Jet cross sections have been measured for the first time in proton-proton collisions at a centre-of-mass energy of 7 TeV using the ATLAS detector. The measurement uses an integrated luminosity of 17 nb-1 recorded at the Large Hadron Collider. The anti-kt algorithm is used to identify jets, with two jet resolution parameters, R = 0.4 and 0.6. The dominant uncertainty comes from the jet energy scale, which is determined to within 7% for central jets above 60 GeV transverse momentum. Inclusive single-jet differential cross sections are presented as functions of jet transverse momentum and rapidity. Dijet cross sections are presented as functions of dijet mass and the angular variable $\chi$. The results are compared to expectations based on next-to-leading-order QCD, which agree with the data, providing a validation of the theory in a new kinematic regime.
Inclusive jet double-differential cross sections in the |rapidity| range 0 to 0.3, using a jet resolution R value of 0.4. The three (sys) errors are respectively, the Absolute JES, the Unfolding and the Luminosity uncertainties.
Inclusive jet double-differential cross sections in the |rapidity| range 0.3 to 0.8, using a jet resolution R value of 0.4. The three (sys) errors are respectively, the Absolute JES, the Unfolding and the Luminosity uncertainties.
Inclusive jet double-differential cross sections in the |rapidity| range 0.8 to 1.2, using a jet resolution R value of 0.4. The three (sys) errors are respectively, the Absolute JES, the Unfolding and the Luminosity uncertainties.
A search for new heavy particles manifested as resonances in two-jet final states is presented. The data were produced in 7 TeV proton-proton collisions by the Large Hadron Collider (LHC) and correspond to an integrated luminosity of 315 nb^-1 collected by the ATLAS detector. No resonances were observed. Upper limits were set on the product of cross section and signal acceptance for excited-quark (q*) production as a function of q* mass. These exclude at the 95% CL the q* mass interval 0.30 < mq* < 1.26 TeV, extending the reach of previous experiments.
The dijet mass distribution (NUMBER OF EVENTS).
95 PCT CL upper limit of the cross section x acceptance.
Dijet angular distributions from the first LHC pp collisions at center-of-mass energy sqrt(s) = 7 TeV have been measured with the ATLAS detector. The dataset used for this analysis represents an integrated luminosity of 3.1 pb-1. Dijet $\chi$ distributions and centrality ratios have been measured up to dijet masses of 2.8 TeV, and found to be in good agreement with Standard Model predictions. Analysis of the $\chi$ distributions excludes quark contact interactions with a compositeness scale $\Lambda$ below 3.4 TeV, at 95% confidence level, significantly exceeding previous limits.
CHI distribution for mass bin 340 to 520 GeV.
CHI distribution for mass bin 520 to 800 GeV.
CHI distribution for mass bin 800 to 1200 GeV.
The first measurements from proton-proton collisions recorded with the ATLAS detector at the LHC are presented. Data were collected in December 2009 using a minimum-bias trigger during collisions at a centre-of-mass energy of 900 GeV. The charged-particle multiplicity, its dependence on transverse momentum and pseudorapidity, and the relationship between mean transverse momentum and charged-particle multiplicity are measured for events with at least one charged particle in the kinematic range |eta|<2.5 and pT>500 MeV. The measurements are compared to Monte Carlo models of proton-proton collisions and to results from other experiments at the same centre-of-mass energy. The charged-particle multiplicity per event and unit of pseudorapidity at eta = 0 is measured to be 1.333 +/- 0.003 (stat.) +/- 0.040 (syst.), which is 5-15% higher than the Monte Carlo models predict.
Average value of charged particle multiplicity per event and unit of pseudorapidity in the pseudorapidity range from -0.2 to 0.2.
Charged particle multiplicity as a function of pseudorapidity.
Charged particle multiplicity as a function of transverse momentum.
This paper describes a search for dark photons ($\gamma_d$) in Higgs boson decay ($H \to \gamma\gamma_d$) produced in proton-proton collisions through the $ZH$ production mode at the Large Hadron Collider at $\sqrt{s}=13$ TeV. The transverse mass of the photon and the missing transverse momentum from the non-interacting $\gamma_d$ would present a distinctive signature at the Higgs boson mass resonance. The results presented use the total Run-2 integrated luminosity of 139 fb$^{-1}$, recorded by the ATLAS detector at the LHC . The dominant reducible background processes have been estimated using data-driven techniques. A Boosted Decision Tree (BDT) technique was adopted to enhance the sensitivity of the search. Given that no excess is observed with respect to the Standard Model predictions, an observed (expected) upper limit on the branching ratio BR$(H\to \gamma\gamma_d)$ of 2.28$\%$ (2.82$^{+1.33}_{-0.84}\%$) is set at 95$\%$ CL for massless $\gamma_d$. For higher dark photons masses up to 40 GeV, the observed (expected) upper limits at 95$\%$ CL are found to be within the [2.19-2.52]$\%$ ([2.71-3.11]$\%$) range.
Distribution of the BDT classifier response for data and for the expected SM background before the background-only fit. The expectations for the signal are also shown for the massless dark photon and for dark photon mass values of 20 GeV and 40 GeV, assuming BR(H$\to\gamma\gamma_d$) = 5%. Uncertainties shown are statistical for data, while for backgrounds include statistical and systematic sources.
Distribution of the BDT classifier response for data and for the expected SM background after the background-only fit. The expectations for the signal are also shown for the massless dark photon and for dark photon mass values of 20 GeV and 40 GeV, assuming BR(H$\to\gamma\gamma_d$) = 5%. Uncertainties shown are statistical for data, while for backgrounds include statistical and systematic sources determined by the multiple-bin fit.
Background, data and signal yields in bins of BDT, in SR and VV$\gamma$ CR, after the background-only fit. The expectations for the signal are shown for the massless dark photon and for dark photon mass values of 20 GeV and 40 GeV, assuming BR(H$\to\gamma\gamma_d$) = 5%. Uncertainties are statistical for data, while for backgrounds include statistical and systematic sources.
This note provides projected sensitivities at the High Luminosity LHC (HL-LHC) using the ATLAS experiment for $t$-channel vector leptoquarks in final states with $b$-jets; and single-production vector-like $T$ quarks (VLTs) decaying to $W(\ell\nu)b$. The projections are obtained from the extrapolation of analyses based on the Run 2 dataset, using 140 fb$^{-1}$ of data at $\sqrt{s}$ = 13 TeV. The datasets with 3 ab$^{-1}$ for the ATLAS only case and 6 ab$^{-1}$ for the ATLAS and CMS combination are considered. Two scenarios for systematic uncertainties are considered. The baseline scenario assumes reduced systematic uncertainties due to increased dataset, while the Run 2 scenario retains the same systematic uncertainties as used in the Run 2 analysis. For the leptoquark signature, the expected limit on the coupling to the third generation left-handed fermion states, $\beta^{33}_L$ , using the baseline (Run 2) scenario of systematic uncertainty improves by about 30% (20%) and 40% (20%) for the case of 3 ab$^{-1}$ and 6 ab$^{-1}$, respectively. Projections for the VLT at 3 ab$^{-1}$ show that the limit on the VLQ coupling $\kappa$ at a given mass improves significantly, being reduced by about 30% at 1.2 TeV up to about 60% at 2.3 TeV, with only modest additional gains achievable through further reduction of systematic uncertainties or the use of the full 6 ab$^{-1}$ dataset. The range of masses that can be studied within the constraints of the narrow-width approximation is extended from 2.3 TeV to over 2.7 TeV.
95% confidence level limits on LQ coupling as a function of LQ mass assuming <em>3 ab<sup>−1</sup></em> (red lines) and <em>6 ab<sup>−1</sup></em> (blue lines). Shown are the expected limits for the Run 2 scenario (dashed lines) and baseline scenario (solid lines), and the region preferred by measurements of <em>B</em>-hadron decays (shaded).
95% confidence level exclusion contour in the (<em>m<sub>VLQ</sub>, κ</em>) plane, for LHC Run 2 (140 fb<sup>−1</sup>), <em>3 ab<sup>−1</sup></em>, and <em>6 ab<sup>−1</sup></em>. Consistent with previous ATLAS studies, parameter points where the VLQ width is more than half the VLQ mass are excluded (the grey region), as, due to the breakdown of the narrow-width approximation, the theoretical calculations become unreliable. The Run 2 <em>T → Wb</em> contour comes from Ref. [<a href="https://arxiv.org/abs/2506.15515">26</a>], and the Run 2 combination refers to Ref. [<a href="https://arxiv.org/abs/2408.08789">27</a>].
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