Results from the aSPECT experiment

We report on a precise measurement of the antineutrino-electron angular correlation (the $a$ coefficient) in free neutron beta-decay from the $a$SPECT experiment. The $a$ coefficient is inferred from the recoil energy spectrum of the protons which are detected in $4\pi$ by the $a$SPECT spectrometer using magnetic adiabatic collimation with an electrostatic filter. Data are presented from a 100 days run at the Institut Laue Langevin in 2013. The sources of systematic errors are considered and included in the final result. We obtain $a = -0.10430(84)$ which is the most precise measurement of the neutron $a$ coefficient to date. From this, the ratio of axial-vector to vector coupling constants is derived giving $\lambda = -1.2677(28)$, in slight tension with other results.\\ \\In collaboration with: F. Ayala Guardia, M. Borg, J. Kahlenberg, R. Munoz Horta, C. Schmidt, A. Wunderle, W. Heil (Institut fuer Physik, Johannes Gutenberg-Universitaet Mainz, 55128 Mainz, Germany), R. Maisonobe, M. Simson, T. Soldner, R. Virot, O. Zimmer (Institut Laue-Langevin, CS 20156, 38042 Grenoble Cedex 9, France), M. Klopf, G. Konrad (Technische Universitaet Wien, Atominstitut , 1020 Wien, Austria), S. Baessler (Department of Physics, University of Virginia, Charlottesville, VA 22904, USA and Oak Ridge National Lab, Bethel Valley Road, Oak Ridge, TN 37831, USA), F. Glueck (Institut fuer Kernphysik (IKP), Karlsruhe Institute of Technology (KIT), 76344 Eggenstein-Leopoldshafen, Germany), U. Schmidt (Physikalisches Institut, Universitaet Heidelberg, 69120 Heidelberg, Germany)