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The Belle II experiment is a particle physics experiment that uses the asymmetric energy electron-positron collider, SuperKEKB, in Tsukuba, Japan. The Belle II detector and SuperKEKB accelerator have been upgraded to study decays of B meson pairs from the Υ(4S) meson with a data sample 50 times larger than the Belle experiment in order to precisely test Standard Model (SM) and search for physics beyond the SM.
The SM forbids the $b \to d/s$ quark transition, which is a flavor-changing neutral current, at the tree level and occurs via penguin or box diagrams. Such decays are sensitive to non-SM particle which may enter into the loop diagram. For example, the two-Higgs-doublet model (2HDM) introduces an additional Higgs doublet and the charged Higgs may appear in the loop instead of a $W$ boson. This new physics contribution affects the branching fraction that depends on the mass of the charged Higgs.
We focus on $B \to K K_S^0 \gamma$ radiative electroweak penguin decays through $b \to d$ quark transition with a high energy photon. $B^0$ meson decays to two identical charge-parity (CP) eigenstate mesons with spin 0 and a high energy real photon for $B^0 \to K_S^0 K_S^0 \gamma$ (“neutral mode”). The angular momentum of an intermediate state decaying into two identical particles $K_S^0 K_S^0$ has the additional requirement that the quantum number be an even number greater than 0. An $f_2$ meson with spin 2 is chosen as the intermediate particle in this study decaying to two $K_S^0$ mesons. The mass of the $f_2$ meson is set to be flat in a range of 1–3 GeV/c$^2$. For $B^+ \to K^+K_S^0\gamma$ (“charged mode”), $B^+$ decays to $K^+$, $K_S^0$, and a prompt photon with three body phase space decay model.
The dominant background is an $e^+ e^- \to qq$ continuum event ($q = u, d, c, s$). In the invariant mass spectrum of $K_S^0 \gamma$ of the charged mode, there is a peaking background from $B^+B^-$. Multivariate analysis (MVA) with the Fast Boosted Decision Tree (FastBDT) package is used to suppress the continuum and peaking backgrounds.
Total systematic uncertainty is calculated to be 6.9% (6.3%) for the neutral (charged) signal mode. The signal reconstruction efficiencies are estimated to be (3.07±0.01)% and (1.72±0.00)% for the neutral and the charged modes, respectively. The expected numbers of the signal and the background are es- timated to be 0.52±0.13 (0.57±0.09) and 11.8±1.6 (30.2±1.9) for the neutral (charged) mode in the case of a data sample of 500 fb$^{−1}$ BB collected at the $\Upsilon(4S)$ resonance.
Using the profile likelihood method, upper limit sensitivities are obtained to be $< 4.4 × 10^{−7} (< 1.2 × 10^{−6})$ at a 90% confidence level for the neutral (charged) signal mode. These results are the first report for $B^0 \to K_S^0 K_S^0 \gamma$ and $B^+ \to K^+ K_S^0 \gamma$.
Supervisor: Prof. Hwanbae Park