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| CMS-TOP-12-039 ; CERN-EP-2016-324 | ||
| Search for associated production of a Z boson with a single top quark and for tZ flavour-changing interactions in pp collisions at $ \sqrt{s} = $ 8 TeV | ||
| CMS Collaboration | ||
| 5 February 2017 | ||
| JHEP 07 (2017) 003 | ||
| Abstract: A search for the production of a single top quark in association with a Z boson is presented, both to identify the expected standard model process and to search for flavour-changing neutral current interactions. The data sample corresponds to an integrated luminosity of 19.7 fb$^{-1}$ recorded by the CMS experiment at the LHC in proton-proton collisions at $ \sqrt{s} = $ 8 TeV. Final states with three leptons (electrons or muons) and at least one jet are investigated. An events yield compatible with tZq standard model production is observed, and the corresponding cross section is measured to be $\sigma(\mathrm{ p }\mathrm{ p } \to \mathrm{ t } \mathrm{ Z } \mathrm{ q } \to \ell \nu \mathrm{ b } \ell^+ \ell^- \mathrm{ q }) = $ 10$^{+8}_{-7}$ fb with a significance of 2.4 standard deviations. No presence of flavour-changing neutral current production of tZq is observed. Exclusion limits at 95% confidence level on the branching fractions of a top quark decaying to a Z boson and an up or a charm quark are found to be $ { \cal{B} }( \mathrm{ t } \rightarrow \mathrm{ Z } \mathrm{u} ) < $ 0.022% and $ { \cal{B} }( \mathrm{ t } \rightarrow \mathrm{ Z c } ) < $ 0.049%. | ||
| Links: e-print arXiv:1702.01404 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Leading-order tZq production Feynman diagrams (all but bottom-right). The initial- and final-state quarks denoted $\mathrm{ q } $ and $\mathrm{ q } ^\prime $ are predominantly first generation quarks, although there are smaller additional contributions from strange- and charm-initiated diagrams. The bottom-right diagram represents the NLO nonresonant contribution to the tZq process. |
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Figure 1-a:
Leading-order tZq production Feynman diagram. The initial- and final-state quarks denoted $\mathrm{ q } $ and $\mathrm{ q } ^\prime $ are predominantly first generation quarks, although there are smaller additional contributions from strange- and charm-initiated diagrams. |
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Figure 1-b:
Leading-order tZq production Feynman diagram. The initial- and final-state quarks denoted $\mathrm{ q } $ and $\mathrm{ q } ^\prime $ are predominantly first generation quarks, although there are smaller additional contributions from strange- and charm-initiated diagrams. |
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Figure 1-c:
Leading-order tZq production Feynman diagram. The initial- and final-state quarks denoted $\mathrm{ q } $ and $\mathrm{ q } ^\prime $ are predominantly first generation quarks, although there are smaller additional contributions from strange- and charm-initiated diagrams. |
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Figure 1-d:
Leading-order tZq production Feynman diagram. The initial- and final-state quarks denoted $\mathrm{ q } $ and $\mathrm{ q } ^\prime $ are predominantly first generation quarks, although there are smaller additional contributions from strange- and charm-initiated diagrams. |
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Figure 1-e:
Leading-order tZq production Feynman diagram. The initial- and final-state quarks denoted $\mathrm{ q } $ and $\mathrm{ q } ^\prime $ are predominantly first generation quarks, although there are smaller additional contributions from strange- and charm-initiated diagrams. |
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Figure 1-f:
Diagram representing the NLO nonresonant contribution to the tZq process. |
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Figure 2:
Feynman diagrams for the production of tZ in tZ-FCNC channels. |
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Figure 2-a:
Feynman diagram for the production of tZ in a tZ-FCNC channel. |
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Figure 2-b:
Feynman diagram for the production of tZ in a tZ-FCNC channel. |
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Figure 2-c:
Feynman diagram for the production of tZ in a tZ-FCNC channel. |
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Figure 2-d:
Feynman diagram for the production of tZ in a tZ-FCNC channel. |
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Figure 3:
Feynman diagram for the production of tZq in the ${\mathrm{ t } \mathrm{ \bar{t} } } $-FCNC channel. |
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Figure 4:
Data-to-prediction comparisons after performing the fit for ${m_{\mathrm {T}}^{\mathrm {W}}}$ distribution in the control region (left) and for the BDT$_{\mathrm {tZq}\text {-SM}}$ responses in the signal region (right). The four lepton channels are combined. The lower panels show the ratio between observed and predicted yields, including the total uncertainty on the prediction. |
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Figure 4-a:
Data-to-prediction comparison after performing the fit for ${m_{\mathrm {T}}^{\mathrm {W}}}$ distribution in the control region. The four lepton channels are combined. The lower panel shows the ratio between observed and predicted yields, including the total uncertainty on the prediction. |
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Figure 4-b:
Data-to-prediction comparison after performing the fit for the BDT$_{\mathrm {tZq}\text {-SM}}$ responses in the signal region. The four lepton channels are combined. The lower panel shows the ratio between observed and predicted yields, including the total uncertainty on the prediction. |
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Figure 5:
Data-to-prediction comparisons for the tZ-FCNC search after performing the fit for ${m_{\mathrm {T}}^{\mathrm {W}}}$ distribution in the control region (top-left), and for the BDT responses in the single top quark (BDT$_{\text {tZ-FCNC}}$) (top-right), and $ {\mathrm{ t } {}\mathrm{ \bar{t} } }$ (BDT$_{\mathrm{ t \bar{t} }\text{-FCNC}} $) (bottom), signal regions. An example of the predicted signal contribution for a value ${ \cal {B} }(\mathrm{ t } \rightarrow \mathrm{ Z } \mathrm{ u } ) = $ 0.1% (FCNC) is shown for illustration. The four channels are combined. The lower panels show the ratio between observed and predicted yields, including the total uncertainty on the prediction. |
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Figure 5-a:
Data-to-prediction comparison for the tZ-FCNC search after performing the fit for ${m_{\mathrm {T}}^{\mathrm {W}}}$ distribution in the control region. An example of the predicted signal contribution for a value ${ \cal {B} }(\mathrm{ t } \rightarrow \mathrm{ Z } \mathrm{ u } ) = $ 0.1% (FCNC) is shown for illustration. The four channels are combined. The lower panel shows the ratio between observed and predicted yields, including the total uncertainty on the prediction. |
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Figure 5-b:
Data-to-prediction comparison for the tZ-FCNC search after performing the fit for the BDT response in the single top quark (BDT$_{\text {tZ-FCNC}}$) signal region. An example of the predicted signal contribution for a value ${ \cal {B} }(\mathrm{ t } \rightarrow \mathrm{ Z } \mathrm{ u } ) = $ 0.1% (FCNC) is shown for illustration. The four channels are combined. The lower panel shows the ratio between observed and predicted yields, including the total uncertainty on the prediction. |
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Figure 5-c:
Data-to-prediction comparison for the tZ-FCNC search after performing the fit for the BDT response in the $ {\mathrm{ t } {}\mathrm{ \bar{t} } }$ (BDT$_{\mathrm{ t \bar{t} }\text{-FCNC}} $) signal region. An example of the predicted signal contribution for a value ${ \cal {B} }(\mathrm{ t } \rightarrow \mathrm{ Z } \mathrm{ u } ) = $ 0.1% (FCNC) is shown for illustration. The four channels are combined. The lower panel shows the ratio between observed and predicted yields, including the total uncertainty on the prediction. |
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Figure 6:
Data-to-prediction comparisons in the background-enriched samples, after applying background normalisation scaling factors as described in the text, of the ${p_{\mathrm {T}}}$ of the lepton from the W boson (top-left), ${ {p_{\mathrm {T}}} ^\text {miss}} $ (top-right), and $m _{\ell \ell }$ (bottom). The four channels are combined. The lower panels show the ratio between observed and predicted yields, including the total uncertainty on the prediction. The distributions shown here are for the tZ-FCNC search, where WZ + h.f. denotes WZ + heavy flavour. |
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Figure 6-a:
Data-to-prediction comparison in the background-enriched samples, after applying background normalisation scaling factors as described in the text, of the ${p_{\mathrm {T}}}$ of the lepton from the W boson. The four channels are combined. The lower panel shows the ratio between observed and predicted yields, including the total uncertainty on the prediction. The distribution shown here are for the tZ-FCNC search, where WZ + h.f. denotes WZ + heavy flavour. |
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Figure 6-b:
Data-to-prediction comparison in the background-enriched samples, after applying background normalisation scaling factors as described in the text, of ${ {p_{\mathrm {T}}} ^\text {miss}} $. The four channels are combined. The lower panel shows the ratio between observed and predicted yields, including the total uncertainty on the prediction. The distribution shown here are for the tZ-FCNC search, where WZ + h.f. denotes WZ + heavy flavour. |
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Figure 6-c:
Data-to-prediction comparison in the background-enriched samples, after applying background normalisation scaling factors as described in the text, of $m _{\ell \ell }$. The four channels are combined. The lower panel shows the ratio between observed and predicted yields, including the total uncertainty on the prediction. The distribution shown here are for the tZ-FCNC search, where WZ + h.f. denotes WZ + heavy flavour. |
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Figure 7:
Data-to-prediction comparisons after performing the fit for the $|\eta |$ distribution of the recoiling jet in the control region (left), and the signal region (right). The four lepton channels are combined. The lower panels show the ratio between observed and predicted yields, including the total uncertainty on the prediction. |
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Figure 7-a:
Data-to-prediction comparison after performing the fit for the $|\eta |$ distribution of the recoiling jet in the control region. The four lepton channels are combined. The lower panel shows the ratio between observed and predicted yields, including the total uncertainty on the prediction. |
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Figure 7-b:
Data-to-prediction comparison after performing the fit for the $|\eta |$ distribution of the recoiling jet in the signal region. The four lepton channels are combined. The lower panel shows the ratio between observed and predicted yields, including the total uncertainty on the prediction. |
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Figure 8:
The expected and observed exclusion limits at 95% CL on ${ \cal {B} } (\mathrm{ t } \rightarrow \mathrm{ Z } \mathrm{c} )$ as a function of the limits on $ {\cal {B}}( \mathrm{ t } \rightarrow \mathrm{ Z } \mathrm{ u } )$. The expected 68% CL is also shown. |
| Tables | |
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Table 1:
The event selections for the signal and control regions for the SM and FCNC analyses. |
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Table 2:
The number of events remaining for each process, after all selections have been applied, in the control and signal regions for the tZq-SM shape analysis. WZ+h.f. denotes WZ + heavy flavour. |
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Table 3:
The systematic sources, variation and type, which represent how the uncertainty is treated in the likelihood fit. |
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Table 4:
The measured cross sections, together with their total uncertainties, for the individual channels and the channels combined for the BDT-based analysis. |
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Table 5:
The measured cross sections for the individual channels and the channels combined for the counting analysis. |
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Table 6:
Expected and observed 95% exclusion limits on the branching fraction of the tZ-FCNC couplings. |
| Summary |
| A search for the associated production of a top quark and a Z boson, as predicted by the standard model was performed with the full CMS data set collected at 8 TeV, corresponding to an integrated luminosity of 19.7 fb$^{-1}$. An events yield compatible with tZq standard model production is observed, and the corresponding cross section is measured to be 10$^{+8}_{-7}$ fb. The corresponding observed and expected significances are 2.4 and 1.8 standard deviations, respectively. A search for tZ production produced via flavour-changing neutral current interactions, either in single-top-quark or $\mathrm{ t \bar{t} }$ production modes, was also performed. For this search the standard model tZq process was considered as a background. No evidence for tZ-FCNC interactions is found, and limits at 95% confidence level are set on the branching fraction for the decay of a top quark into a Z boson and a quark. The limits are ${ \cal{B} } ( \mathrm{ t } \rightarrow \mathrm{ Z } \mathrm{u} ) < $ 0.022% and ${ \cal{B} } ( \mathrm{ t } \rightarrow \mathrm{ Z c }) < $ 0.049%, which improve the previous limits set by the CMS Collaboration by about a factor of two. |
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