Exclusive $p p \to p p π^{0}$ reaction at high energies

Abstract: The amplitudes for the $p p \to p p \pi^{0}$ process applicable at high energy are discussed in detail. Both diffractive bremsstrahlung (Drell-Hiida-Deck--type model), photon-photon and photon-omega exchange mechanisms are included in the calculation. Large cross sections of the order of mb are predicted. The corresponding differential cross sections in rapidities and transverse momenta of outgoing protons and pions as well as relative azimuthal angle between outgoing protons are calculated for RHIC and LHC energies. The hadronic bremsstrahlung contributions dominate at large (forward, backward) pion rapidities. The diffractive nonresonant background contributes at small $\pi^0 p$ invariant mass and could be therefore misinterpreted as the Roper resonance. We predict strong dependence of the slope in $t$ (squared four-momentum transfer between ingoing and outgoing proton) on the mass of the supplementary excited $\pi^{0}p$ system. At high energies and midrapidities, the photon-photon contribution dominates over the diffractive components, however, the corresponding cross section is rather small. The photon-odderon and odderon-photon contributions are included in addition and first estimates (upper limits) of their contributions to rapidity and transverse momentum distribution of neutral pions are presented. We suggest a search for the odderon contribution at midrapidity and at $p_{\perp,\pi^{0}} \sim$ 0.5 GeV. Our predictions are ready for verification at RHIC and LHC. The bremsstrahlung mechanisms discussed here contribute also to the $pp \to p(n \pi^{+})$ reaction. Both channels give a sizable contribution to the low-mass single diffractive cross section and must be included in extrapolating the measured experimental single diffractive cross section.