AtlasPublic Web 
Internal TWiki
|
A Search for New High-Mass Phenomena Producing Top Quarks with the ATLAS Experiment ATLAS-CONF-2011-070 6 May 2011
|
||
| Content | Preview | |
|---|---|---|
| Main document (CDS record) - internal | pdf from CDS | |
| Figures | - | |
| Abstract | ||
| A search for top quark pair resonances in the lepton plus jets final states has been performed with the ATLAS experiment at the LHC. The search uses a data sample corresponding to an integrated luminosity of 33 pb$^{-1}$, and was recorded at a proton-proton centre-of-mass energy of 7 TeV. No evidence of a resonance is found. Using the reconstructed $t\bar{t}$ mass spectrum, limits are set on the production cross-section times branching ratio to $t\bar{t}$ for narrow $Z'$ models. The observed 95\% C.L. limits range from approximately 55 pb to 2.2 pb for masses going from $m=$ 500 GeV to $m=$ 1000 GeV. The analysis is also used to set limits on enhanced top quark production at high $t+X$ mass, using the production of quantum black holes to model the signal. In that context, enhanced $t+X$ production with a mass threshold below 2.35 TeV is excluded. | ||
| Figures | ||
|
Figure 01a: Missing $E_\textrm{T}$ (a) and $M_\textrm{T}(\textrm{electron},\met)$ (b) distributions for the electron channel after fitting the QCD background. No missing $E_\textrm{T}$ requirement is applied in the missing $E_\textrm{T}$ plot, and overflows are included in the last bin. In the left plot, the fractions $\beta$ denote the normalizations of the Standard Model backgrounds determined by the fit, and $f_{\textrm{QCD}}$ gives the fraction of the data due to the QCD background, discussed in the text. png (40kB) pdf (15kB) |
 |
|
|
Figure 01b: Missing $E_\textrm{T}$ (a) and $M_\textrm{T}(\textrm{electron},\met)$ (b) distributions for the electron channel after fitting the QCD background. No missing $E_\textrm{T}$ requirement is applied in the missing $E_\textrm{T}$ plot, and overflows are included in the last bin. In the left plot, the fractions $\beta$ denote the normalizations of the Standard Model backgrounds determined by the fit, and $f_{\textrm{QCD}}$ gives the fraction of the data due to the QCD background, discussed in the text. png (37kB) pdf (15kB) |
 |
|
|
Figure 02a: Correlation between the angular separation to the closest jet $dRmin$ and jet mass $M_j$ for jets matched (a) and not matched (b) to top quark decay products. Jets to the right of the black line are rejected. The absolute color scale is the same in both plots. png (64kB) |
 |
|
|
Figure 02b: Correlation between the angular separation to the closest jet $dRmin$ and jet mass $M_j$ for jets matched (a) and not matched (b) to top quark decay products. Jets to the right of the black line are rejected. The absolute color scale is the same in both plots. png (70kB) |
 |
|
|
Figure 03a: Reconstructed $t\bar{t}$ pair invariant mass (a) and its resolution (b) for three $Z'$ boson masses: $m_{Z'} =$ 500 GeV, $m_{Z'} =$ 700 GeV and $m_{Z'} =$ 1000 GeV. Reconstructed invariant mass distributions for QBH events (c) with mass thresholds at 1500 and 2250 GeV. The last bin contains all events with reconstructed mass larger than 3 TeV. png (35kB) pdf (14kB) |
 |
|
|
Figure 03b: Reconstructed $t\bar{t}$ pair invariant mass (a) and its resolution (b) for three $Z'$ boson masses: $m_{Z'} =$ 500 GeV, $m_{Z'} =$ 700 GeV and $m_{Z'} =$ 1000 GeV. Reconstructed invariant mass distributions for QBH events (c) with mass thresholds at 1500 and 2250 GeV. The last bin contains all events with reconstructed mass larger than 3 TeV. png (37kB) pdf (14kB) |
 |
|
|
Figure 03c: Reconstructed $t\bar{t}$ pair invariant mass (a) and its resolution (b) for three $Z'$ boson masses: $m_{Z'} =$ 500 GeV, $m_{Z'} =$ 700 GeV and $m_{Z'} =$ 1000 GeV. Reconstructed invariant mass distributions for QBH events (c) with mass thresholds at 1500 and 2250 GeV. The last bin contains all events with reconstructed mass larger than 3 TeV. png (38kB) pdf (13kB) |
 |
|
|
Figure 04a: Leading jet (a) and charged lepton (b) \pt\ distributions after all cuts. The electron and muon channels have been added together and all events beyond the range of the histogram have been added to the last bin. png (43kB) pdf (17kB) |
 |
|
|
Figure 04b: Leading jet (a) and charged lepton (b) \pt\ distributions after all cuts. The electron and muon channels have been added together and all events beyond the range of the histogram have been added to the last bin. png (39kB) pdf (15kB) |
 |
|
|
Figure 05a: Reconstructed $t\bar{t}$ mass in linear (a) and logarithmic (b) scale using the $dRmin$ algorithm after all cuts. The electron and muon channels have been added together and all events beyond the range of the histogram have been added to the last bin. png (43kB) pdf (16kB) |
 |
|
|
Figure 05b: Reconstructed $t\bar{t}$ mass in linear (a) and logarithmic (b) scale using the $dRmin$ algorithm after all cuts. The electron and muon channels have been added together and all events beyond the range of the histogram have been added to the last bin. png (45kB) pdf (16kB) |
 |
|
|
Figure 06: Event display for a high-mass event ($m_{t\bar{t}} = 714$ GeV.) The main panel on the left shows the $r-\phi$ view (i.e. looking along the beam axis), and the top right panel is the $\eta-\phi$ view. The top quark boosts lead the decay products to be collimated, albeit still distinguishable using standard reconstruction algorithms. png (343kB) |
 |
|
|
Figure 07a: Impact of the jet energy scale uncertainty on the shape of the Standard Model $t\bar{t}$ mass distribution using the $dRmin$ method for the $t\bar{t}$ and single top backgrounds (a), and a $Z'$ signal with $m_{Z'} = 500$ GeV (b). The bottom panels show the ratio of the upward (red line) and downward (blue line) JES variations to the nominal value, and the shaded region shows the statistical uncertainty on the nominal contribution. png (65kB) pdf (18kB) |
 |
|
|
Figure 07b: Impact of the jet energy scale uncertainty on the shape of the Standard Model $t\bar{t}$ mass distribution using the $dRmin$ method for the $t\bar{t}$ and single top backgrounds (a), and a $Z'$ signal with $m_{Z'} = 500$ GeV (b). The bottom panels show the ratio of the upward (red line) and downward (blue line) JES variations to the nominal value, and the shaded region shows the statistical uncertainty on the nominal contribution. png (58kB) pdf (17kB) |
 |
|
|
Figure 08a: Expected (dashed line) and observed (black points connected by a line) lower limits on $\sigma \times$ BR($Z' \to t \bar{t}$) (top) and the production of quantum black holes (bottom) including statistical uncertainties only (left) and statistical and systematic uncertainties (right), using the $dRmin$ mass reconstruction method. The blue and yellow bands show the range in which the limit is expected to lie in 68\% and 95\% of experiments, respectively, and the red points correspond to the predicted cross-sections in the topcolor (top) and quantum black hole models (bottom). The error bars on the topcolor cross-section curve represent the effect of the PDF uncertainty on the prediction. png (41kB) pdf (16kB) |
 |
|
|
Figure 08b: Expected (dashed line) and observed (black points connected by a line) lower limits on $\sigma \times$ BR($Z' \to t \bar{t}$) (top) and the production of quantum black holes (bottom) including statistical uncertainties only (left) and statistical and systematic uncertainties (right), using the $dRmin$ mass reconstruction method. The blue and yellow bands show the range in which the limit is expected to lie in 68\% and 95\% of experiments, respectively, and the red points correspond to the predicted cross-sections in the topcolor (top) and quantum black hole models (bottom). The error bars on the topcolor cross-section curve represent the effect of the PDF uncertainty on the prediction. png (44kB) pdf (16kB) |
 |
|
|
Figure 08c: Expected (dashed line) and observed (black points connected by a line) lower limits on $\sigma \times$ BR($Z' \to t \bar{t}$) (top) and the production of quantum black holes (bottom) including statistical uncertainties only (left) and statistical and systematic uncertainties (right), using the $dRmin$ mass reconstruction method. The blue and yellow bands show the range in which the limit is expected to lie in 68\% and 95\% of experiments, respectively, and the red points correspond to the predicted cross-sections in the topcolor (top) and quantum black hole models (bottom). The error bars on the topcolor cross-section curve represent the effect of the PDF uncertainty on the prediction. png (39kB) pdf (14kB) |
 |
|
|
Figure 08d: Expected (dashed line) and observed (black points connected by a line) lower limits on $\sigma \times$ BR($Z' \to t \bar{t}$) (top) and the production of quantum black holes (bottom) including statistical uncertainties only (left) and statistical and systematic uncertainties (right), using the $dRmin$ mass reconstruction method. The blue and yellow bands show the range in which the limit is expected to lie in 68\% and 95\% of experiments, respectively, and the red points correspond to the predicted cross-sections in the topcolor (top) and quantum black hole models (bottom). The error bars on the topcolor cross-section curve represent the effect of the PDF uncertainty on the prediction. png (40kB) pdf (14kB) |
 |
|
|
Figure 09a: Reconstructed $t\bar{t}$ pair invariant mass (a) and its resolution (b) using the ``four hardest jets'' algorithm for three $Z'$ boson masses: $m_{Z'} =$ 500 GeV, $m_{Z'} =$ 700 GeV and $m_{Z'} =$ 1000 GeV. png (35kB) pdf (14kB) |
 |
|
|
Figure 09b: Reconstructed $t\bar{t}$ pair invariant mass (a) and its resolution (b) using the ``four hardest jets'' algorithm for three $Z'$ boson masses: $m_{Z'} =$ 500 GeV, $m_{Z'} =$ 700 GeV and $m_{Z'} =$ 1000 GeV. png (38kB) pdf (14kB) |
 |
|
|
Figure 10: Reconstructed invariant mass distributions for QBH events with mass thresholds at 1500 and 2250 GeV using the ``four hardest jets'' algorithm. The last bin contains all events with reconstructed mass larger than 3 TeV. png (38kB) pdf (13kB) |
 |
|
|
Figure 11a: econstructed $t\bar{t}$ mass in linear (a) and logarithmic (b) scale using the ``four hardest jets'' algorithm after all cuts. The electron and muon channels have been added together. png (43kB) pdf (16kB) |
 |
|
|
Figure 11b: econstructed $t\bar{t}$ mass in linear (a) and logarithmic (b) scale using the ``four hardest jets'' algorithm after all cuts. The electron and muon channels have been added together. png (45kB) pdf (16kB) |
 |
|
|
Figure 12a: Expected (dashed line) and observed (black points connected by a line) lower limits on $\sigma \times$ BR($Z' \to t \bar{t}$) (top) and the production of quantum black holes (bottom) including statistical uncertainties only (left) and statistical and systematic uncertainties (right), using the ``four hardest jets'' mass reconstruction method. The blue and yellow bands show the range in which the limit is expected to lie in 68\% and 95\% of experiments, respectively, and the red points correspond to the predicted cross-sections in the topcolor (top) and quantum black hole models (bottom). The error bars on the topcolor cross-section curve represent the effect of the PDF uncertainty on the prediction. png (41kB) pdf (16kB) |
 |
|
|
Figure 12b: Expected (dashed line) and observed (black points connected by a line) lower limits on $\sigma \times$ BR($Z' \to t \bar{t}$) (top) and the production of quantum black holes (bottom) including statistical uncertainties only (left) and statistical and systematic uncertainties (right), using the ``four hardest jets'' mass reconstruction method. The blue and yellow bands show the range in which the limit is expected to lie in 68\% and 95\% of experiments, respectively, and the red points correspond to the predicted cross-sections in the topcolor (top) and quantum black hole models (bottom). The error bars on the topcolor cross-section curve represent the effect of the PDF uncertainty on the prediction. png (43kB) pdf (16kB) |
 |
|
|
Figure 12c: Expected (dashed line) and observed (black points connected by a line) lower limits on $\sigma \times$ BR($Z' \to t \bar{t}$) (top) and the production of quantum black holes (bottom) including statistical uncertainties only (left) and statistical and systematic uncertainties (right), using the ``four hardest jets'' mass reconstruction method. The blue and yellow bands show the range in which the limit is expected to lie in 68\% and 95\% of experiments, respectively, and the red points correspond to the predicted cross-sections in the topcolor (top) and quantum black hole models (bottom). The error bars on the topcolor cross-section curve represent the effect of the PDF uncertainty on the prediction. png (41kB) pdf (14kB) |
 |
|
|
Figure 12d: Expected (dashed line) and observed (black points connected by a line) lower limits on $\sigma \times$ BR($Z' \to t \bar{t}$) (top) and the production of quantum black holes (bottom) including statistical uncertainties only (left) and statistical and systematic uncertainties (right), using the ``four hardest jets'' mass reconstruction method. The blue and yellow bands show the range in which the limit is expected to lie in 68\% and 95\% of experiments, respectively, and the red points correspond to the predicted cross-sections in the topcolor (top) and quantum black hole models (bottom). The error bars on the topcolor cross-section curve represent the effect of the PDF uncertainty on the prediction. png (41kB) pdf (14kB) |
 |
|
