Simone Pacetti

Unexpected features of e+e- \rightarrow p \overline{{p}} and e+e- \rightarrow \Lambda \overline{{\Lambda}} cross-sections near threshold

Unexpected features of the BaBar data on e+e- in baryon-antibaryon cross sections are discussed. These data have been collected, with unprecedented accuracy, by means of the initial state radiation technique, which is particularly suitable in giving good acceptance and energy resolution at threshold. A striking feature observed in the BaBar data is the non-vanishing cross section at threshold for all these processes. This is the expectation due to the Coulomb enhancement factor acting on a charged fermion pair. In the case of e+e- in proton-antiproton it is found that Coulomb final state interactions largely dominate the cross section and the form factor is |G^p(4M^2_p)|~1, which could be a general feature for baryons. In the case of neutral baryons an interpretation of the non-vanishing cross section at threshold is suggested, based on quark electromagnetic interaction and taking into account the asymmetry between attractive and repulsive Coulomb factors. Besides strange baryon cross sections are compared to U-spin invariance predictions.

Search for two photon exchange from e++e−→p+p¯+γ data

Abstract We look for asymmetries in the angular distributions of events from recent data on e + + e − → p + p ¯ + γ from BaBar Collaboration. From first principles, as the C-invariance of the electromagnetic interaction and the crossing symmetry, the presence of two-photon exchange would create a forward backward asymmetry in the data. The analysis of the available data shows no asymmetry, within an error of 2%. Such error is of the order of the asymmetry expected from radiative corrections as calculated from QED. As no systematic deviations are seen, we can conclude that these data do not give any hint of the presence of the two-photon contribution, in all the considered kinematical range.

No Sommerfeld resummation factor in e + e − → pp?

The Sommerfeld rescattering formula is compared to the

A practical guide to unravel time-like transition form factors

e^ + e^ - to pbar p

Two-Photon Exchange: Myth and History

BABAR data at threshold and above. While there is the expected Coulomb enhancement at threshold, two unexpected outcomes have been found: the modulus of the proton form factor is normalized to one at threshold, i.e. |Gp(4Mp2)| = 1, as a pointlike fermion, and, moreover, data show that the resummation factor in the Sommerfeld formula is not needed. Other e+e− → baryon-antibaryon cross-sections show similar behavior near threshold.

Influence of final state radiation in the processe++e−→p¯+p(γ)

The Sommerfeld rescattering formula is compared to the

Analytic pion form factor

e^ + e^ - to pbar p