Search for $B^0 \to K_S^0 K_S^0 \gamma$ Using the Full Data Sample of the Belle Experiment

by Hye Bin Jeon (Ph.D Candidate)

Tuesday Nov 17, 2020, 9:30 AM → 11:30 AM Asia/Seoul

119호 (제1과학관)

The Belle experiment at KEKB has studied decays of $B$-meson pairs in order to test precisely the Standard Model (SM) of particle physics and search new physics beyond the SM. Out of $B$ meson decays, radiative rare decays have very small branching fractions so that they are suitable in searches for new physics. Especially, a $B^0$ decay to two identical CP eigenstate scalar or pseudoscalar mesons with a high-energy photon hardly takes place since this can occur through a $b \to d$ quark transition which is explained by a penguin or a box diagram in the SM.

We report a study based on Monte-Carlo simulation for searching $B^0 \to  K_S^0 K_S^0 \gamma$ decays prior to an analysis of the experimental data of 711 fb$^{−1}$ for $B$-meson pairs collected by the Belle detector. In the $B^0 \to  K_S^0 K_S^0 \gamma$ decay, the system consisting of two $K_S^0$ in the final state can have an even quantum number of the angular momentum greater than 0. Thus the signal MC sample is generated in which the $K_S^0 K_S^0$ system has spin 2 and an uniform invariant mass from 1 GeV/$c^2$ to 3 GeV/$c^2$.

The background events of $10.8 \pm 0.7$ with a few signal events for the full data sample of the Belle are expected in the signal region ($-0.16~\mbox{GeV} < \Delta E < 0.09~\mbox{GeV}$ and $5.272~\mbox{GeV}/c^2 < M_{bc} < 5.290~\mbox{GeV}/c^2$). Suppose that ten events in- cluding the signal and background events are observed in the experimental data, we estimate an upper limit sensitivity for a branching fraction at 90% confidence level as follows:

$\mathcal{B}(B^0 →K_S^0 K_S^0 \gamma) < 2.4 \times 10^{−7}$

In addition, regarding the angular property of the $K_S^0 K_S^0$, the helicity angle is additionally considered as a parameter for further background suppression. Total backgrounds are reduced to $5.5 \pm 0.2$. Assuming five observed events, the upper limit sensitivity is estimated to be $< 1.8 (1.9)$ in an order of $10^{−7}$ without (with) consideration of the systematic uncertainty. These results are consistent with the expected branching fraction of an order of $O(10^{−8})$ which is estimated from the world average of $\mathcal{B}(B^0 \to K^0 \phi \gamma)$.

Thesis Advisor: Prof. Hwanbae Park