Study of the semileptonic decay D + s → f 0 ( 980 ) e + ν and implications for B 0 s → J / ψf 0

Using e+e− → D− s D∗+ s and D∗− s D+ s interactions at 4170 MeV collected with the CLEOc detector, we investigate the semileptonic decays D+ s → f0(980)eν, and D+ s → φe+ν. By examining the decay rates as functions of the four-momentum transfer squared, q2, we measure the ratio [ dB dq2 (D+ s → f0(980)eν)B(f0 → π+π−) ] / [ dB dq2 (D+ s → φe+ν)B(φ → K+K−) ] at q2 of zero to be (42±11)%. This ratio has been predicted to equal the rate ratio [B(Bs → J/ψf0)B(f0 → π+π−)] / [B(Bs → J/ψφ)B(φ → K+K−)], thus indicating that the CP eigenstate J/ψf0 could be useful for measuring CP violation via Bs mixing. Assuming a simple pole model for the form factor |f+(q)| in the f0eν decay, we find a pole mass of (1.7 −0.7±0.2) GeV. We also determine the f0 mass and width as (977 −9 ±1) MeV, and (91 +30 −22±3) MeV, respectively. In addition, we present updated results for B(D+ s → f0(980)eν)B(f0 → π+π−) = (0.20±0.03±0.01)%, and B(D+ s → φe+ν) = (2.36 ± 0.23 ± 0.13)%. Assuming that the f0 wavefunction is a combination of strange and non-strange quark-antiquark components, we use our measurement for B(D+ s → f0(980)eν) to extract a value of the mixing angle that we find consistent with | ss〉 dominance, adding to the mystery as to why the f0 decays predominantly to two pions rather than two kaons. PACS numbers: 13.20.Fc, 12.38.Qk, 14.40.Lb