CMS-PAS-TOP-17-012

CMS-PAS-TOP-17-012
Extraction of CKM matrix elements in single top quark $t$ -channel events in proton-proton collisions at $\sqrt{s} =$ 13 TeV
CMS Collaboration
November 2019

Abstract: This note presents a model-independent extraction of the modulus of the Cabibbo Kobayashi Maskawa matrix elements $\mathrm{V_{\mathrm{tb}}}$ , $\mathrm{V_{\mathrm{td}}}$ , and $\mathrm{V_{\mathrm{ts}}}$ using an event sample enriched in single top quark $t$ -channel events. The analysis uses proton-proton collisions data from the LHC collected during 2016 by the CMS experiment at a centre-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 35.9 fb $^{-1}$ . Processes directly sensitive to the matrix elements $\mathrm{V_{\mathrm{tb}}}$ , $\mathrm{V_{\mathrm{td}}}$ , and $\mathrm{V_{\mathrm{ts}}}$ are considered in both the production and decay vertices of the top quark in single top quark $t$ -channel production, as well as in the background processes containing top quarks. Final states are investigated where a muon or electron stems from the leptonic decay chain of the top quark, and two or three jets are selected, one or two of which are identified as coming from the hadronisation of a b quark. The event sample is divided into categories according to the number of jets. Multivariate classifier variables are built in each category in order to discriminate between signal and other standard model processes, and a simultaneous maximum-likelihood fit to data is performed on all categories. The measured value of $\mathrm{\|V_{\mathrm{tb}}\|}$ is 1.00 $\pm$ 0.03, where the uncertainty includes both statistical and systematic uncertainties, and the upper limit derived on $\mathrm{\|V_{\mathrm{ts}}\|^2}+\mathrm{\|V_{\mathrm{td}}\|^2}$ is 0.17 at 95% confidence level.
Links: CDS record (PDF) ; CADI line (restricted) ; These preliminary results are superseded in this paper, PLB 808 (2020) 135609. The superseded preliminary plots can be found here.

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Leading-order Feynman diagrams for single top quark production via the $t$ -channel featuring: (a) a tWb vertex in production and decay, (b) a tWb vertex in production and a tWq in decay, with q being a s or d quark, (c) a tWq vertex in production and a tWb in decay, and (d) a d quark-initiated process, enhanced thanks to contributions from valence d quarks.
png pdf	Figure 1-a: Leading-order Feynman diagram for single top quark production via the $t$ -channel featuring a tWb vertex in production and decay.
png pdf	Figure 1-b: Leading-order Feynman diagram for single top quark production via the $t$ -channel featuring a tWb vertex in production and a tWq in decay, with q being a s or d quark.
png pdf	Figure 1-c: Leading-order Feynman diagram for single top quark production via the $t$ -channel featuring a tWq vertex in production and a tWb in decay.
png pdf	Figure 1-d: Leading-order Feynman diagram for single top quark production via the $t$ -channel featuring a d quark-initiated process, enhanced thanks to contributions from valence d quarks.
png pdf	Figure 2: The ${m_{\mathrm {T}}^{\mathrm {W}}}$ distributions in the (left) 2j1t and (right) 3j1t regions for the (upper) muon and (lower) electron channels. The lower plots show the ratio of the data to the MC prediction.
png pdf	Figure 2-a: The ${m_{\mathrm {T}}^{\mathrm {W}}}$ distribution in the 2j1t region for the muon channel. The lower plot shows the ratio of the data to the MC prediction.
png pdf	Figure 2-b: The ${m_{\mathrm {T}}^{\mathrm {W}}}$ distribution in the 3j1t region for the muon channel. The lower plot shows the ratio of the data to the MC prediction.
png pdf	Figure 2-c: The ${m_{\mathrm {T}}^{\mathrm {W}}}$ distribution in the 2j1t region for the electron channel. The lower plot shows the ratio of the data to the MC prediction.
png pdf	Figure 2-d: The ${m_{\mathrm {T}}^{\mathrm {W}}}$ distribution in the 3j1t region for the electron channel. The lower plot shows the ratio of the data to the MC prediction.
png pdf	Figure 3: Distributions of the two most discriminant variables in the 2j1t region: the absolute value of the pseudorapidity of the non b-tagged jet ${\eta _{j'}}$ (left) and the invariant mass of the vectorial sum of the lepton and b jet momenta (right), shown for the muon (upper) and electron (lower) channel, respectively.
png pdf	Figure 3-a: Distribution of the absolute value of the pseudorapidity of the non b-tagged jet ${\eta _{j'}}$ in the 2j1t region, shown for the muon channel.
png pdf	Figure 3-b: Distribution of the invariant mass of the vectorial sum of the lepton and b jet momenta in the 2j1t region, shown for the muon channel.
png pdf	Figure 3-c: Distribution of the absolute value of the pseudorapidity of the non b-tagged jet ${\eta _{j'}}$ in the 2j1t region, shown for the electron channel.
png pdf	Figure 3-d: Distribution of the invariant mass of the vectorial sum of the lepton and b jet momenta in the 2j1t region, shown for the electron channel.
png pdf	Figure 4: Distributions of the two most discriminant variables in the 3j1t region: the missing momentum in the transverse plane (left) and the response of the CMVAv2 b-tagger discriminator when applied to the extra jet (right) are shown for the muon (upper) and electron (lower) channel, respectively.
png pdf	Figure 4-a: Distribution of the missing momentum in the transverse plane in the 3j1t region, shown for the muon channel.
png pdf	Figure 4-b: Distribution of the response of the CMVAv2 b-tagger discriminator when applied to the extra jet in the 3j1t region, shown for the muon channel.
png pdf	Figure 4-c: Distribution of the missing momentum in the transverse plane in the 3j1t region, shown for the electron channel.
png pdf	Figure 4-d: Distribution of the response of the CMVAv2 b-tagger discriminator when applied to the extra jet in the 3j1t region, shown for the electron channel.
png pdf	Figure 5: Distributions of the two most discriminant variables in the 3j2t region: the absolute value of the pseudorapidity of the non b-tagged jet ${\eta _{j'}}$ (left) and the invariant mass of the vectorial sum of the lepton and non b-tagged jet (right) are shown for the muon (upper) and electron (lower) channel, respectively.
png pdf	Figure 5-a: Distribution of the absolute value of the pseudorapidity of the non b-tagged jet ${\eta _{j'}}$ in the 3j2t region, shown for the muon channel.
png pdf	Figure 5-b: Distribution of the absolute value of the pseudorapidity of the non b-tagged jet ${\eta _{j'}}$ in the 3j2t region, shown for the muon channel.
png pdf	Figure 5-c: Distribution of the invariant mass of the vectorial sum of the lepton and non b-tagged jet in the 3j2t region, shown for the electron channel.
png pdf	Figure 5-d: Distribution of the invariant mass of the vectorial sum of the lepton and non b-tagged jet in the 3j2t region, shown for the electron channel.
png pdf	Figure 6: Distribution of the multivariate discriminants, comparing data to simulation normalized after the fit procedure for muon channel on the left and for the electron on the right, for 2j1t (top), 3j1t (middle), and 3j2t (bottom). The systematic bands correspond to the profiled uncertainties constrained by the fit procedure.
png pdf	Figure 6-a: Distribution of the multivariate discriminant, comparing data to simulation normalized after the fit procedure for muon channel, for 2j1t. The systematic band corresponds to the profiled uncertainties constrained by the fit procedure.
png pdf	Figure 6-b: Distribution of the multivariate discriminant, comparing data to simulation normalized after the fit procedure for electron channel, for 2j1t. The systematic band corresponds to the profiled uncertainties constrained by the fit procedure.
png pdf	Figure 6-c: Distribution of the multivariate discriminant, comparing data to simulation normalized after the fit procedure for muon channel, for 3j1t. The systematic band corresponds to the profiled uncertainties constrained by the fit procedure.
png pdf	Figure 6-d: Distribution of the multivariate discriminant, comparing data to simulation normalized after the fit procedure for electron channel, for 3j1t. The systematic band corresponds to the profiled uncertainties constrained by the fit procedure.
png pdf	Figure 6-e: Distribution of the multivariate discriminant, comparing data to simulation normalized after the fit procedure for muon channel, for 3j2t. The systematic band corresponds to the profiled uncertainties constrained by the fit procedure.
png pdf	Figure 6-f: Distribution of the multivariate discriminant, comparing data to simulation normalized after the fit procedure for electron channel, for 3j2t. The systematic band corresponds to the profiled uncertainties constrained by the fit procedure.

Tables
png pdf	Table 1: Values of the third-row elements of the CKM matrix inferred from low-energy measurements, taken from [12], with the respective values of the top quark decay branching fractions. The q in ${\mathrm {\|V_{\mathrm {tq}}}\|}$ and ${\mathcal {B}\mathrm {(t\to Wq)}}$ of the first column refers to b, s, and d quarks accordingly to the quark present in the first row.
png pdf	Table 2: Production cross section of signal processes taken into consideration, where the "scale'' component of the uncertainty refers to factorisation and renormalisation scale uncertainties, "pdf'' refers to uncertainties due to the PDFs, and "exp'' denotes the experimental component of the uncertainty from low-energy measurements.
png pdf	Table 3: For each region, the corresponding signal process, the cross section times branching fraction determined, and the specific Feynman diagram from Fig. 1 involved.
png pdf	Table 4: Estimated relative contributions of uncertainty sources in % to the total uncertainties of the measured cross sections for single top quark production.

Summary

A measurement of the CKM matrix elements

${\mathrm{|V_{\mathrm{tb}}}|}$ ,

${\mathrm{|V_{\mathrm{td}}}|}$ , and

${\mathrm{|V_{\mathrm{ts}}}|}$ has been performed in an event sample enriched in

$t$ -channel single top quark events, featuring one muon or electron and jets in the final state. The data sample is from proton-proton collisions at

$\sqrt{s} =$ 13 TeV, acquired at the LHC by the CMS experiment and corresponding to an integrated luminosity of 35.9 fb

$^{-1}$ . The contributions from single top quark processes featuring all three matrix elements in the production vertex have been considered as separate signal processes, as well as contributions from decays of single top quarks involving all three quark families. The yields of the signal processes have been extracted through a simultaneous fit to data in different selected regions, and the values of the CKM matrix elements have been inferred from the signal strengths. All results are in agreement with the SM predictions.

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