arXiv subjects

Mojtaba Mohammadi Najafabadi

Publications and source records attributed to Mojtaba Mohammadi Najafabadi.

At least 19 recordsLinked to original sources

arXiv2026

Constraints on Vector-Like Top Dipole Interactions from Top-Associated Photon Measurements at the LHC

Precision measurements of top-quark production with energetic photons provide a complementary probe of vector-like quarks with nonstandard radiative decays. We reinterpret the CMS differential $t\bar tγ$ measurement and the ATLAS fiducial $t\bar tγγ$ cross section to constrain the electromagnetic and chromomagnetic transition interactions of a charge-$2/3$ vector-like top partner, $T$. We consider effective interactions written in terms of physical fields after electroweak symmetry breaking, retaining only the decays $T\to tγ$ and $T\to tg$. The CMS signal includes both the mixed $(tγ)(tg)$ topology and double-radiative $t\bar tγγ$ events entering the exactly-one-photon fiducial region, while the ATLAS rate is interpreted using a signal-specific detector-response transfer factor. Signal uncertainties are included, and the interpretation is restricted point by point to the domain satisfying the EFT and narrow-width requirements. Denoting the electromagnetic and chromomagnetic transition coefficients by $c_{tγ}$ and $c_{tg}$, respectively, we find that, for $c_{tg}=1.0\times10^{-5}~\GeV^{-1}$ and $m_T=500~\GeV$, the observed Run~2 exclusions at 95\% confidence level extend down to $c_{tγ}\simeq4.7\times10^{-7}~\GeV^{-1}$ for CMS and $2.3\times10^{-6}~\GeV^{-1}$ for ATLAS within the valid domain. At $m_T=1000~\GeV$, the corresponding values are $2.1\times10^{-6}$ and $8.5\times10^{-6}~\GeV^{-1}$. Projections to the HL-LHC indicate improved sensitivity to these radiative transition interactions. These results establish top-associated photon measurements as complementary probes of radiative top-partner interactions beyond searches targeting conventional decay modes.

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arXiv2026

Application of Deep Learning to Jet Charge Discrimination

The Large Hadron Collider (LHC) produces an enormous volume of data in which the identification and characterization of hadronic jets is a central challenge. Determining the electric charge of the parton initiating a light-quark jet; a task known as jet-charge discrimination; is highly valuable for both precision tests of the Standard Model (SM) and searches for physics beyond it. In this work, we benchmark a range of classical and quantum machine-learning models for the task of distinguishing up-quark from anti-up-quark jets in a controlled QCD environment. Among the approaches tested, a Graph Neural Network achieved the best performance, with an AUC of 0.883. Jet-charge tagging of this kind has broad phenomenological applications, from improving measurements of charge asymmetries to enhancing sensitivity in searches for new particles from beyond the SM where quark versus antiquark discrimination is essential. Our study provides a methodological foundation for deploying modern machine-learning techniques in jet-charge analyses at the LHC experiments.

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arXiv2026

W-boson helicity fractions in top decay as probes of dimension-6 and dimension-8 SMEFT operators

Precision measurements of top-quark decays provide powerful probes of physics beyond the Standard Model (SM). While the impact of dimension-6 operators in the SM Effective Field Theory (SMEFT) has been extensively studied, the role of dimension-8 contributions remains largely unexplored, despite their potential importance as experimental precision improves. In this work, we present a combined analysis of dimension-6 and a representative subset of dimension-8 SMEFT effects using the W-boson helicity fractions in top-quark decays. We compute the leading-order contributions of these operators to the top-quark decay width and helicity fractions, and perform one-parameter and selected two-parameter $χ^{2}$ fits to the combined ATLAS and CMS measurements at a reference scale $Λ=1$~TeV. From the fit results, we find that the inclusion of dimension-8 contributions affects the allowed parameter space of several dimension-6 coefficients through non-trivial correlations and degeneracies. Since the leading dimension-8 contributions enter at the same order $\mathcal{O}(Λ^{-4})$ as the squared dimension-6 terms retained in our analysis, this highlights the importance of a consistent treatment of the EFT expansion when interpreting SMEFT constraints.

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arXiv2025

Probing the Chern-Simons Portal at the HL-LHC through Displaced Vertices from W Boson Associated Production

This study explores the Chern-Simons portal model, an extension of the Standard Model that introduces a massive neutral vector boson $X$ associated with a $U_X(1)$ gauge symmetry. Motivated by gauge anomaly cancellation, the model incorporates heavy chiral fermions that induce observable effects through topological Chern-Simons interactions, despite being inaccessible at Large Hadron Collider energies. We investigate the associated production of the $X$ boson with a $W$ boson and jets at the High-Luminosity LHC with a center-of-mass energy of 14 TeV, considering different $X$ masses benchmarks. A multivariate analysis using Boosted Decision Trees is employed to separate signal from background processes. Detector effects are modeled using a fast simulation tuned to the HL-LHC environment, including realistic pile-up conditions with an average of 200 interactions per bunch crossing. We derive expected $95\%$ confidence level exclusion limits in two-dimensional parameter spaces involving the $X$ boson couplings. Our results demonstrate that the HL-LHC can achieve high sensitivity to gauge-anomaly-induced interactions, setting robust constraints on the $X$ boson coupling to the $W$ boson down to $\mathcal{O}(10^{-4})$, depending on $m_X$.

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arXiv2025

Concurrent Exploration of Axion-Like Particle Interactions with Gauge Bosons at the LHC

Axion-like particles (ALPs) are pseudo Nambu-Goldstone bosons associated with spontaneously broken global symmetries incorporated in the Standard Model (SM) Lagrangian in many models beyond the SM. The existence of a light ALP is plausible due to the long-standing problems that the SM has not been able to address, such as the dark matter (DM) problem and the observed matter-antimatter asymmetry. There are many proposals in recent decades considering the ALP as a solution to some of these shortcomings. Motivated by such potential, we search for ALPs with a mass of 1 MeV at the LHC in a model-independent fashion. We explore two complementary production modes: ALP production in association with a pair of electroweak gauge bosons ($ZZ$ or $WW$) and ALP production in association with a single gauge boson ($W$ or $Z$) plus jets. For the $VV+a$ final state, signal and dominant SM backgrounds are generated, and a realistic detector response simulation is performed. A multivariate analysis is employed to discriminate the $VV+a$ signal from background processes, and the expected $95\%$ confidence level (CL) exclusion limits in two-dimensional parameter spaces involving the ALP couplings are subsequently derived. The $V$+jets channel is interpreted using LHC measurements in regimes where the ALP escapes detection, appearing as missing energy. The two analyses are complementary: both the $VV+a$ and the $V$+jets channels probe simultaneously the ALP couplings to gluons and to electroweak gauge bosons. While the $VV+a$ channel offers clean multi-lepton final states and direct reconstruction of the $ZZ$ or $WW$ system, the $V$+jets channel benefits from larger production cross sections, enabling stronger constraints in certain regions of the parameter space.

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arXiv2025

Machine Learning Approaches to Top Quark Flavor-Changing Four-Fermion Interactions in Trilepton Signals at the LHC

We explore the top quark flavor-changing 4-Fermi interactions ($tuee$ and $tcee$) with scalar, vector, and tensor structures using machine learning models to analyze tri-lepton processes at the LHC. The study is performed using $t\bar{t}$ and $tW$ processes, where a top quark decays into $u/c+e^{+}+e^{-}$. The analysis incorporates both reducible and irreducible backgrounds while accounting for realistic detector effects. The dominant backgrounds for these trilepton signatures arise from $t\bar{t}$ production, single top quark production in association with $V$, and $VV$ production (where $V = W, Z$). These backgrounds are significantly reduced using machine learning-based classification models, which optimize event selection and improve signal sensitivity. For an integrated luminosity of 3000 fb$^{-1}$ at the LHC, we find that the expected $95\%$ confidence level (CL) limits on the scale of 4-Fermi FCNC interactions reach $Λ\leq 5.5$ TeV for $tuee$ and $Λ\leq 5.7$ TeV for $tcee$ in the $t\bar{t}$ channel, and $Λ\leq 1.9$ TeV ($tuee$) and $Λ\leq 2.0$ TeV ($tcee$) in the $tW$ channel. We also provide an interpretation of our EFT analysis in the context of a specific $Z'$ model, illustrating how the derived constraints translate into bounds on the parameter space of a heavy neutral gauge boson mediating flavor-changing interactions.

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arXiv2024

Unitarity Constraints and Collider Searches for Dark Photons

Dark photons are predicted by various new physics models, and are being intensively studied in a variety of experiments. In the first part of this paper, we obtain partial wave unitarity constraints on the dark photon parameter space from the allowed $VV\rightarrow VV$ scattering processes in the limit of large center-of-mass energy, where $V=W,Z$. In the second part of the paper, searches are performed using the expected differential rates with a realistic detector simulation including a comprehensive set of background processes on dilepton and dilepton plus a photon events at the High Luminosity LHC. In these searches, sensitive differential distributions are used in an optimized way to determine the sensitivity to dark photon parameter space. It is shown that remarkable sensitivity to the dark photon model is achieved and kinetic mixing strength can be probed down to $(1.4-10)\times 10^{-4}$ for dark photon mass between $15$ GeV to $2$ TeV. We also investigate the sensitivity of a future muon collider suggested by the Muon Accelerator Program (MAP) to the dark photon model at different center-of-mass energies. It is shown that a future muon collider is able to reach a sensitivity to kinetic mixing at the order of $10^{-4}$.

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arXiv2024

Exploring Axion-Like Particle Couplings through Single Top $tW$-Channel and Top Pair Production at the LHC

In this paper, we explore the interactions between light axion-like particles (ALPs) and top quarks, gluons, and W-bosons. Utilizing high-energy probes at the LHC, we probe these couplings in the context of single top quark production associated with a W-boson and an ALP. We analyze the latest LHC Run II data on $t\bar{t}$ spin correlation measurements to establish constraints on these couplings. Additionally, we compare these constraints with those derived from loop-induced couplings and indirect searches involving B-meson decays. Furthermore, we derive two-dimensional limits on the ALP couplings with top quarks and gluons.

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arXiv2024

Exploring non-standard $Hb\bar{b}$ interactions at future electron-proton colliders

In this paper, we use the charged-current Higgs boson production process at future electron-proton colliders, $e^-p \to H j ν_e$, with the subsequent decay of the Higgs boson into a $b\bar{b}$ pair, to probe the Standard Model effective field theory with dimension-six operators involving the Higgs boson and the bottom quark. The study is performed for two proposed future high-energy electron-proton colliders, the Large Hadron Electron Collider (LHeC) and the Future Circular Collider (FCC-he) at the center-of-mass energies of 1.3 TeV and 3.46 TeV, respectively. Constraints on the CP-even and CP-odd $Hb\bar{b}$ couplings are derived by analyzing the simulated signal and background samples. A realistic detector simulation is performed and a multivariate technique using the gradient Boosted Decision Trees algorithm is employed to discriminate the signal from background. Expected limits are obtained at $95\%$ Confidence Level for the LHeC and FCC-he assuming the integrated luminosities of 1, 2 and 10 ab$^{-1}$. We find that using 1 ab$^{-1}$ of data, the CP-even and CP-odd $Hb\bar{b}$ couplings can be constrained with accuracies of the order of $10^{-3}$ and $10^{-2}$, respectively, and a significant region of the unprobed parameter space becomes accessible.

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arXiv2024

Prospect study for measurement of $Hb\bar{b}$ coupling at the LHC and FCC-hh

This paper employs the $H+b+\text{jets}$ signature in proton-proton collisions to explore the structure of the $Hb\bar{b}$ couplings.The focus of the analysis lies in the decay of the Higgs boson into a photon pair, taking into account both reducible and irreducible backgrounds and a realistic simulation of the detector effects. To enhance the discrimination between signal and background, a multivariate analysis is employed to analyse the kinematic variables and optimise the signal-to-background ratio. The results indicate that the $H+b+\text{jets}$ process can significantly contribute to the precise measurement of CP-even and CP-odd couplings between the bottom quark and the Higgs boson at the LHC and FCC-hh. Finally, a novel asymmetry is introduced for the purpose of probing CP violation within the $Hb\bar{b}$ coupling, formulated exclusively based on lab-frame momenta.

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arXiv2023

Probing a light dark sector at future lepton colliders via invisible decays of the SM-like and dark Higgs bosons

A renormalizable UV model for Axion-Like Particles (ALPs) or hidden photons, that may explain the dark matter usually involves a dark Higgs field which is a singlet under the standard model (SM) gauge group. The dark sector can couple to the SM particles via the portal coupling between the SM-like Higgs and dark Higgs fields. Through this coupling, the dark sector particles can be produced in either the early universe or the collider experiments. Interestingly, not only the SM-like Higgs boson can decay into the light dark bosons, but also a light dark Higgs boson may be produced and decay into the dark bosons in a collider. In this paper, we perform the first collider search for invisible decays by taking both the Higgs bosons into account. We use a multivariate technique to best discriminate the signal from the background. We find that a large parameter region can be {probed} at the International Linear Collider (ILC) operating at the center-of-mass energy of 250 GeV. In particular, even when the SM-like Higgs invisible decay is a few orders of magnitude below the planned sensitivity reaches of the ILC and the high luminosity LHC (HL-LHC), the scenario can be probed by the invisible decay of the dark Higgs boson produced via a similar diagram. Measuring the dark Higgs boson decay into the dark sector will be a smoking gun signal of the light dark sector. A similar search of the dark sector would be expected in, e.g., Cool Copper Collider (C$^3$), Circular Electron Positron Collider (CEPC), Compact Linear Collider (CLIC) and {Future Circular electron-positron Collider (FCC-ee).

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arXiv2022

Prospects for probing axionlike particles at a future hadron collider through top quark production

Axionlike particles (ALPs) emerge from spontaneously broken global symmetries in high energy extensions of the Standard Model (SM). This causes ALPs to be among the objectives of future experiments which intend to search for new physics beyond the SM. We discuss the reach of future pp collider FCC-hh in probing the ALP model parameters through top quark pair production associated with ALP ($t\bar{t}+\text{ALP}$) in a model-independent approach. The search is performed in the semileptonic decay mode of $t\bar{t}$ and the analysis is performed using a parametric simulation of the detector response for a projected integrated luminosity of $\rm 30~ab^{-1}$. It is shown that $t\bar{t}+\text{ALP}$ production at the FCC-hh is a promising channel with significant sensitivity to probe the ALP coupling with gluons. The ALP coupling with gluons obtained from HL-LHC and other experiments are presented for comparison.

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arXiv2022

Search for lepton-flavor-violating ALPs at a future muon collider and utilization of polarization-induced effects

Axion-Like Particles (ALPs) are pseudo Nambu-Goldstone bosons associated with spontaneously broken global $U(1)$ symmetries. Such particles can have lepton-flavor-violating (LFV) couplings to the SM charged leptons. LFV ALPs provide the possibility to address some of the SM long-lasting problems. We investigate the sensitivity of a future muon collider suggested by the Muon Accelerator Program (MAP) to the production of LFV ALPs in the ALP mass range $m_a\leq1$ MeV. ALPs are assumed to be produced through the LFV decay $τ\ra\ell a$ ($\ell=e,μ$) of one of the tau leptons produced in the muon-anti muon annihilation. Performing a realistic detector simulation and deploying a multivariate technique, we constrain the LFV couplings $c_{τe}$ and $c_{τμ}$ for both the cases of unpolarized and polarized muon beams. Three different chiral structures are considered for the LFV ALP coupling and the muon collider is assumed to operate at the center-of-mass energies of 126, 350 and 1500 GeV. We present a procedure to search for LFV ALPs at colliders which takes advantage of tau polarization-induced effects. Polarized tau leptons which produce such effects can be produced when the initial muon beams are polarized. Utilizing the properties of polarized tau decays, the main SM background which overwhelms the ALP production in the case of unpolarized muon beams can be significantly suppressed. We present 95$\%$ CL expected limits on the LFV couplings and show that the present analysis can improve current experimental limits on the ALP LFV couplings by roughly one order of magnitude.

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arXiv2022

Search for lepton-flavor-violating decays of the tau lepton at a future muon collider

Tau leptons can have lepton-flavor-violating (LFV) couplings to a muon or an electron and an Axion-Like Particle (ALP). ALPs are pseudo Nambu-Goldstone bosons associated with spontaneously broken global U(1) symmetries. LFV ALPs have been of a great interest in the last several decades as they can address some of the SM long-lasting problems. Assuming a future muon collider proposed by the Muon Accelerator Program (MAP), we search for LFV decays $τ\rightarrow\ell a$ ($\ell=e,μ$) of one of the tau leptons produced in the muon-anti muon annihilation. The ALP mass is assumed to be in the range 100 eV to 1 MeV and three different chiral structures are considered for the LFV coupling. Using a multivariate technique and performing a parameterized simulation based on the ideal target performance, we obtain expected 95$\%$ confidence level upper limits on the LFV couplings tau-electron-ALP and tau-muon-ALP. Limits are computed assuming the center-of-mass energies of 126, 350 and 1500 GeV which the future muon collider is supposed to operate at. We study the two cases of unpolarized and polarized muon beams and show that taking advantage of tau polarization-induced effects, the main background $τ\rightarrow e/μ+ ν\barν$ can be significantly reduced. Results indicate that current limits on the LFV couplings can be improved by roughly one order of magnitude using the present analysis.

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arXiv2020

New Collider Searches for Axion-like Particles Coupling to Gluons

Axion-like particles (ALPs) are pseudo Nambu-Goldstone bosons associated with spontaneously broken global symmetries emerging in many extensions of the Standard Model. Assuming the most general effective Lagrangian up to dimension-5 operators for an ALP interacting with the SM fields, we investigate for the first time the sensitivity of the LHC13 to the ALP production in association with a di-jet. This study is focused on light ALPs which appear as invisible particles at the detector. Performing a realistic detector simulation and deploying a multivariate technique to best discriminate the signal from backgrounds, we set expected upper bounds on the ALP coupling to gluons. A comprehensive set of background processes is considered, and it is shown that this process provides significant sensitivity to the ALP-gluon coupling and the resulting bound is more stringent than those already obtained at the LHC. We also present prospects for the HE-LHC27 and FCC-hh100 and show that these future colliders are able to improve the limits from the LHC by roughly one order of magnitude.

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arXiv2020

Constraining the top quark effective field theory using the top quark pair production in association with a jet at future lepton colliders

Our main aim in this paper is to constrain the effective field theory describing the top quark couplings through the $e^{-} e^{+} \rightarrow t \bar{t}+$jet process. The analysis is carried out considering two different center-of-mass energies of 500 and 3000 GeV including a realistic simulation of the detector response and the main sources of background processes. The expected limits at 95\% CL are derived on the new physics couplings such as $t \bar t γ$, $t \bar t Z$, and $t \bar t g$ for each benchmark scenario using the dileptonic $t \bar{t}$ final state. We show that the 95\% CL limits on dimensionless Wilson coefficients $\bar{c}_i$ considered in this analysis could be probed down to $10^{-4}$.

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arXiv2020

Neutron Decay to a Non-Abelian Dark Sector

According to the Standard Model (SM), we expect to find a proton for each decaying neutron. However, the experiments counting the number of decayed neutrons and produced protons have a disagreement. This discrepancy suggests that neutrons might have an exotic decay to a Dark Sector (DS). In this paper, we explore a scenario where neutrons decay to a dark Dirac fermion $χ$ and a non-abelian dark gauge boson $W'$. We discuss the cosmological implications of this scenario assuming DS particles are produced via freeze-in. In our proposed scenario, DS has three portals with the SM sector: (1) the fermion portal coming from the mixing of the neutron with $χ$, (2) a scalar portal, and (3) a non-renormalizable kinetic mixing between photon and dark gauge bosons which induces a vector portal between the two sectors. We show that neither the fermion portal nor the scalar portal should contribute to the production of the particles in the early universe. Specifically, we argue that the maximum temperature of the universe must be low enough to prevent the production of $χ$ in the early universe. In this paper, we rely on the vector portal to connect the two sectors, and we discuss the phenomenological bounds on the model. The main constraints come from ensuring the right relic abundance of dark matter and evading the neutron star bounds. When dark gauge boson is very light, measurements of the Big Bang Nucleosynthesis impose a stringent constraint as well.

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arXiv2020

Signs of new physics in top quark pair production associated with a neutrino pair at the LHC

In this paper, we examine the interactions of the top quarks with the $Z$ boson using the top quark pair production associated with neutrino pair ($t\bar{t}ν_{l}\bar{ν_{l}}$) at the LHC. In particular, potential constraints on the anomalous electroweak top-quark interactions are determined by considering two opposite-sign charged leptons, missing energy, and two b-tagged jets in the final state. The analysis is performed for a High Luminosity scenario of the LHC with an integrated luminosity of 3 ab$^{-1}$ of proton-proton collisions at a center-of-mass energy of 14 TeV. The $95\%$ confidence intervals are computed on the anomalous couplings considering a realistic detector simulation of an upgraded CMS detector including an average of 200 proton-proton interactions per bunch crossing. We find that the $t\bar{t}ν_{l}\bar{ν_{l}}$ channel can provide stringent bounds on the relevant Wilson coefficients and has the potential to serve as an additional handle beside the $t\bar{t}Z(Z\rightarrow l^{+}l^{-})$ and other channels to search for new physics.

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