arXiv subjects

Shaikh Saad

Publications and source records attributed to Shaikh Saad.

At least 55 records · Page 3Linked to original sources

arXiv2023

Quark-lepton Yukawa ratios and nucleon decay in SU(5) GUTs with type-III seesaw

We consider an extension of the Georgi-Glashow SU(5) GUT model by a 45-dimensional scalar and a 24-dimensional fermionic representation, where the latter leads to the generation of the observed light neutrino masses via a combination of a type I and a type III seesaw mechanism. Within this scenario, we investigate the viability of predictions for the ratios between the charged lepton and down-type quark Yukawa couplings, focusing on the second and third family. Such predictions can emerge when the relevant entries of the Yukawa matrices are generated from single joint GUT operators (i.e. under the condition of single operator dominance). We show that three combinations are viable, (i) $y_τ/y_b=3/2$, $y_μ/y_s=9/2$, (ii) $y_τ/y_b=2$, $y_μ/y_s=9/2$, and (iii) $y_τ/y_b=2$, $y_μ/y_s=6$. We extend these possibilities to three toy models, accounting also for the first family masses, and calculate their predictions for various nucleon decay rates. We also analyse how the requirement of gauge coupling unification constrains the masses of potentially light relic states testable at colliders.

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arXiv2022

BBN Photodisintegration Constraints on Gravitationally Produced Vector Bosons

Gravitational production of massive particles due to cosmic expansion can be significant during the inflationary and reheating period of the Universe. If the particle also has non-gravitational interactions that do not significantly affect its production, numerous observational probes open up, including cosmological probes. In this work, we focus on the gravitational production of light vector bosons that couple feebly to the Standard Model (SM) particles. Due to the very feeble coupling, the light vector bosons never reach thermal equilibrium, and if the Hubble scale at the end of inflation is above $10^8$ GeV, the gravitational production can overwhelm the thermal production via the freeze-in mechanism by many orders of magnitude. As a result, much stronger constraints from the Big Bang Nucleosynthesis (BBN) can be placed on the lifetime and mass of the vector bosons compared to the scenario where only thermal production is considered. As an example, we study the sub-GeV scale dark photons, which couple to the SM only through kinetic mixing, and derive constraints on the mass and kinetic mixing parameter of the dark photon from the photodisintegration effects on the light element abundances relevant at the end of the BBN when the cosmic age was around $10^4$ s.

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arXiv2022

Light thermal relics enabled by a second Higgs

Sub-GeV thermal relic dark matter typically requires the existence of a light mediator particle. We introduce the light two-Higgs-doublet portal, illustrated by a minimal UV-complete model for sub-GeV dark matter with kinematically forbidden annihilations into leptons. All new physics states in this scenario lie at or below the electroweak scale, affecting Higgs physics, the muon anomalous magnetic moment and potentially neutrino masses. Observation of radiative dark matter annihilation by future MeV gamma-ray telescopes would be key to unambiguously identify the scenario.

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arXiv2022

A flavor-inspired radiative neutrino mass model

One of the most important discoveries in particle physics is the observation of nonzero neutrino masses, which dictates that the Standard Model (SM) is incomplete. Moreover, several pieces of evidence of lepton flavor universality violation (LFUV), gathered in the last few years, hint toward physics beyond the SM. TeV-scale scalar leptoquarks are the leading candidates for explaining these flavor anomalies in semileptonic charged and neutral current B-decays, the muon, and the electron magnetic dipole moments that can also participate in neutrino mass generation. In this work, we hypothesize that neutrino masses and LFUV have a common new physics origin and propose a new two--loop neutrino mass model that has the potential to resolve some of these flavor anomalies via leptoquarks and offers rich phenomenology. After deriving the neutrino mass formula for this newly-proposed model, we perform a detailed numerical analysis focusing on neutrino and charged lepton flavor violation phenomenology, where the latter provides stringent constraints on the Yukawa couplings and leptoquark masses. Finally, present and future bounds on the model's parameter space are scrutinized with exemplified benchmark scenarios.

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arXiv2022

Marriage between neutrino mass and flavor anomalies

Experimental hints for lepton flavor universality violation in beauty-quark decay both in neutral- and charged-current transitions require an extension of the Standard Model for which scalar leptoquarks (LQs) are the prime candidates. Besides, these same LQs can resolve the long-standing tension in the muon and the recently reported deviation in the electron $g-2$ anomalies. These tantalizing flavor anomalies have discrepancies in the range of $2.5σ-4.2σ$, indicating that the Standard Model of particle physics may finally be cracking. In this Letter, we propose a resolution to all these anomalies within a unified framework that sheds light on the origin of neutrino mass. In this model, the LQs that address flavor anomalies run through the loops and generate neutrino mass at the two-loop order while satisfying all constraints from collider searches, including those from flavor physics.

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arXiv2022

Leptoquark-vectorlike quark model for the CDF $m_W$, $(g-2)_μ$, $R_{K^{(\ast)}}$ anomalies and neutrino mass

Very recently, a substantial $7σ$ deviation of the $W$-boson mass from the Standard Model (SM) prediction has been reported by the CDF collaboration. Furthermore, the Muon g-2 Experiment recently confirmed the longstanding tension in $(g-2)_μ$. Besides, the updated result from the LHCb collaboration found evidence for the breaking of lepton universality in beauty-quark decays, which shows a $3.1σ$ discrepancy and is consistent with their previous measurements. Motivated by several of these drawbacks of the SM, in this work, we propose a model consisting of two scalar leptoquarks and a vectorlike quark to simultaneously address the $W$-boson mass shift, the $(g-2)_μ$, and anomalies in the neutral current transitions of the $B$-meson decays. The proposed model also sheds light on the origin of neutrino mass and can be fully tested at the future colliders.

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arXiv2022

$W$ boson mass shift and muon magnetic moment in the Zee model

The CDF collaboration at Fermilab has recently reported a new precision measurement of the $W$~boson mass showing a substantial $7σ$ deviation from the Standard Model prediction. Moreover, Fermilab has recently confirmed the longstanding tension in the $(g-2)_μ$ measurement. We propose a unified solution to these deviations within the simplest radiative neutrino mass model: the Zee model. Our analysis establishes non-trivial links between the origin of neutrino mass, the $(g-2)_μ$ anomaly, and the $W$~boson mass shift while being consistent with lepton flavor violation and all other experimental constraints. We find that the mass spectrum of the physical scalars must be hierarchical to be consistent with the $W$~boson mass shift; remarkably, this is also the key to resolving the $(g-2)_μ$ tension. Furthermore, this mass splitting offers a unique same-sign dimuon signal through which our model can be tested at the LHC.

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arXiv2022

Implications of the Zero 1-3 Flavour Mixing Hypothesis: Predictions for $θ_{23}^\mathrm{PMNS}$ and $δ^\mathrm{PMNS}$

We revisit mixing sum rule relations in the lepton and quark sectors under the assumption that the 1-3 elements of the flavour mixing matrices ($V^u_L,V^d_L,V^e_L,V^ν_L$) are zero in the flavour basis. We consider the exact relations resulting from the validity of this "zero 1-3 flavour mixing hypothesis" and analyse their implications based on the current experimental data, including effects from RG running. In particular, we analyse how the existing precise measurement of $θ_{13}^\mathrm{PMNS}$ allows to derive predictions for $θ_{23}^\mathrm{PMNS}$ in models with constrained $θ_{12}^\mathrm{e}$. As examples, we calculate the predictions for $θ_{23}^\mathrm{PMNS}$ which arise in classes of Pati-Salam models and SU(5) GUTs that relate $θ_{12}^\mathrm{e}$ to $θ_{12}^\mathrm{d}$. We also derive a novel "lepton phase sum rule", valid under the additional assumption of small charged lepton mixing contributions. We furthermore point out that, in the context of GUT flavour models, the quark and lepton CP violating phases $δ^\mathrm{CKM}$ and $δ^\mathrm{PMNS}$ can both be predicted from a single imaginary element in the mass matrices.

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arXiv2022

Gravitational Wave Imprints of Left-Right Symmetric Model with Minimal Higgs Sector

We study the gravitational wave imprints of left-right symmetric model equipped with universal seesaw mechanism allowing for the natural generation of hierarchical masses of the Standard Model fermions. The scalar sector of this model is the minimal one, consisting of only two Higgs doublets. Following the construction of the full thermal potential for this model, we perform a scan of the entire parameter space and identify the region in which the cosmic phase transition associated with the left-right symmetry breaking gives gravitational wave signals detectable by a variety of planned space-based interferometers. Then we also discuss the relevant collider implications of this beyond the Standard Model scenario.

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arXiv2022

Viable quark-lepton Yukawa ratios and nucleon decay predictions in $SU(5)$ GUTs with type-II seesaw

We investigate the viability of predictive schemes for quark-lepton Yukawa ratios and nucleon decay in non-supersymmetric SU(5) Grand Unified Theories (GUTs) where neutrino masses are generated by a type II seesaw mechanism. The scalar sector of the considered scenario contains 5-, 24- and 45-dimensional representations plus a 15-dimensional representation for realising the type II seesaw. Predictions for the ratios of the quark and lepton Yukawa couplings emerge when the relevant entries of the Yukawa matrices are generated from single joint GUT operators (i.e. under the condition of "single operator dominance"). Focusing on the 2nd and 3rd family and hierarchical Yukawa matrices, we show that only two sets of predictions, $\frac{y_τ}{y_b}=\frac{3}{2}$, $\frac{y_μ}{y_s}=\frac{9}{2}$ and $\frac{y_τ}{y_b}=2$, $\frac{y_μ}{y_s}=6$ are viable. To further investigate both options, we extend the minimal scenarios to two "toy models", including the 1st family of charged fermions, and calculate the models' predictions, e.g. for the nucleon decay rates and for the masses of the light relics that are potentially within reach of colliders.

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arXiv2022

Dark Matter and $(g-2)_{μ,e}$ in radiative Dirac neutrino mass models

The origin of neutrino mass is a mystery, so is its nature, namely, whether neutrinos are Dirac or Majorana particles. On top of that, hints of large deviations of the muon and the electron anomalous magnetic moments (AMMs) are strong evidence for physics beyond the Standard Model. In this work, piecing these puzzles together, we propose a class of radiative Dirac neutrino mass models to reconcile $(g-2)_{μ,e}$ anomalies with neutrino oscillation data. In this framework, a common set of new physics (NP) states run through the loops that generate non-zero neutrino mass and, due to chiral enhancement, provide substantial NP contributions to lepton AMMs. In addition, one of the three models studied in this work offers a Dark Matter candidate automatically stabilized by the residual symmetry, whose phenomenology is non-trivially connected to the other two puzzles mentioned above. Finally, our detailed numerical analysis reveals a successful resolution to these mysteries while being consistent with all colliders and cosmological constraints.

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arXiv2022

Minimal realization of light thermal Dark Matter

We propose a minimal UV-complete model for kinematically forbidden Dark Matter (DM) leading to a sub-GeV thermal relic. Our crucial realization is that the two-Higgs-doublet model can provide a light mediator through which the DM can annihilate into SM leptons, avoiding indirect detection constraints. The DM mass is predicted to be very close to the mass of the leptons, which can potentially be identified from DM annihilation into gamma-rays. Due to sizable couplings to muons in reproducing the DM relic abundance, this framework naturally favors a resolution to the $(g-2)_μ$ anomaly. Furthermore, by embedding this setup to the Zee model, we show that the phenomenon of neutrino oscillations is inherently connected to the observed relic abundance of DM. All new physics involved in our framework lies at or below the electroweak scale, making it testable at upcoming colliders, beam-dump experiments, and future sub-GeV gamma-ray telescopes.

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arXiv2021

Low-scale Resonant Leptogenesis in $SU(5)$ GUT with $\mathcal{T}_{13}$ Family Symmetry

We investigate low-scale resonant leptogenesis in an $SU(5) \times \mathcal{T}_{13}$ model where a single high energy phase in the complex Tribimaximal seesaw mixing produces the yet-to-be-observed low energy Dirac and Majorana ${CP}$ phases. A fourth right-handed neutrino, required to generate viable light neutrino masses within this scenario, also turns out to be necessary for successful resonant leptogenesis where $CP$ asymmetry is produced by the same high energy phase. We derive a lower bound on the right-handed neutrino mass spectrum in the $\mathrm{GeV}$ range, where part of the parameter space, although in tension with Big Bang Nucleosynthesis constrains, can be probed in planned high intensity experiments like DUNE. We also find the existence of a curious upper bound ($\text{TeV}$-scale) on the right-handed neutrino mass spectrum in majority of the parameter space due to significant washout of the resonant asymmetry by lighter right-handed neutrinos. While in most of the parameter space of our model classical Boltzmann equations are sufficient, when right-handed neutrino masses are below the electroweak sphalerons freeze-out temperature we resort to the more general density matrix formalism to take into account decays and oscillations of the right-handed neutrinos as well as their helicities which are relevant when they are relativistic.

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arXiv2021

Non-Abelian Vector Dark Matter and Lepton $g-2$

The mystery of dark matter remains an unsettled problem of particle physics. On top of that, experiments show a persistent contention of the muon anomalous magnetic moment (AMM) relative to the Standard Model (SM) prediction. In this work, we consider the possibility of extending the SM with a non-Abelian gauge symmetry $SU(2)_X$, under which SM leptons transform non-trivially. SM leptons receive corrections to their AMMs of right order via one-loop processes mediated by beyond SM (BSM) fermions required to cancel anomalies, and BSM gauge bosons that play the role of dark matter. We show that simultaneous explanation of the the muon AMM along with reproducing correct relic abundance allows rather a narrow range of 0.5 - 2 TeV dark matter mass, consistent with current experimental constraints. However, a concurrent description that also includes electron AMM is challenging in this set-up.

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arXiv2021

A Parameter Space Exploration of the Minimal SU(5) Unification

We present phenomenological study of the most minimal realistic $SU(5)$ model that owns its predictivity solely to the gauge symmetry and the representational content. The model is built entirely out of the fields residing in the first five lowest dimensional representations that transform non-trivially under the $SU(5)$ gauge group. It has eighteen real parameters and fourteen phases, all in all, to address experimental observables of the Standard Model fermions and accomplishes that via simultaneous use of three different mass generation mechanisms. Furthermore, it inextricably links the origin of the neutrino mass to the experimentally observed difference between the down-type quark and charged lepton masses. The main predictions of the model are that $(i)$ the neutrinos are Majorana particles, $(ii)$ one neutrino is massless, $(iii)$ the neutrinos have normal mass ordering, and $(iv)$ there are four new scalar multiplets at or below a $120$ TeV mass scale. A one-loop analysis demonstrates that an improvement of the current $p \rightarrow π^0 e^+$ partial lifetime limit by a factor of $2$, $15$, and $96$ would require these four scalar multiplets to reside at or below the $100$ TeV, $10$ TeV, and $1$ TeV mass scales, respectively.

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arXiv2020

Flavor Hierarchies from Clockwork in SO(10) GUT

The clockwork mechanism, which can naturally explain the origin of small numbers, is implemented in $SO(10)$ grand unified theories to address the origin of hierarchies in fermion masses and mixings. We show that a minimal Yukawa sector involving a $10_H$ and $\overline{126}_H$ of Higgs bosons, extended with two clockwork chains consisting of $16+\overline{16}$ vector-like fermions, can explain the hierarchical patterns with all the Yukawa couplings being of order one. Emergence of a realistic mass spectrum does not require any symmetry that distinguishes the three generations. We develop clockwork-extended $SO(10)$ GUTs both in the context of SUSY and non-SUSY frameworks. Implementation of the mechanism in non-SUSY scenario assumes a Peccei-Quinn symmetry realized at an intermediate scale, with the clockwork sector carrying non-trivial charges, which solves the strong CP problem and provides axion as a dark matter candidate.

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arXiv2020

$μ\to e γ$ selecting scalar leptoquark solutions for the $(g-2)_{e,μ}$ puzzles

We investigate all potentially viable scenarios that can produce the chiral enhancement required to simultaneously explain the $(g-2)_{e}$ and $(g-2)_μ$ data with either a single scalar leptoquark or a pair of scalar leptoquarks. We provide classification of these scenarios in terms of their ability to satisfy the existing limits on the branching ratio for the $μ\to e γ$ process. The simultaneous explanation of the $(g-2)_{e,μ}$ discrepancies, coupled with the current experimental data, implies that the $(g-2)_e$ loops are exclusively due to the charm quark propagation whereas the $(g-2)_μ$ loops are due to the top quark propagation. The scenarios where the $(g-2)_e$ loops are due to the top (bottom) quark propagation are, at best, approximately nine (three) orders of magnitude away from the experimental limit on the $μ\to e γ$ branching ratio. All in all, there are only three particular scenarios that can pass the $μ\to e γ$ test and simultaneously create large enough impact on the $(g-2)_{e,μ}$ discrepancies when the new physics is based on the Standard Model fermion content. These are the $S_1$, $R_2$, and $S_1 \& S_3$ scenarios, where the first two are already known to be phenomenologically viable candidates with respect to all other flavor and collider data constraints. We show that the third scenario, where the right-chiral couplings to charged leptons are due to $S_1$, the left-chiral couplings to charged leptons are due to $S_3$, and the two leptoquarks mix through the Standard Model Higgs field, cannot address the $(g-2)_{e}$ and $(g-2)_μ$ discrepancies at the $1σ$ level due to an interplay between $K_L^0 \to e^\pm μ^\mp$, $Z \to e^+ e^-$, and $Z \to μ^+ μ^-$ data despite the ability of that scenario to avoid the $μ\to e γ$ limit.

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arXiv2020

Dark matter assisted lepton anomalous magnetic moments and neutrino masses

We propose a framework that addresses the origin of neutrino mass, explains the observed discrepancies in the electron and the muon anomalous magnetic moments (AMMs) data and incorporates the dark matter (DM) relic abundance. Both the neutrino mass and the lepton AMMs are generated at one-loop level mediated by a common set of beyond the Standard Model (SM) states. In this class of models, the SM is extended with vector-like charged fermion and scalar multiplets, all odd under an imposed $\mathcal{Z}_2$ symmetry, which stabilizes the fermionic or scalar DM candidate residing in one of them. Two scalar multiplets appear in the AMM loops, thus allowing for different signs of their contributions, in agreement with the observed discrepancies which are of opposite sign for electron and muon. The vector-like fermions give rise to large new physics contributions to the lepton AMMs via chirally enhanced terms that are proportional to their mass. To demonstrate the viability of this framework, we perform a detailed study of a particular model for which a fit to the neutrino masses and mixing together with lepton AMMs are provided. Furthermore, DM phenomenology and collider signatures are explored.

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