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Register a new userSpin Transfer in Exclusive Lambda Production from pp Collisions at 3.67 GeV/c
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We report the first polarization transfer measurements for exclusive hyperon production reactions. The normal spin transfer coefficient DNN for pp -> p K+ Lambda is large and negative for forward Lambda production at a beam momentum of 3.67 GeV/c, a result qualitatively consistent with expectations for a mechanism dominated by kaon-exchange and rescattering. The sign of DNN is opposite to that observed in the fragmentation regime for inclusive Lambda production at much higher energies.
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The ratio of the total exclusive production cross sections for $etaprime$ and $eta$ mesons has been measured in the $pp$ reaction at $p_{beam}=3.67$ GeV/c. The observed $etaprime/eta$ ratio is $(0.83pm{0.11}^{+0.23}_{-0.18})times 10^{-2}$ from which the exclusive $etaprime$ meson production cross section is determined to be $(1.12pm{0.15}^{+0.42}_{-0.31})mu b$. Differential cross section distributions have been measured. Their shape is consistent with isotropic $etaprime$ meson production.
The exclusive production cross sections for $omega$ and $phi$ mesons have been measured in proton-proton reactions at $p_{lab}=3.67$ GeV/c. The observed $phi/omega$ cross section ratio is $(3.8pm0.2^{+1.2}_{-0.9})times 10^{-3}$. After phase space corrections, this ratio is enhanced by about an order of magnitude relative to naive predictions based upon the Okubo-Zweig-Iizuka (OZI) rule, in comparison to an enhancement by a factor $sim 3$ previously observed at higher beam momenta. The modest increase of this enhancement near the production threshold is compared to the much larger increase of the $phi/omega$ ratio observed in specific channels of $bar pp$ annihilation experiments. Furthermore, differential cross section results are also presented which indicate that although the $phi$ meson is predominantly produced from a $^3P_1$ proton-proton entrance channel, other partial waves contribute significantly to the production mechanism at this beam momentum.
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 identification, 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 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 implementation of a central calorimeter provided particle identification, energy determination and neutron detection in addition to time-of-flight and angle measurements. Thus all pion production channels were recorded with 1-4 overconstraints. The total and differential cross sections obtained are compared to previous data and theoretical calculations. Main emphasis is put on the discussion of the $pppi^0$ channel, where we obtain angular distributions different from previous experimental results, however, partly in good agreement with recent phenomenological and theoretical predictions. In particular we observe very large anisotropies for the $pi^0$ angular distributions in the kinematical region of small relative proton momenta revealing there a dominance of proton spinflip transitions associated with $pi^0$ $s$- and $d$-partial waves and emphasizing the important role of $pi^0$ d-waves.
The total cross section of the reaction $ppto ppK^+K^-$ has been determined for proton--proton reactions with $p_{beam}=3.67 GeV/c$. This represents the first cross section measurement of the $pp to ppK^-K^+$ channel near threshold, and is equivalent to the inclusive $ppto ppK^-X$ cross section at this beam momentum. The cross section determined at this beam momentum is about a factor 20 lower than that for inclusive $ppto ppK^+X$ meson production at the same CM energy above the corresponding threshold. This large difference in the $K^+$ and $K^-$ meson inclusive production cross sections in proton-proton reactions is in strong contrast to cross sections measured in sub-threshold heavy ion collisions, which are similar in magnitude at the same energy per nucleon below the respective thresholds.
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