Metastable states in antihydrogen formation
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Metastable states in antihydrogen formation S. Jonsell · D. P. van der Werf · M. Charlton
Published online: 6 August 2014 © Springer International Publishing Switzerland 2014
Abstract Formation of antihydrogen atoms from antiprotons immersed in a positron plasma is simulated. Special attention is devoted to the role of metastable states, arising from the near conservation of the energy stored in the cyclotron motion of the positrons. We find that the decay of such states changes the density scaling of the formation rate. Keywords Antihydrogen · Simulations · Formation
At temperatures and densities relevant to current experiments, antihydrogen is mainly ¯ + e+ . The states formed through formed through the three-body process p¯ + e+ + e+ → H this process usually have a binding energy of only a few kelvin. Since this is a three-body process, the formation rate is ∝ n2e , where ne is the positron density. This state is fragile, and will not survive in external electric fields, unless it is first stabilized through further collisions with positrons. Much more often though, these collisions will destroy the weakly bound antiatom, and only a small fraction of the antiatoms formed will gain sufficient binding energy to reach a stable state. The rate of collisions depopulating the initial state is ∝ ni ne , where ni is its population. In steady state, which is formed rapidly after a p¯ enters a e+ plasma, the formation and depopulation processes balance. Hence, the population ni of the initial state becomes ∝ ne . ¯ i → e+ + More deeply bound states j are populaed through two-body collisions (e+ + H ¯ j ). The formation rate of state j , i.e. the rate at which its population initially grows, is H
Proceedings of the 11th International Conference on Low Energy Antiproton Physics (LEAP 2013) held in Uppsala, Sweden, 10–15 June, 2013 This work was supported by the Swedish Research Council (VR) and the EPSRC (U.K.) S. Jonsell () Department of Physics, Stockholm University, Stockholm, Sweden e-mail: [email protected] D. P. van der Werf · M. Charlton Department of Physics, College of Science, Swansea University, Swansea, United Kingdom
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∝ ni ne ∝ n2e (until a steady state is reached also for state j ). Simulations have shown that antiatoms are stable against collisional ionisation once they have reached a binding energy greater than about 8 times the temperature T of the positrons [1–5]. The formation rate of a state with binding energy EB is calculated as the rate of formation of the initial state, times the probability that the antiatom survives collisions until it reaches EB . This rate can be expressed as λ = Cvb 5n2e , where b = e2 /(4π0 kB T ), v is the average velocity of the positrons and C a dimensionless and density independent recombination coefficient. In this paper we use simulations to investigate an alternative mechanism through which ¯ states can be depopulated: a density independent decay. Since this process has a density H scaling which differs from the collisional processes discuss
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