TY - JOUR

T1 - Observation of a Threshold Enhancement in the π+π- Spectrum in ψ (3686) →π+π-J/ψ Decays

AU - BESIII Collaboration

AU - Ачасов, Михаил Николаевич

AU - Мучной, Николай Юрьевич

AU - Николаев, Иван Борисович

N1 - The BESIII Collaboration thanks the staff of BEPCII [37] and the IHEP computing center for their strong support. This work is supported in part by National Key R&D Program of China under Contracts No. 2025YFA1613900, No. 2023YFA1606000, and No. 2023YFA1606704; National Natural Science Foundation of China (NSFC) under Contracts No. 11635010, No. 11935015, No. 11935016, No. 11935018, No. 12025502, No. 12035009, No. 12035013, 12061131003, No. 12192260, No. 12192261, No. 12192262, No. 12192263, No. 12192264, No. 12192265, No. 12221005, No. 12225509, No. 12235017, and No. 12361141819; the Chinese Academy of Sciences (CAS) Large-Scale Scientific Facility Program; the Strategic Priority Research Program of Chinese Academy of Sciences under Contract No. XDA0480600; CAS under Contract No. YSBR-101; 100 Talents Program of CAS; The Institute of Nuclear and Particle Physics (INPAC) and Shanghai Key Laboratory for Particle Physics and Cosmology; ERC under Contract No. 758462; German Research Foundation DFG under Contract No. FOR5327; Istituto Nazionale di Fisica Nucleare, Italy; Knut and Alice Wallenberg Foundation under Contracts No. 2021.0174 and No. 2021.0299; Ministry of Development of Turkey under Contract No. DPT2006K-120470; National Research Foundation of Korea under Contract No. NRF-2022R1A2C1092335; National Science and Technology fund of Mongolia; Polish National Science Centre under Contract No. 2024/53/B/ST2/00975; STFC (United Kingdom); Swedish Research Council under Contract No. 2019.04595; U.S. Department of Energy under Contract No. DE-FG02-05ER41374.

PY - 2026/4/8

Y1 - 2026/4/8

N2 - Based on the (2712.4±14.4)×106 휓⁡(3686) events collected with the BESIII detector, we present a high-precision study of the 휋+⁢휋− mass spectrum in 휓⁡(3686) →휋+⁢휋−⁢퐽/휓 decays. A clear resonancelike structure is observed near the 휋+⁢휋− mass threshold for the first time. A fit with a Breit-Wigner function yields a mass of 285.6±2.5  MeV/푐2 and a width of 16.3 ±0.9  MeV with a statistical significance exceeding 10⁢휎. To interpret the data, we incorporate final-state interactions (FSI) within two theoretical frameworks: chiral perturbation theory (ChPT) and QCD multipole expansion (QCDME). ChPT describes the spectrum above 0.3  GeV/푐2 but fails to reproduce the threshold enhancement. In contrast, the QCDME model—assuming the 휓⁡(3686) is an admixture of S- and D-wave charmonium—reproduces the data well. The pronounced dip near 0.3  GeV/푐2 offers new insight into the interplay between chiral dynamics and low-energy QCD.

AB - Based on the (2712.4±14.4)×106 휓⁡(3686) events collected with the BESIII detector, we present a high-precision study of the 휋+⁢휋− mass spectrum in 휓⁡(3686) →휋+⁢휋−⁢퐽/휓 decays. A clear resonancelike structure is observed near the 휋+⁢휋− mass threshold for the first time. A fit with a Breit-Wigner function yields a mass of 285.6±2.5  MeV/푐2 and a width of 16.3 ±0.9  MeV with a statistical significance exceeding 10⁢휎. To interpret the data, we incorporate final-state interactions (FSI) within two theoretical frameworks: chiral perturbation theory (ChPT) and QCD multipole expansion (QCDME). ChPT describes the spectrum above 0.3  GeV/푐2 but fails to reproduce the threshold enhancement. In contrast, the QCDME model—assuming the 휓⁡(3686) is an admixture of S- and D-wave charmonium—reproduces the data well. The pronounced dip near 0.3  GeV/푐2 offers new insight into the interplay between chiral dynamics and low-energy QCD.

KW - Quantum chromodynamics

KW - Strong interaction

KW - Exotic mesons

KW - Light mesons

KW - Mesons

KW - Pions

KW - Quarkonia

UR - https://www.scopus.com/pages/publications/105036547193

U2 - 10.1103/5h6z-p6nl

DO - 10.1103/5h6z-p6nl

M3 - Article

VL - 136

JO - Physical Review Letters

JF - Physical Review Letters

SN - 0031-9007

IS - 14

M1 - 141902

ER -