Measurements of the $pp to pK^+Lambda$ reaction at $T_p$ = 2.28 GeV have been carried out at COSY-TOF. In addition to the $Lambda p$ FSI and $N^*$ resonance excitation effects a pronounced narrow structure is observed in the Dalitz plot and in its pr
ojection on the $pLambda$-invariant mass. The structure appears at the $pp to $N$K^+Sigma$ threshold and is interpreted as $Sigma$N cusp effect. The observed width of 20 MeV/$c^2$ is substantially broader than anticipated from previous inclusive measurements. Angular distributions of this cusp structure are shown to be dissimilar to those in the residual $pK^+Lambda$ channel, but similar to those observed in the $pK^+Sigma^0$ channel.
Data accumulated recently for the exclusive measurement of the $ppto pppi^+pi^-$ reaction at a beam energy of 0.793 GeV using the COSY-TOF spectrometer have been analyzed with respect to possible events from the $pp to nnpi^+pi^+$ reaction channel. T
he latter is expected to be the only $pipi$ production channel, which contains no major contributions from resonance excitation close to threshold and hence should be a good testing ground for chiral dynamics in the $pipi$ production process. No single event has been found, which meets all conditions for being a candidate for the $pp to nn pi^+pi^+$ reaction. This gives an upper limit for the cross section of 0.16 $mu$b (90% C.L.), which is more than an order of magnitude smaller than the cross sections of the other two-pion production channels at the same incident energy.
The single-pion production reactions $ppto dpi^+$, $ppto nppi^+$ and $ppto pppi^0$ were measured at a beam momentum of 0.95 GeV/c ($T_p approx$ 400 MeV) using the short version of the COSY-TOF spectrometer. The central calorimeter provided particle i
dentification, energy determination and neutron detection in addition to time-of-flight and angle measurements from other detector parts. Thus all pion production channels were recorded with 1-4 overconstraints. Main emphasis is put on the presentation and discussion of the $nppi^+$ channel, since the results on the other channels have already been published previously. The total and differential cross sections obtained are compared to theoretical calculations. In contrast to the $pppi^0$ channel we find in the $nppi^+$ channel a strong influence of the $Delta$ excitation already at this energy close to threshold. In particular we find a $(3 cos^2Theta + 1)$ dependence in the pion angular distribution, typical for a pure s-channel $Delta$ excitation and identical to that observed in the $dpi^+$ channel. Since the latter is understood by a s-channel resonance in the $^1D_2$ $pn$ partial wave, we discuss an analogous scenario for the $pnpi^+$ channel.
The quasifree reaction $npto pppim$ 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 th
e 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.
