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Register a new userIsoscalar off-shell effects in threshold pion production from pd collisions
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L. Canton
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We test the presence of pion-nucleon isoscalar off-shell effects in the $pdto pi^+ t$ reaction around the threshold region. We find that these effects significantly modify the production cross section and that they may provide the missing strength needed to reproduce the data at threshold.
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The production near threshold of isoscalar pion pairs in the pd -> 3He(pi pi)^0 reaction is estimated in a two-step model which successfully describes the production of eta, omega and eta mesons. A virtual pion beam, generated through an NN -> d pi reaction on one of the nucleons in the deuteron, produces a second pion via a pi N -> pi pi N reaction on the other nucleon. Using the same scale factor as for heavy meson production, the model reproduces the total pi^0 pi^0 production rate determined at an excess energy of 37 MeV. There are some indications in the data for a suppression of events with low pi pi masses, as in the pi^- p -> pi^0 pi^0 n reaction, and this is confirmed within the model. The model suggests that a significant fraction of the charged pion production in the p d -> 3He pi^+ pi^- reaction at Q=70 MeV might be associated with isoscalar pion pairs, though this does not explain the strong dependence observed on the pi^+ pi^- relative momentum angle.
We have calculated the proton analyzing power A_{y0} of the pion-production reaction from pd collisions for one energy close to threshold and for another in the region of the $Delta$-resonance. A fair reproduction of the experimental data could be obtained in both cases with a model which includes isoscalar and isovector $pi N$ rescatterings in s waves, as well as the p-wave rescattering mechanisms mediated by the $pi NN$ and $pi N Delta$ vertices. For the analyzing power at threshold we found that the initial-state interaction (ISI) is also quite important.
Previous approaches to the photo- and electro-production of strangeness off the proton, based upon effective hadronic Lagrangians, are extended here to incorporate the so called off-shell effects inherent to the fermions with spin >= 3/2. A formalism for intermediate-state, spin 3/2, nucleonic and hyperonic resonances is presented and applied to the processes $gamma + p ---> K^{+} + Lambda$, for $E_{gamma}^{lab}$ <= 2.5 GeV, $e + p ---> e + K^+ + Lambda$, as well as the branching ratio for the crossed channel reaction $K^- + p ---> gamma + Lambda$, with stopped kaons. The sensitivity, from moderate to significant, of various observables to such effects are discussed.
The reaction $NN to NN pi$ offers a good testing ground for chiral effective field theory at intermediate energies. It challenges our understanding of the first inelastic channel in nucleon-nucleon scattering and of the charge-symmetry breaking pattern in hadronic reactions. In our previous studies, we presented a complete calculation of the pion-production operator for s-wave pions up-to-and-including next-to-next-to-leading order (NNLO) in the formulation of chiral effective field theory, which includes pions, nucleons and $Delta(1232)$ degrees of freedom. In this paper we calculate the near threshold cross section for the $pp to d pi^{+}$ reaction by performing the convolution of the obtained operators with nuclear wave functions based on modern phenomenological and chiral potentials. The available chiral $NN$ wave functions are constructed with a cutoff comparable with the momentum transfer scale inherent in pion production reactions. Hence, a significant portion of the dynamical intermediate-range physics is thereby cut off by them. On the other hand, the NNLO amplitudes evaluated with phenomenological wave functions appear to be largely independent of the $NN$ model used and give corrections to the dominant leading order contributions as expected from dimensional analysis. The result gives support to the counting scheme used to classify the pion production operators, which is a precondition for a reliable investigation of the chirally suppressed neutral pion production. The explicit inclusion of the $Delta(1232)$ is found to be important but smaller than expected due to cancellations.
Pion energy spectra are presented for central collisions of neutron-rich 132Sn+124Sn and neutron-deficient 108Sn+112Sn systems using simulations with Boltzmann-Uehling-Uhlenbeck transport model. These calculations, which incorporate isospin-dependent mean field potentials for relevant baryons and mesons, display a sensitivity to the pion spectra that could allow significant constraints on the density dependence of the symmetry energy and its mean field potential at supra-saturation densities. The predicted sensitivity increases with the isospin asymmetry of the total system and decreases with incident energy.
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