arXiv subjects

Ashok Goyal

Publications and source records attributed to Ashok Goyal.

At least 37 records · Page 2Linked to original sources

arXiv2016

Fermionic Dark Matter in a simple $t$-channel model

We consider a fermionic dark matter (DM) particle in renormalizable Standard Model (SM) gauge interactions in a simple $t$-channel model. The DM particle interactions with SM fermions is through the exchange of scalar and vector mediators which carry colour or lepton number. In the case of coloured mediators considered in this study, we find that if the DM is thermally produced and accounts for the observed relic density almost the entire parameter space is ruled out by the direct detection observations. The bounds from the monojet plus missing energy searches at the Large Hadron Collider are less stringent in this case. In contrast for the case of Majorana DM, we obtain strong bounds from the monojet searches which rule out DM particles of mass less than about a few hundred GeV for both the scalar and vector mediators.

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arXiv2015

Anomalous X-Ray Galactic signal from 7 keV spin-$3/2$ dark matter decay

In order to explain the recently reported peak at $3.55$ keV in the galactic x-ray spectrum, we propose a simple model. In this model, the Standard Model is extended by including a neutral spin-$3/2$ vector-like fermion that transforms like a singlet under SM gauge group. This $7.1$ keV spin-$3/2$ fermion is considered to comprise a portion of the observed dark matter. Its decay into a neutrino and a photon with decay life commensurate with the observed data, fits the relic dark matter density and obeys the astrophysical constraints from the supernova cooling.

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arXiv2015

Measuring anomalous $Wtb$ couplings at $e^-p$ collider

We study the accuracy with which the lowest order $CP$ conserving anomalous $Wtb$ couplings in the single top quark production at the proposed large hadron electron collider (LHeC) can be probed. The one dimensional distribution of various kinematic observables at the parton level MC and their asymmetries arising due to the presence of anomalous couplings both in the hadronic and leptonic $W$ decay is examined. \par We find that at 95 \% C.L. the anomalous coupling associated with the left handed vector current can be measured at an accuracy of the order of $\sim 10^{-2}-10^{-3}$, while those associated with the right handed vector and left as well as right handed tensor currents have sensitivity at the order of $\sim 10^{-1}-10^{-2}$ for the systematic uncertainty varying between 10\%-1\% at an integrated luminosity of 100 fb$^{-1}$. A comprehensive analysis of the combined covariance matrix derived from all one dimensional distributions of kinematical observables is used to compute the errors in anomalous couplings.

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arXiv2013

Top quark physics in the Vector Color-Octet Model

We study and constrain the parameter space of the vector color-octet model from observed data at Tevatron by studying top quark pair production and associated observables afbt and spin-correlation. In particular we study invariant mass and rapidity dependence of afbt at Tevatron. In addition to FC couplings we extend our study to include FV coupling involving first & third generation quarks for both these processes. In order to ensure that we remain within the constraints imposed by LHC data, we analyze the charge asymmetry, p_T spectrum & invariant mass in t\bar t production data at LHC. The constraints from dijet resonance searches performed by LHC are also considered. We also explore the contribution of this model to single top quark production mediated by charged & neutral color-octet vector bosons. FV couplings introduced then induce same-sign top-pair production process which is analyzed for both hadron colliders. We have incorporated effect of finite decay width of color octets on these processes. We find that it is possible to explain the observed afbt anomaly in color-octet vector model without transgressing production cross sections of all these processes both through FC and FV couplings at Tevatron. We predict best point sets in model parameter space for specific choices of color-octet masses corresponding to chi^2_min evaluated using mttb and dyy spectrum of afbt from observed data set at Tevatron (8.7 fb-1). We find that single top quark production is more sensitive to FC and FV couplings in comparison to top-pair production. We provide 95% exclusion contours on the plane of FV chiral couplings from recent data at Tevatron, CMS and ATLAS corresponding to nonobservability of large same-sign dilepton events. The four observed point sets are consistent with the cross section, charge asymmetry and spin-correlation measurements for t\bar t production & dijet searches at LHC.

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arXiv2010

Neutral Trilinear Gauge Boson Couplings in Little Higgs Models

We compute the one loop new physics effects to the CP even triple neutral gauge boson vertices $γ^\star γZ$, $γ^\star Z \,Z$, $Z^\star \,Z\,γ$ and $Z^\star\,Z\,Z$ in the context of Little Higgs models. In addition, we re-examine the MSSM contribution at the chosen point of SPS1a' and compare with the SM and Little Higgs models.

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arXiv2009

New Physics Contribution to Neutral Trilinear Gauge Boson Couplings

We study the one loop new physics effects to the CP even triple neutral gauge boson vertices $γ^\star γZ$, $γ^\star Z Z$, $Z^\star Z γ$ and $Z^\star Z Z$ in the context of Little Higgs models. We compute the contribution of the additional fermions in Littles Higgs model in the framework of direct product groups where $[SU(2)\times U(1)]^2 $ gauge symmetry is embedded in SU(5) global symmetry and also in the framework of simple group where $SU(N)\times U(1)$ gauge symmetry breaks down to $SU(2)_L\times U(1)$. We calculate the contribution of the fermions to these couplings when $T$ parity is invoked. In addition, we re-examine the MSSM contribution at the chosen point of SPS1a' and compare with the SM and Little Higgs models.

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arXiv2008

Constraints on Astro-unparticle Physics from SN 1987A

SN 1987A observations have been used to place constraints on the interactions between standard model particles and unparticles. In this study we calculate the energy loss from the supernovae core through scalar, pseudo scalar, vector, pseudo vector unparticle emission from nuclear bremsstrahlung for degenerate nuclear matter interacting through one pion exchange. In order to examine the constraints on $d_{\cal U}=1$ we considered the emission of scalar, pseudo scalar, vector, pseudo vector and tensor through the pair annihilation process $e^+e^-\to {\cal U} γ$. In addition we have re-examined other pair annihilation processes. The most stringent bounds on the dimensionless coupling constants for $d_{\cal U} =1$ and $Λ_{\cal U}= m_Z$ are obtained from nuclear bremsstrahlung process for the pseudo scalar and pseudo-vector couplings $\bigl|λ^{\cal P}_{0,1}\bigr|\leq 4\times 10^{-11}$ and for tensor interaction, the best limit on dimensionless coupling is obtained from $e^+ e^-\to {\cal U} γ$ and we get $\bigl|λ^{\cal T}\bigr| \leq 6\times 10^{-6}$.

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arXiv2008

Neutrino, Photon Interaction in Unparticle Physics

We investigate the impact of unparticle physics on the annihilation of relic neutrinos with the neutrinos identified as primary source of ultra high energy (UHE) cosmic ray events, producing a cascade of photons and charged particles. We compute the contribution of the unparticle exchange to the cross-sections $ν\barν\toγγ$ and $ν\barν\to f \bar f$ scattering. We estimate the neutrino photon decoupling temperature from the reaction rate of $ν\barν\toγγ$. We find that inclusion of unparticles can in fact account for the flux of UHE cosmic rays and can also result in the lowering of neutrino - photon decoupling temperature below the QCD phase transition for unparticle physics parameters in a certain range. We calculate the mean free path of these high energy neutrinos annihilating themselves with the relic neutrinos to produce vector and tensor unparticles.

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arXiv2006

Lepton flavour violation in The Little Higgs model

Little Higgs models with T-parity have a new source of lepton flavour violation. In this paper we consider the anomalous magnetic moment of the muon \gmtwo and the lepton flavour violating decays \mutoeg and \tautomug in Little Higgs model with T-parity \cite{Goyal:2006vq}. Our results shows that present experimental constraints of \mutoeg is much more useful to constrain the new sources of flavour violation which are present in T-parity models.

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arXiv2006

Lepton Flavor Violation in Little Higgs Model with T-Parity

If neutrino mass and mixing consistent with the neutrino oscillation data are the only source of lepton flavor violation (LFV)in nature, the other LFV decays like the radiative and semileptonic decays would be too small to be observed experimentally in the foreseeable future. These decays have been the objects of recent Belle measurments. We analyze LFV in Little Higgs Model with T-parity and find that with reasonable values of the model parameters, these decays can very well be experimentally observable.

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arXiv2006

Little Higgs model effects in $γγ\to γγ$

Though the predictions of the Standard Model (SM) are in excellent agreement with experiments there are still several theoretical problems associated with the Higgs sector of the SM, where it is widely believed that some ``{\it new physics}'' will take over at the TeV scale. One beyond the SM theory which resolves these problems is the Little Higgs (LH) model. In this work we have investigated the effects of the LH model on $\gggg$ scattering \cite{Choudhury:2006xa}.

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arXiv2006

Little Higgs model effects at $γγ$ collider

Though the predictions of the Standard Model (SM) are in excellent agreement with experiments there are still several theoretical problems, such as fine-tuning and the hierarchy problem. These problems are associated with the Higgs sector of the SM, where it is widely believed that some {\it ``new physics"} will take over at the TeV scale. One beyond the SM theory which resolves these problems is the Little Higgs (LH) model. In this work we shall investigate the effects of the LH model on $\gggg$ scattering; where the process $\gggg$ at high energies occurs in the SM through diagrams involving $W$, charged quark and lepton loops (and is, therefore, particularly sensitive to any new physics

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arXiv2005

Lepton number violation in Little Higgs model

In this note we examine the constraints imposed by muon anomalous magnetic moment ($(g-2)_μ$) and $μ^- \to e^+ e^- e^-$ on lepton number violating (LNV) couplings of the triplet Higgs in Little Higgs (LH) model.

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arXiv2005

Neutrino mass and lepton number violation in the Little Higgs model

We study lepton number violation in Little Higgs model and find that the choice of putting triplet Higgs vev equal to zero so as not to have any tree level neutrino Majorana mass is not natural in the sense that such a term is generated at the one loop level. We investigate the contribution of exotic lepton number violating terms on neutrinoless double beta decay, K meson decay and on trilepton production in $ν$-N scattering.

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arXiv2005

$B_d - \bar{B}_d$ mass difference in Little Higgs model

An alternate solution of hierarchy problem in the Standard Model namely, the Little Higgs model, has been proposed lately. In this work $B_d^0-\bar{B_d^0}$ mass difference in the framework of the Little Higgs model is evaluated. The experimental limits on the mass difference is shown to provide meaningful constraints on the parameter space of the model.

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arXiv2003

Mixed Phase in Compact Starts : M-R relations and radial oscillations

It is believed that quark stars or neutron stars with mixed phase in the core have smaller radii compared to ordinary compact stars. With the recent observation of several low radius objects, typically a radius of $<10 Km.$ for star of mass $< 1M_0$ in low mass X-ray binaries (LMXB), it has become very important to understand the nature of these objects. An accurate determination of mass-radius relationship of these objects provide us with a physical laboratory to study the composition of high density matter and the nature of phase transition. We study the effect of quark and nuclear matter mixed phase on mass radius relationship and radial oscillations of neutron stars. We find that the effect of the mixed phase is to decrease the maximum mass of a stable neutron star and to decrease the radial frequencies .

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arXiv2003

Magnetised Neutron Stars : An Overview

In the presence of strong magnetic field reported to have been observed on the surface of some neutron stars and on what are called Magnetars, a host of physical phenomenon from the birth of a neutron star to free streaming neutrino cooling phase will be modified. In this review I will discuss the effect of magnetic field on the equation of state of high density nuclear matter by including the anomalous magnetic moment of the nucleons into consideration. I would then go over to discuss the neutrino interaction processes in strong as well as in weak magnetic fields. The neutrino processes are important in studying the propagation of neutrinos and in studying the energy loss, Their study is a prerequisite for the understanding of actual dynamics of supernova explosion and on the stabilization of radial pulsation modes through the effect on bulk viscosity. The anisotropy introduced in the neutrino emission and through the modification of the shape of the neutrino sphere may explain the observed pulsar kicks.

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arXiv2003

Hybrid Stars

Recently there have been important developments in the determination of neutron star masses which put severe constraints on the composition and equation of state (EOS) of the neutron star matter. Here we study the effect of quark and nuclear matter mixed phase on mass radius relationship of neutron stars employing recent models from two classes of EOS's and discuss their implications.

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