CMS-TOP-19-004

CMS-TOP-19-004 ; CERN-EP-2020-012
Combination of the W boson polarization measurements in top quark decays using ATLAS and CMS data at $\sqrt{s} = $ 8 TeV
ATLAS and CMS Collaborations
7 May 2020
JHEP 08 (2020) 051
Abstract: The combination of measurements of the W boson polarization in top quark decays performed by the ATLAS and CMS Collaborations is presented. The measurements are based on proton-proton collision data produced at the LHC at a centre-of-mass energy of 8 TeV, and corresponding to an integrated luminosity of about 20 fb$^{-1}$ for each experiment. The measurements used events containing one lepton and having different jet multiplicities in the final state. The results are quoted as fractions of W bosons with longitudinal (${F_{\textrm{0}}}$), left-handed (${F_{\textrm{L}}}$), or right-handed (${F_{\textrm{R}}}$) polarizations. The resulting combined measurements of the polarization fractions are ${F_{\textrm{0}}} = 0.693 \pm 0.014$ and ${F_{\textrm{L}}} = 0.315 \pm 0.011$. The fraction ${F_{\textrm{R}}}$ is calculated from the unitarity constraint to be ${F_{\textrm{R}}} = -0.008 \pm 0.007$. These results are in agreement with the standard model predictions at next-to-next-to-leading order in perturbative quantum chromodynamics and represent an improvement in precision of 25 (29)% for ${F_{\textrm{0}}}$ (${F_{\textrm{L}}}$) with respect to the most precise single measurement. A limit on anomalous right-handed vector ($V_{\textrm{R}}$), and left- and right-handed tensor ($g_{\textrm{L}},g_{\textrm{R}}$) $\mathrm{t}\mathrm{W}\mathrm{b}$ couplings is set while fixing all others to their standard model values. The allowed regions are $[-0.11, 0.16]$ for $V_{\textrm{R}}$, $[-0.08, 0.05]$ for $g_{\textrm{L}}$, and $[-0.04, 0.02]$ for $g_{\textrm{R}}$, at 95% confidence level. Limits on the corresponding Wilson coefficients are also derived.
Links: e-print arXiv:2005.03799 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: The total correlation between the input measurements of the combination.
png pdf	Figure 2: Overview of the four measurements, as well as the results of the combination. The inner and outer error bars correspond to the statistical and the total uncertainties, respectively. The inner bars for the combination include also the background determination uncertainties. The vertical solid line indicates the predictions of NNLO QCD calculations [1].
png pdf	Figure 3: Allowed regions for the $\mathrm{t} \mathrm{W} \mathrm{b} $ anomalous (left) left- and right-handed tensor couplings, and (right) right-handed vector and tensor coupling. The limits are obtained from the ATLAS, CMS, and the combined measurements of the W boson polarization fractions at 68 and 95% CL. The limits from CMS are obtained using the pre-combined result of all CMS input measurements. The anomalous couplings are assumed to be real.
png pdf	Figure 3-a: Allowed region for the $\mathrm{t} \mathrm{W} \mathrm{b} $ anomalous left- and right-handed tensor couplings. The limits are obtained from the ATLAS, CMS, and the combined measurements of the W boson polarization fractions at 68 and 95% CL. The limits from CMS are obtained using the pre-combined result of all CMS input measurements. The anomalous couplings are assumed to be real.
png pdf	Figure 3-b: Allowed region for the $\mathrm{t} \mathrm{W} \mathrm{b} $ anomalous right-handed vector and tensor coupling. The limits are obtained from the ATLAS, CMS, and the combined measurements of the W boson polarization fractions at 68 and 95% CL. The limits from CMS are obtained using the pre-combined result of all CMS input measurements. The anomalous couplings are assumed to be real.

Tables
png pdf	Table 1: Summary of the published ATLAS and CMS measurements for 8 TeV data. The first quoted uncertainty in the ATLAS measurement includes statistical uncertainties and uncertainties in the background determination, and the second uncertainty refers to the remaining systematic contribution. For CMS measurements, the first uncertainty is statistical while the second is the total systematic uncertainty, including that on background determination.
png pdf	Table 2: Summary of the correlation categories considered in the combination. The correlations among the ${F_{\textrm {L}}}$ measurements are not shown for brevity.
png pdf	Table 3: Input correlations across different measurements, as explained in Section xxxxx. The values stand for the correlations $\rho ({F_{\textrm {i}}}, {F_{\textrm {i}}})$, with $\textrm {i}$ being either 0 or L. The correlations of the type $\rho ({F_{\textrm {0}}}, {F_{\textrm {L}}})$ are assumed to be $\rho ({F_{\textrm {0}}}, {F_{\textrm {L}}})=-\rho ({F_{\textrm {0}}}, {F_{\textrm {0}}})=-\rho ({F_{\textrm {L}}}, {F_{\textrm {L}}})$. In case an uncertainty is not applicable, the correlation value is set to zero and marked with an asterisk. The correlations marked with a dagger sign are those that are not precisely determined and checks are performed to test the stability of the results against these assumptions.
png pdf	Table 4: Uncertainties in ${F_{\textrm {0}}}$, ${F_{\textrm {L}}}$ and their corresponding correlations from the ATLAS measurement. The uncertainty that is not applicable to this measurement, or which is included in other categories, is indicated by "n.a.''. The line "Systematic uncertainty'' represents the quadratic sum of all the systematic uncertainty sources except for the uncertainty in the background determination, which is included in the "Stat+bkg'' category. The quoted correlation values are obtained via the procedures described in Section xxxxx.
png pdf	Table 5: Uncertainties in ${F_{\textrm {0}}}$, ${F_{\textrm {L}}}$ and their corresponding correlations from the CMS e+jets and $\mu$+jets measurements. The uncertainty that is not applicable to this measurement, or which is included in other categories, is indicated by "n.a.''. The line "Systematic uncertainty'' represents the quadratic sum of all the systematic uncertainty sources except for the uncertainties in the background determination and the integrated luminosity, which are included in the "Stat+bkg'' category. The quoted correlation values are obtained via the procedures described in Section xxxxx.
png pdf	Table 6: Uncertainties in ${F_{\textrm {0}}}$, ${F_{\textrm {L}}}$ and their corresponding correlations from the CMS (single top) measurement. The uncertainty that is not applicable to this measurement, or which is included in other categories, is indicated by "n.a.''. The line "Systematic uncertainty'' represents the quadratic sum of all the systematic uncertainty sources except for the uncertainties in the background determination and the integrated luminosity, which are included in the "Stat+bkg'' category. The quoted correlation values are obtained via the procedures described in Section xxxxx.
png pdf	Table 7: Results of the ATLAS and CMS combination: W boson polarization fraction values and uncertainties. The combined ${F_{\textrm {0}}}$ and ${F_{\textrm {L}}}$ values are anticorrelated, with $\rho =-0.85$.
png pdf	Table 8: Allowed ranges for the anomalous couplings $V_{\text {R}}$, $g_{\text {L}}$, and $g_{\text {R}}$ at 95% CL. The limit on each coupling is obtained while fixing all other couplings to their SM value. The limits from CMS are obtained using the pre-combined result of all CMS input measurements. The anomalous couplings are assumed to be real.
png pdf	Table 9: Allowed ranges for the Wilson coefficients $C_{\phi \phi}^{\ast}$, $C_{\mathrm{b} \mathrm{W}}^{\ast}$, and $C_{\mathrm{t} \mathrm{W}}$ at 95% CL. The limit on each coefficient is obtained while fixing all other coefficients to their SM values. The limits from CMS are obtained using the pre-combined result of all CMS input measurements. The numerical values are obtained by setting the $\Lambda $ scale to 1 TeV, and the coefficients are assumed to be real.

Summary

The combination of measurements of the W boson polarization in top quark decays performed by the ATLAS and CMS Collaborations is presented. The measurements are based on proton-proton collision data produced at the LHC at a centre-of-mass energy of 8 TeV, and corresponding to an integrated luminosity of about 20 fb$^{-1}$ for each experiment. The fractions of W bosons with longitudinal (${F_{\textrm{0}}}$) and left-handed (${F_{\textrm{L}}}$) polarizations were measured in events containing a single lepton and multiple jets, enhanced in $\mathrm{t\bar{t}}$ or single top quark production processes. The results of the combination are

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