Baryon form factors: Model-independent results

Abstract

Baryon form factors can be analyzed in a largely model-independent fashion in terms of two complementary approaches. These are chiral perturbation theory and dispersion relations. I review the status of dispersive calculations of the nucleon electromagnetic form factors in the light of new data. Then, I present the leading one-loop chiral perturbation theory analysis of the hyperon and the strange nucleon form factors. Open problems and challenges are also discussed.Comment: 10 pp, LaTeX, 10 figures, plenary talk, NUCLEON '99, Frascati, June 1999, to appear in the proceedings (Nucl. Phys. A), typos corrected, references update

Low-energy analysis of the nucleon electromagnetic form factors

“…The resulting electric and magnetic form factors of the proton and the neutron are shown in figs. [8][9][10][11]. Consider the electric form factors first.…”

“…Nevertheless, since there is still some substantial scatter in the data in this range of momentum transfer, we will also use the results of the dispersive analysis for comparison to the ones obtained in the chiral expansion. A recent review on the theory of the form factors is given in [10], the status of the data as of 1999 is discussed in [11].…”

Proton Timelike Form Factors Near Threshold via Antiproton-Nucleus Electromagnetic Annihilation

“…where σ pp→e + e − is defined in (5), s p p = (p p + p p) 2 . The fact that in the annihilation on a free proton the mass of the final e + e − pair is fixed is reflected in (8) in the presence of the delta-function. Comparing this formula with (4), we see that the effect of the nuclear target results in a dilation of the infinitely sharp distribution δ(M − √ s p p) in a distribution of finite width η(M).…”

“…The theoretical models, generally based on dispersion relations [3][4][5] or semi-phenomenological approaches [6,7], predict a smooth behavior of the form factor in the measured regions, but a peaked behavior in the timelike region below the N N threshold (0 < q 2 < 4m 2 , m is the nucleon mass), due to poles in the amplitude (see e.g.fig. 1, taken from [8]). These poles are phenomenological inputs, built from meson exchange, and their properties are fitted to the data in the measured regions.…”

“…Experimental data and predictions for the magnetic proton form factor in the domain −10 GeV 2 ≤ q 2 ≤ 10 GeV 2 . The figure is taken from [8].…”