Probing the electric neutrality of the neutron

Very high precision can be achieved by neutron measurements, due to the fact that neutrons are electrically neutral. Therefore, hypothetical forces and interactions being many orders of magnitude smaller than the electromagnetic force can be searched for, or tiny effects that are otherwise masked can be measured. That the neutron is a particle having zero electric charge has been checked by beam-deflection experiments, where slow neutrons pass through a strong electric field perpendicular to the beam direction. The value of the electric charge of the neutron could be restricted to be smaller than 10-21 times the electric charge of the electron. We propose to test the electric neutrality of neutrons at small distances by a new technique using the spectroscopy of quantum states in the gravity potential above a vertical mirror for ultracold neutrons. The new technique is an application of Ramsey`s method of separated oscillating fields to neutron`s quantum states in the gravity potential of the earth. In the presence of an electric field, the energy of the quantum states changes due to an additional electrostatic potential if a neutron carries a nonvanishing charge. The energy shift differs from state to state due to the special properties of the wave function in a linear potential. The energy difference between two quantum states with and without applying an electric field is measured in the experiment. In the long run our new method has the potential to improve the current limit for the electric charge of the neutron by 2 orders of magnitude.

This project has the aim to test neutrons neutrality at small distances by a new technique using quantum interference. For that purpose we have developed and explored a system consisting of a particle, the ultra-cold neutron, and a macroscopic object, a mirror-system, and its precise measurement of quantum de Broglie phases as a function of an electric field. The readout will be performed by an application of Ramseys method of separated oscillating fields to the spectroscopy of quantum states in the gravity potential above a vertical mirror. The new method has several decisive advantages. First of all, we use a quantum technology providing highest precision through quantum interference. Quantum interference is utilised in a variety of fields; from natural science to medicine. We developed therefore a novel resonant spectroscopy technique devoted to the study of fundamental interactions, and the new method extends the techniques of Purcel, Rabi and Ramsey to neutron quantum states in the gravity potential of the Earth. We named the new technique Gravity Resonance Spectroscopy (GRS) in close analogy to Magnetic Resonance Spectroscopy (MRS). Here a neutron in the gravity potential of the Earth is placed on a reflecting mirror and transitions between the gravitational quantum states are performed by applying mechanical oscillations of the mirror with the proper transition frequency, whereas in MRS technique, an atom, a molecule or a nucleus with a magnetic moment is placed in an outer magnetic field and transitions between the magnetic Zeeman splitting are performed by applying proper oscillations of radiofrequency fields. If the neutron had a non vanishing mass, the application of an electric field in our apparatus would shift the energy levels of the neutron in the gravity potential of the earth and can be detected within measuring accuracy. We would like to mention two further advantages of our experiment. We are able to show that - at small distances - a substantial higher electric field than in previous measurements can be achieved. Furthermore by using ultra cold neutrons, the observation time can be increased significantly. Within the first three years of a six year planning period, we have developed the following decisive components: first of all, the new gravity spectroscopy was built and tested. For the first time resonant transitions between quantum states of the neutron in the gravity potential of the earth have been observed. In parallel we were able achieve significantly higher electric fields than in previous experiment.