18. 6. 2013. Seminar: David B. Cassidy


Tuesday 18 June 2013
"Zvonko Marić" Room, seminar organized by Laboratory for Gaseous Electronics

David B. Cassidy
Department of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT, United Kingdom

Atomic physics experiments with positronium


The fact that an electron and its antiparticle, the positron, can form a metastable bound atomic system (known as positronium, or Ps) was first experimentally demonstrated by Martin Deutsch in 1951 [1], and since that time Ps has been the subject of a wide variety of experimental studies [2]. However, because it is difficult to produce large numbers of these short-lived atoms (<140 ns) there was, until recently, very little work done using lasers to produce excited states of Ps. This situation is now starting to change because of developments in positron trapping technology [3] that make it possible to generate intense bursts of positrons and thereby create a “gas” of positronium, which can easily be probed with pulsed lasers. Moreover, if this is done at a high beam density these Ps atoms may interact with each other and undergo Ps-Ps scattering, or even form Ps2 molecules.

In this talk I will give an overview of several recent experiments that have been done in this way. This will include the formation of molecular positronium [4] and a spin polarized Ps gas [5], spectroscopy [6] and scattering [7] of confined Ps, some optical measurements of the Ps hyperfine interval [8] and the production of Rydberg states of Ps [9]. Prospects for future experiments using these Rydberg states to perform a free-fall Ps gravity measurement will be discussed.


[1] Martin Deutsch“Evidence for the Formation of Positronium in Gases” Phys. Rev. 82, 455 (1951).
[2] A. Rich, “Recent experimental advances in positronium research” Rev. Mod.Phys. 53, 127 (1981).
[3] C. M. Surko and R. G. Greaves, “Emerging science and technology of antimatter plasmas and trap-based beams” Phys. Plasmas 11, 2333 (2004).
[4] D. B. Cassidy and A. P. Mills, Jr., “The production of molecular positronium” Nature (London) 449, 195 (2007).
[5] D. B. Cassidy, V. E. Meligne, and A. P. Mills, Jr., “Production of a Fully Spin-Polarized Ensemble of Positronium Atoms” Phys. Rev. Lett. 104, 173401 (2010).
[6] D. B. Cassidy, M. W. J. Bromley, L. C. Cota, T. H. Hisakado, H. W. K. Tom, and A. P. Mills, Jr., “Cavity Induced Shift and Narrowing of the Positronium Lyman-α Transition” Phys. Rev. Lett. 106, 023401 (2011).
[7] D. B. Cassidy and A. P. Mills, Jr., “Enhanced Ps-Ps Interactions due to Quantum Confinement” Phys. Rev. Lett. 107, 213401 (2011).
[8] D. B. Cassidy, T. H. Hisakado, H. W. K. Tom, and A. P. Mills, Jr. “Positronium Hyperfine Interval Measured via Saturated Absorption Spectroscopy” Phys. Rev. Lett. 109, 073401 (2012).
[9] D. B. Cassidy, T. H. Hisakado, H. W. K. Tom, and A. P. Mills, Jr. Efficient Production of Rydberg Positronium”Phys. Rev. Lett. 108, 043401 (2012).