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| CMS-PAS-TOP-12-039 | ||
| 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 | ||
| August 2016 | ||
| 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 (SM) process and to search for flavour changing neutral current (FCNC) 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. A moderate excess of events compatible with SM tZq production is observed, and the corresponding cross section is measured to be $\sigma ({\rm tZq \rightarrow \ell} \nu {\rm b \ell^+ \ell^- q}) = $ 10$^{+8}_{-7}$ fb with a significance of 2.4 $\sigma$. No presence of FCNC production of tZ(q) is observed and exclusion limits at 95% confidence level on the branching ratios of a top quark decaying to a Z boson and an up or a charm quark are found to be $\mathcal({\rm t\rightarrow Zu}) <$ 0.022% and $\mathcal{B}({\rm t \rightarrow Zu}) <$ 0.049%, respectively. | ||
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Links:
CDS record (PDF) ;
inSPIRE record ;
CADI line (restricted) ;
These preliminary results are superseded in this paper, JHEP 07 (2017) 003. The superseded preliminary plots can be found here. |
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| Figures | |
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Figure 1:
Leading order tZq production diagrams. The initial and final state quarks denoted q and q' are predominantly first generation quarks, although there are smaller additional contributions from strange- and charm-initiated diagrams. Diagram (f) represents the non-resonant contribution to the tZq process. |
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Figure 1-a:
Leading order tZq production diagram. The initial and final state quarks denoted q and q' 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 diagram. The initial and final state quarks denoted q and q' 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 diagram. The initial and final state quarks denoted q and q' 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 diagram. The initial and final state quarks denoted q and q' 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 diagram. The initial and final state quarks denoted q and q' are predominantly first generation quarks, although there are smaller additional contributions from strange- and charm-initiated diagrams. |
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Figure 1-f:
Leading order tZq production diagram. The initial and final state quarks denoted q and q' are predominantly first generation quarks, although there are smaller additional contributions from strange- and charm-initiated diagrams. The diagram represents the non-resonant contribution to the tZq process. |
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Figure 2:
Feynman diagrams for the production of tZ in the single-top-FCNC channels. |
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Figure 2-a:
Feynman diagram for the production of tZ in the single-top-FCNC channels. |
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Figure 2-b:
Feynman diagram for the production of tZ in the single-top-FCNC channels. |
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Figure 2-c:
Feynman diagram for the production of tZ in the single-top-FCNC channels. |
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Figure 2-d:
Feynman diagram for the production of tZ in the single-top-FCNC channels. |
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Figure 3:
Feynman diagram for the production of tZq in the ${\mathrm {t}\overline {\mathrm {t}}} $-FCNC channels. |
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Figure 4:
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), $E_{\mathrm{T}}^{\text{miss}}$ (top right), $\rm m_T^W$ (bottom left) and $\rm m_{\ell \ell }$ (bottom right), shown here for the tZ-FCNC search where WZ + h.f. denotes WZ + heavy flavour. |
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Figure 4-a:
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, shown here for the tZ-FCNC search where WZ + h.f. denotes WZ + heavy flavour. |
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Figure 4-b:
Data-to-prediction comparisons in the background-enriched samples, after applying background normalisation scaling factors as described in the text, of $E_{\mathrm{T}}^{\text{miss}}$, shown here for the tZ-FCNC search where WZ + h.f. denotes WZ + heavy flavour. |
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Figure 4-c:
Data-to-prediction comparisons in the background-enriched samples, after applying background normalisation scaling factors as described in the text, of the $\rm m_T^W$, shown here for the tZ-FCNC search where WZ + h.f. denotes WZ + heavy flavour. |
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Figure 4-d:
Data-to-prediction comparisons in the background-enriched samples, after applying background normalisation scaling factors as described in the text, of $\rm m_{\ell \ell }$, shown here for the tZ-FCNC search where WZ + h.f. denotes WZ + heavy flavour. |
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Figure 5:
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 channels are combined. |
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Figure 5-a:
Data to prediction comparisons after performing the fit for the $\eta $ distribution of the recoiling jet in the control region. The four channels are combined. |
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Figure 5-b:
Data to prediction comparisons after performing the fit for the $\eta $ distribution of the recoiling jet in the signal region. The four channels are combined. |
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Figure 6:
Data to prediction comparisons after performing the fit for $\rm m_T^W$\ distribution in the control region (left), and for the BDT responses in the signal region (right). The four channels are combined. |
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Figure 6-a:
Data to prediction comparisons after performing the fit for $\rm m_T^W$\ distribution in the control region. The four channels are combined. |
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Figure 6-b:
Data to prediction comparisons after performing the fit for the BDT responses in the signal region. The four channels are combined. |
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Figure 7:
Data to prediction comparisons for the tZ-FCNC search after performing the fit for $\rm m_T^W$ distribution in the control region (top left), and for the BDT responses in the single top (top right) and ${\mathrm {t}\overline {\mathrm {t}}} $ (bottom) signal regions. The four channels are combined. An example of the predicted signal contribution for a $\mathcal {B}(\mathrm{ t \rightarrow Zu}) =$ 0.1% is shown for illustration. |
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Figure 7-a:
Data to prediction comparisons for the tZ-FCNC search after performing the fit for $\rm m_T^W$ distribution in the control region. The four channels are combined. An example of the predicted signal contribution for a $\mathcal {B}(\mathrm{ t \rightarrow Zu}) =$ 0.1% is shown for illustration. |
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Figure 7-b:
Data to prediction comparisons for the tZ-FCNC search after performing the fit for the BDT responses in the single top signal region. The four channels are combined. An example of the predicted signal contribution for a $\mathcal {B}(\mathrm{ t \rightarrow Zu}) =$ 0.1% is shown for illustration. |
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Figure 7-c:
Data to prediction comparisons for the tZ-FCNC search after performing the fit for the BDT responses in the ${\mathrm {t}\overline {\mathrm {t}}} $ signal region. The four channels are combined. An example of the predicted signal contribution for a $\mathcal {B}(\mathrm{ t \rightarrow Zu}) =$ 0.1% is shown for illustration. |
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Figure 8:
The expected and observed exclusion limits at the 95% CL on the $\mathcal {B} (\mathrm{ t \rightarrow Zc})$ as a function of the limits on the $\mathcal {B} (\mathrm{ t \rightarrow Zu} )$. The $\pm $1 sigma band is also shown. |
| Tables | |
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Table 1:
Expected and observed 95% exclusion limits on the signal strength of the tZ-FCNC couplings. |
| Summary |
| A search for the associated production of a top quark and a Z boson, predicted by the SM or produced from FCNC interactions, is performed with the full CMS 8 TeV data set. A moderate excess of events over the background is observed and from this the tZq cross section is measured to be $\sigma_{\rm tZq} =$ 10$^{+8}_{-7}$ fb, in good agreement with the SM expectation. The corresponding observed and expected significances are 2.4 and 1.8 respectively. A search for tZ production produced via flavour changing interactions, either in single top or $\mathrm{ t \bar{t} }$ production modes, is also performed. No evidence for tZ-FCNC interactions is found and limits at the 95 % CL are set on the branching ratio for the decay of a top quark into a Z boson and a quark. The limits are found to be $\mathcal{B}( \mathrm{ t\rightarrow Zu } )<$ 0.022 % and $\mathcal{B}( \mathrm{ t \rightarrow Zc }) <$ 0.049%, improving by about a factor of two the previous limits set by the CMS Collaboration [29]. |
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