Abstract
We present the results of the pion production operator calculated up-to-and- including next-to-next-to-leading order (NNLO) in chiral effective field theory. We in- clude explicit Delta degrees of freedom and demonstrate that they provide essential con- tribution required to understand neutral pion production data. Analysis of chiral loops at NNLO reveals new mechanisms which are important, but haven't been considered in phenomenological studies so far.
Highlights
Understanding of near-threshold pion production is of significant importance since it allows a direct test of chiral effective field theory (EFT), probes nucleon-nucleon dynamics at intermediate energies and provides access to isospin violation in few-nucleon processes
Pion-nucleon dynamics at low energies is significantly constrained by the chiral symmetry of QCD
A calculation of the irreducible pion production operator using perturbative chiral EFT and momentum counting scheme (MCS) [8] — a counting scheme which takes into account the initial momentum required to produce a pion
Summary
Understanding of near-threshold pion production is of significant importance since it allows a direct test of chiral effective field theory (EFT), probes nucleon-nucleon dynamics at intermediate energies and provides access to isospin violation in few-nucleon processes (see refs. [1, 2] for review articles). At the energy Tlab = 293.5 MeV, which is just few MeV above the threshold, the total cross section of charged pion production is σtot(pp → dπ+) 42 ± 5 μb, while the cross section of neutral pion production is only σtot(pp → ppπ0) = 2.8 ± 0.2 μb [3]. This experimental evidence is fully in line with the chiral suppression of the leading production operators in the neutral channel and suggests the important role of higher order effects, especially chiral loops, which are discussed in this Contribution.
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