Testing Atom and Neutron Neutrality with Atom Interferometry

Introduction. Charge quantization and atom neutrality in the Standard Model (SM) are mysteries which are automatically solved when the theory is embedded in a Grand Unified group. Even then, Witten has shown [1] that in the presence of CP non-conservation magnetic monopoles acquire an electric charge that is proportional to the amount of CP violation, a non-quantized quantity. This suggests that we have to rethink our notion of atom neutrality even in the presence of a unifying group. The first experiments to test charge cancelation between the constituents of the atom came at the turn of the twentieth century [2]. These experiments placed a bound on e+p e of 10−21, a value that is only an order of magnitude larger than the bound set by recent experiments [2, 3]. Experiments to detect individual neutron charges independently required different technology, took longer to develop, and have eventually reached a sensitivity of 10−21e, similar to that of atom neutrality experiments [2, 3]. Over eighty years after the first precision experiment on atom neutrality was performed, atom interferometry pushes the precision frontier, and provides a new tool for testing fundamental physics by measuring effects on individual atoms [4]. An experiment to test the equivalence principle and modifications of gravity is already under construction [5]. In this Letter, we propose a modification of that experiment based on the scalar Aharonov-Bohm effect [6] that can detect atom and, independently, neutron charges down to 10−28e. Because of the topological nature of the AharonovBohm effect, the atoms are under the influence of pure gauge electromagnetic potentials; all electric and magnetic fields are zero. As a result, there are ideally no forces acting on the atoms, and systematics from the finite atom polarizability are avoided. If the atom carries a small charge ǫe, its wave-function will acquire a phase ǫe h̄ V t, where e is the electron charge, t is the time spent in the region of electric potential V , and ǫ is the ratio of atom charge compared to the electron charge. We begin with the experimental setup, and analyze possible systematics as well as the ultimate sensitivity of the experiment with different upgrades. We end with a discussion on the theoretical motivation behind infinitesimally charged atoms. 2r