Single \pi^- production in np collisions for excess energies up to 90 MeV

TL;DR

This study investigates the quasifree np→ppπ− reaction near threshold energies, analyzing angular distributions, invariant mass spectra, and FSI effects, providing new insights into the reaction mechanism and testing the spectator model.

Contribution

It offers the first comprehensive analysis of np→ppπ− at excess energies up to 90 MeV using a kinematically complete experiment and the spectator model.

Findings

Significant pp FSI effects observed.

Contributions from p and d partial waves identified.

Elementary cross section behavior near threshold discussed.

Abstract

The quasifree reaction $np\to pp\pim$ was studied in a kinematically complete experiment by bombarding a liquid hydrogen target with a deuteron beam of momentum 1.85 GeV/c and analyzing the data along the lines of the spectator model. In addition to the three charged ejectiles the spectator proton was also detected in the large-acceptance time-of-flight spectrometer COSY-TOF. It was identified by its momentum and flight direction thus yielding access to the Fermi motion of the bound neutron and to the effective neutron 4-momentum vector $\mathbb{P}_n$ which differed from event to event. A range of almost 90 MeV excess energy above threshold was covered. Energy dependent angular distributions, invariant mass spectra as well as fully covered Dalitz plots were deduced. Sizeable $pp$ FSI effects were found as were contributions of $p$ and $d$ partial waves. The behavior of the elementary cross section $\sigma_{01}$ close to threshold is discussed in view of new cross section data. In comparison with existing literature data the results provide a sensitive test of the spectator model.