Search for B^{0}→K^{*0}τ^{+}τ^{-} Decays at the Belle II Experiment.

Area of Science:

• Particle Physics
• High-Energy Physics
• Standard Model Physics

Background:

• Flavor-changing neutral-current decays are rare processes predicted by the Standard Model.
• Experimental searches for these decays test the Standard Model's predictions and probe for new physics beyond it.
• The B^{0}→K^{*0}τ^{+}τ^{-} decay is particularly interesting due to the involvement of the τ lepton, which has a large mass.

Purpose of the Study:

• To search for the rare flavor-changing neutral-current decay B^{0}→K^{*0}τ^{+}τ^{-}.
• To set an upper limit on the branching fraction of this decay using data from the Belle II experiment.
• To provide the most stringent constraint to date on the B^{0}→K^{*0}τ^{+}τ^{-} branching fraction.

Main Methods:

• Utilized a 365 fb^{-1} data sample collected by the Belle II experiment at the SuperKEKB electron-positron collider.
• Reconstructed one B meson in a hadronic decay mode and its companion B meson decaying into K^{*0} and two τ leptons.
• Reconstructed τ leptons in final states involving electrons, muons, charged pions, charged ρ mesons, and neutrinos.

Main Results:

• An upper limit on the branching fraction of B(B^{0}→K^{*0}τ^{+}τ^{-})<1.8×10^{-3} was established at the 90% confidence level.
• This result represents the most stringent constraint reported to date for this decay.
• The search provides valuable data for testing theoretical models of rare B meson decays.

Conclusions:

• The Belle II experiment has set a new world's most stringent upper limit on the branching fraction of the B^{0}→K^{*0}τ^{+}τ^{-} decay.
• This result constrains new physics scenarios that could enhance this rare decay.
• Further data collection and analysis by Belle II will continue to improve sensitivity to rare B meson decays.