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Asymptotic form factors of hadrons and nuclei and the continuity of particle and nuclear dynamics

Stanley J. Brodsky*

Benson T. Chertok

  • Stanford Linear Accelerator Center, Stanford University, Stanford, California 94305

  • American University, Washington, D. C. 20016

  • *Research supported by the Energy Research and Development Administration.
  • Research supported by the National Science Foundation under Grant No. MPS75-07325.

Phys. Rev. D 14, 3003 – Published 1 December, 1976

DOI: https://doi.org/10.1103/PhysRevD.14.3003

Abstract

The large-q2 behavior of the elastic form factor of a hadron or nucleus is related by dimensional counting to the number of its elementary constituents. Using the framework of a scale-invariant quark model, dimensional-scaling predictions are derived for the B(q2)A(q2) ratio in the Rosenbluth formula, multiple-photonexchange corrections, and the mass parameters which control the onset of the asymptotic power law in the meson, nucleon, and deuteron form factors. A simple "democratic chain" model predicts that for large q2, F(q2)(1q2mn2)1n, where mn2 is proportional to the number of constituents n. In the case of nuclear targets (or systems with several scales of compositeness), we also define the "reduced" form factor fA(q2)=FA(q2)Πi=1A[Fi(qi2)] in order to remove the minimal falloff of FA due to the nucleon form factors at qi2=(mi2MA2)q2. Dimensional counting predicts (q2)A1fA(q2)const. A systematic comparison of the data for π, p, n, and deuteron form factors with the dimensional-scaling quark-model predictions is given. Predictions are made for the large-spacelike-q2 He3 and α-particle form factors. We also relate the deuteron form factor to (off-shell) fixed-angle np scattering, and show that the experimental results for t5Fd(t) are consistent with the magnitude of the s-wave wave function u(0) obtained from soft-core potentials. The relation of the dynamics of an underlying six-quark state of the deuteron to the nucleon-potential and meson-exchange-current contributions is discussed. The scaling of q2fd(q2) implies that the nuclear potential (after removing the effects of nucleon structure) displays the scale-invariant behavior of a theory without a fundamental length scale. Predictions are also given for the structure functions, fragmentation, and large-angle scattering of a nucleus.

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