Observation of the Radiative Decays of ϒ(1S) to χ_{c1}

Area of Science:

• Particle Physics
• High Energy Physics
• Quantum Chromodynamics

Background:

• The Upsilon(1S) meson is a bottomonium state crucial for studying the strong force.
• Radiative decays provide insights into the internal structure and interactions of heavy mesons.
• Previous searches for Upsilon(1S) radiative decays into charmonium states were inconclusive.

Purpose of the Study:

• To report the first observation of the Upsilon(1S) radiative decay into a charmonium state.
• To measure the branching fraction for the Upsilon(1S) -> chi_c1 decay.
• To set upper limits on branching fractions for other Upsilon(1S) radiative decays.

Main Methods:

• Analysis of a 24.9 fb^-1 data sample collected by the Belle detector at the KEKB collider.
• UtilizedUpsilon(2S) -> Upsilon(1S) pi+ pi- transitions for Upsilon(1S) tagging.
• Searched for radiative decay signals Upsilon(1S) -> chi_c0,2 and eta_c(1S,2S).

Main Results:

• Observed the radiative decay Upsilon(1S) -> chi_c1 with a significance of 6.3 standard deviations.
• Measured the branching fraction B[Upsilon(1S) -> chi_c1] = [4.7 +2.4 -1.8 (stat) -0.5 +0.4 (sys)] x 10^-5.
• Set upper limits on branching fractions for decays into chi_c0,2 and eta_c(1S,2S).

Conclusions:

• The first observation of Upsilon(1S) radiative decay into a charmonium state (chi_c1) is confirmed.
• The measured branching fraction provides a precise test of theoretical models of quarkonium decays.
• The results contribute to a comprehensive understanding of heavy quarkonium spectroscopy and transitions.