Measurements of azimuthal anisotropies of jet production in Pb+Pb collisions at sNN\sqrt{^{s}NN} = 5.02 TeV with the ATLAS detector

The azimuthal variation of jet yields in heavy-ion collisions provides information about the path-length dependence of the energy loss experienced by partons passing through the hot, dense nuclear matter known as the quark–gluon plasma. This paper presents the azimuthal anisotropy coefficients v$_{2}$, v$_{3}$ and v$_{4}$ measured for jets in Pb + Pb collisions at $\sqrt{^{s}NN}$ = 5.02 TeV using the ATLAS detector at the LHC. The measurement uses data collected in 2015 and 2018, corresponding to an integrated luminosity of 2.2 nb$^{-1}$. The v$_{n}$ values are measured as a function of the transverse momentum of the jets between 71 and 398 GeV and the event centrality. A nonzero value of v$_{2}$ is observed in all but the most central collisions. The value of v$_{2}$ is largest for jets with lower transverse momentum, with values up to 0.05 in mid-central collisions. A smaller, nonzero value of v$_{3}$ of approximately 0.01 is measured with no significant dependence on jet p$_{T}$ or centrality, suggesting that fluctuations in the initial state play a small but distinct role in jet energy loss. No significant deviation of v$_{4}$ from zero is observed in the measured kinematic region

Search for associated production of a Z boson with an invisibly decaying Higgs boson or dark matter candidates at s\sqrt{s} = 13 TeV with the ATLAS detector

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 fb$^{-1}$ 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

Constraints on spin-0 dark matter mediators and invisible Higgs decays using ATLAS 13 TeV pp collision data with two top quarks and missing transverse momentum in the final state

This paper presents a statistical combination of searches targeting final states with two top quarks and invisible particles, characterised by the presence of zero, one or two leptons, at least one jet originating from a b-quark and missing transverse momentum. The analyses are searches for phenomena beyond the Standard Model consistent with the direct production of dark matter in pp collisions at the LHC, using 139 fb$^{-1}$ of data collected with the ATLAS detector at a centre-of-mass energy of 13 TeV. The results are interpreted in terms of simplified dark matter models with a spin-0 scalar or pseudoscalar mediator particle. In addition, the results are interpreted in terms of upper limits on the Higgs boson invisible branching ratio, where the Higgs boson is produced according to the Standard Model in association with a pair of top quarks. For scalar (pseudoscalar) dark matter models, with all couplings set to unity, the statistical combination extends the mass range excluded by the best of the individual channels by 50 (25) GeV, excluding mediator masses up to 370 GeV. In addition, the statistical combination improves the expected coupling exclusion reach by 14% (24%), assuming a scalar (pseudoscalar) mediator mass of 10 GeV. An upper limit on the Higgs boson invisible branching ratio of 0.38 (0.30$_{-0.09}^{+0.13}$) is observed (expected) at 95% confidence level

Modelling and computational improvements to the simulation of single vector-boson plus jet processes for the ATLAS experiment

This paper presents updated Monte Carlo configurations used to model the production of single electroweak vector bosons $(W,Z/\gamma ^{*})$ in association with jets in proton-proton collisions for the ATLAS experiment at the Large Hadron Collider. Improvements pertaining to the electroweak input scheme, parton-shower splitting kernels and scale-setting scheme are shown for multi-jet merged configurations accurate to next-to-leading order in the strong and electroweak couplings. The computational resources required for these set-ups are assessed, and approximations are introduced resulting in a factor three reduction of the per-event CPU time without affecting the physics modelling performance. Continuous statistical enhancement techniques are introduced by ATLAS in order to populate low cross-section regions of phase space and are shown to match or exceed the generated effective luminosity. This, together with the lower per-event CPU time, results in a 50% reduction in the required computing resources compared to a legacy set-up previously used by the ATLAS collaboration. The set-ups described in this paper will be used for future ATLAS analyses and lay the foundation for the next generation of Monte Carlo predictions for single vector-boson plus jets production

Search for invisible Higgs-boson decays in events with vector-boson fusion signatures using 139fb−1139fb^{-1} of proton-proton data recorded by the ATLAS experiment

A direct search for Higgs bosons produced via vector-boson fusion and subsequently decaying into invisible particles is reported. The analysis uses $139 fb^{-1}$ of pp collision data at a centre-of-mass energy of $\sqrt{s}=13$ TeV recorded by the ATLAS detector at the LHC. The observed numbers of events are found to be in agreement with the background expectation from Standard Model processes. For a scalar Higgs boson with a mass of 125 GeV and a Standard Model production cross section, an observed upper limit of 0.145 is placed on the branching fraction of its decay into invisible particles at 95% confidence level, with an expected limit of 0.103. These results are interpreted in the context of models where the Higgs boson acts as a portal to dark matter, and limits are set on the scattering cross section of weakly interacting massive particles and nucleons. Invisible decays of additional scalar bosons with masses from 50 GeV to 2 TeV are also studied, and the derived upper limits on the cross section times branching fraction decrease with increasing mass from 1.0 pb for a scalar boson mass of 50 GeV to 0.1 pb at a mass of 2 TeV

Measurement of Higgs boson decay into b-quarks in associated production with a top-quark pair in pp collisions at s\sqrt{s} = 13 TeV with the ATLAS detector

The associated production of a Higgs boson and a top-quark pair is measured in events characterised by the presence of one or two electrons or muons. The Higgs boson decay into a b-quark pair is used. The analysed data, corresponding to an integrated luminosity of 139 fb$^{-1}$, were collected in proton-proton collisions at the Large Hadron Collider between 2015 and 2018 at a centre-of-mass energy of $\sqrt{s}$ = 13 TeV. The measured signal strength, defined as the ratio of the measured signal yield to that predicted by the Standard Model, is 0.35$_{-0.34}^{+0.36}$. This result is compatible with the Standard Model prediction and corresponds to an observed (expected) significance of 1.0 (2.7) standard deviations. The signal strength is also measured differentially in bins of the Higgs boson transverse momentum in the simplified template cross-section framework, including a bin for specially selected boosted Higgs bosons with transverse momentum above 300 GeV

Search for doubly charged Higgs boson production in multi-lepton final states using 139 fb−1^{-1} of proton–proton collisions at s\sqrt{s} = 13 TeV with the ATLAS detector

2902 - łączna liczba autorów publikacjiA search for pair production of doubly charged Higgs bosons (H$^{\pm \pm}$), each decaying into a pair of prompt, isolated, and highly energetic leptons with the same electric charge, is presented. The search uses a proton–proton collision data sample at a centre-of-mass energy of 13 TeV corresponding to an integrated luminosity of 139 fb$^{-1}$ recorded by the ATLAS detector during Run 2 of the Large Hadron Collider (LHC). This analysis focuses on same-charge leptonic decays, H$^{\pm \pm} \to \ell^{\pm} \ell '^{\pm}$ where $\ell, \ell'$ = e, μ, τ, in two-, three-, and four-lepton channels, but only considers final states which include electrons or muons. No evidence of a signal is observed. Corresponding upper limits on the production cross-section of a doubly charged Higgs boson are derived, as a function of its mass m(H$^{\pm \pm}$), at 95% confidence level. Assuming that the branching ratios to each of the possible leptonic final states are equal, $\mathcal{B}(H^{\pm\pm} \to e^{\pm }e^{\pm })$ = $\mathcal{B}(H^{\pm\pm} \to e^{\pm }\mu^{\pm })$ = $\mathcal{B}(H^{\pm\pm} \to \mu^{\pm }\mu^{\pm })$ = $\mathcal{B}(H^{\pm\pm} \to e^{\pm }\tau^{\pm })$ = $\mathcal{B}(H^{\pm\pm} \to \mu^{\pm }\tau^{\pm })$ = $\mathcal{B}(H^{\pm\pm} \to \tau^{\pm }\tau^{\pm })$ = 1/6, the observed (expected) lower limit on the mass of a doubly charged Higgs boson is 1080 GeV (1065 GeV) within the left-right symmetric type-II seesaw model, which is the strongest limit to date produced by the ATLAS Collaboration. Additionally, this paper provides the first direct test of the Zee–Babu neutrino mass model at the LHC, yielding an observed (expected) lower limit of m(H$^{\pm\pm}$) = 900 GeV (880 GeV)

Exclusive dielectron production in ultraperipheral Pb + Pb collisions at SNN\sqrt{^{S}NN} = 5.02 TeV with ATLAS

Exclusive production of dielectron pairs, γγ $\to e^{+}e^{-}$, is studied using $\mathcal{L}_{int}$ = 1.72 nb$^{-1}$ of data from ultraperipheral collisions of lead nuclei at $\sqrt{^{S}NN}$ = 5.02 TeV recorded by the ATLAS detector at the LHC. The process of interest proceeds via photon–photon interactions in the strong electromagnetic fields of relativistic lead nuclei. Dielectron production is measured in the fiducial region defined by following requirements: electron transverse momentum p$_{T}^{e}$ > 2.5 GeV, absolute electron pseudorapidity $\left|\eta ^{e} \right|$ 5 GeV, and dielectron transverse momentum p$_{T}^{ee}$ > 2 GeV. Differential cross-sections are measured as a function of m$_{ee}$, average p$_{T}^{e}$, absolute dielectron rapidity $\left| y_{ee} \right|$, and scattering angle in the dielectron rest frame, $\left| cos \theta^{*} \right|$, in the inclusive sample, and also with a requirement of no activity in the forward direction. The total integrated fiducial cross-section is measured to be 215 $\pm$ 1(stat. )$_{-20}^{+23}$(syst. ) $\pm$ 4(lumi. ) μb. Within experimental uncertainties the measured integrated cross-section is in good agreement with the QED predictions from the Monte Carlo programs STARLIGHT and SUPERCHIC, confirming the broad features of the initial photon fluxes. The differential cross-sections show systematic differences from these predictions which are more pronounced at high $\left| y_{ee} \right|$ and $\left| cos \theta^{*} \right|$ values

Search for Higgs boson pair production in the two bottom quarks plus two photons final state in pp collisions at s\sqrt{s} = 13 TeV with the ATLAS detector

Searches are performed for nonresonant and resonant di-Higgs boson production in the $b\bar{b}\gamma \gamma$ final state. The dataset used corresponds to an integrated luminosity of 139 fb$^{-1}$ of proton–proton collisions at a center-of-mass energy of 13 TeV recorded by the ATLAS detector at the CERN Large Hadron Collider. No excess above the expected background is found and upper limits on the di-Higgs boson production cross sections are set. A 95% confidence-level upper limit of 4.2 times the cross section predicted by the Standard Model is set on pp $\to$ HH nonresonant production, where the expected limit is 5.7 times the Standard Model predicted value. The expected constraints are obtained for a background hypothesis excluding pp $\to$ HH production. The observed (expected) constraints on the Higgs boson trilinear coupling modifier $\kappa _{\lambda }$ are determined to be [-1.5, 6.7] ([-2.4, 7.7]) at 95% confidence level, where the expected constraints on $\kappa _{\lambda }$ are obtained excluding pp $\to$ HH production from the background hypothesis. For resonant production of a new hypothetical scalar particle X (X $\to$ HH $\to$ $b\bar{b}\gamma \gamma$), limits on the cross section for pp $\to$ X $\to$ HH are presented in the narrow-width approximation as a function of $m_{X}$ in the range 251 GeV $\leq m_{X} \leq$ 1000 GeV. The observed (expected) limits on the cross section for pp $\to$ X $\to$ HH range from 640 fb to 44 fb (391 fb to 46 fb) over the considered mass range