A combination of the searches for pair-produced vector-like partners of the top and bottom quarks in various decay channels (T→Zt/W b/Ht, B→Z b/Wt/Hb) is performed using 36.1 fb −1 of pp collision data at √ s = 13 TeV with the ATLAS detector at the Large Hadron Collider. The observed data are found to be in good agreement with the Standard Model background prediction in all individual searches. Therefore, combined 95% confidence-level upper limits are set on the production cross-section for a range of vector-like quark scenarios, significantly improving upon the reach of the individual searches. Model-independent limits are set assuming the vector-like quarks decay to Standard Model particles. A singlet T is excluded for masses below 1.31 TeV and a singlet B is excluded for masses below 1.22 TeV. Assuming a weak isospin (T, B) doublet and |V T b | |V t B |, T and B masses below 1.37 TeV are excluded.

A direct search for lepton-flavour-violating H → µτ decays of the recently discovered Higgs boson with the ATLAS detector at the LHC is presented. The analysis is performed in the H → µτ had channel, where τ had is a hadronically decaying τ -lepton. The search is based on the data sample of proton-proton collisions collected by the ATLAS experiment corresponding to an integrated luminosity of 20.3 fb −1 at a centre-of-mass energy of √ s = 8 TeV. No statistically significant excess of data over the predicted background is observed. The observed (expected) 95% confidence-level upper limit on the branching fraction, Br(H → µτ ), is 1.85% (1.24%). There are three possibilities for LFV effects mediated via virtual Higgs bosons: µ-e, τ -µ, and τ -e transitions. Indirect experimental constraints are reviewed and translated into constraints on Br(H → eµ, µτ, eτ ) in recent papers [4,19]. Searches for µ → eγ [20] place a very stringent constraint on H → eµ decays: Br(H → eµ) < O(10 −8 ) [4,19]. The indirect constraints on H → µτ, eτ decays mostly come from searches for τ → µγ, eγ [21][22][23] or other rare τ -lepton decays [24], as well as from measurements of the anomalous magnetic moment of the muon and the electron [25] and are much less stringent: Br(H → µτ, eτ ) < O(10%) [4,19]. A relatively large Br(H → µτ ) can be achieved without any particular tuning of the effective couplings, while a large Br(H → eτ ) is possible only at the cost of some fine-tuning of the corresponding couplings [19]. It is also important to note that the presence of a H → µτ signal would essentially exclude the presence of a -1 - Keywords JHEP11(2015)211H → eτ signal, and vice versa, at an experimentally observable level at the LHC due to strong experimental bounds on µ → eγ decays [19]. The CMS Collaboration has recently performed the first direct search for LFV H → µτ decays [26] and reported a slight excess (2.4 standard deviations) of data over the predicted background. Their results give a 1.57% upper limit on Br(H → µτ ) at the 95% confidence level (CL). This paper presents a search for lepton-flavour-violating H → µτ decays of the recently discovered Higgs boson with one muon and one hadronically decaying τ -lepton (τ had ) in the final state. All Higgs boson production processes are considered. The analysis is based on the data sample of pp collisions which was collected at a centre-of-mass energy of √ s = 8 TeV and corresponds to an integrated luminosity of 20.3 fb −1 . The ATLAS detector and object reconstructionThe ATLAS detector 1 is described in detail in ref. [27]. ATLAS consists of an inner tracking detector (ID) covering the range |η| < 2.5, surrounded by a superconducting solenoid providing a 2 T magnetic field, electromagnetic (|η| < 3.2) and hadronic calorimeters (|η| < 4.9) and a muon spectrometer (MS) (|η| < 2.7) with a toroidal magnetic field. The signature of LFV H → µτ decays used in this search is characterised by the presence of an energetic muon, originating directly from a Higgs boson decay and carryin...

Direct searches for lepton flavour violation in decays of the Higgs and Z bosons with the ATLAS detector at the LHC are presented. The following three decays are considered: , , and . The searches are based on the data sample of proton–proton collisions collected by the ATLAS detector corresponding to an integrated luminosity of 20.3  at a centre-of-mass energy of  TeV. No significant excess is observed, and upper limits on the lepton-flavour-violating branching ratios are set at the 95 confidence level: Br, Br, and Br.