Abstract: The ratio of the cross sections for inclusive isolated-photon production in pp collisions at centre-of-mass energies of 13 and 8 TeV is measured using the ATLAS detector at the LHC. The integrated luminosities of the 13 TeV and 8 TeV datasets are 3.2 fb(-1) and 20.2 fb(-1), respectively. The ratio is measured as a function of the photon transverse energy in different regions of the photon pseudorapidity. The predictions from next-to-leading-order perturbative QCD calculations are compared with the measured ratio. The experimental systematic uncertainties as well as the uncertainties affecting the predictions are evaluated taking into account the correlations between the two centre-of-mass energies, resulting in a reduction of up to a factor of 2.5 (5) in the experimental (theoretical) systematic uncertainties. The predictions based on several parameterisations of the proton parton distribution functions agree with the data within the reduced experimental and theoretical uncertainties. In addition, this ratio to that of the fiducial cross sections for Z boson production at 13 and 8 TeV using the decay channels Z e(+)e(-) and Z (+-) is made and compared with the theoretical predictions. In this double ratio, a further reduction of the experimental uncertainty is obtained because the uncertainties arising from the luminosity measurement cancel out. The predictions describe the measurements of the double ratio within the theoretical and experimental uncertainties.

Abstract: Measurements of CP observables in B-+/- -> D(*) K-+/- and B-+/- -> D(*) pi(+/-) decays are presented, where D(*) indicates a neutral D or D* meson that is an admixture of meson and anti-meson states. Decays of the D(*) meson to the D pi(0) and D gamma final states are partially reconstructed without inclusion of the neutral pion or photon. Decays of the D meson are reconstructed in the K-+/-pi(-/+), K+K-, and pi(+)pi(-) final states. The analysis uses a sample of charged B mesons produced in proton-proton collisions and collected with the LHCb experiment, corresponding to integrated luminosities of 2.0, 1.0, and 5.7 fb(-1) taken at centre-of-mass energies of 7, 8, and 13TeV, respectively. The measurements of partially reconstructed B-+/- -> D(*) K-+/- and B-+/- -> D(*)pi(+/-) with D -> K--/+pi(+/-) decays are the first of their kind, and a first observation of the B-+/- -> (D pi(0)) D*pi(+/-) decay is made with a significance of 6.1 standard deviations. All CP observables are measured with world-best precision, and in combination with other LHCb results will provide strong constraints on the CKM angle gamma.

Abstract: A search for new phenomena with top quark pairs in final states with one isolated electron or muon, multiple jets, and large missing transverse momentum is performed. Signal regions are designed to search for two-, three-, and four-body decays of the directly pair-produced supersymmetric partner of the top quark (stop). Additional signal regions are designed specifically to search for spin-0 mediators that are produced in association with a pair of top quarks and decay into a pair of dark-matter particles. The search is performed using the Large Hadron Collider proton-proton collision dataset at a centre-of-mass energy of s = 13 TeV recorded by the ATLAS detector from 2015 to 2018, corresponding to an integrated luminosity of 139 fb(-1). No significant excess above the Standard Model background is observed, and limits at 95% confidence level are set in the stop-neutralino mass plane and as a function of the mediator mass or the dark-matter particle mass. Stops are excluded up to 1200 GeV (710 GeV) in the two-body (three-body) decay scenario. In the four-body scenario stops up to 640 GeV are excluded for a stop-neutralino mass difference of 60 GeV. Scalar and pseudoscalar dark-matter mediators are excluded up to 200 GeV when the coupling strengths of the mediator to Standard Model and dark-matter particles are both equal to one and when the mass of the dark-matter particle is 1 GeV.

Abstract: We perform a search for near-threshold I (b) (0) resonances decaying to I (b) (-) pi (+) in a sample of proton-proton collision data corresponding to an integrated luminosity of 3 fb(-1) collected by the LHCb experiment. We observe one resonant state, with the following properties: m(Xi b*0) – m (Xi b-) – m (pi+) = 15.727 +/- 0.068 (stat) +/- 0.023 (syst) MeV/c2, Gamma(Xi b*0) = 0.90 +/- 0.16 (stat) +/- 0.08 (syst) MeV. This confirms the previous observation by the CMS collaboration. The state is consistent with the J (P) = 3/2(+)aEuro integral I (b) (au 0) resonance expected in the quark model. This is the most precise determination of the mass and the first measurement of the natural width of this state. We have also measured the ratio sigma(pp -> Xi b*0 X)B(Xi b*0 -> Xi b-pi+)/sigma(pp -> Xi b- X) = 0.28 +/- 0.03 (stat.) +/- 0.01 (syst).

Abstract: The production of W and Z bosons in association with jets is studied in the forward region of proton-proton collisions collected at a centre-of-mass energy of 8 TeV by the LHCb experiment, corresponding to an integrated luminosity of 1.98 +/- 0.02 fb(-1). The W boson is identified using its decay to a muon and a neutrino, while the Z boson is identified through its decay to a muon pair. Total cross-sections are measured and combined into charge ratios, asymmetries, and ratios of W+jet and Z+jet production cross-sections. Differential measurements are also performed as a function of both boson and jet kinematic variables. All results are in agreement with Standard Model predictions.