Anomalies in decays of B mesons, in which a bottom quark changes flavour to become a charmed quark, reported by the LHCb, Belle and Babar collaborations, have triggered considerable excitement in the particle-physics community (see "Beauty quarks test lepton universality"). The combined results of these experiments suggest that the decay rates of B → D τ ν and B → D* τ ν differ by more than four standard deviations from the Standard Model (SM) predictions.

Several phenomenological studies have suggested that these differences could be explained by the existence of hypothetical new particles called leptoquarks (LQs), which couple to both leptons and quarks. Such particles appear naturally in several scenarios of new physics, including models inspired by grand unified theories or Higgs-compositeness models. Leptoquarks that couple to the third generation of SM fermions (top and bottom quarks, and the tau lepton and its associated neutrino) are considered to be of particular interest to explain these flavour anomalies.

Leptoquarks coupling to fermions of the first and also the second generation of the SM have been the target of many searches by collider experiments at the successive energy frontiers (SPS, LEP, HERA, Tevatron). The most sensitive searches have been performed at the LHC, resulting in the exclusion of LQs with masses below 1.1 TeV. Searches for third-generation LQs were first performed at the Tevatron, and the baton has now been passed to the LHC.

The first investigation by the CMS collaboration used events recorded at an energy of 8 TeV during LHC Run 1, and targeted LQ pair production via the strong interaction with the decay channel of the LQ to a top quark and a tau lepton. The result of this search, reported by CMS in 2015, was that third-generation LQs with masses below 0.685 TeV were excluded. These early results have now been extended using the 2016 dataset at 13 TeV, employing more sophisticated analysis methods. The new search investigates final states containing an electron or a muon, one or two tau leptons that decay to hadrons and additional jets. To achieve sensitivity to the largest possible range of LQ masses, the analysis uses several event categories in which deviations from the SM predictions are searched for. The SM backgrounds mainly consist of top-quark pair production and W+ jets events, whose contributions are derived from the data rather than from simulation.

No significant indication of the existence of third-generation LQs has yet been found in any of the categories studied (see left-hand figure). The collaboration was therefore able to place exclusion limits on the product of the production cross section and branching fraction as small as 0.01 pb, which translate into lower limits on LQ masses extending above 1 TeV.

Combining the result of a search for the pair-production of supersymmetric bottom squarks, which can be reinterpreted as a search for LQs in the decay mode of a bottom quark and a tau neutrino, results in limits that probe the TeV mass range over all possible LQ branching ratios (see figure, right). Another recent search targets different LQs that decay into a bottom quark and a tau lepton. Using a smaller dataset at 13 TeV, this search excludes masses below 0.85 TeV for a unity branching fraction.

This is the first time that searches at the LHC have achieved sufficient sensitivity to explore the mass range favoured by phenomenological analyses of LQs and the current flavour anomalies. No hints of these states have been found, but analyses are under way using larger datasets and including additional signatures.

Further reading

CMS Collaboration 2015 JHEP 7 42.

CMS Collaboration 2017 JHEP 7 121.

CMS Collaboration 2018 arXiv:1803.02864.