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CMS-PAS-BTV-15-002
Identification of double-b quark jets in boosted event topologies
Abstract: Searches for new physics at the LHC necessitate to distinguish the merged decay products of resonances produced with high transverse momentum, from jets that originate from single partons. We present an algorithm that aims to reconstruct the $\mathrm{b\bar{b}}$ decay mode of such resonances. The algorithm is applicable to any resonance with the mass close to the W/Z/H boson mass and with high enough energy for its decay products to be clustered in a single jet within a cone of size $R= $ 0.8 . An example is the 125 GeV Higgs boson which is the focus of this document. The efficiency and the mistag rate from top quark jets have been measured using the 2.6 fb$^{-1}$ dataset collected with the CMS experiment at 13 TeV in 2015.
Figures & Tables Summary References CMS Publications
Figures

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Figure 1:
Schematic comparison of the fat jet and subjet b tagging approaches and the presented double-b tagger.

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Figure 2-a:
Distributions of 2D IP significance for the most displaced track raising the SV invariant mass above the bottom quark threshold, number of secondary vertices associated to the AK8 jet, the vertex energy ratio for SV$_0$, and the $z$ variable. Comparison between H$\rightarrow \mathrm {b\bar {b}}$ jets from simulated samples of KK-Graviton decaying to HH and QCD jets containing zero, one or two b quarks are used. AK8 jets are selected with $ {p_{\mathrm {T}}} > $ 300 GeV and pruned jet mass 70 $ < m < $ 200 GeV. The distributions are normalized to unit area.

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Figure 2-b:
Distributions of 2D IP significance for the most displaced track raising the SV invariant mass above the bottom quark threshold, number of secondary vertices associated to the AK8 jet, the vertex energy ratio for SV$_0$, and the $z$ variable. Comparison between H$\rightarrow \mathrm {b\bar {b}}$ jets from simulated samples of KK-Graviton decaying to HH and QCD jets containing zero, one or two b quarks are used. AK8 jets are selected with $ {p_{\mathrm {T}}} > $ 300 GeV and pruned jet mass 70 $ < m < $ 200 GeV. The distributions are normalized to unit area.

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Figure 2-c:
Distributions of 2D IP significance for the most displaced track raising the SV invariant mass above the bottom quark threshold, number of secondary vertices associated to the AK8 jet, the vertex energy ratio for SV$_0$, and the $z$ variable. Comparison between H$\rightarrow \mathrm {b\bar {b}}$ jets from simulated samples of KK-Graviton decaying to HH and QCD jets containing zero, one or two b quarks are used. AK8 jets are selected with $ {p_{\mathrm {T}}} > $ 300 GeV and pruned jet mass 70 $ < m < $ 200 GeV. The distributions are normalized to unit area.

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Figure 2-d:
Distributions of 2D IP significance for the most displaced track raising the SV invariant mass above the bottom quark threshold, number of secondary vertices associated to the AK8 jet, the vertex energy ratio for SV$_0$, and the $z$ variable. Comparison between H$\rightarrow \mathrm {b\bar {b}}$ jets from simulated samples of KK-Graviton decaying to HH and QCD jets containing zero, one or two b quarks are used. AK8 jets are selected with $ {p_{\mathrm {T}}} > $ 300 GeV and pruned jet mass 70 $ < m < $ 200 GeV. The distributions are normalized to unit area.

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Figure 3-a:
Comparison of the performance of the double-b tagger, the minimum CSVv2 value among the two subjets b tag scores, and fat jet b tag which exploits CSVv2 algorithm. The tagging efficiency for signal is evaluated using boosted H$\rightarrow {\mathrm{ b \bar{b} } } $ jets from simulation. The mistag rate is evaluated for simulated QCD jets containing zero, one or two b quarks. \textbf {a} for all jets with 300 $< {p_{\mathrm {T}}} <$ 500 GeV, \textbf {b} for 500 $< {p_{\mathrm {T}}} <$ 800 GeV, \textbf {c} for ${p_{\mathrm {T}}} >$ 800 GeV. \textbf {d} shows for ${p_{\mathrm {T}}} >$ 800 GeV the mistag rate evaluated for $g\rightarrow {\mathrm{ b \bar{b} } } $.

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Figure 3-b:
Comparison of the performance of the double-b tagger, the minimum CSVv2 value among the two subjets b tag scores, and fat jet b tag which exploits CSVv2 algorithm. The tagging efficiency for signal is evaluated using boosted H$\rightarrow {\mathrm{ b \bar{b} } } $ jets from simulation. The mistag rate is evaluated for simulated QCD jets containing zero, one or two b quarks. \textbf {a} for all jets with 300 $< {p_{\mathrm {T}}} <$ 500 GeV, \textbf {b} for 500 $< {p_{\mathrm {T}}} <$ 800 GeV, \textbf {c} for ${p_{\mathrm {T}}} >$ 800 GeV. \textbf {d} shows for ${p_{\mathrm {T}}} >$ 800 GeV the mistag rate evaluated for $g\rightarrow {\mathrm{ b \bar{b} } } $.

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Figure 3-c:
Comparison of the performance of the double-b tagger, the minimum CSVv2 value among the two subjets b tag scores, and fat jet b tag which exploits CSVv2 algorithm. The tagging efficiency for signal is evaluated using boosted H$\rightarrow {\mathrm{ b \bar{b} } } $ jets from simulation. The mistag rate is evaluated for simulated QCD jets containing zero, one or two b quarks. \textbf {a} for all jets with 300 $< {p_{\mathrm {T}}} <$ 500 GeV, \textbf {b} for 500 $< {p_{\mathrm {T}}} <$ 800 GeV, \textbf {c} for ${p_{\mathrm {T}}} >$ 800 GeV. \textbf {d} shows for ${p_{\mathrm {T}}} >$ 800 GeV the mistag rate evaluated for $g\rightarrow {\mathrm{ b \bar{b} } } $.

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Figure 3-d:
Comparison of the performance of the double-b tagger, the minimum CSVv2 value among the two subjets b tag scores, and fat jet b tag which exploits CSVv2 algorithm. The tagging efficiency for signal is evaluated using boosted H$\rightarrow {\mathrm{ b \bar{b} } } $ jets from simulation. The mistag rate is evaluated for simulated QCD jets containing zero, one or two b quarks. \textbf {a} for all jets with 300 $< {p_{\mathrm {T}}} <$ 500 GeV, \textbf {b} for 500 $< {p_{\mathrm {T}}} <$ 800 GeV, \textbf {c} for ${p_{\mathrm {T}}} >$ 800 GeV. \textbf {d} shows for ${p_{\mathrm {T}}} >$ 800 GeV the mistag rate evaluated for $g\rightarrow {\mathrm{ b \bar{b} } } $.

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Figure 4-a:
Signal efficiency (a) and mistag rate (b) distribution with respect to jet ${p_{\mathrm {T}}}$ after a selection on the double-b tagger for Loose, Medium and Tight operating points. Simulated H$\rightarrow {\mathrm{ b \bar{b} } } $ jets from KK-Graviton decaying to HH (a) and QCD jets containing zero, one or two b quarks (b) are used. AK8 jets are selected with $ {p_{\mathrm {T}}} > $ 300 GeV and pruned jet mass 70 $ < m < $ 200 GeV.

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Figure 4-b:
Signal efficiency (a) and mistag rate (b) distribution with respect to jet ${p_{\mathrm {T}}}$ after a selection on the double-b tagger for Loose, Medium and Tight operating points. Simulated H$\rightarrow {\mathrm{ b \bar{b} } } $ jets from KK-Graviton decaying to HH (a) and QCD jets containing zero, one or two b quarks (b) are used. AK8 jets are selected with $ {p_{\mathrm {T}}} > $ 300 GeV and pruned jet mass 70 $ < m < $ 200 GeV.

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Figure 5-a:
Distributions of 2D IP significance for the most displaced track raising the SV invariant mass above the b quark threshold, number of secondary vertices associated to the AK8 jet, the vertex energy ratio for SV$_0$, and the $z$ variable. Data and simulated events are shown for the double-muon tagged jets selection. Simulated events are normalized to the yield observed in data, the overflow is in last bin. The bottom panel in each figure shows the ratio of the number of events observed in data to that of the MC prediction.

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Figure 5-b:
Distributions of 2D IP significance for the most displaced track raising the SV invariant mass above the b quark threshold, number of secondary vertices associated to the AK8 jet, the vertex energy ratio for SV$_0$, and the $z$ variable. Data and simulated events are shown for the double-muon tagged jets selection. Simulated events are normalized to the yield observed in data, the overflow is in last bin. The bottom panel in each figure shows the ratio of the number of events observed in data to that of the MC prediction.

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Figure 5-c:
Distributions of 2D IP significance for the most displaced track raising the SV invariant mass above the b quark threshold, number of secondary vertices associated to the AK8 jet, the vertex energy ratio for SV$_0$, and the $z$ variable. Data and simulated events are shown for the double-muon tagged jets selection. Simulated events are normalized to the yield observed in data, the overflow is in last bin. The bottom panel in each figure shows the ratio of the number of events observed in data to that of the MC prediction.

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Figure 5-d:
Distributions of 2D IP significance for the most displaced track raising the SV invariant mass above the b quark threshold, number of secondary vertices associated to the AK8 jet, the vertex energy ratio for SV$_0$, and the $z$ variable. Data and simulated events are shown for the double-muon tagged jets selection. Simulated events are normalized to the yield observed in data, the overflow is in last bin. The bottom panel in each figure shows the ratio of the number of events observed in data to that of the MC prediction.

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Figure 6:
Double-b tagger discriminant distribution in data and simulated samples for the double-muon tagged jets selection. Simulated events are normalized to the yield observed in data. The loose, medium and tight operating points are also reported. The bottom panel shows the ratio of the number of events observed in data to that of the MC prediction.

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Figure 7-a:
Comparison of the JP discriminant distribution for the data and the sum of the fitted templates for all selected jets (a) and those jets passing the loose double-b tagger requirement (b) with ${p_{\mathrm {T}}}$ between 500 and 600 GeV. The shaded area represents the statistical and systematic (refer to the text for details) uncertainties on MC templates. Double-muon tagged AK8 jets are used for this measurement. The overflow is included in the last bin.

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Figure 7-b:
Comparison of the JP discriminant distribution for the data and the sum of the fitted templates for all selected jets (a) and those jets passing the loose double-b tagger requirement (b) with ${p_{\mathrm {T}}}$ between 500 and 600 GeV. The shaded area represents the statistical and systematic (refer to the text for details) uncertainties on MC templates. Double-muon tagged AK8 jets are used for this measurement. The overflow is included in the last bin.

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Figure 8-a:
Data/MC efficiency ratio (SF) for loose, medium and tight double-b tagger requirement obtained with single and double-muon tagged selections. Central values of scale factors are artificially shifted along the x-axis for better visibility.

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Figure 8-b:
Data/MC efficiency ratio (SF) for loose, medium and tight double-b tagger requirement obtained with single and double-muon tagged selections. Central values of scale factors are artificially shifted along the x-axis for better visibility.

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Figure 8-c:
Data/MC efficiency ratio (SF) for loose, medium and tight double-b tagger requirement obtained with single and double-muon tagged selections. Central values of scale factors are artificially shifted along the x-axis for better visibility.

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Figure 9:
Double-b tagger discriminant distribution for the jet associated to the boosted top quark hadronic decay in selected semi-leptonic $ {\mathrm{ t \bar{t} } } $ events. Simulated events are normalized to the yield observed in data. The bottom panel shows the ratio of the number of events observed in data to that of the MC prediction.

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Figure 10-a:
Comparison of the performance of the double-b tagger, the minimum CSVv2 value among the two subjets b tag scores, and fat jet b tag which exploits CSVv2 algorithm. The tagging efficiency for signal is evaluated using boosted H$\rightarrow {\mathrm{ b \bar{b} } } $ jets from simulation. The mistag rate is evaluated for simulated QCD jets containing zero b quark. \textbf {a} for all jets with 300 $< {p_{\mathrm {T}}} <$ 500 GeV, \textbf {b} for 500 $< {p_{\mathrm {T}}} <$ 800 GeV, \textbf {c} for ${p_{\mathrm {T}}} >$ 800 GeV.

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Figure 10-b:
Comparison of the performance of the double-b tagger, the minimum CSVv2 value among the two subjets b tag scores, and fat jet b tag which exploits CSVv2 algorithm. The tagging efficiency for signal is evaluated using boosted H$\rightarrow {\mathrm{ b \bar{b} } } $ jets from simulation. The mistag rate is evaluated for simulated QCD jets containing zero b quark. \textbf {a} for all jets with 300 $< {p_{\mathrm {T}}} <$ 500 GeV, \textbf {b} for 500 $< {p_{\mathrm {T}}} <$ 800 GeV, \textbf {c} for ${p_{\mathrm {T}}} >$ 800 GeV.

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Figure 10-c:
Comparison of the performance of the double-b tagger, the minimum CSVv2 value among the two subjets b tag scores, and fat jet b tag which exploits CSVv2 algorithm. The tagging efficiency for signal is evaluated using boosted H$\rightarrow {\mathrm{ b \bar{b} } } $ jets from simulation. The mistag rate is evaluated for simulated QCD jets containing zero b quark. \textbf {a} for all jets with 300 $< {p_{\mathrm {T}}} <$ 500 GeV, \textbf {b} for 500 $< {p_{\mathrm {T}}} <$ 800 GeV, \textbf {c} for ${p_{\mathrm {T}}} >$ 800 GeV.

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Figure 11-a:
Comparison of the performance of the double-b tagger, the minimum CSVv2 value among the two subjets b tag scores, and fat jet b tag which exploits CSVv2 algorithm. The tagging efficiency for signal is evaluated using boosted H$\rightarrow {\mathrm{ b \bar{b} } } $ jets from simulation. The mistag rate is evaluated for simulated QCD jets containing two b quarks. \textbf {a} for all jets with 300 $< {p_{\mathrm {T}}} <$ 500 GeV, \textbf {b} for 500 $< {p_{\mathrm {T}}} <$ 800 GeV.

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Figure 11-b:
Comparison of the performance of the double-b tagger, the minimum CSVv2 value among the two subjets b tag scores, and fat jet b tag which exploits CSVv2 algorithm. The tagging efficiency for signal is evaluated using boosted H$\rightarrow {\mathrm{ b \bar{b} } } $ jets from simulation. The mistag rate is evaluated for simulated QCD jets containing two b quarks. \textbf {a} for all jets with 300 $< {p_{\mathrm {T}}} <$ 500 GeV, \textbf {b} for 500 $< {p_{\mathrm {T}}} <$ 800 GeV.

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Figure 12-a:
Comparison of the performance of the double-b tagger, the minimum CSVv2 value among the two subjets b tag scores, and fat jet b tag which exploits CSVv2 algorithm. The tagging efficiency for signal is evaluated using boosted H$\rightarrow {\mathrm{ b \bar{b} } } $ jets from simulation. The mistag rate is evaluated for simulated QCD jets containing one b quark. \textbf {a} for all jets with 300 $< {p_{\mathrm {T}}} <$ 500 GeV, \textbf {b} for 500 $< {p_{\mathrm {T}}} <$ 800 GeV, \textbf {c} for ${p_{\mathrm {T}}} >$ 800 GeV.

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Figure 12-b:
Comparison of the performance of the double-b tagger, the minimum CSVv2 value among the two subjets b tag scores, and fat jet b tag which exploits CSVv2 algorithm. The tagging efficiency for signal is evaluated using boosted H$\rightarrow {\mathrm{ b \bar{b} } } $ jets from simulation. The mistag rate is evaluated for simulated QCD jets containing one b quark. \textbf {a} for all jets with 300 $< {p_{\mathrm {T}}} <$ 500 GeV, \textbf {b} for 500 $< {p_{\mathrm {T}}} <$ 800 GeV, \textbf {c} for ${p_{\mathrm {T}}} >$ 800 GeV.

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Figure 12-c:
Comparison of the performance of the double-b tagger, the minimum CSVv2 value among the two subjets b tag scores, and fat jet b tag which exploits CSVv2 algorithm. The tagging efficiency for signal is evaluated using boosted H$\rightarrow {\mathrm{ b \bar{b} } } $ jets from simulation. The mistag rate is evaluated for simulated QCD jets containing one b quark. \textbf {a} for all jets with 300 $< {p_{\mathrm {T}}} <$ 500 GeV, \textbf {b} for 500 $< {p_{\mathrm {T}}} <$ 800 GeV, \textbf {c} for ${p_{\mathrm {T}}} >$ 800 GeV.

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Figure 13-a:
Distributions of 2D IP significance for the most displaced track raising the SV invariant mass above the b quark threshold, number of secondary vertices associated to the AK8 jet, the vertex energy ratio for SV$_0$, and the $z$ variable. Data and simulated events are shown for the single-muon tagged jets selection. Simulated events are normalized to the yield observed in data, the overflow is in last bin. The bottom panel in each figure shows the ratio of the number of events observed in data to that of the MC prediction.

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Figure 13-b:
Distributions of 2D IP significance for the most displaced track raising the SV invariant mass above the b quark threshold, number of secondary vertices associated to the AK8 jet, the vertex energy ratio for SV$_0$, and the $z$ variable. Data and simulated events are shown for the single-muon tagged jets selection. Simulated events are normalized to the yield observed in data, the overflow is in last bin. The bottom panel in each figure shows the ratio of the number of events observed in data to that of the MC prediction.

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Figure 13-c:
Distributions of 2D IP significance for the most displaced track raising the SV invariant mass above the b quark threshold, number of secondary vertices associated to the AK8 jet, the vertex energy ratio for SV$_0$, and the $z$ variable. Data and simulated events are shown for the single-muon tagged jets selection. Simulated events are normalized to the yield observed in data, the overflow is in last bin. The bottom panel in each figure shows the ratio of the number of events observed in data to that of the MC prediction.

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Figure 13-d:
Distributions of 2D IP significance for the most displaced track raising the SV invariant mass above the b quark threshold, number of secondary vertices associated to the AK8 jet, the vertex energy ratio for SV$_0$, and the $z$ variable. Data and simulated events are shown for the single-muon tagged jets selection. Simulated events are normalized to the yield observed in data, the overflow is in last bin. The bottom panel in each figure shows the ratio of the number of events observed in data to that of the MC prediction.

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Figure 14:
Double-b tagger discriminant distribution in data and simulated samples for the single-muon tagged jets selection. Simulated events are normalized to the yield observed in data. The loose, medium and tight operating points are also reported. The bottom panel shows the ratio of the number of events observed in data to that of the MC prediction.
Tables

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Table 1:
Loose double-b tag efficiency ($\epsilon $) and Data/MC efficiency ratio (SF). Uncertainties are both statistical and systematic for the SF and data efficiency, while for the MC efficiency only the statistical uncertainty is reported. Jets with ${p_{\mathrm {T}}} >$700 GeV are included in the last bin.

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Table 2:
Medium double-b tag efficiency ($\epsilon $) and Data/MC efficiency ratio (SF). Uncertainties are both statistical and systematic for the SF and data efficiency, while for the MC efficiency only the statistical uncertainty is reported. Jets with ${p_{\mathrm {T}}} >$ 700 GeV are included in the last bin.

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Table 3:
Tight double-b tag efficiency ($\epsilon $) and Data/MC efficiency ratio (SF). Uncertainties are both statistical and systematic for the SF and data efficiency, while for the MC efficiency only the statistical uncertainty is reported. Jets with ${p_{\mathrm {T}}} >$500 GeV are included in the last bin.

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Table 4:
Mistag scale factors from top quark jets for the three operating points of the double-b tagger and for different ${p_{\mathrm {T}}}$ ranges. The reported uncertainties are statistical only. For the ${p_{\mathrm {T}}} $-inclusive scale factor ($ {p_{\mathrm {T}}} >$ 300 GeV) both statistical and systematic uncertainties are reported. Jets with ${p_{\mathrm {T}}} >$ 700 (500) GeV are included in the last bin.
Summary
We have presented the ``double-b tagging" algorithm aiming at identifying the $\mathrm{b\bar{b}}$ decay mode of resonances produced with high transverse momentum and detected as single fat jets, and to distinguish them from jets initiated by single partons. An example is the Higgs boson which is the focus of this document but with general applicability to any resonance with the mass close to the W/Z/H mass and $p_\mathrm{T}$ above 300 GeV. We show that this new tagger outperforms the previous techniques to distinguish H jets from the QCD background. At the same signal efficiency, the mistag rate is lower by a factor of 2 compared to, for example, the subjet b tagging approach [4,DP-2014/031]. Given the different kinematic properties expected for a $\mathrm{ b \bar{b} }$ pair originating from the decay of a massive resonance compared to gluon splitting, the mistag rate for the gluon splitting background reduces from 60% to 50% for the loose operating point and from 20% to 10% for the tight operating point compared to the subjet approach. The efficiency and mistag rate from top quark jets have been measured in data and correction factors for simulated jets have been derived for three different operating points for jets with $p_\mathrm{T}$ between 300 and 700 GeV (500 GeV for the tight working point). The uncertainty on the scale factor measurements will be improved with the increased integrated luminosity in 2016, as well as a higher $p_\mathrm{T}$ range will be covered.
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