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Abstract:

Jets with different radius parameters R are an important tool for probing quantum chromodynamics processes at different angular scales. Jets with small R=0.2$$R=0.2$$ are instrumental in measurements of the substructure of large-R jets resulting from collimated hadronic decays of energetic W,  Z,  and Higgs bosons, top quarks, and of potential new resonances. This paper presents measurements of the energy scale, resolution, and associated uncertainties of jets with radius parameters R=0.2$$R=0.2$$ and 0.6,  obtained using the ATLAS detector. The results are based on 37fb-1$$37~\text {fb}^{-1}$$ of proton–proton collision data from the Large Hadron Collider at a centre-of-mass energy of s=13$$\sqrt{s}=13$$ TeV. A new in situ method for measuring jet energy scale differences between data and Monte Carlo simulations is presented. The systematic uncertainties in the jet energy scale for central jets (|η|<1.2)$$(|\eta | < 1.2)$$ typically vary from 1% to about 5% as a function of |η|$$|\eta |$$ at very low transverse momentum, pT,$$p_{\text {T}},$$ of around 20 GeV for both R=0.2$$R=0.2$$ and 0.6 jets. The relative energy resolution ranges from (35±6)%$$(35 \pm 6)\%$$ at pT=20$$p_{\text {T}} = 20$$ GeV to (6±0.5)%$$(6 \pm 0.5)\%$$ at pT=300$$p_{\text {T}} = 300$$ GeV for central R=0.2$$R=0.2$$ jets, and is found to be slightly worse for R=0.6$$R=0.6$$ jets. Finally, the effect of close-by hadronic activity on the jet energy scale is investigated and is found to be well modelled by the ATLAS Monte Carlo simulations.

Abstract:

We determine the value of the strong coupling αs and study its running over a wide range of scales as probed by the dijet production process at hadron colliders, based on an NNLO QCD analysis of LHC dijet data. From a large subset of these data a value of αs(mZ) =0.1178 ± 0.0022 is obtained for the strong coupling at the scale of the Z-boson mass mZ, using the invariant mass of the dijet system to select the scale where is probed. The combination of different data sets enhances the reach and precision of the analysis in the multi-TeV range and allows for the first determination of up to scales of 7. Complementing the LHC data with dijet cross sections measured at the HERA electron–proton collider, the kinematic range is extended to test the running of the strong coupling towards smaller scales. Our results exhibit excellent agreement with predictions based on the renormalization group equation of QCD, and represent a comprehensive test of the asymptotic behavior of QCD, spanning more than three orders of magnitude in energy scale.

Abstract:

A search is performed for long-lived heavy neutral leptons (HNLs), produced through the decay of a W boson along with a muon or electron. Two channels are explored: a leptonic channel, in which the HNL decays into two leptons and a neutrino, and a semi-leptonic channel, in which the HNL decays into a lepton and a charged pion. The search is performed with 140 fb−1 of s = 13 TeV proton-proton collision data collected by ATLAS during Run 2 of the Large Hadron Collider. No excess of events is observed; Dirac-like and Majorana-like HNLs with masses below 14.5 GeV and mixing coefficients as small as 10−7 are excluded at the 95% confidence level. The results are interpreted under different assumptions on the flavour of the leptons from the HNL decays.

Abstract:

This paper presents a search for massive, charged, long-lived particles with the ATLAS detector at the Large Hadron Collider using an integrated luminosity of 140 fb−1 of proton-proton collisions at s = 13 TeV. These particles are expected to move significantly slower than the speed of light. In this paper, two signal regions provide complementary sensitivity. In one region, events are selected with at least one charged-particle track with high transverse momentum, large specific ionisation measured in the pixel detector, and time of flight to the hadronic calorimeter inconsistent with the speed of light. In the other region, events are selected with at least two tracks of opposite charge which both have a high transverse momentum and an anomalously large specific ionisation. The search is sensitive to particles with lifetimes greater than about 3 ns with masses ranging from 200 GeV to 3 TeV. The results are interpreted to set constraints on the supersymmetric pair production of long-lived R-hadrons, charginos and staus, with mass limits extending beyond those from previous searches in broad ranges of lifetime.

Abstract:

A set of measurements for the production of a W-boson in association with high-transverse-momentum jets is presented using 140 fb-1 of proton–proton collision data at a centre-of-mass energy of s=13 TeV collected by the ATLAS detector at the LHC. The measurements are performed in final states in which the W-boson decays into an electron or muon plus a neutrino and is produced in association with jets with pTEMPTY>30 GeV, where the leading jet has pTEMPTY>500 GeV. The angular separation between the lepton and the closest jet with pTEMPTY>100 GeV is measured and used to define a collinear phase space, wherein measurements of kinematic properties of the W-boson and the associated jet are performed. The collinear phase space is populated by dijet events radiating a W-boson and events with a W-boson produced in association with several jets and it serves as an excellent data sample to probe higher-order theoretical predictions. Measured differential distributions are compared with predictions from state-of-the-art next-to-leading order multi-leg merged Monte Carlo event generators and a fixed-order calculation of the W+1-jet process computed at next-to-next-to-leading order in the strong coupling constant.