TY - JOUR

T1 - Observation of Λ+c → nπ+η and search for Λ+c → na0(980)+

AU - BESIII Collaboration

AU - Ачасов, Михаил Николаевич

AU - Мучной, Николай Юрьевич

AU - Николаев, Иван Борисович

N1 - The BESIII Collaboration thanks the staff of BEPCII (https://cstr.cn/31109.02.BEPC) 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 Nos. 2023YFA1606000, 2023YFA1606704; National Natural Science Foundation of China (NSFC) under Contracts Nos. 11635010, 11935015, 11935016, 11935018, 12025502, 12035009, 12035013, 12061131003, 12192260, 12192261, 12192262, 12192263, 12192264, 12192265, 12221005, 12225509, 12235017, 12342502, 12361141819; the Chinese Academy of Sciences (CAS) Large-Scale Scientific Facility Program; CAS Project for Young Scientists in Basic Research No. YSBR-117; the Strategic Priority Research Program of Chinese Academy of Sciences under Contract No. XDA0480600; CAS under Contract No. YSBR-101; Postdoctoral Fellowship Program of China Postdoctoral Science Foundation under Grant No. GZC20261827; 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 Nos. 2021.0174, 2021.0299, 2023.0315; 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 - 2026/8/3

Y1 - 2026/8/3

N2 - By analysing 6.1 fb−1 of data collected at centre-of-mass energies between√s = 4.600 and 4.843 GeV with the BESIII detector at the BEPCII collider, we observe the decay Λ+c → nπ+η for the first time with a statistical significance of 9.5σ. The ratio of branching fractions B(Λ+c → nπ+η)/B(Λ+c → Λπ+η) is measured to be 0.155 ± 0.031stat. ± 0.012syst. Taking the world average of B(Λ+c → Λπ+η) as reference, the absolute branching fraction is calculated to be B(Λ+c → nπ+η) = (2.94 ± 0.59stat. ± 0.23syst. ± 0.13ref.) × 10−3. The intermediate process Λ+c → na0(980)+ is also searched for in the π+η invariant mass spectrum. Since no significant signal is found, the upper limit on B(Λ+c → na0(980)+) × B(a0(980)+ → π+η) is set to 8.4 × 10−4 at 90% confidence level. A sophisticated deep learning approach using a Transformer-based architecture is employed to distinguish signals from prevalent hadronic backgrounds, complemented by thorough validation and systematic uncertainty quantification.

AB - By analysing 6.1 fb−1 of data collected at centre-of-mass energies between√s = 4.600 and 4.843 GeV with the BESIII detector at the BEPCII collider, we observe the decay Λ+c → nπ+η for the first time with a statistical significance of 9.5σ. The ratio of branching fractions B(Λ+c → nπ+η)/B(Λ+c → Λπ+η) is measured to be 0.155 ± 0.031stat. ± 0.012syst. Taking the world average of B(Λ+c → Λπ+η) as reference, the absolute branching fraction is calculated to be B(Λ+c → nπ+η) = (2.94 ± 0.59stat. ± 0.23syst. ± 0.13ref.) × 10−3. The intermediate process Λ+c → na0(980)+ is also searched for in the π+η invariant mass spectrum. Since no significant signal is found, the upper limit on B(Λ+c → na0(980)+) × B(a0(980)+ → π+η) is set to 8.4 × 10−4 at 90% confidence level. A sophisticated deep learning approach using a Transformer-based architecture is employed to distinguish signals from prevalent hadronic backgrounds, complemented by thorough validation and systematic uncertainty quantification.

KW - Branching fraction

KW - Charm Physics

KW - e +-e − Experiments

KW - Particle and Resonance Production

UR - https://www.scopus.com/pages/publications/105047841981

U2 - 10.1007/JHEP08(2026)009

DO - 10.1007/JHEP08(2026)009

M3 - Article

VL - 2026

JO - Journal of High Energy Physics

JF - Journal of High Energy Physics

SN - 1029-8479

IS - 8

M1 - 9

ER -