Extra fermions inE6superstring theories

In ${\mathrm{E}}_6$ superstring theories, there are new fermion fields which lie in the 27-dimensional representation along with the known fermions. We examine possible production modes of these exotic fermions and their likely decays. Whereas $e^{+}$ $e^{\mathrm{-}}$ collider data exclude new charged fermions with masses below 23 GeV, it appears that present pp¯ collider data for dimuons and for isolated single electrons exclude exotic quarks with masses below 30 GeV and considerable regions of lepton mass assignments in the range 20–45 GeV (assuming &). We place constraints on the mixings of the exotic fermions from experimental limits on flavor-changing neutral currents and the observed weak universality of charged-current interactions. The decays of the exotic h quark (singlet, charge -(1/3)) satisfy the bound Γ(h→${\mathrm{de}}^{+}$ $e^{\mathrm{-}}$)/Γ(h→ueν)≳12%. Production and decays of very heavy exotics are also considered.