Renewed relevance of new tests of the equivalence principle involving intrinsic properties of particles and antiparticles

Abstract

The equivalence of the Shapiro delay for photons and (anti)neutrinos is the currently accepted lone test of the equivalence principle concerning intrinsic properties and quantum numbers of relativistic particles and seemingly constitute the only direct test for antiparticles. We show that due to the overwhelming contribution of the kinetic energy to the effective passive gravitational mass, Shapiro delay neither tests the equivalence principle nor anomalous long-range couplings related to the intrinsic properties and quantum numbers of the particles. In effect, the internal properties of the test particle are irrelevant in the context of weak equivalence principle (WEP) due to the relativistic suppression factor. The general result, independent of the nature of the long-range WEP-violating interaction assumed, underlines the potential significance of new tests.