A search for cascade decays of charged sleptons and sneutrinos using final states characterized by three leptons (electrons or muons) and missing transverse momentum is presented. The analysis is based on a dataset with <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mn>140</mn><mtext> </mtext><mtext> </mtext><msup><mi>fb</mi><mrow><mo>−</mo><mn>1</mn></mrow></msup></math> of proton-proton (pp) collisions at a center-of-mass energy of <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msqrt><mi>s</mi></msqrt><mo>=</mo><mn>13</mn><mtext> </mtext><mtext> </mtext><mi>TeV</mi></math> recorded by the ATLAS detector at the Large Hadron Collider. This paper focuses on a supersymmetric scenario that is motivated by the muon anomalous magnetic moment observation, dark-mattter relic density abundance, and electroweak naturalness. A mass spectrum involving light Higgsinos and heavier sleptons with a bino at intermediate mass is targeted. No significant deviation from the Standard Model expectation is observed. This search enables us to place stringent constraints on this model, excluding at the 95% confidence level charged slepton and sneutrino masses up to 450 GeV when assuming a lightest neutralino mass of 100 GeV and mass-degenerate selectrons, smuons and sneutrinos.

France

G. Aad

Norway

E. Aakvaag

United States

B. Abbott

United Arab Emirates

S. Abdelhameed

Germany

K. Abeling

N.J. Abicht

S.H. Abidi

M. Aboelela

Morocco

A. Aboulhorma

Israel

H. Abramowicz

Y. Abulaiti

Italy

B.S. Acharya

A. Ackermann

C. Adam Bourdarios

Poland

L. Adamczyk

S.V. Addepalli

United Kingdom

M.J. Addison

J. Adelman

Türkiye

A. Adiguzel

T. Adye

A.A. Affolder

Y. Afik

Spain

M.N. Agaras

A. Aggarwal

Romania

C. Agheorghiesei

F. Ahmadov

S. Ahuja

China

X. Ai

G. Aielli

A. Aikot

M. Ait Tamlihat

B. Aitbenchikh

M. Akbiyik

Sweden

T.P.A. Åkesson

A.V. Akimov

Japan

D. Akiyama

N.N. Akolkar

S. Aktas

G.L. Alberghi

Canada

J. Albert

P. Albicocco

G.L. Albouy

S. Alderweireldt

Z.L. Alegria

CERN

M. Aleksa

I.N. Aleksandrov

C. Alexa

Greece

T. Alexopoulos

F. Alfonsi

Switzerland

M. Algren

M. Alhroob

Czechia

B. Ali

H.M.J. Ali

S. Ali

S.W. Alibocus

South Africa

M. Aliev

G. Alimonti

W. Alkakhi

C. Allaire

B.M.M. Allbrooke

J.S. Allen

J.F. Allen

P.P. Allport

A. Aloisio

Argentina

F. Alonso

C. Alpigiani

Z.M.K. Alsolami

A. Alvarez Fernandez

M. Alves Cardoso

M.G. Alviggi

M. Aly

Brazil

Y. Amaral Coutinho

A. Ambler

C. Amelung

M. Amerl

C.G. Ames

T. Amezza

D. Amidei

B. Amini

K. Amirie

A. Amirkhanov

Portugal

S.P. Amor Dos Santos

K.R. Amos

D. Amperiadou

S. An

C. Anastopoulos

T. Andeen

J.K. Anders

A.C. Anderson

A. Andreazza

S. Angelidakis

A. Angerami

A.V. Anisenkov

A. Annovi

C. Antel

E. Antipov

M. Antonelli

F. Anulli

M. Aoki

T. Aoki

Search for cascade decays of charged sleptons and sneutrinos in final states with three leptons and missing transverse momentum in <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>p</mi><mi>p</mi></math> collisions at <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msqrt><mi>s</mi></msqrt><mo>=</mo><mn>13</mn><mtext> </mtext><mtext> </mtext><mi>TeV</mi></math> with the ATLAS detector

Measurements of jet substructure are key to probing the energy frontier at colliders, and many of them use track-based observables which take advantage of the angular precision of tracking detectors. Theoretical calculations of track-based observables require ‘track functions’, which characterize the transverse momentum fraction <math altimg='si2.svg'><msub><mrow><mi>r</mi></mrow><mrow><mi>q</mi></mrow></msub></math> carried by charged hadrons from a fragmenting quark or gluon. This letter presents a direct measurement of <math altimg='si2.svg'><msub><mrow><mi>r</mi></mrow><mrow><mi>q</mi></mrow></msub></math> distributions in dijet events from the 140 fb &#8722;1 of proton&#8211;proton collisions at <math altimg='si1.svg'><msqrt><mrow><mi>s</mi></mrow></msqrt><mo linebreak='goodbreak' linebreakstyle='after'>=</mo><mn>13</mn></math> TeV recorded with the ATLAS detector. The data are corrected for detector effects using machine-learning methods. The scale evolution of the moments of the <math altimg='si2.svg'><msub><mrow><mi>r</mi></mrow><mrow><mi>q</mi></mrow></msub></math> distribution is sensitive to non-linear renormalization group evolution equations of QCD, and is compared with analytic predictions. When incorporated into future theoretical calculations, these results will enable a precision program of theory-data comparison for track-based jet substructure observables.

H. Abreu

J. Agarwala

W.S. Ahmed

K. Al Khoury

Chile

C.A. Allendes Flores

M. Alvarez Estevez

V. Ananiev

S.Y. Andrean

Measurement of jet track functions in <italic>pp</italic> collisions at <math altimg='si1.svg'><msqrt><mrow><mi>s</mi></mrow></msqrt><mo linebreak='goodbreak' linebreakstyle='after'>=</mo><mn>13</mn></math> TeV with the ATLAS detector

This paper presents a new $$\tau $$ -lepton reconstruction and identification procedure at the ATLAS detector at the Large Hadron Collider, which leads to significantly improved performance in the case of physics processes where a highly boosted pair of $$\tau $$ -leptons is produced and one $$\tau $$ -lepton decays into a muon and two neutrinos ( $$\tau _\mathrm {\mu }$$ ), and the other decays into hadrons and one neutrino ( $$\tau _\textrm{had}$$ ). By removing the muon information from the signals used for reconstruction and identification of the $$\tau _\textrm{had}$$ candidate in the boosted pair, the efficiency is raised to the level expected for an isolated $$\tau _\textrm{had}$$ . The new procedure is validated by selecting a sample of highly boosted $$Z\rightarrow \tau _\mathrm {\mu }\tau _\textrm{had}$$ candidates from the data sample of 140 $$\textrm{fb}^{-1}$$ of proton–proton collisions at 13 TeV recorded with the ATLAS detector. Good agreement is found between data and simulation predictions in both the $$Z\rightarrow \tau _\mathrm {\mu }\tau _\textrm{had}$$ signal region and in a background validation region. The results presented in this paper demonstrate the effectiveness of the $$\tau _\textrm{had}$$ reconstruction with muon removal in enhancing the signal sensitivity of the boosted $$\tau _\mathrm {\mu }\tau _\textrm{had}$$ channel at the ATLAS detector.

C. Bourdarios

S. Addepalli

M. Addison

A. Affolder

M. Agaras

Azerbaijan

M. Tamlihat

T. Åkesson

A. Akimov

N. Akolkar

G. Alberghi

G. Albouy

Z. Alegria

I. Aleksandrov

Saudi Arabia

H. Ali

S. Alibocus

B. Allbrooke

J. Allen

C. Flores

P. Allport

Z. Alsolami

A. Fernandez

M. Cardoso

M. Alviggi

Y. Coutinho

C. Ames

S. Santos

K. Amos

J. Anders

A. Anderson

A. Anisenkov

Improved reconstruction of highly boosted $$\tau $$ -lepton pairs in the $$\tau \tau \rightarrow (\mu \nu _{\mu }\nu _{\tau })(\text {hadrons}+\nu _{\tau })$$ decay channels with the ATLAS detector

A search for decays of the Higgs boson into a <italic>Z</italic> boson and a light resonance, with a mass of 0.5&#8211;3.5 GeV, is performed using the full 140 fb &#8722;1 dataset of 13 TeV proton&#8211;proton collisions recorded by the ATLAS detector during LHC Run 2. Leptonic decays of the <italic>Z</italic> boson and hadronic decays of the light resonance are considered. The resonance can be interpreted as a <math altimg='si2.svg'><mi>J</mi><mo stretchy='false'>/</mo><mi>&#968;</mi></math> or <math altimg='si3.svg'><msub><mrow><mi>&#951;</mi></mrow><mrow><mi>c</mi></mrow></msub></math> meson, an axion-like particle, or a light pseudoscalar predicted in two-Higgs-doublet models. Due to its low mass, this resonance is produced with a high Lorentz boost in the laboratory frame and therefore reconstructed as a single small-radius jet of hadrons. A neural network is used to correct the Monte Carlo simulation of the total expected background using data from sideband regions. Two additional neural networks are used to distinguish signal from background, enhancing the purity of the signal region. A binned profile-likelihood fit is performed on the final-state invariant mass distribution. No significant excess of events relative to the expected background is observed, and upper limits at 95% confidence level are set on the Higgs boson's branching fraction to a <italic>Z</italic> boson and a light resonance. The exclusion limit is &#8764;10% for the lower masses, and increases for higher masses. Upper limits on the effective coupling <math altimg='si4.svg'><msubsup><mrow><mi>C</mi></mrow><mrow><mi>Z</mi><mi>H</mi></mrow><mrow><mtext>eff</mtext></mrow></msubsup><mo stretchy='false'>/</mo><mi mathvariant='normal'>&#923;</mi></math> of an axion-like particle to a Higgs boson and <italic>Z</italic> boson are also set at 95% confidence level, and range from 0.9 to 2 TeV &#8722;1.

Search for Higgs boson decays into a <italic>Z</italic> boson and a light hadronically decaying resonance in <italic>pp</italic> collisions at <math altimg='si1.svg'><msqrt><mrow><mi>s</mi></mrow></msqrt></math>=13 TeV with the ATLAS detector

The mass of the top quark is measured using top-quark-top-antiquark pair events with high transverse momentum top quarks. The dataset, collected with the ATLAS detector in proton–proton collisions at <math altimg='si2.svg'><msqrt><mrow><mi>s</mi></mrow></msqrt><mo linebreak='goodbreak' linebreakstyle='after'>=</mo><mn>13</mn></math> TeV delivered by the Large Hadron Collider, corresponds to an integrated luminosity of 140 fb &#8722;1. The analysis targets events in the lepton-plus-jets decay channel, with an electron or muon from a semi-leptonically decaying top quark and a hadronically decaying top quark that is sufficiently energetic to be reconstructed as a single large-radius jet. The mean of the invariant mass of the reconstructed large-radius jet provides the sensitivity to the top quark mass and is simultaneously fitted with two additional observables to reduce the impact of the systematic uncertainties. The top quark mass is measured to be <math altimg='si3.svg'><msub><mrow><mi>m</mi></mrow><mrow><mi>t</mi></mrow></msub><mo linebreak='goodbreak' linebreakstyle='after'>=</mo><mn>172.95</mn><mo>&#177;</mo><mn>0.53</mn></math> GeV, which is the most precise ATLAS measurement from a single channel.

Measurement of the top quark mass with the ATLAS detector using <math altimg='si1.svg'><mi>t</mi><mover accent='true'><mrow><mi>t</mi></mrow><mrow><mo stretchy='false'>&#175;</mo></mrow></mover></math> events with a high transverse momentum top quark

A search for the production of three massive vector bosons, <math altimg='si2.svg'><mi>V</mi><mi>V</mi><mi>Z</mi><mo stretchy='false'>(</mo><mi>V</mi><mo linebreak='badbreak' linebreakstyle='after'>=</mo><mi>W</mi><mo>,</mo><mi>Z</mi><mo stretchy='false'>)</mo></math>, in proton&#8211;proton collisions at <math altimg='si1.svg'><msqrt><mrow><mi>s</mi></mrow></msqrt><mo linebreak='goodbreak' linebreakstyle='after'>=</mo><mn>13</mn></math> TeV is performed using data with an integrated luminosity of 140 fb &#8722;1 recorded by the ATLAS detector at the Large Hadron Collider. Events produced in the leptonic final states <math altimg='si3.svg'><mi>W</mi><mi>W</mi><mi>Z</mi><mo stretchy='false'>&#8594;</mo><mi>&#8467;</mi><mi>&#957;</mi><mi>&#8467;</mi><mi>&#957;</mi><mi>&#8467;</mi><mi>&#8467;</mi></math> ( <math altimg='si15.svg'><mi>&#8467;</mi><mo linebreak='goodbreak' linebreakstyle='after'>=</mo><mi>e</mi><mo>,</mo><mi>&#956;</mi></math>), <math altimg='si5.svg'><mi>W</mi><mi>Z</mi><mi>Z</mi><mo stretchy='false'>&#8594;</mo><mi>&#8467;</mi><mi>&#957;</mi><mi>&#8467;</mi><mi>&#8467;</mi><mi>&#8467;</mi><mi>&#8467;</mi></math>, <math altimg='si6.svg'><mi>Z</mi><mi>Z</mi><mi>Z</mi><mo stretchy='false'>&#8594;</mo><mi>&#8467;</mi><mi>&#8467;</mi><mi>&#8467;</mi><mi>&#8467;</mi><mi>&#8467;</mi><mi>&#8467;</mi></math>, and the semileptonic final states <math altimg='si7.svg'><mi>W</mi><mi>W</mi><mi>Z</mi><mo stretchy='false'>&#8594;</mo><mi>q</mi><mi>q</mi><mi>&#8467;</mi><mi>&#957;</mi><mi>&#8467;</mi><mi>&#8467;</mi></math> and <math altimg='si8.svg'><mi>W</mi><mi>Z</mi><mi>Z</mi><mo stretchy='false'>&#8594;</mo><mi>&#8467;</mi><mi>&#957;</mi><mi>q</mi><mi>q</mi><mi>&#8467;</mi><mi>&#8467;</mi></math>, are analysed. The measured cross section for the <math altimg='si149.svg'><mi>p</mi><mi>p</mi><mo stretchy='false'>&#8594;</mo><mi>V</mi><mi>V</mi><mi>Z</mi></math> process is <math altimg='si10.svg'><msubsup><mrow><mn>660</mn></mrow><mrow><mo linebreak='badbreak' linebreakstyle='after'>&#8722;</mo><mn>90</mn></mrow><mrow><mo linebreak='badbreak' linebreakstyle='after'>+</mo><mn>93</mn></mrow></msubsup><msubsup><mrow><mo stretchy='false'>(</mo><mtext>stat.</mtext><mo stretchy='false'>)</mo></mrow><mrow><mo linebreak='badbreak' linebreakstyle='after'>&#8722;</mo><mn>81</mn></mrow><mrow><mo linebreak='badbreak' linebreakstyle='after'>+</mo><mn>88</mn></mrow></msubsup><mo stretchy='false'>(</mo><mtext>syst.</mtext><mo stretchy='false'>)</mo></math> fb, and the observed (expected) significance is 6.4 (4.7) standard deviations, representing the observation of <italic>VVZ</italic> production. In addition, the measured cross section for the <math altimg='si11.svg'><mi>p</mi><mi>p</mi><mo stretchy='false'>&#8594;</mo><mi>W</mi><mi>W</mi><mi>Z</mi></math> process is <math altimg='si12.svg'><mn>442</mn><mo>&#177;</mo><mn>94</mn><msubsup><mrow><mo stretchy='false'>(</mo><mtext>stat.</mtext><mo stretchy='false'>)</mo></mrow><mrow><mo linebreak='badbreak' linebreakstyle='after'>&#8722;</mo><mn>52</mn></mrow><mrow><mo linebreak='badbreak' linebreakstyle='after'>+</mo><mn>60</mn></mrow></msubsup><mo stretchy='false'>(</mo><mtext>syst.</mtext><mo stretchy='false'>)</mo></math> fb, and the observed (expected) significance is 4.4 (3.6) standard deviations, representing evidence of <italic>WWZ</italic> production. The measured cross sections are consistent with the Standard Model predictions. Constraints on physics beyond the Standard Model are also derived in the effective field theory framework by setting limits on Wilson coefficients for dimension-8 operators describing anomalous quartic gauge boson couplings.

Observation of <italic>VVZ</italic> production at <math altimg='si1.svg'><msqrt><mrow><mi>s</mi></mrow></msqrt><mo linebreak='goodbreak' linebreakstyle='after'>=</mo><mn>13</mn></math> TeV with the ATLAS detector

A search is conducted for a new scalar boson <italic>S</italic>, with a mass distinct from that of the Higgs boson, decaying promptly into four leptons ( <math altimg='si1.svg'><mi>&#8467;</mi><mo linebreak='goodbreak' linebreakstyle='after'>=</mo><mi>e</mi></math>, <italic>&#956;</italic>) via an intermediate state containing two on-shell, promptly decaying new spin-1 bosons <math altimg='si2.svg'><msub><mrow><mi>Z</mi></mrow><mrow><mtext>d</mtext></mrow></msub></math>: <math altimg='si20.svg'><mi>S</mi><mo stretchy='false'>&#8594;</mo><msub><mrow><mi>Z</mi></mrow><mrow><mtext>d</mtext></mrow></msub><msub><mrow><mi>Z</mi></mrow><mrow><mtext>d</mtext></mrow></msub><mo stretchy='false'>&#8594;</mo><mn>4</mn><mi>&#8467;</mi></math>, where the <math altimg='si2.svg'><msub><mrow><mi>Z</mi></mrow><mrow><mtext>d</mtext></mrow></msub></math> boson has a mass between 15 and 300 GeV, and the <italic>S</italic> boson has a mass between either 30 and 115 GeV or 130 and 800 GeV. The search uses proton&#8211;proton collision data collected with the ATLAS detector at the Large Hadron Collider with an integrated luminosity of 139 fb &#8722;1 at a centre-of-mass energy of <math altimg='si101.svg'><msqrt><mrow><mi>s</mi></mrow></msqrt><mo linebreak='goodbreak' linebreakstyle='after'>=</mo><mn>13</mn></math> TeV. No significant excess above the Standard Model background expectation is observed. Upper limits at 95% confidence level are set on the production cross-section times branching ratio, <math altimg='si5.svg'><mi>&#963;</mi><mo stretchy='false'>(</mo><mi>g</mi><mi>g</mi><mo stretchy='false'>&#8594;</mo><mi>S</mi><mo stretchy='false'>)</mo><mo>&#215;</mo><mi mathvariant='script'>B</mi><mo stretchy='false'>(</mo><mi>S</mi><mo stretchy='false'>&#8594;</mo><msub><mrow><mi>Z</mi></mrow><mrow><mtext>d</mtext></mrow></msub><msub><mrow><mi>Z</mi></mrow><mrow><mtext>d</mtext></mrow></msub><mo stretchy='false'>&#8594;</mo><mn>4</mn><mi>&#8467;</mi><mo stretchy='false'>)</mo></math>, as a function of the mass of both particles, <math altimg='si6.svg'><msub><mrow><mi>m</mi></mrow><mrow><mi>S</mi></mrow></msub></math> and <math altimg='si7.svg'><msub><mrow><mi>m</mi></mrow><mrow><mi>Z</mi><mtext>d</mtext></mrow></msub></math>.

A.C. Abusleme Hoffman

L. Adamek

A. Ahmad

I. Aizenberg

Netherlands

A. Alfonsi

Taiwan

M.T. Anthony

D.J.A. Antrim

Search for a new scalar decaying into new spin-1 bosons in four-lepton final states with the ATLAS detector

Charged Higgs bosons produced either in top-quark decays or in association with a top quark, subsequently decaying via <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>H</mi><mo>±</mo></msup><mo stretchy="false">→</mo><msup><mi>τ</mi><mo>±</mo></msup><msub><mi>ν</mi><mi>τ</mi></msub></math>, are searched for in <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mn>140</mn><mtext> </mtext><mtext> </mtext><msup><mi>fb</mi><mrow><mo>−</mo><mn>1</mn></mrow></msup></math> of proton-proton collision data at <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msqrt><mi>s</mi></msqrt><mo>=</mo><mn>13</mn><mtext> </mtext><mtext> </mtext><mi>TeV</mi></math> recorded with the ATLAS detector. Depending on whether the top quark is produced together with the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>H</mi><mo>±</mo></msup></math> decays hadronically or semileptonically, the search targets <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>τ</mi><mo>+</mo><mtext>jets</mtext></math> or <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>τ</mi><mo>+</mo><mi>lepton</mi></math> final states, in both cases with a <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>τ</mi></math>-lepton decaying into a neutrino and hadrons. No significant excess over the Standard Model background expectation is observed. For the mass range of <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mn>80</mn><mo>≤</mo><msub><mi>m</mi><msup><mi>H</mi><mo>±</mo></msup></msub><mo>≤</mo><mn>3000</mn><mtext> </mtext><mtext> </mtext><mi>GeV</mi></math>, upper limits at 95% confidence level are set on the production cross section of the charged Higgs boson times the branching fraction <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi mathvariant="script">B</mi><mo stretchy="false">(</mo><msup><mi>H</mi><mo>±</mo></msup><mo stretchy="false">→</mo><msup><mi>τ</mi><mo>±</mo></msup><msub><mi>ν</mi><mi>τ</mi></msub><mo stretchy="false">)</mo></math> in the range 4.5 pb–0.4 fb. In the mass range 80–160 GeV, assuming the Standard Model cross section for <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>t</mi><mover accent="true"><mi>t</mi><mo stretchy="false">¯</mo></mover></math> production, this corresponds to upper limits between 0.27% and 0.02% on <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi mathvariant="script">B</mi><mo stretchy="false">(</mo><mi>t</mi><mo stretchy="false">→</mo><mi>b</mi><msup><mi>H</mi><mo>±</mo></msup><mo stretchy="false">)</mo><mo>×</mo><mi mathvariant="script">B</mi><mo stretchy="false">(</mo><msup><mi>H</mi><mo>±</mo></msup><mo stretchy="false">→</mo><msup><mi>τ</mi><mo>±</mo></msup><msub><mi>ν</mi><mi>τ</mi></msub><mo stretchy="false">)</mo></math>.

Search for charged Higgs bosons produced in top-quark decays or in association with top quarks and decaying via <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>H</mi><mo>±</mo></msup><mo stretchy="false">→</mo><msup><mi>τ</mi><mo>±</mo></msup><msub><mi>ν</mi><mi>τ</mi></msub></math> in 13 TeV <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>p</mi><mi>p</mi></math> collisions with the ATLAS detector

Top-quark pair production is observed in lead–lead (<math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mi>Pb</mi><mo>+</mo><mi>Pb</mi></mrow></math>) collisions at <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><msqrt><mrow><msub><mrow><mi>s</mi></mrow><mrow><mi>NN</mi></mrow></msub></mrow></msqrt><mo>=</mo><mn>5.02</mn><mtext> </mtext><mtext> </mtext><mi>TeV</mi></mrow></math> at the Large Hadron Collider with the ATLAS detector. The data sample was recorded in 2015 and 2018, amounting to an integrated luminosity of <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mn>1.9</mn><mtext> </mtext><mtext> </mtext><msup><mi>nb</mi><mrow><mo>−</mo><mn>1</mn></mrow></msup></math>. Events with exactly one electron and one muon and at least two jets are selected. Top-quark pair production is measured with an observed (expected) significance of 5.0 (4.1) standard deviations. The measured top-quark pair production cross section is <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><msub><mrow><mi>σ</mi></mrow><mrow><mi>t</mi><mover accent="true"><mrow><mi>t</mi></mrow><mrow><mo stretchy="false">¯</mo></mrow></mover></mrow></msub><mo>=</mo><mn>3.6</mn><msubsup><mrow><mtext> </mtext></mrow><mrow><mo>−</mo><mn>0.9</mn></mrow><mrow><mo>+</mo><mn>1.0</mn></mrow></msubsup><mo stretchy="false">(</mo><mi>stat</mi><mo stretchy="false">)</mo><msubsup><mrow><mtext> </mtext></mrow><mrow><mo>−</mo><mn>0.5</mn></mrow><mrow><mo>+</mo><mn>0.8</mn></mrow></msubsup><mo stretchy="false">(</mo><mi>syst</mi><mo stretchy="false">)</mo><mtext> </mtext><mtext> </mtext><mrow><mi mathvariant="normal">μ</mi><mi mathvariant="normal">b</mi></mrow></mrow></math>, with a total relative uncertainty of 31%, and is consistent with theoretical predictions using a range of different nuclear parton distribution functions. The observation of this process consolidates the evidence of the existence of all quark flavors in the preequilibrium stage of the quark-gluon plasma at very high energy densities, similar to the conditions present in the early Universe.

Observation of <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mi>t</mi><mover accent="true"><mrow><mi>t</mi></mrow><mrow><mo stretchy="false">¯</mo></mrow></mover></mrow></math> Production in Pb+Pb Collisions at <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><msqrt><mrow><msub><mrow><mi>s</mi></mrow><mrow><mi>NN</mi></mrow></msub></mrow></msqrt><mo>=</mo><mn>5.02</mn><mtext> </mtext><mtext> </mtext><mi>TeV</mi></mrow></math> with the ATLAS Detector

A search is performed for dark matter particles produced in association with a resonantly produced pair of <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>b</mi></math>-quarks with <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mn>30</mn><mo>&lt;</mo><msub><mrow><mi>m</mi></mrow><mrow><mi>b</mi><mi>b</mi></mrow></msub><mo>&lt;</mo><mn>150</mn><mtext> </mtext><mtext> </mtext><mi>GeV</mi></mrow></math> using <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mn>140</mn><mtext> </mtext><mtext> </mtext><msup><mrow><mi>fb</mi></mrow><mrow><mo>−</mo><mn>1</mn></mrow></msup></mrow></math> of proton-proton collisions at a center-of-mass energy of 13 TeV recorded by the ATLAS detector at the LHC. This signature is expected in extensions of the standard model predicting the production of dark matter particles, in particular those containing a dark Higgs boson <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>s</mi></math> that decays into <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>b</mi><mover accent="true"><mi>b</mi><mo stretchy="false">¯</mo></mover></math>. The highly boosted <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>s</mi><mo stretchy="false">→</mo><mi>b</mi><mover accent="true"><mi>b</mi><mo stretchy="false">¯</mo></mover></math> topology is reconstructed using jet reclustering and a new identification algorithm. This search places stringent constraints across regions of the dark Higgs model parameter space that satisfy the observed relic density, excluding dark Higgs bosons with masses between 30 and 150 GeV in benchmark scenarios with <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>Z</mi><mo>′</mo></msup></math> mediator masses up to 4.8 TeV at 95% confidence level.

Search for Dark Matter Produced in Association with a Dark Higgs Boson in the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mi>b</mi><mover accent="true"><mrow><mi>b</mi></mrow><mrow><mo stretchy="false">¯</mo></mrow></mover></mrow></math> Final State Using <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>p</mi><mi>p</mi></math> Collisions at <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><msqrt><mrow><mi>s</mi></mrow></msqrt><mo>=</mo><mn>13</mn><mtext> </mtext><mtext> </mtext><mi>TeV</mi></mrow></math> with the ATLAS Detector