Untagged B -> X_(s+d) gamma CP asymmetry as a probe for new physics
T. Hurth, E. Lunghi, W. Porod
TL;DR
This work investigates the untagged CP asymmetry in $\bar{B} \to X_{s+d} \gamma$ as a clean probe of new physics beyond the SM. It develops model-independent NLL formulas for inclusive $\bar{B} \to X_q \gamma$ decays, incorporating NP in dipole operators $O_7$, $O_8$ (and their right-handed counterparts) and performing two-loop RG running, while updating SM predictions and exploring correlations with the tagged asymmetries and $A_{\rm CP}$ in $\bar{B} \to X_d \gamma$. The analysis then specializes to MFV scenarios in SUSY, both with and without additional CP-violating phases, and includes a comprehensive numerical study under EDM constraints. It provides explicit expressions and numerical results for branching ratios and CP asymmetries, and derives a practical relation for the untagged asymmetry in terms of the tagged ones, clarifying how new CP phases could manifest in $B$-system observables. The work highlights the untagged asymmetry as a sensitive discriminator of non-CKM CP violation and demonstrates how EDM bounds shape the viable parameter space for MFV and NP effects.
Abstract
The direct CP asymmetry in the untagged inclusive channel B -> X_(s+d) gamma provides a strict test of the standard model. It has been shown beyond the partonic level that this asymmetry is negligibly small thanks to U-spin relations and to the unitarity of the CKM matrix. In the present paper we investigate this relation beyond the SM; in particular, we analyse to which extent deviations from this prediction are possible in supersymmetric scenarios. We analyse the minimal flavour violation scenario, including tan(beta) enhanced terms and using the complete two-loop renormalization group running. Our analysis fully takes into account also the EDM constraints on the supersymmetric phases. We investigate possible correlations between the tagged and the untagged CP asymmetries and the indirect sensitivity of the latter to the B -> X_d gamma CP asymmetry. Furthermore, we derive general model-independent formulae for the branching ratios and CP asymmetries for the inclusive B -> X_d gamma and B -> X_s gamma modes, and update the corresponding SM predictions.