arXiv subjects

Farzana Ahmad

Publications and source records attributed to Farzana Ahmad.

3 recordsLinked to original sources

arXiv2026

Probing Singlet Vector-Like Top Quarks in the Hadronic tZ Channel at the HL-LHC using Machine and Deep Learning Architectures

In this work, we study the single production of a vector-like singlet top partner $ T $ at the 14 TeV HL-LHC in the channel $ pp \to T j $ with $ T \to t Z $, $ t \to b W \to b j j $, and $ Z \to ν\barν $. Signal and background samples are generated with MadGraph5\_aMC@NLO v3.5.11, showered with Pythia 8, and passed through Delphes. The dominant backgrounds are $ t \bar{t} $, $ t Z j $, $ ZZ j j $, and $ W Z j j $ (including charge conjugates). A hadronic pre-selection ($ N_j \geq 3 $, $ N_b \geq 1 $, $ N_\ell = 0 $) is imposed as trigger, followed by optimized kinematic cuts. We perform multivariate classification with Extreme Gradient Boosting (XGBoost) and a Graph Neural Network (GNN) based on jet-level features. Sensitivities at 3000 fb$^{-1}$ are quoted using the Asimov significance, $ S / \sqrt{S + B} $, and an Asimov variant with a 20\% background systematic. The model parameters $ g^* $ and $ R_L $ are defined in Sec.~2, and a single global working point is used to avoid per-mass tuning bias. In the $ (g^*, m_T) $ scan, we present 2$σ$ exclusion and 5$σ$ discovery contours for $ R_L = 0 $ and $ R_L = 0.5 $. For $ R_L = 0 $, 2$σ$ exclusion corresponds to $ g^* \in [0.17, 0.49] $ ($ 0.16, 0.43 $) over $ m_T \in [1.8, 2.7] $ TeV, while 5$σ$ discovery corresponds to $ g^* \in [0.27, 0.44] $ ($ 0.26, 0.40 $) over $ m_T \in [1.8, 2.2] $ TeV for XGBoost and GNN respectively. For $ R_L = 0.5 $, the 2$σ$ reach is $ g^* \in [0.21, 0.48] $ ($ 0.20, 0.43 $) over $ m_T \in [1.8, 2.5] $ TeV, and the 5$σ$ reach is $ g^* \in [0.33, 0.43] $ ($ 0.31, 0.49 $) over $ m_T \in [1.8, 2.2] $ TeV, with the GNN yielding slightly stronger and smoother limits across the scan.

hep-ph↗

arXiv2026

Probing Extended Higgs Sectors via Multi-Top Events from Higgs Pair Decays in 2HDM Type-I at the HL-LHC

This study investigates the production of multi-top quark events final states containing up to four top quarks as a probe for new physics beyond the Standard Model (SM) within the framework of the Two Higgs Doublet Model (2HDM) Type-I. Focusing on the High-Luminosity Large Hadron Collider (HL-LHC) at a center-of-mass energy of $\sqrt{s} = 14$TeV, we analyze associated production processes including $pp \to t\bar{t}H$, $pp \rightarrow t\bar{t}A$, $pp \rightarrow HA$, $pp \rightarrow HH^{\pm}$, and $pp \rightarrow AH^{\pm}$. The simulation pipeline incorporates theoretical constraints and branching ratios. By targeting these high-multiplicity final states, the research aims to establish the sensitivity of the HL-LHC to an extended Higgs sector, utilizing the top quark's unique coupling to the scalar field as a primary discovery channel. The analysis is centered on two benchmark points characterized by a degenerate mass spectrum ($m_H = m_A = m_{H^{\pm}} = 500$GeV) in the alignment limit ($\sin(β- α) \to 1$), with $\tanβ$ varied between 1 and 3. Event selection was performed with stringent cuts on jet multiplicity to suppress dominant SM backgrounds, such as $t\bar{t}W$, $t\bar{t}Z$, and $WWZ$. Our results indicate that signal significance improves substantially as the integrated luminosity increases from $3000~fb^{-1}$ to $4000~fb^{-1}$, with all investigated channels exceeding the $5σ$ discovery threshold.

hep-ph↗

arXiv2026

Probing the Scalar Sector: Discovery Reach for Heavy Higgs Pairs at a $\sqrt{s} = 6$ TeV Muon Collider in the 2HDM Alignment Limit

This study provides a comprehensive phenomenological investigation into the discovery potential of heavy Higgs boson pairs ($HH, HA, AA, H^+H^-$) at a $\sqrt{s}=6$~TeV Muon Collider. Utilizing the Two-Higgs-Doublet Model (2HDM) Type-I within the alignment limit ($\sin(β-α) \approx 1$), we evaluate two primary benchmarks with degenerate scalar masses of 1000~GeV (BP1) and 2000~GeV (BP2). Theoretical calculations performed reveal that Type-I branching fractions to third-generation fermions remain uniquely independent of $\tanβ$, providing a stable signal across the investigated parameter space. We demonstrate that the Muon Collider environment allows for the precise identification of high-multiplicity hadronic final states. A key finding of this research is that the signal processes yield distinctive topological signatures: an 8-jet state ($4j+4b$) for charged pairs and a highly complex 12-jet state ($8j+4b$) for neutral pairs ($HA/AA$). These signatures, combined with hard transverse momentum distributions and central pseudorapidity ($|η| \le 3$), allow for nearly absolute suppression of Standard Model backgrounds like $t\bar{t}$, $W^+W^-Z$, and $ZZZ$. At an integrated luminosity of 10~ab$^{-1}$, we report a staggering statistical significance of 104,000 for the $H^+H^-$ channel and 3343 for the $HA$ channel in the BP1 scenario. Furthermore, total selection efficiencies were found to increase from approximately 20\% at BP1 to 47\% at BP2, suggesting that the decay products of heavier scalars are kinematically easier to resolve. We conclude that a 6~TeV Muon Collider offers an unparalleled discovery reach for the extended scalar sector, providing a definitive facility for probing physics beyond the Standard Model.

hep-ph↗