CP-violating top-Higgs coupling in gauge theories
Faculty of Education
Zheng Yajuan
Theoretical particle physics

Abstract
Phenomenological consequences of CP-violating (CPV) top-Higgs coupling at a future muon collider
process, a top pair associated production with Higgs particle and missing neutrinos has been studied. We
found sharp increase of the production rate with energy and identified its origin as the gauge noninvariance of the traditional CPV interactions. We proposed a gauge invariant SMEFT framework for thestudy and obtained consistent theoretical predictions. In addition, novel unitarity bound on the coefficientof SMEFT operator is obtained by studying the 2 to 2 and 2 to 3 cross sections in the J=0 channel.

Research background:
The particle physics standard model (SM) as an effective filed theory (EFT) has been successful ininterpreting all high energy experimental data, but the matter-antimatter asymmetry of our universe tellsthat there should be new CP-violating (CPV) interactions. One promising possibility is the CPV top-Higgs
coupling, which is the strongest coupling of the Higgs particle. Phenomenological consequences of CPV
top-Higgs couplings has been studied at various hadron collider and electron-positron collider processes.
A future muon circular collider is supposed to reach very high center-of-mass energies beyond 10 TeVand provides a new realm for the CPV study.

Research contents:
We study consequences of the CPV top-Higgs interactions through a muon collider process: a top pairassociated production with Higgs particle and missing neutrinos.

Research achievements:
Abnormal growth of the cross section at high energies is found when the CPV phase is nonzero andits cause is identified as a violation of gauge invariance in the sub-amplitudes of the left figure. We propose
a gauge invariant description in the SMEFT framework, and obtain the first consistent predictions of themiddle figure. Since rising cross section with energies leads to unitarity violation, we obtain the upperbound on the coefficient of the SMEFT operator by using a novel method of calculating all 2 to 2 and 2 to
3 cross sections, as shown in the right figure.

Future plan:
I would like to explore the possibility of observing new CP violation phenomena in the top-Higgsinteractions at all present and future collides and uncover the origin of the matter-antimatter asymmetryof our universe.

Published paper:
Title：Authors：
Journal：CP-violating top-Higgs coupling in SMEFT
Vernon Barger, Kaoru Hagiwara, Ya-Juan Zheng
Physics Letters B

Publication date：28 February 2024
Scientific Terms
・CPV: Violation of the CP symmetry, or the symmetry between matter and antimatter, is a necessary
condition to explain the observed matter dominated universe.
・SMEFT: Effective Field Theory (EFT) which respects the gauge symmetries of the Standard Model(SM) of particle physics
・Gauge invariance: Invariance under the gauge symmetries allows the SM to have predictions, whichshould be confronted against experimental data.
・Unitarity: Conservation of probability is an essential principle of all physical theories, which is expressedas the unitarity of scattering amplitudes in the quantum field theory.

This research was obtained as a part of the following research project:
・ The Ministry of Education, Culture, Sports, Science and Technology (MEXT) Grants-in-Aid for
Scientific Research (B) 「21H01077」 Principal Investigator: Mawatari Kentarou・ The Ministry of Education, Culture, Sports, Science and Technology (MEXT) Grants-in-Aid for
Scientific Research (C) 「23K03403」 Principal Investigator: Zheng Yajuan

Inquiries toZheng Yajuan
Faculty of Education, Iwate University
Tel：019-621-6561
email：yjzheng@iwate-u.ac.jp