arXiv subjects

Bruce Mellado

Publications and source records attributed to Bruce Mellado.

At least 19 recordsLinked to original sources

arXiv2026

Radiative Signature of New Scalar Boson Decays in the $m_{\ell \ell \gamma}$ Spectrum at the LHC

We investigate the radiative decay $S \to W^+W^-\gamma$ in the context of the multi-lepton anomalies and recent indications of a narrow scalar resonance near $m_S = 152 \pm 1~\text{GeV}$ in the $\gamma\gamma$, $Z\gamma$, and $W^+W^-$ channels at the Large Hadron Collider. These excesses arise in final states containing leptons, missing transverse momentum, and associated $b$-jets, and motivate a search for a corresponding localized excess in the invariant-mass spectrum of the dilepton--photon system, $m_{\ell\ell\gamma}$, in events with associated $b$-jets. We use recent CMS measurements of the ${t\bar{t}}\gamma$ differential cross sections~\cite{CMS:2025zbe} to study the $m_{\ell\ell\gamma}$ spectrum and perform a search for a scalar-resonance contribution. A localized excess is observed, compatible with the scalar-resonance hypothesis, with a global significance of $2.7\sigma$ at $m_S = 152~\text{GeV}$. This result provides additional support for the hypothesis of a narrow resonance. The ratio $\sigma(S \to W^+W^-\gamma)/\sigma(S \to W^+W^-) = (2.14 \pm 0.77)$\% is extracted. This value is compatible with an enhanced radiative contribution that could arise in scenarios beyond the Standard Model.

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arXiv2026

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

Excesses in di-photon, $Z\gamma$, 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 $Y = 0$ ($\Delta$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: $6.4\sigma$ (observed) vs $4.7\sigma$ (expected) in the $VVZ$ (where $V = W$ or $Z$) channel and $4.4\sigma$ vs $3.6\sigma$ in $WWZ$, suggesting the possibility that these signals may be manifestations of an extended Higgs sector. We investigate whether the $\Delta$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 ($2.6\sigma$), the $\Delta$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

AI Agents, Language, Deep Learning and the Next Revolution in Science

Modern science is reaching a critical inflection point. Instruments across disciplines, from particle physics and astronomy to genomics and climate modeling, now produce data of such scale, diversity, and interdependence that traditional analytical methods can no longer keep pace. This growing imbalance between data generation and data understanding signals the need for a new scientific paradigm. We propose that intelligent, human-supervised AI agents operating over deep-learning algorithms, represent the next evolution of the scientific method. Built upon large language models and multimodal learning, these agents can interpret scientific intent, design and execute analytical workflows, and ensure traceability through domain-specific languages that preserve human oversight and accountability. Particle physics, a historic incubator of computational innovation, offers the ideal testbed for this transition. At the Institute of High Energy Physics of the Chinese Academy of Sciences, the Dr. Sai system embodies this vision, a multi-agent reasoning framework deployed within collider research at the CEPC. This emerging approach does not replace human scientists but extends their cognitive reach, enabling discovery to scale with complexity and redefining how knowledge itself is produced in the age of intelligent machines. The significance of this paradigm transcends particle physics, offering a blueprint for all data-driven sciences facing the same complexity ceiling.

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arXiv2025

Emerging Narrow Resonance at 152 GeV

The discovery of the Higgs boson at the LHC completed the Standard Model (SM), yet the possibility of additional scalars remains open, provided their contributions to electroweak symmetry breaking are sufficiently small. Recent analyses of LHC data have revealed statistically significant anomalies in multi-lepton final states - events characterized by multiple leptons, missing transverse energy, and ($b$-)jets. These anomalies provide intriguing hints of physics beyond the SM. In this work, we present the signature of growing excesses for a new scalar resonance with a mass of $152 \pm 1$ GeV, observed in the $\gamma\gamma$, $Z\gamma$, and $WW$ channels. The combined global significance reaches a level that points toward the growing signature of this resonance. The findings align with a simplified model in which a heavy scalar boson decays into two lighter scalars, thus providing a consistent framework explaining the observed multi-lepton anomalies. These results significantly advance the search for new scalar bosons at the electroweak scale. Future investigations, including precision studies with upcoming HL-LHC data, will be crucial for confirming the nature of this resonance and exploring its implications for extending the SM.

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arXiv2025

Exploring the Discovery Reach for a 95 GeV Scalar in Future $e^+e^-$ Collisions

The observed indications for a new scalar resonance with a mass around 95\,GeV, initially reported by LEP and supported by CMS and ATLAS in di-photon, $\tau \tau$, and $W^+ W^-$ channels, motivate exploring its discovery potential at future electron-positron colliders. This study focuses on the production of the new scalar ($S$) via $e^+ e^- \rightarrow ZS $ with $Z \rightarrow \mu^+ \mu^- $ and $S \rightarrow b \bar{b}$ and optimizes the signal recognition using the recoil-mass method. By employing deep neural networks for signal-background discrimination, we demonstrate that a 95\,GeV scalar, mixing with the Standard Model Higgs by an angle of $\sim$0.1, can be observed with a 5$\sigma$ significance at $\sqrt{s}$ = 250\,GeV or 200\,GeV with 5~ab$^{-1}$ of integrated luminosity.

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arXiv2025

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 $\sigma(A\to ZH)\times {\rm Br} (H\to t\bar t)$ are between $0.12$ pb and $0.62$ pb. Interestingly, we observe these stringent limits despite a preference (up to $2.5\sigma$) 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 $\sigma(A \to ZH) \times {\rm Br}(H \to t\bar t) \approx 0.3$ pb. This cross section can be accommodated within a top-philic 2HDM for a top-Yukawa coupling of the second Higgs doublet of $\mu_t \gtrsim 0.16$.

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arXiv2025

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

The Real Higgs Triplet model, known as the $\Delta$SM, is a minimal extension of the Standard Model (SM) obtained by adding a hypercharge 0 triplet ($\Delta$). This simple model is motivated by the multi-lepton anomalies and excesses in di-photon, $Z\gamma$, and $WW$ spectra at $\approx152$ GeV. The model contains, in addition to the SM particle content, a $CP$-even neutral Higgs ($\Delta^0$) and a charged state ($\Delta^\pm$), 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 $\tau\nu$. In this article, we consider the discovery prospects of the charged Higgs with mass 150 GeV at future electron-positron colliders. Taking into account $e^+e^- \to \gamma^*,Z^* \to \Delta^\pm \Delta^\mp$ 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: $\ge 3\ell + \tau_{\text{had}}$, SR3: $\ge 4j + \tau_{\text{had}}$. For $m_{\Delta^\pm}=150\text{ GeV}$, a $5\sigma$ 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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arXiv2025

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 $\tau\nu$, $cs$, 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 $2\sigma$ preference for a non-zero value. Interpreted within the hypercharge $Y=0$ Higgs triplet model, this translates into a stringent constraint on the triplet Higgs vacuum expectation value of $v_\Delta\lesssim 2$ GeV, which is stronger than those from the $cs,\tau\nu$ modes and even surpasses electroweak precision constraints from the $\rho$ parameter. Moreover, the $2\sigma$ 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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arXiv2025

Analyzing $t\bar{t}Z$-couplings at the future $e^-p$ collider

The proposed Large Hadron Electron Collider (LHeC), with center-of-mass energy of $\sqrt{s}\approx 1.3$ TeV, provides a clean and sensitive environment to probe the top quark's neutral current interactions with the $Z$ boson via the process $e^- p \to e^- t \bar{t}$. We investigate the precision with which the Standard Model (SM) $t\bar{t}Z$ couplings-the vector and axial-vector components ($\Delta C_{1V}$, $\Delta C_{1A}$)-can be measured, along with possible new physics effects parameterized by higher-dimensional operators inducing weak electric and magnetic dipole-like interactions ($C_{2V}$, $C_{2A}$). Focusing on the semileptonic decay channel, where either the top quark or anti-top decays leptonically to a positively charged lepton ($\ell^+ = e^+, \mu^+$), we utilize the azimuthal angle difference $\Delta \phi$ between the scattered electron and the charged lepton as the key observable. Using a one-parameter multi-bin $\chi^2$-analysis of this differential distribution, we find that constraints on $\Delta C_{1V}$ and $\Delta C_{1A}$ improve from order $10^{-1}$ at 50 fb$^{-1}$ to order $10^{-2}$ at 1000 fb$^{-1}$, corresponding to approximately 50% and 6% precision relative to their SM values. The anomalous tensor couplings $C_{2V}$ and $C_{2A}$ are constrained at the $10^{-1}$ level even at low luminosity and improve moderately with high luminosity. While the two-parameter analysis broadens the allowed regions due to parameter correlations, it retains competitive sensitivity, particularly for SM-like couplings. A systematic uncertainty of 5% is assumed throughout. These results highlight the LHeC's potential to provide complementary and competitive sensitivity to top-$Z$ couplings compared to current and future hadron and lepton collider capabilities.

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arXiv2025

Emerging Excess Consistent with a Narrow Resonance at 152 GeV in High-Energy Proton-Proton Collisions

The Higgs boson discovery at the Large Hadron Collider (LHC) at CERN confirmed the existence of the last missing particle of the Standard Model (SM). The existence of new fundamental constituents of matter beyond the SM is of great importance for our understanding of Nature. In this context, indirect (non-resonant) indications for new scalar bosons were found in the data from the first run of the LHC, taken between 2010 and 2012 at CERN: an excess in the invariant mass of muon-electron pairs, consistent with a new Higgs boson ($S$) with a mass of $150\pm5$ GeV. Other processes with multiple leptons in the final state, moderate missing energy, and possibly (bottom quark) jets exhibit deviations from the SM predictions. These anomalies can be explained within a simplified model in which a new heavy Higgs boson $H$ decays into two lighter Higgses $S$. This lighter Higgs $S$ subsequently decays to $W$ bosons, bottom quarks and has also an invisible decay mode. Here, we demonstrate that using this model we can identify narrow excesses in di-photon and $Z$-photon spectra around 152 GeV. By incorporating the latest measurements of di-photons in association with leptons, we obtain a combined global significance of $5.4\sigma$. This represents the highest significance ever reported for an excess consistent with a narrow resonance beyond the SM (BSM) in high-energy proton-proton collision data at the LHC. Such findings have the potential to usher in a new era in particle physics - the BSM epoch - offering crucial insights into unresolved puzzles of nature.

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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 $\Delta$SM. It contains a $CP$-even neutral Higgs ($\Delta^0$) and two charged Higgs bosons ($\Delta^\pm$), 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 ($\gamma\gamma$) and $Z\gamma$. In the limit of a small mixing between the SM Higgs and $\Delta^0$, 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 $pp\to \Delta^+\Delta^-\to \tau^+\tau^-\nu\bar\nu$, $(ii)$ multi-lepton final states from $pp\to \gamma^*\to \Delta^+\Delta^-\to W^+W^-ZZ$ and $pp\to W^{*} \to \Delta^\pm\Delta^0\to W^\pm Z W^+W^-$ as well as $(iii)$ associated di-photon production from $pp\to W^{*} \to \Delta^\pm(\Delta^0\to\gamma\gamma)$. Concerning $(i)$, the reinterpretation of the recent supersymmetric tau partner search by ATLAS and CMS excludes $m_{\Delta^\pm}<110$ 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$(\Delta^0\to\gamma\gamma)$ 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 $4\sigma$.

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arXiv2024

Discovery Potential of Future Electron-Positron Colliders for a 95 GeV Scalar

The Large Electron Positron collider observed an indication for a new Higgs boson with a mass around $95$\,GeV-$100$\,GeV in the process $e^+e^-\to Z^*\to ZS$ with $S\to b\bar b$. The interest in this excess re-emerged with the di-photon signature at $\approx$\,95\,GeV at the Large Hadron Collider. In fact, a combined global significance of $3.4\sigma$ is obtained once $WW$ and $\tau\tau$ signals are included in addition. In this article, we perform a feasibility study for discovering such a new scalar $S$ at future electron-positron colliders using the recoil-mass method applied to $e^{+} e^{-} \to ZS$ with $Z \rightarrow \mu^{+} \mu^{-}$ and $S \to b \bar{b}$. For this, we employ a Deep Neural Network to enhance the separation between the Standard Model background and the signal, reducing the required integrated luminosity necessary for discovery by a factor of two to three. As a result, an $SU(2)_L$ singlet Higgs with a mass of $\approx$\,95\,GeV can be observed with more than 5$\sigma$ significance at a 250\,GeV centre-of-mass energy collider with $5~ {\rm ab}^{-1}$ integrated luminosity if it has a mixing angle of at least $0.1$ with the Standard Model Higgs, which means that a discovery can be achieved within the whole 95\% confidence-level region preferred by Large Electron Positron excess. Furthermore, including more decay channels such as $S\to \tau\tau$ and $Z\to e^+e^-$ further enhances the discovery potential of future $e^+e^-$ accelerators, like CEPC, CLIC, FCC-ee and ILC.

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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

Trials Factor for Semi-Supervised NN Classifiers in Searches for Narrow Resonances at the LHC

To mitigate the model dependencies of searches for new narrow resonances at the Large Hadron Collider (LHC), semi-supervised Neural Networks (NNs) can be used. Unlike fully supervised classifiers these models introduce an additional look-elsewhere effect in the process of optimising thresholds on the response distribution. We perform a frequentist study to quantify this effect, in the form of a trials factor. As an example, we consider simulated $Z\gamma$ data to perform narrow resonance searches using semi-supervised NN classifiers. The results from this analysis provide substantiation that the look-elsewhere effect induced by the semi-supervised NN is under control.

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arXiv2024

Anomalies in Particle Physics

The currently accepted mathematical description of the fundamental constituents and interactions of matter is the Standard Model of particle physics. Its last missing particle, the famous Higgs boson, was observed at the Large Hadron Collider at CERN in 2012. However, it is clear that the Standard Model cannot be the ultimate theory of Nature, and e.g. cannot account for Dark Matter or non-vanishing neutrino masses (and does not include gravity). In fact, searches for physics beyond the SM have been intensified since the Higgs boson discovery. In this article, we review the hints for new physics, called ``anomalies'', obtained in particle physics experiments within the last years. We consider both direct high-energy searches for new resonances at the LHC and indirect low-energy precision experiments. These anomalies range from the nuclear scale (approximately the mass of the proton) to the electroweak scale (i.e. the mass of the Higgs boson) to the TeV scale (the highest scale directly accessible at the LHC), therefore spanning over four orders of magnitude. After discussing the experimental and theoretical status of the anomalies, we summarize possible explanations in terms of new particles and new interactions. In particular, new Higgs bosons and leptoquarks are promising candidates. Discovery prospects and implications for future colliders are discussed.

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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

Combined Explanation of LHC Multi-Lepton, Di-Photon and Top-Quark Excesses

The LHC analyses of processes containing two or more leptons and missing energy, possibly in association with b-jets, show strong tensions with the Standard Model predictions and are known as multi-lepton anomalies. In particular, top-quark differential distributions point towards the associated production of new Higgs bosons decaying into bottom quarks and W bosons ($>5\sigma$) with masses consistent with the di-photon excesses at 95GeV and 152GeV ($3.8\sigma$ and $4.9\sigma$, respectively). Furthermore, CMS found indications for resonant top-quark pair production at 400GeV ($3.5\sigma$) and both ATLAS and CMS reported elevated four-top and ttW cross-sections. In this article, we propose a combined explanation of these excesses by supplementing the SM Higgs with a second scalar doublet, a real scalar singlet ($S$) and a Higgs triplet with $Y=0$ ($\Delta$); the $\Delta$2HDMS. We fix the masses of the neutral triplet-like and the singlet-like scalars by the di-photon excesses, i.e. $m_{\Delta^0}=152$GeV and $m_S=95$GeV, respectively. Here, H, the CP-even component of the second doublet, is produced via gluon fusion from a top-loop and decays dominantly to $S+\Delta^0$ whose subsequent decays to WW and bb explain the differential top-quark distributions for $\sigma(pp\to H\to S\Delta^0)\approx6$pb. Fixing the top-Yukawa accordingly, the CP-odd Higgs boson A turns out to have the right production cross-section to account for the resonant top-pair excess at 400GeV, while the top-associated production of H and A results in new physics pollution of Standard Model ttW and four-top cross sections, as preferred by the data. Furthermore, a positive shift in the W mass is naturally induced by the vacuum expectation value of the triplet and we show that the most relevant signal strengths of the 152GeV boson are compatible with the process $pp\to H\to \Delta^0S$ if S is allowed to decay invisibly.

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arXiv2023

Searches for Additional Scalars at Future $e^{+} e^{-}$ Colliders

We present growing excesses consistent with a 95 GeV scalar. We provide a comprehensive analysis of the Two Higgs Doublet Model and an additional singlet (2HDM+S) at future $e^{+} e^{-}$ collider. In particular, we provide a precise mass reconstruction measurement for the scalar, $m_{S}$, using the recoil mass method through $e^{+} e^{-} \to Z S$ where $Z \to μ_{+} μ_{-}$ and $S \to b \bar{b}$ at $\sqrt{s} = 250$~GeV and $\sqrt{s} = 200$~GeV. Furthermore, we employ Deep Neural Network to analyze the properties and behaviour of the scalar particle with a mass most importantly to provide enhanced resolution for the separation between beyond the Standard Model (SM) signal and SM background in the region 95 - 96 GeV in the $S \to b \bar{b}$ for $μ_{+} μ_{-}$ channel. A 95 GeV scalar can be observed with $5σ$ significance at $15(10)$ fb$^{-1}$ integrated luminosity for $\sqrt{s} = 250(200)$~GeV. This strengthens the discovery of the potential of the future $e^{+} e^{-}$ collider.

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