Abstract
We study Higgs boson production associated with single top or anti-top via t-channel weak boson exchange at the LHC. The process is an ideal probe of the top quark Yukawa coupling, because we can measure the relative phase of htt and hWW couplings, thanks to the significant interference between the two amplitudes. By choosing the emitted W momentum along the polar axis in the th ( overline{t}h ) rest frame, we obtain the helicity amplitudes for all the contributing subprocesses analytically, with possible CP phase of the Yukawa coupling. We study the azimuthal asymmetry between the W emission and the Wb ( overline{b} ) → t( overline{t} ) h scattering planes, as well as several t and overline{t} polarization asymmetries as a signal of CP violating phase in the htt coupling. Both the azimuthal asymmetry and the polarization perpendicular to the scattering plane are found to have the opposite sign between the top and anti-top events. We identify the origin of the sign of asymmetries, and propose the possibility of direct CP violation test in pp collisions by comparing the top and anti-top polarization at the LHC.
Highlights
We can measure the phase of the htt coupling with respect to that of the hW W coupling, whose magnitude and phase have already been constrained rather well [5,6,7] and will be determined precisely in the HL-LHC era at the level of 4% or better [8]
We study Higgs boson production associated with single top or anti-top via t-channel weak boson exchange at the LHC
We study the azimuthal asymmetry between the W emission and the W b ( ̄b) → t(t) h scattering planes, as well as several t and tpolarization asymmetries as a signal of CP violating phase in the htt coupling
Summary
In the SM, four subprocess contribute to single top plus Higgs production in the leading order ub → dth (cb → sth) db → uth (sb → cth). Because both A and B terms are positive definite, their magnitudes are smallest at ξ = 0 (SM), where the W exchange term A and the t-exchange term B interfere destructively, whereas they become largest at |ξ| = π where the two terms interfere constructively, explaining the order of magnitude difference in the total cross section between ξ = 0 and |ξ| = π shown in figure 1 This strong interference between the two amplitudes gives the opportunity to accurately measure the htt Yukawa coupling with respect to the hW W coupling. Another important observation from the above approximation is that the CP-violation (CPV) effects proportional to sin ξ are significant only in the amplitude of the right-handed helicity top quark, M+, because M− is proportional to e−iξ + e+iξ = 2 cos ξ at large W where δ = δ. We find M− to have significant imaginary part proportional to sin ξ, whereas M+ is almost proportional to cos ξ, which are opposite of what we find for the single t and h production amplitudes
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