Why are some BL Lacs detected by \fermi, but others not ?
  By cross-correlating an archival sample of 170 BL Lacs with 2 year \fermilat
AGN sample, we have compiled a sample of 100 BL Lacs with \fermi detection
(FBLs), and a sample of 70 non-\fermi BL Lacs (NFBLs). We compared various
parameters of FBLs with those of NFBLs, including the redshift, the low
frequency radio luminosity at 408 MHz ($L_{\rm 408MHz}$), the absolute
magnitude of host galaxies ($M_{\rm host}$), the polarization fraction from
NVSS survey ($P_{\rm NVSS}$), the observed arcsecond scale radio core flux at 5
GHz ($F_{\rm core}$) and jet Doppler factor; all the parameters are directly
\textbf{measured} or derived from available data from literatures. We found
that the Doppler factor is on average larger in FBLs than in NFBLs, and the
$Fermi~ \gamma$-ray detection rate is higher in sources with higher Doppler
factor. In contrast, there are no significant differences in terms of the
intrinsic parameters of redshift, $ L_{\rm 408MHz}$, $ M_{\rm host}$ and $
P_{\rm NVSS}$. FBLs seem to have a higher probability of exhibiting measurable
proper motion. These results strongly indicate a higher beaming effect in FBLs
compared to NFBLs. The radio core flux is found to be strongly correlated with
$\gamma$-ray flux, which remains after excluding the common dependence of the
Doppler factor. At the fixed Doppler factor, FBLs have systematically larger
radio core flux than NFBLs, implying lower $\gamma$-ray emission in NFBLs since
the radio and $\gamma$-ray flux are significantly correlated. Our results
indicate that the Doppler factor is an important parameter of $\gamma$-ray
detection, the non-detection of $\gamma$-ray emission in NFBLs is likely due to
low beaming effect, and/or low intrinsic $\gamma$-ray flux, and the gamma-rays
are likely produced co-spatially with the arcsecond-scale radio core radiation
and mainly through the SSC process.
