Eur. Phys. J. D (2018) 72: 53 https://doi.org/10.1140/epjd/e2018-80721-y THE EUROPEAN PHYSICAL JOURNAL D Colloquium Experimental progress in positronium laser physics David B. Cassidya Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK Received 23 November 2017 / Received in final form 29 December 2017 Published online 27 March 2018 c The Author(s) 2018. This article is published with open access at Springerlink.com Abstract. The field of experimental positronium physics has advanced significantly in the last few decades, with new areas of research driven by the development of techniques for trapping and manipulating positrons using Surko-type buffer gas traps. Large numbers of positrons (typically ≥106) accumulated in such a device may be ejected all at once, so as to generate an intense pulse. Standard bunching techniques can produce pulses with ns (mm) temporal (spatial) beam profiles. These pulses can be converted into a dilute Ps gas in vacuum with densities on the order of 107 cm−3 which can be probed by standard ns pulsed laser systems. This allows for the efficient production of excited Ps states, including long-lived Rydberg states, which in turn facilitates numerous experimental programs, such as precision optical and microwave spectroscopy of Ps, the application of Stark deceleration methods to guide, decelerate and focus Rydberg Ps beams, and studies of the interactions of such beams with other atomic and molecular species. These methods are also applicable to antihydrogen production and spectroscopic studies of energy levels and resonances in positronium ions and molecules. A summary of recent progress in this area will be given, with the objective of providing an overview of the field as it currently exists, and a brief discussion of some future directions. 1 Introduction author learned from Professor Arthur Ruark that he had previously envisaged the existence of the particular entity The modern (cf. [1]) concept of antimatter was first composed of one electron and one positron. Dr. Ruark revealed in the form of anti-electrons (positrons), pre- has discussed the optical spectrum and the life time of dicted to exist by the relativistic quantum theory of Dirac this two-particle system in a note dated September 23, [2{5]. These particles were soon observed experimentally 1945, which he intends to submit for publication to the in cloud chamber experiments by Anderson [6,7], and then physical review in the form of a \Letter to the Editor". A also by Blackett and Ochlialini [8]. Almost immediately reference to unpublished work by L. Landau on the prop- Mohoroviˇci´csuggested that a positron and an electron erties of the bi-electron has been made by Alichanian, A. could form a hydrogen-like bound state (which he called and T. Asatiani. 1945. J. Phys. USSR. 9: 56." \electrum"), evidence of which might be found in astro- Thus, one could say that a prediction of the basic con- physical observations of spectral recombination lines [9]. cept of an electron{positron bound state can be indepen- This suggestion was either unappreciated, or unknown dently attributed to at least five different people (probably (opinions vary [10,11]), but in any case was not mentioned more), starting with Mohoroviˇci´c.The experimental real- in several later works that independently predict the exis- ization of Ps, however, belongs entirely to Martin Deutsch. tence of what we now call positronium, and describe in In a series of remarkable experiments beginning in 1951 more detail some of its properties. The first of these is evidence for Ps production was obtained by observing the Ph.D. thesis of Pirenne [12,13]. As mentioned by Beck changes in annihilation lifetimes due to interactions with in 1946 [14], Pirenne performed calculations of Ps decay various gases [17]. This result was confirmed shortly rates in Paris in 1942, but this work was not widely known thereafter by Pond [18]. At the time there were several at the time because of war in Europe. three-photon decay rate calculations that did not agree. Two more independent and simultaneous predictions of The measurements were able to resolve the conflict [19], the existence of Ps were made in the US by Ruark [15] and proving that a calculation by Ore and Powell [20] which Wheeler [16]. Wheeler's paper, which also describes Ps found a lifetime of 1:4 × 10−7 s was indeed correct. As ions and molecules (polyleptons), contains the following pointed out by Deutsch, this was one of the first instances footnote: in which \experimental verification of a theoretical result \On October 5, four days after the present paper was has been possible for a third-order radiation process" submitted to the New York Academy of Sciences, the [19]. The next experiment in this remarkable series was a measurement of the hyperfine splitting, observed via the a e-mail: [email protected] magnetic field dependence of singlet{triplet mixing [21]. Page 2 of 72 Eur. Phys. J. D (2018) 72: 53 This result, subsequently verified by Pond and Dicke [22], relatively little work on Ps laser spectroscopy was done was obtained by observing the fraction of two and three- for decades as a result. The development of positron traps photon decays in different magnetic fields. Realizing the has changed this situation since it is now much easier to limitations of this method, a more accurate technique was generate intense pulsed positron beams in smaller-scale developed [23{25] which remains the standard method for laboratories: at one point Surko traps could even be pur- precision hyperfine interval measurements. Deutsch later chased commercially, although this is no longer the case. went on to perform positron annihilation in flight mea- Pulsed sources of Ps atoms, ions, and even molecules surements [26] which also became antecedents of modern are ideally suited to laser spectroscopy (as discussed in day measurements (e.g., [27,28]). Sects. 3 and 4), and hence many of the impediments to The work of Deutsch and colleagues did not imme- such work have been removed. diately provoke widespread experimental Ps research, The purpose of this colloquium is to provide a com- although in the following decades some measurements plete overview of the experimental research conducted in were conducted, including studies of magnetic quench- this burgeoning field to date. This might appear to be a ing [29{31] and electric field effects on Ps in gases [32], daunting task but, as is evident from Table 1, most of the (unsuccessful) searches for Ps Lyman alpha lines [33], this work was been done in the last 8 years. Some excellent measurements of Ps formation [34] and diffusion [35] in reviews of Ps physics are already available, particularly solid materials, powders [36], and liquids [37], positron those by Rich [70] and Berko and Pendleton [71]. These polarization measurements [38], symmetry tests [39] and reviews focus mainly on fundamental Ps physics and QED more refined hyperfine interval measurements [40], as well tests, and have served as introductions to Ps physics for as Ps decay rate measurements [41]. All of this work generations of researchers. However, they are both now utilized positrons emitted from radioactive sources and more than 35 years old (older than most PhD students). captured in the material in which Ps formation occurred. Their continuing utility is in part a testament to the excel- A significant breakthrough was the development of slow lence of these articles, but they both predate all Ps laser positron beams following positron moderation; creating experiments. Some recent publications cover various areas an energy tunable slow positron beam had been a focus of Ps laser spectroscopy, (e.g., [72{76]), and Nagashima of study for some time [42], with some success [43], but has written a thorough review on experiments with Ps the first instance in which a slow positron beam was ions, including photodetachment and optical excitation of used to perform a measurement was in the determination shape resonances [77]. of positron-helium scattering cross sections at University Of course, much work has been done in other (non- College London (UCL) [44]. In this work a \smoked MgO" optical) areas of experimental positron and positronium moderator was employed, producing around 2 positrons research (e.g., [78{80]). Some examples include new mea- per second. At around the same time a beam of 0.5 slow surements of the Ps ground state hyperfine interval that positrons per second was produced using Au on mica as may solve the long-standing discrepancy between the- a moderator [45]. However, moderation efficiencies were ory and experiment [81], the resolution of the (perhaps soon improved by orders of magnitude [46], and useful related) positronium \lifetime puzzle" [82,83], advances slow positron beams became an experimental reality [47]. in positron scattering from atoms and molecules [84{86], Experiments conducted using slow positron beams led antihydrogen research [87{90], positron beam [91{93] and to an improved understanding of positron-solid interac- trap [66,69] development, materials science [65,94], and tions [48], which in turn made it easier to generate higher surface physics [95] (including positron diffraction [96,97] intensity slow positron beams (e.g., [49{51]) as well as and positron induced Auger emission [98,99]). These more efficient ways to generate Ps atoms (e.g., [47,52]). areas, and others, will not be discussed here. In 1986 the solid neon moderator was introduced [53], and this methodology still underpins the state-of-the-art in slow positron beam production using radioactive iso- 2 Properties of positronium topes today [54{61]. The availability of positron beams has facilitated a great deal of work on experimental atomic The intrinsic properties of Ps atoms (that is, atomic positron and Ps physics [62{65], and positron-gas scat- structure and decay modes) are determined by the elec- tering measurements have informed the development of tromagnetic interaction: the Coulomb force binding a positron buffer-gas traps [66{69].