Formation of long-lived gas-phase antiprotonic helium atoms and quenching by H2

Metastable antiprotonic atoms were observed to endure for several microseconds in gaseous helium. Although the metastability of antiprotons in liquid helium had been previously verified, this is the first evidence that the particles are equally long-lasting in gas-phase helium. Furthermore, this metastability was terminated abruptly and completely by introducing hydrogen gas in minute quantities.