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ATLAS Collaboration

Publications and source records attributed to ATLAS Collaboration.

At least 325 records · Page 18Linked to original sources

arXiv2024

Electron and photon energy calibration with the ATLAS detector using LHC Run 2 data

This paper presents the electron and photon energy calibration obtained with the ATLAS detector using 140 fb$^{-1}$ of LHC proton-proton collision data recorded at $\sqrt{s}=13$ TeV between 2015 and 2018. Methods for the measurement of electron and photon energies are outlined, along with the current knowledge of the passive material in front of the ATLAS electromagnetic calorimeter. The energy calibration steps are discussed in detail, with emphasis on the improvements introduced in this paper. The absolute energy scale is set using a large sample of $Z$-boson decays into electron-positron pairs, and its residual dependence on the electron energy is used for the first time to further constrain systematic uncertainties. The achieved calibration uncertainties are typically 0.05% for electrons from resonant $Z$-boson decays, 0.4% at $E_\text{T}\sim 10$ GeV, and 0.3% at $E_\text{T}\sim 1$ TeV; for photons at $E_\text{T}\sim 60$ GeV, they are 0.2% on average. This is more than twice as precise as the previous calibration. The new energy calibration is validated using $J/ψ\to ee$ and radiative $Z$-boson decays.

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arXiv2024

Measurement of the $H \to γγ$ and $H \to ZZ^* \to 4 \ell$ cross-sections in $pp$ collisions at $\sqrt{s}=13.6$ TeV with the ATLAS detector

The inclusive Higgs boson production cross-section is measured in the di-photon and the $ZZ^* \to 4 \ell$ decay channels using 31.4 and 29.0 fb$^{-1}$ of $pp$ collision data respectively, collected with the ATLAS detector at a centre-of-mass energy of $\sqrt{s}=13.6$ TeV. To reduce the model dependence, the measurement in each channel is restricted to a particle-level phase space that closely matches the channel's detector-level kinematic selection, and it is corrected for detector effects. These measured fiducial cross-sections are $σ_{\mathrm{fid},γγ} = 76^{+14}_{-13}$ fb, and $σ_{\mathrm{fid},4 \ell} = 2.80 \pm 0.74$ fb, in agreement with the corresponding Standard Model predictions of $67.6 \pm 3.7 $ fb and $3.67 \pm 0.19 $ fb. Assuming Standard Model acceptances and branching fractions for the two channels, the fiducial measurements are extrapolated to the full phase space yielding total cross-sections of $σ(pp \to H) = 67^{+12}_{-11}$ pb and $46 \pm 12$ pb at $13.6$ TeV from the di-photon and $ZZ^* \to 4 \ell$ measurements respectively. The two measurements are combined into a total cross-section measurement of $σ(pp \to H)= 58.2 \pm 8.7$ pb, to be compared with the Standard Model prediction of $σ(pp \to H)_\mathrm{SM} = 59.9 \pm 2.6 $ pb.

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arXiv2024

Measurement of the production cross-section of $J/ψ$ and $ψ(2$S$)$ mesons in $pp$ collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector

Measurements of the differential production cross-sections of prompt and non-prompt $J/ψ$ and $ψ(2$S$)$ mesons with transverse momenta between 8 and 360 GeV and rapidity in the range $|y|<2$ are reported. Furthermore, measurements of the non-prompt fractions of $J/ψ$ and $ψ(2$S$)$, and the prompt and non-prompt $ψ(2$S$)$-to-$J/ψ$ production ratios, are presented. The analysis is performed using 140 fb$^{-1}$ of $\sqrt{s}=13$ TeV $pp$ collision data recorded by the ATLAS detector at the LHC during the years 2015-2018.

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arXiv2024

Evidence of pair production of longitudinally polarised vector bosons and study of CP properties in $ZZ \to 4\ell$ events with the ATLAS detector at $\sqrt{s} = 13$ TeV

A study of the polarisation and CP properties in $ZZ$ production is presented. The used data set corresponds to an integrated luminosity of 140 fb$^{-1}$ of proton-proton collisions at a centre-of-mass energy of $13$ TeV recorded by the ATLAS detector at the Large Hadron Collider. The $ZZ$ candidate events are reconstructed using two same-flavour opposite-charge electron or muon pairs. The production of two longitudinally polarised $Z$ bosons is measured with a significance of 4.3 standard deviations, and its cross-section is measured in a fiducial phase space to be $2.45 \pm 0.60$ fb, consistent with the next-to-leading-order Standard Model prediction. The inclusive differential cross-section as a function of a CP-sensitive angular observable is also measured. The results are used to constrain anomalous CP-odd neutral triple gauge couplings.

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arXiv2024

Search for new phenomena in two-body invariant mass distributions using unsupervised machine learning for anomaly detection at $\sqrt{s} = 13$ TeV with the ATLAS detector

Searches for new resonances are performed using an unsupervised anomaly-detection technique. Events with at least one electron or muon are selected from 140 fb$^{-1}$ of $pp$ collisions at $\sqrt{s} = 13$ TeV recorded by ATLAS at the Large Hadron Collider. The approach involves training an autoencoder on data, and subsequently defining anomalous regions based on the reconstruction loss of the decoder. Studies focus on nine invariant mass spectra that contain pairs of objects consisting of one light jet or $b$-jet and either one lepton ($e$, $μ$), photon, or second light jet or $b$-jet in the anomalous regions. No significant deviations from the background hypotheses are observed.

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arXiv2024

Performance and calibration of quark/gluon-jet taggers using 140 fb$^{-1}$ of $pp$ collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector

The identification of jets originating from quarks and gluons, often referred to as quark/gluon tagging, plays an important role in various analyses performed at the Large Hadron Collider, as Standard Model measurements and searches for new particles decaying to quarks often rely on suppressing a large gluon-induced background. This paper describes the measurement of the efficiencies of quark/gluon taggers developed within the ATLAS Collaboration, using $\sqrt{s} = 13$ TeV proton-proton collision data with an integrated luminosity of 140 fb$^{-1}$ collected by the ATLAS experiment. Two taggers with high performances in rejecting jets from gluon over jets from quarks are studied: one tagger is based on requirements on the number of inner-detector tracks associated with the jet, and the other combines several jet substructure observables using a boosted decision tree. A method is established to determine the quark/gluon fraction in data, by using quark/gluon-enriched subsamples defined by the jet pseudorapidity. Differences in tagging efficiency between data and simulation are provided for jets with transverse momentum between 500 GeV and 2 TeV and for multiple tagger working points.

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arXiv2024

Search for non-resonant Higgs boson pair production in the $2b + 2\ell + E_\mathrm{T}^\mathrm{miss}$ final state in $pp$ collisions $\sqrt{s} = 13\,\mathrm{TeV}$ with the ATLAS detector

A search for non-resonant Higgs boson pair ($HH$) production is presented, in which one of the Higgs bosons decays to a b-quark pair ($b\bar b$) and the other decays to $WW^*$, $ZZ^*$, or $τ^+τ^-$, with in each case a final state with $\ell^+\ell^- +$ neutrinos ($\ell = e, μ$). The analysis targets separately the gluon-gluon fusion and vector boson fusion production modes. Data recorded by the ATLAS detector in proton-proton collisions at a centre-of-mass energy of 13 TeV at the Large Hadron Collider, corresponding to an integrated luminosity of $140\mathrm{fb}^{-1}$, are used in this analysis. Events are selected to have exactly two $b$-tagged jets and two leptons with opposite electric charge and missing transverse momentum in the final state. These events are classified using multivariate analysis algorithms to separate the $HH$ events from other Standard Model processes. No evidence of the signal is found. The observed (expected) upper limit on the cross-section for non-resonant Higgs boson pair production is determined to be 9.7 (16.2) times the Standard Model prediction at 95% confidence level. The Higgs boson self-interaction coupling parameter $κ_λ$ and the quadrilinear coupling parameter $κ_{2V}$ are each separately constrained by this analysis to be within the ranges ${[-6.2, 13.3]}$ and ${[-0.17, 2.4]}$, respectively, at 95% confidence level, when all other parameters are fixed.

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arXiv2024

Search for heavy Majorana or Dirac neutrinos and right-handed $W$ gauge bosons in final states with charged leptons and jets in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

A search for heavy right-handed Majorana or Dirac neutrinos $N_{\mathrm{R}}$ and heavy right-handed gauge bosons $W_{\mathrm{R}}$ is performed in events with energetic electrons or muons, with the same or opposite electric charge, and energetic jets. The search is carried out separately for topologies of clearly separated final-state products (``resolved'' channel) and topologies with boosted final states with hadronic and/or leptonic products partially overlapping and reconstructed as a large-radius jet (``boosted'' channel). The events are selected from $pp$ collision data at the LHC with an integrated luminosity of 139 fb$^{-1}$ collected by the ATLAS detector at $\sqrt{s}$ = 13 TeV. No significant deviations from the Standard Model predictions are observed. The results are interpreted within the theoretical framework of a left-right symmetric model, and lower limits are set on masses in the heavy right-handed $W_{\mathrm{R}}$ boson and $N_{\mathrm{R}}$ plane. The excluded region extends to about $m(W_{\mathrm{R}}) = 6.4$ TeV for both Majorana and Dirac $N_{\mathrm{R}}$ neutrinos at $m(N_{\mathrm{R}})<1$ TeV. $N_{\mathrm{R}}$ with masses of less than 3.5 (3.6) TeV are excluded in the electron (muon) channel at $m(W_{\mathrm{R}})=4.8$ TeV for the Majorana neutrinos, and limits of $m(N_{\mathrm{R}})$ up to 3.6 TeV for $m(W_{\mathrm{R}}) = 5.2$ (5.0) TeV in the electron (muon) channel are set for the Dirac neutrinos. These constitute the most stringent exclusion limits to date for the model considered.

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arXiv2024

Studies of new Higgs boson interactions through nonresonant $HH$ production in the $b\bar{b}γγ$ final state in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

A search for nonresonant Higgs boson pair production in the $b\bar{b}γγ$ final state is performed using 140 fb$^{-1}$ of proton-proton collisions at a centre-of-mass energy of 13 TeV recorded by the ATLAS detector at the CERN Large Hadron Collider. This analysis supersedes and expands upon the previous nonresonant ATLAS results in this final state based on the same data sample. The analysis strategy is optimised to probe anomalous values not only of the Higgs ($H$) boson self-coupling modifier $κ_λ$ but also of the quartic $HHVV$ ($V=W,Z$) coupling modifier $κ_{2V}$. No significant excess above the expected background from Standard Model processes is observed. An observed upper limit $μ_{HH}<4.0$ is set at 95% confidence level on the Higgs boson pair production cross-section normalised to its Standard Model prediction. The 95% confidence intervals for the coupling modifiers are $-1.4<κ_λ<6.9$ and $-0.5<κ_{2V}<2.7$, assuming all other Higgs boson couplings except the one under study are fixed to the Standard Model predictions. The results are interpreted in the Standard Model effective field theory and Higgs effective field theory frameworks in terms of constraints on the couplings of anomalous Higgs boson (self-)interactions.

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arXiv2024

Search for flavor-changing neutral $tqH$ interactions with $H\rightarrow γγ$ in $pp$ collisions at $\sqrt{s}$ = 13 TeV using the ATLAS detector

A search for flavour-changing neutral interactions involving the top quark, the Higgs boson and an up-type quark $q$ ($q = c, u$) is presented. The proton-proton collision data set used, with an integrated luminosity of 139 fb$^{-1}$, was collected at $\sqrt{s} = 13$TeV by the ATLAS experiment at the Large Hadron Collider. Both the decay process $t \to qH$ and the production process $pp \to tH$, with the Higgs boson decaying into two photons, are investigated. No significant excess is observed and upper limits are set on the $t\rightarrow cH$ and the $t\rightarrow uH$ branching ratios of $4.3\times 10^{-4}$ and $3.8\times 10^{-4}$, respectively, at the 95% confidence level, while the expected limits in the absence of signal are $4.7\times 10^{-4}$ and $3.9\times 10^{-4}$. Combining this search with ATLAS searches in the $H \to τ^+τ^-$ and $H \to b\bar{b}$ final states yields observed (expected) upper limits on the $t\to cH$ branching ratio of $5.8\times 10^{-4}\ (3.0\times 10^{-4})$ at the 95% confidence level. The corresponding observed (expected) upper limit on the $t\rightarrow uH$ branching ratio is $4.0 \times 10^{-4}\ (2.4 \times 10^{-4})$

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arXiv2024

Search for magnetic monopoles and stable particles with high electric charges in $\sqrt{s}=$13 TeV $pp$ collisions with the ATLAS detector

We present a search for magnetic monopoles and high-electric-charge objects using LHC Run 2 $\sqrt{s} =$13 TeV proton$-$proton collisions recorded by the ATLAS detector. A total integrated luminosity of 138 fb$^{-1}$ was collected by a specialized trigger. No highly ionizing particle candidate was observed. Considering the Drell-Yan and photon-fusion pair production mechanisms as benchmark models, cross-section upper limits are presented for spin-0 and spin-$\frac{1}{2}$ magnetic monopoles of magnetic charge $1g_\textrm{D}$ and $2g_\textrm{D}$ and for high-electric-charge objects of electric charge $20 \leq |z| \leq 100$, for masses between 200 GeV and 4000 GeV. The search improves by approximately a factor of three the previous cross-section limits on the Drell-Yan production of magnetic monopoles and high-electric charge objects. Also, the first ATLAS limits on the photon-fusion pair production mechanism of magnetic monopoles and high-electric-charge objects have been obtained.

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arXiv2024

Measurement of exclusive pion pair production in proton-proton collisions at $\sqrt{s} = 7$ TeV with the ATLAS detector

The exclusive production of pion pairs in the process $pp\to ppπ^+π^-$ has been measured at $\sqrt{s}$ = 7 TeV with the ATLAS detector at the LHC, using 80 $μ$b$^{-1}$ of low-luminosity data. The pion pairs were detected in the ATLAS central detector while outgoing protons were measured in the forward ATLAS ALFA detector system. This represents the first use of proton tagging to measure an exclusive hadronic final state at the LHC. A cross-section measurement is performed in two kinematic regions defined by the proton momenta, the pion rapidities and transverse momenta, and the pion-pion invariant mass. Cross section values of $4.8 \pm 1.0 \text{(stat.)} + {}^{+0.3}_{-0.2} \text{(syst.)}μ$b and $9 \pm 6 \text{(stat.)} + {}^{+2}_{-2}\text{(syst.)}μ$b are obtained in the two regions; they are compared with theoretical models and provide a demonstration of the feasibility of measurements of this type.

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arXiv2024

Observation of $WZγ$ Production in $pp$ Collisions at $\sqrt{s}=13$ TeV with the ATLAS Detector

This Letter reports the observation of $WZγ$ production and a measurement of its cross-section using 140.1 $\pm$ 1.2 fb$^{-1}$ of proton-proton collision data recorded at a center-of-mass energy of 13 TeV by the ATLAS detector at the Large Hadron Collider. The $WZγ$ production cross-section, with both the $W$ and $Z$ bosons decaying leptonically, $pp \rightarrow WZγ\rightarrow {\ell'}^{\pm}ν\ell^{+}\ell^{-}γ$ ($\ell^{(')} = e, μ$), is measured in a fiducial phase-space region defined such that the leptons and the photon have high transverse momentum and the photon is isolated. The cross-section is found to be 2.01 $\pm$ 0.30 (stat.) $\pm$ 0.16 (syst) fb. The corresponding Standard Model predicted cross-section calculated at next-to-leading order in perturbative quantum chromodynamics and at leading order in the electroweak coupling constant is 1.50 $\pm$ 0.06 fb. The observed significance of the $WZγ$ signal is 6.3$σ$, compared with an expected significance of 5.0$σ$.

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arXiv2024

Search for heavy Higgs bosons with flavour-violating couplings in multi-lepton plus $b$-jets final states in $pp$ collisions at 13 TeV with the ATLAS detector

A search for new heavy scalars with flavour-violating decays in final states with multiple leptons and $b$-tagged jets is presented. The results are interpreted in terms of a general two-Higgs-doublet model involving an additional scalar with couplings to the top-quark and the three up-type quarks ($ρ_{tt}$, $ρ_{tc}$, and $ρ_{tu}$). The targeted signals lead to final states with either a same-sign top-quark pair, three top-quarks, or four top-quarks. The search is based on a data sample of proton-proton collisions at $\sqrt{s}=13$ TeV recorded with the ATLAS detector during Run 2 of the Large Hadron Collider, corresponding to an integrated luminosity of 139f b$^{-1}$. Events are categorised depending on the multiplicity of light charged leptons (electrons or muons), total lepton charge, and a deep-neural-network-based categorisation to enhance the purity of each of the signals. Masses of an additional scalar boson $m_{H}$ between $200-630$ GeV with couplings $ρ_{tt}=0.4$, $ρ_{tc}=0.2$, and $ρ_{tu}=0.2$ are excluded at 95% confidence level. Additional interpretations are provided in models of $R$-parity violating supersymmetry, motivated by the recent flavour and $(g-2)_μ$ anomalies.

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arXiv2024

Search for heavy particles in the $b$-tagged dijet mass distribution with additional $b$-tagged jets in proton-proton collisions at $\sqrt{s} = 13$ TeV with the ATLAS experiment

A search optimized for new heavy particles decaying to two $b$-quarks and produced in association with additional $b$-quarks is reported. The sensitivity is improved by $b$-tagging at least one lower-$p_{\rm{T}}$ jet in addition to the two highest-$p_{\rm{T}}$ jets. The data used in this search correspond to an integrated luminosity of 103 $\text{fb}^{-1}$ collected with a dedicated trijet trigger during the 2017 and 2018 $\sqrt{s} = 13$ TeV proton-proton collision runs with the ATLAS detector at the LHC. The search looks for resonant peaks in the $b$-tagged dijet invariant mass spectrum over a smoothly falling background. The background is estimated with an innovative data-driven method based on orthonormal functions. The observed $b$-tagged dijet invariant mass spectrum is compatible with the background-only hypothesis. Upper limits at 95% confidence level on a heavy vector-boson production cross section times branching ratio to a pair of $b$-quarks are derived.

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arXiv2024

Observation of $Wγγ$ triboson production in proton-proton collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector

This letter reports the observation of $W(\ellν)γγ$ production in proton-proton collisions. This measurement uses the full Run 2 sample of events recorded at a center-of-mass energy of $\sqrt{s} = 13$ TeV by the ATLAS detector at the LHC, corresponding to an integrated luminosity of 140 fb$^{-1}$. Events with a leptonically-decaying $W$ boson and at least two photons are considered. The background-only hypothesis is rejected with an observed and expected significance of $5.6$ standard deviations. The inclusive fiducial production cross section of $W(eν)γγ$ and $W(μν)γγ$ events is measured to be $σ_{\mathrm{fid}} = 13.8 \pm 1.1 (\mathrm{stat}) \substack{+2.1 \\ -2.0} (\mathrm{syst}) \pm 0.1 (\mathrm{lumi})$ fb, in agreement with the Standard Model prediction.

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arXiv2024

Search for vector-boson resonances decaying into a top quark and a bottom quark using $pp$ collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector

A search for a new massive charged gauge boson, $W'$, is performed with the ATLAS detector at the LHC. The dataset used in this analysis was collected from proton-proton collisions at a centre-of-mass energy of $\sqrt{s} =13$ TeV, and corresponds to an integrated luminosity of 139 fb$^{-1}$. The reconstructed $tb$ invariant mass is used to search for a $W'$ boson decaying into a top quark and a bottom quark. The result is interpreted in terms of a $W'$ boson with purely right-handed or left-handed chirality in a mass range of 0.5-6 TeV. Different values for the coupling of the $W'$ boson to the top and bottom quarks are considered, taking into account interference with single-top-quark production in the $s$-channel. No significant deviation from the background prediction is observed. The results are expressed as upper limits on the $W' \rightarrow tb$ production cross-section times branching ratio as a function of the $W'$-boson mass and in the plane of the coupling vs the $W'$-boson mass.

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arXiv2024

Measurement of cross-sections for production of a $Z$ boson in association with a flavor-inclusive or doubly $b$-tagged large-radius jet in proton-proton collisions at $\sqrt{s} = 13$ TeV with the ATLAS experiment

We present measurements of cross-sections for production of a leptonically decaying $Z$ boson in association with a large-radius jet in 13 TeV proton-proton collisions at the LHC, using $36~\mathrm{fb}^{-1}$ of data from the ATLAS detector. Integrated and differential cross-sections are measured at particle-level in both a flavor-inclusive and a doubly $b$-tagged fiducial phase-space. The large-radius jet mass and transverse momentum, its kinematic relationship to the $Z$ boson, and the angular separation of $b$-tagged small-radius track-jets within the large-radius jet are measured. This measurement constitutes an important test of perturbative quantum chromodynamics in kinematic and flavor configurations relevant to several Higgs boson and beyond-Standard-Model physics analyses. The results highlight issues with modeling of additional hadronic activity in the flavor-inclusive selection, and a distinction between flavor-number schemes in the $b$-tagged phase-space.

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