A search for second-generation leptoquarks in $pp$ collisions at $\sqrt{s}$ = 7 TeV with the ATLAS detector

The apparent symmetry between the quark and lepton families of the Standard Model (SM) are, at the very least, suggestive of a more fundamental relationship between them. In some Beyond the Standard Model theories, such interactions are mediated by leptoquarks (LQs): hypothetical color-triplet bosons with both lepton and baryon number and fractional electric charge. The results of a search for pair-production of second-generation scalar LQs are presented for a final state consisting of one muon, missing transverse energy, and at least two jets. The search is performed on 1.03 fb$^{-1}$ of integrated luminosity of proton-proton collision data produced by the Large Hadron Collider at a center-of mass energy of 7 TeV, as recorded by the ATLAS detector. Observed event yields in selected signal regions are found to be consistent with SM background expectations. Therefore, limits are set: LQs with mass $m_{LQ} <$ 545 GeV are excluded at 95% confidence level (CL), assuming the branching ratio of a LQ to a muon and a quark $\beta \equiv \text{BR}(LQ \rightarrow \mu q) = $0.5. When these results are combined with those of a complementary search in the $\mu \mu j j$ final state, LQs with mass $m_{LQ} <$ 594 (685) GeV are excluded at 95% CL for $\beta =$0.5 (1.0). These are currently the world’s most stringent limits on second-generation scalar LQ production.