Comprehensive study of leptoquark bounds.

Abstract

We make a comprehensive study of indirect bounds on scalar leptoquarks that couple chirally and diagonally to the first generation by examining available data from low-energy experiments as well as from high energy ${e}^{+}{e}^{\ensuremath{-}}$ and $p\overline{p}$ accelerators. The strongest bounds turn out to arise from low-energy data: For leptoquarks that couple to right-handed quarks, the most stringent bound comes from atomic parity violation. For leptoquarks that couple to left-handed quarks, there are two mass regions: At low masses the bounds arise from atomic parity violation or from universality in leptonic $\ensuremath{\pi}$ decays. At masses above a few hundred GeV's the dominant bounds come from the flavor-changing neutral current (FCNC) processes that are unavoidable in these leptoquarks: The FCNC bound of the up sector, that arises from ${D}^{0}\ensuremath{-}{\overline{D}}^{0}$ mixing, combines with the FCNC bounds from the down sector, that arise from rare $K$ decays and ${\overline{K}}^{0}\ensuremath{-}{\overline{K}}^{0}$ mixing, to a bound on the flavor conserving coupling to the first generation. The bounds restrict leptoquarks that couple with electromagnetic strength to lie above 600 or 630 GeV for leptoquarks that couple to right-handed quarks, and above 1040, 440, and 750 GeV for the ${\mathrm{SU}(2)}_{W}$ scalar, doublet, and triplet leptoquarks that couple to left-handed quarks. These bounds are considerably stronger than the first results from the direct searches at the DESY $\mathrm{ep}$ collider HERA. Our bounds also already exclude large regions in the parameter space that could be examined by various methods proposed for indirect leptoquark searches.