Unbroken B-L symmetry

The difference between baryon number Band lepton number Lis the only anomaly-free global symmetry of the Standard Model, easily promoted to a local symmetry by introducing three right-handed neutrinos, which automatically make neutrinos massive. The non-observation of any (B-L)-violating processes leads us to scrutinize the case of unbroken gauged B-L; besides Dirac neutrinos, the model contains only three parameters, the gauge coupling strengthg g, the Stuckelber mass M-z , and the kinetic mixing angle X. The new force could manifest itself at any scale, and we collect and derive bounds on g' over the entire testable range M-Z' = 0-1013eV, also of interest for the more popular case of spontaneously broken B-Lor other new light forces. We show in particular that successful Big Bang nucleosynthesis provides strong bounds for masses 10eV< MZ < 10GeVdue to resonant enhancement of the rate ff..R.R. The strongest limits typically arise from astrophysics and colliders, probing scales MZ /g from TeV up to 1010GeV. (C) 2014 The Author. Published by Elsevier B. V.