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 Majorana nature of neutrinos is one of the most fundamental questions in particle physics.It is directly related to the violation of accidental lepton number symmetry. This motivated enormous efforts into the search of such process and among them, one conventional experiment is the neutrinoless double-beta decay (0νββ). On the other hand, there have been proposals of future electron-positron colliders as "Higgs factory" for the precise measurement of Higgs boson properties and it has been proposed to convert such machine into an electron-electron collider. This option enables a new way to probe TeV Majorana neutrino via the inverse 0νββ decay process (e − e − → W − W − ) as an alternative and complementary test to the conventional 0νββ decay experiments. In this paper, we investigate the collider search for e − e − → W − W − in different decay channels at future electron colliders. We find the pure hadronic channel, semi-leptonic channel with muon and pure leptonic channel with dimuon have the most discovery potential. 1Enormous neutrino oscillation experiments in the last two decades have provided definite evidence for non-zero neutrino masses and the mixing between different flavors [1][2][3]. Even though the recent discovery of a Higgs-like boson has significantly improved our knowledge over generation of SM fermion masses, being tiny but electric neutral, the origin of neutrino mass may remain an open question. Firs of all, if neutrino masses arise from Yukawa couplings as the same mechanicsm as quarks and charged leptons, one immediately encounters the O(10 −12 ) hierarchy in y ν /y t . A second argument arises from the prediction of electric charge quantization. Anomaly-free conditions determine U (1) Y as the unique U (1) gauge symmetry in SM up to a normalization factor [7]. Though extending SM with milli-charged Dirac neutrino does not explicitly violate the anomaly-free conditions, the hyper-charge assignment is no longer uniquely determined unless the neutrino is a Majorana particle [8]. On the other hand, the bound on neutrino electric charge Q ν is |Q ν | (0.5 ± 2.9) × 10 −21 e (68% CL) by assuming charge conservation in β-decay n → p+e − +ν e [4,5], and |Q ν | < 2×10 −15 e from SN1987A astrophysics observation [6]. These facts motivate the study of Majorana neutrinos.Taking the effective theory approach, Majorana mass term is from the non-renormalizablewith dimensionless coupling y ij . This dimensionfive operator breaks lepton number by two units (∆L = 2) and indicates new physics at some specific Λ However, they're still weaker than the electroweak precision observable(EWPO) bound from constraining the non-unitarity of leptonic mixing matrix [37]More detailed analyses are available in [38][39][40].As an alternative, e − e − → W − W − scattering process in Fig.1 mediated by Majorana neutrino exchange is sensitive to the TeV-seesaw scenario. The intriguing feature of this process is that it could be regarded as the inverse of 0νββ decay with LNV but could occur at colliders. In addition...

[1] The Bayesian probability inversion and a Markov chain Monte Carlo (MCMC) technique were applied to a terrestrial ecosystem model to analyze uncertainties of estimated carbon (C) transfer coefficients and simulated C pool sizes. This study used six data sets of soil respiration, woody biomass, foliage biomass, litterfall, C content in the litter layers, and C content in mineral soil measured under both ambient CO 2 (350 ppm) and elevated CO 2 (550 ppm) plots from 1996 to 2000 at the Duke Forest FreeAir CO 2 Experiment (FACE) site. A Metropolis-Hastings algorithm was employed to construct a posterior probability density function (PPDF) of C transfer coefficients on the basis of prior information of model parameters, model structure, and the six data sets. The constructed PPDFs indicated that the transfer coefficients from pools of nonwoody biomass, woody biomass, and structural litter were well constrained by the six data sets under both ambient and elevated CO 2 . The data sets also gave moderate information to the transfer coefficient from the slow soil C pool. However, the transfer coefficients from pools of metabolic litter, microbe, and passive soil C were poorly constrained. The poorly constrained parameters were attributable to either the lack of experimental data or the mismatch of timescales between the available data and the parameters to be estimated. Cumulative distribution functions were constructed for simulated C pool sizes on the basis of the six data sets, showing that on average the ecosystem would store 16,616 g C m À2at elevated CO 2 by the year 2010, significantly higher than 13,426 g C m À2 at ambient CO 2 with 95% confidence. This study shows that the combination of a Bayesian approach and MCMC inversion technique is an effective method to synthesize information from various sources for assessment of ecosystem responses to elevated CO 2 .

Search for squarks and gluinos in final states with jets and missing transverse momentum using 36 fb − 1 of ffiffi s p = 13 TeV pp collision data with the ATLAS detector A search for the supersymmetric partners of quarks and gluons (squarks and gluinos) in final states containing hadronic jets and missing transverse momentum, but no electrons or muons, is presented. The data used in this search were recorded in 2015 and 2016 by the ATLAS experiment in ffiffi ffi s p ¼ 13 TeV proton-proton collisions at the Large Hadron Collider, corresponding to an integrated luminosity of 36.1 fb −1 . The results are interpreted in the context of various models where squarks and gluinos are pair produced and the neutralino is the lightest supersymmetric particle. An exclusion limit at the 95% confidence level on the mass of the gluino is set at 2.03 TeV for a simplified model incorporating only a gluino and the lightest neutralino, assuming the lightest neutralino is massless. For a simplified model involving the strong production of mass-degenerate first-and second-generation squarks, squark masses below 1.55 TeV are excluded if the lightest neutralino is massless. These limits substantially extend the region of supersymmetric parameter space previously excluded by searches with the ATLAS detector.

Search for heavy neutral leptons in decays of W bosons produced in 13 TeV p p collisions using prompt and displaced signatures with the ATLAS detectorThe ATLAS CollaborationThe problems of neutrino masses, matter-antimatter asymmetry, and dark matter could be successfully addressed by postulating right-handed neutrinos with Majorana masses below the electroweak scale. In this work, leptonic decays of W bosons extracted from 32.9 fb −1 to 36.1 fb −1 of 13 TeV proton-proton collisions at the LHC are used to search for heavy neutral leptons (HNLs) that are produced through mixing with muon or electron neutrinos. The search is conducted using the ATLAS detector in both prompt and displaced leptonic decay signatures. The prompt signature requires three leptons produced at the interaction point (either µµe or eeµ) with a veto on same-flavour opposite-charge topologies. The displaced signature comprises a prompt muon from the W boson decay and the requirement of a dilepton vertex (either µµ or µe) displaced in the transverse plane by 4-300 mm from the interaction point. The search sets constraints on the HNL mixing to muon and electron neutrinos for HNL masses in the range 4.5-50 GeV.