Form factors of pions, kaons & protons at the highest timelike momentum transfers & first measurements of hyperons

Abstract

Abstract. Electromagnetic form factors of hadrons for timelike momentum transfers can be measured by particle-antiparticle annihilation into hadron-antihadron pairs. We have made the world's first high precision measurements of timelike form factors of ? and K mesons, and ground states of the baryons, protons, and the hyperons PreambleAn elementary particle is defined, or should be defined, as one which is not composite. So, all we want to know about an elementary particle is its static properties, mass, charge, spin, etc. When, almost exactly 100 years ago, Rutherford discovered the nucleus, we had just two elementary particles, the electron, and the proton, and there was no question of their constituents. While the electron has remained elementary, we know today that nucleons, proton and neutron, are not. The nucleons are composite of three light, spin 1/2 (m<10 MeV each, charge +2/3 and -1/3) up and down quarks, which are bound by massless spin 1 gluons. Mesons are simpler, being composite of a quark and an antiquark, with the quarks being up, down, and strange. Hyperons are baryons like the nucleons, with one or more up/down quarks replaced by strange quarks, which are still rather light. Now that mesons and baryons are composite, we have to find answers to a lot of questions. Among them, the important ones are: a) How does one account for the masses of light quark mesons and baryons, their spins and their spatial and momentum distributions? b) How do kaons differ from pions, and hyperons differ from nucleons? c) What happens to the structure of a hadron when lots of momentum or energy is pumped into it from the outside? The questions are of fundamental importance, and they can only be answered by experimental measurements. The measurements in question are form factor measurements.We are all familiar with the fact that measurement of elastic scattering of electrons from nuclei and nucleons is related to spacelike electric and magnetic form factors, and they have provided direct insight into the spatial distribution of charges and currents in the nuclei, and in the nucleon. Unfortunately, this can only be done for protons and nuclei which are available as targets.