A new observable to probe $CP$ violation in $H\gamma\gamma$ coupling
by
B105
CHEP
The diphoton invariant mass distribution of interference between $gg\to H \to \gamma\gamma$ and $gg\to \gamma\gamma$ at leading order is an almost antisymmetric lineshape around Higgs mass $M_H$. We propose an integral odd around $M_H$ ($\int^{M_H}-\int_{M_H}$) to get the contribution of interference and divide it by the total cross section of Higgs signal, which makes a new observable $A_{int}$. We investigate the $A_{int}$ in Standard Model and various $CP$-violating $H\gamma\gamma$ cases.
It could reach about $10\%$ in SM, and the significance of deviation caused by $CP$ violation could be large as $5\sim10\sigma$, which could constrain the $CP$ violation phase $\xi_\gamma\notin[\pi/2,3\pi/2]$ at $99.9\%$ confidence level.
The $A_{int}$ with both $CP$-violating $H\gamma\gamma$ and $Hgg$ couplings are also studied and the significance is given.
