Figure 1

Constraints on $g_{\nu }$ and $m_V$ from $K$ decays (gray, solid line), $W$ decays (gray, dashed line), and BBN. The two former are reproduced from Ref. 2, and the sample region in Fig. 3 of Ref. 1 is also presented (light gray). The hashed region bounded by the thick solid curve is excluded by $\Delta N_{\nu }<1$ at 95% C.L. [8]. The excluded region will shrink for $\Delta N_{\nu }<1.5$ (see, e.g., Ref. 9), and even further for the “maximally conservative” limit $\Delta N_{\nu }\lesssim 1.58$ for $m_V>0.05\mathrm{MeV}$.Reuse & Permissions