  Pairing in the Weyl semi - metal appearing on the surface of topological
insulator is considered. It is shown that due to an "ultra-relativistic"
dispersion relation there is a quantum critical point governing the zero
temperature transition to a superconducting state. Starting from the
microscopic Hamiltonian with local attraction, we calculated using the Gor'kov
equations, the phase diagram of the superconducting transition at arbitrary
chemical potential, its magnetic properties and critical exponents close to the
quantum critical point. The Ginzburg - Landau effective theory is derived for
small chemical potential allowing to consider effects of spatial dependence of
order parameters in magnetic field. The GL equations are very different from
the conventional ones reflecting the chiral universality class of the quantum
phase transition. The order parameter distribution of a single vortex is found
to be different as well. The magnetization near the upper critical field is
found to be quadratic, not linear as usual. We discuss the application of these
results to recent experiments in which surface superconductivity was found that
some 3D topological insulators and estimate feasibility of the phonon pairing.
