Forty years of antiprotons

Abstract

The discovery of the antiproton some 40 years ago and the almost synchronous fall of parity (P) and charge conjugation (C) symmetries were soon followed by the realization that CPT rather than C invariance is the fundamental symmetry connecting matter and antimatter, and that consequently any measurement of the antiproton's properties can be interpreted as a test of that symmetry. It is the latter view of the antiproton, as an object of study in its own right, rather than as a means to such other ends as the production of gauge bosons and meson resonances, that is presented here. The authors review the technical steps that have led from the handful of antiprotons observed by Chamberlain, Segrè , Wiegand, and Ypsilantis to the intense, high-quality beams available today and show how the state of rest and isolation required for high precision measurements of their properties can be achieved by confining them in electromagnetic traps or in their microscopic counterparts, exotic atoms. The test bench role of antiprotons and antihydrogen atoms for both CPT symmetry and the gravitational weak equivalence principle is discussed, and the body of experimental results obtained since 1955 critically reviewed from this standpoint. Future experiments are then discussed in the light of the closure of the CERN Low Energy Antiproton Ring (LEAR), its replacement in 1999 by the Antiproton Decelerator (AD), and the likely antiproton source at the Japan Hadron Facility.[S0034-6861(99)00401-8] CONTENTS

Exotic atoms in superintense laser fields

“…Another interesting type of hadronic atoms involves antiprotons, e.g. antiprotonic hydrogen ("protonium") or helium [5,28].…”

Gross properties of exotic nuclei investigated at storage rings and ion traps

“…After the first generation of experiments with exotic nuclei at storage rings and ion traps new experimental facilities are coming up like the already mentioned AD experiments [1] and the HITRAP [32] project. At both facilities the ions are decelerated in storage rings and then captured, bunched, cooled and finally trapped.…”

“…New precision experiments will become possible with ion traps capturing decelerated beams extracted from storage rings, for instance at the antiproton decelerator AD [1] at CERN or at the heavy ion trap experiment HITRAP [32] downstream of the ESR. In the latter experiment the ESR is used for accumulation and subsequent deceleration of highly or even fully stripped heavy ions from primary or secondary beams down to specific kinetic energies of the order of few MeV/u.…”

“…Beams of exotic heavy nuclei and of antiprotons are used for a large variety of investigations in physics ranging from the test of fundamental symmetries and physical constants [1,2] to structure studies of complex nuclei [3][4][5]. The common feature of these experiments is that the particles under investigation are not available from conventional ion sources but have to be produced in dedicated nuclear reactions [1,4,6].…”

“…The common feature of these experiments is that the particles under investigation are not available from conventional ion sources but have to be produced in dedicated nuclear reactions [1,4,6]. Therefore, it was necessary to develop techniques and devices for production, separation, accumulation, bunching, storage, and cooling of the reaction products: electromagnetic separators, storage rings, and ion traps [7].…”