Hunting a Light U(1)_B Gauge Boson Coupled to Baryon Number in Collider Experiments

We analyze several signals at HERA and the Tevatron of a light $U(1)_B$ gauge boson ($gamma_B$) coupling to baryon number. We show that the study of the production of $b bar{b}$ pairs at the (upgraded) Tevatron can exclude $gamma_B$ with masses ($m_B$) in the range $40 lesssim m_B lesssim 300$ GeV for $gamma_B$ couplings ($alpha_B$) greater than $2 times 10^{-2}$ ($3 times 10^{-3}$). We also show that the HERA experiments cannot improve the present bounds on $gamma_B$. Moreover, we demonstrate that the production at HERA and the Tevatron of di--jet events with large rapidity gaps between the jets cannot be explained by the existence of a light $gamma_B$.