LEAR's physics legacy

By providing an intense and clean source of antiprotons for the first time, LEAR has opened many fields of research. Although it is not easy to summarise its versatile physics programme in a few lines, this article is an attempt to do justice to the 27 experiments performed during 14 years of running and to their successful results.

 

LEAR complex, September 1983.

In the early days of LEAR operation, the antiproton intensity was quite limited and the machine was mostly used for studying antiprotonic atoms and interactions of low-energy antiprotons with nuclei. Physicists were also performing measurements of cross-sections (total, elastic, charge exchange) as a function of the antiproton momentum. These first experiments provided valuable insights into the properties of nucleon-antinucleon interactions, including their spin structure, and ruled out the existence of hypothesized multiquark states of matter referred to as “baryonium”. The ASTERIX experiment pioneered the detection of X-rays in coincidence with annihilations, which had not been possible in previous bubble chamber experiments: this helped to disentangle the quantum state of the produced resonances.

The six-fold increase of available antiprotons in 1988 (read more in "An accelerator ahead of its time") opened the way to more “antiproton-hungry” experiments. The PS170 experiment probed the proton structure by studying the ppbar → e+ereaction. Experiment PS185 was able to study hyperon production over a wide range of beam energies, in particular around the various production thresholds. 

Nucleon-antiproton annihilation provides a potentially rich source of exotic and rare meson resonance production (for example glueballs). For this reason, several large meson spectroscopy experiments were set up with complementary detection performance: the Crystal Barrel (PS197) experiment included a CsI(Tl) calorimeter for precise detection of neutral particles; the OBELIX (PS201) experiment reused the Open Axial Field Spectrometer magnet to search for mesons decaying mostly into charged particles, with good identification of strange particles. These experiments made it possible to establish or measure the quantum numbers of many mesonic states like the a0(1450), f0(1370) or f0(1500), etc. The JETSET (PS202) experiment used collisions of higher energy antiprotons with an internal hydrogen gas target to search for exotic states decaying to φφ in the 2-2.4 GeV/c2 mass region.

Another important part of the programme was dedicated to the study of fundamental symmetries, in particular matter-antimatter symmetry. By studying specific reactions (ppbar → π+K-K0 and ppbar → π-K+K0bar), the CPLEAR (PS195) experiment was able to produce “beams” of tagged flavour neutral kaons, allowing detailed measurements of CP violation – the still unique direct evidence for time reversal violation –– and allows stringent limits to be placed on CPT conservation in weak interactions. The experimental method used by CPLEAR for tagging flavour can be considered as a pioneer for some of the tagging methods used today in LHCb.

Experiments PS189, PS196 and PS205 were set-up to study the mass difference between the proton and the antiproton, another test of the CPT theorem. While PS189 (a mass spectrometer) was limited by the difficulty in slowing down antiprotons, PS196 was the first experiment which succeeded in trapping antiprotons and keeping them for extended periods (up to 2 months!). This technique allowed a very precise test of the equality of the antiproton and proton charge-over-mass ratio to be performed with an accuracy of 1 part in 1010.  The study of antiprotonic helium by PS205 allowed the independent verification of the mass and charge equalities with an accuracy of a few parts in 107. Further precision atomic measurements, this time of the X-ray transitions of the anti-proton - proton atom, were carried out by PS207, and for other anti-protonic atoms by PS209.

The dream of LEAR physicists was not only to study antiparticles but also to produce antimatter. This became a reality in 1995, with the observation of the first ever produced anti-hydrogen atoms (nine of them) obtained by colliding antiprotons with a Xenon gas target in the PS210 experiment. These successes were at the root of the decision to continue fundamental research on antimatter at CERN at the Antiproton Decelerator (AD). The recent decision to upgrade the AD complex by adding the ELENA decelerator, which will improve the antiproton trapping efficiency by two orders of magnitude, is clear proof that this field pioneered at LEAR is still of prime interest to the particle physics community.

by Philippe Bloch