Search for B^{+}→K^{+}νν[over ¯] Decays Using an Inclusive Tagging Method at Belle II

Area of Science:

• Particle Physics
• High-Energy Physics
• Experimental Physics

Background:

• Flavor-changing neutral-current (FCNC) decays are rare processes predicted by the Standard Model.
• Observing FCNC decays can reveal new physics beyond the Standard Model.
• The decay B+→K+νν̄ is a prime channel to search for New Physics.

Purpose of the Study:

• To search for the FCNC decay B+→K+νν̄ at the Belle II experiment.
• To set an upper limit on the branching fraction of this decay.
• To constrain new physics models by searching for this rare decay.

Main Methods:

• Utilized data from the Belle II experiment at the SuperKEKB collider.
• Employed a novel inclusive tagging approach to suppress backgrounds.
• Analyzed a data sample of 63 fb⁻¹ at the ϒ(4S) resonance and 9 fb⁻¹ below resonance.

Main Results:

• No significant signal for the B+→K+νν̄ decay was observed.
• An upper limit on the branching fraction was set at 4.1×10⁻⁵ at the 90% confidence level.
• The inclusive tagging method demonstrated improved signal efficiency.

Conclusions:

• The search for B+→K+νν̄ did not yield a significant signal.
• The obtained upper limit provides constraints on new physics scenarios.
• Further data collection at Belle II will improve sensitivity to this rare decay.