On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of ∼ 1.7 s with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg2 at a luminosity distance of 40 − 8 + 8 Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 M ⊙ . An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at ∼ 40 Mpc ) less than 11 hours after the merger by the One-Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over ∼10 days. Following early non-detections, X-ray and radio emission were discovered at the transient’s position ∼ 9 and ∼ 16 days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC 4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by the radioactive decay of r-process nuclei synthesized in the ejecta.

Measurement of the Z/γ * boson transverse momentum distribution in pp collisions at √ s = 7 TeV with the ATLAS detectorThe ATLAS collaboration E-mail: atlas.publications@cern.chAbstract: This paper describes a measurement of the Z/γ * boson transverse momentum spectrum using ATLAS proton-proton collision data at a centre-of-mass energy of √ s = 7 TeV at the LHC. The measurement is performed in the Z/γ * → e + e − and Z/γ * → µ + µ − channels, using data corresponding to an integrated luminosity of 4.7 fb −1 . Normalized differential cross sections as a function of the Z/γ * boson transverse momentum are measured for transverse momenta up to 800 GeV. The measurement is performed inclusively for Z/γ * rapidities up to 2.4, as well as in three rapidity bins. The channel results are combined, compared to perturbative and resummed QCD calculations and used to constrain the parton shower parameters of Monte Carlo generators. The ATLAS collaboration 31 Keywords: Hadron-Hadron Scattering IntroductionThe transverse momentum distribution of W and Z bosons produced in hadronic collisions is a traditional probe of strong interaction dynamics. The low transverse momentum (p T ) range is governed by initial-state parton radiation (ISR) and the intrinsic transverse momentum of the initial-state partons inside the proton, and modeled using soft-gluon resummation [1] or parton shower models [2,3]. Quark-gluon scattering dominates at high p T and is described by perturbative QCD [4][5][6]. The correct modelling of the vector boson p T distribution is important in many physics analyses at the LHC for which the production of W or Z bosons constitutes a significant background. Moreover, it is crucial for a precise measurement of the W boson mass. The transverse momentum distribution also probes the gluon density of the proton [7]. Vector boson p T distribution measurements were published by ATLAS [8, 9] and CMS [10] based on 35-40 pb −1 of proton-proton collisions at a centre-of-mass energy of √ s = 7 TeV. The typical precision of these measurements is 4% to 10%.-1 - JHEP09(2014)145This paper presents a measurement of the normalized Z boson transverse momentum distribution (p Z T ) with the ATLAS detector, in the Z/γ * → e + e − and Z/γ * → µ + µ − channels, using LHC proton-proton collision data taken in 2011 at a centre-of-mass energy of √ s = 7 TeV and corresponding to an integrated luminosity of 4.7 fb −1 [11]. The large integrated luminosity allows the measurement to be performed in three different Z boson rapidity (y Z ) bins, probing the transverse momentum dynamics over a wide range of the initial-state parton momentum fraction. With respect to previous results, the present analysis aims at reduced uncertainties, finer binning and extended measurement range.Reconstructed from the final-state lepton kinematics, p Z T is affected by lepton energy and momentum measurement uncertainties. To minimize the impact of these uncertainties, the φ η observable 1 was introduced as an alternative probe of p Z T [12], pioneered at the Tev...

A search for the direct production of charginos and neutralinos in final states with three leptons and missing transverse momentum is presented. The analysis is based on 20.3 fb −1 of √ s = 8 TeV proton-proton collision data delivered by the Large HadronCollider and recorded with the ATLAS detector. Observations are consistent with the Standard Model expectations and limits are set in R-parity-conserving phenomenological Minimal Supersymmetric Standard Models and in simplified supersymmetric models, significantly extending previous results. For simplified supersymmetric models of direct chargino (χ ± 1 ) and next-to-lightest neutralino (χ 0 2 ) production with decays to lightest neutralino (χ 0 1 ) via either all three generations of sleptons, staus only, gauge bosons, or Higgs bosons,χ ± 1 andχ 0 2 masses are excluded up to 700 GeV, 380 GeV, 345 GeV, or 148 GeV respectively, for a masslessχ 0 1 . Keywords: Hadron-Hadron ScatteringArXiv ePrint: 1402.7029Open Access, Copyright CERN, for the benefit of the ATLAS Collaboration. The ATLAS collaboration 30 IntroductionSupersymmetry (SUSY) [1][2][3][4][5][6][7][8][9] proposes the existence of supersymmetric particles, with spin differing by one-half unit with respect to that of their Standard Model (SM) partners. Charginos,χ ± 1,2 , and neutralinos,χ 0 1,2,3,4 , collectively referred to as electroweakinos, are the ordered mass eigenstates formed from the linear superposition of the SUSY partners of the Higgs and electroweak gauge bosons (higgsinos, winos and binos). Based on naturalness arguments [10,11], the lightest electroweakinos are expected to have mass of order 100 GeV and be accessible at the Large Hadron Collider (LHC). In the R-parity-conserving minimal supersymmetric extension of the SM (MSSM) [12][13][14][15][16], SUSY particles are pair-produced and the lightest SUSY particle (LSP), assumed in many models to be theχ 0 1 , is stable. Charginos and neutralinos can decay into leptonic final states via superpartners of neutrinos (ν, sneutrinos) or charged leptons (l, sleptons), or via W , Z or Higgs (h) bosons (χ ± i → ℓ ±ν , νl ± , W ±χ 0 j , Zχ JHEP04(2014)169µ or τ , referred to as leptons in the following) and missing transverse momentum originating from the two undetected LSPs and the neutrinos. The analysis is based on 20.3 fb −1 of proton-proton collision data recorded by ATLAS at a centre-of-mass energy of √ s = 8 TeV.Previous searches for charginos and neutralinos are documented in refs [17][18][19] by ATLAS, and in ref. [20] by CMS. Similar searches were conducted at the Tevatron [21, 22]. At LEP [23][24][25][26][27], searches for direct chargino production set a model-independent lower limit of 103.5 GeV at 95% confidence level (CL) on the mass of promptly decaying charginos. SUSY scenariosAmong the electroweakino pair-production processes leading to three leptons in the final state,χ ± 1χ 0 2 production has the largest cross-section in most of the MSSM parameter space. Several simplified supersymmetric models ("simplified models" [28])...

Results of a search for H → τ τ decays are presented, based on the full set of proton-proton collision data recorded by the ATLAS experiment at the LHC during 2011 and 2012. The data correspond to integrated luminosities of 4.5 fb −1 and 20.3 fb −1 at centre-of-mass energies of √ s = 7 TeV and √ s = 8 TeV respectively. All combinations of leptonic (τ → νν with = e, µ) and hadronic (τ → hadrons ν) tau decays are considered. An excess of events over the expected background from other Standard Model processes is found with an observed (expected) significance of 4.5 (3.4) standard deviations. This excess provides evidence for the direct coupling of the recently discovered Higgs boson to fermions. The measured signal strength, normalised to the Standard Model expectation, of µ = 1.43 +0.43−0.37 is consistent with the predicted Yukawa coupling strength in the Standard Model. The ATLAS collaboration 58 Keywords: Hadron-Hadron ScatteringThe investigation of the origin of electroweak symmetry breaking and, related to this, the experimental confirmation of the Brout-Englert-Higgs mechanism [1-6] is one of the prime goals of the physics programme at the Large Hadron Collider (LHC) [7]. With the discovery of a Higgs boson with a mass of approximately 125 GeV by the ATLAS [8] and CMS [9] collaborations, an important milestone has been reached. More precise measurements of the properties of the discovered particle [10,11] as well as tests of the spin-parity quantum numbers [12][13][14] continue to be consistent with the predictions for the Standard Model (SM) Higgs boson.These measurements rely predominantly on studies of the bosonic decay modes, H → γγ, H → ZZ * and H → W W * . To establish the mass generation mechanism for fermions as implemented in the SM, it is of prime importance to demonstrate the direct coupling of the Higgs boson to fermions and the proportionality of its strength to mass [15]. The most promising candidate decay modes are the decays into tau leptons, H → τ τ , and bottom quarks (b-quarks), H → bb. Due to the high background, the search for decays to bb is restricted to Higgs bosons produced in modes which have a more distinct signature but a lower cross-section, such as H production with an associated vector boson. The smaller rate of these processes in the presence of still large background makes their detection challenging. More favourable signal-to-background conditions are expected for H → τ τ decays. Recently, the CMS Collaboration published evidence for H → τ τ decays at a significance in terms of standard deviations of 3.2σ [16], and an excess corresponding to a significance of 2.1σ in the search for H → bb decays [17]. The combination of channels provides evidence for fermionic couplings with a significance of 3.8σ [18]. The yield of events in the search for H → bb decays observed by the ATLAS Collaboration has a signal significance of 1.4σ [19]. The Tevatron experiments have observed an excess corresponding to 2.8σ in the H → bb search [20].In this paper, the results of a search for...