Abstract
We report the first observation of the radiative decay of the Upsilon(1S) into a charmonium state. The significance of the observed signal of Upsilon(1S) -> gamma chi(c1) is 6.3 standard deviations including systematics. The branching fraction is calculated to be B[Upsilon(1S) -> gamma chi(c1)] = [4.7(-1.8)(+2.4) (stat)(-0.5)(+0.4) (sys) x 10(-5)]. We also searched for Upsilon(1S) radiative decays into chi(c0.2) and eta(c)(1S, 2S), and set upper limits on their branching fractions. These results are obtained from a 24.9 fb(-1) data sample collected with the Belle detector at the KEKB asymmetric-energy e(+)e(-) collider at a center-of-mass energy equal to the Upsilon(2S) mass using Upsilon(1S) tagging by the Upsilon(2S) -> Upsilon(1S)pi(+)pi(-) transitions.
We thank the KEKB group for excellent operation of the accelerator; the KEK cryogenics group for efficient solenoid operations; and the KEK computer group, the NII, and PNNL/EMSL for valuable computing and SINET5 network support. We acknowledge support from MEXT, JSPS and Nagoya's TLPRC (Japan); ARC (Australia); FWF (Austria); NSFC and CCEPP (China); MSMT (Czechia); CZF, DFG, EXC153, and VS (Germany); DST (India); INFN (Italy); MOE, MSIP, NRF, RSRI, FLRFAS project, GSDC of KISTI and KREONET/GLORIAD (Korea); MNiSW and NCN (Poland); RSF, Grant No. 18-12-00226 (Russia); ARRS (Slovenia); IKERBASQUE (Spain); SNSF (Switzerland); MOE and MOST (Taiwan); and DOE and NSF (USA).