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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Spontaneous recombination of antihydrogen in a magnetic field: a new approach

Vladislav Viktorovich Serov, Vladimir Leonardovich Derbov

Published Jul 13, 2004• 2 citations
Publisher

Abstract

We propose a new mechanism that can contribute to the spontaneous recombination of antihydrogen atoms in traps containing cold positron-antiproton plasma in the presence of a constant magnetic field. The latter plays a principal role restricting the motion of the positron in the transverse plane and, finally, prolonging the time during which the positron can lose its energy due to spontaneous radiation. Classical and quantum mechanical estimates are presented under the conditions close to those experimentally realized in the existing traps.

DOI
10.1117/12.578938

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We propose a new mechanism that can contribute to the spontaneous recombination of antihydrogen atoms in traps containing cold positron-antiproton plasma in the presence of a constant magnetic field. The latter plays a principal role restricting the motion of the positron in the transverse plane and, finally, prolonging the time during which the positron can lose its energy due to spontaneous radiation. Classical and quantum mechanical estimates are presented under the conditions close to those experimentally realized in the existing traps.
To increase the efficiency of laser-induced recombination of antihydrogen from cold antihydrogen—positron plasma in a trap, it is proposed to use a new resonance mechanism with the participation of positron quasi-stationary states, arising under the joint action of an antiproton Coulomb field and a strong magnetic field of the trap. The recombination rate is expressed through the atomic laser ionization cross section whose frequency dependence is nonmonotonic due to the presence of quasi-stationary states against the background of the continuum. The estimates with the use of the ionization cross sections calculated earlier demonstrate the possibility of improving the efficiency of the laser-induced recombination at an optimally selected laser frequency.
We have simulated the formation of antihydrogen through three-body recombination using classical trajectories of antiprotons and positrons. The simulations include several effects which are important in current antihydrogen experiments: the full motion of the antiproton repeatedly passing into and out of the positron plasma, the energy loss of antiprotons due to the interaction with the positron plasma, and the field-ionization of antihydrogen en route from the plasma to the detector. We find that whereas the overall simulated rate of formation of antihydrogen has a density dependence close to n 2 e , the rate of antihydrogen detection follows a power law less than 2. The difference is due to the effect of density dependent field ionization.
The formation of antihydrogen atoms from antiprotons injected into a positron plasma is simulated, focussing on the fraction that fulfil the conditions necessary for confinement of anti-atoms in a magnetic minimum trap. Trapping fractions of around 10 −4 are found under conditions similar to those used in recent experiments, and in reasonable accord with their results. We have studied the behaviour of the trapped fraction at various positron plasma densities and temperatures and found that collisional effects play a beneficial role via a redistribution of the antihydrogen magnetic moment, allowing enhancements of the yield of low-field seeking states that are amenable to trapping.
The merging of antiprotons with a positron plasma is the predominant and highest efficient method for cold antihydrogen formation used to date. We present experimental evidence that this method has serious disadvantages for producing antihydrogen cold enough to be trapped. Antihydrogen is neutral but may be trapped in a magnetic field minimum. However, the depth of such traps are of order 1 K, shallow compared to the kinetic energies in current antihydrogen experiments. Studying the spatial distribution of the antihydrogen emerging from the ATHENA positron plasma we have, by comparison with a simple model, extracted information about the temperature of the antihydrogen formed. We find that antihydrogen is formed before thermal equilibrium is attained between the antiprotons and the positrons, and thus that further positron cooling may not be sufficient for producing antihydrogen cold enough to be trapped. We discuss the implications for trapping of antihydrogen in a magnetic trap, important for ongoing work by the ALPHA collaboration.
To enhance the laser-stimulated recombination of antihydrogen from cold antiproton-positron plasma in a trap we propose to use a new resonance mechanism involving the quasi-stationary states of the positron that arise from the joint action of the Coulomb field of the antiproton and the strong magnetic field of the trap. The recombination rate is expressed via the cross-section of laser ionization of the atom that has strongly non-monotonic frequency dependence due to the presence of quasi-stationary states merged into the continuum background. The estimates using previously calculated ionization cross-section show the possibility to enhance the laser-stimulated recombination by means of the optimal laser frequency choice.