Probing the flavour structure of dimension-6 EFT operators in multilepton final states in proton-proton collisions at $\sqrt{s}$ = 13 TeV

Abstract

An analysis of the flavour structure of dimension-6 effective field theory (EFT) operators in multilepton final states is presented, focusing on the interactions of quarks with Z bosons. For the first time, the flavour structure of these operators is disentangled by simultaneously probing the interactions with different quark generations. The analysis targets the associated production of a top quark pair and a Z boson, as well as diboson processes in final states with at least three leptons, which can be electrons or muons. The data were recorded by the CMS experiment in the years 2016$-$2018 in proton-proton collisions at a centre-of-mass energy of 13 TeV and correspond to an integrated luminosity of 138 fb$^{-1}$. Consistency with the standard model of particle physics is observed and limits are set on the selected Wilson coefficients, split into couplings to light- and heavy-quark generations.

Figures (10)

• Figure 1: Representative Feynman diagrams showing the leading-order contributions to the ${ \mathup{{{t}}{} _{ {}} ^{ {}}} }\xspace{ \mathup{{ \overline{ {{ \mathup{{{t}}{} _{ {}} ^{ {}}} }\xspace}}}{} _{ {}} ^{ {}}} }\xspace{ \mathup{{{Z}}{} _{ {}} ^{ {}}} }\xspace$ production, with the $\mathup{{{Z}}{} _{ {}} ^{ {}}}$ boson radiated from the initial-state quarks (left) and from one of the top quarks (middle). The ${ \mathup{{{W}}{} _{ {}} ^{ {}}} }\xspace{ \mathup{{{Z}}{} _{ {}} ^{ {}}} }\xspace$ and/or ${ \mathup{{{Z}}{} _{ {}} ^{ {}}} }\xspace{ \mathup{{{Z}}{} _{ {}} ^{ {}}} }\xspace$ production is also shown (right). The vertices affected by the EFT operators probed in this analysis are highlighted with red dots.
• Figure 2: Distributions of the $\mathup{{{Z}}{} _{ {}} ^{ {}}}$ boson $p_{\mathrm{T}}$ in the three signal regions of this analysis. Shown are $\text{SR}_{{ \mathup{{{t}}{} _{ {}} ^{ {}}} }\xspace{ \mathup{{ \overline{ {{ \mathup{{{t}}{} _{ {}} ^{ {}}} }\xspace}}}{} _{ {}} ^{ {}}} }\xspace{ \mathup{{{Z}}{} _{ {}} ^{ {}}} }\xspace\xspace}$ (upper left), $\text{SR}_{{ \mathup{{{W}}{} _{ {}} ^{ {}}} }\xspace{ \mathup{{{Z}}{} _{ {}} ^{ {}}} }\xspace\xspace}$ (upper right), and $\text{SR}_{{ \mathup{{{Z}}{} _{ {}} ^{ {}}} }\xspace{ \mathup{{{Z}}{} _{ {}} ^{ {}}} }\xspace\xspace}$ (lower). In each region, the target process (${ \mathup{{{t}}{} _{ {}} ^{ {}}} }\xspace{ \mathup{{ \overline{ {{ \mathup{{{t}}{} _{ {}} ^{ {}}} }\xspace}}}{} _{ {}} ^{ {}}} }\xspace{ \mathup{{{Z}}{} _{ {}} ^{ {}}} }\xspace$, ${ \mathup{{{W}}{} _{ {}} ^{ {}}} }\xspace{ \mathup{{{Z}}{} _{ {}} ^{ {}}} }\xspace$, or ${ \mathup{{{Z}}{} _{ {}} ^{ {}}} }\xspace{ \mathup{{{Z}}{} _{ {}} ^{ {}}} }\xspace$) is shown at the SM point (coloured areas) and various EFT hypotheses (lines). The hashed band includes only uncertainties in the renormalisation and factorisation scales ($\mu_{\text{R}}$ and $\mu_{\text{F}}$). The upper, middle, and lower ratio panels show the ratio of EFT hypotheses for light-quark, heavy-quark, and EW boson couplings, respectively. The bin content is divided by the bin width.
• Figure 3: Distributions of the $\mathup{{{Z}}{} _{ {}} ^{ {}}}$ boson $p_{\mathrm{T}}$ in the control regions of this analysis. Shown are $\text{CR}_{{ \mathup{{{t}}{} _{ {}} ^{ {}}} }\xspace{ \mathup{{ \overline{ {{ \mathup{{{t}}{} _{ {}} ^{ {}}} }\xspace}}}{} _{ {}} ^{ {}}} }\xspace{ \mathup{{{Z}}{} _{ {}} ^{ {}}} }\xspace\xspace}$ (left) and $\text{CR}_{{ \mathup{{{W}}{} _{ {}} ^{ {}}} }\xspace{ \mathup{{{Z}}{} _{ {}} ^{ {}}} }\xspace\xspace}$ (right). Predictions are all obtained from simulation and are displayed as coloured areas. The hashed area shows the statistical uncertainty in the prediction. Data are displayed as markers, where the vertical bars represent the statistical uncertainty. The bin content is divided by the bin width.
• Figure 4: Schematic representation of the SRs and CRs used in this analysis. The application of the estimated nonprompt lepton background from the CRs into the SRs is illustrated with arrows.
• Figure 5: Distributions of the $\mathup{{{Z}}{} _{ {}} ^{ {}}}$ boson $p_{\mathrm{T}}$ in the two validation regions $\text{VR}_{{ \mathup{{{t}}{} _{ {}} ^{ {}}} }\xspace{ \mathup{{ \overline{ {{ \mathup{{{t}}{} _{ {}} ^{ {}}} }\xspace}}}{} _{ {}} ^{ {}}} }\xspace{ \mathup{{{Z}}{} _{ {}} ^{ {}}} }\xspace\xspace}$ (left) and $\text{VR}_{{ \mathup{{{W}}{} _{ {}} ^{ {}}} }\xspace{ \mathup{{{Z}}{} _{ {}} ^{ {}}} }\xspace\xspace}$ (right) after a fit including only uncertainties in the nonprompt lepton estimation. The data (markers) are compared to the prediction from simulation and the data-driven estimate of nonprompt leptons (coloured areas). The lower panel displays the ratio to the predictions after the fit. The hashed area displays the uncertainties in the estimation of the nonprompt lepton background after the fit. The bin content is divided by the bin width.