The BESIII collaboration recently reported the observation of the W-Annihilation Process D_s⁺→ωρ⁺ and the measurement of D_s⁺→ϕρ⁺. The results have been published in Physics Review Letters on May 22, 2025 [PhysRevLett.134.201902].
The polarization information of heavy-flavor mesons decaying into two vector particles(V) has attracted the attention of physicists for decades because of its unique advantage in the probe of new physics and novel phenomena in hadron structures. In the charm sector, it is generally predicted that the transverse polarization dominates over the longitudinal one in D to VV decays, while quantitative discrepancies show in many measurements, which is known as “polarization puzzle”. Among the decays, the comparison between the pure W-annihilation decay D_s⁺→ωρ⁺ and pure W-external decay D_s⁺→ϕρ⁺ offer an ideal approach to investigate the mechanism behind the polarization puzzle. Figure 1 shows the topological diagrams for D_s⁺→ωρ⁺ and D_s⁺→ϕρ⁺.
 

Figure 1: The topological diagram for (a) D_s⁺→ωρ⁺ and (b) D_s⁺→ϕρ⁺

Based on 7.33 fb^-1 of e⁺e^- annihilation data collected at center-of-mass energies between 4.128 and 4.226 GeV with the BESIII detector, the first observation of the pure W-annihilation decay D_s⁺→ωρ⁺, and the anomalous dominance of the D wave over the S and P waves, with a fraction of (51.85±7.28_-7.90^+4.83)% are reported. The branching fraction of D_s⁺→ϕρ⁺ is measured to be (3.98±0.33_-0.19^+0.21)%. Figure 2 shows the comparison between the data and the obtained fit result.

Figure 2: The comparison between the data and the obtained fit result

The measured branching fraction provides the first direct experimental determination on a W-annihilation process in D to VV decays. In comparison to the dominance of the S wave and the low significance of the D wave in the pure external W-emission decay D_s⁺→ϕρ⁺, the dominance of D wave inD_s⁺→ωρ⁺ deviates from the expectation of the naïve factorization model, and is expected to offer important insights for unraveling the “polarization puzzle”. Furthermore, the R_ϕ=B(ϕ→π⁺ π^- π⁰)/B(ϕ→K⁺ K^-) is measured to be (0.222±0.019_-0.016^+0.016), which is consistent with the previous measurement in charm-strange meson but deviates from the world average value by more than 3σ.

Reference: BESIII Collaboration, Phys. Rev. Lett. 134. 201902(2025)
Journal publication: https://link.aps.org/doi/10.1103/PhysRevLett.134.201902
DOI: 10.1103/PhysRevLett.134.20190