Could Vector Leptoquarks be Rather Light?

Abstract

A possible Standard Model extension of the Pati-Salam type with a lepton number as the fourth color is reexamined. A new type of mixing in the interaction of the $SU(4)_V$--leptoquark with quarks and leptons is shown to be required. An additional arbitrariness of the mixing parameters could allow to decrease noticeably the lower bound on the leptoquark mass $M_X$ originated from the $\pi$ and $K$ decays and the $\mu e$ conversion. The only mixing independent bound emerging from the cosmological limit on the $\pi^0 \rightarrow \nu \bar{\nu}$ decay width is $M_X > 18~TeV$.