arXiv subjects

Dilip Kumar Ghosh

Publications and source records attributed to Dilip Kumar Ghosh.

At least 37 records · Page 2Linked to original sources

arXiv2024

The $N_{\rm eff}$ at CMB challenges $U(1)_X$ light gauge boson scenarios

The relativistic degrees of freedom ($N_{\rm eff}$) is one of the crucial cosmological parameters. The precise measurement of $N_{\rm eff}$ at the time of cosmic microwave background formation, by Planck 2018 can be used to understand the new fundamental interactions, in particular involving light mediators. Presence of any new particle with sufficient energy density and sizeable interactions with Standard Model particles at the temperature around $\sim$ MeV can significantly alter the neutrino decoupling and hence $N_{\rm eff}$. Thus the bound on $N_{\rm eff}$ can place stringent constraints on various beyond Standard Model paradigms involving light particles. $U(1)_X$ models are among such scenarios and are widely studied in several aspects. In this work, we consider several popular $U(1)_X$ models with light $Z'$ boson like $U(1)_{B-L}$, $U(1)_{B - 3L_i}$, $U(1)_{B_i - 3 L_j}$, $U(1)_{L_i - L_j}$; $i,j =1,2,3$ being the flavour indices and study their impact on $N_{\rm eff}$. We also examine the constraints from ground based experiments like Xenon1T, Borexino, trident, etc. Our analysis shows that for light mass $M_{Z'} \lesssim \mathcal{O} (\rm{MeV})$ the $N_{\rm eff}$ provides the most stringent constraints on the $Z'$ mass and coupling, far exceeding the existing constraints from other experiments.

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arXiv2024

Searching for exotic Higgs bosons from top quark decays at the HL-LHC

Exotic spin-$0$ states with unusual couplings with the gauge and matter fields of the Standard Model are worth exploring at the CERN LHC. Though our approach is largely model independent, we take inspiration from flavor models based on some discrete symmetries which predict a set of a scalar and a pseudoscalar having purely off-diagonal Yukawa interactions with quarks and leptons. In a previous paper, some of us explored how to decipher such exotic scalar and pseudoscalar states whose off-diagonal Yukawa couplings involve light quarks. In this work we follow a complementary path and focus on the Yukawa couplings that necessarily involve a top quark. If one such spin-$0$ state is lighter than the top quark, then the rare decay of the latter, on account of the high yield of the $t\bar t$ events, could provide a potential hunting ground of those exotic states particularly during the high luminosity phase of the LHC run. We carry out an exhaustive collider analysis of some promising signatures of those exotic states using sophisticated Machine Learning techniques and obtain considerable signal significance.

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arXiv2023

Axion-like particle (ALP) portal freeze-in dark matter confronting ALP search experiments

The relic density of Dark Matter (DM) in the freeze-in scenario is highly dependent on the evolution history of the universe and changes significantly in a non-standard (NS) cosmological framework prior to Big Bang Nucleosynthesis (BBN). In this scenario, an additional species dominates the energy budget of the universe at early times (before BBN), resulting in a larger cosmological expansion rate at a given temperature compared to the standard radiation-dominated (RD) universe. To investigate the production of DM in the freeze-in scenario, we consider both standard RD and NS cosmological picture before BBN and perform a comparative analysis. We extend the Standard Model (SM) particle content with a SM singlet DM particle $χ$ and an axion-like particle (ALP) $a$. The interactions between ALP, SM particles, and DM are generated by higher dimensional effective operators. This setup allows the production of DM $χ$ from SM bath through the mediation of ALP, via ALP-portal processes. These interactions involve non-renormalizable operators, leading to ultraviolet (UV) freeze-in, which depends on the reheating temperature ($T_{RH}$) of the early universe. In the NS cosmological scenario, the faster expansion rate suppresses the DM production processes, allowing for enhanced effective couplings between the visible and dark sectors to satisfy the observed DM abundance compared to RD scenario. This improved coupling increases the detection prospects for freeze-in DM via the ALP-portal, which is otherwise challenging to detect in RD universe due to small couplings involved. Using an effective field theory set-up, we show that various ALP searches such as in FASER, DUNE, and SHiP, etc. will be able to probe significant parameter space depending on the different model parameters.

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arXiv2023

Flavon Signatures at the HL-LHC

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 $3σ-5σ$ significance through these channels.

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arXiv2023

FIMP Dark Matter from Flavon Portals

We investigate the phenomenology of a non-thermal dark matter (DM) candidate in the context of flavor models that explain the hierarchy in the masses and mixings of quarks and leptons via the Froggatt-Nielsen (FN) mechanism. A flavor-dependent $U(1)_{\rm FN}$ symmetry explains the fermion mass and mixing hierarchy, and also provides a mechanism for suppressed interactions of the DM, assumed to be a Majorana fermion, with the Standard Model (SM) particles, resulting in its FIMP (feebly interacting massive particle) character. Such feeble interactions are mediated by a flavon field through higher dimensional operators governed by the $U(1)_{\rm FN}$ charges. We point out a natural stabilizing mechanism for the DM within this framework with the choice of half-integer $U(1)_{\rm FN}$ charge $n$ for the DM fermion, along with integer charges for the SM fermions and the flavon field. In this flavon portal scenario, the DM is non-thermally produced from the decay of the flavon in the early universe which becomes a relic through the freeze-in mechanism. We explore the allowed parameter space for this DM candidate from relic abundance by solving the relevant Boltzmann equations. We find that reproducing the correct relic density requires the DM mass to be in the range $(100-300)$ keV for $n=7.5$ and $(3-10)$ MeV for $n=8.5$ where $n$ is the $U(1)_{\rm FN}$ charge of the DM fermion.

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arXiv2023

CMB signature of non-thermal Dark Matter produced from self-interacting dark sector

The basic idea of this work is to achieve the observed relic density of a non-thermal dark matter(DM) and its connection with Cosmic Microwave Background (CMB) via additional relativistic degrees of freedom which are simultaneously generated during the period $T_{\rm BBN}~{\rm to}~T_{\rm CMB}$ from a long-lived dark sector particle. To realize this phenomena we minimally extend the type-I seesaw scenario with a Dirac fermion singlet($χ$) and a complex scalar singlet ($φ$) which transform non-trivially under an unbroken symmetry $\mathcal{Z}_3$. $χ$ being the lightest particle in the dark sector acts as a stable dark matter candidate while the next to lightest state $φ$ operates like a long lived dark scalar particle. The initial density of $φ$ can be thermally produced through either self-interacting number changing processes ($3 φ\to 2 φ$) within dark sector or the standard annihilation to SM particles ($2 φ\to 2~ {\rm SM}$). The late time (after neutrino decoupling) non-thermal decay of $φ$ can produce dark matter in association with active neutrinos. The presence of extra relativistic neutrino degrees of freedom at the time of CMB can have a significant impact on $Δ\rm N_{eff}$. Thus the precise measurement of $Δ\rm N_{ eff}$ by current PLANCK 2018 collaboration and future experiments like SPT-3G and CMB-S4 can indirectly probe this non-thermal dark matter scenario which is otherwise completely secluded due to its tiny coupling with the standard model.

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arXiv2022

Muon $g-2$ and $W$-mass in a framework of colored scalars: an LHC perspective

A color octet isodoublet can have esoteric origins and it complies with minimal flavour violation. In this study, we take a scenario where the well known Type-X Two-Higgs doublet model is augmented with a color octet isodoublet. We shed light on how such a setup can predict the recently observed value for the $W$-boson mass. We also evaluate the two-loop Barr-Zee contributions to muon $g-2$ stemming from the colored scalars. The parameter space compatible with the observed muon $g-2$ gets relaxed w.r.t. what it is in the pure Type-X 2HDM by virtue of the contribution from the colored scalars. The extended parameter region therefore successfully accounts for both the $W$-mass and muon $g-2$ anomalies successfully. Finally, a collider signature leading to $τ^+ τ^- b \bar{b}$ final state is explored at the 14 TeV LHC using both cut-based and multivariate techniques. Such a signal can confirm the existence of both colorless as well colored scalars that are introduced by this framework.

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arXiv2022

Long lived inert Higgs in fast expanding universe and its imprint on cosmic microwave background

Presence of any extra radiation energy density at the time of cosmic microwave background formation can significantly impact the measurement of the effective relativistic neutrino degrees of freedom or ${\rm ΔN_{eff}}$ which is very precisely measured by the Planck collaboration. Here, we propose a scenario where a long lived inert scalar, which is very weakly coupled to dark sector, decays to a fermion dark matter via freeze-in mechanism plus standard model neutrinos at very low temperature $(T< T_{\rm BBN})$. We explore this model in the fast expanding universe, where it is assumed that the early epoch $(T > T_{\rm BBN})$ of the universe is dominated by a non-standard species $Φ$ instead of the standard radiation. In this non-standard cosmological picture, such late time decay of the inert scalar can inject some entropy to the neutrino sector after it decouples from the thermal bath and this will make substantial contribution to ${\rm Δ{N_{eff}}}$. Besides, in this scenario, the new contribution to $ΔN_{\rm eff}$ is highly correlated with the dark matter sector. Thus, one can explore such feebly interacting dark matter particles (FIMP) by the precise measurement of $ΔN_{\rm eff}$ using the current (Planck2018) and forthcoming (CMB-S4 \& SPT3G) experiments.

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arXiv2022

Concealing Dirac neutrinos from cosmic microwave background

The existence of prolonged radiation domination prior to the Big Bang Nucleosynthesis (BBN), starting just after the inflationary epoch, is not yet established unanimously. If instead, the universe undergoes a non-standard cosmological phase, it will alter the Hubble expansion rate significantly and may also generate substantial entropy through non-adiabatic evolution. This leads to a thumping impact on the properties of relic species decoupled from the thermal bath before the revival of the standard radiation domination in the vicinity of the BBN. In this work, considering the Dirac nature of neutrinos, we have studied decoupling of ultra-relativistic right-handed neutrinos ($ν_R$s) in presence of two possible non-standard cosmological phases. While in both cases we have modified Hubble parameters causing faster expansions in the early universe, one of the situations predicts a non-adiabatic evolution and thereby a slower redshift of the photon temperature due to the expansion. Considering the most general form of the collision term with Fermi-Dirac distribution and Pauli blocking factors, we have solved the Boltzmann equation numerically to obtain $Δ{\rm N}_{\rm eff}$ for the three right-handed neutrinos. We have found that for a large portion of parameter space, the combined effect of early decoupling of $ν_R$ as well as the slower redshift of photon bath can easily hide the signature of right-handed neutrinos, in spite of precise measurement of $Δ{\rm N}_{\rm eff}$, at the next generation CMB experiments like CMB-S4, SPT-3G etc. This however will not be applicable for the scenarios with only fast expansion.

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arXiv2022

Searching exotic Higgs bosons at the LHC

We analyse in a model independent way the possibilities of digging out neutral exotic Higgs states, should they exist endowed with unconventional couplings with ordinary matter and gauge fields, at the 14 TeV run of the Large Hadron Collider (LHC), adding some comparative studies for 13.6 and 13 TeV runs. Flavor models, based on some discrete symmetry groups, with extended scalar sectors are known to yield exotic spin-0 states, both CP-even and CP-odd, with purely flavor off-diagonal Yukawa couplings. The gauge interaction of one such CP-even state is also unusual that, unlike the Standard Model Higgs boson, it does not couple to gauge boson pairs. Such unconventional properties immune these exotic states from receiving traditional collider and electroweak constraints, and hence those states could be light. Without committing to any specific model, exploiting their peculiar Yukawa and gauge properties, we explore the discovery potential of those exotic Higgs states through some interesting topologies by figuring out some specific kinematic variables that suppress the backgrounds.

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arXiv2022

Signals for vector-like leptons in an $S_3$-symmetric 2HDM at ILC

In this work, we explore the signals of an $S_3$-symmetric two Higgs doublet model with two generations of vector-like leptons (VLLs) at the proposed International Linear Collider (ILC). The lightest neutral component of the VLL in this model provides a viable dark matter (DM) candidate satisfying the current relic density data as well as circumventing all direct and indirect DM search constraints. Some representative benchmark points have been selected with low, medium and high DM masses, satisfying all theoretical and experimental constraints of the model and constraints coming from the DM sector. The VLLs (both neutral and charged) will be produced in pair leading to multi-lepton and multi-jet final states. We show that the ILC will prove to be a much more efficient and useful machine to hunt for such signals compared to LHC. Using traditional cut-based analysis as well as sophisticated multivariate analysis, we perform a detailed analysis of some promising channels containing mono-lepton plus di-jet, di-lepton, four-lepton and four jets along with missing transverse energy in the final state at 1 TeV ILC.

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arXiv2021

Dark Matter and Collider Searches in $S_3$-Symmetric 2HDM with Vector Like Leptons

We study the $S_3$-symmetric two Higgs doublet model by adding two generations of vector like leptons (VLL) which are odd under a discrete $Z_2$ symmetry. The lightest neutral component of the VLL acts as a dark matter (DM) whereas the full VLL set belongs to a dark sector with no mixings allowed with the standard model fermions. We analyse the model in light of dark matter and collider searches. We show that the DM is compatible with the current relic density data as well as satisfying all direct and indirect dark matter search constraints. We choose some representative points in the model parameter space allowed by all aforementioned dark matter constraints and present a detailed collider analysis of multi-lepton signal viz. the mono-lepton, di-lepton, tri-lepton and four-lepton along with missing transverse energy in the final state using both the cut-based analysis and multivariate analysis respectively at the high luminosity 14 TeV LHC run.

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arXiv2021

Gamma-ray and Synchrotron Radiation from Dark Matter annihilations in Ultra-faint Dwarf Galaxies

The very large (100-1000) mass-to-light ratio applicable to the ultra-faint dwarf galaxies (UFDs) implies a high concentration of dark matter, thus rendering them ideal theatres for indirect signatures of dark matter. In this paper, we consider 14 recently discovered UFDs and study the electromagnetic radiation emanating from them over a wide range, from gamma ray down to radio frequencies. We analyze the Fermi-LAT data on high energy gamma rays and radio fluxes at the GMRT and VLA to obtain upper limits on annihilation cross section $\langleσv\rangle$ in a model independent way. We further discuss the sensitivity of the Square Kilometer Array radio telescope in probing the synchrotron radiation from the aforementioned UFDs. We also investigate the dependences of the said upper limits on the uncertainties in the determination of various astrophysical parameters.

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arXiv2020

Fingerprinting the contribution of colored scalars to the $H^+ W^- Z(γ)$ vertex

Color-octet scalars arise in various Grand Unification scenarios and also in other models of new physics. They are also postulated for minimal flavour violation. Purely phenomenological imprints of such scalars are therefore worth looking at. Motivated by this, we perform a complete one-loop calculation of the $H^+ \to W^+ Z (\g)$ decay in a two Higgs doublet model augmented by a color-octet $SU(2)_L$ scalar doublet. The computation is conveniently segregated into colorless and colored components. The color-octet part of the amplitude, being scaled by the color-factor, provides an overall enhancement to the form factors. Crucial constraints from perturbative unitarity, positivity of the scalar potential, oblique parameters, Higgs signal strengths and direct search of a charged Higgs and color-octet scalars are folded-in into the analysis. Sensitivity of the loop-induced $H^+ \to W^+ Z (\g)$ vertex to other model parameters is elucidated. Finally, the prospect of observing a loop-induced $H^+ \to W^+ Z (\g)$ interaction at the future hadronic collisions is also discussed.

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arXiv2020

Sub-TeV $H^+$ Boson Production as Probe of Extra Top Yukawa Couplings

We suggest searching for the charged Higgs boson at the Large Hadron Collider (LHC) via $cg \to b H^+ \to b t \bar b$. In the general two Higgs Doublet Model, extra top Yukawa couplings $ρ_{tc}$ and $ρ_{tt}$ can drive the disappearance of antimatter from the Universe, while $\bar cbH^+$ and $\bar tbH^+$ couple with strength $ρ_{tc}V_{tb}$ and $ρ_{tt}V_{tb}$, respectively. For $ρ_{tc},\, ρ_{tt} \sim 0.5$ and $m_{H^+} \sim 300$--500 GeV, evidence could emerge from LHC Run 2 data at hand, and discovery by adding Run 3 data in the near future.

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arXiv2020

Associated $Z^\prime$ production in the flavorful $U(1)$ scenario for $R_{K^{(*)}}$

The flavorful $Z^\prime$ model with its couplings restricted to the left-handed second generation leptons and third generation quarks can potentially resolve the observed anomalies in $R_K$ and $R_{K^*}$. After examining the current limits on this model from various low-energy processes, we probe this scenario at 14 TeV high-luminosity run of the LHC using two complementary channels: one governed by the coupling of $Z'$ to $b$-quarks and the other to muons. We also discuss the implications of the latest LHC high mass resonance searches in the dimuon channel on the model parameter space of our interest.

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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

Collider signature of $U_1$ Leptoquark and constraints from $b\to c$ observables

One of the most popular models that is known to be able to solve the lepton flavour universality violating charged ($b\to c$) and neutral current ($b\to s$) anomalies is the Leptoquark Model. In this work we examine the {\it multijet} + $\mET$ collider signature of a vector leptoquark ($U_1$) which has the potential to mediate both the charged and neutral current processes at tree level. From our collider analysis we derive the exclusion limits on mass for the $U_1$ leptoquark at 95\% C.L. at the current and future experiment of the Large Hadron Collider. We also calculate the effect of such a leptoquark in $b\to c$ observables.

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