Biomedical subjects

Xiangdong Ji

Publications and source records attributed to Xiangdong Ji.

7 recordsLinked to original sources

PubMed2006

Unified picture for single transverse-spin asymmetries in hard-scattering processes.

Using lepton-pair production in hadron-hadron collisions as an example, we explore the relation between two well-known mechanisms for single-transverse-spin asymmetries in hard processes: twist-three quark-gluon correlations when the pair's transverse momentum is large, q perpendicular >> Lambda QCD, and time-reversal-odd and transverse-momentum-dependent parton distributions when q perpendicular is much less than the pair's mass. We find that, although the two mechanisms each have their own domain of validity, they describe the same physics in the kinematic region where they overlap. This unifies the two mechanisms and imposes an important constraint on phenomenological studies.

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PubMed2004

Probing quark-distribution amplitudes through generalized parton distributions at large momentum transfer.

In the large momentum transfer limit, generalized parton distributions can be calculated through a QCD factorization theorem which involves perturbatively calculable hard kernels and light-cone parton distribution amplitudes of hadrons. We illustrate this through the H(q)(x,xi,t) distribution for the pion and the proton, presenting the hard kernels at leading order. As a result, experimental data on the generalized parton distributions in this regime can be used to determine the functional form of the parton distribution amplitudes which has thus far been quite challenging to obtain. Our result can also be used as a constraint in phenomenological generalized parton distribution parametrizations.

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PubMed2003

Perturbative QCD analysis of the nucleon's Pauli form factor F2(Q2).

We perform a perturbative QCD analysis of the nucleon's Pauli form factor F2(Q2) in the asymptotically large Q2 limit. We find that the leading contribution to F2(Q2) has a 1/Q6 power behavior, consistent with the well-known result in the literature. Its coefficient depends on the leading- and subleading-twist light-cone wave functions of the nucleon, the latter describing the quarks with one unit of orbital angular momentum. We also derive at the logarithmic accuracy the asymptotic scaling F2(Q2)/F(1)(Q2) approximately (log2Q2/Lambda2)/Q2 which describes recent Jefferson Lab data well.

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PubMed2003

Viewing the proton through "color" filters.

While the form factors and parton distributions provide separately the shape of the proton in coordinate and momentum spaces, a more powerful imaging of the proton structure can be obtained through phase-space distributions. Here we introduce the Wigner-type quark and gluon distributions which depict a full-3D proton at every fixed light-cone momentum, like what is seen through momentum ("color") filters. After appropriate phase-space reductions, the Wigner distributions are related to the generalized parton distributions (GPDs) and transverse-momentum dependent parton distributions, which are measurable in high-energy experiments. The new interpretation of GPDs provides a classical way to visualize the orbital motion of the quarks, which is known to be the key to the spin and magnetic moment of the proton.

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PubMed2003

Generalized counting rule for hard exclusive processes.

We derive a generalized counting rule for hard exclusive processes involving parton orbital angular momentum and hadron helicity flip. We start with a systematic way to enumerate the Fock components of a hadronic light-cone wave function with n partons and orbital angular momentum projection l(z). We show that the wave-function amplitude psi(n)(x(i),k(i perpendicular ),l(zi)) has a leading behavior 1/(k(2)( perpendicular ))[n(+|l(z)|+min(n(')+|l(')(z)|)]/2-1) when all parton transverse momenta are uniformly large, where n(') and l(')(z) are the number of partons and orbital angular momentum projection, respectively, of an amplitude that mixes under renormalization. Besides the generalized counting rule, the result can be used as a constraint in modeling the hadronic light-cone wave functions.

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PubMed2002

Delta-->Ngamma in large-N(c) QCD.

The decay Delta(+)-->pgamma is studied in the 1/N(c) expansion of QCD. The ratio of the helicity amplitudes is determined to be A(3/2)/A(1/2)=sqrt[3]+O(1/N(2)(c)). Equivalently, the ratio E2/M1 of the multipole amplitudes is predicted to be order 1/N(2)(c).

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PubMed2002

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 into the role of pions in the spin structure of the nucleon and can serve as a guide for extrapolating lattice QCD calculations at large quark masses to the chiral limit.

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