Pendellösung Interferometric Measurement of the Neutron Charge Radius and Constraints on BSM Forces*

Pendellösung interference occurs between internal neutron wave states in a Bragg-diffracting crystal. The absolute phase of the interference fringes is directly related to the neutron-crystalline potential. Recent measurements of silicon Bragg reflections with Miller indices (111), (220), and (400) have resulted in a determination of the neutron mean square charge radius of -0.1101 ± 0.0089 fm² and placed limits on a Yukawa modification to gravity on atomic length scales. Further measurements of other Bragg reflections and crystal species can improve the precision of the charge radius measurement, as well as further constrain Beyond the Standard Model forces.