Precision measurements of the pp\to \pi^+pn and pp\to \pi^+d reactions: importance of long-range and tensor force effects

TL;DR

This study measures pion production in proton-proton collisions at 400 and 600 MeV, highlighting the significant roles of long-range interactions and tensor forces in the final state, and compares results with theoretical predictions.

Contribution

It provides new high-resolution measurements of pion production channels and demonstrates the importance of long-range and tensor force effects in theoretical models.

Findings

Deviations from predictions are influenced by long-range production terms.

Tensor forces significantly affect the $pn$ final state.

A semi-quantitative agreement with theory is achieved.

Abstract

Inclusive measurements of pion production in proton--proton collisions in the forward direction were undertaken at 400 and 600 MeV at COSY using the Big Karl spectrograph. The high resolution in the $\pi^+$ momentum ensured that there was an unambiguous separation of the $pp\to {\pi}^+d/\pi^+pn$ channels. Using these and earlier data, the ratio of the production cross sections could be followed through the $\Delta$ region and compared with the predictions of final state interaction theory. Deviations are strongly influenced by long-range terms in the production operator and the tensor force in the final $pn$ system. These have been investigated in a realistic $pp\to\pi^+d/\pi^+pn$ calculation that includes $S \rightleftharpoons D$ channel coupling between the final nucleons. A semi-quantitative understanding of the observed effects is achieved.