We have seen that the mean field picture can explain many of the basic observations about nuclei: their sizes and shapes, their cross sections for elastic scattering, many of the excited states of odd-mass nuclei, and most of the liquid-drop binding energy formula. Now we have to go beyond the mean field picture. Our first step in this direction will explain the origin of the term in the mass formula (4.3.2) which describes the striking difference in binding energies of odd and even numbers of neutrons and protons: the pairing term () B P = [ ( − 1 ) N + ( − 1 ) Z ]     δ A 1 / 2 , https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780429493904/5a3d5767-683d-4d23-8f8e-5cb099237056/content/eq770.tif"/>