arXiv subjects

Subhaditya Bhattacharya

Publications and source records attributed to Subhaditya Bhattacharya.

At least 19 recordsLinked to original sources

arXiv2026

High energy probes of Higgs self-coupling via $W$ boson fusion at future lepton colliders

We investigate the sensitivity to the Higgs self-coupling through $W$ boson fusion di-Higgs production at CLIC with a center-of-mass energy of $\sqrt{s}=3$ TeV. We study the interplay between the Higgs self-coupling modifier ($\kappa_{\lambda}$) and Higgs-gauge coupling modifiers ($\kappa_{V}$ and $\kappa_{2V}$) within the $\kappa$ framework. To enhance the separation between signal and background, we develop a graph neural network (GNN) based classifier that achieves a signal significance of $\mathscr{Z}\approx 20~\sigma$ at $5~\mathrm{ab}^{-1}$, substantially exceeding projected HL-LHC sensitivity. Our results demonstrate that high-energy lepton colliders, combined with graph-based machine learning, provide excellent sensitivity to the Higgs self-coupling and offer a powerful probe of new physics in the electroweak sector, disentangling linearly and non-linearly realized electroweak symmetry breaking.

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arXiv2025

Exploring Leptogenesis, WIMP Dark Matter, and Gravitational Waves in an extended Scalar Framework

We explore extensions of type I seesaw framework with a scalar mediator ($\Phi$) connecting to a complex scalar dark field ($S$), and right handed neutrinos ($N_i$), with an aim to correlate neutrino mass generation, leptogenesis, and dark matter. $\mathcal{Z}_4\times CP$ turns out to be a phenomenologically viable choice of the extended symmetry, which can accommodate a dimension five effective interaction $\bar{l}_L^\alpha \tilde{H}\Phi N_i$, involving the SM lepton isodoublet ${l}_L$, and Higgs $H$; prohibiting the canonical Yukawa term $\bar{l}_L^\alpha \tilde{H} N_i$. The $\mathcal{Z}_{4}$ symmetry is spontaneously broken via the vacuum expectation value (VEV) of the $\Phi$ filed, which directly affects neutrino mass generation and leptogenesis; while the $CP$ symmetry stabilises one component of $S$, making it a viable dark matter candidate. The discrete symmetry breaking creates domain wall, which needs to be annihilated before the over-closure of the Universe. This paves the way for gravitational wave signal associated with the model set up, which probes the symmetry breaking scale, and indirectly connects to the other phenomena.

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arXiv2025

Probing CP-violating Higgs-Gauge couplings with Higgsstrahlung at $e^-e^+$ collider

We investigate the sensitivity of a future high-luminosity $e^-e^+$ collider operating at $\sqrt{s}=250~\text{GeV}$ to CP-violating and CP-conserving anomalous $hVV$ interactions via the Higgsstrahlung process. The effects of new physics are parameterized in the Standard Model Effective Field Theory~(SMEFT) framework through six dimension-6 operators modifying the $hVV$ vertices. Using polarized beams and exploiting polarization and spin correlation asymmetries reconstructed from Higgs decay products, we perform a comprehensive analysis across the three dominant decay modes, $h\to b\bar{b}$, $WW^\star$, and $ZZ^\star$. The $h\to WW^\star$ channel exhibits the highest sensitivity to $\mathcal{O}_{HW}$ and $\mathcal{O}_{H\widetilde{W}}$, while the $b\bar{b}$ mode constrains the remaining operators with high statistical precision. Sensitivity studies incorporating luminosity scaling and systematic uncertainties show that the projected bounds improve significantly with increasing integrated luminosity, but saturate once experimental systematics exceed the few-percent level. These results highlight the crucial role of spin-based observables and beam polarization in achieving sub-percent precision on SMEFT coefficients at future lepton colliders.

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arXiv2025

Lepton Collider as a Window to Reheating via Freezing Out Dark Matter Detection

We investigate a particle dark matter (DM) scenario where the DM interaction with the Standard Model are mediated by a leptophilic effective operator. Unlike conventional WIMP scenarios where thermal freeze-out occurs in a radiation-dominated Universe, we consider DM freeze-out during a prolonged reheating epoch driven by inflaton decay. The resulting departure from standard cosmology alters the thermal evolution of the dark matter abundance, making it sensitive to the reheating temperature and the history of entropy injection. The leptophilic nature of the interaction, motivated by the absence of DM signals in the current LHC searches, suppresses couplings to quarks and gluons and instead enables viable DM-lepton interactions that remain largely unconstrained. Within this setup, we analyze the mono-Higgs plus missing energy channel at future lepton colliders where the same operator responsible for setting the relic abundance can be directly probed. We perform a detailed signal-background analysis using both polarized and unpolarized beams. Additionally, our results illustrate how collider experiments, when interpreted jointly with relic density constraints, can provide indirect hints of the Universe's thermal history, offering potential insights into the reheating temperature and the dynamics preceding Big Bang Nucleosynthesis.

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arXiv2025

Optimal estimation of Higgs-Gauge Boson couplings at the future $e^+e^-$ colliders

The proposed $e^+e^-$ collider offers an ideal environment for precise estimation of Higgs boson properties which are of utmost importance to validate the Standard Model of particle physics. We investigate $hVV$ couplings, where $V\in \{Z,\gamma\}$ with single Higgs production associated with $Z$ boson at the proposed $e^+e^-$ machine with $\sqrt{s}=250$ GeV, within the Standard Model Effective Field Theory (SMEFT) framework. We employ the recoil mass of the dilepton system, to select the signal phase space, i.e, $Zh \to l^+l^-b\Bar{b}$ events. The constraints on the Wilson coefficients (WCs) are obtained using the optimal observable technique (OOT). On comparison with the current experimental limits at $68\%$ CL with $138$ fb$^{-1}$ luminosity, our limits are tighter by a factor ranging from $1.5-10$ for CP even operators, while CP-odd WCs shows comparable limits.

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arXiv2025

Probing $\Delta L=2$ lepton number violating SMEFT operators at the same-sign muon collider

We investigate lepton number violation (LNV) induced by $\Delta L = 2$ dimension-seven Standard Model Effective Field Theory (SMEFT) operators in the context of same-sign muon colliders. Specifically, we study $\mu^+ \mu^+ \rightarrow W^+W^+/\;W^+qq'$ production at the $\mu$TRISTAN at $\sqrt{s} =$ 2 TeV with an integrated luminosity of 1 ab$^{-1}$, using the final-state signature comprising two fat jets. This process is sensitive to eight distinct SMEFT operators, providing a unique avenue for testing LNV beyond the Standard Model. We determine the maximal sensitivity to these operators and compare our results with existing LHC constraints and future projections from the FCC. Our findings highlight the potential of same-sign muon colliders to serve as powerful probes of LNV and New Physics (NP) at the TeV scale.

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arXiv2025

Probing ALP-portal fermionic dark matter at the $e^+e^-$ colliders

Axion-like particles (ALPs) are promising candidates for mediating interactions between a dark sector and the Standard Model (SM). In this work, considering the effective interactions of ALPs with the SM gauge bosons and a fermion dark matter (DM), we explore the DM relic satisfied parameter space and assess its testability through indirect searches. The effect of early kinetic decoupling is also discussed in the resonant regime. The potential of probing such ALP-portal fermionic DM at the electron-positron colliders is investigated with the mono-photon plus missing energy final states. We show that a spectacular distinction between the signal and SM background is possible via the missing energy variable, the seed of which lies in the ALP-photon interaction, which also governs the relic density of DM. We further discuss the sensitivity of ALP-photon coupling using the $\chi^2$ analysis at the future electron-positron collider specifications.

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arXiv2025

Up-type FCNC in presence of Dark Matter

Dark Matter (DM) is a known unknown. Apart, current experimental constraints on flavor-changing neutral current (FCNC) processes involving up-type quarks also provide scope to explore physics beyond the Standard Model (SM). In this article, we establish a connection between the flavor sector and the DM sector with minimal extension of the SM. Here a singlet complex scalar field, stable under $\mathbb{Z}_3$ symmetry, acts as DM and couples to SM up-type quarks through a heavy Dirac vector-like quark (VLQ), which shares the same $\mathbb{Z}_3$ charge as of the DM. The model thus addresses the observed $D^0-\bar{D^0}$ mixing, top-FCNC interactions, and $D^0$ meson decays, together with DM relic density, while evading the direct and indirect DM search bounds. The model can be probed at the future high-energy muon collider, through distinctive signatures of VLQ production, where the VLQ decays into DM and SM particles, abiding by the existing bounds.

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arXiv2025

Direct Search signal of two-component Dark Matter

How do we know if the dark sector consists of more than one dark matter (DM) component is an important question, for which the answer is not very definite. In this article we study such a possibility in context of direct DM search. It was pointed out earlier in a model independent analysis that a kink in the nuclear recoil energy spectrum may indicate to the presence of two DM components. However, realising one such model was difficult due to experimental constraints. Here we propose and study a model containing a vector boson DM and a scalar DM, aided by a light scalar mediator, where a kink in the nuclear recoil spectrum arises after addressing individual relic densities, direct search limits, collider constraints and theoretical limits. We find out the allowed parameter space of the model and those regions likely to show such distinctive signal.

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arXiv2024

Two-component Dark Matter and low scale Thermal Leptogenesis

The observable cosmos exhibits sizable baryon asymmetry, small active neutrino masses, and the presence of dark matter (DM). To address these phenomena together, we propose a two component DM scenario in an extension of Scotogenic model, imposing $\mathbb{Z}_2 \otimes \mathbb{Z}_2^{\prime}$ symmetry. The electroweak sphaleron process converts the $\rm Y_{B-L}^{}$ yield, generated through the Leptogenesis mechanism, into the baryon asymmetry ($\rm Y_{\Delta B}^{}$) at $\rm T_{\rm sph}\sim 130$ GeV, the sphalerons decoupling temperature. In this framework, the CP asymmetry as well as the radiative neutrino mass generation explicitly involve the two DM particles, thus establishing a correlation between the baryon asymmetry, DM and observed active neutrino masses. We study in details the allowed parameter space available after considering all the constraints from the three phenomena as well as from the collider search limits, and outline the region which could potentially be tested in future DM detection experiments through direct or indirect detection searches, lepton flavor-violating decays, etc.

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arXiv2024

Pseudo-FIMP dark matter in presence of a SIMP

Pseudo-feebly Interacting Massive Particle (pFIMP) has been postulated in two component dark matter (DM) scenarios, where it has feeble interaction with the visible sector, but sizeable one with a thermal bath partner. In this work, we study the possibility and dynamics of pFIMP in presence of a Strongly Interacting Massive Particle (SIMP), which is well known to solve too-big-to-fail and core-vs-cusp problems. Our analysis is primarily model-independent via solving coupled Boltzmann equations, with negligible DM-DM conversion adhering to pure SIMP-FIMP limit, and then with larger DM-DM conversion rate pertaining to SIMP-pFIMP limit. We also illustrate the simplest model yielding pFIMP-SIMP set-up having two scalars stabilised under $\mathbb{Z}_2\otimes \mathbb{Z}_3$ symmetry, and explore the accessible parameter space after addressing relic density, unitarity, self interaction constraints etc. pFIMP detectability is limited in such circumstances, but possible via a thermal DM loop when the SIMP has a visible sector interaction via light mediator.

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arXiv2024

Lepton collider as a window to reheating via freezing in dark matter detection. Part II

Dark matter (DM) genesis via Ultraviolet (UV) freeze-in embeds the seed of reheating temperature and dynamics in its relic density. Thus, discovery of such a DM candidate can possibly open the window for post-inflationary dynamics. However, there are several challenges in this exercise, as freezing-in DM possesses feeble interaction with the visible sector and therefore very low production cross-section at the collider. We show that mono-photon (and dilepton) signal at the ILC, arising from DM effective operators connected to the SM field strength tensors, can still warrant a signal discovery. We study both the scalar and fermionic DM production during reheating via UV freeze-in, when the inflaton oscillates at the bottom of a general monomial potential. Interestingly, we see, right DM abundance can be achieved only in the case of bosonic reheating scenario, satisfying bounds from big bang nucleosynthesis (BBN). This provides a unique correlation between collider signal and the post-inflationary dynamics of the Universe within single-field inflationary models.

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arXiv2024

Multiparticle scalar dark matter with $\mathbb{Z}_N$ symmetry

More than one dark sector particle transforming under the same symmetry provides one stable dark matter (DM) component which undergoes co-annihilation with the heavier particle(s) decaying to DM. Specific assumptions on the kinematics and on the coupling parameters may render the heavier component(s) stable and contribute as DM. The choices of the charges of the dark sector fields under transformation play a crucial role in the resultant phenomenology. In this paper, we systematically address the possibility of obtaining two scalar DM components under $\mathbb{Z}_N$ symmetry. We consider both the possibilities of DM being weakly interacting massive particle (WIMP) or pseudofeebly interacting massive particle (pFIMP). We elaborate upon $\mathbb{Z}_3$ symmetric model, confronting the relic density allowed parameter space with recent most direct and indirect search bounds and prospects. We also highlight the possible distinction of the allowed parameter space in single component and two component cases, as well as between WIMP-WIMP and WIMP-pFIMP scenarios.

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arXiv2024

Flavor-Specific Dark Matter Signatures through the Lens of Neutrino Oscillations

We investigate the flavor-specific properties of leptophilic dark matter in neutrino mass models, where dark matter signals are directly correlated with the neutrino oscillation data, providing complementary insights into the neutrino mass hierarchy and CP phases. Notably, this can be accomplished without introducing a flavor-specific portal to dark matter, imposing any new flavor symmetry, or involving flavon fields. As a case study, we analyze the correlation between the flavor-philic nature of dark matter and neutrino oscillation data in the type-II seesaw and Zee-Babu models, and extend this discussion to other neutrino mass models. We analyze the indirect signatures of such leptophilic dark matter, specifically examining the spectrum of the cosmic ray electron/positron flux resulting from the pair annihilation of dark matter in the Galactic halo, and explore correlated lepton-specific signals at collider experiments sensitive to neutrino oscillation data.

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arXiv2024

Lepton collider as a window to reheating via freezing in dark matter detection. Part I

We propose a methodology to infer the reheat temperature ($T_{\rm RH}$) of the Universe from the collider signal of freezing in dark matter (DM). We demonstrate it for the mono-$\gamma$ signal at the electron-positron colliders, which indicates to a low-scale $T_{\rm RH}$, after addressing observed DM abundance, BBN, and other relevant constraints. The method can be used to correlate different reheating dynamics, DM models, and collider signals.

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arXiv2024

Leptogenesis, Dark Matter and Gravitational Waves from Discrete Symmetry Breaking

We analyse a model that connects the neutrino sector and the dark sector of the universe via a mediator $Φ$, stabilised by a discrete $Z_4$ symmetry that breaks to a remnant $Z_2$ upon $Φ$ acquiring a non-zero vacuum expectation value ($v_ϕ$). The model accounts for the observed baryon asymmetry of the universe via additional contributions to the canonical Type-I leptogenesis. The $Z_4$ symmetry breaking scale ($v_ϕ$) in the model not only establishes a connection between the neutrino sector and the dark sector, but could also lead to gravitational wave signals that are within the reach of current and future experimental sensitivities.

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arXiv2024

Higgs couplings in SMEFT via Zh production at the HL-LHC

We study the Higgs couplings involved in the $Zh$ associated production mode at the Large Hadron Collider (LHC) in presence of Higgs-gauge boson coupling modifiers via $\kappa$ framework, and dimension 6 Standard Model Effective Theory (SMEFT) operators. The analysis is performed mainly in context of the HL-LHC (with $\sqrt{s}=$14 TeV and luminosity 3000 $fb^{-1}$) setup using cut based as well as machine learning techniques. The analysis shows significant betterment in the signal significance by using the machine learning technique. We also do a $\chi^2$ analysis, which reveals an appreciable change in the sensitivity of the coupling modifiers due to the presence of effective operators, in particular due to the $qqZh$ Higgs-current interactions. The dipole operators concerning the same vertex, combine quarks of different chiralities and does not interfere with the SM diagrams, hence contributing only at $\mathcal{O} (\Lambda^{-4})$. In order to observe better sensitivity for this class of operators, we need to move to higher center-of-mass energy, where the effects of both Higgs-current and dipole 4-point interactions are more prominent.

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arXiv2023

Probing flavor constrained SMEFT operators through $tc$ production at the Muon collider

We investigate flavour violating four Fermi Standard Model Effective Field Theory (SMEFT) operators of dimension-six that can be probed via $tc ~(\bar{t}c+t\bar{c})$ production at the multi-TeV muon collider. We study different FCNC and FCCC processes related to $B$, $B_s$, $K$ and $D$ decays and mixings, sensitive to these operators and constrain the corresponding couplings. The tensor operator turns out to be most tightly bound. We perform event simulation of the final state signal from $tc$ production together with the SM background to show that operators after flavour constraint can reach the discovery limit at 10 TeV muon collider. We further adopt the optimal observable technique (OOT) to determine the optimal statistical sensitivity of the Wilson coefficients and compare them with the flavour constraints. We use the limits to predict the observational sensitivities of the rare processes like $K_L \to \pi_0 \ell \ell$, $D_0 \to \mu\mu$, $t \to c\ell\ell$.

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