School of Physics

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Measurement of the branching fraction of B0 → π0 π0 decays using the final Belle dataset

JULIUS, T'MIR ( 2015)

This thesis presents a measurement of the branching fraction of B0 → π0 π0 using data collected by the Belle experiment based at the KEKB accelerator in Tsukuba, Japan. This study is performed using the final and complete Belle dataset of 711 fb^−1, comprising about 771 × 10^6 BB pairs collected at the Υ(4S) resonance. The measurement of B0 → π0 π0 is a vital component of the study into the B → ππ system, through which a measurement of the CP violation parameter φ2 (also called α) can be made. This analysis makes use of an artificial neural network to improve continuum suppression as well as making use of timing information pertaining to photons detected in the Belle electromagnetic calorimeter to help distinguish between signal and background originating from QED backgrounds. Photon reconstruction was enhanced through the recovery of photons that converted within the inner detector. The timing information for 20% of the Belle dataset was recovered specifically for the use of this analysis. This is the first time a branching fraction for B → π0 π0 has been presented using an artificial neural network and such QED suppression techniques. The branching fraction of B → π0 π0 is measured to be Br(B → π0 π0 ) = (1.22 +0.28 −0.27,+0.10 −0.12 ) × 10^−6.