No abstract

The smallness of the quark sector parameters and the hierarchy between them could be the result of a horizontal symmetry broken by a small parameter. Such an explicitly broken symmetry can arise from an exact symmetry which is spontaneously broken. Constraints on the scales of new physics arise from new flavor changing interactions and from Landau poles, but do not exclude the possibility of observable signatures at the TeV scale. Such a horizontal symmetry could also lead to many interesting results: (i) quark -squark alignment that would suppress, without squark degeneracy, flavor changing neutral currents induced by supersymmetric particles, (ii) exact relations between mass ratios and mixing angles, (iii) a solution of the µ-problem and (iv) a natural mechanism for obtaining hierarchy among various symmetry breaking scales. 10/93We would like to make two comments about the estimates in (1.3):

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 − and pp 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 π decays. At masses above a few hundred GeV's the dominant bounds come from the FCNC processes that are unavoidable in these leptoquarks: The FCNC bound of the up sector, that arises from D 0 −D 0 mixing, combines with the FCNC bounds from the down sector, that arise from rare K decays and K 0 −K 0 mixing, to a bound on the flavour conserving coupling to the first generation.The bounds restrict leptoquarks that couple with electromagnetic strength to lie above 600 GeV or 630 GeV for leptoquarks that couple to RH quarks, and above 1040 GeV, 440 GeV, and 750 GeV for the SU(2) W scalar, doublet and triplet leptoquarks that couple to LH quarks. These bounds are considerably stronger than the first results from the direct searches at HERA. Our bounds also already exclude large regions in the parameter space that could be examined by various methods proposed for indirect leptoquark searches.

We derive new bounds on scalar leptoquark couplings from K°-K°, D°-D°, and B°'B° mixing. Although leptoquarks contribute to these processes only at one loop, their contribution is large, due to the lack of Glashow-Iliopoulos-Maiani cancellation. Our bounds have two important features: (i) They bound g 4 /M 2 , in contrast to the hitherto known bounds on g 2 /M 2 , and are consequently stronger at high masses, (ii) The bound from D°-D° mixing is the first flavor changing neutral current bound in the up sector for chirally coupled leptoquarks, and is similar in strength to the K°-K° and B°-B° bounds. Together, these bounds strongly constrain any leptoquark that couples to left-handed quarks.

We derive bounds on vector leptoquarks coupling to the first generation, using data from low energy experiments as well as from high energy accelerators. Similarly to the case of scalar leptoquarks, we find that the strongest indirect bounds arise from atomic parity violation and universality in leptonic 1~ decays. These bounds are considerably stronger than the first direct bounds of the DESY ep collider HERA, restricting vector leptoquarks that couple with electromagnetic strength to right-handed quarks to lie above 430 GeV or 460 GeV, and leptoquarks that couple with electromagnetic strength to lefthanded quarks to lie above 1.3 TeV, 1.2 TeV, and 1.5 TeV for the SU(2)w singlet, doublet, and triplet, respectively.PACS number(s): 14.80. -j, 12.60.-i