The precision measurement of the electron- antineutrino correlation in neutron beta decay

The electron-antineutrino angular correlation (a) in the neutron beta decay is being measured by the aCORN experiment which is aiming to measure the correlation coefficient to a relative accuracy of about 1% using a novel technique. An electrostatic mirror is used to make two groups of protons in this method. These two groups of protons are distinguished by time of flight. The counting asymmetry between these two groups of protons is measured which is proportional to the a coefficient. Precision measurement of a will improve the determination of the nucleon axial vector coupling constant gA, and along with other decay parameters can be used to test the validity and self-consistency of the Standard Model and to explore the physics beyond it. After taking data for two years from 2013 to 2014 on the NG-6 beamline at NIST, aCORN has now moved to a new higher flux NG-C beamline. The detailed description of the apparatus, precision alignment techniques and data analysis for the data taken in the 2013-14 period will be presented. We were able to determine the most precise value of the a coefficient from this dataset. The preliminary result for the measured a coefficient is -0.1066 +/-0.0040.