}^{*}l^{+}l^{-} (l=mu,tau) is made. It is shown that the values of these physical observables are greatly influenced by the effects of extra sequential quark generation in the SM. It is found that the zero position of the forward-backward asymmetry in B_{c} --> D_{s}^{*} mu^{+} mu^{-} deviates significantly from that of the SM value while the effects in the decay rate, helicity fractions of D_s^{*} meson and the lepton polarization asymmetries are also quite promising. In B_{c} --> D_{s}^{*} tau^{+} tau^{-} decay, the SM4 the NP effects are somewhat mild in the decay rate and forward-backward asymmetry but they have shown them up in the longitudinal and transverse lepton polarization and also in the helicity fractions of the D_{s}^{*} meson which differ distinctively from the SM values. Thus the study of these physical observables will provide us useful information to probe new physics and helps us to search for the fourth generation of quarks (t',b') in an indirect way.

hep-ph↗

arXiv2011

Analysis of Discrete Symmetries in b-Baryon Decays

A study of the decay channels Lambda_{b}-->Lambda V, where V is a vector meson (1^-), has been done by putting together kinematical and dynamical analysis. An intensive use of the helicity formalism is involved on the kinematical side, while on the dynamical side, Heavy Quark Effective Theory (HQET) is applied to calculate the hadronic matrix elements between the baryons Lambda_{b} and Lambda. The branching ratios(BR) and helicity asymmetry parameters (alpha_{AS}) for Lambda_{b} --> Lambda J/psi, Lambda_{b} --> Lambda rho^0 and Lambda_{b} --> Lambda omega have been calculated. Since both the decay products are polarized, so they offer interesting opportunities to perform tests of time reversal, CP violation and of CPT invariance. A model independent parametrization is done via spin density matrix of the angular distribution, polarizations and some of polarization correlations of the decay products. The transverse component of the polarization and two polarization correlations are sensitive to time reversal violations. Moreover several CP- and CPT-odd observables are pointed out.

hep-ph↗