A search for heavy Majorana neutrinos in events containing a pair of high-p T leptons of the same charge and high-p T jets is presented. The search uses 20.3 fb −1 of pp collision data collected with the ATLAS detector at the CERN Large Hadron Collider with a centre-of-mass energy of √ s = 8 TeV. The data are found to be consistent with the background-only hypothesis based on the Standard Model expectation. In the context of a Type-I seesaw mechanism, limits are set on the production cross-section times branching ratio for production of heavy Majorana neutrinos in the mass range between 100 and 500 GeV. The limits are subsequently interpreted as limits on the mixing between the heavy Majorana neutrinos and the Standard Model neutrinos. In the context of a left-right symmetric model, limits on the production cross-section times branching ratio are set with respect to the masses of heavy Majorana neutrinos and heavy gauge bosons W R and Z . Conclusions 20The ATLAS collaboration 28 IntroductionThe discovery of mixing between generations of neutrinos [1] has established that at least two of the neutrinos have small non-zero masses. A unique feature of neutrinos compared to other fermions in the Standard Model (SM) is that neutrinos could be their own antiparticles, so-called Majorana fermions. If this is realised in nature, then the unusually low mass scale of the light neutrinos could be generated by a seesaw mechanism [2][3][4][5][6][7] which -1 -JHEP07(2015)162 Figure 1. The tree-level diagram for the production of a heavy Majorana neutrino (N ) in the mTISM model. Lepton flavour is denoted by α and β. Lepton flavour is assumed to be conserved, such that α = β. The W boson produced from the N decay is on-shell and, in this case, decays hadronically.would imply the existence of yet unobserved heavy Majorana neutrino states. The nature of Majorana neutrinos would explicitly allow for lepton number violation.In this paper, a search is presented for heavy Majorana neutrinos using the ATLAS detector at the Large Hadron Collider (LHC). The data sample was collected in 2012 during √ s = 8 TeV pp collisions and corresponds to an integrated luminosity of 20.3 fb −1 . Heavy Majorana neutrinos with masses above 50 GeV are considered. In this regime, the production and subsequent decay of heavy Majorana neutrinos could lead to a final state containing exactly two charged leptons, where the leptons may have the same or opposite charge in equal fractions of the heavy neutrino decays. Only lepton pairs of the same charge (same-sign) are considered as there is a smaller expected SM background for pairs of same-sign leptons than for pairs of leptons of opposite charge (opposite-sign). The search includes the ee and µµ final states.The search is guided by two theoretical models. In the first model, the SM is extended in the simplest way to include right-handed neutrinos [8], such that light neutrino masses are generated by a Type-I seesaw mechanism or by radiative corrections [9]. In this minimal Type-I seesaw mechanism...

The ATLAS detector at the Large Hadron Collider is used to search for high-mass resonances decaying to dielectron or dimuon final states. Results are presented from an analysis of proton-proton (pp) collisions at a center-of-mass energy of 8 TeV corresponding to an integrated luminosity of 20.3 fb −1 in the dimuon channel. A narrow resonance with Standard Model Z couplings to fermions is excluded at 95% confidence level for masses less than 2.79 TeV in the dielectron channel, 2.53 TeV in the dimuon channel, and 2.90 TeV in the two channels combined. Limits on other model interpretations are also presented, including a grand-unification model based on the E 6 gauge group, Z Ã bosons, minimal Z 0 models, a spin-2 graviton excitation from Randall-Sundrum models, quantum black holes, and a minimal walking technicolor model with a composite Higgs boson.

A search for pair production of vector-like quarks, both up-type (T ) and downtype (B), as well as for four-top-quark production, is presented. The search is based on pp collisions at √ s = 8 TeV recorded in 2012 with the ATLAS detector at the CERN Large Hadron Collider and corresponding to an integrated luminosity of 20.3 fb −1 . Data are analysed in the lepton-plus-jets final state, characterised by an isolated electron or muon with high transverse momentum, large missing transverse momentum and multiple jets. Dedicated analyses are performed targeting three cases: a T quark with significant branching ratio to a W boson and a b-quark (TT → W b+X), and both a T quark and a B quark with significant branching ratio to a Higgs boson and a third-generation quark (TT → Ht+X and BB → Hb+X respectively). No significant excess of events above the Standard Model expectation is observed, and 95% CL lower limits are derived on the masses of the vector-like T and B quarks under several branching ratio hypotheses assuming contributions from T → W b, Zt, Ht and B → W t, Zb, Hb decays. The 95% CL observed lower limits on the T quark mass range between 715 GeV and 950 GeV for all possible values of the branching ratios into the three decay modes, and are the most stringent constraints to date. Additionally, the most restrictive upper bounds on four-top-quark production are set in a number of new physics scenarios.

Using 1:8 fb À1 of pp collisions at a center-of-mass energy of 7 TeV recorded by the ATLAS detector at the Large Hadron Collider, we present measurements of the production cross sections of Çð1S; 2S; 3SÞ mesons. Ç mesons are reconstructed using the dimuon decay mode. Total production cross sections for p T < 70 GeV and in the rapidity interval jy Ç j <

The production of W bosons in association with two jets in proton-proton collisions at a centre-of-mass energy of √ s = 7 TeV has been analysed for the presence of double-parton interactions using data corresponding to an integrated luminosity of 36 pb −1 , collected with the ATLAS detector at the Large Hadron Collider. The fraction of events arising from double-parton interactions, f (D) DP , has been measured through the p T balance between the two jets and amounts to f (D) DP = 0.08 ± 0.01 (stat.) ± 0.02 (sys.) for jets with transverse momentum p T > 20 GeV and rapidity |y| < 2.8. This corresponds to a measurement of the effective area parameter for hard double-parton interactions of σ eff = 15 ± 3 (stat.) +5 −3 (sys.) mb.