Kaonium is an exotic atom consisting of a bound state of a positively charged and a negatively charged kaon. Kaonium has not been observed experimentally and is expected to have a short lifetime on the order of 10−18seconds.[1][2]
^Krewald, S.; Lemmer, R. H.; Sassen, F. P. (2004). "Lifetime of kaonium". Physical Review D. 69 (1): 016003. arXiv:hep-ph/0307288. Bibcode:2004PhRvD..69a6003K. doi:10.1103/PhysRevD.69.016003. S2CID 119336701.