AtlFast3: The Next Generation of Fast Simulation in ATLAS

Computing and Software for Big Science · 2022 · 106 citations

• Computer Science
• Computer Science
• Computational science

Abstract The ATLAS experiment at the Large Hadron Collider has a broad physics programme ranging from precision measurements to direct searches for new particles and new interactions, requiring ever larger and ever more accurate datasets of simulated Monte Carlo events. Detector simulation with Geant4 is accurate but requires significant CPU resources. Over the past decade, ATLAS has developed and utilized tools that replace the most CPU-intensive component of the simulation—the calorimeter shower simulation—with faster simulation methods. Here, AtlFast3, the next generation of high-accuracy fast simulation in ATLAS, is introduced. AtlFast3 combines parameterized approaches with machine-learning techniques and is deployed to meet current and future computing challenges, and simulation needs of the ATLAS experiment. With highly accurate performance and significantly improved modelling of substructure within jets, AtlFast3 can simulate large numbers of events for a wide range of physics processes.

Jet energy scale and resolution measured in proton–proton collisions at $$\sqrt{s}=13$$ TeV with the ATLAS detector

The European Physical Journal C · 2021 · 248 citations

• Physics
• Nuclear physics
• Particle physics

Abstract Jet energy scale and resolution measurements with their associated uncertainties are reported for jets using 36–81 fb $$^{-1}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mrow/><mml:mrow><mml:mo>-</mml:mo><mml:mn>1</mml:mn></mml:mrow></mml:msup></mml:math> of proton–proton collision data with a centre-of-mass energy of $$\sqrt{s}=13$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msqrt><mml:mi>s</mml:mi></mml:msqrt><mml:mo>=</mml:mo><mml:mn>13</mml:mn></mml:mrow></mml:math> $${\text {Te}}{\text {V}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mtext>TeV</mml:mtext></mml:math> collected by the ATLAS detector at the LHC. Jets are reconstructed using two different input types: topo-clusters formed from energy deposits in calorimeter cells, as well as an algorithmic combination of charged-particle tracks with those topo-clusters, referred to as the ATLAS particle-flow reconstruction method. The anti- $$k_t$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>k</mml:mi><mml:mi>t</mml:mi></mml:msub></mml:math> jet algorithm with radius parameter $$R=0.4$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>R</mml:mi><mml:mo>=</mml:mo><mml:mn>0.4</mml:mn></mml:mrow></mml:math> is the primary jet definition used for both jet types. This result presents new jet energy scale and resolution measurements in the high pile-up conditions of late LHC Run 2 as well as a full calibration of particle-flow jets in ATLAS. Jets are initially calibrated using a sequence of simulation-based corrections. Next, several in situ techniques are employed to correct for differences between data and simulation and to measure the resolution of jets. The systematic uncertainties in the jet energy scale for central jets ( $$|\eta |&lt;1.2$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mo>|</mml:mo><mml:mi>η</mml:mi><mml:mo>|</mml:mo><mml:mo>&lt;</mml:mo><mml:mn>1.2</mml:mn></mml:mrow></mml:math> ) vary from 1% for a wide range of high- $$p_{{\text {T}}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>p</mml:mi><mml:mtext>T</mml:mtext></mml:msub></mml:math> jets ( $$250&lt;p_{{\text {T}}} &lt;2000~{\text {Ge}}{\text {V}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mn>250</mml:mn><mml:mo>&lt;</mml:mo><mml:msub><mml:mi>p</mml:mi><mml:mtext>T</mml:mtext></mml:msub><mml:mo>&lt;</mml:mo><mml:mn>2000</mml:mn><mml:mspace/><mml:mtext>GeV</mml:mtext></mml:mrow></mml:math> ), to 5% at very low $$p_{{\text {T}}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>p</mml:mi><mml:mtext>T</mml:mtext></mml:msub></mml:math> ( $$20~{\text {Ge}}{\text {V}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mn>20</mml:mn><mml:mspace/><mml:mtext>GeV</mml:mtext></mml:mrow></mml:math> ) and 3.5% at very high $$p_{{\text {T}}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>p</mml:mi><mml:mtext>T</mml:mtext></mml:msub></mml:math> ( $$&gt;2.5~{\text {Te}}{\text {V}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mo>&gt;</mml:mo><mml:mn>2.5</mml:mn><mml:mspace/><mml:mtext>TeV</mml:mtext></mml:mrow></mml:math> ). The relative jet energy resolution is measured and ranges from ( $$24 \pm 1.5$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mn>24</mml:mn><mml:mo>±</mml:mo><mml:mn>1.5</mml:mn></mml:mrow></mml:math> )% at 20 $${\text {Ge}}{\text {V}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mtext>GeV</mml:mtext></mml:math> to ( $$6 \pm 0.5$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mn>6</mml:mn><mml:mo>±</mml:mo><mml:mn>0.5</mml:mn></mml:mrow></mml:math> )% at 300 $${\text {Ge}}{\text {V}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mtext>GeV</mml:mtext></mml:math> .

Search for new phenomena in three- or four-lepton events in $pp$ collisions at $\sqrt s$ =13 TeV with the ATLAS detector

2021 · 1 citations

• Physics
• Particle physics
• Nuclear physics

A search with minimal model dependence for physics beyond the Standard Model in events featuring three or four charged leptons (3ℓ and 4ℓ, ℓ=e,μ) is presented. The analysis aims to be sensitive to a wide range of potential new-physics theories simultaneously. This analysis uses data from pp collisions delivered by the Large Hadron Collider at a centre-of-mass energy of s=13 TeV and recorded with the ATLAS detector, corresponding to the full Run 2 dataset of 139 fb−1. The 3ℓ and 4ℓ phase space is divided into 22 event categories according to the number of leptons in the event, the missing transverse momentum, the invariant mass of the leptons, and the presence of leptons originating from a Z-boson candidate. These event categories are analysed independently for the presence of deviations from the Standard Model. No statistically significant deviations from the Standard Model predictions are observed. Upper limits for all signal regions are reported in terms of the visible cross-section.

Measurements of top-quark pair spin correlations in the $$e\mu $$ channel at $$\sqrt{s} = 13$$ TeV using pp collisions in the ATLAS detector

The European Physical Journal C · 2020 · 60 citations

• Physics
• Particle physics
• Nuclear physics

Abstract A measurement of observables sensitive to spin correlations in $$t\bar{t}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>t</mml:mi><mml:mover><mml:mrow><mml:mi>t</mml:mi></mml:mrow><mml:mrow><mml:mo>¯</mml:mo></mml:mrow></mml:mover></mml:mrow></mml:math> production is presented, using 36.1 $$\hbox {fb}^{-1}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mtext>fb</mml:mtext><mml:mrow><mml:mo>-</mml:mo><mml:mn>1</mml:mn></mml:mrow></mml:msup></mml:math> of pp collision data at $$\sqrt{s} = 13$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msqrt><mml:mi>s</mml:mi></mml:msqrt><mml:mo>=</mml:mo><mml:mn>13</mml:mn></mml:mrow></mml:math> TeV recorded with the ATLAS detector at the Large Hadron Collider. Differential cross-sections are measured in events with exactly one electron and one muon with opposite-sign electric charge as a function of the azimuthal opening angle and the absolute difference in pseudorapidity between the electron and muon candidates in the laboratory frame. The azimuthal opening angle is also measured as a function of the invariant mass of the $$t\bar{t}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>t</mml:mi><mml:mover><mml:mrow><mml:mi>t</mml:mi></mml:mrow><mml:mrow><mml:mo>¯</mml:mo></mml:mrow></mml:mover></mml:mrow></mml:math> system. The measured differential cross-sections are compared to predictions by several NLO Monte Carlo generators and fixed-order calculations. The observed degree of spin correlation is somewhat higher than predicted by the generators used. The data are consistent with the prediction of one of the fixed-order calculations at NLO, but agree less well with higher-order predictions. Using these leptonic observables, a search is performed for pair production of supersymmetric top squarks decaying into Standard Model top quarks and light neutralinos. Top squark masses between 170 and 230 GeV are largely excluded at the 95% confidence level for kinematically allowed values of the neutralino mass.

ATLAS data quality operations and performance for 2015–2018 data-taking

Journal of Instrumentation · 2020 · 256 citations

• Computer Science
• Computer Science
• Computer Security

The ATLAS detector at the Large Hadron Collider reads out particle collision data from over 100 million electronic channels at a rate of approximately 100 kHz, with a recording rate for physics events of approximately 1 kHz. Before being certified for physics analysis at computer centres worldwide, the data must be scrutinised to ensure they are clean from any hardware or software related issues that may compromise their integrity. Prompt identification of these issues permits fast action to investigate, correct and potentially prevent future such problems that could render the data unusable. This is achieved through the monitoring of detector-level quantities and reconstructed collision event characteristics at key stages of the data processing chain. This paper presents the monitoring and assessment procedures in place at ATLAS during 2015-2018 data-taking. Through the continuous improvement of operational procedures, ATLAS achieved a high data quality efficiency, with 95.6% of the recorded proton-proton collision data collected at s = 13 TeV certified for physics analysis.

Searches for electroweak production of supersymmetric particles with compressed mass spectra in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msqrt><mml:mrow><mml:mi>s</mml:mi></mml:mrow></mml:msqrt><mml:mo>=</mml:mo><mml:mn>13</mml:mn><mml:mtext> </mml:mtext><mml:mtext> </mml:mtext><mml:mi>TeV</mml:mi></mml:mrow></mml:math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>p</mml:mi><mml:mi>p</mml:mi></mml:math> collisions with the ATLAS detector

Physical review. D/Physical review. D. · 2020 · 207 citations

• Physics
• Particle physics
• Nuclear physics

This paper presents results of searches for the electroweak production of supersymmetric particles in models with compressed mass spectra. The searches use 139 fb -1 of ffiffi ffi s p 13 TeV proton-proton collision data collected by the ATLAS experiment at the Large Hadron Collider. Events with missing transverse momentum and two same-flavor, oppositely charged, low-transverse-momentum leptons are selected, and are further categorized by the presence of hadronic activity from initial-state radiation or a topology compatible with vector-boson fusion processes. The data are found to be consistent with predictions from the Standard Model. The results are interpreted using simplified models of R-parity-conserving supersymmetry in which the lightest supersymmetric partner is a neutralino with a mass similar to the lightest chargino, the second-to-lightest neutralino, or the slepton. Lower limits on the masses of charginos in different simplified models range from 193 to 240 GeV for moderate mass splittings, and extend down to mass splittings of 1.5 to 2.4 GeV at the LEP chargino bounds (92.4 GeV). Similar lower limits on degenerate light-flavor sleptons extend up to masses of 251 GeV and down to mass splittings of 550 MeV. Constraints on vector-boson fusion production of electroweak SUSY states are also presented.

A search for the dimuon decay of the Standard Model Higgs boson with the ATLAS detector

Physics Letters B · 2020 · 147 citations

• Physics
• Particle physics
• Nuclear physics

A search

Charged-lepton-flavour violation at the LHC: a search for $Z\to e\tau/\mu\tau$ decays with the ATLAS detector

arXiv (Cornell University) · 2020 · 2 citations

• Particle physics
• Physics
• Nuclear physics

In the Standard Model of particle physics, leptons are key building blocks of matter and come in three families (flavours). Leptons of different flavours have the same properties, except for their mass. In addition, the number of leptons in each family is conserved in interactions. Such conservation is known as lepton flavour conservation, and no fundamental principles impose it. Since the formulation of the Standard Model, the observation of flavour oscillations among neutrinos (the neutral leptons) has demonstrated that neutrinos have mass and in neutrino weak interactions the lepton flavour is not conserved. To date, there is no experimental evidence that lepton flavour violation occurs in interactions between charged leptons, and an observation of such a phenomenon would be an exciting sign of new particles or new type of interactions beyond the Standard Model. The ATLAS experiment at the Large Hadron Collider at CERN sets a new constraint on lepton-flavour-violating effects in weak interactions, searching for $Z$-boson decays into a $\tau$-lepton and another lepton of different flavour ($e$ or $\mu$) with opposite electric charge. The branching fractions for these decays are now measured by the ATLAS experiment to be less than $8.1\times10^{-6}$ ($e\tau$) and $9.5\times10^{-6}$ ($\mu\tau$) at 95% confidence level, using 139 fb$^{-1}$ of proton-proton collision data at centre-of-mass energy $\sqrt{s}=13$ TeV and 20.3 fb$^{-1}$ at $\sqrt{s}=8$ TeV. These results supersede the best limits set by the LEP experiments more than two decades ago.

Search for direct production of electroweakinos in final states with one lepton, missing transverse momentum and a Higgs boson decaying into two b-jets in $$pp$$ collisions at $$\sqrt{s}=13$$ TeV with the ATLAS detector

The European Physical Journal C · 2020 · 85 citations

• Computer Science
• Algorithm
• Database

Abstract The results of a search for electroweakino pair production $$pp \rightarrow \tilde{\chi }^\pm _1 \tilde{\chi }^0_2$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>p</mml:mi><mml:mi>p</mml:mi><mml:mo>→</mml:mo><mml:msubsup><mml:mover><mml:mi>χ</mml:mi><mml:mo>~</mml:mo></mml:mover><mml:mn>1</mml:mn><mml:mo>±</mml:mo></mml:msubsup><mml:msubsup><mml:mover><mml:mi>χ</mml:mi><mml:mo>~</mml:mo></mml:mover><mml:mn>2</mml:mn><mml:mn>0</mml:mn></mml:msubsup></mml:mrow></mml:math> in which the chargino ( $$\tilde{\chi }^\pm _1$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mover><mml:mi>χ</mml:mi><mml:mo>~</mml:mo></mml:mover><mml:mn>1</mml:mn><mml:mo>±</mml:mo></mml:msubsup></mml:math> ) decays into a W boson and the lightest neutralino ( $$\tilde{\chi }^0_1$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mover><mml:mi>χ</mml:mi><mml:mo>~</mml:mo></mml:mover><mml:mn>1</mml:mn><mml:mn>0</mml:mn></mml:msubsup></mml:math> ), while the heavier neutralino ( $$\tilde{\chi }^0_2$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mover><mml:mi>χ</mml:mi><mml:mo>~</mml:mo></mml:mover><mml:mn>2</mml:mn><mml:mn>0</mml:mn></mml:msubsup></mml:math> ) decays into the Standard Model 125 GeV Higgs boson and a second $$\tilde{\chi }^0_1$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mover><mml:mi>χ</mml:mi><mml:mo>~</mml:mo></mml:mover><mml:mn>1</mml:mn><mml:mn>0</mml:mn></mml:msubsup></mml:math> are presented. The signal selection requires a pair of b -tagged jets consistent with those from a Higgs boson decay, and either an electron or a muon from the W boson decay, together with missing transverse momentum from the corresponding neutrino and the stable neutralinos. The analysis is based on data corresponding to 139 $$\mathrm {fb}^{-1}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mrow><mml:mi>fb</mml:mi></mml:mrow><mml:mrow><mml:mo>-</mml:mo><mml:mn>1</mml:mn></mml:mrow></mml:msup></mml:math> of $$\sqrt{s}=13$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msqrt><mml:mi>s</mml:mi></mml:msqrt><mml:mo>=</mml:mo><mml:mn>13</mml:mn></mml:mrow></mml:math> TeV pp collisions provided by the Large Hadron Collider and recorded by the ATLAS detector. No statistically significant evidence of an excess of events above the Standard Model expectation is found. Limits are set on the direct production of the electroweakinos in simplified models, assuming pure wino cross-sections. Masses of $$\tilde{\chi }^{\pm }_{1}/\tilde{\chi }^{0}_{2}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msubsup><mml:mover><mml:mi>χ</mml:mi><mml:mo>~</mml:mo></mml:mover><mml:mn>1</mml:mn><mml:mo>±</mml:mo></mml:msubsup><mml:mo>/</mml:mo><mml:msubsup><mml:mover><mml:mi>χ</mml:mi><mml:mo>~</mml:mo></mml:mover><mml:mn>2</mml:mn><mml:mn>0</mml:mn></mml:msubsup></mml:mrow></mml:math> up to 740 GeV are excluded at 95% confidence level for a massless $$\tilde{\chi }^{0}_{1}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mover><mml:mi>χ</mml:mi><mml:mo>~</mml:mo></mml:mover><mml:mn>1</mml:mn><mml:mn>0</mml:mn></mml:msubsup></mml:math> .