First measurement of mixing-induced CP violation in B0→π0π0 decays with Belle II

About

Belle II at the SuperKEKB asymmetric-energy e+e− collider is pushing the intensity frontier with unprecedented instantaneous luminosities. Having recently surpassed the Belle data sample, Belle II is entering a new phase in which growing statistics, state-of-the-art detector performance, and novel analysis techniques are opening new opportunities for precision flavour physics.

We present the first measurement of mixing-induced charge-parity (CP) violation in B0→π0π0 decays, a key input to the isospin determination of the CKM angle φ2/α from B→ππ transitions. Accessing decay-time-dependent CP violation in B0→π0π0 decays had long been considered remote: the conventional method would require rare π0→e+e−γ topologies to reconstruct the signal decay position, and therefore data samples of O(10) ab−1. We overcome this limitation with a new approach that exploits the quantum entanglement of the B0B0-bar pair produced in Υ(4S) decays, using only the tag-side B decay time to access the signal time evolution.

With 0.4 ab−1, we determine the mixing-induced CP-violating parameter in B0→π0π0 decays and significantly improve the φ2/α constraint from B→ππ. Enabled by the SuperKEKB nanobeam scheme and the excellent vertex resolution of the Belle II pixel detector, the method opens a new route to time-dependent CP-violation studies in final states without a reconstructible decay vertex.

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