arXiv subjects

Mukesh Kumar

Publications and source records attributed to Mukesh Kumar.

At least 55 records · Page 3Linked to original sources

arXiv2020

Transitioning Universe with hybrid scalar field in Bianchi I space-time

In this paper we investigate a Bianchi type I transitioning Universe in Brans-Dicke theory. To get an explicit solution of the field equations, we assume scalar field as $ϕ= ϕ_{0}\left[t^αexp(βt)\right]^δ$ with $ϕ_{0}$, $α$, $β$ and $δ$ as constants. The values of $α$ and $β$ are obtained by probing the proposed model with recent observational Hubble data (OHD) points. The interacting and non-interacting scenarios between dark matter and dark energy of the derived Universe within the framework of Brans-Dicke gravity are investigated. The $om(z)$ analysis of the Universe in derived model shows that the Universe is filled with dynamical dark energy with its equation of state parameter $ω_{de} > -1$. Hence the Universe behaves like a quintessence model at present epoch. Some physical properties of the Universe are also discussed.

gr-qc↗

arXiv2020

Heat driven transport in serial double quantum dot devices

Studies of thermally induced transport in nanostructures provide access to an exciting regime where fluctuations are relevant, enabling the investigation of fundamental thermodynamic concepts and the realization of thermal energy harvesters. We study a serial double quantum dot formed in an InAs/InP nanowire coupled to two electron reservoirs. By means of a specially designed local metallic joule-heater, the temperature of the phonon bath in the vicinity of the double quantum dot can be enhanced. This results in phonon-assisted transport, enabling the conversion of local heat into electrical power in a nano-sized heat engine. Simultaneously, the electron temperatures of the reservoirs are affected, resulting in conventional thermoelectric transport. By detailed modelling and experimentally tuning the interdot coupling we disentangle both effects. Furthermore, we show that phonon-assisted transport gives access to the energy of excited states. Our findings demonstrate the versatility of our design to study fluctuations and fundamental nanothermodynamics.

cond-mat.mes-hall↗

arXiv2020

Hot-Carrier Separation in Heterostructure Nanowires observed by Electron-Beam Induced Current

The separation of hot carriers in semiconductors is of interest for applications such as thermovoltaic photodetection and third-generation photovoltaics. Semiconductor nanowires offer several potential advantages for effective hot-carrier separation such as: a high degree of control and flexibility in heterostructure-based band engineering, increased hot-carrier temperatures compared to bulk, and a geometry well suited for local control of light absorption. Indeed, InAs nanowires with a short InP energy barrier have been observed to produce electric power under global illumination, with an open-circuit voltage exceeding the Shockley-Queisser limit. To understand this behaviour in more detail, it is necessary to maintain control over the precise location of electron-hole pair-generation in the nanowire. In this work we perform electron-beam induced current measurements with high spatial resolution, and demonstrate the role of the InP barrier in extracting energetic electrons. We interprete the results in terms of hot-carrier separation, and extract estimates of the hot carrier mean free path.

cond-mat.mes-hall↗

arXiv2020

Connecting the Muon Anomalous Magnetic Moment and the Multi-lepton Anomalies at the LHC

A number of predictions were made in von Buddenbrock et al (2016 Eur. Phys. J. C 76, 10, 580) pertaining to the anomalous production of multiple leptons at the Large Hadron Collider. Discrepancies in multi-lepton final states have now become statistically compelling with the available Run~2 data. These could be connected with a heavy boson, $H$, decaying predominantly into a SM Higgs boson, $h$, and a singlet scalar, $S$, where $m_H\approx 270$\,GeV and $m_S\approx 150$\,GeV. These can be embedded into a scenario where a Two Higgs Doublet is considered with an additional singlet scalar, 2HDM+S. The long-standing discrepancy in the muon anomalous magnetic moment, $Δa_μ$, is interpreted in the context of the 2HDM+S type-II and type-X, along with additional fermionic degrees of freedom. The 2HDM+S model alone with the constraints from the LHC data does not seem to explain the $Δa_μ$ anomaly. However, adding fermions with mass of order $\mathcal{O}(100)$\,GeV can explain the discrepancy for low enough values of fermion-scalar couplings.

hep-ph↗

arXiv2019

Dark matter in the Randall-Sundrum model with non-universal coupling

We consider simplified dark matter models (DM) interacting gravitationally with the standard model (SM) particles in a Randall-Sundrum (RS) framework. In this framework, the DM particles interact through the exchange of spin-2 Kaluza-Klein (KK) gravitons in the $s$-channel with the SM particles. The parameter space of the RS model with universal couplings to SM particles is known to be strongly constrained from the LHC data. We are thus led to consider models with non-universal couplings. The first model we consider in this study is a top-philic graviton model in which only the right-handed top quarks are taken to interact strongly with the gravitons. In the second, the lepto-philic model, we assume that only the right-handed charged leptons interact strongly with the gravitons. We extend the study to include not only the scalar, vector and spin-1/2 fermions but also spin-3/2 fermionic dark matter. We find that there is a large parameter space in these benchmark models where it is possible to achieve the observed relic density consistent with the direct and indirect searches and yet not to be constrained from the LHC data.

hep-ph↗

arXiv2019

The emergence of multi-lepton anomalies at the LHC and their compatibility with new physics at the EW scale

A recent study [1] has shown that a simplified model predicting a heavy scalar of mass 270 GeV ($H$) that decays to a Standard Model (SM) Higgs boson in association with a scalar singlet of mass 150 GeV ($S$) can accommodate several anomalous multi-lepton results in proton-proton collisions at the Large Hadron Collider (LHC). With this in mind, the goal of this article is to provide a more formal study of a wider set of LHC results pertaining to the production of multiple leptons. We find that a combination of such results lead to strong discrepancies between the data and SM Monte Carlo predictions. These discrepancies appear in corners of the phase-space where different SM processes dominate, indicating that the potential mismodeling of a single SM process is unlikely to explain them. Systematic uncertainties from the prediction of SM processes evaluated with currently available tools seem unable to explain away these discrepancies. A combination is able to constrain the simplified model's single degree of freedom $β_g^2$, related to the size of the Yukawa coupling of $H$ to the top quark, to a value of $2.92\pm 0.35$. This is in contrast to the absence of signal, where $β_g=0$. This result is discussed in the independent contexts of both potential for new physics in the existing LHC data as well as the limitations of our current understanding of the SM. That being said, QCD NNLO and EW NLO corrections in di-lepton final states are not expected to change the conclusions of this study. New results pertaining to the production of two opposite sign different flavour charged leptons with a full jet veto further confirm the presence of anomalies in similar corners of the leptonic phase-space.

hep-ph↗

arXiv2019

Constraints on a 2HDM with a singlet scalar and implications in the search for heavy bosons at the LHC

We study a two-Higgs doublet model extended with an additional singlet scalar (2HDM+S), and provide a brief introduction to the model and its parameters. Constraints are applied to the parameter space of this model in order to accommodate a number of features in the data that have been interpreted in von Buddenbrock et al (2018 J. Phys. G 45 115003) as the result of the $H\rightarrow Sh$ decay produced via gluon-gluon fusion and in association with top quarks. Implications on the phenomenology of the heavy pseudo-scalar ($A$) and charged scalar ($H^+$) are discussed. In particular, the decays $A\rightarrow ZH$ and $H^+\rightarrow W^+H$ become prominent. This leads to final states with multiple leptons and $b$-quarks. The decay $A\rightarrow ZH\rightarrow ZSh$ results in the production of a high transverse momentum $Z$ produced in association with a lepton and two $b$-quarks with little additional jet activity. These predictions are compared to the data with model's benchmark points. With the parameters obtained here the model is able to accommodate the features at the LHC reported in von Buddenbrock et al (2018 J. Phys. G 45 115003). Without varying these parameters additional excesses in the $Zb\overline{b}$ and $t\overline{t}$ invariant mass spectra, and the production of $3$ leptons plus two $b$-tagged jets can be explained assuming $m_A\approx 600$\,GeV.

hep-ph↗

arXiv2019

Dark matter in the Randall-Sundrum model

In Ref. \cite{Goyal:2019vsw}, we had considered simplified dark matter models interacting gravitationally with the Standard Model particles in a Randall-Sundrum frame work where models are considered with non-universal couplings. In the top-philic graviton model the right-handed top quarks are taken to interact strongly with the gravitons and in the lepton-philic model, we assume that only the right-handed charged leptons interact strongly with the gravitons. We extend the study to include not only the scalar, vector and spin-1/2 fermions but also spin-3/2 fermionic dark matter. We find that there is a large parameter space in these benchmark models where it is possible to achieve the observed relic density consistent with the direct and indirect searches, which are also consistent with the data from Large Hadron Collider.

hep-ph↗

arXiv2019

Fingerprinting the Top quark FCNC via anomalous $Ztq$ couplings at the LHeC

A study of the top quark \emph{Flavour Changing Neutral Current} (FCNC) through $Z$-boson has been performed in the proposed future $e^-p$ collider for the energy, $E_{e(p)} = 60~(7000)$~GeV. We considered an effective theory where the anomalous FCNC couplings are of vector and tensor nature. The effect of these couplings is probed in the single top production along with the scattered electron. The polar angle $θ$ of the electrons coming out of the primary vertex in association with the top quark polarization asymmetries constructed from the angular distribution of the secondary lepton arising from the top decay, allow to distinguish the Lorentz structure of the coupling. From a multi-parameter analysis, we obtain a reach of ${\cal O} (10^{-2})$ in the case of $Ztu$ and $Ztc$ couplings at an integrated luminosity of 2~ab$^{-1}$ at 95\% C.L.

hep-ph↗

arXiv2019

$k_T$-factorization approach to the Higgs boson production in $ZZ^*\to 4\ell$ channel at the LHC

We calculated a differential cross section of the Higgs boson production in the $h\to ZZ^*\to 4\ell$ decay channel within the framework of $k_T$-factorization. Results are obtained using an off-shell matrix element for the $g^*g^*\to h\to ZZ^*\to 4\ell$ process together with Ciafaloni-Catani-Fiorani-Marchesini (CCFM) evolution equations for an unintegrated gluon distribution function. We have presented a comparison of our results with the latest experimental measurements at $\sqrt{S}$ = 8 TeV and $\sqrt{S}$ = 13 TeV from the ATLAS and CMS collaborations at the LHC. In addition to this, we have compared our results with the results from the collinear factorization formalism calculated up to next-to-next-to-leading order plus next-to-next-to-leading logarithm (NNLO + NNLL) accuracy obtained using the HRes code for the Higgs boson production in the gluon-gluon fusion process. Our estimates are consistently close to NNLO + NNLL results obtained using a collinear factorization formalism and are also in agreement with experimental measurements.

hep-ph↗

arXiv2018

Spin-3/2 dark matter in a simple $t$-channel model

We consider a spin-3/2 fermionic dark matter (DM) particle interacting with the Standard Model quarks through the exchange of a charged and coloured scalar or vector mediator in a simple $t$-channel model. It is found that for the vector mediator case, almost the entire parameter space allowed by the observed relic density is already ruled out by the direct detection LUX data. No such bounds exist on the interaction mediated by scalar particles. Monojet + missing energy searches at the Large Hadron Collider provide the most stringent bounds on the parameters of the model for this case. The collider bounds put a lower limit on the allowed DM masses.

hep-ph↗

arXiv2018

Silver plasmonic density tuned polarity switching and anomalous behaviour of high performance self-powered \b{eta}-gallium oxide solar-blind photodetector

Deep understanding of plasmonic nanoparticles (PNPs)-light interaction over semiconductors surface shows great promises in enhancing their optoelectronic devices efficiency beyond the conventional limit. However, PNP-light interaction critically decided by the distribution density of PNPs over the semiconductor surface which is not entirely understood. Here, a systematic study depicting how the interparticle gap between Silver (Ag) NPs influences the performance of the \b{eta}-Ga2O3 based solar-blind photodetector. Interestingly, a remarkable transition is observed, where the varied interparticle gap not only changes the polarity but also reverses the traditional photodetector behaviour. The positive transient response of bare \b{eta}-Ga2O3 photodetector with feeble DUV light switches its behaviour remarkably to 20 times enhance negative-photoresponse when decorated by sparsely-spaced Ag-PNPs with ultra-high responsivity of 107.47 A/W at moderate power and an incredible report-highest responsivity of 4.29 mA/W on single semiconducting \b{eta}-Ga2O3 layer at self-powered mode. Moreover, as the density of the Ag-PNPs was further increased, the photocurrent decreases with illumination which dynamically reverses the traditional photodetector to unnatural anomalous effect. In particular, our study represents the first demonstration of plasmonic tuning effect to two active dynamic switching modes; i.e. reverse switchable and anomalous behaviour, the fundamentals of which have not studied experimentally yet. Finally, we propose a unified well-explained model to rationalize all observed experimental trends while set-up fundamental basis for establishing potential applications.

physics.app-ph↗

arXiv2018

Multi-lepton signatures of additional scalar bosons beyond the Standard Model at the LHC

Following a prediction made in Refs.~\cite{vonBuddenbrock:2015ema,Kumar:2016vut,vonBuddenbrock:2016rmr}, this paper focuses on multi-lepton signatures arising from two new hypothetical scalar bosons, $H$ and $S$, at the Large Hadron Collider (LHC). These two new bosons are an extension to the Standard Model (SM) and interact with the SM Higgs boson, $h$. We consider two production modes for $H$, one being gluon fusion and the other being in association with top quarks. The $H \to S h$ decay mode is considered, where leptonic final states are studied. The CP properties of $S$ are characterised by considering effective couplings derived from dimension six operators through $SWW$ vertices. The nature of the $S$ boson is considered in two separate contexts. Firstly in a simplified model, it is considered to have Higgs-like couplings. Secondly, we consider a heavy neutrino model and its interactions with the $Z, W$ and $S$ bosons. The predictions of the models are compared both to ATLAS and CMS results at $\sqrt{s} = 8$ and $13$~TeV, where appropriate. The data is interpreted using a simplified model where all the signal comes from $H \to S h$, assuming $S$ to be Higgs-like, $m_H=270$~GeV and $m_S=150$~GeV. The combined result yields gives a best fit value for the parameter $β_g$ (the strength of the Yukawa coupling of $H$ to top quarks), $β_g^2=1.38\pm 0.22$. A number of regions of the phase space are suggested to the experiments for further exploration.

hep-ph↗

arXiv2017

The Madala hypothesis with Run 1 and 2 data at the LHC

The Madala hypothesis postulates a new heavy scalar, H, which explains several independent anomalous features seen in ATLAS and CMS data simultaneously. It has already been discussed and constrained in the literature by Run 1 results, and its underlying theory has been explored under the interpretation of a two Higgs doublet model coupled with a scalar singlet, $S$. When applying the hypothesis to Run 2 results, it can be shown that the constraints from the data are compatible with those obtained using Run 1 results.

hep-ph↗

arXiv2017

Exploring CP-even scalars of a Two Higgs-doublet model in future $e^-\,p$ colliders

In this proceeding we shall explore the potential of a future $e^- \,p$ collider to probe the CP-even scalars in a two Higgs doublet model. We consider Type-I in this study. The mass of the lighter scalar particle is considered to be the Higgs-boson, $m_h = 125$ GeV, and a heavy scalar $m_H = 270$ GeV. The centre of mass energy for the $e^-\,p$ collision is considered as in the Large Hadron Electron Collider and the Future Circular Hadron Electron Collider configurations, by fixing the proton beam energy to be $E_p = 7$ and 50 TeV, respectively, and an electron beam energy of $E_e = 60$ GeV. Production cross sections of these scalars are also shown at higher electron beam energies. Future prospects of these studies are also discussed.

hep-ph↗

arXiv2017

Impact of additional bosons on the exploration of the Higgs boson at the LHC

The LHC is making strides in the exploration of the properties of the newly discoverd Higgs boson, $h$. In Refs.~\cite{vonBuddenbrock:2015ema,Kumar:2016vut,vonBuddenbrock:2016rmr} the compatibility of the proton-proton data reported in the Run I period with the presence of a heavy scalar, $H$, with a mass around 270\,GeV and its implications were explored. This boson would decay predominantly to $H\rightarrow Sh$, where $S$, is a lighter scalar boson. The production cross-section of $pp\rightarrow H(\rightarrow Sh) + X$ is considerable and it would significantly affect the inclusive rate of $h$. The contamination from this new production mechanism would depend strongly on the final state used to measure the rate of $h$. The contamination in the rate measurement of $Vh(\rightarrow b\overline{b}), V=Z,W$ is estimated to be small. This statement does not depend strongly on assumptions made on the decay of $S$.

hep-ph↗

arXiv2017

Measuring CP nature of top-Higgs couplings at the future Large Hadron electron collider

We investigate the sensitivity of top-Higgs coupling by considering the associated vertex as CP phase ($ζ_t$) dependent through the process $p\, e^- \to \bar t \,h \,ν_e$ in the future Large Hadron electron collider. In particular the decay modes are taken to be $h \to b\bar b$ and $\bar t \to$ leptonic mode. Several distinct $ζ_t$ dependent features are demonstrated by considering observables like cross sections, top-quark polarisation, rapidity difference between $h$ and $\bar t$ and different angular asymmetries. Luminosity ($L$) dependent exclusion limits are obtained for $ζ_t$ by considering significance based on fiducial cross sections at different $σ$-levels. For electron and proton beam-energies of 60 GeV and 7 TeV respectively, at $L = 100$~fb$^{-1}$, the regions above $π/5 < ζ_t \leq π$ are excluded at 2$σ$ confidence level, which reflects better sensitivity expected at the Large Hadron Collider. With appropriate error fitting methodology we find that the accuracy of SM top-Higgs coupling could be measured to be $κ= 1.00 \pm 0.17 (0.08)$ at $\sqrt{s} = 1.3 (1.8)$ TeV for an ultimate $L = 1 \,\rm{ab}^{-1}$.

hep-ph↗

arXiv2017

The production of the heavy scalar H in association with top and anti-top quarks

Following arXiv:1506.00612, an effective field theory approach has been introduced for two additional real scalars, $H$ and $χ$, to study the associated Higgs boson ($h$) production with top quarks at the Large Hadron Collider with centre of mass energy $\sqrt s = 8$ TeV. In all studies, one benchmark scenario is considered where the parameters of the model are constrained for $m_H = 275$ GeV and $m_χ= 60$ GeV. A comparative study has been performed for the production of the Higgs boson in association with a single top and top-pairs in the Standard Model with respect to the processes where $h$ is replaced with the heavy scalar $H$, which further decays via $H \to h χχ$. The analysis has been performed at two stages where the top quarks are studied through a single-lepton channel firstly, and then in the second case with same-sign leptons, electrons or muons in di-lepton events. Different observables, like $b$ and $c$-tagged jet multiplicities, total missing energy, and total scalar sum of transverse momenta of all jets and leptons, has been studied.

hep-ph↗