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.

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Study of the rare decays of B 0 s and B 0 mesons into muon pairs using data collected during 2015 and 2016 with the ATLAS detector The ATLAS Collaboration A study of the decays B 0 s → µ + µ − and B 0 → µ + µ − has been performed using 26.3 fb −1 of 13 TeV LHC proton-proton collision data collected with the ATLAS detector in 2015 and 2016. Since the detector resolution in µ + µ − invariant mass is comparable to the B 0 s -B 0 mass difference, a single fit determines the signal yields for both decay modes. This results in a measurement of the branching fraction B(B 0 s → µ + µ − ) = 3.2 +1.1 −1.0 × 10 −9 and an upper limit B(B 0 → µ + µ − ) < 4.3 × 10 −10 at 95% confidence level. The result is combined with the Run 1 ATLAS result, yielding B(B 0 s → µ + µ − ) = 2.8 +0.8 −0.7 ×10 −9 and B(B 0 → µ + µ − ) < 2.1×10 −10 at 95% confidence level. The combined result is consistent with the Standard Model prediction within 2.4 standard deviations in the B(B 0 → µ + µ − )-B(B 0 s → µ + µ − ) plane.

A QCD analysis is reported of ATLAS data on inclusive W(±) and Z boson production in pp collisions at the LHC, jointly with ep deep-inelastic scattering data from HERA. The ATLAS data exhibit sensitivity to the light quark sea composition and magnitude at Bjorken x∼0.01. Specifically, the data support the hypothesis of a symmetric composition of the light quark sea at low x. The ratio of the strange-to-down sea quark distributions is determined to be 1.00(-0.28)(+0.25) at absolute four-momentum transfer squared Q(2)=1.9  GeV(2) and x=0.023.

Measurements of the production of jets of particles in association with a Z boson in pp collisions at √ s = 7 TeV are presented, using data corresponding to an integrated luminosity of 4.6 fb −1 collected by the ATLAS experiment at the Large Hadron Collider. Inclusive and differential jet cross sections in Z events, with Z decaying into electron or muon pairs, are measured for jets with transverse momentum p T > 30 GeV and rapidity |y| < 4.4. The results are compared to next-to-leading-order perturbative QCD calculations, and to predictions from different Monte Carlo generators based on leading-order and next-to-leading-order matrix elements supplemented by parton showers. Conclusions 24The ATLAS collaboration 35 IntroductionThe production of jets of particles in association with a Z boson 1 at hadron colliders provides an important test of perturbative quantum chromodynamics (pQCD). Such events also constitute a non-negligible background for studies of the Higgs boson candidate [1,2] and searches for new phenomena. In these searches, the multiplicity and kinematics of jets in Z + jets events are exploited to achieve a separation of signal from background. This procedure often introduces scales larger than the mass of the Z boson, resulting in large logarithmic contributions in the calculation of higher-order QCD corrections to the predicted Z + jets cross section [3,4]. The measured Z + jets cross section can be compared 1 The notation Z refers to the complete Z/γ * interference.-1 - JHEP07(2013)032directly to fixed-order predictions at next-to-leading-order (NLO) in pQCD [5][6][7] and to Monte Carlo (MC) generators based on next-to-leading-order or leading-order (LO) matrix elements supplemented by parton showers [8][9][10]. The simulations based on LO matrix elements are affected by large uncertainties in the factorization and renormalization scales and need to be tuned and validated using data. Measurements of the Z + jets cross section have been reported for lower jet energies and lower jet multiplicities in proton-antiproton collisions at a center-of-mass energy of √ s = 1.96 TeV [11][12][13] and in proton-proton collisions based on a data set of 0.036 fb −1 collected at √ s = 7 TeV [14, 15]. This article extends these measurements, using 4.6 fb −1 of proton-proton collision data collected by the ATLAS experiment in 2011 at √ s = 7 TeV. Complementary measurements in Z + jets final states in proton-proton collisions at √ s = 7 TeV have been reported in ref. [16]. The large data set allows cross sections to be measured for the production of up to seven jets in association with a Z boson. Differential jet cross sections are accessible for large jet multiplicities and for energy regimes up to 1 TeV, which allows the modelling of the Z + jets process to be probed for typical phase-space regimes expected from new phenomena and from Higgs boson production, for example via vector-boson-fusion (VBF).Selected events contain a Z boson decaying into a pair of electrons or muons. Associated jets are identifi...