In any Lorentz-invariant local field theory, particle and antiparticle masses must be identical. That equality has been verified to high precision for leptons and hadrons, but not for quarks. With one exception, it’s impossible to measure quark masses directly because a newly created quark “dresses itself” in other quarks and gluons to form a hadron within 10−22 seconds. And hadron masses yield, at best, only rough estimates of the quark masses. The exception is the top quark. Almost 200 times heavier than the proton, the top is by far the most massive quark. Its lifetime of 10−24 seconds is much too brief to form a hadron. Thus by measuring its decay products, experiments at Fermilab’s Tevatron collider have determined the top mass (173 GeV) with a precision of better than 1%. Those experiments were based on the production of top–antitop pairs, and the analyses assumed that the masses...

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