Observation of an Altered a₀(980) Line shape in D⁺ → π⁺ηη

@misc{collaboration2026observation,
  title = {{Observation of an Altered $a_0(980)$ Line shape in $D^+ \rightarrow π^+ηη$}},
  author = {BESIII Collaboration and M. Ablikim and M. N. Achasov and P. Adlarson and X. C. Ai and R. Aliberti and A. Amoroso and Q. An and Y. Bai and O. Bakina and Y. Ban and H. -R. Bao and V. Batozskaya and K. Begzsuren and N. Berger and M. Berlowski and M. Bertani and D. Bettoni and F. Bianchi and E. Bianco and A. Bortone and I. Boyko and R. A. Briere and A. Brueggemann and H. Cai and M. H. Cai and X. Cai and A. Calcaterra and G. F. Cao and N. Cao and S. A. Cetin and X. Y. Chai and J. F. Chang and G. R. Che and Y. Z. Che and C. H. Chen and Chao Chen and G. Chen and H. S. Chen and H. Y. Chen and M. L. Chen and S. J. Chen and S. L. Chen and S. M. Chen and T. Chen and X. R. Chen and X. T. Chen and X. Y. Chen and Y. B. Chen and Y. Q. Chen and Y. Q. Chen and Z. J. Chen and Z. K. Chen and S. K. Choi and X. Chu and G. Cibinetto and F. Cossio and J. Cottee-Meldrum and J. J. Cui and H. L. Dai},
  howpublished = {arXiv},
  year = {2026},
  month = oct,
  doi = {10.48550/arxiv.2606.17724},
  eprint = {2606.17724},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2606.17724},
  note = {CC BY 4.0},
}

TY  - UNPB
TI  - Observation of an Altered a_0(980) Line shape in D^+ → π^+ηη
AU  - BESIII Collaboration
AU  - M. Ablikim
AU  - M. N. Achasov
AU  - P. Adlarson
AU  - X. C. Ai
AU  - R. Aliberti
AU  - A. Amoroso
AU  - Q. An
AU  - Y. Bai
AU  - O. Bakina
AU  - Y. Ban
AU  - H. -R. Bao
AU  - V. Batozskaya
AU  - K. Begzsuren
AU  - N. Berger
AU  - M. Berlowski
AU  - M. Bertani
AU  - D. Bettoni
AU  - F. Bianchi
AU  - E. Bianco
AU  - A. Bortone
AU  - I. Boyko
AU  - R. A. Briere
AU  - A. Brueggemann
AU  - H. Cai
AU  - M. H. Cai
AU  - X. Cai
AU  - A. Calcaterra
AU  - G. F. Cao
AU  - N. Cao
AU  - S. A. Cetin
AU  - X. Y. Chai
AU  - J. F. Chang
AU  - G. R. Che
AU  - Y. Z. Che
AU  - C. H. Chen
AU  - Chao Chen
AU  - G. Chen
AU  - H. S. Chen
AU  - H. Y. Chen
AU  - M. L. Chen
AU  - S. J. Chen
AU  - S. L. Chen
AU  - S. M. Chen
AU  - T. Chen
AU  - X. R. Chen
AU  - X. T. Chen
AU  - X. Y. Chen
AU  - Y. B. Chen
AU  - Y. Q. Chen
AU  - Y. Q. Chen
AU  - Z. J. Chen
AU  - Z. K. Chen
AU  - S. K. Choi
AU  - X. Chu
AU  - G. Cibinetto
AU  - F. Cossio
AU  - J. Cottee-Meldrum
AU  - J. J. Cui
AU  - H. L. Dai
PY  - 2026
DA  - 2026/10/15
PB  - arXiv
DO  - 10.48550/arxiv.2606.17724
UR  - https://arxiv.org/abs/2606.17724
AB  - Using 20.3 fb^−1 of e^+e^- collision data collected with the BESIII detector at √s=3.773 GeV, we perform the first amplitude analysis of the decay D^+→π^+ηη. The intermediate process D^+→ a_0(980)^+η, a_0(980)^+→π^+η, is observed as the only significant component in the amplitude analysis, and its branching fraction is measured to be (3.67±0.12_stat±0.06_syst)×10^−3. The π^+η mass spectrum associated with a_0(980)^+η production exhibits a line shape that differs substantially from those observed in D_(s)→ a_0(980)π and D^0→ a_0(980)^-e^+ν_e decays. We examine several conventional descriptions of the a_0(980) amplitude, including Flatté, dispersively modified Flatté, T-matrix, and K-matrix parameterizations. With reference a_0(980) parameters, neither these models nor their extensions including additional small resonant or non-resonant amplitudes reproduce the observed line shape satisfactorily. When the a_0(980) parameters are allowed to float, satisfactory fits can be obtained, but the pole mass is driven well above the KK̅ threshold, inconsistent with the near-threshold character of the a_0(980). The results reveal a tension between fit quality and the physical pole position in conventional direct-production amplitude models.
ER  -