This paper presents for the first time a precise measurement of the production properties of the Z boson in the full phase space of the decay leptons. The measurement is obtained from proton-proton collision data collected by the ATLAS experiment in 2012 at $\sqrt s$ = 8 TeV at the LHC and corresponding to an integrated luminosity of 20.2 fb$^{-1}$. The results, based on a total of 15.3 million Z-boson decays to electron and muon pairs, extend and improve a previous measurement of the full set of angular coefficients describing Z-boson decay. The double-differential cross-section distributions in Z-boson transverse momentum p$_T$ and rapidity y are measured in the pole region, defined as 80 $<$ m $<$ 100 GeV, over the range $|y| <$ 3.6. The total uncertainty of the normalised cross-section measurements in the peak region of the p$_T$ distribution is dominated by statistical uncertainties over the full range and increases as a function of rapidity from 0.5-1.0% for $|y| <$ 2.0 to 2-7% at higher rapidities. The results for the rapidity-dependent transverse momentum distributions are compared to state-of-the-art QCD predictions, which combine in the best cases approximate N$^4$LL resummation with N$^3$LO fixed-order perturbative calculations. The differential rapidity distributions integrated over p$_T$ are even more precise, with accuracies from 0.2-0.3% for $|y| <$ 2.0 to 0.4-0.9% at higher rapidities, and are compared to fixed-order QCD predictions using the most recent parton distribution functions. The agreement between data and predictions is quite good in most cases.
Measured $p_T$ cross sections in full-lepton phase space for |y| < 0.4.
Measured $p_T$ cross sections in full-lepton phase space for 0.4 < |y| < 0.8.
Measured $p_T$ cross sections in full-lepton phase space for 0.8 < |y| < 1.2.
A search for new particles that decay into top quark pairs is reported. The search is performed with the ATLAS experiment at the LHC using an integrated luminosity of 20.3 fb$^{-1}$ of proton-proton collision data collected at a centre-of-mass energy of $\sqrt{s}=8$ TeV. The lepton-plus-jets final state is used, where the top pair decays to $W^+bW^-\bar{b}$, with one $W$ boson decaying leptonically and the other hadronically. The invariant mass spectrum of top quark pairs is examined for local excesses or deficits that are inconsistent with the Standard Model predictions. No evidence for a top quark pair resonance is found, and 95% confidence-level limits on the production rate are determined for massive states in benchmark models. The upper limits on the cross-section times branching ratio of a narrow $Z'$ boson decaying to top pairs range from 4.2 pb to 0.03 pb for resonance masses from 0.4 TeV to 3.0 TeV. A narrow leptophobic topcolour $Z'$ boson with mass below 1.8 TeV is excluded. Upper limits are set on the cross-section times branching ratio for a broad colour-octet resonance with $\Gamma/m =$ 15% decaying to $t\bar{t}$. These range from 2.5 pb to 0.03 pb for masses from 0.4 TeV to 3.0 TeV. A Kaluza-Klein excitation of the gluon in a Randall-Sundrum model is excluded for masses below 2.2 TeV.
Selection efficiency x Acceptance for a Z' resonance.
Selection efficiency x Acceptance for a KK gluon resonance.
Selection efficiency x Acceptance for a KK graviton resonance.
A search for high-mass resonances decaying into $\tau^{+}\tau^{-}$ final states using proton-proton collisions at $\sqrt{s}= 8$ TeV produced by the Large Hadron Collider is presented. The data were recorded with the ATLAS detector and correspond to an integrated luminosity of 19.5-20.3 fb$^{-1}$. No statistically significant excess above the Standard Model expectation is observed; 95% credibility upper limits are set on the cross section times branching fraction of $Z^{\prime}$ resonances decaying into $\tau^+\tau^-$ pairs as a function of the resonance mass. As a result, $Z^{\prime}$ bosons of the Sequential Standard Model with masses less than 2.02 TeV are excluded at 95% credibility. The impact of the fermionic couplings on the $Z^{\prime}$ acceptance is investigated and limits are also placed on a $Z^{\prime}$ model that exhibits enhanced couplings to third-generation fermions.
Signal acceptance times efficiency (ACC*EFF) for Z'L, Z'R, Z'narrow and Z'wide divided by ACC*EFF for Z'SSM as a function of the Z' mass, separately for the had-had and lep-had channels.
Ratio of the Z'NU to Z'SSM cross section times tau+tau- branching fraction (SIG*BR) as a function of sin^2phi and the Z' mass.
Ratio of the Z'NU to Z'SSM acceptance times efficiency (ACC*EFF) in the had-had channel as a function of sin^2phi and the Z' mass.
This paper reviews and extends searches for the direct pair production of the scalar supersymmetric partners of the top and bottom quarks in proton-proton collisions collected by the ATLAS collaboration during the LHC Run 1. Most of the analyses use 20 fb$^{-1}$ of collisions at a centre-of-mass energy of $\sqrt{s}$ = 8 TeV, although in some case an additional 4.7 fb$^{-1}$ of collision data at $\sqrt{s}$ = 7 TeV are used. New analyses are introduced to improve the sensitivity to specific regions of the model parameter space. Since no evidence of third-generation squarks is found, exclusion limits are derived by combining several analyses and are presented in both a simplified model framework, assuming simple decay chains, as well as within the context of more elaborate phenomenological supersymmetric models.
Summary of the ATLAS Run 1 searches for direct stop pair production in models where no supersymmetric particle other than the $\tilde t_1$ and the $\tilde \chi_1^0$ is involved in the $\tilde t_1$ decay. Lines for $\Delta m(\tilde t_1, \chi_1^0 ) > m_{t}$ - t0L/t1L combined observed limit hepdata.cedar.ac.uk/view/ins1380183/d63 - t0L/t1L combined expected limit hepdata.cedar.ac.uk/view/ins1380183/d64 - t2L observed limit hepdata.cedar.ac.uk/view/ins1286444/d19 - t2L expected limit hepdata.cedar.ac.uk/view/ins1286444/d20 - SC observed limit $m_t< m_{\tilde t_1} < 198$ GeV - SC expected limit $m_t< m_{\tilde t_1} < 184$ GeV Lines for $m_b + m_W < \Delta m(\tilde t_1, \chi_1^0 ) < m_{t}$ - t1L observed limit hepdata.cedar.ac.uk/view/ins1304456/d22 - t1L expected limit hepdata.cedar.ac.uk/view/ins1304456/d23 - t2L observed limit hepdata.cedar.ac.uk/view/ins1286444/d22 - t2L expected limit hepdata.cedar.ac.uk/view/ins1286444/d23 - WW observed limit hepdata.cedar.ac.uk/view/ins1380183/d47 - WW expected limit hepdata.cedar.ac.uk/view/ins1380183/d48 Lines for $0 < \Delta m(\tilde t_1, \chi_1^0 ) < m_b + m_W $ - tc observed limit hepdata.cedar.ac.uk/view/ins1304459 (root macro) - tc expected limit hepdata.cedar.ac.uk/view/ins1304459 (root macro) - t1L observed limit hepdata.cedar.ac.uk/view/ins1304456/d22 - t1L expected limit hepdata.cedar.ac.uk/view/ins1304456/d23 - WW observed limit hepdata.cedar.ac.uk/view/ins1380183/d47 - WW expected limit hepdata.cedar.ac.uk/view/ins1380183/d48.
Upper limits on the stop pair production cross sections for different values of the branching ratios for the decays $\tilde{t}_1 \rightarrow c\tilde{\chi}_1^0$ and $\tilde{t}_1 \rightarrow ff'b\tilde{\chi}_1^0$, where BR$(\tilde{t}_1 \rightarrow c\tilde{\chi}_1^0)$ + BR$(\tilde{t}_1 \rightarrow ff'b\tilde{\chi}_1^0)$ = 1. Signal points with $\Delta m (\tilde{t}_1, \tilde{\chi}_1^0)$ of 10 GeV are shown. The limits quoted are taken from the best performing, based on expected exclusion CLs, signal regions from the tc-M, tc-C, t1L-bCa_low and WW analyses at each mass point. - Theoretical cross section from twiki.cern.ch/twiki/bin/view/LHCPhysics/SUSYCrossSections8TeVstopsbottom.
Upper limits on the stop pair production cross sections for different values of the branching ratios for the decays $\tilde{t}_1 \rightarrow c\tilde{\chi}_1^0$ and $\tilde{t}_1 \rightarrow ff'b\tilde{\chi}_1^0$, where BR$(\tilde{t}_1 \rightarrow c\tilde{\chi}_1^0)$ + BR$(\tilde{t}_1 \rightarrow ff'b\tilde{\chi}_1^0)$ = 1. Signal points with $\Delta m (\tilde{t}_1, \tilde{\chi}_1^0)$ of 80 GeV are shown. The limits quoted are taken from the best performing, based on expected exclusion CLs, signal regions from the tc-M, tc-C, t1L-bCa_low and WW analyses at each mass point. - Theoretical cross section from twiki.cern.ch/twiki/bin/view/LHCPhysics/SUSYCrossSections8TeVstopsbottom.
An analysis is presented of events containing jets including at least one $b$-tagged jet, sizeable missing transverse momentum, and at least two leptons including a pair of the same electric charge, with the scalar sum of the jet and lepton transverse momenta being large. A data sample with an integrated luminosity of 20.3 fb$^{-1}$ of $pp$ collisions at $\sqrt{s} = 8$ TeV recorded by the ATLAS detector at the Large Hadron Collider is used. Standard Model processes rarely produce these final states, but there are several models of physics beyond the Standard Model that predict an enhanced rate of production of such events; the ones considered here are production of vector-like quarks, enhanced four-top-quark production, pair production of chiral $b^\prime$-quarks, and production of two positively charged top quarks. Eleven signal regions are defined; subsets of these regions are combined when searching for each class of models. In the three signal regions primarily sensitive to positively charged top quark pair production, the data yield is consistent with the background expectation. There are more data events than expected from background in the set of eight signal regions defined for searching for vector-like quarks and chiral $b^\prime$-quarks, but the significance of the discrepancy is less than two standard deviations. The discrepancy reaches 2.5 standard deviations in the set of five signal regions defined for searching for four-top-quark production. The results are used to set 95% CL limits on various models.
Observed and expected number of events with statistical (first) and systematic (second) uncertainties for the positively charged top pair signal selection. The p-values for agreement between the observed yield and the expected background in each signal region are reported.
Observed and expected number of events with statistical (first) and systematic (second) uncertainties for five of the signal regions defined for VLQ, chiral bprime-quark and four-top-quark production searches. The p-values for agreement between the observed yield and the expected background in each signal region are reported.
Observed and expected number of events with statistical (first) and systematic (second) uncertainties for three of the signal regions defined for VLQ, chiral bprime-quark and four-top-quark production searches. The p-values for agreement between the observed yield and the expected background in each signal region are reported.
This paper presents measurements of distributions of charged particles which are produced in proton--proton collisions at a centre-of-mass energy of $\sqrt{s} = 8$ TeV and recorded by the ATLAS detector at the LHC. A special dataset recorded in 2012 with a small number of interactions per beam crossing (below 0.004) and corresponding to an integrated luminosity of $160 \mathrm{\mu b^{-1}}$ was used. A minimum-bias trigger was utilised to select a data sample of more than 9 million collision events. The multiplicity, pseudorapidity, and transverse momentum distributions of charged particles are shown in different regions of kinematics and charged-particle multiplicity, including measurements of final states at high multiplicity. The results are corrected for detector effects and are compared to the predictions of various Monte Carlo event generator models which simulate the full hadronic final state.
Central primary-charged-particle density 1/Nev dNch/deta at eta = 0 for five different phase spaces. The results are given for the fiducial definition tau > 300 ps, as well as for the previously used fiducial definition tau > 30 ps using an extrapolation factor of 1.012 +- 0.004 (for pT > 100 MeV) or 1.025 +- 0.008 (for pT > 500 MeV), which accounts for the fraction of charged strange baryons predicted by Epos LHC simulation.
Charged-particle multiplicity distributions in proton-proton collisions at a centre-of mass energy of 8000 GeV for events with the number of charged particles >=2 having transverse momentum >100 MeV and absolute(pseudorapidity) <2.5.
Charged-particle multiplicity distributions in proton-proton collisions at a centre-of mass energy of 8000 GeV for events with the number of charged particles >=1 having transverse momentum >500 MeV and absolute(pseudorapidity) <2.5.
This Letter presents the first study of $W^{\pm}W^{\pm}jj$, same-electric-charge diboson production in association with two jets, using 20.3 fb$^{-1}$ of proton--proton collision data at $\sqrt{s}=8$ TeV recorded by the ATLAS detector at the Large Hadron Collider. Events with two reconstructed same-charge leptons ($e^\pm e^\pm$, $e^\pm \mu^\pm$, and $\mu^\pm \mu^\pm$) and two or more jets are analyzed. Production cross sections are measured in two fiducial regions, with different sensitivities to the electroweak and strong production mechanisms. First evidence for $W^{\pm}W^{\pm}jj$ production and electroweak-only $W^{\pm}W^{\pm}jj$ production is observed with a significance of $4.5$ and $3.6$ standard deviations respectively. The measured production cross sections are in agreement with Standard Model predictions. Limits at 95% confidence level are set on anomalous quartic gauge couplings.
The fiducial cross sections for the production of W+- W+- JET JET in the inclusive and VBS regions.
This Letter reports evidence of triple gauge boson production $pp\to W(\ell\nu)\gamma\gamma + X$, which is accessible for the first time with the 8 TeV LHC data set. The fiducial cross section for this process is measured in a data sample corresponding to an integrated luminosity of 20.3 fb$^{-1}$, collected by the ATLAS detector in 2012. Events are selected using the $W$ boson decay to $e\nu$ or $\mu\nu$ as well as requiring two isolated photons. The measured cross section is used to set limits on anomalous quartic gauge couplings in the high diphoton mass region.
The measured inclusive ($N_{jet}\geq$ 0) fiducial cross section in the e$\nu\gamma\gamma$, $\mu\nu\gamma\gamma$ channels together with the combined $\ell\nu\gamma\gamma$ cross section. The first uncertainty shown is the statistical uncertainty on the measurement, the second one is the total systematic uncertainty (excluding the term coming from the luminosity), the third one is the systematic uncertainty coming from the luminosity. A parton to particle correction factors of 0.99 is applied to the MCFM prediction.
The measured exclusive ($N_{jet}$ = 0) fiducial cross section in the e$\nu\gamma\gamma$, $\mu\nu\gamma\gamma$ channels together with the combined $\ell\nu\gamma\gamma$ cross section. The first uncertainty shown is the statistical uncertainty on the measurement, the second one is the total systematic uncertainty (excluding the term coming from the luminosity), the third one is the systematic uncertainty coming from the luminosity. A parton to particle correction factor of 0.87 is applied to the MCFM prediction.
Observed and expected 95\% CL limits obtained for the $f_{\mathrm{T0}}/\Lambda^4$, $f_{\mathrm{M2}}/\Lambda^4$ and $f_{\mathrm{M3}}/\Lambda^4$ aQGC parameters for the combination of the two channels. The values of $n = 0,\,1,\,2$ are the exponential choices of the form factor, $\Lambda_{\rm{FF}}$ is fixed to $600$ GeV for $f_{\mathrm{T0}}/\Lambda^4$ and to $500$ GeV for the other parameters. The $n=0$ choice produces the limits without the form factor applied.
Measurements of fiducial and differential cross sections of Higgs boson production in the ${H \rightarrow ZZ ^{*}\rightarrow 4\ell}$ decay channel are presented. The cross sections are determined within a fiducial phase space and corrected for detection efficiency and resolution effects. They are based on 20.3 fb$^{-1}$ of $pp$ collision data, produced at $\sqrt{s}$=8 TeV centre-of-mass energy at the LHC and recorded by the ATLAS detector. The differential measurements are performed in bins of transverse momentum and rapidity of the four-lepton system, the invariant mass of the subleading lepton pair and the decay angle of the leading lepton pair with respect to the beam line in the four-lepton rest frame, as well as the number of jets and the transverse momentum of the leading jet. The measured cross sections are compared to selected theoretical calculations of the Standard Model expectations. No significant deviation from any of the tested predictions is found.
Measured differential fiducial cross sections in Higgs transverse momentum (second column). The given uncertainty includes statistical and systematic components. The third (fourth) column gives the theoretical prediction of Higgs production in the fiducial volume using Powheg Minlo HJ (HRes) for the ggF process, Powheg for the VBF process, and Pythia 8 for the VH and ttH process. The uncertainty includes PDF, scale, and branching fraction uncertainty. The fifth column gives the non-ggF prediction (total minus ggF). All predicted distributions were normalized to the best predicted inclusive Higgs production cross sections available at the time of the publication.
Measured differential fiducial cross sections in the absolute value of the Higgs rapidity (second column). The given uncertainty includes statistical and systematic components. The third (fourth) column gives the theoretical prediction of Higgs production in the fiducial volume using Powheg Minlo HJ (HRes) for the ggF process, Powheg for the VBF process, and Pythia 8 for the VH and ttH process. The uncertainty includes PDF, scale, and branching fraction uncertainty. The fifth column gives the non-ggF prediction (total minus ggF). All predicted distributions were normalized to the best predicted inclusive Higgs production cross sections available at the time of the publication.
Measured differential fiducial cross sections in m34, which corresponds to the invariant mass of the off-shell Z boson (second column). The given uncertainty includes statistical and systematic components. The third column gives the theoretical prediction of Higgs production in the fiducial volume using Powheg Minlo HJ for the ggF process, Powheg for the VBF process, and Pythia 8 for the VH and ttH process. The uncertainty includes PDF, scale, and branching fraction uncertainty. The fourth column gives the non-ggF prediction (total minus ggF). All predicted distributions were normalized to the best predicted inclusive Higgs production cross sections available at the time of the publication.
The dynamics of isolated-photon plus one-, two- and three-jet production in $pp$ collisions at a centre-of-mass energy of 8 TeV are studied with the ATLAS detector at the LHC using a data set with an integrated luminosity of 20.2 fb$^{-1}$. Measurements of isolated-photon plus jets cross sections are presented as functions of the photon and jet transverse momenta. The cross sections as functions of the azimuthal angle between the photon and the jets, the azimuthal angle between the jets, the photon-jet invariant mass and the scattering angle in the photon-jet centre-of-mass system are presented. The pattern of QCD radiation around the photon and the leading jet is investigated by measuring jet production in an annular region centred on each object; enhancements are observed around the leading jet with respect to the photon in the directions towards the beams. The experimental measurements are compared to several different theoretical calculations, and overall a good description of the data is found.
Measured cross sections for isolated-photon plus 1jet production as a function of $E_{\rm T}^{\gamma}$.
Measured cross sections for isolated-photon plus 1jet production as a function of $p_{\rm T}^{\rm jet1}$.
Measured cross sections for isolated-photon plus 1jet production as a function of $m^{\gamma-\rm jet1}$.
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