  Among active galactic nuclei, BL Lac objects show extreme properties that
have been interpreted as the effect of relativistic beaming on the emission
from a plasma jet oriented close to the line of sight. The Doppler
amplification of the jet emission makes them ideal targets for studying jet
physics. In particular, low-power BL Lacs (LPBL) are very interesting because
they probe the jet formation and emission processes at the lowest levels of
accretion. However, they are difficult to identify since their emission is
swamped by the radiation from the host galaxy in most observing bands. In this
paper we propose a new LPBL selection method based on the mid-infrared
emission, in addition to the traditional optical indices. We considered the
radio-selected sample of Best & Heckman (2012, MNRAS, 421, 1569) and
cross-matched it with the WISE all-sky survey. In a new diagnostic plane
including the W2-W3 color and the Dn(4000) index, LPBL are located in a region
scarcely populated by other sources. By filtering objects with small emission
line equivalent width, we isolated 36 LPBL candidates up to redshift 0.15.
Their radio luminosity at 1.4 GHz spans the range log L_r = 39.2-41.5 [erg/s].
Considering the completeness of our sample, we analyzed the BL Lac luminosity
function (RLF), finding a dramatic paucity of LPBL with respect to the
extrapolation of the RLF toward low power. This requires a break in the RLF
located at log L_r~40.6 [erg/s]. The consequent peak in the BL Lacs number
density is possibly the manifestation of a minimum power required to launch a
relativistic jet.
