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Shaaban Khalil

Publications and source records attributed to Shaaban Khalil.

At least 19 recordsLinked to original sources
arXiv2026

Z'-Mediated Slepton and Sneutrino Signatures at HL-LHC

We investigate the discovery potential of a heavy $Z'$ boson as a portal to charged sleptons and right-handed sneutrinos within the $B-L$ Supersymmetric Standard Model (BLSSM) at the High-Luminosity LHC. Using a detector-level simulation and Boosted Decision Trees to suppress Standard Model backgrounds, we find that the direct slepton decay $pp \to Z' \to \tilde{\ell}^+\tilde{\ell}^- \to 2\ell + E_{\rm T}^{\rm miss}$ yields a significance exceeding $5\sigma$ at 3000 fb$^{-1}$, while the cascade $(\mbox{multileptons} + E_{\rm T}^{\rm miss})$ mode is unobservable due to rate suppression. For right-handed sneutrinos, the hadronic channel $pp \to Z' \to \tilde{\nu}_R\tilde{\nu}_I \to 2\ell + 4j + E{\rm T}^{\rm miss}$ emerges as the most promising signature. These results establish the $2\ell + E_{\rm T}^{\rm miss}$ and $2\ell + 4j + E_{\rm T}^{\rm miss}$ final states as the principal discovery channels for the scalar leptonic sector of the BLSSM at the HL-LHC.

hep-ph↗
arXiv2026

Axion-Sourced Gravitational Waves in $\mathrm{B\!-\!L}$ Hybrid Inflation

Minimal supersymmetric $\mathrm{B\!-\!L}$ hybrid inflation predicts a negligible vacuum tensor-to-scalar ratio $r_{\rm vac}\sim 10^{-8}$ due to its extremely flat potential. In this letter, we show that adding a spectator pseudoscalar axion-like field coupled to the $\rm {U}(1)_{\mathrm{B\!-\!L}}$ gauge sector via a Chern-Simons term circumvents this suppression. The rolling axion triggers a tachyonic instability for one gauge field helicity, exponentially amplifying gauge fluctuations. These sourced modes generate a stochastic gravitational-wave background with tensor power spectrum ${\cal P}_{T}^{\rm src}\propto e^{4\pi\xi}/\xi^{6}$, where $\xi = \alpha \dot{\phi} / (2 f_a H)$ is the gauge-field instability parameter. For $\xi\sim3.3$--$3.6$, the tensor-to-scalar ratio reaches $r\sim10^{-3}$, within the sensitivity of LiteBIRD. For $\xi\sim3.6$, the gravitational-wave spectrum develops a peaked shape that enters the LISA sensitivity band (peaking at frequencies around $10^{-3}$ Hz) with an amplitude $h^2\Omega_{\rm GW}\sim2.5\times10^{-13}$. The mechanism predicts chiral gravitational waves, a broken consistency relation $r=16\epsilon$, and a distinctive $r$--$\Omega_{\rm GW}$ correlation that could be tested by future CMB and interferometer observations.

astro-ph.CO↗
arXiv2026

Explaining the $B \to K\mu^+\mu^-$ Anomaly in the Left-Right Inverse Seesaw Model

We investigate the long-standing anomaly in the rare decay B into Kll within the Left-Right Inverse Seesaw (LRIS) model. Global analyses of the B into s mu mu data consistently indicate a significant negative shift in the vector Wilson coefficient, $\Delta C{9} \approx -1$, while the axial coefficient $\Delta C{10}$ remains consistent with zero. We show that a charged-scalar/heavy-neutrino box diagram in the LRIS model naturally generates this pattern through a \emph{non-decoupling} mechanism: the right-handed coupling produces a contribution to $\Delta C{9}$ that is unsuppressed in the heavy-neutrino limit, while the simultaneous presence of a comparable left-handed Dirac Yukawa coupling ensures the automatic cancellation $\Delta C{10} \approx 0$. The otherwise large contribution to $B_s$--$\bar{B}_s$ mixing is suppressed by several orders of magnitude through a GIM-like phase structure in the right-handed quark mixing matrix. A numerical scan over the model parameter space identifies a viable region, consistent with all current flavor and collider constraints. The $b \to s\gamma$ constraint is satisfied with two orders of magnitude to spare throughout the viable band. These results motivate correlated searches for the charged scalar and the heavy right-handed neutrinos at the LHC and future high-luminosity experiments.

hep-ph↗
arXiv2026

Forward backward CP asymmetry in $\tau^- \to K \pi \nu_{\tau}$ in the Left-Right Inverse seesaw model

The Left--Right Inverse Seesaw (LRIS) model, in which TeV-scale right-handed neutrinos can carry $\mathcal{O}(1)$ Yukawa couplings while light neutrino masses remain protected by a small lepton-number-violating scale, provides a testable link between the neutrino-mass mechanism and flavor and collider observables: the same non-decoupling dynamics has recently been shown to enable viable TeV-scale non-thermal leptogenesis and to explain the $B\to K\mu^+\mu^-$ anomaly. Here we examine a further, independent test of the same scalar sector in the semileptonic decay $\tau \to K\pi\nu_\tau$. We identify a distinct, unsuppressed signal in the \emph{differential} forward-backward CP asymmetry $A_{\rm CP}^{\rm FB}(s)$, driven by interference between the SM vector current and a non-decoupling scalar operator $g_S$ generated by a top-quark flavor-changing neutral-current box diagram with internal heavy neutrinos and charged Goldstone bosons. We derive the effective $|\Delta S|=1$ Hamiltonian, verify consistency with $K^0$--$\bar K^0$ and $B_d^0$--$\bar B_d^0$ mixing, $B\to X_s\gamma$, and neutrino non-unitarity constraints, and show numerically that $A_{\rm CP}^{\rm FB}(s)$ is resonantly enhanced near the $K_0^*(1430)$ state, reaching $\mathcal{O}(10^{-4})$ -- within reach of Belle~II.

hep-ph↗
arXiv2026

Mono-Z' Signatures in the B-L Supersymmetric Standard Model at the LHC

The B-L Supersymmetric Standard Model with Inverse Seesaw (BLSSM-IS) extends the Minimal Supersymmetric Standard Model (MSSM) by incorporating a gauged B-L symmetry, right-handed neutrinos and an additional neutral gauge boson Z'. Searches at the Large Hadron Collider (LHC) constrain the mass of this gauge boson to be as low as only ~ 2.2 TeV in the BLSSM-IS, owing to interference effects with the SM. In this framework, mono-Z' events can arise from the associated production of a Z' boson and a singlet Higgs boson h', where h' subsequently decays into missing energy carried by a pair of the Lightest Supersymmetric Particle (LSP) - either a neutralino or a right-handed sneutrino - which serves as a Dark Matter (DM) candidate. Focusing on leptonic decays of the Z' (electrons and muons), we analyse the kinematic distributions of the final-state leptons and the missing transverse energy in order to extract a signal for this process which is independent of the nature of the BLSSM-IS DM.

hep-ph↗
arXiv2026

CP Violation in $D \to KK$ Decays: A Comparative Analysis of Triplet and Sextet Diquarks

Recent measurements of the CP asymmetry in the decay $D^0 \rightarrow K_S^0 K_S^0$ by the CMS collaboration, $A_{CP}(K_S^0 K_S^0) = (6.2 \pm 3.0 \pm 0.2 \pm 0.8)\%$, and by LHCb, $A_{CP}(D^0 \to K_S^0 K_S^0) = (1.86 \pm 1.04 \pm 0.41)\%$, suggest possible deviations from Standard Model (SM) expectations, which predict asymmetries below the percent level. This singly Cabibbo-suppressed decay is particularly sensitive to new physics, as the leading amplitudes vanish in the exact U-spin symmetry limit and the process is dominated by W-exchange topologies. We investigate scalar diquark contributions to this decay, comparing color-sextet and color-triplet representations. We find that the color-sextet diquark, characterized by a symmetric color structure $(C_1^{\mathrm{NP}} = C_2^{\mathrm{NP}})$, avoids color suppression and can generate CP asymmetries in the range $0.5\%$--$1.5\%$ for a diquark mass of order 1~TeV. In contrast, the color-triplet contribution is strongly suppressed due to destructive interference from its antisymmetric color structure. We further show that a flavor hierarchy in the sextet couplings, with $\lambda_{ud} > \lambda_{us}$, can simultaneously account for the observed deviation from the U-spin sum rule in $D^0 \to K^+ K^-$ and $D^0 \to \pi^+ \pi^-$ and the measured CP asymmetry in $D^0 \to K_S^0 K_S^0$. These results identify color-sextet scalar diquarks as viable candidates for explaining enhanced CP violation in charm decays.

hep-ph↗
arXiv2026

Non-Thermal Leptogenesis in the BLSM with Inverse Seesaw Mechanism

We investigate the viability of non-thermal leptogenesis in the gauged $U(1)_{B-L}$ extension of the Standard Model (BLSM) with an inverse seesaw (ISS) mechanism for neutrino mass generation. In this framework, right-handed neutrinos typically have $\mathcal{O}(1)$ Yukawa couplings, which induce strong washout effects and render conventional thermal leptogenesis ineffective. We demonstrate that a successful baryogenesis scenario can nevertheless be realized through non-thermal leptogenesis, where right-handed neutrinos are produced from the decay of the heavy $B\!-\!L$ Higgs boson $\chi$. We explicitly analyze the interplay between the dilution factor $T_R/M_\chi$ and the washout parameter characteristic of the ISS, highlighting the tension between suppressing washout effects and maintaining sufficient reheating. We show that a viable lepton asymmetry can be generated provided the scalar mass spectrum is appropriately tuned --- a condition that requires a fine-tuning of order one part in $10^5$ against one-loop radiative corrections --- allowing for a reduced reheating temperature while keeping washout under control. The resulting lepton asymmetry is efficiently converted into the observed baryon asymmetry of the Universe via sphaleron processes. Our results establish that the inverse-seesaw $B\!-\!L$ model remains a predictive and robust framework for non-thermal leptogenesis and baryogenesis.

hep-ph↗
arXiv2025

Predictions for $\Lambda_b\to \Lambda_c\tau\bar\nu$ in BLSSM with Inverse seesaw

The persistent deviations observed in semileptonic $B$ decays, in particular the lepton flavor universality ratios $\mathcal{R}(D^{(*)})$ and $\mathcal{R}(\Lambda_c)$, provide intriguing hints of physics beyond the Standard Model (SM). While current measurements remain limited by experimental uncertainties, their lower central values compared to SM expectations motivate further theoretical scrutiny. In this work we study these observables within the $B-L$ Supersymmetric Standard Model with an inverse seesaw (BLSSM-IS). We emphasize the role of penguin diagrams involving charginos, neutralinos, and right-handed sneutrinos, which induce flavor-dependent loop corrections to the effective $W\ell\nu$ vertex. These corrections can suppress the light-lepton decay rates relative to the $\tau$ mode, leading to a modest enhancement of $\mathcal{R}(D^{(*)})$ and, through the sum rule, $\mathcal{R}(\Lambda_c)$. We present updated numerical results illustrating the correlation between mesonic and baryonic observables, showing that the BLSSM-IS framework provides a natural and testable explanation of the current data. Our findings underline the importance of upcoming precision measurements at Belle II and the LHCb upgrade in clarifying the possible role of supersymmetry in lepton flavor universality violation.

hep-ph↗
arXiv2025

Probing Right Handed Neutrino assisted Reheating with Gravitational Waves and Leptogenesis

We investigate a non-instantaneous reheating period in the early Universe, where the inflaton field decays exclusively to right-handed neutrinos (RHNs). The subsequent decay of these RHNs into Standard Model particles not only drives the transition to a radiation-dominated era but also generates the baryon asymmetry of the Universe via leptogenesis. In this typical reheating scenario, gravitational waves (GWs) can be produced during inflaton decay, both through bremsstrahlung and inflaton scattering processes. While GW production via bremsstrahlung dominates near the end of the reheating phase, inflaton scattering leads to a non-negligible GW contribution near the maximum temperature of the Universe. The combined GW spectrum from both decay and scattering processes lies within the sensitivity range of proposed resonant cavity experiments. This framework thus offers a compelling and unified approach to addressing neutrino mass generation, the baryon asymmetry of the Universe via leptogenesis, and probing the dynamics of a non-instantaneous reheating era.

hep-ph↗
arXiv2025

Primordial Black Holes and Gravitational Waves in Extensions of the Standard Model

We investigate the phenomenology of a Standard Model extension incorporating an inert scalar doublet and a gauged $U(1)_{B-L}$ symmetry. Our analysis reveals regions of the parameter space that support strong first-order phase transitions, including cases featuring two successive transitions. Each transition can generate a stochastic gravitational wave background within the sensitivity reach of upcoming experiments. Remarkably, the high-scale transition may also produce primordial black holes with appreciable abundance.

hep-ph↗
arXiv2025

t-channel dark matter at the LHC -- a whitepaper

This report, summarising work achieved in the context of the LHC Dark Matter Working Group, investigates the phenomenology of $t$-channel dark matter models, spanning minimal setups with a single dark matter candidate and mediator to more complex constructions closer to UV-complete models. For each considered class of models, we examine collider, cosmological and astrophysical implications. In addition, we explore scenarios with either promptly decaying or long-lived particles, as well as featuring diverse dark matter production mechanisms in the early universe. By providing a unified analysis framework, numerical tools and guidelines, this work aims to support future experimental and theoretical efforts in exploring $t$-channel dark matter models at colliders and in cosmology.

hep-ph↗
arXiv2024

Unveiling E$_6$SSM Scalar Diquarks at the HL-LHC

We investigate the phenomenology of scalar diquarks with sub-TeV masses within the framework of the $E_6$ Supersymmetric Standard Model (E$_6$SSM) at the Large Hadron Collider (LHC). Focusing on the lightest of the six diquarks predicted by the model, we select some representative low masses for them in a parameter space region consistent with experimental constraints from direct searches for additional Higgs boson(s), Cold Dark Matter (CDM), and supersymmetry, as well as from flavor physics analyses. Using Monte Carlo ($MC$) simulations, we assess these benchmark points against the latest LHC results corresponding to an integrated luminosity of 140 fb$^{-1}$. We further evaluate the signal significance of the pair-production of these diquarks, when each of them decays into $tb$ pairs, at the $\sqrt{s}=13$ TeV LHC Run 3 with design integrated luminosity of 300 fb$^{-1}$, and also at the 3000 fb$^{-1}$ High-Luminosity LHC (HL-LHC). Our analysis yields a statistical significance exceeding $3\sigma$ at the HL-LHC for diquark masses up to 1 TeV, indicating promising prospects for their discovery.

hep-ph↗
arXiv2024

Exploring $Z'$ and Right-Handed Neutrinos in the BLSM at the Large Hadron Collider

We study the collider phenomenology of the $B$-$L$ extension of the Standard Model (BLSM), focusing on the production and decay of a heavy neutral gauge boson (\( Z' \)) at the Large Hadron Collider (LHC). In this framework, the \( Z' \) can decay into pairs of heavy right-handed neutrinos (\( \nu_R \)), which subsequently decay into charged leptons and \( W \) bosons. These processes give rise to three distinctive final states: (i) two leptons plus four jets (\( 2\ell + 4j \)), (ii) four leptons plus missing transverse energy (\( 4\ell + \text{MET} \)), and (iii) three leptons plus two jets and MET (\( 3\ell + 2j + \text{MET} \)). % To enhance signal sensitivity and suppress Standard Model backgrounds, we employ multivariate analysis techniques based on Boosted Decision Trees (BDTs), as well as selection optimizations using the \texttt{XGBOOST} framework. The classifiers are trained on kinematic observables sensitive to the masses of the \( Z' \) and \( \nu_R \). We demonstrate that all three final states offer significant discovery potential for both the \( Z' \) and heavy \( \nu_R \) at the High-Luminosity LHC. Our results highlight the testability of the BLSM at current and future collider experiments, and provide a promising avenue for probing the origin of neutrino masses and the baryon asymmetry of the Universe.

hep-ph↗
arXiv2024

Flavor Violations in $B$-Mesons within Non-Minimal SU(5)

Recent anomalies in $B$-meson decays, such as deviations in $R_{D^{(*)}}$ and $B\to K\nu{\bar\nu}$, suggest possible lepton flavor universality violation and new exotic interactions. In this work, we explore these anomalies within a non-minimal SU(5) grand unified theory (GUT) framework, which introduces a 45-dimensional Higgs representation predicting exotic scalar particles, including the leptoquark $R_2$ and diquark $S_6$. The $R_2$ leptoquark addresses charged current anomalies in $b\to c\tau\nu$ transitions, the $S_6$ diquark contributes to nonleptonic neutral current processes, such as $B\to K\pi$, while at the loop level, diagrams involving the exchanges of the leptoquark, diquark, and the Standard Model particles contribute to $B\to K\nu{\bar\nu}$, offering solutions to longstanding puzzles.

hep-ph↗
arXiv2024

Non-thermal Dark Matter in $U(1)_{B-L}$ Extension of Inert Doublet Model

We propose an extension of the Inert Doublet Model (IDM) that explains both neutrino masses and dark matter (DM) in the intermediate-mass range by incorporating a $U(1)_{B-L}$ gauge symmetry. This additional symmetry enables the inclusion of right-handed neutrinos, providing a natural mechanism for neutrino mass generation. While the CP-even component of the inert doublet can serve as a DM candidate, its thermal relic abundance is insufficient to match the observed DM density. To address this, we introduce a non-thermal production mechanism, where a heavy scalar associated with the $U(1)_{B-L}$ symmetry decays into the inert doublet scalar, yielding a viable relic abundance at low reheating temperatures. We also examine both direct and indirect detection prospects for this DM candidate and assess the model against current experimental constraints.

hep-ph↗
arXiv2024

Primordial Black Holes and Gravitational Waves in the $U(1)_{B-L}$ Extended Inert Doublet Model: A First-Order Phase Transition Perspective

We conduct an analysis of a $U(1)_{B-L}$ extended inert doublet model and obtained the parameter space allowing strong first order phase transitions. We show that a large part of the parameter space can cause double first-order phase transitions. Whereas both of these phase transitions can generate a detectable stochastic gravitational wave background, one of them can create primordial black holes with appreciable abundance. The primordial black holes generated at the high scale transition can account for the dark matter maintaining the correct relic abundance. We also show specific benchmark cases and their consequences from the aspect of primordial black holes and gravitational waves.

hep-ph↗
arXiv2023

Higgs Quadruplet Impact on $W$ Mass Shift, Dark Matter, and LHC Signatures

The addition of a Higgs quadruplet to the standard model (SM) of quarks and leptons would shift the $W$ boson mass upward. It could also facilitate the production of dark matter through the conventional thermal freeze-out scenario via Yukawa interaction with the Higgs quadruplet or freeze-in production from the decay of SM Higgs. We investigate the same-sign lepton smoking gun signature of the double-charged scalar component of the Quadruplet Higgs at the LHC.

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