Scalar leptoquark contributions to the $gg\rightarrow Zh$ process

TL;DR

The study assesses one-loop scalar leptoquark effects on the rare process $gg\rightarrow Zh$, showing that nonzero contributions require off-diagonal Higgs and Z couplings to LQs and give rise to new CP-odd tensor structures. It proves that such effects vanish in one-field and two-field LQ models but can appear in three-field models, with the $S_1+\widetilde{R}_2+S_3$ realization providing a concrete CP-violating example; however, the heavy LQ mass scale suppresses the magnitude, aligning with SMEFT that these effects are captured by dimension-ten operators. The authors provide explicit amplitude expressions, gauge-independence checks, and a thorough heavy-mass expansion, mapping the results to EFT operators and highlighting the negligible phenomenological impact at current colliders. Overall, the work offers a quantitative baseline for scalar LQ contributions to $gg\rightarrow Zh$ and emphasizes the strong screening effect of color-triplet scalars in this channel, while illuminating the potential for CP-odd tensor structures in beyond-SM scenarios.

Abstract

In this manuscript, we study the general scalar leptoquark contributions to the $gg\rightarrow Zh$ process at one-loop level. We find that the contributions only show up for the off-diagonal Higgs and $Z$ boson interactions with scalar leptoquarks, which can lead to new tensor structures different from the standard model. In the one-field and two-field models, the contributions vanish exactly; thus, the contributions are possible at least in the three-field models. We exemplify the contributions in the $S_1+\widetilde{R}_2+S_3$ model, which can appear in the presence of scalar leptoquark mass splittings and $CP$ violation. In view of the heavy scalar leptoquark mass suppression, the leading contributions are typically small and difficult to observe at hadron colliders. However, this work offers a quantitative and systematic investigation of scalar leptoquark contributions, also making it valuable for the new physics studies in other models.

Figures (4)

• Figure 1: Triangle (a, b, c) and box (d, e, f) Feynman diagrams contributing to the $gg\rightarrow Zh$ process. Both the clockwise and counter-clockwise diagrams should be included for each diagram. The Feynman diagrams are generated by FeynArts Hahn:2000kx.
• Figure 2: Feynman diagrams without contributing to the $gg\rightarrow Zh$ process. In the two rows of upper panel (a, b, c, d, e), the diagrams wrapped by the square frame vanish automatically because the color trace is zero. In the row of lower panel (f, g), the diagrams vanish automatically because the diagonal $G^0-\mathrm{LQ}-\mathrm{LQ}$ interactions are absent.
• Figure 3: Feynman diagrams without contributing to the $gg\rightarrow Zh$ process. In the row of upper panel (a), both the clockwise and counter-clockwise triangle diagrams should be included, which make the zero total contribution. In the lower panel (b, c, d), the bubble-like diagrams vanish after loop integration.
• Figure 4: Typical triangle (left) and box (right) diagrams contributing to the $ggZh$ contact effective operators. Here, the indices $i,j,k$ label three different fields. The diagrams are generated by JaxoDraw Binosi:2008ig.