arXiv subjects

M. Ahmadi

Publications and source records attributed to M. Ahmadi.

8 recordsLinked to original sources

arXiv2026

Rare Exclusive Top Decays t , b M and Vector-Meson Helicity

We present a leading-order study of the rare exclusive decays $t \to bM$, where $M$ denotes a pseudoscalar or vector meson. Starting from the tree-level transition $t \to bW^* \to b(q\bar q')$ and using a factorization approximation, we derive compact expressions for the amplitudes and partial widths in which the long-distance QCD dynamics is encoded in the meson decay constants $f_M$. We provide numerical branching-ratio predictions for representative modes such as $t \to b\pi^+$, $t \to bK^+$, $t \to bD_s^{(*)}$, and $t \to bB_c^{(*)}$, finding $\mathrm{Br} \sim 10^{-8}$--$10^{-7}$. For vector final states, we derive helicity amplitudes and Standard-Model polarization fractions, and we outline the sensitivity of these observables to anomalous chiral $tWb$ couplings in the limit $m_b \to 0$. Our results provide an updated Standard Model (SM) baseline for rare exclusive top-quark decays and a starting point for EFT interpretations of non-standard charged-current interactions.

hep-ph↗

arXiv2026

Phenomenology of Hypothetical Single-Top Hadronic States

We present a comprehensive theoretical study of the masses of possible baryonic and mesonic configurations containing a single top quark. Our analysis includes the baryons $\Lambda_t$, $\Xi_t$, $\Sigma_t$, $\Xi'_t$, $\Omega_t$, $\Omega_{tcc}$, and $\Omega_{tbb}$, together with the pseudoscalar and vector mesons $T_{t\bar n}^{\mathrm{Ps}}$, $T_{t\bar n}^{\mathrm{V}}$, $T_{t\bar s}^{\mathrm{Ps}}$, $T_{t\bar s}^{\mathrm{V}}$, $T_{t\bar c}^{\mathrm{Ps}}$, $T_{t\bar c}^{\mathrm{V}}$, $T_{t\bar b}^{\mathrm{Ps}}$, and $T_{t\bar b}^{\mathrm{V}}$. Motivated in part by recent experimental indications of a pseudoscalar enhancement near the $t\bar t$ threshold reported by the CMS and ATLAS collaborations, this study is carried out within the framework of two-point QCD sum rules to determine the corresponding ground-state masses by including perturbative contributions and nonperturbative condensates up to dimension eight. For several channels, including the $\Lambda_t$, $\Xi_t$, $\Sigma_t$, $T_{t\bar b}^{\mathrm{Ps}}$, and $T_{t\bar b}^{\mathrm{V}}$ states, the extracted central masses lie slightly below the corresponding sums of constituent quark masses, which may indicate nontrivial binding dynamics or near-threshold multiquark configurations within the uncertainties of the method. Moreover, when the full theoretical uncertainties are taken into account in a conservative manner, a larger subset of the investigated states exhibits a consistent tendency toward weak binding behavior, suggesting that the possibility of loosely bound configurations cannot be excluded for most of the considered baryonic and mesonic channels. These results provide useful first-principles theoretical benchmarks for possible top-containing hadronic systems, which may support future searches at the LHC, along with sensitivity analyses for next-generation facilities such as the FCC.

hep-ph↗

arXiv2025

Study of the semileptonic decay $\Lambda \to p\,\ell\,\bar{\nu}_{\ell}$ in QCD

We conduct a comprehensive study of the semileptonic decay process $\Lambda \to p\,\ell\,\bar{\nu}_{\ell}$, focusing on the determination of all six vector and axial-vector form factors that govern the low-energy hadronic matrix elements of the underlying theory. These invariant form factors constitute the essential inputs for describing the decay, and their dependence on the momentum transfer $q^{2}$ is analyzed across the entire physical kinematic region. To parametrize the $q^{2}$dependence, we adopt both the $z$-expansion formalism and a polynomial fitting approach. Utilizing parametrizations, we compute the exclusive decay widths for both the electron and muon channels, and subsequently extract the corresponding branching ratios. Furthermore, we evaluate the ratio of decay widths between the muon and electron channels defined as $R^{\mu e} \equiv \frac{\Gamma(\Lambda \to p\,\mu\,\bar{\nu}_{\mu})}{\Gamma(\Lambda \to p\,e\,\bar{\nu}_{e})}$ obtaining $R^{\mu e} = 0.196^{+0.009}_{-0.012}$ from the polynomial fit and $R^{\mu e} = 0.174^{+0.002}_{-0.005}$ from the $z$ expansion. While both ratios are compatible with previously reported values in the literature, the result from the $z$ expansion exhibits particularly strong agreement with the averages reported by the Particle Data Group.

hep-ph↗

arXiv2025

QCD sum rule study of the tensor $\Delta^0\Delta^0$ dibaryon state

We study the exotic $\Delta^{0}\Delta^{0}$ dibaryon state with quantum numbers $J^{P} = 2^{+}$ within the framework of QCD sum rules. A tensor interpolating current is constructed to determine the mass and residue of this state, including explicit calculations of contributions from quark, gluon, and mixed vacuum condensates up to dimension nine. The numerical analysis yields a mass of $m_{\Delta^{0}\Delta^{0}} = 2426^{+101}_{-108}~\mathrm{MeV}$ and a residue of $f_{\Delta^{0}\Delta^{0}} = \left( 2.30^{+0.49}_{-0.45} \right) \times 10^{-4}~\mathrm{GeV}^{8}$. The predicted mass lies about $38~\mathrm{MeV}$ below the $2m_{\Delta^{0}}$ threshold, which indicates the possible existence of a bound molecular configuration. Consequently, the tensor $\Delta^{0}\Delta^{0}$ dibaryon may serve as a promising candidate for future experimental searches.

hep-ph↗

arXiv2020

A Novel Regularization Scheme for Nucleon-Nucleon Lattice Simulations with Effective Field Theory

We propose a new regularization scheme to study the bound state of two-nucleon systems in Lattice Effective Field Theory. Inspired by continuum EFT calculation, we study an exponential regulator acting on the leading-order (LO) and next-to-leading order (NLO) interactions, consisting of local contact terms. By fitting the low-energy coefficients (LECs) to deuteron binding energy and the asymptotic normalization coefficient (ANC) on a lattice simulation, we extract the effective range expansion (ERE) parameters in the $^3S_1$ channel to order $p^2$. We explore the impact of different powers of the regulator on the extracted ERE parameters for the lattice spacing $a=1.97$ fm. Moreover, we investigate how the implementation of the regularization scheme improves the predicted ERE parameters on the lattice spacing in the range of $1.4 \le a \le 2.6$ fm. Our numerical analysis indicates that for lattice spacing greater than $2$ fm, the predicted observables are very close to the experimental data.

nucl-th↗

arXiv2014

Semi-definite programming and functional inequalities for Distributed Parameter Systems

We study one-dimensional integral inequalities, with quadratic integrands, on bounded domains. Conditions for these inequalities to hold are formulated in terms of function matrix inequalities which must hold in the domain of integration. For the case of polynomial function matrices, sufficient conditions for positivity of the matrix inequality and, therefore, for the integral inequalities are cast as semi-definite programs. The inequalities are used to study stability of linear partial differential equations.

math.OC↗

arXiv2012

Finite 2-groups of Class 2 with Specific Automorphism Group

In this paper we classify all finite 2-groups of class 2 for which every automorphism of order 2 leaving the Frattini subgroup elementwise fixed is inner. We prove that every such group G is isomorphic to Q(n; r) = for some positive integers r; n such that 2 < 2r <= n; and every automorphism of Q(n; r) of order 2 leaving the Frattini subgroup elementwise fixed is inner.

math.GR↗

arXiv2004

Mean field theory for a balanced hypercolumn model of orientation selectivity in primary visual cortex

We present a complete mean field theory for a balanced state of a simple model of an orientation hypercolumn. The theory is complemented by a description of a numerical procedure for solving the mean-field equations quantitatively. With our treatment, we can determine self-consistently both the firing rates and the firing correlations, without being restricted to specific neuron models. Here, we solve the analytically derived mean-field equations numerically for integrate-and-fire neurons. Several known key properties of orientation selective cortical neurons emerge naturally from the description: Irregular firing with statistics close to -- but not restricted to -- Poisson statistics; an almost linear gain function (firing frequency as a function of stimulus contrast) of the neurons within the network; and a contrast-invariant tuning width of the neuronal firing. We find that the irregularity in firing depends sensitively on synaptic strengths. If Fano factors are bigger than 1, then they are so for all stimulus orientations that elicit firing. We also find that the tuning of the noise in the input current is the same as the tuning of the external input, while that for the mean input current depends on both the external input and the intracortical connectivity.

q-bio.NC↗