Single Leptoquark Production at Hadron Colliders

Leptoquarks can be produced in pairs by gluon--gluon fusion and quark--antiquark annihilation at hadron colliders. While HERA is the proper machine for single production of $(eu)$ and $(ed)$ type leptoquarks, the flavor species of ($\mu u$), ($\mu d$) and ($\tau u$), ($\tau d$) type leptoquarks can be produced at hadron colliders very efficiently. Besides exploiting gluon-quark collisions, leptoquarks can also be produced singly by colliding the quarks in one proton beam with leptons $e,\mu,\tau$ generated by splitting photons which are radiated off the quarks in the other proton beam. For Yukawa couplings of the size $\alpha$ leptoquark masses up to about 300~GeV can be generated at the Tevatron while the LHC can produce leptoquarks with masses up to about 3~TeV. [Leptoquarks involving heavy quarks can be produced singly at a lower rate, determined by the heavy flavor flux in the proton beam.]