Neutron-electron interaction in neutron interferometry

The neutron consists of three charged quarks and has therefore a nontrivial charge distribution. The second moment of the charge distribution is called charge radius, it is a fundamental parameter characterizing the internal neutron structure. The neutron`s charge radius is expected to be negative due to the cloud of negative pions at about 1 fm distance from the centre. The charge radius is accessible from the neutron-electron interaction process which is characterized by the neutron- electron scattering length bne . Because of the absence of neutron targets, it is the neutron scattering on atomic electrons that provides a possibility for an experimental determination of bne . Hitherto, most experiments for determining bne involve the measurement of the total neutron cross-section but in spite of many years of experimental attempts there is no final agreement in the experimental values of bne . We propose an alternative method which directly yields bne of a light silicon atom instead of the total cross sections of heavy atoms. Our technique employs a perfect silicon crystal interferometer whose lattice structure and atomic density is known with high accuracy. The neutron beam is generated at the ILL high-flux reactor and the neutron target is a perfect silicon crystal plate rigidly connected with the interferometer crystal because it is cut from the same ingot. In front of the target plate we refract the neutron beam by prism deflection. Although the beam deflection is only in the region of sec of arc this yields a detectable phase shift in the target plate which directly relates to bne . The proposed neutron interferometry method is extremely sensitive to a precise determination of the neutron- electron scattering length and it is to a high degree free of systematic uncertainties. The knowledge of bne and the neutron`s charge radius has an impact on a broad range of topics, which vary from the modelling of the internal quark structure in quantum chromodynamics, to the determination of the slope of the electric form factor at zero momentum transfer, and the verification of the Dirac-Pauli model for the neutron in an external electromagnetic field.

A new method has been tested for measuring the neutron interaction with an electrical charge distribution. Despite the fact that the neutron is an electrically neutral particle, its intrinsic charge distribution yields a small electrostatic interaction in addition to the dominating nuclear interaction. In our experiments the interaction takes place inside a perfect silicon crystal which is part of the crystal interferometer. Two middle plates inside the interferometer are therefore acting as neutron targets where the neutron interacts with the charge distribution of silicon, i.e., the fourteen protons in the nucleus and fourteen electrons in the atomic shell. The interferometer`s sensitivity can be manipulated by deflecting the neutron beam in front of the target. The electrostatic interaction becomes most pronounced near the Bragg angle. By selecting different neutron energies the electrostatic and nuclear scattering becomes distinguishable. The most prominent interaction parameters like the nuclear and the neutron-electron scattering length can be derived from phase shifts which appear if one neutron beam is slightly deflected in front of the crystal target. Diffraction effects in the lattice enhance the phase sensitivity to electrostatic neutron interaction. First measurements are promising and confirm the high phase sensitivity and angular resolution of the new instrument. Aiming at precision experiments the interferometer crystal and its environment have to be stabilized to only a few milliKelvin which is currently in progress at instrument ILL-S18 in Grenoble. The most interesting outcome of this work is the first direct measurement of the so-called Laue phase, a phase induced by crystal diffraction in Laue geometry which has been introduced theoretically and experimentally verified as well. It turns out that the Laue phase is extremely sensitive to angular beam deviation inside the interferometer. In order to fully exploit the angular resolution a more dedicated interferometer concept has been designed which has the potential to become among the best in angular precision aiming at the order of one micro- second of arc. The splitting of a single neutron state over macroscopic distances, 5-6 cm beam separation along a path length of 25-32 cm in the two largest crystal interferometers, remains unrivalled in matter wave interferometry. New interferometry methods become feasible in fundamental physics, like the sensing of electrostatic, short-range and gravitational interactions, a direct measurement of Coriolis deflection due to Earth rotation, and the search for unorthodox phenomena where path length, beam separation and angular shift are essential.