Abstract: This article presents a new set of proton parton distribution functions, ATLASepWZVjet20, produced in an analysis at next-to-next-to-leading order in QCD. The new data sets considered are the measurements of W+ and W− boson and Z boson production in association with jets in pp collisions at s√ = 8 TeV performed by the ATLAS experiment at the LHC with integrated luminosities of 20.2 fb−1 and 19.9 fb−1, respectively. The analysis also considers the ATLAS measurements of differential W± and Z boson production at s√ = 7 TeV with an integrated luminosity of 4.6 fb−1 and deep-inelastic-scattering data from e±p collisions at the HERA accelerator. An improved determination of the sea-quark densities at high Bjorken x is shown, while confirming a strange-quark density similar in size to the up- and down-sea-quark densities in the range x ≲ 0.02 found by previous ATLAS analyses

Abstract: A search for invisible decays of the Higgs boson as well as searches for dark matter candidates, produced together with a leptonically decaying Z boson, are presented. The analysis is performed using proton−proton collisions at a centre-of-mass energy of 13 TeV, delivered by the LHC, corresponding to an integrated luminosity of 139 and recorded by the ATLAS experiment. Assuming Standard Model cross-sections for ZH production, the observed (expected) upper limit on the branching ratio of the Higgs boson to invisible particles is found to be 19% (19%) at the 95% confidence level. Exclusion limits are also set for simplified dark matter models and two-Higgs-doublet models with an additional pseudoscalar mediator

Abstract: A measurement of the energy asymmetry in jet-associated top-quark pair production is presented using 139fb−1 of data collected by the ATLAS detector at the Large Hadron Collider during pp collisions at s√=13TeV . The observable measures the different probability of top and antitop quarks to have the higher energy as a function of the jet scattering angle with respect to the beam axis. The energy asymmetry is measured in the semileptonic tt ̄ decay channel, and the hadronically decaying top quark must have transverse momentum above 350GeV . The results are corrected for detector effects to particle level in three bins of the scattering angle of the associated jet. The measurement agrees with the SM prediction at next-to-leading-order accuracy in quantum chromodynamics in all three bins. In the bin with the largest expected asymmetry, where the jet is emitted perpendicular to the beam, the energy asymmetry is measured to be −0.043±0.020 , in agreement with the SM prediction of −0.037±0.003 . Interpreting this result in the framework of the Standard Model effective field theory (SMEFT), it is shown that the energy asymmetry is sensitive to the top-quark chirality in four-quark operators and is therefore a valuable new observable in global SMEFT fits

Abstract: We propose a novel way to search for feebly interacting massive particles, exploiting two properties of systems involving collisions between high energy electrons and intense laser pulses. The first property is that the electron-laser collision results in a large flux of hard photons, as the laser behaves effectively as a thick medium. The second property is that the emitted photons free-stream inside the laser and thus for them the laser behaves effectively as a very thin medium. Combining these two features implies that the electron-intense-laser collision is an apparatus, which can efficiently convert O ( 10 GeV ) electrons to a large flux of hard, collinear photons. The photons are directed onto a solid dump in which feebly interacting massive particles may be produced. With the much smaller backgrounds induced by the photon beam compared to those expected in electron- or proton-beam dump experiments and combined with a relatively shorter dump used here, the sensitivity to short lifetimes is unparalleled. We denote this novel apparatus as "optical dump" or NPOD (new physics search with optical dump). The proposed LUXE experiment at the European XFEL has all the basic required ingredients to realize this experimental concept for the first time. Moreover, the NPOD extension of LUXE is essentially parasitic to the main experiment and thus, practically it does not have any bearing on its main program. We discuss how the NPOD concept can be realized in practice by adding a detector after the last physical dump of the experiment to reconstruct the two-photon decay of a new spin-0 particle. We show that even with a relatively short dump, the search can still be background-free. Remarkably, even with a few days of data taking with a 40 TW laser corresponding to its initial run, LUXE-NPOD will be able to probe an uncharted territory of models with pseudoscalars and scalars. Furthermore, with a 350 TW laser of the main run, LUXE-NPOD will have a unique reach for these models. In particular it can probe natural scalar theories for masses above 100 MeV. We note that the new NPOD concept may be ported to other existing or future facilities worldwide, including, e.g., future lepton colliders

Abstract: A measurement of inclusive and differential fiducial cross-sections for the production of the Higgs boson decaying into two photons is performed using 139 fb−1 of proton-proton collision data recorded at s√ = 13 TeV by the ATLAS experiment at the Large Hadron Collider. The inclusive cross-section times branching ratio, in a fiducial region closely matching the experimental selection, is measured to be 67 ± 6 fb, which is in agreement with the state-of-the-art Standard Model prediction of 64 ± 4 fb. Extrapolating this result to the full phase space and correcting for the branching ratio, the total cross-section for Higgs boson production is estimated to be 58 ± 6 pb. In addition, the cross-sections in four fiducial regions sensitive to various Higgs boson production modes and differential cross-sections as a function of either one or two of several observables are measured. All the measurements are found to be in agreement with the Standard Model predictions. The measured transverse momentum distribution of the Higgs boson is used as an indirect probe of the Yukawa coupling of the Higgs boson to the bottom and charm quarks. In addition, five differential cross-section measurements are used to constrain anomalous Higgs boson couplings to vector bosons in the Standard Model effective field theory framework