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Register a new userObservation of strong final-state effects in pi+ production in pp collisions at 400 MeV
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Differential cross sections of the reactions $pp to dpi^+$ and $pp to pnpi^+$ have been measured at $T_p = 400$ MeV by detecting the charged ejectiles in the angular range $4^0 leq Theta_{Lab} leq 21^circ$. The deduced total cross sections agree well with those published previously for neighbouring energies. The invariant mass spectra are observed to be strongly affected by $Delta$ production and $NN$ final-state interaction. The data are well described by Monte Carlo simulations including both these effects. The ratio of $pp to pnpi^+$ and $pp to dpi^+$ cross sections also compares favourably to a recent theoretical prediction which suggests a dominance of $np$-production in the relative $^3S_1$-state.
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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. The 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 reaction pp -> pp pi0 was studied with the WASA detector at the CELSIUS storage ring. The center of mass angular distribution of the pi0 was obtained by detection of the gamma decay products together with the two outgoing protons, and found to be anisotropic with a negative second derivative slope, in agreement with the theoretical predictions from a microscopic calculation.
The spin correlation coefficient combinations Axx + Ayy, Axx - Ayy and the analyzing powers Ay(theta) were measured for pp-->pnpi+ at beam energies of 325, 350, 375 and 400 MeV. A polarized internal atomic hydrogen target and a stored, polarized proton beam were used. These polarization observables are sensitive to contributions of higher partial waves. A comparison with recent theoretical calculations is provided.
The production of omega-mesons in the pp->pp omega reaction has been investigated with the COSY-ANKE spectrometer for excess energies of 60 and 92MeV by detecting the two final protons and reconstructing their missing mass. The large multipion background was subtracted using an event-by-event transformation of the proton momenta between the two energies. Differential distributions and total cross sections were obtained after careful studies of possible systematic uncertainties in the overall ANKE acceptance. The results are compared with the predictions of theoretical models. Combined with data on the phi-meson, a more refined estimate is made of the Okubo-Zweig-Iizuka rule violation in the phi/omega production ratio.
The NA60 experiment has studied J/psi production in p-A collisions at 158 and 400 GeV, at the CERN SPS. Nuclear effects on the J/psi yield have been estimated from the A-dependence of the production cross section ratios sigma_{J/psi}^{A}/sigma_{J/psi}^{Be} (A=Al, Cu, In, W, Pb, U). We observe a significant nuclear suppression of the J/psi yield per nucleon-nucleon collision, with a larger effect at lower incident energy, and we compare this result with previous observations by other fixed-target experiments. An attempt to disentangle the different contributions to the observed suppression has been carried out by studying the dependence of nuclear effects on x_2, the fraction of nucleon momentum carried by the interacting parton in the target nucleus.
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