arXiv subjects

S. Atashbar Tehrani

Publications and source records attributed to S. Atashbar Tehrani.

At least 37 records · Page 2Linked to original sources

arXiv2016

QCD analysis of nucleon structure functions in deep-inelastic neutrino-nucleon scattering: Laplace transform and Jacobi polynomials approach

We present a detailed QCD analysis of nucleon structure functions $xF_3 (x, Q^2)$, based on Laplace transforms and Jacobi polynomials approach. The analysis corresponds to the next-to-leading order and next-to-next-to-leading order approximation of perturbative QCD. The Laplace transform technique, as an exact analytical solution, is used for the solution of nonsinglet Dokshitzer-Gribov-Lipatov-Altarelli-Parisi evolution equations at low- and large-$x$ values. The extracted results are used as input to obtain the $x$ and Q$^2$ evolution of $xF_3(x, Q^2)$ structure functions using the Jacobi polynomials approach. In our work, the values of the typical QCD scale $Λ_{\overline{\rm MS}}^{(n_f)}$ and the strong coupling constant $α_s(M_Z^2)$ are determined for four quark flavors ($n_f=4$) as well. A careful estimation of the uncertainties shall be performed using the Hessian method for the valence-quark distributions, originating from the experimental errors. We compare our valence-quark parton distribution functions sets with those of other collaborations; in particular with the {\tt BBG}, {\tt CT14}, {\tt MMHT14} and {\tt NNPDF} sets, which are contemporary with the present analysis. The obtained results from the analysis are in good agreement with those from the literature.

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arXiv2016

Next-to-next-to-leading order QCD analysis of spin-dependent parton distribution functions and their uncertainties: Jacobi polynomials approach

We present a first QCD analysis of next-to-next-leading-order (NNLO) contributions of the spin-dependent parton distribution functions (PPDFs) in the nucleon and their uncertainties using the Jacobi polynomial approach. Having the NNLO contributions of the quark-quark and gluon-quark splitting functions in perturbative QCD (Nucl. Phys. B 889 (2014) 351-400), one can obtain the evolution of longitudinally polarized parton densities of hadrons up to NNLO accuracy of QCD. A very large sets of recent and up-to-date experimental data of spin structure functions of the proton $g_1^p$, neutron $g_1^n$, and deuteron $g_1^d$ have been used in this analysis. The predictions for the NNLO calculations of the polarized parton distribution functions as well as the proton, neutron and deuteron polarized structure functions are compared with the corresponding results of the NLO approximation. We form a mutually consistent set of polarized PDFs due to the inclusion of the most available experimental data including the recently high-precision measurements from {\tt COMPASS16} experiments (Phys. Lett. B 753 (2016) 18-28). We have performed a careful estimation of the uncertainties using the most common and practical method, the Hessian method, for the polarized PDFs originating from the experimental errors. The proton, neutron and deuteron structure functions and also their first moments, $Γ^{\rm p, n, d}$, are in good agreement with the experimental data at small and large momentum fraction of $x$. We will discuss how our knowledge of spin-dependence structure functions can improve at small and large value of $x$ by the recent {\tt COMPASS16} measurements at CERN, the {\tt PHENIX} and {\tt STAR} measurements at RHIC, and at the future proposed colliders such as Electron-Ion collider (EIC).

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arXiv2016

Global analysis of nuclear parton distribution functions and their uncertainties at next-to-next-to-leading order

We perform a next-to-next-to-leading order (NNLO) analysis of nuclear parton distribution functions (nPDFs) using neutral current charged-lepton ($\ell ^\pm$ + nucleus) deeply inelastic scattering (DIS) data and Drell-Yan (DY) cross-section ratios $σ_{DY}^{A}/σ_{DY}^{A^\prime}$ for several nuclear targets. We study in detail the parametrizations and the atomic mass (A) dependence of the nuclear PDFs at this order. The present nuclear PDFs global analysis provides us a complete set of nuclear PDFs, $f_i^{(A,Z)}(x,Q^2)$, with a full functional dependence on $x$, A, Q$^2$. The uncertainties of the obtained nuclear modification factors for each parton flavour are estimated using the well-known Hessian method. The nuclear charm quark distributions are also added into the analysis. We compare the parametrization results with the available data and the results of other nuclear PDFs groups. We found our nuclear PDFs to be in reasonably good agreement with them. The estimates of errors provided by our global analysis are rather smaller than those of other groups. In general, a very good agreement is achieved. We also briefly review the recent heavy-ion collisions data including the first experimental data from the LHC proton+lead and lead+lead run which can be used in the global fits of nuclear PDFs. We highlight different aspects of the high luminosity Pb--Pb and p--Pb data which have been recorded by the CMS Collaboration.

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arXiv2014

Nuclear dynamics in the EMC effect at Next-to-Next-to-Leading order

We study in details the parameterizations of the nuclear parton distributions at the next-to-next-to-leading order (NNLO) of $α_s$. In low $x$ and $Q_0^2$, we observe negative gluon distribution at this order which signals the saturation condition or the quark-gluon plasma condition. Our study also shows the gluon distribution at (NNLO) is less than next-to-leading order (NLO) of $α_s$, and the sea quark distribution at (NNLO) is larger than (NLO).

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arXiv2012

Symmetry breaking effect on determination of polarized and unpolarized parton distributions

We perform a new extraction for unpolarized and polarized parton distribution functions considering a flavor decompositions for sea quarks and applying very recent deep inelastic scattering (DIS) and semi inclusive deep inelastic scattering (SIDIS) data in the fixed flavor number scheme (FFNS) framework. In the new symmetry breaking scenario the light quark and antiquark densities are extracted separately and new parametrization forms are determined for them. The heavy flavors contribution, including charm and bottom quarks, are also taken to be account for unpolarized distributions.

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arXiv2012

New parton distributions in fixed flavour factorization scheme from recent deep-inelastic-scattering data

We present our QCD analysis of the proton structure function $F_2^p(x,Q^2)$ to determine the parton distributions at the next-to-leading order (NLO). The heavy quark contributions to $F_2^i(x,Q^2)$, with $i$ = $c$, $b$ have been included in the framework of the `fixed flavour number scheme' (FFNS). The results obtained in the FFNS are compared with available results such as the general-mass variable-flavour-number scheme (GM-VFNS) and other prescriptions used in global fits of PDFs. In the present QCD analysis, we use a wide range of the inclusive neutral-current deep-inelastic-scattering (NC DIS) data, including the most recent data for charm $F_2^c$, bottom $F_2^b$, longitudinal $F_L$ structure functions and also the reduced DIS cross sections $σ_{r,NC}^\pm$ from HERA experiments. The most recent HERMES data for proton and deuteron structure functions are also added. We take into account ZEUS neutral current $e^ \pm p$ DIS inclusive jet cross section data from HERA together with the recent Tevatron Run-II inclusive jet cross section data from CDF and DØ. The impact of these recent DIS data on the PDFs extracted from the global fits are studied. We present two families of PDFs, {\tt KKT12} and {\tt KKT12C}, without and with HERA `combined' data sets on $e^{\pm}p$ DIS. We find these are in good agreement with the available theoretical models.

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arXiv2012

A global analysis of diffractive events at HERA

We extract diffractive parton distribution functions (DPDFs) and diffractive structure functions from the most recent H1 and ZEUS diffractive DIS data obtained by various methods. We consider Pomeron as an object with parton distribution function, evolving according to the next-to-leading order (NLO) DGLAP equations within the framework of the `Fixed Flavour Number Scheme' (FFNS). Having performed a global fit analysis, we achieve a very good description of all available measurements by introducing a new set of quark distribution form for the Pomeron. We predict longitudinal and charm proton diffractive structure function as well. Our results are compared with other analysis from the literature.

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arXiv2011

Polarized Deeply Inelastic Scattering (DIS) Structure Functions for Nucleons and Nuclei

We extract parton distribution functions (PDFs) and structure functions from recent experimental data of polarized lepton-DIS on nucleons at next-to-leading order (NLO) Quantum Chromodynamics. We apply the Jacobi polynomial method to the DGLAP evolution as this is numerically efficient. Having determined the polarized proton and neutron spin structure, we extend this analysis to describe 3He and 3H polarized structure functions, as well as various sum rules. We compare our results with other analyses from the literature.

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arXiv2010

Nonsinglet parton distribution functions from the precise next-to-next-to-next-to leading order QCD fit

We present the results of our QCD analysis for non-singlet unpolarized quark distributions and structure function $F_2(x,Q^2)$ up to N$^3$LO. In this regards 4-loop anomalous dimension can be obtain from the Padé approximations. The analysis is based on the Jacobi polynomials expansion of the structure function. New parameterizations are derived for the non-singlet quark distributions for the kinematic wide range of $x$ and $Q^2$. Our calculations for non-singlet unpolarized quark distribution functions up to N$^3$LO are in good agreement with available theoretical models. The higher twist contributions of $F_2^{p,d}(x,Q^2)$ are extracted in the large $x$ region in N$^3$LO analysis. The values of $Λ_{QCD}$ and $α_s(M_z^2)$ are determined.

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arXiv2008

NNLO QCD contributions to the flavor non-singlet sector of ${\bf {F_2(x,Q^2)}}$

We present the results of our QCD analysis for nonsinglet unpolarized quark distributions and structure function $F_2(x,Q^2)$. New parameterizations are derived for the nonsinglet quark distributions for the kinematic wide range of $x$ and $Q^2$. The analysis is based on the Jacobi polynomials expansion of the structure function. The higher twist contributions of proton and deuteron structure function are obtained in the large $x$ region. Our calculations for nonsinglet unpolarized quark distribution functions based on the Jacobi polynomials method are in good agreement with the other theoretical models. The values of $Λ_{QCD}$ and $α_s(M_z^2)$ are determined.

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arXiv2008

QCD Analysis for Nuclear Parton Distributions in the Next to Leading Order

A QCD analysis of the nuclear parton distributions and structure functions in the NLO is performed by using the world data. By having bounded parton distributions for a nuclear with atomic number A, we can obtain the nuclear structure function in x space. Our results for nuclear structure function ratio $F^A_2 /F^D_2$ for some different values of A, are in good agreement with the experimental data. We compare our results for LO and NLO approximation.

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arXiv2007

Simple model for QCD analysis of the proton helicity structure

In this paper we use the experimental data to obtain the polarized parton distribution functions (PPDFs) in the LO and NLO approximations. The analysis is based on the Jacobi polynomials expansion of the polarized structure function (PSF). Our calculations for polarized parton distribution functions based on the Jacobi polynomials method are in good agreement with the other theoretical models.

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arXiv2007

The Jacobi Polynomials QCD analysis for the polarized structure function

We present the results of our QCD analysis for polarized quark distribution and structure function $xg_1 (x,Q^2)$. We use very recently experimental data to parameterize our model. New parameterizations are derived for the quark and gluon distributions for the kinematic range $x ε[10^{-8},1]$, $Q^2 ε[1,10^6]$ GeV^2. The analysis is based on the Jacobi polynomials expansion of the polarized structure functions. Our calculations for polarized parton distribution functions based on the Jacobi polynomials method are in good agreement with the other theoretical models. The values of $Λ_{QCD}$ and $α_s(M_z)$ are determined.

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arXiv2007

The NNLO non-singlet QCD analysis of parton distributions based on Bernstein polynomials

A non-singlet QCD analysis of the structure function $xF_3$ up to NNLO is performed based on the Bernstein polynomials approach. We use recently calculated NNLO anomalous dimension coefficients for the moments of the $xF_3$ structure function in $νN$ scattering. In the fitting procedure, Bernstein polynomial method is used to construct experimental moments from the $xF_3$ data of the CCFR collaboration in the region of $x$ which is inaccessible experimentally. We also consider Bernstein averages to obtain some unknown parameters which exist in the valence quark densities in a wide range of $x$ and $Q^2$. The results of valence quark distributions up to NNLO are in good agreement with the available theoretical models. In the analysis we determined the QCD-scale $Λ^ {\bar{MS}}_{QCD, N_{f}=4}=211$ MeV (LO), 259 MeV (NLO) and 230 MeV (NNLO), corresponding to $α_s(M_Z^2)=0.1291$ LO, $α_s(M_Z^2)=0.1150$ NLO and $α_s(M_Z^2)=0.1142$ NNLO. We compare our results for the QCD scale and the $α_s(M_Z^2)$ with those obtained from deep inelastic scattering processes.

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arXiv2007

An approach to NNLO QCD analysis of non-singlet structure function

We use the next-to-next-to-leading order (NNLO) contributions to anomalous dimension governing the evolution of non-singlet quark distributions. We use the xF3 data of the CCFR collaboration to obtain some unknown parameters which exist in the non-singlet quark distributions in the NNLO approximation. In the fitting procedure, Bernstein polynomial method is used. The results of valence quark distributions in the NNLO, are in good agreement with the available theoretical model.

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arXiv2006

EMC effect and nuclear structure functions

We analyze experimental data of nuclear structure function ratios $F_2^A/F_2^D$ for obtaining optimum parton distribution functions (PDFs) in nuclei. Then, uncertainties of the nuclear PDFs are estimated by the Hessian method. Parametrization of nuclear parton distribution is investigated in the leading order of $α_s$. The parton distribution are provided at $Q^2=1 GeV^2$ with a number of parameters, which are determined by a $χ^2$ analysis of the data on nuclear structure function. From the analysis, we propose parton distributions at $Q^2=1 GeV^2$ for nuclei from deuteron to heavy ones with a mass number $A\sim 208$.

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arXiv2006

The Role of Polarized Valons in the Flavor Symmetry Breaking of Nucleon Sea

Next-to-leading order approximation of the quark helicity distributions are used in the frame work of polarized valon model. The flavor-asymmetry in the light-quark sea of the nucleon can be obtained from the contributions of unbroken sea quark distributions. We employ the polarized valon model and extract the flavor-broken light sea distributions which are modeled with the help of a Pauli-blocking ansatz. Using this ansatz, we can obtain broken polarized valon distributions. From there and by employing convolution integral, broken sea quark distributions are obtainable in this frame work. 0ur results for $δu$, $δd$, $δ\bar u$, $δ\bar d$ and $δ\bar s$ are in good agreement with recent experimental data for polarized parton distribution from HERMES experimental group and also with GRSV model. Some information on orbital angular momentum as a main ingredient of total nucleon spin are given. The $Q^2$ evolution of this quantity, using the polarized valon model is investigated.

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arXiv2004

Next-to-Leading order approximation of polarized valon and parton distributions

Polarized parton distributions and structure functions of the nucleon are analyzed in the improved valon model. The valon representation provides a model to represent hadrons in terms of quarks, providing a unified description of bound state and scattering properties of hadrons. Polarized valon distributions are seen to play an important role in describing the spin dependence of parton distributions in the leading order (LO) and next-to-leading order (NLO) approximations. In the polarized case, a convolution integral is derived in the framework of the valon model. The Polarized valon distribution in a proton and the polarized parton distributions inside the valon are necessary to obtain the polarized parton distributions in a proton. Bernstein polynomial averages are used to extract the unknown parameters of the polarized valon distributions by fitting to the available experimental data. The predictions for the NLO calculations of the polarized parton distributions and proton structure functions are compared with the LO approximation. It is shown that the results of the calculations for the proton structure function, $xg_1^p$, and its first moment, $Γ_{1}^p$, are in good agreement with the experimental data for a range of values of $Q^{2}$. Finally the spin contribution of the valons to the proton is calculated.

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