Antiproton Decelerator

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File:Experimental area at CERNs Antiproton Decelerator (AD) Hall.jpg
CERNs AD with the ALPHA, ASACUSA and ATRAP collaborations.

The Antiproton Decelerator (AD) is a storage ring at the CERN laboratory in Geneva. It was built as a successor to the Low Energy Antiproton Ring (LEAR) and started operation in the year 2000. The decelerated antiprotons are ejected to one of several connected experiments.

AD experiments

AD experiments
AD1 ATHENA Alberto Rotondi Antihydrogen production and precision experiments  ?? 12 Jun 1997  ?? 16 Nov 2004 SPIRES
Grey Book
AD2 ATRAP Gerald Gabrielse Cold antihydrogen for precise laser spectroscopy  ?? 12 Jun 1997  ?? Running SPIRES
Grey Book
AD3 ASACUSA Ryugo Hayano Atomic spectroscopy and collisions using slow antiprotons 7 Oct 1997 20 Nov 1997  ?? Running SPIRES
Grey Book
AD4 ACE Michael Holzscheiter Relative biological effectiveness and peripheral damage of antiproton annihilation  ?? 6 Feb 2003  ?? Running Grey Book Website
AD5 ALPHA Jeffrey Hangst Antihydrogen laser physics apparatus  ?? 2 Jun 2005  ?? Running Grey Book Website
AD6 AEGIS Michael Doser Antihydrogen experiment gravity interferometry spectroscopy 8 Jun 2007 5 Dec 2008 Not yet N/A Grey Book Website


ATHENA was an antimatter ((cellular)) research project that took place at the Antiproton Decelerator. In August 2002, it was the first experiment to produce 50,000 low-energy antihydrogen atoms, as reported in Nature. In 2005, ATHENA was disbanded and many of the former members worked on the subsequent ALPHA experiment.

ATHENA physics

For antihydrogen to be created, antiprotons and positrons (also called antielectrons) must first be prepared. The antiprotons are provided by the Antiproton Decelerator, while positrons are obtained from a positron accumulator. Both are then led into a recombination trap, where they bind together and form an antihydrogen atom. After preparation, a high-resolution detector confirms that antihydrogen was created. It then looks at the antihydrogen spectrum in order to compare it against "normal" hydrogen spectrum.

ATHENA collaboration

The ATHENA collaboration comprised the following institutions: Template:Columns-list


The ATRAP collaboration at CERN developed out of TRAP, a collaboration whose members pioneered cold antiprotons, cold positrons, and first made the ingredients of cold antihydrogen to interact. ATRAP members also pioneered accurate hydrogen spectroscopy and first observed hot antihydrogen atoms. The motto of the ATRAP collaboration is "It's a trap!". [citation needed]

ATRAP physics

Template:Expand section ATRAP is a collaboration between physicists around the world with the goal of creating and experimenting with antihydrogen. ATRAP accumulates positrons with using a radioactive sodium-22 source. These positrons are trapped in a Penning trap and then combined with antiprotons to create antihydrogen. The long-term goal is to trap antihydrogen in a Ioffe trap and collect enough antihydrogen to perform accurate laser spectroscopy on it.

ATRAP collaboration

The ATRAP collaboration comprises the following institutions: Template:Columns-list


ASACUSA (Atomic Spectroscopy And Collisions Using Slow Antiprotons) is an experiment testing for CPT-symmetry by laser spectroscopy of antiprotonic helium and microwave spectroscopy of the hyperfine structure of antihydrogen. It also measures atomic and nuclear cross sections of antiprotons on various targets at extremely low energies. The spokesperson for the experiment is Ryugo S. Hayano from the University of Tokyo. It was originally proposed in 1997.


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The ALPHA experiment is designed to trap neutral antihydrogen in a magnetic trap, and conduct experiments on them. The ultimate goal of this endeavour is to test CPT symmetry through comparison of the atomic spectra of hydrogen and antihydrogen (see hydrogen spectral series). The ALPHA collaboration consists of some former members of the ATHENA collaboration (the first group to produce cold antihydrogen, in 2002), as well as a number of new members.

ALPHA physics

ALPHA faces several challenges. Magnetic traps – wherein neutral atoms are trapped using their magnetic moments – are notoriously weak; only atoms with kinetic energies equivalent to less than one kelvin may be trapped. The cold antihydrogen created first in 2002 by the ATHENA and the ATRAP collaborations was produced by merging cold plasmas of positrons (also called antielectrons) and antiprotons. While this method has been quite successful, it creates antiatoms with kinetic energies too large to be trapped. Furthermore, to do laser spectroscopy on these anti-atoms, it is important that they are in their ground state, something which does not seem to be the case for the majority of the anti-atoms created thus far.

Antiprotons are received by the Antiproton Decelerator and are 'mixed' with positrons from a specially-designed positron accumulator in a versatile Penning trap. The central region where the mixing and thus antihydrogen formation takes place is surrounded by a superconducting octupole magnet and two axially separated short solenoids "mirror-coils" to form a "minimum-B" magnetic trap. Once trapped antihydrogen can be subjected to detailed study and be compared to hydrogen.

In order to detect trapped antihydrogen atoms ALPHA also comprises a silicon vertex detector. This cylindrically shaped detector consists of three layers of silicon panels (strips). Each panel acts as a position sensitive detector for charged particles passing through. By recording how the panels are excited ALPHA can reconstruct the tracks of charged particles traveling through their detector. When an antiproton annihilates (disintegrates) the process typically results in the emission of 3-4 charged pions. These can be observed by the ALPHA detector and by reconstructing their tracks through the detector their origin, and thus the location of the annihilation, can be determined. These tracks are quite distinct from the tracks of cosmic rays which are also detected but are of high energy and pass straight through the detector. By carefully analyzing the tracks ALPHA distinguishes between cosmic rays and antiproton annihilations.

To detect successful trapping the ALPHA trap magnet that created the minimum B-field was designed to allow it to be quickly and repeatedly de-energized. The currents' decay during de-energization has a characteristic time of 9 ms, orders of magnitude faster than similar systems. This fast turn-off and the ability to suppress false signal from cosmic rays should allow ALPHA to detect the release of even a single trapped antihydrogen atom during de-energization of the trap.

In order to make antihydrogen cold enough to be trapped the ALPHA collaboration has implemented a novel technique, well known from atomic physics, called evaporative cooling.<ref>Template:Cite web</ref> The motivation for this is that one of the main challenges of trapping antihydrogen is to make it cold enough. State-of-the art minimum-B traps like the one ALPHA comprises have depths in temperature units of order one Kelvin. As no readily available techniques exist to cool antihydrogen, the constituents must be cold and kept cold for the formation. Antiprotons and positrons are not easily cooled to cryogenic temperatures and the implementation of evaporative cooling is thus an important step towards antihydrogen trapping.

ALPHA collaboration

The ALPHA collaboration comprises the following institutions: Template:Columns-list


AEGIS (Antimatter experiment: Gravity, interferometry, spectroscopy), is a proposed experiment to be set up at the Antiproton Decelerator.

AEGIS physics

AEGIS would attempt to determine if gravity affects antimatter in the same way it affects matter by testing its effect on an antihydrogen beam. By sending a stream of antihydrogen through a series of diffraction gratings, the pattern of light and dark patterns would allegedly enable the position of the beam to be pinpointed with up to 1% accuracy. It was originally proposed in 2007.

AEGIS collaboration

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See also

  • Antimatter
  • Antihydrogen
  • Gravitational interaction of antimatter



Further reading

External links


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es:Proyecto Athena fr:ALPHA (expérience) fr:AEGIS (Physique des particules) pt:ATRAP simple:ALPHA Collaboration