Abstract
We demonstrate how probes of $CP$-violating observables in Higgs boson ditau decays at prospective future lepton colliders could provide a test of weak scale baryogenesis with significant discovery potential. Measurements at the Circular Electron Positron Collider, for example, could exclude a $CP$ phase larger than 2.9\ifmmode^\circ\else\textdegree\fi{} (5.6\ifmmode^\circ\else\textdegree\fi{}) at 68% (95%) C.L. assuming the Standard Model value for magnitude of the tau lepton Yukawa coupling. Conversely, this sensitivity would allow for a $5\ensuremath{\sigma}$ discovery for 82% of the $CP$ phase range $[0,2\ensuremath{\pi})$. The reaches of the Future Circular Collider for electrons and positrons (FCC-ee) and International Linear Collider are comparable. As a consequence, future lepton colliders could establish the presence of $CP$ violation required by lepton flavored electroweak baryogenesis with at least $3\ensuremath{\sigma}$ sensitivity. Our results illustrate that Higgs factories are not just precision machines, but can also make $\mathcal{O}(1)$ measurement of the new physics beyond the Standard Model.
Highlights
The discovery of the Higgs boson at the Large Hadron Collider (LHC) [1,2] and subsequent measurements of its properties strongly favor the mechanism of electroweak symmetry breaking (EWSB) given by the Standard Model (SM) of particle physics
We focus on the possibility that the observation of CP-violating effects in Higgs-tau lepton interactions at a future lepton collider could provide new insight into the BAU problem
At the high luminosity (HL)-LHC, the 1σ uncertainty of μττ can be further improved to 5% [60,61], which is further combined with the CP measurement [38] that is shown as the black contour
Summary
The discovery of the Higgs boson at the Large Hadron Collider (LHC) [1,2] and subsequent measurements of its properties strongly favor the mechanism of electroweak symmetry breaking (EWSB) given by the Standard Model (SM) of particle physics. We focus on the possibility that the observation of CP-violating effects in Higgs-tau lepton interactions at a future lepton collider could provide new insight into the BAU problem. Our present focus is on the possible modification of the τ Yukawa coupling by a nonzero CP phase Δ as defined in Eq (2.1) below and the resulting impact on Higgs decay into a pair of τ leptons In this context, it has been known for some time that the Δ phase can be measured at colliders [20,21,22,23,24,25,26,27,28,29,30,31,32].
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