Outside the Lyman-break box: detecting Lyman continuum emitters at
  $3.5<z<5.1$ with CLAUDS
  Identifying non-contaminated sample of high-redshift galaxies with escaping
Lyman continuum (LyC) flux is important for understanding the sources and
evolution of cosmic reionization. We present CLAUDS $u$-band photometry of the
COSMOS field to probe LyC radiation from spectroscopically confirmed galaxies
at $z\geq3.5$ and outside the standard Lyman-break galaxy colour selection
expectations. Complementary to the CLAUDS data, we use Subaru multi-filter
photometry, \textit {Hubble Space Telescope} ($HST$) multi-filter imaging, and
the spectroscopic surveys D10K, VUDS and 3D-HST. We present a sample of Lyman
continuum galaxy (LCG) candidates in the redshift range $3.5\lesssim
z\lesssim5.1$. Here, we introduce 5 LCG candidates, where two are flagged
quality 1 and three quality 2. The estimated $f_{\rm esc}^{\rm abs}$ for
quality 1 candidates are in the range $\sim5\% - 73\%$ and $\sim30\% - 93\%$.
These estimates are based on our derived parameters from individual galaxies as
inputs to a range of BPASS models as well as mean intergalactic medium (IGM)
and maximal intergalactic and circumgalactic media (IGM+CGM) transmission. We
conclude that our search for LCGs is most likely biased to lines of sight with
low HI densities or free from Lyman limit systems. Our two best LCG candidates
have EW (Ly$\alpha)\leq50$\AA\ and we find no correlation or anti-correlation
between EW (Ly$\alpha$), $f_{\rm esc}^{\rm abs}$, and $R_{\rm obs}$, the ratio
of ionizing to non-ionizing observed flux in the measured passbands. Stacking
candidates without solid LyC detections ($S/N<3$) results in an estimated
$f_{\rm esc}^{\rm abs}$ from galaxies not greater than $1\%$.
