pp→ppπ0reaction near threshold: A chiral power counting approach

Abstract

We use power-counting arguments as an organizing principle to apply chiral perturbation theory, including an explicit ⌬, to the pp→pp 0 reaction near threshold. There are two lowest-order leading mechanisms expected to contribute to the amplitude with similar magnitudes: an impulse term, and a ⌬-excitation mechanism. We examine formally subleading but potentially large mechanisms, including pion-rescattering and short-ranged contributions. We show that the pion-rescattering contribution is enhanced by off-shell effects and has a sign opposite to that of a recent estimate based on a PCAC pion interpolating field. Our result is that the impulse term interferes destructively with the pion-rescattering and ⌬-excitation terms. In addition, we have modeled the short-ranged interaction using and exchange mechanisms. A recoil correction to the impulse approximation is small. The total amplitude obtained including all of these processes is found to yield cross sections substantially smaller than the measured ones. ͓S0556-2813͑96͒01306-4͔

Charge symmetry violation in pn→dπ0 and chiral effective field theory

“…(5) and (6) reveals the ratio m π /m N between the two effects. Note that the counting rules used here are consistent with those of previous works in pion production [18]. We calculate A f b following the procedure described in Ref.…”

Toy model for pion production in nucleon-nucleon collisions. II. The role of three-particle singularities

“…For example, in Ref. [7] a propagator similar to that of Eq. (4.3) is used, but with E ′ + ≡ m π /2 and E ′′ ≡ 0.…”

“…[6]. However, not only is such a procedure difficult to implement, arguments based on effective field theories suggest that, at least close to the production threshold, most of the diagrams generated within a coupled channel approach are suppressed [7,8]. Those arguments indicate that the only piece that needs to be treated non-perturbatively is the nucleon-nucleon (NN) interaction, whereas the transition amplitude NN → NNπ can be treated perturbatively.…”

Spin correlation coefficients in from 325 to 400 MeV

“…Modern chiral perturbation work on this reaction has been discussed recently in [14,15], but this approach has not yet explained the data. It has become apparent that for pp → pnπ + the first order terms dominate and delta and complicating heavy meson exchange contribution are relatively small making this reaction theoretically important.…”