arXiv subjects

Xiangdong Ji

Publications and source records attributed to Xiangdong Ji.

At least 253 records · Page 14Linked to original sources

arXiv2001

A Lattice Study of the Magnetic Moment and the Spin Structure of the Nucleon

Using an approach free from momentum extrapolation, we calculate the nucleon magnetic moment and the fraction of the nucleon spin carried by the quark angular momentum in the quenched lattice QCD approximation. Quarks with three values of lattice masses, 210, 124 and 80 MeV, are formulated on the lattice using the standard Wilson approach. At every mass, 100 gluon configurations on 16^3 x 32 lattice with β=6.0 are used for statistical averaging. The results are compared with the previous calculations with momentum extrapolation. The contribution of the disconnected diagrams is studied at the largest quark mass using noise theory technique.

hep-lat↗

arXiv2001

Leading Chiral Contributions to the Spin Structure of the Proton

The leading chiral contributions to the quark and gluon components of the proton spin are calculated using heavy-baryon chiral perturbation theory. Similar calculations are done for the moments of the generalized parton distributions relevant to the quark and gluon angular momentum densities. These results provide useful insight about the role of pions in the spin structure of the nucleon, and can serve as a guidance for extrapolating lattice QCD calculations at large quark masses to the chiral limit.

hep-ph↗

arXiv2001

Validity Of The Vafa-Witten Proof On Absence Of Spontaneous Parity Breaking In QCD

The vacuum energy response of a quantum field theory is studied as a function of complex external fields coupled to the symmetry-breaking observables of the theory. It is found that the energy density is insensitive to spontaneous symmetry breaking effects along a special direction in the complex plane. In particular, in the case of vector-like gauge theories such as QCD, the change in energy generated by coupling to pure-gluonic parity-odd operators is always a growing function of the external fields along the real axis, independent of the mode in which symmetry is realized. In light of this, although there is little doubt that parity is not spontaneously broken in massive QCD, the validity of the Vafa and Witten proof is, however, questionable.

hep-ph↗

arXiv2001

Constructing Parton Convolution in Effective Field Theory

Parton convolution models have been used extensively in describing the sea quarks in the nucleon and explaining quark distributions in nuclei (the EMC effect). From effective field theory point of view, we construct the parton convolution formalism which has been the underlying conception of all convolution models. We explain the significance of scheme and scale dependence of the auxiliary quantities such as the pion distributions in a nucleon. As an application, we calculate the complete leading nonanalytic chiral contribution to the isovector component of the nucleon sea.

hep-ph↗

arXiv2001

Is the Sullivan Process Compatible with QCD Chiral Dynamics?

We calculate the leading non-analytic quark-mass dependence in the moments of isovector quark distributions using heavy-baryon chiral perturbation theory. The results differ from those obtained from the Sullivan process in which hard scattering occurs through the virtual pion cloud of the nucleon. Our results provide useful guidance for improving upon the meson-cloud model and can be used to constrain the extrapolations of the existing lattice QCD results to the physical quark masses.

hep-ph↗

arXiv2001

Large-N_c Quark Distributions in the Delta and Chiral Logarithms in Quark Distributions of the Nucleon

In a world with two quark flavors and a large number of colors (N_c), the polarized and unpolarized quark distributions in the delta are completely determined by those in the nucleon up to {\cal O}(1/N_c). In particular, we find q_Δ(x) =[(1\pm 2T_z)u_N(x)+ (1\mp 2T_z)d_N(x)]/2 and Δq_Δ(x) =[(5\pm 2T_z)Δu_N(x) + (5\mp 2T_z)Δd_N(x)]/10, where q = u, d and $T_z$ the charge state of a delta. The result can be used to estimate the leading chiral-logarithmic corrections to the quark distributions in the nucleon.

hep-ph↗

arXiv2001

Photon Structure Functions from Quenched Lattice QCD

We calculate the first moment of the photon structure function, $ ^γ=\int^1_0 dx F^γ_2(x,Q^2)$, on the quenched lattices with $β=6.0$ using the formalism developed by the authors recently. In this exploratory study, we take into account only the connected contractions. The result is compared with the experimental data as well as model predictions.

hep-lat↗

arXiv2001

An Analysis of the Next-to-Leading Order Corrections to the g_T(=g_1+g_2) Scaling Function

We present a general method for obtaining the quantum chromodynamical radiative corrections to the higher-twist (power-suppressed) contributions to inclusive deep-inelastic scattering in terms of light-cone correlation functions of the fundamental fields of quantum chromodynamics. Using this procedure, we calculate the previously unknown ${\cal O}(α_s)$ corrections to the twist-three part of the spin scaling function $g_T(x_B,Q^2) (=g_1(x_B,Q^2)+g_2(x_B,Q^2))$ and the corresponding forward Compton amplitude $S_T(ν,Q^2)$. Expanding our result about the unphysical point $x_B=\infty$, we arrive at an operator product expansion of the nonlocal product of two electromagnetic current operators involving twist-two and -three operators valid to ${\cal O}(α_s)$ for forward matrix elements. We find that the Wandzura-Wilczek relation between $g_1(x_B,Q^2)$ and the twist-two part of $g_T(x_B,Q^2)$ is respected in both the singlet and non-singlet sectors at this order, and argue its validity to all orders. The large-$N_c$ limit does not appreciably simplify the twist-three Wilson coefficients.

hep-ph↗

arXiv2001

Measuring the P-odd Pion-Nucleon Coupling h_{πNN}^{(1)} in π^+ Photoproton Production Near the Threshold

We show that polarized gamma p -> π^+ n in the threshold region is an excellent candidate for measuring the leading parity-violating pion-nucleon coupling h_{πNN}^{(1)} to an uncertainty of 20% if it has a natural size from dimensional analysis. The conclusion is based on a large unpolarized cross section, a new low-energy theorem for the photon polarization asymmetry at the threshold A_γ|_{th} = \sqrt{2} f_π(μ_p-μ_n) h_{πNN}^{(1)}/g_A m_N \sim h_{πNN}^{(1)}/2, and its strong dominance at forward and backward angles in the threshold region.

hep-ph↗

arXiv2001

Comment on "Subleading Corrections to Parity Violating Pion Photoproduction"

In a recent preprint, hep-ph/0012253, Zhu et al. calculated a part of the next-to-leading order corrections to parity-violating pion photoproduction (pol)gamma p-> pi^+ n in heavy-baryon chiral perturbation theory. They claim they have found the contribution as large as the leading-order. If correct, the process will not be a clean way to extract the longest range PV pion-nucleon coupling constant h_{πNN}^(1), as was asserted in our previous publications (hep-ph/0011230, nucl-th/0011100). In this comment we show that there is no solid evidence to support Zhu et al.'s claim. Moreover, we show that the subleading parity-violating coupling h_V cannot be extracted from observables at the order of interest based on the formalism of effective field theory.

hep-ph↗

arXiv2001

Studying Hadronic Structure of the Photon in Lattice QCD

We show that the matrix element of a local quark-gluon operator in the photon state, $<γ(kλ)|\hat O| γ(kλ)>$, can be calculated in lattice QCD. The result is generalized to other quantities involving space-like photons, including the transition form factor $γγ^*\to π^0$ and the virtual-photon-nucleon Compton amplitude $<γ^*N |γ^*N>$ which can be used to define the generalized Drell-Hearn-Gerasimov and Bjorken sum rules.

hep-lat↗

arXiv2001

The Spin Structure of the Nucleon

We present an overview of recent experimental and theoretical advances in our understanding of the spin structure of protons and neutrons.

hep-ph↗

arXiv2000

Key Issues in Hadronic Physics

A group of fifty physicists met in Duck, NC, Nov. 6-9 to discuss the current status and future goals of hadronic physics. The main purpose of the meeting was to define the field by identifying its key issues, challenges, and opportunities. The conclusions, incorporating considerable input from the community at large, are presented in this white paper.

hep-ph↗

arXiv2000

Counting Form Factors of Twist-Two Operators

We present a simple method to count the number of hadronic form factors based on the partial wave formalism and crossing symmetry. In particular, we show that the number of independent nucleon form factors of spin-n, twist-2 operators (the vector current and energy-momentum tensor being special examples) is n+1. These generalized form factors define the generalized (off-forward) parton distributions that have been studied extensively in the recent literature. In proving this result, we also show how the J^{PC} rules for onium states arise in the helicity formalism.

hep-ph↗

arXiv2000

Parity-Violating Pion-Nucleon Coupling h_{πNN}^{(1)} from π^+ Electroproton Production Near the Threshold

We study the possibility of measuring parity-violating pion-nucleon coupling h_{πNN}^{(1)} from π^+ electroproton production in the threshold region. We calculate the electron-helicity asymmetry in terms of h_{πNN}^{(1)} and the Z-boson exchange between the electron and proton in leading-order heavy-baryon chiral perturbation theory. From the result, we demonstrate that the coupling can be determined to an accuracy of about 2 10^{-7} in an experiment with incident electron energy 250 MeV, luminosity 10^{38} sec^{-1} cm^{-2}, and a running time of 10^7 sec.

nucl-th↗

arXiv2000

Comment on "Complete One-Loop Analysis of the Nucleon's Spin Polarisabilities"

We comment on the paper of Gellas et al, nucl-th/0002027, in which heavy baryon chiral perturbation theory results for the spin polarisabilities of the nucleon are given. That paper uses a definition which is different from the one used in papers by us, hep-ph/9908526, hep-ph/9909442 and hep-ph/0002133. In this comment we show that it is the latter definition, and not that of Gellas et al, that should be compared with the values of the polarisabilities extracted from dispersion relation analyses.

nucl-th↗

arXiv2000

Residual Chiral Symmetry Breaking in Domain-Wall Fermions

We study the effective quark mass induced by the finite separation of the domain walls in the domain-wall formulation of chiral fermion as the function of the size of the fifth dimension ($L_s$), the gauge coupling ($β$) and the physical volume ($V$). We measure the mass by calculating the small eigenvalues of the hermitian domain-wall Dirac operator ($H_{\rm DWF}(m_0=1.8))$ in the topologically-nontrivial quenched SU(3) gauge configurations. We find that the induced quark mass is nearly independent of the physical volume, decays exponentially as a function of $L_s$, and has a strong dependence on the size of quantum fluctuations controlled by $β$. The effect of the choice of the lattice gluon action is also studied.

hep-lat↗

arXiv2000

The Singlet g_2 Structure Function in the Next-to-Leading Order

Following a previous study of the one-loop factorization of the nonsinglet $g_2$ structure function of the nucleon, we present in this paper the next-to-leading order coefficient functions in the singlet sector. To obtain the result, the partonic processes of virtual Compton scattering off two and three "on-shell" gluons are calculated. A key step in achieving the correct factorization is to separate the correct twist-two contribution. The Burkardt-Cottingham sum rule is nominally satisfied at this order.

hep-ph↗