Answer: kaon

AnswerCrossword Clue
KAONUnstable particle
KAONUnstable subatomic particle
KAONParticle named for a letter of the alphabet
KAONUnstable meson
kaona meson that has a positive or negative charge and a rest mass of about 966 electron masses, or no charge and a rest mass of 974 electron masses
kaonone of a subatomic particle