Progress in the study of the positronium negative ion

  Electrons can bind to their anti-particles, positrons, to form positronium (Ps). It is the lightest atom composed of two leptons and used for atomic physics and material sciences. It also provides testing ground for quantum electrodynamics.

  Ps can also bind to another electron weakly to form a positronium negative ion (Ps^-). It is one of a series of exotic systems composed of three particles with equal masses and bound through Coulomb interaction, and is the only one produced experimentally so far.

  The existence of Ps^- was theoretically discussed by Wheeler [1] in 1946 and first produced by Mills [2] in 1981 using the beam-foil method. Its production efficiency was low and researches on Ps^- were limited to the measurement of its decay rate. However, in 2008, the production efficiency was improved by double digits by using an alkali metal coated tungsten surface [3] and we are entering a new era of Ps^- research.

  In this presentation, I will talk recent experimental developments concerning Ps^- including:

(i) photodetachment of Ps^- [4],

(ii) studies of shape resonance of Ps^- [5],

(iii) determination of the Ps^- binding energy [6],

(iv) production of an energy-tunable Ps beam via photodetachment of accelerated Ps^- [7], and

(v) resonant coherent excitation of Ps [8] studied using the energy-tunable Ps beam.

  In addition to the efficient Ps^- production technology, these studies use notable technologies in this field, such as linac-based pulsed slow positron beam [9] and a buffer gas positron trap (Surko-trap) [10].