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ATHENA - Scientific description

Physics Motivation

The goal of ATHENA is the direct comparison of the properties of antihydrogen and hydrogen atoms with very high precision.

The apparent absence of antimatter in the Universe together with the Big Bang theory suggest that antihydrogen atoms - made of an antiproton and a positron - have never existed in our Universe. Although the Big Bang initially produced equal amounts of antimatter and matter, a small asymmetry in favour of matter evolved within a tiny fraction of a second. In the subsequent annihilation of matter and antimatter, only this very small surplus of matter did not transform into radiation. This is the matter our Universe is made of today - and antihydrogen atoms never formed part of it.

By comparing atoms of matter and antimatter, we verify if the symmetry between matter and antimatter really is complete - to any level of precision - or not. Even a tiny difference e.g. in the energy spectrum of antihydrogen and hydrogen would be of enormous consequence for our understanding of the fundamental principles of nature. A possible reason could be gravity. The gravitational attraction between matter particle has been verified experimentally to great precision, but a direct measurement of antimatter-matter gravitational effects is missing.

As a by-product, we will study the interaction of the antihydrogen atom with atoms like hydrogen or oxygen. This is of interest for the continuing search for antimatter galaxies in the Universe. If they existed, there would be regions in space where such antiatom-atom collisions would lead to an observable signal. Our experiment could give information on the strength of this signal.

Experimental method.

Cold antihydrogen atoms are made according to the following prescription: 1) capture antiprotons into a 'trap' and cool them to milli-eV energies; 2) capture positrons into a similar way; 3) bring the two plasmas into contact without increasing their respective temperature.

How do we realise this in practice ? How do we know that we have produced antihydrogen atoms? And how do we compare antihydrogen with hydrogen? The design of the ATHENA (AnTiHydrogEN Apparatus) experiment addresses these questions.

Antiprotons coming from the AD with a kinetic energy of 5.3 MeV are further decelerated, using thin degrader foils. They enter a Penning-Malmberg trap (2.5 cm diameter, 50 cm long) - made of several cylindrically symmetric electrodes - in which charged particles can be confined axially by a series of electric potentials and radially by a 3 Tesla magnetic solenoid field. Antiprotons with kinetic energies below 15 keV are captured by reflecting them from the exit electrode and by switching the entrance electrode to -15 kV after the beam has entered the trap. These antiprotons are then cooled by the interaction with a cloud of cold electrons, which has been injected into the trap before the arrival of the antiprotons. Electrons in a strong magnetic field emit synchrotron radiation and quickly reach thermal equilibrium with the environment. Hence the trapped antiprotons are also cooled down to the ambient temperature of 4 K. The apparatus is designed to capture and to cool up to 10^6 antiprotons.

Positrons are accumulated separately and continuously in the 'positron accumulator'. After emission from a Na-22 source, the positrons are moderated by a thin layer of solid neon and propagate along a magnetic guiding field into another Penning-Malmberg trap. Inside they are accumulated with high efficiency using a buffer gas moderation technique. About 108 positrons per minute can be accumulated, cooled to room temperature, compressed and then transferred into the UHV region of the experiment.

The two oppositely charged plasmas are then brought into contact inside the recombination trap. Several techniques for achieving a good overlap of antiprotons and positron clouds have been proposed and will be explored in the first phase of the experiment. With 10^6 antiprotons in contact with 10^8 positrons, we expect the formation rate of antihydrogen in its ground state to be in the range of 10-100 per second.

After antihydrogen atoms are formed, they drift to a near-by electrode and annihilate, emitting a characteristic signal which is detected by a sophisticated detector surrounding the formation region. It consists of of 2 layers of 16 Si-strip detectors, surrounded by a cylindrical layer of 192 small CsI crystals, which are read out by photodiodes. The detector measures the annihilation vertex using the charged pion tracks from the antiproton-nucleon annihilation, and also both 511 keV gammas from positron-electron annihilation, thus allowing a complete reconstruction of the antihydrogen annihilation in space and time. The high granularity of the detector is essential to distinguish antihydrogen from antiproton annihilations.

For the second phase of the experiment, we will construct a magnetic bottle to store antihydrogen atoms with sufficiently low kinetic energy. Stable laser beams with very well defined wave length will then be tuned to excite transitions between different energy levels of the atom. The crucial question is if a different wavelength is needed for the excitation of antihydrogen or hydrogen atoms. We are also investigating the possibility to produce beams of antihydrogen for other types of experiments.

 

Selected scientific publications of ATHENA members relevant to antihydrogen

: Trapping and Cooling Antiprotons

  • M. Holzscheiter et al., Phys. Lett. A214, 279 (1996)
  • X. Feng et al., J. Appl. Phys. 79, 8 (1996)

Ultrahigh precision two-photon laser spectroscopy of trapped atomic hydrogen:

  • C.L. Cesar et al., Phys.Rev. Lett. 77, 255-258 (1996).

Ultra-low energy antihydrogen production:

  • M.H. Holzscheiter and M. Charlton, Rep. Prog. Phys. 62, 1 (1999)

 

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