We report on laser cooling of a large fraction of positronium (Ps) in
free-flight by strongly saturating the 13S-23P transition with a
broadband, long-pulsed 243 nm alexandrite laser. The ground state Ps cloud is
produced in a magnetic and electric field-free environment. We observe two
different laser-induced effects. The first effect is an increase in the number
of atoms in the ground state after the time Ps has spent in the long-lived
33P states. The second effect is the one-dimensional Doppler cooling of Ps,
reducing the cloud's temperature from 380(20) K to 170(20) K. We demonstrate a
58(9) % increase in the coldest fraction of the Ps ensemble.Comment: 6 pages, 5 figure