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Using data samples collected with the BESIII detector at the BEPCII collider at center-of-mass energies ranging from 3.80 to 4.95 GeV, corresponding to an integrated luminosity of 20 fb−1, a measurement of Born cross sections for the e+e−→D0D¯0 and D+D− processes is presented with unprecedented precision. Many clear peaks in the line shape of e+e−→D0D¯0 and D+D− around the mass range of G(3900), ψ(4040), ψ(4160), Y(4260), and ψ(4415), etc., are foreseen. These results offer crucial experimental insights into the nature of hadron production in the open-charm region.
First study of reaction Ξ⁰n → Ξ⁻ p using Ξ⁰-nucleus scattering at an electron-positron collider
(2023)
Using ð1.0087 0.0044Þ × 1010 J=ψ events collected with the BESIII detector at the BEPCII storage ring, the process Ξ0n → Ξ−p is studied, where the Ξ0 baryon is produced in the process J=ψ → Ξ0Ξ¯ 0 and the neutron is a component of the 9 Be, 12C, and 197Au nuclei in the beam pipe. A clear signal is observed with a statistical significance of 7.1σ. The cross section of the reaction Ξ0 þ 9 Be → Ξ− þ p þ 8 Be is determined to be σðΞ0 þ 9 Be → Ξ− þ p þ 8 BeÞ¼ð22.1 5.3stat 4.5sysÞ mb at the Ξ0 momentum of 0.818 GeV=c, where the first uncertainty is statistical and the second is systematic. No significant H-dibaryon signal is observed in the Ξ−p final state. This is the first study of hyperon-nucleon interactions in electron-positron collisions and opens up a new direction for such research.
Based on a data sample of (27.08±0.14)×108 ψ(3686) events collected with the BESIII detector at the BEPCII collider, the M1 transition ψ(3686)→γηc(2S) with ηc(2S)→KK¯π is studied, where KK¯π is K+K−π0 or K0SK±π∓. The mass and width of the ηc(2S) are measured to be (3637.8±0.8(stat)±0.2(syst)) MeV/c2 and (10.5±1.7(stat)±3.5(syst)) MeV, respectively. The product branching fraction B(ψ(3686)→γηc(2S))×B(ηc(2S)→KK¯π) is determined to be (0.97±0.06(stat)±0.09(syst))×10−5. Using BR(ηc(2S)→KK¯π)=(1.86+0.68−0.49)%, we obtain the branching fraction of the radiative transition to be BR(ψ(3686)→γηc(2S))=(5.2±0.3(stat)±0.5(syst)+1.9−1.4(extr))×10−4, where the third uncertainty is due to the quoted BR(ηc(2S)→KK¯π).
The J/ψ→Ξ0Ξ¯0 process and subsequent decays are investigated using (10087±44)×106 J/ψ events collected at the BESIII experiment. The decay parameters of Ξ0 and Ξ¯0 are measured with greatly improved precision over previous measurements to be αΞ=−0.3750±0.0034±0.0016, α¯Ξ=0.3790±0.0034±0.0021, ϕΞ=0.0051±0.0096±0.0018~rad, ϕ¯Ξ=−0.0053±0.0097±0.0019~rad, where the first and the second uncertainties are statistical and systematic, respectively. From these measurements, precise CP symmetry tests in Ξ0 decay are performed, and AΞCP=(−5.4±6.5±3.1)×10−3 and ΔϕΞCP=(−0.1±6.9±0.9)×10−3~rad are consistent with CP conservation. The sequential decay also enables a separation of weak and strong phase differences, which are found for the first time to be ξP−ξS=(0.0±1.7±0.2)×10−2~rad and δP−δS=(−1.3±1.7±0.4)×10−2~rad, respectively. In addition, we measure the Λ decay parameters and test CP symmetry in Λ decays.
Based on (448.1±2.9)×106 ψ(3686) events collected with the BESIII detector operating at the BEPCII collider, the decay ψ(3686)→ϕK0SK0S is observed for the first time. Taking the interference between ψ(3686) decay and continuum production into account, the branching fraction of this decay is measured to be B(ψ(3686)→ϕK0SK0S) = (3.53 ± 0.20 ± 0.21)×10−5, where the first uncertainty is statistical and the second is systematic. Combining with the world average value for B(J/ψ→ϕK0SK0S), the ratio B(ψ(3686)→ϕK0SK0S)/B(J/ψ→ϕK0SK0S) is determined to be (6.0±1.6)%, which is suppressed relative to the 12% rule.
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 Cabbibo-favored decay Λ+c→Ξ0K+π0 is studied for the first time using 6.1 fb−1 of e+e− collision data at center-of-mass energies between 4.600 and 4.840 GeV, collected with the BESIII detector at the BEPCII collider. With a double-tag method, the branching fraction of the three-body decay Λ+c→Ξ0K+π0 is measured to be (7.79±1.46±0.71)×10−3, where the first and second uncertainties are statistical and systematic, respectively. The branching fraction of the two-body decay Λ+c→Ξ(1530)0K+ is (5.99±1.04±0.29)×10−3, which is consistent with the previous result of (5.02±0.99±0.31)×10−3. In addition, the upper limit on the branching fraction of the doubly Cabbibo-suppressed decay Λ+c→nK+π0 is 7.1×10−4 at the 90% confidence level. The upper limits on the branching fractions of Λ+c→Σ0K+π0 and ΛK+π0 are also determined to be 1.8×10−3 and 2.0×10−3, respectively.
Using initial-state radiation events from a total integrated luminosity of 11.957 fb−1 of 𝑒+𝑒− collision data collected at center-of-mass energies between 3.773 and 4.258 GeV with the BESIII detector at BEPCII, the cross section for the process 𝑒+𝑒−→Λ¯Λ is measured in 16 Λ¯Λ invariant mass intervals from the production threshold up to 3.00 GeV/𝑐2. The results are consistent with previous results from BABAR and BESIII, but with better precision and with narrower Λ¯Λ invariant mass intervals than BABAR.
The process e+e−→D∗+sD∗−s is studied with a semi-inclusive method using data samples at center-of-mass energies from threshold to 4.95 GeV collected with the BESIII detector operating at the Beijing Electron Positron Collider. The Born cross sections of the process are measured for the first time with high precision in this energy region. Two resonance structures are observed in the energy-dependent cross sections around 4.2 and 4.4 GeV. By fitting the cross sections with a coherent sum of three Breit-Wigner amplitudes and one phase-space amplitude, the two significant structures are assigned masses of (4186.5±9.0±30) MeV/c2 and (4414.5±3.2±6.0) MeV/c2, widths of (55±17±53) MeV and (122.6±7.0±8.2) MeV, where the first errors are statistical and the second ones are systematic. The inclusion of a third Breit-Wigner amplitude is necessary to describe a structure around 4.79 GeV.
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.