The results of a search for direct pair production of heavy top-quark partners in 4.7 fb-1 of integrated luminosity from pp collisions at sqrt(s) = 7 TeV collected by the ATLAS detector at the LHC are reported. Heavy top-quark partners decaying into a top quark and a neutral non-interacting particle are searched for in events with two leptons in the final state. No excess above the Standard Model expectation is observed. Limits are placed on the mass of a supersymmetric scalar top and of a spin-1/2 top-quark partner. A spin-1/2 top-quark partner with a mass between 300 GeV and 480 GeV, decaying to a top quark and a neutral non-interacting particle lighter than 100 GeV, is excluded at 95% confidence level.
(1) Number of generated MC events for the scalar top signal grid (2) Relative Cross section uncertainties for the scalar top signal grid.
(1) Acceptance of the same flavour selection for the scalar top signal grid (2) Selection efficiency of the same flavour selection for the scalar top signal grid (3) Product of the acceptance and efficiency of the same flavour selection for the scalar top signal grid (4) Relative experimental uncertainties on the acceptance times efficiency of the same flavour selection for the scalar top signal grid.
(1) Acceptance of the different flavour selection for the scalar top signal grid (2) Selection efficiency of the different flavour selection for the scalar top signal grid (3) Product of the acceptance and efficiency of the different flavour selection for the scalar top signal grid (4) Relative experimental uncertainties on the acceptance times efficiency of the different flavour selection for the scalar top signal grid.
The results of a search for pair production of light top squarks are presented, using 4.7 fb^-1 of sqrt(s) = 7 TeV proton-proton collisions collected with the ATLAS detector at the Large Hadron Collider. This search targets top squarks with masses similar to, or lighter than, the top quark mass. Final states containing exclusively one or two leptons (e, mu), large missing transverse momentum, light-jets and b-jets are used to reconstruct the top squark pair system. Global mass scale variables are used to separate the signal from a large ttbar background. No excess over the Standard Model expectations is found. The results are interpreted in the framework of the Minimal Supersymmetric Standard Model, assuming the top squark decays exclusively to a chargino and a b-quark. Light top squarks with masses between 123-167 GeV are excluded for neutralino masses around 55 GeV.
Expected 95 PCT exclusion limit in the M(stop), M(neutralino) plane in gaugino universality scenario.
Observed 95 PCT exclusion limit in the M(stop), M(neutralino) plane in gaugino universality scenario.
Expected 95 PCT exclusion limit in the M(chargino), M(neutralino) plane in the scenario where M(stop) = 180 GEV.
A measurement of single top-quark production in the s-channel is performed in proton$-$proton collisions at a centre-of-mass energy of 13 TeV with the ATLAS detector at the CERN Large Hadron Collider. The dataset corresponds to an integrated luminosity of 139 fb$^{-1}$. The analysis is performed on events with an electron or muon, missing transverse momentum and exactly two $b$-tagged jets in the final state. A discriminant based on matrix element calculations is used to separate single-top-quark s-channel events from the main background contributions, which are top-quark pair production and $W$-boson production in association with jets. The observed (expected) signal significance over the background-only hypothesis is 3.3 (3.9) standard deviations, and the measured cross-section is $\sigma=8.2^{+3.5}_{-2.9}$ pb, consistent with the Standard Model prediction of $\sigma^{\mathrm{SM}}=10.32^{+0.40}_{-0.36}$ pb.
Result of the s-channel single-top cross-section measurement, in pb. The statistical and systematic uncertainties are given, as well as the total uncertainty. The normalisation factors for the $t\bar{t}$ and $W$+jets backgrounds are also shown, with their total uncertainties.
Distribution of ${E}_{T}^{miss}$ after the fit of the multijet backgrounds, in the electron channel, in the signal region, without applying the cut on ${E}_{T}^{miss}$. Simulated events are normalised to the expected number of events given the integrated luminosity, after applying the normalisation factors obtained in the multijet fit. The last bin includes the overflow. The uncertainty band indicates the simulation's statistical uncertainty, the normalisation uncertainties for different processes ($40$ % for $W$+jets production, $30$ % for multijet background and $6$ % for top-quark processes) and the multijet background shape uncertainty in each bin, summed in quadrature. The lower panel of the figure shows the ratio of the data to the prediction.
Distribution of ${E}_{T}^{miss}$ after the fit of the multijet backgrounds, in the electron channel, in the $W$+jets VR, without applying the cut on ${E}_{T}^{miss}$. Simulated events are normalised to the expected number of events given the integrated luminosity, after applying the normalisation factors obtained in the multijet fit. The last bin includes the overflow. The uncertainty band indicates the simulation's statistical uncertainty, the normalisation uncertainties for different processes ($40$ % for $W$+jets production, $30$ % for multijet background and $6$ % for top-quark processes) and the multijet background shape uncertainty in each bin, summed in quadrature. The lower panel of the figure shows the ratio of the data to the prediction.
This Letter reports the observation of single top quarks produced together with a photon, which directly probes the electroweak coupling of the top quark. The analysis uses 139 fb$^{-1}$ of 13 TeV proton-proton collision data collected with the ATLAS detector at the Large Hadron Collider. Requiring a photon with transverse momentum larger than 20 GeV and within the detector acceptance, the fiducial cross section is measured to be 688 $\pm$ 23 (stat.) $^{+75}_{-71}$ (syst.) fb, to be compared with the standard model prediction of 515 $^{+36}_{-42}$ fb at next-to-leading order in QCD.
This table shows the values for $\sigma_{tq\gamma}\times\mathcal{B}(t\rightarrow l\nu b)$ and $\sigma_{tq\gamma}\times\mathcal{B}(t\rightarrow l\nu b)+\sigma_{t(\rightarrow l\nu b\gamma)q}$ obtained by a profile-likelihood fit in the fiducial parton-level phase space (defined in Table 1) and particle-level phase space (defined in Table 2), respectively.
Distribution of the reconstructed top-quark mass in the $W\gamma\,$CR before the profile-likelihood fit. The "Total" column corresponds to the sum of the expected contributions from the signal and background processes. The uncertainty represents the sum of statistical and systematic uncertainties in the signal and background predictions. The first and last bins include the underflow and overflow, respectively.
Distribution of the NN output in the 0fj$\,$SR in data and the expected contribution of the signal and background processes after the profile-likelihood fit. The "Total" column corresponds to the sum of the expected contributions from the signal and background processes. The uncertainty represents the sum of statistical and systematic uncertainties in the signal and background predictions considering the correlations of the uncertainties as obtained by the fit.
A search is presented for direct top-squark pair production in final states with two leptons (electrons or muons) of opposite charge using 20.3fb-1 of pp collision data at sqrt(s)=8TeV, collected by the ATLAS experiment at the Large Hadron Collider in 2012. No excess over the Standard Model expectation is found. The results are interpreted under the separate assumptions (i) that the top squark decays to a b-quark in addition to an on-shell chargino whose decay occurs via a real or virtual W boson, or (ii) that the top squark decays to a t-quark and the lightest neutralino. A top squark with a mass between 150 GeV and 445 GeV decaying to a b-quark and an on-shell chargino is excluded at 95% confidence level for a top squark mass equal to the chargino mass plus 10 GeV, in the case of a 1 GeV lightest neutralino. Top squarks with masses between 215 (90) GeV and 530 (170) GeV decaying to an on-shell (off-shell) t-quark and a neutralino are excluded at 95% confidence level for a 1 GeV neutralino.
Distribution of mT2 for events passing all the signal candidate selection requirements, except that on mT2 of the L90 and L120 selections, for SF events.
Distribution of mT2 for events passing all the signal candidate selection requirements, except that on mT2 of the L90 and L120 selections, for DF events.
Distribution of mT2 for events passing all the signal candidate selection requirements, except that on mT2 of the L100 selection, for SF events.
The results of a search for direct pair production of the scalar partner to the top quark using an integrated luminosity of $20.1 \rm{fb}^{-1}$ of proton-proton collision data at $\sqrt{s}=8$ TeV recorded with the ATLAS detector at the LHC are reported. The top squark is assumed to decay via $\tilde{t} \rightarrow t \tilde{\chi}_{1}^{0}$ or $\tilde{t}\rightarrow b\tilde{\chi}_{1}^{\pm} \rightarrow b W^{\left(\ast\right)} \tilde{\chi}_{1}^{0}$, where $\tilde{\chi}_{1}^{0}$ ($\tilde{\chi}_{1}^{\pm}$) denotes the lightest neutralino (chargino) in supersymmetric models. The search targets a fully-hadronic final state in events with four or more jets and large missing transverse momentum. No significant excess over the Standard Model background prediction is observed, and exclusion limits are reported in terms of the top squark and neutralino masses and as a function of the branching fraction of $\tilde{t} \rightarrow t \tilde{\chi}_{1}^{0}$. For a branching fraction of 100%, top squark masses in the range 270-645 GeV are excluded for $\tilde{\chi}_{1}^{0}$ masses below 30 GeV. For a branching fraction of 50% to either $\tilde{t} \rightarrow t \tilde{\chi}_{1}^{0}$ or $\tilde{t}\rightarrow b\tilde{\chi}_{1}^{\pm}$, and assuming the $\tilde{\chi}_{1}^{\pm}$ mass to be twice the $\tilde{\chi}_{1}^{0}$ mass, top squark masses in the range 250-550 GeV are excluded for $\tilde{\chi}_{1}^{0}$ masses below 60 GeV.
Etmiss distribution for SRA1 and SRA2 after all selection requirements except those on Etmiss.
Etmiss distribution for SRA3 and SRA4 after all selection requirements except those on Etmiss.
Etmiss distribution for SRB after all selection requirements except those on Etmiss.
The results of a search for top squark (stop) pair production in final states with one isolated lepton, jets, and missing transverse momentum are reported. The analysis is performed with proton--proton collision data at $\sqrt{s} = 8$ TeV collected with the ATLAS detector at the LHC in 2012 corresponding to an integrated luminosity of $20$ fb$^{-1}$. The lightest supersymmetric particle (LSP) is taken to be the lightest neutralino which only interacts weakly and is assumed to be stable. The stop decay modes considered are those to a top quark and the LSP as well as to a bottom quark and the lightest chargino, where the chargino decays to the LSP by emitting a $W$ boson. A wide range of scenarios with different mass splittings between the stop, the lightest neutralino and the lightest chargino are considered, including cases where the $W$ bosons or the top quarks are off-shell. Decay modes involving the heavier charginos and neutralinos are addressed using a set of phenomenological models of supersymmetry. No significant excess over the Standard Model prediction is observed. A stop with a mass between $210$ and $640$ GeV decaying directly to a top quark and a massless LSP is excluded at $95$ % confidence level, and in models where the mass of the lightest chargino is twice that of the LSP, stops are excluded at $95$ % confidence level up to a mass of $500$ GeV for an LSP mass in the range of $100$ to $150$ GeV. Stringent exclusion limits are also derived for all other stop decay modes considered, and model-independent upper limits are set on the visible cross-section for processes beyond the Standard Model.
Expected and observed $H_{T,sig}^{miss}$ distribution for tN_med SR, before applying the $H_{T,sig}^{miss}>12$ requirement. The uncertainty includes statistical and all experimental systematic uncertainties. The last bin includes overflows.
Expected and observed large-R jet mass distribution for tN_boost SR, before applying the large-R jet mass$>75$ GeV requirement. The uncertainty includes statistical and all experimental systematic uncertainties. The last bin includes overflows.
Expected and observed b-jet multiplicity distribution for bCc_diag SR, before applying the b-jet multiplicity$=0$ requirement. The uncertainty includes statistical and all experimental systematic uncertainties. The last bin includes overflows.
A search for direct pair production of the supersymmetric partner of the top quark, decaying via a scalar tau to a nearly massless gravitino, has been performed using 20 fb$^{-1}$ of proton-proton collision data at $\sqrt{s}=8$ TeV. The data were collected by the ATLAS experiment at the LHC in 2012. Top squark candidates are searched for in events with either two hadronically decaying tau leptons, one hadronically decaying tau and one light lepton, or two light leptons. No significant excess over the Standard Model expectation is found. Exclusion limits at 95% confidence level are set as a function of the top squark and scalar tau masses. Depending on the scalar tau mass, ranging from the 87 GeV LEP limit to the top squark mass, lower limits between 490 GeV and 650 GeV are placed on the top squark mass within the model considered.
Distribution of $m_{\rm T}^{\rm sum}(\tau_{\mathrm{had}},\tau_{\mathrm{had}})$ for the events passing all the hadron-hadron signal region requirements, except that on the variable itself. The SM background process have been normalised using a fit to the data observed in CRs.
Distribution of $m_{{\rm T}2}(\tau_{\mathrm{had}},\tau_{\mathrm{had}})$ for the events passing all the hadron-hadron signal region requirements, except that on the variable itself. The SM background process have been normalised using a fit to the data observed in CRs.
Distribution of $m_{{\rm T}2}(b \ell, b \tau_{\mathrm{had}})$ for events passing all the lepton-hadron LM signal region requirements, except that on the variable itself. The SM background process have been normalised using a fit to the data observed in CRs.
A search is presented for direct top squark pair production using events with at least two leptons including a same-flavour opposite-sign pair with invariant mass consistent with the $Z$ boson mass, jets tagged as originating from $b$-quarks and missing transverse momentum. The analysis is performed with proton-proton collision data at $\sqrt{s}=8$ TeV collected with the ATLAS detector at the LHC in 2012 corresponding to an integrated luminosity of 20.3 fb$^{-1}$. No excess beyond the Standard Model expectation is observed. Interpretations of the results are provided in models based on the direct pair production of the heavier top squark state ($\tilde{t}_2$) followed by the decay to the lighter top squark state ($\tilde{t}_1$) via $\tilde{t}_2 \to Z\tilde{t}_1$, and for $\tilde{t}_1$ pair production in natural gauge-mediated supersymmetry breaking scenarios where the neutralino ($\tilde{\chi}^0_1$) is the next-to-lightest supersymmetric particle and decays producing a $Z$ boson and a gravitino ($\tilde{G}$) via the $\tilde{\chi}^0_1 \to Z\tilde{G}$ process.
PT(ll) distribution in SR2A for stop GMSB model (data+/-stat, bkg+/-(stat+syst), signal+/-stat), m(stop1)=500 GeV, m(neutralino1)=400 GeV.
njet_3l (data+/-stat, bkg+/-(stat+syst), signal+/-stat).
Missing ET distribution in SR2A for stop GMSB model (data+/-stat, bkg+/-(stat+syst), signal+/-stat), m(stop1)=500 GeV, m(neutralino1)=400 GeV.
A search for a massive $W'$ gauge boson decaying to a top quark and a bottom quark is performed with the ATLAS detector in $pp$ collisions at the LHC. The dataset was taken at a centre-of-mass energy of $\sqrt{s} = 8$ TeV and corresponds to 20.3 fb$^{-1}$ of integrated luminosity. This analysis is done in the hadronic decay mode of the top quark, where novel jet substructure techniques are used to identify jets from high-momentum top quarks. This allows for a search for high-mass $W'$ bosons in the range $1.5 - 3.0$ TeV. $b$-tagging is used to identify jets originating from $b$-quarks. The data are consistent with Standard Model background-only expectations, and upper limits at 95% confidence level are set on the $W' \rightarrow tb$ cross section times branching ratio ranging from $0.16$ pb to $0.33$ pb for left-handed $W'$ bosons, and ranging from $0.10$ pb to $0.21$ pb for $W'$ bosons with purely right-handed couplings. Upper limits at 95% confidence level are set on the $W'$-boson coupling to $tb$ as a function of the $W'$ mass using an effective field theory approach, which is independent of details of particular models predicting a $W'$ boson.
m_tb distributions in data in the one b-tag and the two b-tag category, together with background-only fits excluding the region 4-5 TeV which is beyond the range considered for this analysis. Potential WPRIME_L signal shapes in the hadronic top-quark decay channel with gPRIME = gSM are also given for resonance masses of 1.5, 2.0, 2.5 and 3.0 TeV.
Limits at 95% CL on the cross section times branching ratio to TOP BOTTOM for the left-handed and for the right-handed WPRIME model. The expected cross section for WPRIME production with gprime = gSM is also shown.
Observed and expected 95% CL limits on the ratio of coupling gWPRIME_L/gSM (gWPRIME_R/gSM) of the WPRIME_L (WPRIME_R) model as a function of the WPRIME mass.
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