Constraints on two-lepton, two quark operators

This manuscript replaces arXiv:0908.4594. The presentation has been modified, and some additional constraints included. 23 pages, 13 tablesPhysics from beyond the Standard Model, such as leptoquarks, can induce four fermion operators involving a quark, an anti-quark, a lepton and an anti-lepton. We update the constraints on the coefficients of such operators, arising from collider searches for contact interactions, and certain leptonic and semi-leptonic meson decays. We then make naive estimates for the magnitude of the coefficients, as could arise in texture models or from inverse hierarchies in the kinetic term coefficients. These estimates suggest that rare Kaon decays could be a good place to look for such operators