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Register a new userProduction of isoscalar pion pairs in the pd -> 3He pi pi reaction near 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.
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The cross section of the pd -> 3He pi+ pi - reaction has been measured at the MOMO facility in a kinematically complete experiment at a c. m. excess energy of Q = 70 MeV. The energy and angular distributions show that the reaction is dominated by p-wave pi+ pi- pairs. This is in complete contrast to the results of inclusive measurements at somewhat higher energies which show a strong s-wave ABC enhancement at low pi pi masses. There are however indications of p-wave pion pairs from other experiments at low Q.
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.
New experimental data on the pd -> 3He pi+ pi- reaction obtained with the COSY-MOMO detector below the three-pion threshold are presented. The reaction was also studied in inverse kinematics with a deuteron beam and the higher counting rates achieved were especially important at low excess energies. The comparison of these data with inclusive pd -> 3He+X rates allowed estimates also to be made of pi0 pi0 production. The results confirm our earlier findings that close to threshold there is no enhancement at low excitation energies in the pi+ pi- system, where the data seem largely suppressed compared to phase space. Possible explanations for this behavior, such as strong p- waves in the pi+pi- -system or the influence of two-step processes, are explored.
The contributions of three different types of driving terms are included in the estimation of the pd -> pd eta reaction at low energies. Near threshold, it is predicted that a two-step model involving an intermediate pion should be the most important but, as the energy approaches the threshold for eta production in the free nucleon--nucleon reaction, a pick-up mechanism with a spectator proton should become dominant. The total cross sections are underestimated by about a factor of two compared to experimental data but the discrepancies in the angular distributions are more serious, with no sign in the data for the peaks corresponding to the pick--up diagram.
The near-threshold n p -> d pi0 cross section is calculated in chiral perturbation theory to next-to-leading order in the expansion parameter sqrt{M m_pi}/Lambda_chi. At this order irreducible pion loops contribute to the relevant pion-production operator. While their contribution to this operator is finite, considering initial-and final-state distortions produces a linear divergence in its matrix elements. We renormalize this divergence by introducing a counterterm, whose value we choose in order to reproduce the threshold n p -> d pi0 cross section measured at TRIUMF. The energy-dependence of this cross section is then predicted in chiral perturbation theory, being determined by the production of p-wave pions, and also by energy dependence in the amplitude for the production of s-wave pions. With an appropriate choice of the counterterm, the chiral prediction for this energy dependence converges well.
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