Speaker
Description
Beyond the Standard Model physics may have new sources of CP violation. In particular, CP violation in the Yukawa interactions is welcome for electroweak baryogenesis. The most general CP violating $H\psi\psi$ Yukawa interaction can be written as $ -\frac{m_\psi}{v}\bar\psi ( a + ib\gamma_5)\psi$, where $a,b$ are real valued parameters ($a^{SM} = 1, b^{SM}=0$). If both $a\neq 0$ and $b\neq 0$, it implies CP violation in $H\psi\psi$ Yukawa interaction.
The parameters $b_\psi$ are strongly constrained by the experimental bounds on the electron and neutron Electric Dipole Moments. The $\tau$ lepton Yukawa coupling is of interest as it is large and the EDM bound, $b_\tau < 0.3$, is weak enough for it to play a role in electroweak baryogenesis. CP violation in the $\tau$ Yukawa has also been searched for at the LHC. The recent study by CMS probing $H \to \tau^+ \tau^-$ gives $|b_\tau | <0.34$ at 1$\sigma$ level. Majority of the experimental studies concentrate on measurements of the angle between $\tau$ decay planes determined by the directions of particles produced in $\tau$ lepton decays, such as in $H \to \tau^+ \tau^- \to \pi^+ \pi^- \nu_\tau \bar\nu_\tau $ or $H \to \tau^+ \tau^- \to \rho^+ \rho^- \nu_\tau \bar\nu_\tau$.
We propose to measure the forward-backward asymmetry of $\tau$ lepton angular distribution in the $H \to \tau^+ \tau^-\gamma$ decay, as a measure of the CP violation in $H \tau \tau$ Yukawa interaction. The source of such asymmetry is the interference of the CP-even loop-level contribution coming from $H \to Z\gamma \to \tau^+ \tau^-\gamma$ decay channel with the contribution from tree-level CP-odd Yukawa interaction. We find that the CP violating effect is maximum when the invariant mass of the $\tau^+ \tau^-$ pair is equal to the mass of the $Z$ boson. We propose and utilise various Dalitz plot asymmetries to quantify the maximal size of the asymmetry and perform Monte Carlo simulations to study the feasibility of measuring it in the high luminosity phase of the Large Hadron Collider. The lower branching ratio than the $H \to \tau^+ \tau^-$ is partially compensated by the fact that one only requires to reconstruct the 4-momenta of the $\tau$ leptons and not the full spatial distributions of the final $\tau$ decay products. Our heuristic Monte Carlo simulations for the HL-LHC show that one can possibly probe $b_\tau$ using our proposed methodology.
