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.
Exclusion limits as a function of the stop2 branching ratio for decays into Z, Higgs and neutralino. m(t2)=350 GeV and m(chi1)=20 GeV (top plot). This table is for the t1L/t0L expected limit. - t2t1Z observed limit hepdata.cedar.ac.uk/view/ins1286622/d14 - t2t1Z expected limit hepdata.cedar.ac.uk/view/ins1286622/d15.
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.
Charged-particle multiplicities in proton-proton collisions at a centre-of mass energy of 8000 GeV as a function of transverse momentum for events with the number of charged particles >=2 having transverse momentum >100 MeV and absolute(pseudorapidity) <2.5.
Charged-particle multiplicities in proton-proton collisions at a centre-of mass energy of 8000 GeV as a function of transverse momentum for events with the number of charged particles >=1 having transverse momentum >500 MeV and absolute(pseudorapidity) <2.5.
Charged-particle multiplicities in proton-proton collisions at a centre-of mass energy of 8000 GeV as a function of transverse momentum for events with the number of charged particles >=6 having transverse momentum >500 MeV and absolute(pseudorapidity) <2.5.
A measurement of the forward-backward asymmetry A[FB] of oppositely charged lepton pairs (mu mu and e e) produced via Z/gamma* boson exchange in pp collisions at sqrt(s) = 8 TeV is presented. The data sample corresponds to an integrated luminosity of 19.7 inverse femtobarns collected with the CMS detector at the LHC. The measurement of A[FB] is performed for dilepton masses between 40 GeV and 2 TeV and for dilepton rapidity up to 5. The A[FB] measurements as a function of dilepton mass and rapidity are compared with the standard model predictions.
Covariance matrix of Drell-Yan AFB in electron channel for |y|<1 for cos(theta)<0 and cos(theta)>0. The covariance matrix is built in 2 dimensions, 14x14(mass) for cos(theta), --, -+, +-, ++ combination where + means positive cos(theta) and - implies the negative cos(theta).
This paper presents a measurement of the $W$ boson production cross section and the $W^{+}/W^{-}$ cross-section ratio, both in association with jets, in proton--proton collisions at $\sqrt{s}=8$ TeV with the ATLAS experiment at the Large Hadron Collider. The measurement is performed in final states containing one electron and missing transverse momentum using data corresponding to an integrated luminosity of 20.2 fb$^{-1}$. Differential cross sections for events with one or two jets are presented for a range of observables, including jet transverse momenta and rapidities, the scalar sum of transverse momenta of the visible particles and the missing transverse momentum in the event, and the transverse momentum of the $W$ boson. For a subset of the observables, the differential cross sections of positively and negatively charged $W$ bosons are measured separately. In the cross-section ratio of $W^{+}/W^{-}$ the dominant systematic uncertainties cancel out, improving the measurement precision by up to a factor of nine. The observables and ratios selected for this paper provide valuable input for the up quark, down quark, and gluon parton distribution functions of the proton.
Statistical correlation between bins in data for the differential cross section for the production of W bosons as a function of the leading jet rapidity for events with N<sub> jets</sub> ≥ 1.
This Letter presents the measurement of differential cross sections of isolated prompt photons produced in association with a b-jet or a c-jet. These final states provide sensitivity to the heavy-flavour content of the proton and aspects related to the modelling of heavy-flavour quarks in perturbative QCD. The measurement uses proton-proton collision data at a centre-of-mass energy of 8 TeV recorded by the ATLAS detector at the LHC in 2012 corresponding to an integrated luminosity of up to 20.2 fb$^{-1}$. The differential cross sections are measured for each jet flavour with respect to the transverse energy of the leading photon in two photon pseudorapidity regions: $|\eta^\gamma|<1.37$ and $1.56<|\eta^\gamma|<2.37$. The measurement covers photon transverse energies $25 < E_\textrm{T}^\gamma<400$ GeV and $25 < E_\textrm{T}^\gamma<350$ GeV respectively for the two $|\eta^\gamma|$ regions. For each jet flavour, the ratio of the cross sections in the two $|\eta^\gamma|$ regions is also measured. The measurement is corrected for detector effects and compared to leading-order and next-to-leading-order perturbative QCD calculations, based on various treatments and assumptions about the heavy-flavour content of the proton. Overall, the predictions agree well with the measurement, but some deviations are observed at high photon transverse energies. The total uncertainty in the measurement ranges between 13% and 66%, while the central $\gamma+b$ measurement exhibits the smallest uncertainty, ranging from 13% to 27%, which is comparable to the precision of the theoretical predictions.
Signed shifts of the individual systematic uncertainties on the $\gamma+b$ cross section for $|\eta^\gamma|<1.37$. The numbers after the name of the uncertainty source refer to the individual component in that uncertainty. Each bin of the MC statistical uncertainty is independent of any other bin. The first four components of the photon energy scale uncertainty are specific to this $|\eta^\gamma|$ region and are independent of the components in the other region. The region is indicated as part of their name to indicate the independence between the $|\eta^\gamma|$ regions. The uncertainties on the prompt photon modelling, non-perturbative QCD models and particle-level migration effects are only varied once and not up and down by their nature, but are symmetrised for the final results. Only uncertainties which have at least a 1% variation in at least one bin of the $\gamma+b$ and $\gamma+c$ cross section measurements, including the ratios, are listed. The others are summed in quadrature and listed as a single entry.
Signed shifts of the individual systematic uncertainties on the $\gamma+b$ cross section for $1.56<|\eta^\gamma|<2.37$. The numbers after the name of the uncertainty source refer to the individual component in that uncertainty. Each bin of the MC statistical uncertainty is independent of any other bin. The first four components of the photon energy scale uncertainty are specific to this $|\eta^\gamma|$ region and are independent of the components in the other region. The region is indicated as part of their name to indicate the independence between the $|\eta^\gamma|$ regions. The uncertainties on the prompt photon modelling, non-perturbative QCD models and particle-level migration effects are only varied once and not up and down by their nature, but are symmetrised for the final results. Only uncertainties which have at least a 1% variation in at least one bin of the $\gamma+b$ and $\gamma+c$ cross section measurements, including the ratios, are listed. The others are summed in quadrature and listed as a single entry.
Signed shifts of the individual systematic uncertainties on the $\gamma+c$ cross section for $|\eta^\gamma|<1.37$. The numbers after the name of the uncertainty source refer to the individual component in that uncertainty. Each bin of the MC statistical uncertainty is independent of any other bin. The first four components of the photon energy scale uncertainty are specific to this $|\eta^\gamma|$ region and are independent of the components in the other region. The region is indicated as part of their name to indicate the independence between the $|\eta^\gamma|$ regions. The uncertainties on the prompt photon modelling, non-perturbative QCD models and particle-level migration effects are only varied once and not up and down by their nature, but are symmetrised for the final results. Only uncertainties which have at least a 1% variation in at least one bin of the $\gamma+b$ and $\gamma+c$ cross section measurements, including the ratios, are listed. The others are summed in quadrature and listed as a single entry.
This paper describes a measurement of fiducial and differential cross sections of gluon-fusion Higgs boson production in the $H{\rightarrow\,}WW^{\ast}{\rightarrow\,}e\nu\mu\nu$ channel, using 20.3 fb$^{-1}$ of proton-proton collision data. The data were produced at a centre-of-mass energy of $\sqrt{s} = 8$ TeV at the CERN Large Hadron Collider and recorded by the ATLAS detector in 2012. Cross sections are measured from the observed $H{\rightarrow\,}WW^{\ast}{\rightarrow\,}e\nu\mu\nu$ signal yield in categories distinguished by the number of associated jets. The total cross section is measured in a fiducial region defined by the kinematic properties of the charged leptons and neutrinos. Differential cross sections are reported as a function of the number of jets, the Higgs boson transverse momentum, the dilepton rapidity, and the transverse momentum of the leading jet. The jet-veto efficiency, or fraction of events with no jets above a given transverse momentum threshold, is also reported. All measurements are compared to QCD predictions from Monte Carlo generators and fixed-order calculations, and are in agreement with the Standard Model predictions.
Correction factors from inclusive parton level to fiducial particle level for the jet-veto efficiency with different jet pT thresholds derived with POWHEG NNLOPS+Pythia8. The asterisk on the 25 GeV bin label indicates that the results are for a mixed pT threshold, which is raised from 25 GeV to 30 GeV for jets with 2.5 < |eta| < 4.5, corresponding to the selection used to define the signal regions for the analysis.
Jet multiplicity distributions in top quark pair (t t-bar) events are measured in pp collisions at a centre-of-mass energy of 8 TeV with the CMS detector at the LHC using a data set corresponding to an integrated luminosity of 19.7 inverse femtobarns. The measurement is performed in the dilepton decay channels (e+ e-, mu+ mu-, and e+/- mu-/+). The absolute and normalized differential cross sections for t t-bar production are measured as a function of the jet multiplicity in the event for different jet transverse momentum thresholds and the kinematic properties of the leading additional jets. The differential t t-bar b and t t-bar b b-bar cross sections are presented for the first time as a function of the kinematic properties of the leading additional b jets. Furthermore, the fraction of events without additional jets above a threshold is measured as a function of the transverse momenta of the leading additional jets and the scalar sum of the transverse momenta of all additional jets. The data are compared and found to be consistent with predictions from several perturbative quantum chromodynamics event generators and a next-to-leading order calculation.
Normalized differential ttbar cross sections as a function of pt of the leading additional jet j1 in the event (not coming from the top quark decay products), along with their statistical, systematic, and total uncertainties. The results are presented at the particle level in the full phase space of the ttbar system, corrected for acceptance and branching fractions.
Measurements of four-lepton (4$\ell$, $\ell=e,\mu$) production cross sections at the $Z$ resonance in $pp$ collisions at the LHC with the ATLAS detector are presented. For dilepton and four-lepton invariant mass region $m_{\ell^+\ell^-} > 5$ GeV and $80 < m_{4\ell} < 100$ GeV, the measured cross sections are $76 \pm 18 \text { (stat) } \pm 4 \text { (syst) } \pm 1.4 \text { (lumi) }$ fb and $107 \pm 9 \text{ (stat) } \pm 4 \text{ (syst) } \pm 3.0 \text { (lumi) }$ fb at $\sqrt s$ = 7 and 8 TeV, respectively. By subtracting the non-resonant 4$\ell$ production contributions and normalizing with $Z\rightarrow \mu^+\mu^-$ events, the branching fraction for the $Z$ boson decay to $4\ell$ is determined to be $\left( 3.20 \pm 0.25\text{ (stat)} \pm 0.13\text{ (syst)} \right) \times 10^{-6}$, consistent with the Standard Model prediction.
The measured individual cross sections in the fiducial region and the combined cross sections for 4-muon and 4-electron final states at a centre-of-collision energy of 7 TeV.
The measured individual cross sections in the fiducial region and the combined cross sections for 2-muon-2-electron final states at a centre-of-collision energy of 7 TeV.
The measured cross section for four-lepton final states at a centre-of-collision energy of 7 TeV.
The normalized differential cross section for top quark pair (tt-bar) production is measured in pp collisions at a centre-of-mass energy of 8 TeV at the CERN LHC using the CMS detector in data corresponding to an integrated luminosity of 19.7 inverse femtobarns. The measurements are performed in the lepton + jets (e/mu + jets) and in the dilepton (e+e-, mu+mu-, and e+-mu-+) decay channels. The tt-bar cross section is measured as a function of the kinematic properties of the charged leptons, the jets associated to b quarks, the top quarks, and the tt-bar system. The data are compared with several predictions from perturbative quantum chromodynamics up to approximate next-to-next-to-leading-order precision. No significant deviations are observed relative to the standard model predictions.
Statistical covariance matrix for the normalized differential tt cross section (from l+jets channel) as a function of the rapidity y of the top quark or antiquark.
Measurements are reported of the normalized differential cross sections for top quark pair production with respect to four kinematic event variables: the missing transverse energy; the scalar sum of the jet transverse momentum (pT); the scalar sum of the pT of all objects in the event; and the pT of leptonically decaying W bosons from top quark decays. The data sample, collected using the CMS detector at the LHC, consists of 5.0 inverse femtobarns of proton-proton collisions at sqrt(s) = 7 TeV and 19.7 inverse femtobarns at sqrt(s) = 8 TeV. Top quark pair events containing one electron or muon are selected. The results are presented after correcting for detector effects to allow direct comparison with theoretical predictions. No significant deviations from the predictions of several standard model event simulation generators are observed.
Normalized $t\bar{t}$ differential cross section measurements with respect to the $E^{miss}_{T}$ variable at a center-of-mass energy of 7 TeV (combination of electron and muon channels).
Normalized $t\bar{t}$ differential cross section measurements with respect to the $H_T$ variable at a center-of-mass energy of 7 TeV (combination of electron and muon channels).
Normalized $t\bar{t}$ differential cross section measurements with respect to the $S_T$ variable at a center-of-mass energy of 7 TeV (combination of electron and muon channels).
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