A search is conducted for new resonant and non-resonant high-mass phenomena in dielectron and dimuon final states. The search uses 36.1 fb −1 of proton-proton collision data, collected at √ s = 13 TeV by the ATLAS experiment at the LHC in 2015 and 2016. No significant deviation from the Standard Model prediction is observed. Upper limits at 95% credibility level are set on the cross-section times branching ratio for resonances decaying into dileptons, which are converted to lower limits on the resonance mass, up to 4.1 TeV for the E 6 -motivated Z χ . Lower limits on the qq contact interaction scale are set between 2.4 TeV and 40 TeV, depending on the model. Conclusion 21A Dilepton invariant mass tables 22The ATLAS collaboration 44 IntroductionThis article presents a search for resonant and non-resonant new phenomena, based on the analysis of dilepton final states (ee and µµ) in proton-proton (pp) collisions with the ATLAS detector at the Large Hadron Collider (LHC) operating at √ s = 13 TeV. The data set was collected during 2015 and 2016, and corresponds to an integrated luminosity of 36.1 fb −1 . In the search for new physics carried out at hadron colliders, the study of -1 - JHEP10(2017)182dilepton final states provides excellent sensitivity to a large variety of phenomena. This experimental signature benefits from a fully reconstructed final state, high signal-selection efficiencies and relatively small, well-understood backgrounds, representing a powerful test for a wide range of theories beyond the Standard Model (SM).Models with extended gauge groups often feature additional U(1) symmetries with corresponding heavy spin-1 bosons. These bosons, generally referred to as Z , would manifest as a narrow resonance through its decay, in the dilepton mass spectrum. Among these models are those inspired by Grand Unified Theories, which are motivated by gauge unification or a restoration of the left-right symmetry violated by the weak interaction. Examples considered in this article include the Z bosons of the E 6 -motivated [1,2] theories as well as Minimal models [3]. The Sequential Standard Model (SSM) [2] is also considered due to its inherent simplicity and usefulness as a benchmark model. The SSM manifests a Z SSM boson with couplings to fermions equal to those of the SM Z boson.The most sensitive previous searches for a Z boson decaying into the dilepton final state were carried out by the ATLAS and CMS collaborations [4,5]. Using 3.2 fb −1 of pp collision data at √ s = 13 TeV collected in 2015, ATLAS set a lower exclusion limit at 95% credibility level (CL) on the Z SSM pole mass of 3.4 TeV for the combined ee and µµ channels. Similar limits were set by CMS using the 2015 data sample.This search is also sensitive to a series of other models that predict the presence of narrow dilepton resonances. These models include the Randall-Sundrum (RS) model [6] with a warped extra dimension giving rise to spin-2 graviton excitations, the quantum black-hole model [7], the Z * model [8], and the minimal wal...

Search for high-mass dilepton resonances using139 fb −1 of p p collision data collected at √ s = 13 TeV with the ATLAS detectorThe ATLAS Collaboration A search for high-mass dielectron and dimuon resonances in the mass range of 250 GeV to 6 TeV is presented. The data were recorded by the ATLAS experiment in proton-proton collisions at a centre-of-mass energy of √ s = 13 TeV during Run 2 of the Large Hadron Collider and correspond to an integrated luminosity of 139 fb −1 . A functional form is fitted to the dilepton invariant-mass distribution to model the contribution from background processes, and a generic signal shape is used to determine the significance of observed deviations from this background estimate. No significant deviation is observed and upper limits are placed at the 95% confidence level on the fiducial cross-section times branching ratio for various resonance width hypotheses. The derived limits are shown to be applicable to spin-0, spin-1 and spin-2 signal hypotheses. For a set of benchmark models, the limits are converted into lower limits on the resonance mass and reach 4.5 TeV for the E 6 -motivated Z ψ boson. Also presented are limits on Heavy Vector Triplet model couplings.ATLAS [14-16] is a multipurpose detector with a forward-backward symmetric cylindrical geometry with respect to the LHC beam axis.1 The innermost layers consist of tracking detectors in the pseudorapidity range |η| < 2.5. This inner detector (ID) is surrounded by a thin superconducting solenoid that provides a 1 ATLAS uses a right-handed coordinate system with its origin at the nominal interaction point (IP) in the centre of the detector and the z-axis along the beam pipe. The x-axis points from the IP to the centre of the LHC ring, and the y-axis points upwards. Cylindrical coordinates (r, φ) are used in the transverse plane, φ being the azimuthal angle around the z-axis. The pseudorapidity is defined in terms of the polar angle θ as η = − ln tan(θ/2). Angular distance is measured in units of ∆R ≡ (∆η) 2 + (∆φ) 2 .

The ATLAS CollaborationThe observation of Higgs boson production in association with a top quark pair (ttH), based on the analysis of proton-proton collision data at a centre-of-mass energy of 13 TeV recorded with the ATLAS detector at the Large Hadron Collider, is presented. Using data corresponding to integrated luminosities of up to 79.8 fb −1 , and considering Higgs boson decays into bb, WW * , τ + τ − , γγ, and Z Z * , the observed significance is 5.8 standard deviations, compared to an expectation of 4.9 standard deviations. Combined with the ttH searches using a dataset corresponding to integrated luminosities of 4.5 fb −1 at 7 TeV and 20.3 fb −1 at 8 TeV, the observed (expected) significance is 6.3 (5.1) standard deviations. Assuming Standard Model branching fractions, the total ttH production cross section at 13 TeV is measured to be 670 ± 90 (stat.) +110 −100 (syst.) fb, in agreement with the Standard Model prediction.

Dijet events are studied in the proton-proton collision data set recorded at ffiffi ffi s p ¼ 13 TeV with the ATLAS detector at the Large Hadron Collider in 2015 and 2016, corresponding to integrated luminosities of 3.5 fb −1 and 33.5 fb −1 respectively. Invariant mass and angular distributions are compared to background predictions and no significant deviation is observed. For resonance searches, a new method for fitting the background component of the invariant mass distribution is employed. The data set is then used to set upper limits at a 95% confidence level on a range of new physics scenarios. Excited quarks with masses below 6.0 TeV are excluded, and limits are set on quantum black holes, heavy W 0 bosons, W Ã bosons, and a range of masses and couplings in a Z 0 dark matter mediator model. Model-independent limits on signals with a Gaussian shape are also set, using a new approach allowing factorization of physics and detector effects. From the angular distributions, a scale of new physics in contact interaction models is excluded for scenarios with either constructive or destructive interference. These results represent a substantial improvement over those obtained previously with lower integrated luminosity.

Search for new phenomena in high-mass diphoton final states using 37 fb −1 of proton-proton collisions collected at √ s = 13 TeV with the ATLAS detectorThe ATLAS Collaboration Searches for new phenomena in high-mass diphoton final states with the ATLAS experiment at the LHC are presented. The analysis is based on pp collision data corresponding to an integrated luminosity of 36.7 fb −1 at a centre-of-mass energy √ s = 13 TeV recorded in 2015 and 2016. Searches are performed for resonances with spin 0, as predicted by theories with an extended Higgs sector, and for resonances with spin 2, using a warped extra-dimension model as a benchmark model, as well as for non-resonant signals, assuming a large extradimension scenario. No significant deviation from the Standard Model is observed. Upper limits are placed on the production cross section times branching ratio to two photons as a function of the resonance mass. In addition, lower limits are set on the ultraviolet cutoff scale in the large extra-dimensions model. c 2017 CERN for the benefit of the ATLAS Collaboration. 1 The ATLAS experiment uses a right-handed coordinate system with its origin at the nominal interaction point (IP) in the centre of the detector and the z-axis along the beam pipe. The x-axis points from the IP to the centre of the LHC ring, and the y-axis points upward. Cylindrical coordinates (r, φ) are used in the transverse plane, φ being the azimuthal angle around the z-axis. The pseudorapidity is defined in terms of the polar angle θ as η = − ln tan(θ/2). The transverse energy is defined as3 Simulated Monte Carlo (MC) events are used for optimizing the search strategy [23], and for the signal and background modelling studies detailed in Sections 5 and 6, respectively. Interference effects between the resonant signal and the background processes are neglected.The spin-0 signal MC samples were generated using the effective-field-theory approach implemented in MadGraph5_aMC@NLO [24] version 2.3.3 at next-to-leading order (NLO) in quantum chromodynamics (QCD). From the Higgs characterization framework [25], CP-even dimension-five operators coupling the new resonance to gluons and photons were included. Samples were generated with the NNPDF3.0 NLO parton distribution functions (PDFs) [26], using the A14 set of tuned parameters (tune) of Pythia 8.186 [27,28] for the parton-shower and hadronization simulation. Simulated samples were produced for fixed values of the mass and width of the assumed resonance, spanning the range 200-2400 GeV for the mass, and the range from 4 MeV to 10% of the mass for the decay width. Choosing an improved signal model with an event generator different from the one used in Ref.[1] provides a description of the signal which is less sensitive to modelling effects from the off-shell region. The impact of this change is only visible in scenarios with a large signal decay width, with mass values at the TeV scale.Spin-2 signal samples for the RS1 model were generated using Pythia 8.186, with the NNPDF23LO PDF set [29] and the A1...