Abstract
Positronium in the $2^3S$ metastable state exhibits a low electrical polarizability and a long lifetime (1140 ns) making it a promising candidate for interferometry experiments with a neutral matter-antimatter system. In the present work, $2^3S$ positronium is produced - in absence of electric field - via spontaneous radiative decay from the $3^3P$ level populated with a 205nm UV laser pulse. Thanks to the short temporal length of the pulse, 1.5 ns full-width at half maximum, different velocity populations of a positronium cloud emitted from a nanochannelled positron/positronium converter were selected by delaying the excitation pulse with respect to the production instant. $ 2^3S $ positronium atoms with velocity tuned between $ 7 \cdot 10^4 $ m/s and $ 10 \cdot 10^4 $ m/s were thus produced. Depending on the selected velocity, a $2^3S$ production effciency ranging from $\sim 0.8 \%$ to $\sim 1.7%$, with respect to the total amount of emitted positronium, was obtained. The observed results give a branching ratio for the $3^3P$-$2^3S$ spontaneous decay of $(9.7 \pm 2.7) \% $. The present velocity selection technique could allow to produce an almost monochromatic beam of $\sim 1 \cdot 10^3 $ $2^3S$ atoms with a velocity spread $ < 10^4 $ m/s and an angular divergence of $\sim$ 50 mrad.
Highlights
Positronium (Ps) is one of the few matter-antimatter systems being considered for probing experimentally the gravitational interaction [1,2,3,4,5].Several experimental schemes based on long-lived Ps beams have been proposed, either letting the atoms free-fall in a drift tube [6,7] or using a matter-wave atom interferometer [8] to measure their vertical displacement with a position-sensitive detector
14Kirchhoff-Institute for Physics, Heidelberg University, Im Neuenheimer Feld 227, 69120
Ps Rydberg sublevels with large electrical dipoles can be guided and focused [12,13] while the selective excitation to sublevels with low dipole moments has been proposed [14] as a method to minimize the deflection of Rydberg Ps in interferometric measurements with physical gratings [15]
Summary
Positronium (Ps) is one of the few matter-antimatter systems (with antihydrogen and muonium) being considered for probing experimentally the gravitational interaction [1,2,3,4,5].Several experimental schemes based on long-lived Ps beams have been proposed, either letting the atoms free-fall in a drift tube [6,7] or using a matter-wave atom interferometer [8] to measure their vertical displacement with a position-sensitive detector. Atoms in Rydberg states are, in general, sensitive to electric-field gradients [11] which can modify their trajectories This is due to their large electrical polarizability (up to ∼10−32 C m2 V−1 for Ps in n = 15) [12,13]. An alternative way to produce a beam of long-lived Ps with lower electrical polarizability (∼10−38 C m2 V−1) consists of laser exciting the atoms to their 2 3S metastable level [15], whose lifetime is 1.14 μs in vacuum and in the absence of an electric field [16]. A beam of 2 3S Ps atoms (of known average velocity) has been shown to be suitable for improving the inertial sensitivity in proposed matter-wave interferometric layouts [8]. The availability of 2 3S Ps with an average velocity of
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