Probing Top-quark Operators with Precision Electroweak Measurements

Abstract

In the Standard Model Effective Field Theory (SMEFT), operators involving the top quark are generally difficult to probe, and can generate sizable loop contributions to the electroweak precision observables, measured by past and future lepton colliders. Could the high precision of the electroweak measurements compensate the loop suppression and provide competitive reaches on these operators? Would the inclusion of these contributions introduce too many additional parameters for a meaningful global electroweak analysis to be done? In this paper, we perform a detailed phenomenological study to address these two important questions. Focusing on eight dimension-6 operators that generate anomalous couplings between the electroweak gauge bosons and the third-generation quarks, we calculate their one loop contributions to the e + e − → f f processes both on and off the Z-pole and the e − e + → W W process. A global analysis is performed with these eight operators and the ones that contribute to the above processes at tree level, using the measurements at LEP, SLC and several low energy experiments. We find that, while the current electroweak precision measurements are sensitive to the one-loop effects of the topquark operators, it is difficult to separate them from the operators that contribute at the tree level, making a global analysis rather challenging. Under more assumptions (for instance that the new physics contribute only to the third generation quark operators and the S, T parameters), competitive reaches could be obtained in a global fit. Another important finding of our study is that the two operators that generate dipole interactions of the bottom quark have significant impacts in the Z-pole measurements and should not be omitted. We also discuss the implication of the recently reported W-boson mass measurement at CDF to our results. Finally, we estimate the reaches of future lepton colliders in probing the top-quark operators with precision electroweak measurements.

Electroweak precision tests for triplet scalars

“…24 ± 0.02 with χ 2 min (d.o.f ) = 17.20(14) as shown in the right panel of Table-II. Our results are consistent within 1σ with Ref [8,66,69,70,75,86,88,90,106]…”

“…Due to CDF new excess in W mass measurement, we can expect some observables in the global fits may suffer from new tension since the EW parameters in the SM are closely related to each other. An impressive amount of activity has already been generated by the CDF experiment and global EW fit with the new input for m W [8,66,67,70,72,75,86,88,90,93,106,107]. However, these fits are mainly performed by the oblique parameters S, T and U [108,109] or Wilson coefficients in EFT.…”

SMEFT analysis of mW

“…1 The SMEFiT collaboration subsequently made a global analysis that included SMEFT effects at quadratic order [7], assuming that the precision electroweak data are consistent with the SM. Other global fits have been performed for various combinations of electroweak, diboson and Higgs data, e.g., [8][9][10][11][12][13][14][15], as well as separate fits to mainly top measurements, e.g., [16][17][18][19][20][21][22][23][24][25][26][27].…”