arXiv subjects

Siddharth P. Maharathy

Publications and source records attributed to Siddharth P. Maharathy.

14 recordsLinked to original sources

arXiv2026

Searching for a Charged Higgs Boson in Top-Quark Decays via the $WZ$ Mode

Top-quark decays are sensitive probes of light charged Higgs bosons ($H^\pm$) due to the sizable $t\bar t$ production cross section at the LHC in conjunction with their distinct experimental signatures. While dedicated ATLAS and CMS searches considered only $H^\pm$ decays into $τν$, $cs$, or $cb$ for $m_{H^\pm}<m_t$, the $WZ$ channel remains unexplored, despite being the dominant mode in $SU(2)_L$ triplet models. Since, top-quark pair production with $t \to H^\pm b$ and $H^\pm \to WZ$ gives rise to $t\bar{t}Z$-like signatures, we recast existing $t\bar{t}Z$ analyses to search for signs of charged Higgs bosons and set novel limits on the product of branching fractions Br$(t\to H^\pm b) \times $Br$(H^\pm\to WZ)$. These constraints turn out to be at the sub-permille level, despite the observed $2σ$ preference for a non-zero value. Interpreted within the Georgi--Machacek model, these constraints probe the $Y=0$ triplet vacuum expectation value at the ${\cal O}(10)$ GeV level, providing stronger sensitivity than the existing $cs$ and $τν$ searches while remaining compatible with electroweak precision data. Moreover, the $2σ$ preference for a non-zero cross section further strengthens the cumulative case for a $\approx152$ GeV boson as suggested, in particular, by di-photon excesses.

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arXiv2026

Multi-Lepton Probes of the Drell-Yan Production of Triplet Higgses

Excesses in di-photon, $Zγ$, and $WW$ spectra indicate the existence of a new Higgs boson with mass $152 \pm 1$ GeV. However, no excess is observed in the $ZZ$ channel. This pattern aligns with a Real Higgs Triplet model with hypercharge $Y = 0$ ($Δ$SM). A prediction of this model is the Drell--Yan production of scalars at the LHC, which dominantly decay to electroweak bosons, thus enhancing the cross sections of triboson channels such as $WWZ$, $WZZ$, and $WWW$. Interestingly, both ATLAS and CMS have reported higher-than-expected significances for such processes: $6.4σ$ (observed) vs $4.7σ$ (expected) in the $VVZ$ (where $V = W$ or $Z$) channel and $4.4σ$ vs $3.6σ$ in $WWZ$, suggesting the possibility that these signals may be manifestations of an extended Higgs sector. We investigate whether the $Δ$SM can account for these triboson excesses through electroweak production and decay of triplet scalars. We find that while current data prefers a non-zero new physics signal ($2.6σ$), the $Δ$SM predicts more events than observed, such that it is consistent with data but not preferred over the SM. However, this tension could be clarified with Run~3 and HL-LHC data.

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arXiv2026

Discovery Prospects for the 150 GeV charged scalar at Future $e^+e^-$ Colliders

The Real Higgs Triplet model, known as the $Δ$SM, is a minimal extension of the Standard Model (SM) obtained by adding a hypercharge 0 triplet ($Δ$). This simple model is motivated by the multi-lepton anomalies and excesses in di-photon, $Zγ$, and $WW$ spectra at $\approx152$ GeV. The model contains, in addition to the SM particle content, a $CP$-even neutral Higgs ($Δ^0$) and a charged state ($Δ^\pm$), which are quasi-degenerate in mass. Observing the charged scalar at the LHC and measuring its mass is very challenging, since it dominantly decays to $WZ$, $tb$, and $τν$. In this article, we consider the discovery prospects of the charged Higgs with mass 150 GeV at future electron-positron colliders. Taking into account $e^+e^- \to γ^*,Z^* \to Δ^\pm Δ^\mp$ as the production mechanism and the dominant decay modes, we define three signal regions (SR) to study the 150 GeV charged Higgs properties: SR1: $\ge 3j + 1\ell$, SR2: $\ge 3\ell + τ_{\text{had}}$, SR3: $\ge 4j + τ_{\text{had}}$. For $m_{Δ^\pm}=150\text{ GeV}$, a $5σ$ significance can be achieved in SR1 with an integrated luminosity of less than $1\text{ fb}^{-1}$. SR2 is very clean with leptonic final states having low background and small systematic uncertainties. Furthermore, SR3 is crucial for reconstructing the charged scalar invariant mass, which can be measured with $\mathcal{O}(1)$ GeV accuracy with an integrated luminosity of $500\text{ fb}^{-1}$.

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arXiv2026

Indications for New Higgs Bosons

After the Higgs discovery, the question of whether particles beyond those of the Standard Model exist is more pressing than ever. In this context, the scalar sector is particularly promising, since it lies at the core of the internal problems of the Standard Model, while extensions of it allow us to resolve them and can provide explanations for Dark matter, non-zero neutrino masses, inflation etc. In these proceedings, we review the indications for new Higgs bosons at the electroweak scale with masses of $\approx$95 GeV and $\approx$152 GeV. These excesses are most significant in the di-photon channel but are supported by weaker-than-expected limits in other decay modes. While for the 95 GeV candidate the production mechanism is mostly unknown, the (hypothetical) 152 GeV Higgs is dominantly produced in association with leptons, $(b)$ jets and missing energy, pointing towards the Drell-Yan production of an $SU(2)_L$ triplet with $Y=0$. Interestingly, this model predicts $t\to H^\pm b$ with $H^\pm\to WZ$, which resembles the signature of $t\bar{t}Z$ production in the Standard Model and is in fact preferred by current data. Finally, we investigate the possibility that the significant tensions between the Standard Model predictions and the measurements in differential top-quark distributions are due to contamination from new physics involving both the 152 GeV and the 95 GeV scalar.

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arXiv2026

Vacuum structure of the Babu-Nandi-Tavartkiladze model of neutrino mass generation

We analyze the vacuum structure of the Babu--Nandi--Tavartkiladze (BNT) model of neutrino mass generation, in which the Standard Model is extended by an $SU(2)_L$ scalar quadruplet with hypercharge $Y=3/2$ and a vector-like $SU(2)_L$ triplet fermion with $Y=1$, generating neutrino masses via an effective dimension-seven operator. We delineate the theoretical constraints on the model, requiring the scalar potential to be bounded from below in all field directions, ensuring perturbative unitarity of scattering amplitudes, and demanding that the electroweak vacuum corresponds to the global minimum of the potential. We find that the electroweak vacuum is not generically guaranteed to be the global minimum: several charge-breaking stationary points may coexist with -- and potentially lie below -- it in potential depth. For the electroweak-like vacuum with vanishing quadruplet expectation value, the condition of global stability reduces to two simple mass inequalities involving the doubly- and triply-charged scalars. In contrast, for the general electroweak vacuum with nonzero doublet and quadruplet expectation values -- compatible with neutrino-mass generation -- no comparably simple analytic condition emerges, and the stability must in general be assessed for specific choices of scalar couplings. In the special case where the interaction responsible for neutrino-mass generation vanishes, both electroweak configurations coexist, and the bounded-from-below conditions ensure a definite ordering between them. In this limit, the mass inequalities alone are sufficient to guarantee that the general electroweak vacuum is the global minimum. In the physically relevant regime, the results provide practical sufficient criteria and a systematic framework for assessing vacuum stability in the BNT model.

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arXiv2026

Constraining $A\to ZH$ with $H\to t\bar t$ in the Low-Mass Region

The decay $A\to ZH$ is a characteristic signal of two-Higgs-doublet models (2HDMs), where $A$ and $H$ lie primarily within the same $SU(2)_L$ multiplet, leading to a coupling of order $g_2$ to the $Z$ boson. The subsequent decay $H\to tt^{(*)}$ is particularly promising, as it gives rise to distinct final states involving multiple leptons and $b$-jets. The required splitting between $m_A$ and $m_H$ can naturally occur near the electroweak scale while being consistent with perturbative unitarity. Whereas dedicated ATLAS and CMS searches focused on the region with both top-quarks on-shell, we cover lower masses where one top quark is off-shell by recasting Standard Model $t\bar{t}Z$ measurements of ATLAS and CMS. The obtained limits on $σ(A\to ZH)\times {\rm Br} (H\to t\bar t)$ are between $0.12$ pb and $0.62$ pb. Interestingly, we observe these stringent limits despite a preference (up to $2.5σ$) for a non-zero new physics signal, most pronounced around for $m_A \approx 450-460$ GeV and $m_H\approx 290$ GeV, with a best-fit value of $σ(A \to ZH) \times {\rm Br}(H \to t\bar t) \approx 0.3$ pb. This cross section can be accommodated within a top-philic 2HDM for a top-Yukawa coupling of the second Higgs doublet of $μ_t \gtrsim 0.16$.

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arXiv2025

LHC Signatures of the Generic Georgi-Machacek Model

Vector-boson fusion production of new Higgs bosons decaying into pairs of electroweak gauge bosons ($W^\pm W^\pm$, $WZ$ and $ZZ$) is a smoking-gun signature of the Georgi-Machacek (GM) Model. Notably, ATLAS has observed a $3.3σ$ excess in $W^\pm W^\pm$ at $\approx 450\,$GeV and a $2.8σ$ excess in the $WZ$ channel at $\approx 375\,$GeV, while CMS reported weaker-than-expected limits at these masses. However, the canonical custodial-symmetric GM Model cannot accommodate these signals, as it predicts mass degeneracy among the new gauge-philic Higgs bosons. To overcome this obstacle, we consider a generalized version of the GM Model without the custodial $SU(2)_C$ symmetry in the scalar potential. In the limit of small mixing among the Higgs bosons, the $W^\pm W^\pm$ and $WZ$ excesses can be explained by the doubly and singly-charged Higgs bosons originating primarily from the $Y=1$ triplet, while respecting the bounds from $ZZ$ searches. Furthermore, the neutral Higgs boson mostly contained in the $Y=0$ triplet can account for the excess at $\approx 152\,$GeV in associated di-photon production, while being consistent with constraints from vacuum stability and the Standard Model Higgs signal strength measurements.

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arXiv2024

Anatomy of the Real Higgs Triplet Model

In this article, we examine the Standard Model extended by a $Y=0$ real Higgs triplet, the $Δ$SM. It contains a $CP$-even neutral Higgs ($Δ^0$) and two charged Higgs bosons ($Δ^\pm$), which are quasi-degenerate in mass. We first study the theoretical constraints from vacuum stability and perturbative unitarity and then calculate the Higgs decays, including the loop-induced modes such as di-photons ($γγ$) and $Zγ$. In the limit of a small mixing between the SM Higgs and $Δ^0$, the latter decays dominantly to $WW$ and can have a sizable branching ratio to di-photon. The model predicts a positive definite shift in the $W$ mass, which agrees with the current global electroweak fit. At the Large Hadron Collider, it leads to a $(i)$ stau-like signature from $pp\to Δ^+Δ^-\to τ^+τ^-ν\barν$, $(ii)$ multi-lepton final states from $pp\to γ^*\to Δ^+Δ^-\to W^+W^-ZZ$ and $pp\to W^{*} \to Δ^\pmΔ^0\to W^\pm Z W^+W^-$ as well as $(iii)$ associated di-photon production from $pp\to W^{*} \to Δ^\pm(Δ^0\toγγ)$. Concerning $(i)$, the reinterpretation of the recent supersymmetric tau partner search by ATLAS and CMS excludes $m_{Δ^\pm}<110$ GeV at 95% CL. From $(ii)$, some of the signal regions of multi-lepton searches lead to bounds close to the predicted cross-section, but electroweak scale masses are still allowed. For $(iii)$, the recast of the associated di-photon searches by ATLAS and a combined log-likelihood fit of signal and background to data find that out of the 25 signal regions, 10 provide relevant limits on Br$(Δ^0\toγγ)$ at the per cent level. Interestingly, 6 signal regions show excesses at around 152 GeV, leading to a preference for a non-zero di-photon branching ratio of about 0.7% with the corresponding significance amounting to about $4σ$.

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arXiv2024

Growing Evidence for a Higgs Triplet

Despite intensive searches at the LHC, no new fundamental particle has been discovered since the discovery of the 125 GeV Higgs boson. In general, a new physics discovery is challenging without a UV-complete model because different channels and observables cannot be combined directly and unambiguously. Moreover, without indirect hints for new particles, the parameter space to be searched is huge, resulting in diminished significance due to the look-elsewhere effect. Several LHC searches with multiple leptons in the final state point towards the existence of a new Higgs boson with a mass in the 140-160 GeV range, decaying mostly to a pair of W bosons. This dominant decay mode motivates a Higgs triplet with zero hypercharge, which also predicts a heavier-than-expected $W$-boson as indicated by the CDF-II measurement. Within this simple and predictive model, we simulate and combine channels of associated di-photon production. Considering the run-2 results of ATLAS, including those presented recently at the Moriond conference, a significance of 4.3$σ$ is obtained for a mass of 152 GeV. This is the largest statistical evidence for a new narrow resonance observed at the LHC.

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arXiv2024

Explaining the $γγ+X$ Excesses at $\approx$151.5 GeV via the Drell-Yan Production of a Higgs Triplet

The multi-lepton anomalies and searches for the associated production of a narrow resonance indicate the existence of a $\approx$151 GeV Higgs with a significance of $>5σ$ and $>3.9σ$, respectively. On the one hand, these anomalies require a sizable branching fraction of the new scalar to $WW$, while on the other hand, no $ZZ$ signal at this mass has been observed. This suggests that the new boson is the neutral component of an $SU(2)_L$ triplet with zero hypercharge. This field leads to a positive definite shift in the $W$ mass, as preferred by the current global fit, and is produced via the Drell-Yan process $pp\to W^*\to Δ^0Δ^\pm$. We use the side-bands of the ATLAS analysis \cite{ATLAS:2023omk} of the associated production of the Standard Model Higgs in the di-photon channel to search for this production mode of the triplet. Since the dominant decays of $Δ^\pm$ depend only on its mass, the effect in the 22 signal categories considered by ATLAS is completely correlated. We find that the ones most sensitive to the Drell-Yan production of the triplet Higgs show consistent excesses at a mass of $\approx$151.5 GeV. Combining these channels in a likelihood ratio test, a non-zero Br$[Δ^0\toγγ] = 0.66\%$ is preferred by $\approx$3$σ$, supporting our conjecture.

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arXiv2023

Probing compressed mass spectra in the type-II seesaw model at the LHC

Despite a great deal of effort in searching for the triplet-like Higgses in the type-II seesaw model, evidence for their production is yet to be found at the LHC. As such, one might be in the balance regarding this model's relevance at the electroweak scale. In this work, we peruse a scenario, akin to compressed mass spectra in Supersymmetry, which might have eluded the experimental searches thus far. We perform a multivariate analysis to distinguish signals with a pair of same-sign leptons with low invariant mass from the SM processes, including those accruing from fake leptons and electron charge misidentification, and find that a significant part of the hitherto unconstrained parameter space could be probed with the already collected Run 2 13 TeV LHC and future HL-LHC data.

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arXiv2023

Type-II see-saw at $μ^+$$μ^-$ collider

Doubly-charged Higgs bosons have extensively been searched at the LHC. In this work, we study the sensitivity reach of the doubly-charged scalar ($H^{\pm\pm}$) in muon collider for the well-known Type-II seesaw scenario. First, we perform a cut-based analysis to predict the discovery prospect in the muon collider operating with 3 TeV center of mass energy. In addition to this, we have also performed a multivariate analysis and compare the cut-based result with the result obtained from the multivariate analysis. We find that the cut-based analysis is more significant as compared to the multivariate analysis in the large doubly-charged scalar mass region. We predict that a doubly-charged scalar mass, $M_{H^{\pm\pm}}$, upto 1450 GeV can be probed with $5σ$ significance for center of mass $\sqrt{s}= 3$ TeV and integrated luminosity $\mathcal{L} = 1000\,\textrm{fb}^{-1}$.

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arXiv2022

An Alternate Left-Right Symmetric Model with Dirac Neutrinos

We study a different variant of Left-Right Symmetric Model, incorporating Dirac type neutrinos. In the absence of the bi-doublet scalars, the possibility of a universal seesaw type of mass generation mechanism for all the Standard Model charged fermions have been discussed. The model has been constructed by extending the Standard Model particle spectrum with heavy vector-like fermions as well as different scalar multiplets. We have shown that this model can generate non zero neutrino mass through loop mediated processes. The parameters which are involved in neutrino mass generation mechanism can satisfy the neutrino oscillation data for both normal and inverted hierarchy. The lightest charged Higgs plays a crucial role in neutrino mass generation mechanism and can have mass of $\mathcal{O}[\text{GeV}]$. We have systematically studied different constraints which are relevant for the charged Higgs phenomenology. In addition to that we also briefly discuss discovery prospects of the charged Higgs at different colliders.

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arXiv2021

Interference effect in lepton number violating and conserving meson decays for a left-right symmetric model

We study the effect of interference on the lepton number violating~(LNV) and lepton number conserving~(LNC) three-body meson decays $M_1^{+}\to l_i^{+} l_j^{\pm}π^{\mp}$, that arise in a TeV scale Left Right Symmetric model~(LRSM) with degenerate or nearly degenerate right handed~(RH) neutrinos. LRSM contains three RH neutrinos and a RH gauge boson. The RH neutrinos with masses in the range of $M_N \sim$ (MeV - few GeV) can give resonant enhancement in the semi-leptonic LNV and LNC meson decays. In the case, where only one RH neutrino contributes to these decays, the predicted new physics branching ratio of semi-leptonic LNV and LNC meson decays $M_1^{+}\to l_i^{+} l_j^{+}π^{-}$ and $M_1^{+}\to l_i^{+} l_j^{-}π^{+}$ are equal. We find that with at least two RH neutrinos contributing to the process, the LNV and LNC decay rates can differ. Depending on the neutrino mixing angles and $CP$ violating phases, the branching ratios of LNV and LNC decay channels mediated by the heavy neutrinos can be either enhanced or suppressed, and the ratio of these two rates can differ from unity.

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