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The decays J/ψ→ηΣ+Σ¯− and ψ(3686)→ηΣ+Σ¯− are observed for the first time, using (10087±44)×106 J/ψ and (448.1±2.9)×106 ψ(3686) events collected with the BESIII detector at the BEPCII collider. We determine the branching fractions of these two decays to be B(J/ψ→ηΣ+Σ¯−)=(6.34±0.21±0.37)×10−5 and B(ψ(3686)→ηΣ+Σ¯−)=(9.59±2.37±0.61)×10−6, where the first uncertainties are statistical and the second are systematic. The ratio of these two branching fractions is determined to be B(ψ(3686)→ηΣ+Σ¯−)B(J/ψ→ηΣ+Σ¯−)=(15.1±3.8)%, which is in agreement with the "12\% rule."
Using a data sample corresponding to an integrated luminosity of 10.9 fb−1 collected at center-of-mass energies from 4.16 to 4.34 GeV with the BESIII detector, we search for the decay χc1(3872)→π+π−χc1 in the radiative production e+e−→γχc1(3872). No significant signal is observed, and the ratio for the branching fraction of χc1(3872)→π+π−χc1 to χc1(3872)→π+π−J/ψ is measured as R≡B[χc1(3872)→π+π−χc1]B[χc1(3872)→π+π−J/ψ]<0.18 at 90% confidence level. The upper limit on the product of the cross section σ[e+e−→γχc1(3872)] and the branching fraction B[χc1(3872)→π+π−χc1] at each center-of-mass energy is also given. These measurements favor the non-conventional charmonium nature of the χc1(3872) state.
Using data samples collected at center-of-mass energies between 2.000 and 3.080 GeV with the BESIII detector operating at the BEPCII collider, a partial-wave analysis is performed on the process e+e−→ηπ+π−. In addition to the dominant e+e−→ρη component, the e+e−→a2(1320)π process is also sizeable, contributing up to 24% of the total reaction. The measured cross sections of the process e+e−→ηπ+π− are systematically higher than those of BaBar by more than 3σ at center-of-mass energies between 2.000 and 2.300 GeV. In the cross section lineshape for e+e−→a2(1320)π, a resonant structure is observed with a significance of 5.5σ, with M=(2044±31±4) MeV/c2, Γ=(163±69±24) MeV and BR⋅ΓRe+e−=(34.6±17.1±6.0) eV or (137.1±73.3±2.1) eV. In the cross section lineshape for e+e−→ρη, an evidence of a dip structure around 2180 MeV/c2 is observed with statistical significance of 3.0σ.
The measurement of the Cabibbo-favored semileptonic decay Λ+c→Λμ+νμ is reported using 4.5 fb−1 of e+e− annihilation data collected at center-of-mass energies ranging from 4.600~GeV to 4.699~GeV. The branching fraction of the decay is measured to be B(Λ+c→Λμ+νμ)=(3.48±0.14stat.±0.10syst.)%, three times more precise than the prior world average result. Tests of lepton flavor universality using Λ+c→Λℓ+νℓ (ℓ=e,μ) decays are reported for the first time, based on measurements of the differential decay rates and the forward-backward asymmetries in separate four-momentum transfer regions. The results are compatible with Standard Model predictions. Furthermore, we improve the determination of the form-factor parameters in Λ+c→Λℓ+νℓ decays, which provide stringent tests and calibration for lattice quantum chromodynamics (LQCD) calculations.
We report the measurement of the inclusive cross sections for e+e−→nOCH (where nOCH denotes non-open charm hadrons) with improved precision at center-of-mass (c.m.) energies from 3.645 to 3.871 GeV. We observe three resonances: R(3760), R(3780), and R(3810) with significances of 8.1σ, 13.7σ, and 8.8σ, respectively. The R(3810) state is observed for the first time, while the R(3760) and R(3780) states are observed for the first time in the nOCH cross sections. Two sets of resonance parameters describe the energy-dependent line shape of the cross sections well. In set I [set II], the R(3810) state has mass (3805.7±1.1±2.7) [(3805.7±1.1±2.7)] MeV/c2, total width (11.6±2.9±1.9) [(11.5±2.8±1.9)] MeV, and an electronic width multiplied by the nOCH decay branching fraction of (10.9±3.8±2.5) [(11.0±3.4±2.5)] eV. In addition, we measure the branching fractions B[R(3760)→nOCH]=(25.2±16.1±30.4)%[(6.4±4.8±7.7)%] and B[R(3780)→nOCH]=(12.3±6.6±8.3)%[(10.4±4.8±7.0)%] for the first time. The R(3760) state can be interpreted as an open-charm (OC) molecular state, but containing a simple four-quark state component. The R(3810) state can be interpreted as a hadrocharmonium state.
Using a sample of about 10 billion J/ψ events with the BESIII detector, we search for the weak decays of J/ψ→D¯0π0+c.c., J/ψ→D¯0η+c.c., J/ψ→D¯0ρ0+c.c., J/ψ→D−π++c.c., and J/ψ→D−ρ++c.c.. Since no significant signal is observed, we set the upper limits of the branching fractions of these decays to be B(J/ψ→D¯0π0+c.c.)<4.7×10−7, B(J/ψ→D¯0η+c.c.)<6.8×10−7, B(J/ψ→D¯0ρ0+c.c.)<5.2×10−7, B(J/ψ→D−π++c.c.)<7.0×10−8, and B(J/ψ→D−ρ++c.c.)<6.0×10−7 at the 90\% confidence level.
Based on 4.4 fb−1 of e+e− annihilation data collected at the center-of-mass energies between 4.60 and 4.70 GeV with the BESIII detector at the BEPCII collider, the pure \textit{W}-boson-exchange decay Λ+c→Ξ0K+ is studied with a full angular analysis. The corresponding decay asymmetry is measured for the first time to be αΞ0K+=0.01±0.16(stat.)±0.03(syst.). This result reflects the non-interference effect between the S- and P-wave amplitudes. The phase shift between S- and P-wave amplitudes has two solutions, which are δp−δs=−1.55±0.25(stat.)±0.05(syst.) rad or 1.59±0.25(stat.)±0.05(syst.) rad.
Improved measurement of the decays η′ → π⁺π⁻π⁺⁽⁰⁾π⁻⁽⁰⁾ and search for the rare decay η′ → 4π⁰
(2024)
Using a sample of 10 billion J/ψ events collected with the BESIII detector, the decays η′→π+π−π+π−, η′→π+π−π0π0 and η′→4π0 are studied via the process J/ψ→γη′. The branching fractions of η′→π+π−π+π− and η′→π+π−π0 π0 are measured to be (8.56±0.25(stat.)±0.23(syst.))×10−5 and (2.12±0.12(stat.)±0.10(syst.))×10−4, respectively, which are consistent with previous measurements but with improved precision. No significant η′→4π0 signal is observed, and the upper limit on the branching fraction of this decay is determined to be less than 1.24×10−5 at the 90% confidence level. In addition, an amplitude analysis of η′→π+π−π+π− is performed to extract the doubly virtual isovector form factor α for the first time. The measured value of α=1.22±0.33(stat.)±0.04(syst.), is in agreement with the prediction of the VMD model.
Based on data samples collected with the BESIII detector at the BEPCII collider, the process e+e−→Σ+Σ¯− is studied at center-of-mass energies s√ = 2.3960, 2.6454, and 2.9000 GeV. Using a fully differential angular description of the final state particles, both the relative magnitude and phase information of the Σ+ electromagnetic form factors in the timelike region are extracted. The relative phase between the electric and magnetic form factors is determined to be sinΔΦ = -0.67~±~0.29~(stat)~±~0.18~(syst) at s√ = 2.3960 GeV, ΔΦ = 55∘~±~19∘~(stat) ±~14∘~(syst) at s√ = 2.6454 GeV, and 78∘~±~22∘~(stat) ±~9∘~(syst) at s√ = 2.9000 GeV. For the first time, the phase of the hyperon electromagnetic form factors is explored in a wide range of four-momentum transfer. The evolution of the phase along with four-momentum transfer is an important input for understanding its asymptotic behavior and the dynamics of baryons.
Based on 4.4 fb−1 of e+e− annihilation data collected at the center-of-mass energies between 4.60 and 4.70 GeV with the BESIII detector at the BEPCII collider, the pure \textit{W}-boson-exchange decay Λ+c→Ξ0K+ is studied with a full angular analysis. The corresponding decay asymmetry is measured for the first time to be αΞ0K+=0.01±0.16(stat.)±0.03(syst.). This result reflects the non-interference effect between the S- and P-wave amplitudes. The phase shift between S- and P-wave amplitudes has two solutions, which are δp−δs=−1.55±0.25(stat.)±0.05(syst.) rad or 1.59±0.25(stat.)±0.05(syst.) rad.