  Most of the extragalactic sources detected at TeV energies are BL Lac
objects. They belong to the subclass of "high frequency peaked BL Lacs" (HBLs)
exhibiting spectral energy distributions with a lower energy peak in the X-ray
band; this is widely interpreted as synchrotron emission from relativistic
electrons. The X-ray spectra are generally curved, and well described in terms
of a log-parabolic shape. In a previous investigation of TeV HBLs (TBLs) we
found two correlations between their spectral parameters. (1) The synchrotron
peak luminosity L_p increases with its peak energy E_p; (2) the curvature
parameter b decreases as E_p increases. The first is consistent with the
synchrotron scenario, while the second is expected from statistical/stochastic
acceleration mechanisms for the emitting electrons. Here we present an
extensive X-ray analysis of a sample of HBLs observed with XMM-Newton and SWIFT
but undetected at TeV energies (UBLs), to compare their spectral behavior with
that of TBLs. Investigating the distributions of their spectral parameters and
comparing the TBL X-ray spectra with that of UBLs, we develop a criterion to
select the best HBLs candidates for future TeV observations.
