Measurement of $t$-channel production of single top quarks and antiquarks in $pp$ collisions at 13 TeV using the full ATLAS Run 2 data sample

The ATLAS collaboration
JHEP 05 (2024) 305, 2024.

Abstract (data abstract)
- CERN-LHC. The production of single top quarks and top antiquarks via the $t$-channel exchange of a virtual $W$ boson are measured in proton--proton collisions at a centre-of-mass energy of 13 TeV at the LHC. The full Run 2 dataset recorded with the ATLAS detector in the years 2015--2018 is used. The total cross-sections are determined to be $\sigma(tq)=137^{+8}_{-8}\,\mathrm{pb}$ and $\sigma(\bar{t}q)=84^{+6}_{-5}\,\mathrm{pb}$ for top-quark and top-antiquark production, respectively. The combined cross-section is found to be $\sigma(tq+\bar{t}q)=221^{+13}_{-13}\,\mathrm{pb}$ and the cross-section ratio is $R_{t}=\sigma(tq)/\sigma(\bar{t}q)=1.636^{+0.036}_{-0.034}$. The measured cross-sections are in good agreement with predictions made at next-to-next-to-leading order in quantum chromodynamics. The predicted value of $R_{t}$ using different sets of parton distribution functions is compared to the measured value, demonstrating the potential to further constrain the functions when using this result in global fits. The measurements of $\sigma(tq)$, $\sigma(\bar{t}q)$, and $\sigma(tq+\bar{t}q)$ are interpreted in an effective field theory approach, setting limits at the 95\% confidence level on the strength of a four-quark operator and an operator coupling the third quark generation to the Higgs boson doublet: $-0.37 < C_{qQ}^{(1,3)}/\Lambda^2 < 0.06$ and $-0.87 < C_{\phi Q}^{(3)}/\Lambda^2 < 1.42$. The measured total cross-section is further used to derive the constraint $|V_{tb}|>0.95$ at the 95% confidence level. In a more general approach, pairs of CKM matrix elements involving top quarks are simultaneously constrained, leading to confidence contours in the corresponding two-dimensional parameter spaces.

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