Hadron Production Processes

Abstract

The experimental search for the pion -- proposed in 1935 by Hideki Yukawa as the force carrier of the strong nucleon-nucleon interaction -- was rewarded in 1947 when in cosmic ray photographic emulsion data a charged particle was identified with the proper mass of about 300 times the electron mass, completed three years later by the discovery of the neutral pion. Since then, accelerator-driven pion and meson (photo-)production on the nucleon and the associated production of new baryons have become the key elements for ground-breaking discoveries in numerous areas of particle and nuclear physics, from fundamental symmetries and their breaking to low-energy QCD dynamics, laying also foundations for modern elementary particle physics and the Standard Model. This article is an overview of eight decades of experimental and theoretical meson production physics, from isospin to charm and beyond, forming our understanding of hadrons and their interactions.

Figures (33)

• Figure 1: Feynman diagram describing the production of a charged pion (dashed line) and a neutron (full line) by an incident photon $\gamma$ (wavy line), incident on a proton (full line) .
• Figure 2: The light hadron spectrum of QCD as obtained by QCD. Horizontal lines and bands are the experimental values with their decay widths. QCD results are shown by solid circles. Vertical error bars represent combined statistical systematic error estimates. The pion ($\pi$), $S=-1$ kaon ($K$), and the $S=-2$ cascade baryon $\Xi$ have no error bars, because they were used to set the $u, d, s$ quark masses and the overall scale, respectively. The figure is adapted from Ref. Lenske:2018bgr using the LQCD results of Durr et al.Durr:2008zz.
• Figure 3: Diagram illustrating the Bethe-Salpeter equation of the T-matrix for scattering of a hadron or photon (dashed line) on a nucleon or any other baryon (full line). The bare interactions, i.e. the Born terms, are denoted by $V$; the scattering amplitude $T$ includes the full scattering series, summed to all orders. The evolution of the intermediate system is described by the Bethe-Salpeter propagator $\mathcal{G}_{BS}$ (see text).
• Figure 4: Structure of the tree-level interaction potential $V$. $s-$, $u-$, and $t-$ channel interactions defining the non-resonant background contributions are shown in the first line, including contact terms which are chosen such that gauge invariance is assured. The so-called z-diagrams, displayed in the second line, are generic for the double-pion channels. $s-$ channel resonance interactions are depicted in the last line. Time is running from left to right.
• Figure 5: The $e^+e^- \to \pi^+\pi^-\pi^0$ cross section measured by the SND Collaboration and collected in Achasov:2006xb. The curve is the fit with the $\rho^0$, $\omega$, $\phi$, $\omega^\prime$, and $\omega^{\prime\prime}$ resonances. Red arrows shown $\rho^0$, $\omega$, and $\phi$ thresholds.