arXiv subjects

Shahin Atashbar Tehrani

Publications and source records attributed to Shahin Atashbar Tehrani.

13 recordsLinked to original sources

arXiv2026

Laplace-Space Analysis of $xF_3$ Including Nuclear Effects and Gegenbauer-Polynomial Parton Distributions

We present a next-to-leading order (NLO) and next-to-next-to-leading order (NNLO) QCD analysis of the non-singlet structure function $xF_3$, utilizing a Gegenbauer-polynomial representation for the input parton distribution functions (PDFs). The main objective is to assess how this flexible parameterization improves the extraction of valence quark distributions in the presence of nuclear effects. To this end, we incorporate nuclear modification factors into the input PDFs for heavy nuclear targets and solve the DGLAP evolution equations analytically in Laplace space. The structure function in Bjorken-$x$ space is then reconstructed using a Jacobi polynomial expansion. This combined framework enables a systematic investigation of the $xF_3$ data from the CCFR, NuTeV, and CHORUS experiments at both NLO and NNLO accuracies. We further examine the sensitivity of the extracted distributions to nuclear corrections and discuss their implications for the Gross--Llewellyn Smith, Bjorken, and Adler sum rules. Our results demonstrate that the Gegenbauer-polynomial PDF formalism provides a flexible and efficient framework for describing nuclear $xF_3$ data, yielding improved phenomenological consistency across a wide kinematic range.

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arXiv2026

Machine Learning for Predicting the Proton Structure Function $F_2^P$ in QCD

We present a comparative study of four supervised machine learning regression algorithms -- Support Vector Regression (SVR), Gradient Boosting Regression (GBR), Gaussian Process Regression (GPR), and Multilayer Perceptron (MLP) -- for predicting the proton structure function $F_2^p(x, Q^2)$ using high-precision BCDMS experimental data. Unlike conventional methods that solve the DGLAP evolution equations, our data-driven framework directly captures the complex nonlinear dynamics of partonic structure. To ensure statistical robustness, we employ $k$-fold cross-validation and perform thorough hyperparameter optimization. Our results show that the MLP and GPR models achieve superior predictive accuracy. In particular, MLP exhibits the highest sensitivity to nonlinear gradients, while SVR proves most stable against experimental uncertainties. The close convergence of training and validation metrics confirms that the models learn the underlying QCD physics without overfitting to statistical fluctuations. This work highlights the potential of ML-based regression as a complementary tool for structure function analysis and kinematic extrapolation in high-energy physics.

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arXiv2026

New analysis for Nucleon Form Factors from GPDs

Generalized Parton Distributions (GPDs) provide a comprehensive framework for describing the three-dimensional structure of the nucleon. Extracting GPDs from experimental data requires flexible and physically motivated ansatz. In this study, we introduce a new ansatz, AMA25, designed to address the limitations of the previous model, GSAMA24 (Phys. Rev. C 111 (2025) 2, 025203). We conduct a fast and efficient comparison by fitting AMA25 models and other relevant data using the iMinuit optimization package within a Jupyter Notebook environment. The AMA25 ansatz demonstrates superior fit quality, achieving a reduced $χ^2$, while better satisfying theoretical constraints. Additionally, AMA25 exhibits enhanced stability when extrapolated to the exclusive region. Our analysis highlights the power of modern computational tools for rapid model validation and underscores the importance of innovative ansatz in advancing nucleon structure studies.

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arXiv2025

Nucleon Form Factors from GPDs

In this study, we introduce a novel ansatz for Generalized Parton Distributions (GPDs), named GSAMA24. This ansatz aims to provide a more accurate and comprehensive description of the internal structure of hadrons by incorporating advanced parameterizations and fitting techniques. We compare the performance of the GSAMA24 ansatz with three established models: the Extended Regge (ER), Modified Gaussian (MG), and M-HS22 ansatz. The GSAMA24 ansatz is designed to address limitations observed in previous models by offering improved flexibility in the ( $t$ )-dependence and skewness parameter $ξ$. Our analysis involves fitting the GSAMA24 ansatz to experimental form factor data and evaluating its predictive power against the ER, MG, and M-HS22 models. The comparison is based on key metrics such as the accuracy of form factor predictions, the consistency with known GPD properties, and the computational efficiency of the fitting process. Results indicate that the GSAMA24 ansatz provides a superior fit to the experimental data, particularly in the high momentum transfer region, where it outperforms the ER and MG models. Additionally, the GSAMA24 ansatz demonstrates better agreement with the theoretical expectations of GPD behavior compared to the M-HS22 model. These findings suggest that the GSAMA24 ansatz is a promising tool for future studies of hadronic structure and could significantly enhance our understanding of the spatial and momentum distributions of quarks and gluons within hadrons.

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arXiv2024

AAK24: Global QCD analysis on polarized parton distribution in the presence of $A_2$ asymmetry measurements

This article introduces {\tt AAK24}, a Next-to-Leading Order (NLO) QCD analysis of polarized data from both polarized Deep Inelastic Scattering (DIS) and Semi-Inclusive Deep Inelastic Scattering (SIDIS) experiments on the nucleon. The {\tt AAK24} QCD analysis incorporates SU(2) and SU(3) symmetry breaking, specifically $δ\bar{u} \neq δ\bar{d} \neq δ\bar{s}$, while assuming $δ\bar{s}$ and $δs$ are equal. Emphasizing the significance of the semi-inclusive data, the study explores the determination of polarized sea quark distributions. Recent experimental data from {\tt JLAB17}, {\tt COMPASS16}, and {\tt COMPASS17}, including the $A_2$ asymmetry measurements along with SIDIS observables, are thoroughly examined for their impact on the central values of polarized PDFs, their uncertainties, and overall fit quality. Additionally, we include the nonperturbative target mass corrections (TMC) as well as higher-twist terms (HT) which are particularly important. In this work, the uncertainties are quantified using the standard Hessian method. The main results and findings of the {\tt AAK24} QCD analysis show overall good agreement with the analyzed experimental data, aligning well with other polarized PDF determinations, particularly {\tt DSSV14}, {\tt LSS10}, {\tt JAM17}, and {\tt AKS14}, all considering SU(2) and SU(3) symmetry breaking.

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arXiv2024

QCD analysis of $xF_3$ structure functions in deep-inelastic scattering: Mellin transform by Gegenbauer polynomial up to N$^3$LO approximation

This paper provides a thorough examination of the $xF_3$ structure functions in deep-inelastic scattering through a comprehensive QCD analysis. Our approach harnesses sophisticated mathematical techniques, namely the Mellin transform combined with Gegenbauer polynomials. We have employed the Jacobi polynomials approach for analysis, conducting investigations at three levels of precision: Next-to-Leading Order (NLO), Next-to-Next-to-Leading Order (N$^2$LO), and Next-Next-Next-to-Leading Order (N$^3$LO). We have performed a comparison of our sets of valence-quark parton distribution functions with those of recent research groups, specifically CT18 and MSHT20 at NLO and N$^2$LO, and MSTH23 at N$^3$LO, which are concurrent with our current analysis. The combination of Mellin transforms with Gegenbauer polynomials proves to be a powerful tool for investigating the $xF_3$ structure functions in deep-inelastic scattering and the results obtained from our analysis demonstrate a favorable alignment with experimental data.

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arXiv2024

Self-Similar Properties of the Proton Structure at Low x within the \textbf{xFitter} framework

The structure of the proton exhibits Fractal behavior at low \textit{x}, where \textit{x} is the fraction of the proton's momentum carried by the interacting partons. This Fractal behavior is characterized by self-similar properties at different scales and can be quantified using the concept of Fractal dimension. An investigation into the Fractal properties of the proton structure at low \textit{x} is critical for understanding the fundamental properties of the strong force and developing a more comprehensive understanding of the hadron structure. Fractals, characterized by self-similar patterns across scales, demonstrate a direct correlation between their Fractal dimension and entropy, where higher Fractal dimensions correspond to increased informational content. Furthermore, it is essential for designing high-energy physics experiments and developing more accurate models of subatomic particle interactions. This paper has a fresh look at the self-similar properties of the proton structure at low \textit{x}. Our study involves the use of the \textbf{xFitter} framework to parameterize the proton structure functions with a Fractal formalism at low \textit{x}. We also examine how the inclusion of new data affects the results of our analysis.

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arXiv2023

Proton structure functions at low $x$: the Fractal distributions

In this paper, we present the extraction of the Parton Distribution Functions (PDFs) at small momentum fractions x and at the next-to-leading order (NLO) accuracy in perturbative QCD. We show that the "sea quark distribution functions" have "Fractal" or self-similar behaviour with fixed exponent at $x<0.01$. To this end, a simple parametrization for the Parton Distribution Functions and especially for the sea quarks PDFs based on the "Fractal" approach is considered. The small $x$ experimental datasets on electron-proton ($e^-p$) and positron-proton ($e^+p$) in DIS processes at HERA for the range of $1.5< Q^2 < 650$ ($GeV^2$) and $x<0.01$ are included in this analysis. The estimations of the uncertainty in the present analysis are carried out using the standard "Hessian" method. In total, considering the overall value of $χ^2/{\rm dof}$ and theory/data comparisons, the results indicate nice agreements between the experimental datasets and the theory predictions at small momentum fractions $x$. Finally, we present detailed comparisons between predictions for the relevant small-$x$ observables obtained with various recent models of proton PDFs available in literature.

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arXiv2023

Effect of symmetry breaking of polarized light sea quarks on the nucleon and nuclear structure functions, and sum rules

In this study, we performed calculations and analyses of the structure functions of polarized nucleons and light nuclei, specifically $^3$He and $^3$H, using second-order Feynman diagrams. Our investigation focused on two main aspects: Firstly, we examined the symmetry properties of polarized light sea quarks. Secondly, we conducted a detailed investigation into the impacts of symmetry breaking on the structure functions of both nucleons and nuclei. To achieve this, we utilized the existing polarized Parton Distribution Functions (polarized PDFs) available in the literature. These PDFs were used to calculate and compare the polarized structure functions $g_1$ and $g_2$ of the nuclei. Additionally, we examined and analyzed the Bjorken and Efremov-Leader-Teryaev sum rules by utilizing the moments of the polarized structure functions. The Lorentz color force components, namely $F_E^{y,n}$ and $F_B^{y,n}$, are determined using the twist-2, twist-3, and twist-4 matrix elements. When symmetry breaking is applied, it is observed that they have similar magnitudes but opposite signs. Our theoretical predictions for the polarized structure functions of nucleons and light nuclei, taking into account the symmetry breaking of light sea quarks, exhibit better agreement with experimental data.

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arXiv2023

Recent data analysis to revisit the spin structure function of nucleon in Laplace space

Considering a fixed-flavor number scheme and based on laplace transdormation, we perform a leading-order and next-to-leading-order QCD analysis which are including world data on polarized structure functions $g_1$ and $g_2$. During our analysis, taking the DGLAP evolution, we employ the Jacobi polynomials expansion technique. In our recent analysis we utilize the recent available data and consequently include more data than what we did in our previous analysis. we obtain good agreements between our results for the polarized parton densities and nucleon structure functions with all available experimental data and some common parametrization models.

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arXiv2022

The effect of light sea quark symmetry breaking on polarized nucleus and sum rules

The polarized structure functions of $^3He$ and $^3H$ nuclei are calculated in NLO approximation, considering and disregarding the light sea quark symmetry breaking. We employ the polarized structure function of the nucleons within the nucleus extracted from our two recent analysis on polarized DIS data and on polarized DIS+SIDIS data. Since the data of the second analysis cover a bigger range of Bjorken variable, both SU(2) and SU(3) symmetry breaking is considered within the analysis. Then we calculate and compare the polarized structures of nuclei extracted from both scenarios. Also the Bjorken and ELT sum rules are calculated using the moments of structure functions. The results are compared with experimental data and the differences are investigated.

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arXiv2022

Impacts of EMC effects on the D meson modification factor in equilibrating QGP

In this article we employ the nuclear EMC effect to extract the parton distribution functions (PDFs) inside the Lead (Pb) and Gold (Au) nuclei. Extracted PDFs are utilized to obtain the transverse momentum dependent (TMD) ones, using the computing codes like Pythia 8 or MCFM-10. Through this procedure TMDPDFs for charm and bottom quarks in Au at $\sqrt{s_{NN}}=200\;GeV$, Pb at $\sqrt{s_{NN}}=2.76\;TeV$ and $\sqrt{s_{NN}}=5.02\;TeV$ are calculated. To evaluate the validity of results and investigate the influence of nuclear EMC effect, the numerated TMDs are used as input to estimate heavy quark modification factor $R_{AA}$ at transverse plane $P_T$. This observable is calculated through numerical solution of the Fokker-Planck equation. For this purpose we need to extract the drag and diffusion coefficients, using the hard thermal loop correction. It is done in the frame work of the relativistic hydrodynamics up to the third order approximation of gradient expansion. The results are compared with same solutions when the input PFDs are considered inside the unbounded protons where the nuclear effect is not included. The comparison indicates a significant improvement of computed $R_{AA}$ with available experimental data when the EMC effect is considered.

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arXiv2022

Nucleon spin structure functions, considering target mass correction and higher twist effects at the NNLO accuracy and their transverse momentum dependence

Using recent and updated world data on polarized structure functions $g_1$ and $g_2$ we perform QCD analysis at the next-next-to-leading-order (NNLO) accuracy. We include also target mass correction and higher twist effect to get more precise results in our fitting procedure. To confirm the validity of our fitting results several sum rules are examined and we do a comparison for them with results from other models. In our analysis we employ Jacobi polynomials approach to obtain analytical solutions of the DGLAP evolution equations for parton distribution functions (PDFs). Using the extracted PDFs from our data analysis as input we also compute the $x$- and $\mathbf{p}_{T}$-dependence of some transverse momentum dependence (TMD) PDFs in polarized case, based on covariant parton model. These functions are naively even time-reversal (T-even) at twist-2 approximation. The results for TMDs are indicating proper and acceptable behaviour with respect to what are presented in other literatures.

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