Najimuddin Khan
@hri.res.in
POSTDOCTORAL FELLOW, Physics
Harish-Chandra Research Institute
RESEARCH INTERESTS
Stability analysis, dark matter, neutrino and collider
phenomenology of physics beyond the Standard Model
in the context of high-energy high-luminosity colliders
(HE-LHC, HE-HL-LHC, ILC etc.).
Scopus Publications
Scholar Citations
Scholar h-index
Scholar i10-index
Scopus Publications
M. A. Arroyo-Ureña, Amit Chakraborty, J. Lorenzo Díaz-Cruz, Dilip Kumar Ghosh, Najimuddin Khan, and Stefano Moretti
American Physical Society (APS)
The detection of a single Higgs boson at the Large Hadron Collider (LHC) has allowed one to probe some properties of it, including the Yukawa and gauge couplings. However, in order to probe the Higgs potential, one has to rely on new production mechanisms, such as Higgs pair production. In this paper, we show that such a channel is also sensitive to the production and decay of a so-called `Flavon' field ($H_F$), a new scalar state that arises in models that attempt to explain the hierarchy of the Standard Model (SM) fermion masses. Our analysis also focuses on the other decay channels involving the Flavon particle, specifically the decay of the Flavon to a pair of $Z$ bosons ($H_F \\to Z Z$) and the concurrent production of a top quark and charm quark ($H_F\\to tc$), having one or more leptons in the final states. In particular, we show that, with 3000 fb$^{-1}$ of accumulated data at 14 TeV (the Run 3 stage) of the LHC an heavy Flavon $H_F$ with mass $M_{H_F} \\simeq 2m_t $ can be explored with $3\\sigma -5\\sigma $ significance through these channels.
Pritam Das and Najimuddin Khan
American Physical Society (APS)
We consider a new physics model, where the Standard Model (SM) is extended by hyperchargeless $Y=0$ triplet fermions and Higgs triplet with hypercharge $Y=2$. The first two generation fermion triplets are even under the $Z_2$ transformation. In contrast, the third fermion triplet and scalar triplet are odd under the same $Z_2$ transformation. It is a unifying framework for the simultaneous explanation of neutrino mass and mixing, dark matter, baryogenesis, inflation, and reheating temperature of the Universe. The two $Z_2$ even neutral fermions explain the neutrino low energy variables, whereas the third one can serve as a viable dark matter candidate, explaining the exact relic density. The scalar triplet is coupled nonminimally to gravity and forms the inflaton. We calculate the inflationary parameters and find them consistent with the new Planck-2018 constraints. We also do the reheating analysis for the inflaton decays/annihilations to relativistic SM particles. The triplet fermions associated with $Z_2$ even sector can provide the observed baryon asymmetry of the Universe at the TeV scale.
Najimuddin Khan
Elsevier BV
Pritam Das, M. K. Das and Najimuddin Khan
Pritam Das, M. K. Das and Najimuddin Khan
Pritam Das, Mrinal Kumar Das, and Najimuddin Khan
Springer Nature Singapore
Pritam Das, M. K. Das and Najimuddin Khan
We perform an exclusive study on the Feebly Interacting Massive Particle (FIMP) dark matter candidate in an extended hyperchargeless (Y = 0) Higgs triplet model. The additional Z2 odd neutral fermion singlet plays the role of dark matter with support from two other vector-like fermion doublets. The mixing between the neutral component of a doublet and singlet fermions controls the current relic density through the Freeze-in mechanism, whereas the additional doublet fermion helps to get the neutrino mass and mixing angles. We obtain a broad region of the parameter spaces satisfying the current relic density and neutrino mass and mixing angles. ∗ prtmdas9@gmail.com † mkdas@tezu.ernet.in ‡ psnk2235@iacs.res.in 1 ar X iv :2 10 7. 01 57 8v 2 [ he pph ] 2 7 N ov 2 02 1
A. Adhikary, B. Bhattacherjee, R. Godbole, Najimuddin Khan and S. Kulkarni
Pritam Das, M. K. Das and Najimuddin Khan
R. K. Barman, B. Bhattacherjee, Indranil Chakraborty, A. Choudhury and Najimuddin Khan
The projected reach of direct electroweakino searches at the HL-LHC ($\\sqrt{s}=14~{\\rm TeV}, ~3000~{\\rm fb^{-1}}$ LHC) in the framework of simplified models with R-parity violating (RPV) operators: $\\lambda_{112}^{\\prime \\prime}u^{c}d^{c}s^{c}$ and $\\lambda_{113}^{\\prime\\prime}u^{c}d^{c}b^{c}$, is studied. Four different analysis channels are chosen: $Wh$ mediated $1l+2b+jets+\\rm E{\\!\\!\\!/}_T$, $Wh$ mediated $1l+2\\gamma+jets+\\rm E{\\!\\!\\!/}_T$, $WZ$ mediated $3l+jets+\\rm E{\\!\\!\\!/}_T$ and $WZ$ mediated $3l+2b+jets+\\rm E{\\!\\!\\!/}_T$ and the projected exclusion/discovery reach of direct wino searches in these channels is analyzed by performing a detailed cut based collider analysis. The projected exclusion contour reaches up to $600-700~{\\rm GeV}$ for a massless bino-like $\\chi_{1}^{0}$ from searches in the $Wh$ mediated $1l+2b+jets+\\rm E{\\!\\!\\!/}_T$, $Wh$ mediated $1l+2\\gamma +jets+\\rm E{\\!\\!\\!/}_T$ and $WZ$ mediated $3l+jets+\\rm E{\\!\\!\\!/}_T$ channels, while the $WZ$ mediated $3l+2b+jets+\\rm E{\\!\\!\\!/}_T$ search channel is found to have a projected exclusion reach up to $600~{\\rm GeV}$ for $150~{\\rm GeV} < M_{\\chi_{1}^{0}} < 250~{\\rm GeV}$. The baryon number violating simplified scenario considered in this work is found to furnish a weaker projected reach (typically by a factor of $\\sim 1/2$) than the R-parity conserving (RPC) case. The projected reach at the HL-LHC in these four channels is also recasted for realistic benchmark scenarios.
S. Goswami, K. N. Vishnudath and Najimuddin Khan
Ila Garg, S. Goswami, K. N. Vishnudath and Najimuddin Khan
Pritam Das, M. K. Das and Najimuddin Khan
G. Chakravarty, Najimuddin Khan and S. Mohanty
We construct an unified model of inflation and PeV dark matter with an appropriate choice of no-scale Kahler potential, superpotential and gauge kinetic function in terms of MSSM fields and hidden sector Polonyi field. The model is consistent with the CMB observations and can explain the PeV neutrino flux observed at IceCube HESE. A Starobinsky like Higgs-sneutrino plateau inflation is obtained from the $D$-term SUGRA potential while $F$-term being subdominant during inflation. To get PeV dark matter, SUSY breaking at PeV scale is achieved through Polonyi field. This sets the scale for soft SUSY breaking parameters $m_0, m_{1/2}, A_0 $ at the GUT scale in terms of the parameters of the model. The low energy particles spectrum is obtained by running the RGEs. We show that the $\\sim$125 GeV higgs and the gauge coupling unification can be obtained in this model. The $6$ PeV bino-type dark matter is a subdominant fraction ($\\sim 11\\%$) of the relic density and its decay gives the PeV scale neutrino flux observed at IceCube by appropriately choosing the couplings of the $R$-parity violating operators. Also we find that there is a degeneracy in scalar field parameters $(\\gamma, \\beta)$ and coupling $\\zeta$ value in producing the correct amplitude of CMB power spectrum. However the value of parameter $\\tan(\\beta)=1.8$, which is tightly fixed from the requirement of PeV scale SUSY breaking, removes the degeneracy in the values of the scalar field parameters to provide a unique solution for inflation. In this way it brings the explanation for dark matter, PeV neutrinos and inflation within the same framework.
B. Bhattacherjee, Najimuddin Khan and Ayon Patra
We perform a detailed study of the various decay channels of the heavy charged and neutral gauge bosons ($W_R$ and $Z_R$ respectively) in a left-right supersymmetric (LRSUSY) framework. The decay branching ratios of the $W_R$ and $Z_R$ bosons depend significantly on the particle spectrum and composition of the SUSY states. We show several combinations of mass spectrum for the SUSY particles to facilitate the decay of these heavy gauge bosons into various combinations of final states. Finally, we choose two benchmark points and perform detailed collider simulations for these heavy gauge bosons in the context of the high energy and high luminosity run of the large hadron collider. We analyze two SUSY cascade decay channels -- mono-$W$ + $\\slashed{E}_T$ and mono-$Z$ + $\\slashed{E}_T$ along with the standard dilepton and dijet final states. Our results show that the existence of these heavy gauge bosons can be ascertained in the direct decay channels of dilepton and dijet whereas the other two channels will be required to establish the supersymmetric nature of this model.
S. Goswami, K. N. Vishnudath and Najimuddin Khan
We study the minimal type-III seesaw model in which we extend the standard model by adding two SU(2)(L) triplet fermions with zero hypercharge to explain the origin of the nonzero neutrino masses. We show that the naturalness conditions and the limits from lepton flavor violating decays provide very stringent bounds on the model parameters along with the constraints from the stability/metastability of the electroweak vacuum. We perform a detailed analysis of the model parameter space including all the constraints for both normal as well as inverted hierarchies of the light neutrino masses. We find that most of the regions that are allowed by lepton flavor violating decays and naturalness fall in the stable/metastable region depending on the values of the standard model parameters.
Najimuddin Khan
Hindawi Limited
We extend the standard model with three right-handed singlet neutrinos and a real singlet scalar. We impose two Z2 and Z2′ symmetries. We explain the tiny neutrino mass-squared differences with two Z2- and Z2′-even right-handed neutrinos using type I seesaw mechanism. The Z2-odd fermion and the Z2′-odd scalar can both serve as viable dark matter candidates. We identify new regions in the parameter space which are consistent with relic density of the dark matter from recent direct search experiments LUX-2016 and XENON1T-2017 and LHC data.
Ila Garg, S. Goswami, N. VishnudathK. and Najimuddin Khan
We consider singlet extensions of the standard model, both in the fermion and the scalar sector, to account for the generation of neutrino mass at the TeV scale and the existence of dark matter respectively. For the neutrino sector we consider models with extra singlet fermions which can generate neutrino mass via the so called inverse or linear seesaw mechanism whereas a singlet scalar is introduced as the candidate for dark matter. We show that although these two sectors are disconnected at low energy, the coupling constants of both the sectors get correlated at high energy scale by the constraints coming from the perturbativity and stability/metastability of the electroweak vacuum. The singlet fermions try to destabilize the electroweak vacuum while the singlet scalar aids the stability. As an upshot, the electroweak vacuum may attain absolute stability even upto the Planck scale for suitable values of the parameters. We delineate the parameter space for the singlet fermion and the scalar couplings for which the electroweak vacuum remains stable/metastable and at the same time giving the correct relic density and neutrino masses and mixing angles as observed.
Amitava Datta, Nabanita Ganguly, Najimuddin Khan, and Subhendu Rakshit
American Physical Society (APS)
We revisit the multilepton $(ml)+{\\overline{)E}}_{T}+X$ signatures of the inert doublet model (IDM) of dark matter in future LHC experiments for $m=3$, 4 and simulate, for the first time, the $m=5$ case. Here $X$ stands for any number of jets. We illustrate these signals with benchmark points consistent with the usual constraints like unitarity, perturbativity, the precision electroweak data, the observed dark matter relic density of the Universe and, most importantly, the stringent LHC constraints from the post\\char21{}Higgs ($h$) discovery era like the measured ${M}_{h}$ and the upper bound on the invisible width of $h$ decay, which were not included in earlier analyses of multilepton signatures. We find that if the IDM is embedded in a grand desert scenario so that the unitarity constraint holds up to a very high scale, the whole of the highly restricted parameter space allowed by the above constraints can be probed at the LHC via the $3l$ signal for an integrated luminosity $\\ensuremath{\\sim}3000\\text{ }\\text{ }{\\mathrm{fb}}^{\\ensuremath{-}1}$. On the other hand, if any new physics shows up at a scale $\\ensuremath{\\sim}10\\text{ }\\text{ }\\mathrm{TeV}$, only a part of the enlarged allowed parameter space can be probed. The $4l$ and $5l$ signals can help to discriminate among different IDM scenarios as and when sufficient integrated luminosity accumulates.
Najimuddin Khan and Subhendu Rakshit
American Physical Society (APS)
The inert scalar doublet model of dark matter can be valid up to the Planck scale. We briefly review the bounds on the model in such a scenario and identify parameter spaces that lead to absolute stability and metastability of the electroweak vacuum.
A. Chaudhuri, Najimuddin Khan, B. Mukhopādhyāẏa and S. Rakshit
The type III seesaw mechanism for neutrino mass generation usually makes use of at least two $Y = 0$, $SU(2)_L$ lepton triplets. We augment such a model with a third triplet and a sterile neutrino, both of which are odd under a conserved $\\Z_2$ symmetry. With all new physics confined to the $\\Z_2$-odd sector, whose low energy manifestation is in some higher-dimensional operators, a fermionic dark matter candidate is found to emerge. We identify the region of the parameter space of the scenario, which is consistent with all constraints from relic density and direct searches, and allows a wide range of masses for the dark matter candidate.
Najimuddin Khan and Subhendu Rakshit
American Physical Society (APS)
We study several aspects of electroweak vacuum metastability when an extra gauge singlet scalar, a viable candidate for a dark matter particle, is added to the standard model of particle physics, which is assumed to be valid up to the Planck scale. Phase diagrams are drawn for different parameter spaces, and based on that, we graphically demonstrate how the confidence level, at which stability of electroweak vacuum is excluded, depends on such new physics parameters.