Search for exotic light-flavor quark partners in pp collisions at $\sqrt{s} = 8~\mathrm{TeV}$

A search is presented for vector-like quarks (VLQ) that only couple to light generation quarks in proton-proton collisions at $\sqrt{s}=8~\mathrm{TeV}$ at the Large Hadron Collider (LHC). The data were collected by the CMS experiment during the 2012 LHC run, corresponding to an integrated luminosity of $19.7~\mathrm{fb}^{-1}$. Both single and pair production of VLQ are considered. The single production search is restricted to down-type vector-like quarks (electric charge $-1/3$), while the pair production search is sensitive to up-type (charge $+2/3$) and down-type heavy quarks. Final states with at least one muon or one electron are considered. No significant excess over standard model expectations is observed, and lower limits on the mass of VLQ are derived. Combining all processes together (including single production), the lower mass limits range from $400~\mathrm{GeV}$ to $1800~\mathrm{GeV}$, depending on the VLQ branching fractions (BF) to W, Z or H bosons and the electroweak single-production strength. For pair production only, at the usual benchmark assuming $\mathrm{BF(W)}=0.5$, $\mathrm{BF(Z)}=0.25$ and $\mathrm{BF(H)}=0.25$, vector-like quarks with a mass below $685~\mathrm{GeV}$ are excluded at $95\%$ confidence level (with an expected limit of $720~\mathrm{GeV}$). For $\mathrm{BF(W)}=1.0$ the observed lower limit is $845~\mathrm{GeV}$ (expected limit $825~\mathrm{GeV}$). The results are more stringent than those previously obtained in single and pair production of VLQ coupling to light generation quarks at 7 and $8~\mathrm{TeV}$.