  We study collider phenomenology of a leptophobic $Z'$ boson existing in eight
scenarios of the $E_6$ grand unified theory, differing in particle embeddings.
We first review the current bound on the $Z'$ mass $m_{Z'}$ based upon the LHC
data of $pp\to t\bar{t}$ process at 8 TeV collisions with an integrated
luminosity of 19.6 fb$^{-1}$. Most scenarios have a lower bound of about 1 TeV.
However, this constraint does not apply to the case where $m_{Z'} < 2 m_t$, and
other methods need to be employed for this lower mass regime. Using existing
UA2 constraints and dijet data at the LHC, we find that only one of the eight
scenarios is excluded at 95\% confidence level. No bound can be obtained from
$Wjj$ and $Zjj$ measurements. We propose to use the photon associated
production of the $Z'$ boson that subsequently decays into a pair of bottom
quarks, $pp\to Z'\gamma \to b\bar{b}\gamma$, at the LHC to explore the
constraints in the lower mass regime. We compute the expected signal
significance as a function of $m_{Z'}$ using detailed simulations of signal and
irreducible background events. We find constraints for two more scenarios using
the 8-TeV data and taking appropriate kinematical cuts. We also show the
discovery reach for each scenario at the 14-TeV LHC machine.
