Cross-sections for the production of a $Z$ boson in association with two photons are measured in proton$-$proton collisions at a centre-of-mass energy of 13 TeV. The data used correspond to an integrated luminosity of 139 fb$^{-1}$ recorded by the ATLAS experiment during Run 2 of the LHC. The measurements use the electron and muon decay channels of the $Z$ boson, and a fiducial phase-space region where the photons are not radiated from the leptons. The integrated $Z(\rightarrow\ell\ell)\gamma\gamma$ cross-section is measured with a precision of 12% and differential cross-sections are measured as a function of six kinematic variables of the $Z\gamma\gamma$ system. The data are compared with predictions from MC event generators which are accurate to up to next-to-leading order in QCD. The cross-section measurements are used to set limits on the coupling strengths of dimension-8 operators in the framework of an effective field theory.
Measured fiducial-level integrated cross-section. NLO predictions from Sherpa 2.2.10 and MadGraph5_aMC@NLO 2.7.3 are also shown. The uncertainty in the predictions is divided into statistical and theoretical uncertainties (scale and PDF+$\alpha_{s}$).
Measured unfolded differential cross-section as a function of the leading photon transverse energy $E^{\gamma1}_{\mathrm{T}}$. NLO predictions from Sherpa 2.2.10 and MadGraph5_aMC@NLO 2.7.3 are also shown. The uncertainty in the predictions is divided into statistical and theoretical uncertainties (scale and PDF+$\alpha_{s}$).
Measured unfolded differential cross-section as a function of the subleading photon transverse energy $E^{\gamma2}_{\mathrm{T}}$. NLO predictions from Sherpa 2.2.10 and MadGraph5_aMC@NLO 2.7.3 are also shown. The uncertainty in the predictions is divided into statistical and theoretical uncertainties (scale and PDF+$\alpha_{s}$).
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.
First measurements of the W -> lnu and Z/gamma* -> ll (l = e, mu) production cross sections in proton-proton collisions at sqrt(s) = 7 TeV are presented using data recorded by the ATLAS experiment at the LHC. The results are based on 2250 W -> lnu and 179 Z/gamma* -> ll candidate events selected from a data set corresponding to an integrated luminosity of approximately 320 nb-1. The measured total W and Z/gamma*-boson production cross sections times the respective leptonic branching ratios for the combined electron and muon channels are $\stotW$ * BR(W -> lnu) = 9.96 +- 0.23(stat) +- 0.50(syst) +- 1.10(lumi) nb and $\stotZg$ * BR(Z/gamma* -> ll) = 0.82 +- 0.06(stat) +- 0.05(syst) +- 0.09(lumi) nb (within the invariant mass window 66 < m_ll < 116 GeV). The W/Z cross-section ratio is measured to be 11.7 +- 0.9(stat) +- 0.4(syst). In addition, measurements of the W+ and W- production cross sections and of the lepton charge asymmetry are reported. Theoretical predictions based on NNLO QCD calculations are found to agree with the measurements.
Measured fiducial cross section times leptonic branching ratio for W+ production in the W+ -> e+ nu final state.
Measured fiducial cross section times leptonic branching ratio for W- production in the W- -> e- nubar final state.
Measured fiducial cross section times leptonic branching ratio for W+/- production in the combined W+ -> e+ nu and W- -> e- nubar final state.
In a special run of the LHC with $\beta^\star = 2.5~$km, proton-proton elastic-scattering events were recorded at $\sqrt{s} = 13~$TeV with an integrated luminosity of $340~\mu \textrm{b}^{-1}$ using the ALFA subdetector of ATLAS in 2016. The elastic cross section was measured differentially in the Mandelstam $t$ variable in the range from $-t = 2.5 \cdot 10^{-4}~$GeV$^{2}$ to $-t = 0.46~$GeV$^{2}$ using 6.9 million elastic-scattering candidates. This paper presents measurements of the total cross section $\sigma_{\textrm{tot}}$, parameters of the nuclear slope, and the $\rho$-parameter defined as the ratio of the real part to the imaginary part of the elastic-scattering amplitude in the limit $t \rightarrow 0$. These parameters are determined from a fit to the differential elastic cross section using the optical theorem and different parameterizations of the $t$-dependence. The results for $\sigma_{\textrm{tot}}$ and $\rho$ are \begin{equation*} \sigma_{\textrm{tot}}(pp\rightarrow X) = \mbox{104.7} \pm 1.1 \; \mbox{mb} , \; \; \; \rho = \mbox{0.098} \pm 0.011 . \end{equation*} The uncertainty in $\sigma_{\textrm{tot}}$ is dominated by the luminosity measurement, and in $\rho$ by imperfect knowledge of the detector alignment and by modelling of the nuclear amplitude.
The measured total cross section. The systematic uncertainty includes experimental and theoretical uncerainties.
The measured total cross section. The systematic uncertainty includes experimental and theoretical uncerainties.
The rho-parameter, i.e. the ratio of the real to imaginary part of the elastic scattering amplitude extrapolated to t=0. The systematic uncertainty includes experimental and theoretical uncerainties.
The exclusive production of pion pairs in the process $pp\to pp\pi^+\pi^-$ has been measured at $\sqrt{s}$ = 7 TeV with the ATLAS detector at the LHC, using 80 $\mu$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 $4.8 \pm 1.0 \text{(stat.)} + {}^{+0.3}_{-0.2} \text{(syst.)}\mu$b and $9 \pm 6 \text{(stat.)} + {}^{+2}_{-2}\text{(syst.)}\mu$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.
The measured fiducial cross sections. The first systematic uncertainty is the combined systematic uncertainty excluding luminosity, the second is the luminosity
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.
This paper presents measurements of charged-hadron spectra obtained in $pp$, $p$+Pb, and Pb+Pb collisions at $\sqrt{s}$ or $\sqrt{s_{_\text{NN}}}=5.02$ TeV, and in Xe+Xe collisions at $\sqrt{s_{_\text{NN}}}=5.44$ TeV. The data recorded by the ATLAS detector at the LHC have total integrated luminosities of 25 pb${}^{-1}$, 28 nb${}^{-1}$, 0.50 nb${}^{-1}$, and 3 $\mu$b${}^{-1}$, respectively. The nuclear modification factors $R_{p\text{Pb}}$ and $R_\text{AA}$ are obtained by comparing the spectra in heavy-ion and $pp$ collisions in a wide range of charged-particle transverse momenta and pseudorapidity. The nuclear modification factor $R_{p\text{Pb}}$ shows a moderate enhancement above unity with a maximum at $p_{\mathrm{T}} \approx 3$ GeV; the enhancement is stronger in the Pb-going direction. The nuclear modification factors in both Pb+Pb and Xe+Xe collisions feature a significant, centrality-dependent suppression. They show a similar distinct $p_{\mathrm{T}}$-dependence with a local maximum at $p_{\mathrm{T}} \approx 2$ GeV and a local minimum at $p_{\mathrm{T}} \approx 7$ GeV. This dependence is more distinguishable in more central collisions. No significant $|\eta|$-dependence is found. A comprehensive comparison with several theoretical predictions is also provided. They typically describe $R_\text{AA}$ better in central collisions and in the $p_{\mathrm{T}}$ range from about 10 to 100 GeV.
Charged-hadron cross-section in pp collisions. The systematic uncertainties are described in the section 7 of the paper. The total systematic uncertainties are determined by adding the contributions from all relevant sources in quadrature.
Charged-hadron spectrum in the centrality interval 0-5% for p+Pb, divided by 〈TPPB〉. The systematic uncertainties are described in the section 7 of the paper. The total systematic uncertainties are determined by adding the contributions from all relevant sources in quadrature.
Charged-hadron spectrum in the centrality interval 5-10% for p+Pb, divided by 〈TPPB〉. The systematic uncertainties are described in the section 7 of the paper. The total systematic uncertainties are determined by adding the contributions from all relevant sources in quadrature.
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