arXiv subjects

Nilanjana Kumar

Publications and source records attributed to Nilanjana Kumar.

At least 19 recordsLinked to original sources

arXiv2026

Warm Inflation Beyond the Markovian Limit

Warm inflation is commonly studied under the assumption that the stochastic force sourcing inflaton fluctuations is Markovian. Realistic thermal systems, however, possess finite relaxation times and can therefore generate colored noise with non-zero correlation time. In this work, we investigate warm inflation beyond the Markovian limit and determine how finite correlation time modifies the primordial scalar power spectrum. We show that memory effects suppress the scalar spectrum relative to the standard white-noise result and derive a simple expression for this correction in terms of the background thermal dynamics. In particular, we relate the size of the non-Markovian effect directly to the thermal ratio between the bath temperature and the Hubble scale, thereby establishing a transparent link between warm-inflation background quantities and the validity of the Markovian approximation. We also derive the corresponding modification of the tensor-to-scalar ratio and the induced shifts in the scalar spectral index and the running of the scalar spectral index. Our results provide a simple and practical diagnostic for identifying when finite correlation-time effects become relevant in warm-inflation model building.

astro-ph.CO↗

arXiv2025

Search for Quadruplet Scalars using Boosted Decision Trees at the LHC

Beyond the Standard Model scenarios introduce additional scalar and fermion multiplets, which influence neutrino mass generation mechanisms and yield distinctive collider signatures. This work focuses on a particular scenario involving a fermion quintuplet and a scalar quadruplet. The study examines the production and decay of the scalar quadruplet components at the Large Hadron Collider (LHC), emphasizing how their decay patterns, fermiophobic versus fermiophilic, depend on mass differences and Yukawa couplings with the fermion multiplets. This study provides an overview of possible signals at the LHC, along with a detailed collider analysis focused on final states containing at least four leptons and two jets, in which the masses of the scalars and fermions are reconstructed successfully. Standard Model backgrounds are also incorporated in the study, with multivariate techniques leveraged via Boosted Decision Trees. Results indicate discovery potential for scalar masses around 600-700 GeV and exclusion sensitivity extending beyond 1 TeV, highlighting the promising experimental signatures of the model and its role in probing new physics at colliders.

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arXiv2025

Fatjet Signatures of Quintuplet Fermions at the LHC

This paper explores a simplified extension of the standard model featuring a neutral fermion quintuplet and a scalar quadruplet, which together generate neutrino masses through tree and loop level mechanisms. The quintuplet fermions decay into standard model gauge bosons via the scalars, producing unique collider signatures at the LHC characterized by multilepton and multijet final states. The study focuses on the pair production of quintuplet fermions in the 700-1200 GeV mass range, where their decays produce highly boosted W and Z bosons identifiable as fatjets. Emphasis is placed on the production and decay of doubly charged fermions due to their higher cross section. Advanced jet substructure and kinematic techniques are applied to enhance sensitivity by reducing standard model backgrounds. A detailed analysis of signal significance is performed in the two lepton, two fatjet and three lepton, one fatjet channels for different masses of the fermion and the scalars, optimizing selection cuts to maximize signal efficiency over standard model backgrounds. The study found that both channels exhibit excellent performance, with significance exceeding $5\sigma$ under realistic conditions including a 50\% background uncertainty at integrated luminosity up to 3000 fb$^{-1}$.

hep-ph↗

arXiv2025

Vectorlike $\tau$ production through leptoquarks

Numerous phenomenological studies and collider searches have probed for the existence of new physics by looking for signatures of leptoquarks (LQs) or vectorlike leptons (VLLs). We consider a new possibility that can arise in theories with enhanced gauge symmetries: both particles are simultaneously present, and LQ-mediated processes enhance the VLL production at the LHC. We study the effect of non-standard interactions of LQs that contribute to novel production and decay signatures. We obtain the HL-LHC prospects of this framework in the mono-and di-lepton final states, and discuss other potentially relevant channels.

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arXiv2025

Exotic Decays and Collider Signatures of pNGB Scalars in the $SU(5)/SO(5)$ Composite Higgs Model

The nature of the Higgs boson, whether it is elementary or composite, will be investigated through precision measurements at the collider experiments. In composite Higgs scenarios, the Higgs may manifest as a pseudo Nambu-Goldstone boson (pNGB) arising from a strongly interacting sector. The $SU(5)/SO(5)$ Composite Higgs Model features a rich scalar sector, with the decay patterns of the scalars being heavily influenced by the way fermions are embedded in various representations of $SU(5)$. We discuss how masses of the pNGB scalars and their couplings depend functionally on the compositeness scale and the parameters of the strong sector. Unique decay modes of the scalars emerge from the model when the mixing among the various pNGB scalars is non-negligible. We present a comprehensive and thorough analysis of the fermiophilic and fermiophobic decay modes of the pNGB scalars. Significant differences are observed in the decay patterns of the two singly charged scalars. Further, the decay of one pNGB to another on-shell pNGB when masses exceed about $1$ TeV presents a rich phenomenology, leading to distinctive signatures at the colliders. In this context, the future muon collider offers a promising avenue for detecting pNGB scalars with masses larger than $1$ TeV, especially in final states involving $W/Z$ fatjets.

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arXiv2024

Unconventional Searches for Exotic Particles at Future Lepton Colliders

The main aim of the the Large Hadron Collider (LHC) experiments is to search for exotic particles with masses in the TeV range as predicted by Beyond Standard Model (BSM) theories. However, there is no hint of BSM around TeV scale so far. Hence, it is possible that the exotic particles are heavier and larger centre of mass energy is needed to observe them. Alternatively, the future lepton colliders offer a comparatively cleaner environment than the LHC which is advantageous to detect light exotic particles. Lepton colliders, like the International Linear Collider, provide the opportunity to detect exotic particles at energies below the TeV scale. The Muon Collider, once fully operational, will have the capability to observe exotic particles at and beyond the TeV scale. The search for BSM particles typically assumes a minimal scenario where only one type of BSM particle couples with the Standard Model (SM) sector. But there are theories which involve such interactions of multiple BSM particles. Here I discusses a specific model featuring a fermionic quintuplet and a scalar quartet that interact before decaying into SM particles. This model yields distinctive signatures characterized by high lepton and jet multiplicities, making it a promising candidate for detection at future lepton colliders.

hep-ph↗

arXiv2024

Attenuation of Boosted Dark Matter in Two Component Dark Matter Scenario

Boosted dark matter constitutes a small fraction of the total dark matter in the Universe, with mass ranging from eV to MeV and often exhibiting (semi)relativistic velocity. Hence the likelihood of detecting boosted dark matter in Earth-based direct detection experiments is relatively high. There is more than one explanation for the origin of the boosted dark matter including the two-component dark matter models where the heavier dark matter species(dominant) annihilates to nearly monoenergetic light dark matter particles (subdominant) in the galactic halo. If the dominant dark matter species is heavier (MeV-GeV), the subdominant light dark matter achieves (semi)relativistic velocity or {\it boost}. These boosted dark matter particles suffer from scattering with electrons and nuclei while crossing the atmosphere and the Earth's crust before reaching underground experiments and hence the kinetic energy of the dark matter is attenuated. In the two-component dark matter framework, we examine how the boost of the dark matter influences the attenuation of kinetic energy across a broad spectrum of dark matter masses. We perform a detailed study at various DM-electron and DM-nucleus cross sections including the effect of nuclear form factor and elastic and inelastic scattering (for large kinetic energy). For a 10 MeV boosted dark matter with boost $\sim$ 10-100, the effect of DM-electron scattering is found to be severe than the DM-nucleus scattering (with form factor) if DM-nucleon scattering cross section is $10^{-29}$cm$^2$. We also show how the peak position of the boosted dark matter flux shifts due to the attenuation of its kinetic energy.

hep-ph↗

arXiv2023

Viability of Boosted Light Dark Matter in a Two-Component Scenario

We study the boosted dark matter (BDM) scenario in a two-component model. We consider a neutrinophilic two-Higgs doublet model ($\nu$2HDM), which consists of one extra Higgs doublet and a light right-handed neutrino. This model is extended with a light ($\sim 10$~MeV) singlet scalar DM $\phi_3$, which is stabilized under an extra dark $Z_2^{\rm DM}$ symmetry and can only effectively annihilate through the CP even scalar $H$. Although oblique parameters put tight constraints on the model, introduction of vectorlike leptons (VLL) can potentially salvage the issue. The vectorlike doublet $N$ and singlet $\chi$ are also stabilized through dark $Z_2^{\rm DM}$ symmetry. The lightest vectorlike mass eigenstate ($\chi_1 \sim 100$~GeV) is the 2nd DM component of the model. The fermion DM is restricted in a narrow mass region while a somewhat broader mass region is allowed for the scalar DM. However, when two DM sectors are coupled, the annihilation channel $\chi_1 \chi_1 \to \phi_3 \phi_3$ opens up. As a result, the fermionic relic density decreases, and paves way for broader fermionic DM mass region with under-abundant relic: a region of $[30-65]$ GeV compared to a narrower $[40-50]$ GeV window for the single component case. On the other hand, the light DM $\phi_3$ acquires significant boost from the annihilation of $\chi_1$, causing a dilution in the resonant annihilation of $\phi_3$. This in turn increases the scalar DM relic, allowing for a smaller mass region compared to the individual case. The exact and underabundant relic is achievable in a significant parameter space of the two-component model where the total DM relic is mainly dominated by the fermionic DM contribution. The scalar DM is found to be sub-dominant or equally dominant ($\sim 30 \% - 80 \%$ of total DM) with significant boost which can be detected in experiments.

hep-ph↗

arXiv2023

Composite pseudo Nambu Goldstone Quintessence

A pseudo-Nambu Goldstone Boson (pNGB) arising from the breaking of a global symmetry ($G\rightarrow H$) can be one of the most promising candidates for the quintessence model, to explain the late-time acceleration of our universe. Motivated from the Composite Higgs scenario, we have investigated the case where the pNGB associated with $SO(N)/ SO(N-1)$ develops a potential through its couplings with the particles that do not form the complete representations of $G$. The Coleman Weinberg (CW) potential is generated via the external particles in the loop which are linked with the strongly interacting dynamics and can be computed predicatively. The model of Dark Energy (DE) is tested against several latest cosmological observations such as supernovae data of Pantheon, Baryon Acoustic Oscillation (BAO), Redshift-space distortion (RSD) data, etc. We have found that the fit prefers the sub-Planckian value of the pNGB field decay constant. Moreover, we have found that the model predicts cosmological parameters well within the allowed range of the observation and thus gives a well-motivated model of quintessence.

astro-ph.CO↗

arXiv2023

Veltman Criteria in Beyond Standard Model Effective Field Theory of Complex Scalar Triplet

The Higgs mass is not protected by any symmetry in the Standard Model. Hence, the self-energy corrections to the Higgs mass become large due to the quadratic divergence terms. Veltman condition (V.C.) ensures that the coefficient of the quadratic divergent term either vanishes or becomes negligible. The non-observation of new physics has pushed the new physics scale to be larger than 1 TeV, making it impossible to satisfy the Veltman condition in the Standard Model without very large fine-tuning. Many attempts are made to satisfy the V.C. in Beyond Standard Model theories, but the V.C. is hard to achieve at a very large scale ($\Lambda$). Alternatively, it is possible that the new physics appears much above the Electroweak scale, and the effect of the new physics is observed in terms of the Wilson coefficients of the Standard Model Effective Field Theory (SMEFT) operators. The V.C. can be addressed in the SMEFT framework. In this paper, some specific new physics scenarios are considered at a very large scale. Below that scale, the effect of the new physics is observed as Beyond Standard Model Effective Field Theory (BSM-EFT). We particularly study the type-II seesaw model with the complex scalar triplet ($Y=1$) in the context of V.C. We found that this particular model is the minimal model to generate all SMEFT operators that appear in V.C. and satisfies V.C. We also examine the model parameter dependence of the Wilson coefficients in detail and show how the cancellation of the Wilson coefficients is highly dependent on some specific values of the model parameters.

hep-ph↗

arXiv2022

A Brief Review on Jet Substructure in Connection with Collider Phenomenology

It is a challenge for the theoretical particle physicists to perform the phenomenology of the Beyond Standard Model (BSM) theories using advanced simulations which can mimic the experimental environment at the colliders as closely as possible. In collider phenomenology jet substructure is a concept that is used frequently to analyse the properties of the jets, characterised as a cluster of hadrons, which is often the end result of particle collisions at the colliders, such as Large Hadron Collider (LHC). A vast literature on jet substructure exists both from the theory and experimental point of view. But, even with the knowledge of Quantum Chromodynamics (QCD), it is hard to cope up with the vastness of the applicability of the subject for a new researcher in this field. In this review, an attempt has been made to bridge the gap between the concept of jet substructure and its application in the collider phenomenology. However, for detailed understanding, one should look at the references.

hep-ph↗

arXiv2022

A multi-charged particle model with local $U(1)_{μ-τ}$ to explain muon $g-2$, flavor physics, and possible collider signature

We consider a model with multi-charged particles including vector-like fermions and a charged scalar under a local $U(1)_{μ- τ}$ symmetry. We search for allowed parameter region explaining muon anomalous magnetic moment (muon $g-2$) and $b \to s \ell^+ \ell^-$ anomalies, satisfying constraints from the lepton flavor violations, $Z$ boson decays, meson anti-meson mixing and collider experiments. Carrying out numerical analysis, we explore the typical size of the muon $g-2$ and Wilson coefficients to explain $b \to s \ell^+ \ell^-$ anomalies in our model when all other experimental constraints are satisfied. We then discuss the collider physics of the multicharged vectorlike fermions, considering some benchmark points in the allowed parameter space.

hep-ph↗

arXiv2022

The physics case of a 3 TeV muon collider stage

In the path towards a muon collider with center of mass energy of 10 TeV or more, a stage at 3 TeV emerges as an appealing option. Reviewing the physics potential of such muon collider is the main purpose of this document. In order to outline the progression of the physics performances across the stages, a few sensitivity projections for higher energy are also presented. There are many opportunities for probing new physics at a 3 TeV muon collider. Some of them are in common with the extensively documented physics case of the CLIC 3 TeV energy stage, and include measuring the Higgs trilinear coupling and testing the possible composite nature of the Higgs boson and of the top quark at the 20 TeV scale. Other opportunities are unique of a 3 TeV muon collider, and stem from the fact that muons are collided rather than electrons. This is exemplified by studying the potential to explore the microscopic origin of the current $g$-2 and $B$-physics anomalies, which are both related with muons.

hep-ph↗

arXiv2022

Alternative signatures of the quintuplet fermions at the LHC and future linear colliders

Large fermionic multiplets appear in different extensions of the Standard Model (SM), which are essential to predict small neutrino masses, relic abundance of the dark matter (DM) and the measured value of muon anomalous magnetic moment (muon (g-2)). Models containing quintuplet of fermions ($Σ$), along with other scalar multiplets, can address recent anomalies in the flavor sector while satisfying the constraints from the electroweak physics. In standard scenarios, the exotic fermions couple with the SM particles directly and there exists a strong limit on their masses from collider experiments such as the Large Hadron Collider (LHC). In this paper, we choose a particular scenario where the quintuplet fermions are heavier than the scalars, which is naturally motivated from the muon (g-2) data. A unique nature of these models is that they predict non-standard signatures at the colliders as the quintuplet fermions decay via the scalars once produced at the colliders. We study these non-standard interactions and provide alternative search strategies for these exotic fermions at the LHC and future linear colliders (such as $e^+e^-$ colliders). We also discuss their exclusion and discovery limits. For the doubly charged quintuplet fermion ($Σ^{\pm\pm}$), discovery is possible with 5$σ$ significance at integrated luminosity of 3000 fb$^{-1}$ at 14 TeV LHC if $M_Σ\leq 980$ GeV. For the singly charged quintuplet fermion ($Σ^\pm$), the discovery is challenging at LHC but there might be a possibility of 5 $σ$ discovery with 1000 fb$^{-1}$ luminosity at $e^+e^-$ collider for $M_Σ\leq 700$ GeV.

hep-ph↗

arXiv2021

Emergent 2HDM in LSS Little-Higgs: Musings from Flavor and Electroweak Physics

The low energy effective theory ($\sim$ TeV) of the little-Higgs model with $SU(6)/Sp(6)$, as proposed by Low, Skiba and Smith (LSS), exhibits a two-Higgs doublet model (2HDM) structure. The symmetry dictates interesting Yukawa patterns, translating to non-trivial fermion couplings with both of the Higgs doublets. The couplings of the scalars with the fermions can induce flavor changing neutral currents (FCNC), which get constraints from flavor physics observables such as BR$(B\rightarrow X_sγ)$, $B_s - \bar{B}_s$ mixing etc. The precision measurement of $Z b \bar{b}$ vertex, the top and Higgs mass along with other Higgs coupling measurements at the Large Hadron Collider (LHC) also enforce severe restrictions on the LSS model. Direct LHC search results of beyond the Standard Model (BSM) particles also impose bounds on the masses. We probe the LSS model in view of the above constraints through a random scan in the multi-dimensional parameter space. We observe, on contrary to the general 2HDM scenario, the emergent 2HDM from the LSS model is less constrained from the flavor data and the $Z b \bar{b}$ measurement but is severely constrained form the electroweak (EW) searches at the LHC. From the flavor data and $Z b \bar{b}$, we find that the charged Higgs mass is relaxed with $\tanβ$ being restricted to $0.5-5$, whereas the charged Higgs mass is pushed to larger than 1 TeV along with $\tanβ$ being further restricted to $< 3$ when the LHC bounds are incorporated.

hep-ph↗

arXiv2021

Looking for a vectorlike B quark at LHC using jet substructure

Vectorlike quarks have been shown to resolve certain long-standing discrepancies pertaining to the bottom sector. We investigate, here, the prospects of identifying the existence of a topless vectorlike doublet $(B,~Y)$, as is preferred by the electroweak precision measurements. Concentrating on single production, $viz.$ $B \bar b$ with $B \to b + Z/H$ subsequently, we find that the fully hadronic decay-channel is susceptible to discovery provided jet substructure observables are used. At the 13 TeV LHC with an integrated luminosity of 300 fb$^{-1}$, a modest value of the chromomagnetic transition moments allows for the exclusion of $M \lesssim 1.8(2.2)$ TeV in the $Z$ and $H$ channels respectively.

hep-ph↗

arXiv2020

Flavor violation at LHC in events with two opposite sign leptons and a $b$-jet

Hints of flavor violation at both charged and neutral current decays of mesons have been observed in experiments such as LHCb, Belle, and BaBar. The anomalies in the result can be addressed in the effective field theory (EFT) framework. The effective operators predict different beyond standard model (BSM) signatures and the four point interaction vertices can be probed at Large Hadron Collider (LHC). In this context, the discovery projection of two opposite sign leptons and a $b$-jet signature is studied in this paper at 13 TeV LHC.

hep-ph↗

arXiv2020

Scotogenic neutrino mass with large $SU(2)_L$ multiplet fields

We construct a scotogenic neutrino mass model introducing large $SU(2)_L$ multiplet fields without adding an extra symmetry. We have introduced extra scalar fields such as a septet, quintet and quartet where we make the vacuum expectation value of quartet scalar to be zero while septet and quintet develop non-zero ones. Then the neutrino mass is generated at one-loop level by introducing quintet fermion. We analyze the neutrino mass matrix taking constraints from lepton flavor violation into account and discuss collider physics regarding charged fermions from large multiplet fields. We have analysed the production and the decays of the quintet fermions, as well as the discovery reach at 14 TeV and 27 TeV LHC.

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