arXiv subjects

Ipsita Saha

Publications and source records attributed to Ipsita Saha.

35 records · Page 2Linked to original sources

arXiv2021

Muon Anomalous Magnetic Moment in Two Higgs Doublet Models with Vector-Like Leptons

We show that inclusion of a single generation of vector-like leptons in the Two-Higgs Doublet Models significantly enlarges the allowed parameter space consistent with the muon anomalous magnetic moment, as well as with other theoretical and experimental constraints. While previously $(g -2)_μ$ could only be resolved in Type-X scenario by requiring a light pseudoscalar Higgs boson and large $\tan β$, contributions of vector-like leptons via two-loop Barr Zee diagrams broaden the allowed parameter space, allowing $\tan β$ as low as 10 and pseudoscalar masses as large as $\cal{O}$(1 TeV) while fulfilling the stringent constraints from precision and flavor observable. Similar results are obtained for Type-II scenarios, but there the parameter space is more restricted by flavor observables.

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arXiv2020

Role of Future Lepton Colliders for Fermionic Z-portal Dark Matter

The fermionic Z-portal dark matter model suffers from severe constraints from direct detection experiments. However, a narrow parameter space around the Z-funnel region is beyond the reach due to the resonance annihilation. In this paper, we provide an intriguing collider prospect for probing the Z-funnel dark matter mass range at the future lepton colliders including the beam polarization feature. We have done a comprehensive analysis for mono-photon signal at the colliders for such a dark matter. A realistic estimation for the 90% C.L. constraints with the systematic beam uncertainties has also been provided.

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arXiv2020

Charged Higgs searches in the Georgi-Machacek model at the LHC

The Georgi-Machacek Model allows for a large triplet vacuum expectation value (vev) with a custodial symmetric scalar potential. This manifestly leads to large charged Higgs coupling to SM fermions thus enhancing the detection probability of the model at the Large hadron collider. We show that the latest bound from LHC searches on singly charged Higgs scalar decaying to top and bottom quark restricts the triplet vev from above. We demonstrate that the combined limit from theoretical constraints and the latest Higgs data together with this charged Higgs bound can already curb the model parameter space significantly. Further, we propose the charged Higgs decaying to $W^{\pm}$ boson and a SM-like Higgs as a prospective channel to probe the yet unbounded model parameter space at the future run of the LHC. A simple cut-based analysis for this channel has been made showing the parameter region that can be probed with $5σ$ significance at the high luminosity LHC run.

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arXiv2019

Alignment limit in three Higgs-doublet models

The LHC Higgs data is showing a gradual inclination towards the SM result and realization of an SM-like limit becomes essential for BSM scenarios to survive. Considering the accuracy that can be achieved in future colliders, BSMs that acquire the alignment limit with an SM-like Higgs boson coupling can surpass others in the long run. Using a convenient parametrization, we demonstrate that the alignment limit for CP-conserving 3HDMs takes on the same analytic structure as that in the case of 2HDMs. Using the example of a $Z_3$-symmetric 3HDM, we illustrate how such alignment conditions can be efficiently implemented for numerical analysis in a realistic scenario.

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arXiv2018

Cornering variants of Georgi-Machacek model using Higgs precision data

We show that in the absence of trilinear terms in the scalar potential of Georgi-Machacek model, heavy charged scalars do not necessarily decouple from the $h \to γγ$ decay amplitude. In such scenarios, measurement of the Higgs to diphoton signal strength can place stringent constraints in the parameter space. Using the projected precisions at the High Luminosity LHC (HL-LHC) and the ILC, we find that the upper bound on the triplet vacuum expectation value can be as low as 10 GeV. We also found that when combined with the theoretical constraints from perturbative unitarity and stability, such variants may be ruled out altogether.

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arXiv2018

Revisiting the high-scale validity of Type-II seesaw model with novel LHC signature

The Type-II seesaw model is a well-motivated new physics scenario to address the origin of the neutrino mass issue. We show that this model can easily accommodate an absolutely stable vacuum until the Planck scale, however with strong limit on the exotic scalar masses and the corresponding mixing angle. We examine the model prediction at the current and future high luminosity run of the Large Hadron Collider (LHC) for the scalar masses and mixing angles fixed at such high-scale valid region. Specifically, we device the associated and pair production of the charged scalars as a new probe of the model at the LHC. We show that for a particular signal process the model can be tested with $5σ$ signal significance even at the present run of the LHC.

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arXiv2018

Can measurements of 2HDM parameters provide hints for high scale supersymmetry?

Two-Higgs-doublet models (2HDMs) are minimal extensions of the Standard Model (SM) that may still be discovered at the LHC. The quartic couplings of their potentials can be determined from the measurement of the masses and branching ratios of their extended scalar sectors. We show that the evolution of these couplings through renormalization group equations can determine whether the observed 2HDM is a low energy manifestation of a more fundamental theory, as for instance, supersymmetry, which fixes the quartic couplings in terms of the gauge couplings. At leading order, the minimal supersymmetric extension of the SM (MSSM) dictates all the quartic couplings, which can be translated into a predictive structure for the scalar masses and mixings at the weak scale. Running these couplings to higher scales, one can check if they converge to their MSSM values, and more interestingly, whether one can infer the supersymmetry breaking scale. Although we study this question in the context of supersymmetry, this strategy could be applied to any theory whose ultraviolet completion unambiguously predicts all scalar quartic couplings.

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arXiv2018

Higgs data does not rule out a sequential fourth generation with an extended scalar sector

Contrary to the common perception, we show that the current Higgs data does not eliminate the possibility of a sequential fourth generation that get their masses through the same Higgs mechanism as the first three generations. The inability to fix the sign of the bottom-quark Yukawa coupling from the available data plays a crucial role in accommodating a chiral fourth generation which is consistent with the bounds on the Higgs signal strengths. We show that effects of such a fourth generation can remain completely hidden not only in the production of the Higgs boson through gluon fusion but also to its subsequent decay to $γγ$ and $ Zγ$. This, however, is feasible only if the scalar sector of the Standard Model is extended. We also provide a practical example illustrating how our general prescription can be embedded in a realistic model.

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arXiv2017

Dark Matter and Collider Studies in the Left-Right Symmetric Model with Vector-Like Leptons

In the context of a left-right symmetric model, we introduce one full generation of vectorlike lepton doublets (both left and right-handed) together with their mirror doublets. We show that the lightest vectorlike neutrino in the model is right-handed, and can serve as the dark matter candidate. We find that the relic density, as well as the direct and indirect DM detection bounds, are satisfied for a large range of the parameter space of the model. In accordance with the parameter space, we then explore the possibility of detecting signals of the model at both the LHC and the ILC, in the pair production of the associated vectorlike charged leptons which decay into final states including dark matter. A comprehensive analysis of signal and backgrounds shows that the signals at the ILC, especially with polarized beams are likely to be visible for light vectorlike leptons, even with low luminosity, rendering our model highly predictable and experimentally testable.

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arXiv2016

Little Higgs after the little one

At the LHC, the Littlest Higgs Model with $T$-parity is characterised by various production channels. If the $T$-odd quarks are heavier than the exotic partners of the $W$ and the $Z$, then associated production can be as important as the pair-production of the former. Studying both, we look for final states comprising at least one lepton, jets and missing transverse energy. We consider all the SM processes that could conspire to contribute as background to our signals, and perform a full detector level simulation of the signal and background to estimate the discovery potential at the current run as well as at the scheduled upgrade of the LHC. We also show that, for one of the channels, the reconstruction of two tagged $b$-jets at the Higgs mass $(M_h = 125~{\rm GeV})$ provides us with an unambiguous hint for this model.

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arXiv2015

Distinguishing Neutrino Mass Hierarchies using Dark Matter Annihilation Signals at IceCube

We explore the possibility of distinguishing neutrino mass hierarchies through the neutrino signal from dark matter annihilation at neutrino telescopes. We consider a simple extension of the standard model where the neutrino masses and mixing angles are obtained via the type-II seesaw mechanism as an explicit example. We show that future extensions of IceCube neutrino telescope may detect the neutrino signal from DM annihilation at the Galactic Center and inside the Sun, and differentiate between the normal and inverted mass hierarchies, in this model.

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arXiv2015

Dark matter, neutrino masses and high scale validity of an inert Higgs doublet model

We consider a two-Higgs doublet scenario containing three $SU(2)_L$ singlet heavy neutrinos with Majorana masses. The second scalar doublet as well as the neutrinos are odd under a $Z_2$ symmetry. This scenario not only generates Majorana masses for the light neutrinos radiatively but also makes the lighter of the neutral $Z_2$-odd scalars an eligible dark matter candidate, in addition to triggering leptogenesis at the scale of the heavy neutrino masses. Taking two representative values of this mass scale, we identify the allowed regions of the parameter space of the model, which are consistent with all dark matter constraints. At the same time, the running of quartic couplings in the scalar potential to high scales is studied, thus subjecting the regions consistent with dark matter constraints to further requirements of vacuum stability, perturbativity and unitarity. It is found that part of the parameter space is consistent with all of these requirements all the way up to the Planck scale, and also yields the correct signal strength in the diphoton channel for the scalar observed at the Large Hadron Collider.

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arXiv2015

Search for a 'stable alignment limit' in two Higgs-doublet models

We study the conditions required to make the 2HDM scalar potential stable up to the Planck scale. The lightest CP-even scalar is assumed to have been found at the LHC and the {\em alignment limit} is imposed in view of the LHC Higgs data. We find that ensuring stability up to scales $\gtrsim 10^{10}$~GeV necessitates the introduction of a soft breaking parameter in the theory. Even then, some interesting correlations between the nonstandard masses and the soft breaking parameter need to be satisfied. Consequently, a 2HDM becomes completely determined by only two nonstandard parameters, namely, $\tb$ and a mass parameter, $m_0$, with $\tb \gtrsim 3$. These observations make a 2HDM, in the {\em stable alignment limit}, more predictive than ever.

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arXiv2015

Confronting the Galactic Center Gamma Ray Excess With a Light Scalar Dark Matter

The Fermi Large Area Telescope observed an excess in gamma ray emission spectrum coming from the center of the Milky Way galaxy. This data reveals that a light Dark Matter (DM) candidate of mass in the range 31-40 GeV, dominantly decaying into $b\bar b$ final state, can explain the presence of the observed bump in photon energy. We try to interpret this observed phenomena by sneutrino DM annihilation into pair of fermions in the Supersymmetric Inverse Seesaw Model (SISM). This model can also account for tiny non-zero neutrino masses satisfying existing neutrino oscillation data. We show that a Higgs portal DM in this model is in perfect agreement with this new interpretation besides satisfying all other existing collider, cosmological and low energy experimental constraints.

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arXiv2014

Left-right supersymmetry after the Higgs boson discovery

We perform a thorough analysis of the parameter space of the minimal left-right supersymmetric model in agreement with the LHC data. The model contains left- and right-handed fermionic doublets, two Higgs bidoublets, two Higgs triplet representations, and one singlet, insuring a charge-conserving vacuum. We impose the condition that the model complies with the experimental constraints on supersymmetric particles masses and on the doubly-charged Higgs bosons, and require that the parameter space of the model satisfy the LHC data on neutral Higgs signal strengths at $2σ$. We choose benchmark scenarios by fixing some basic parameters and scanning over the rest. The LSP in our scenarios is always the lightest neutralino. We find that the signals for $H\to γγ$ and $H \to VV^\star$ are correlated, while $H \to b \bar b$ is anti-correlated with all the other decay modes, and also that the contribution from singly-charged scalars dominate that of the doubly-charged scalars in $H\to γγ$ and $H \to Zγ$ loops, contrary to Type-II seesaw models. We also illustrate the range for mass spectrum of the LRSUSY model in light of planned measurements of the branching ratio of $H\to γγ$ to 10% level.

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arXiv2014

Neutrino Mass and Dark Matter in light of recent AMS-02 results

We study a simple extension of the Standard Model supplemented by an electroweak triplet scalar field to accommodate small neutrino masses by the type-II seesaw mechanism, while an additional singlet scalar field can play the role of cold dark matter (DM) in our Universe. This DM candidate is leptophilic for a wide range of model parameter space, and the lepton flux due to its annihilation carries information about the neutrino mass hierarchy. Using the recently released high precision data on positron fraction and flux from the AMS-02 experiment, we examine the DM interpretation of the observed positron excess in our model for two kinematically distinct scenarios with the DM and triplet scalar masses (a) non-degenerate ($m_{\rm DM}\gg m_Δ$), and (b) quasi-degenerate ($m_{\rm DM} \simeq m_Δ$). We find that a good fit to the AMS-02 data can be obtained in both cases (a) and (b) with a normal hierarchy of neutrino masses, while the inverted hierarchy case is somewhat disfavored. Although we require a larger boost factor for the normal hierarchy case, this is still consistent with the current upper limits derived from Fermi-LAT and IceCube data for case (a). Moreover, the absence of an excess anti-proton flux as suggested by PAMELA data sets an indirect upper limit on the DM-nucleon spin-independent elastic scattering cross section which is stronger than the existing DM direct detection bound from LUX in the AMS-02 preferred DM mass range.

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arXiv2013

125 GeV Higgs Boson and the Type-II Seesaw Model

We study the vacuum stability and unitarity conditions for a 125 GeV Standard Model (SM)-like Higgs boson mass in the type-II seesaw model. We find that, as long as the seesaw scale is introduced below the SM vacuum instability bound, there exists a large parameter space predicting a 125 GeV Higgs mass, irrespective of the exact value of the seesaw scale, satisfying both stability and unitarity conditions up to the Planck scale. We also study the model predictions for the Higgs partial decay widths in the diphoton and Z+photon channels with respect to their SM expectations and find that the decay rates for these two processes are correlated. We further show that for any given enhancement in the Higgs-to-diphoton rate over its SM expectation, there exists an upper bound on the type-II seesaw scale, and hence, on the masses of the associated doubly- and singly-charged Higgs bosons in the allowed parameter space. For instance, if more than 10% enhancement persists in the Higgs-to-diphoton channel, the upper limit on the type-II seesaw scale is about 450 GeV which is completely within the reach of the 14 TeV LHC. We believe this to be an encouraging result for the experimental searches of the singly- and doubly-charged Higgs bosons which, in combination with improved sensitivity in the Higgs-to-diphoton and Higgs-to-Z+photon signal strengths, could probe the entire allowed parameter space of the minimal type-II seesaw model, and establish/eliminate it as a single viable extension of the SM.

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