Abstract
In this paper, a detailed study to probe the top quark Flavour-Changing Neutral Currents (FCNC) tqγ and tqZ at the future e−e+ collider FCC-ee in two different center-of-mass energies of 240 and 350 GeV is presented. A set of useful variables are proposed and used in a multivariate technique to separate signal e−e+→Z/γ→tq¯ (t¯q) from Standard Model background processes. The study includes a fast detector simulation based on the delphes package to consider the detector effects. The upper limits on the FCNC branching ratios at 95% confidence level (CL) in terms of the integrated luminosity are presented. It is shown that with 300 fb−1 of integrated luminosity of data, FCC-ee would be able to exclude the effective coupling strengths above O(10−4−10−5) which is corresponding to branching fraction of O(0.01−0.001)%. We show that moving to a high-luminosity regime leads to a significant improvement on the upper bounds on the top quark FCNC couplings to a photon or a Z boson.
Highlights
The top quark with its large mass and very short life time is one of the most interesting discovered particles in the Standard Model (SM)
Several extensions of the SM such as Technicolor, SUSY models, two Higgs doublet models predict much higher branching ratios up to 108 − 109 order of magnitude larger than SM predictions [2,3,4,5,6,7,8,9,10]. Any observation of these rare Flavour-Changing Neutral Currents (FCNC) transitions would be a clear signal of new physics beyond the SM
In order to investigate the sensitivity to b-tagging efficiency and mis-tagging rates, we present the 95% C.L upper limits on the branching ratios of t → qγ and t → q Z for 85% of b-tagging efficiency and a 5% mistagging rate for charm quark jets and 1% mistagging rate for the light flavor jets
Summary
The top quark with its large mass and very short life time is one of the most interesting discovered particles in the Standard Model (SM). [55,56], an analysis has been performed to probe the sensitivity of a future e−e+ collider to top quark FCNC to the photon and a Z boson in the e−e+ → Z /γ → tq (tq) channel. This analysis has been done at the center-of-mass energies of 500 GeV and 800 GeV with the integrated luminosity of up to 1 ab−1 without including the effects of parton showering, hadronization, and decay of unstable particles.
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