Abstract
We study the top-Higgs coupling with a CP violating phase ξ at a future multi-TeV muon collider. We focus on processes that are directly sensitive to the top quark Yukawa coupling: \( t\overline{t}h \), tbhμν, and \( t\overline{t} h\nu \overline{\nu} \) with \( h\to b\overline{b} \) and semileptonic top decays. At different energies, different processes dominate the cross section, providing complementary information. At and above an energy of \( \mathcal{O}(10) \) TeV, vector boson fusion processes dominate. As we show, in the Standard Model there is destructive interference in the vector boson fusion processes \( t\overline{t} h\nu \overline{\nu} \) and tbhμν between the top quark Yukawa and Higgs-gauge boson couplings. A CP-violating phase changes this interference, and the cross section measurement is very sensitive to the size of the CP-violating angle. Although we find that the cross sections are measured to \( \mathcal{O}\left(50\%\right) \) statistical uncertainty at 1σ, a 10 and 30 TeV muon collider can bound the CP-violating angle |ξ| ≲ 9.0° and |ξ| ≲ 5.4°, respectively. However, cross section measurements are insensitive to the sign of the CP-violating angle. To determine that the coupling is truly CP violating, observables sensitive to CP-violation must be measured. We find in the \( t\overline{t}h \) process the azimuthal angle between the \( t+\overline{t} \) plane and the initial state muon+Higgs plane shows good discrimination for ξ = ±0.1π. For the tbhμν and \( t\overline{t} h\nu \overline{\nu} \) processes, the operator proportional to \( \left({\overrightarrow{p}}_{\mu}\times {\overrightarrow{p}}_h\right) \) · \( {\overrightarrow{p}}_t \) is sensitive to the sign of CP phase ξ. From these observables, we construct asymmetry parameters that show good distinction between different values and signs of the CP violating angle.
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Acknowledgments
The authors would like to thank J. Kanzaki and K. Mawatari for instructions on new HELAS, and D. Gonçalves and K. Hagiwara for useful discussions. IML would like to thank the University of Pittsburgh Particle Physics Astrophysics and Cosmology Center and the Aspen Center for Physics, which is supported by National Science Foundation grant PHY-2210452, for their hospitality during the completion of this manuscript. MEC is supported in part by the Kenneth P. Dietrich School of Arts and Science Whittington Fellowship at the University of Pittsburgh. MEC and YZ are supported in part by the State of Kansas EPSCoR grant program. ZD is supported in part by College of Liberal Arts and Sciences Research Fund at the University of Kansas. KK is supported in part by US DOE DE-SC0024407. IML is supported in part by DE-SC0017988. YJZ is supported by JSPS KAKENHI Grant No. 21H01077 and 23K03403. The data to reproduce the plots is available upon request.
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Cassidy, M.E., Dong, Z., Kong, K. et al. Probing the CP structure of the top quark Yukawa at the future muon collider. J. High Energ. Phys. 2024, 176 (2024). https://doi.org/10.1007/JHEP05(2024)176
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DOI: https://doi.org/10.1007/JHEP05(2024)176