The Low Energy Anti-Proton Ring (LEAR) was a

particle accelerator A particle accelerator is a machine that uses electromagnetic fields to propel electric charge, charged particles to very high speeds and energies to contain them in well-defined particle beam, beams. Small accelerators are used for fundamental ...

CERN The European Organization for Nuclear Research, known as CERN (; ; ), is an intergovernmental organization that operates the largest particle physics laboratory in the world. Established in 1954, it is based in Meyrin, western suburb of Gene ...

which operated from 1982 until 1996. The ring was designed to decelerate and store

antiproton The antiproton, , (pronounced ''p-bar'') is the antiparticle of the proton. Antiprotons are stable, but they are typically short-lived, since any collision with a proton will cause both particles to be annihilated in a burst of energy. The exis ...

s, to study the properties of

antimatter In modern physics, antimatter is defined as matter composed of the antiparticles (or "partners") of the corresponding subatomic particle, particles in "ordinary" matter, and can be thought of as matter with reversed charge and parity, or go ...

and to create atoms of

antihydrogen Antihydrogen () is the antimatter counterpart of hydrogen. Whereas the common hydrogen atom is composed of an electron and proton, the antihydrogen atom is made up of a positron and antiproton. Scientists hope that studying antihydrogen may sh ...

. Antiprotons for the ring were created by the CERN Proton Synchrotron via the Antiproton Collector and the Antiproton Accumulator (AA). The creation of at least nine atoms of antihydrogen were confirmed by the PS210 experiment in 1995.

Experimental setup

LEAR is a multipurpose storage ring located in the South Hall of the Proton Synchrotron (PS), with a circumference of 78.5 m. Four straight sections are alternated with compact 90° bending magnets, along with eight

quadrupole A quadrupole or quadrapole is one of a sequence of configurations of things like electric charge or current, or gravitational mass that can exist in ideal form, but it is usually just part of a multipole expansion of a more complex structure re ...

doublets. The straight sections each consist of an 8m long section, where equipment such as apparatus for internal beams and electron cooling can be stored, and two short sections of 1m. The C-type (bending) magnets used are open to the outside of the ring for injection and ejection. The vacuum system used for LEAR is designed for baking at 300 °C. Bunches of usually a few 10⁹ antiprotons are skimmed off the AA and then decelerated by the PS from 3.5 GeV/c to 0.6 GeV/c. The bunch was transferred to LEAR where it could be decelerated to a minimum 100 MeV/c or accelerated to generally 1000 MeV/c. For most experiments, a "beam stretcher mode" was used, where an ultra-slow extraction provided a high-

duty A duty (from "due" meaning "that which is owing"; , past participle of ; , whence "debt") is a commitment or expectation to perform some action in general or if certain circumstances arise. A duty may arise from a system of ethics or morality, e ...

(continuous) amount of antiprotons. Another mode, "internal target", kept a beam circulating for hours, or even days, until most particles were consumed by a gas jet target. LEAR_magnet

Stochastic cooling is implemented at several stages of the LEAR experimental setup, at different momenta. The focus of stochastic cooling is to restrict the motion of particles in the beam and control their energies close to a certain value. From 1987, the setup included electron cooling, using the electron cooler from the Initial Cooling Experiment (ICE) to complement the stochastic cooling. Using cooling, high quality beams at low energies and low emittances could be produced.

Results

There was a total of 27 experiments performed during LEAR's 14 years of running. Several meson spectroscopy experiments were setup at LEAR to analyse the rare meson resonances produced in nucleon-antiproton annilhilation. These included the Crystal Barrel, OBELIX and JETSET experiments. Furthermore, matter-antimatter symmetry was investigated by studying specific proton-antiproton interactions, resulting in detailed measurements of CP violation. The mass difference between the proton and antiproton was also studied at LEAR with an accuracy in 1 part in 10¹⁰.

Conversion to LEIR

In 1996, LEAR was converted into the Low Energy Ion Ring, which has since been used in the

lead Lead () is a chemical element; it has Chemical symbol, symbol Pb (from Latin ) and atomic number 82. It is a Heavy metal (elements), heavy metal that is density, denser than most common materials. Lead is Mohs scale, soft and Ductility, malleabl ...

ion injection process for the

Large Hadron Collider The Large Hadron Collider (LHC) is the world's largest and highest-energy particle accelerator. It was built by the CERN, European Organization for Nuclear Research (CERN) between 1998 and 2008, in collaboration with over 10,000 scientists, ...

. Low energy antiproton research continues at CERN using the Antiproton Decelerator. It was built as a successor for LEAR and started operation in 2000.

References

{{CERN CERN accelerators Particle accelerators CERN facilities Particle physics facilities