Reference papers for Leptoquarks :

• ``Leptoquarks in lepton-quark Collisions", W. Buchmuller, R. Ruckl, D. Wyler, DESY-86-150, Nov 1986. 21pp.
  Phys.Lett.B191:442,1987, Erratum-ibid.B448:320,1999
  The reference paper for LQs at HERA. There you find the expressions for the matrix elements. Note that some of the interference terms with DIS had the wrong sign in the original publication. The corrected signs can be found in the Erratum.

• ``Model Independent Constraints on Leptoquarks from Rare Processes", S. Davidson, D. Bailey, B.A. Campbell, Published in Z.Phys.C61:613-644,1994.
  Indirect limits for LQs with or without Lepton Flavor Violation.

• ``Formation and Decay of Scalar Leptoquarks/Squarks in ep Collisions", T. Plehn, H. Spiesberger, M. Spira, P. M. Zerwas, hep-ph/9703433, Published in Z.Phys. C74 (1997) 611-614.