Antihydrogen TRAP Collaboration - ATRAP

The year 2000 marks the inauguration of CERN's new antimatter factory.  Antiprotons with energies of 6 MeV are now being sent to experiments.  ATRAP is one of these experiments.  The goal of ATRAP is to produce cold antihydrogen atoms for the first time, to store these atoms in a magnetic trap, and to use extremely precise laser spectroscopy to see just how similar are the properties of antihydrogen and hydrogen atoms.  Antihydrogen is the simplest antimatter atom; it consists of a positron in an orbit around an antiproton.  It is the antimatter counterpart of the hydrogen, the simplest matter atom, in which an electron orbits around a proton.

     Antimatter particles have been available at CERN for a long time, and the TRAP Collaboration (the predecessor of ATRAP), working at CERN's LEAR facility,  developed the crucial techniques that all the collaborations working at the AD intend to use in one way or another.  These techniques allow extremely cold antiprotons to be accumulated in the small volume of an ion trap.  In particular,
 

1986 --	TRAP demonstrates first slowing of LEAR antiprotons to energies below 3 keV.
1986 --	TRAP demonstrates the first capture of LEAR antiprotons into the tiny volume of an ion trap.
1989 --	TRAP holds 100,000 antiprotons for 2 months, demonstrating an incredibly good vacuum (< 3 x 10-17 Torr).
1989 --	TRAP demonstrates the first electron cooling of antiprotons in an ion trap. The cold antiprotons in our ion trap are more than ten billion times lower in energy than are the antiprotons received from CERN's antimatter factory.
1989 --	TRAP demonstrates the first stacking of antiprotons in a ion trap.
1990 --	TRAP shows that the magnitude of the charge-to-mass ratio of the antiproton and proton are the same to 4 parts in 108 or better.
1992 --	TRAP observes the radio signal from a single antiproton isolated by itself in a trap.
1995 --	TRAP shows that the magnitude of the charge-to-mass ratio of the antiproton and proton are the same to 1 parts in 109 or better.
1999 --	TRAP shows that the magnitude of the charge-to-mass ratio of the antiproton and proton are the same to 9 parts in 1011 or better.  This is by far the most sensitive test of CPT invariance for a baryon system.
1999 --	TRAP reports the fist simultaneous confinement of the ingredients of cold antihydrogen.

     The strategy of CERN's new antimatter factory is built upon the TRAP demonstration that antiprotons can be accumulated (i.e. stacked) in the small volume of an ion trap more economically than in large storage rings.  CERN was thus able to shut down three large storage rings, and replace them with one storage ring, thus saving resources and allowing the quest for cold antihydrogen to continue.

     ATRAP members have clearly been preparing for this opportunity for a long time.  The antihydrogen goal was clearly enunciated shortly after TRAP captured antiprotons in an ion trap for the first time.

``For me, the most attractive way ... would be to capture the antihydrogen in a neutral particle trap ... The objective would be to then study the properties of a small number of [antihydrogen] atoms confined in the neutral trap for a long time."

Gerald Gabrielse, 1986 Erice Lecture (shortly after first trapping of antiprotons)
"Penning Traps, Masses and Antiprotons", in Fundamental Symmetries,
edited by P. Bloch, P. Paulopoulos and R. Klapisch, p. 59 (Plenum, New York, 1987).

Photo by P. Horowitz

The initial ATRAP experiments will take place in the world's most intricate Penning trap structure, pictured to the left.  The trap consists of a long series of gold-plated, copper rings, each of which has an interior diameter of 1.2 centimeters.  The electrodes are cooled to 4 degrees above absolute zero (-269 degrees Celsius) during operation. The apparatus is located within a very strong (6 Tesla) magnetic field that is directed along the direction of the central axis of the trap.      Antiprotons enter the trap apparatus from below, and are captured in the lower section of the trap (below the place where the diameter of the support structure becomes larger.)  When captured they oscillate within this lower structure, until collisions with cold electrons stored within lower their energy dramatically.  They end up in the center of the lower region with a temperature that is also only 4 degrees above absolute zero.      Positrons from a carefully shielded radioactive source enter the trap apparatus from above.  They will be accumulated in the upper section of the trap, at the same time that antiprotons are being accumulated in the lower section.       When cold antiprotons and cold positrons have both been accumulated, then a mechanical valve that separates the lower and upper regions of the trap will be opened, and the ingredients of cold antihydrogen will be allowed to interact.  If antihydrogen is formed it will no longer be trapped in the Penning trap.  It will drift to the walls of the trap and annihilate.  We will look for simultaneous annihilations of an antiproton and a positron (in detectors not pictured) as the signature that cold antihydrogen has been formed for the first time.      The trap shown was constructed by ATRAP collaborators from Harvard University; this group is also coordinating the ATRAP apparatus and operation.  A scintillating fiber detector built at the Institute for Nuclear Physics in Juelich, and a BGO detector built at the University of Bonn, will establish that cold antihydrogen has been formed.  The beam is steered using a parallel plate avalanche detector built at IMEP in Vienna.       Once cold antihydrogen is formed, and its formation is optimized, then a new apparatus (under construction) will replace the trap shown.  It will allow include a magnetic trap intended to capture the cold antihydrogen, and will allow the introduction of lasers for precise spectroscopy.

     With the first well steered antiprotons available from the Antiproton Decelerator (AD), CERN's new antimatter factory, ATRAP was able to make significant progress.  The figure below left shows the first antiprotons trapped in an ion trap at the AD.  The figure below right shows the first electron-cooling of trapped antiprotons at the AD. These signals were observed by slowly lowering the trapping potential to let the trapped antiprotons escape from the trap.  The annihilations of the escaping antiprotons was detected when they hit the walls of the trap.  More details and any new developments are available at http://hussle.harvard.edu/~atrap.
 
 
 

"Precision Mass Spectroscopy of the Antiproton and Proton Using Simultaneously Trapped Particles",
G. Gabrielse, A. Khabbaz, D.S. Hall, C. Heimann, H. Kalinowsky and W. Jhe;
Phys. Rev. Lett. 82, 3198 (1999).
        The most stingent test of CPT invariance with baryons also demonstrates the sensitive radiofrequency
          diagnostics that will be used to monitor the ingredients of cold antihydrogen.

"The Ingredients of Cold Antihydrogen: Simultaneous Confinement of Antiprotons and Positrons at 4K",
G. Gabrielse, D.S. Hall, T. Roach, P. Yesley, A. Khabbaz, J. Estrada, C. Heimann, and H. Kalinowsky;
Phys. Lett. B 455, 311 (1999).
        First time that cold antiprotons and positrons have occupied the same vacuum container and been made to interact.

"Observing the Quantum Limit of an Electron Cyclotron: QND Measurements of Quantum Jumps Between Fock States",
S. Peil and G. Gabrielse;
Phys. Rev. Lett. 83, 1287 (1999).
        Demonstration (with electrons) of techniques that will be used to confine and cool antiprotons and positrons at 70 mK temperatures,
          the first time that charged particles have ever been stored at temperatures below 4.2 K.

"Continuous Wave Coherent Lyman-alpha Radiation",
K.S.E. Eikema, J. Walz, and T.W. Haensch
Phys. Rev. Lett. 83, 3828 (1999).
        First demonstration of a coherent source of Lyman alpha radiation.
          This source was developed for antihydrogen spectroscopy and cooling.

"Field ionization of Strongly Magnetized Rydberg Positronium: A New Physical Mechanism for Positron Accumulation",
J. Estrada, T. Roach, J.N. Tan, P. Yesley and G. Gabrielse;
Phys. Rev. Lett. 84, 859 (2000).
        Most efficient mechanism yet demonstrated for accumulating 4.2 K positrons
          in the cyrogenic vacuum desired for the production of cold antihydrogen.

"Field-Induced Electron-Ion Recombination, a Novel Route Towards Neutral (Anti-)Matter"
C. Wesdorp, F. Robicheaux, L.D. Noordam
Phys. Rev. Lett. 84, 3799 (2000).
        Demonstration (with electrons and ions) of a recombination mechanism
          that may allow the efficient production of cold antihydrogen.