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Based on a sample of (10.09±0.04)×109 J/ψ events collected with the BESIII detector operating at the BEPCII storage ring, a partial wave analysis of the decay J/ψ→γηη′ is performed. An isoscalar state with exotic quantum numbers JPC=1−+, denoted as η1(1855), has been observed for the first time with statistical significance larger than 19σ. Its mass and width are measured to be (1855±9+6−1)~MeV/c2 and (188±18+3−8)~MeV, respectively. The product branching fraction B(J/ψ→γη1(1855)→γηη′) is measured to be (2.70±0.41+0.16−0.35)×10−6. The first uncertainties are statistical and the second are systematic. In addition, an upper limit on the branching ratio B(f0(1710)→ηη′)/B(f0(1710)→ππ) is determined to be 1.61×10−3 at 90\% confidence level, which lends support to the hypothesis that the f0(1710) has a large glueball component.
Based on (10087±44)×106 J/ψ events collected with the BESIII detector, the process J/ψ→γπ+π−η′ is studied using two dominant decay channels of the η′ meson, η′→γπ+π− and η′→ηπ+π−,η→γγ. The X(2600) is observed with a statistical significance larger than 20σ in the π+π−η′ invariant mass spectrum, and it has a strong correlation to a structure around 1.5 GeV/{\it c}2 in the π+π− invariant mass spectrum. A simultaneous fit on the π+π−η′ and π+π− invariant mass spectra with the two η′ decay modes indicates that the mass and width of the X(2600) state are 2617.8±2.1+18.2−1.9 MeV/{\it c}2 and 200±8+20−17 MeV, respectively. The corresponding branching fractions are measured to be B(J/ψ→γX(2600))⋅B(X(2600)→f0(1500)η′)⋅B(f0(1500)→π+π−) = (3.39±0.18+0.91−0.66)×10−5 and B(J/ψ→γX(2600))⋅B(X(2600)→f′2(1525)η′)⋅B(f′2(1525)→π+π−) = (2.43±0.13+0.31−1.11)×10−5, where the first uncertainties are statistical, and the second systematic.
Using a sample of about 1010 𝐽/𝜓 events collected at a center-of-mass energy √𝑠=3.097 GeV with the BESIII detector, the electromagnetic Dalitz decays 𝐽/𝜓→𝑒+𝑒−𝜋+𝜋−𝜂′, with 𝜂′→𝛾𝜋+𝜋− and 𝜂′→𝜋+𝜋−𝜂, have been studied. The decay 𝐽/𝜓→𝑒+𝑒−𝑋(1835) is observed with a significance of 15𝜎, and also an 𝑒+𝑒− invariant-mass dependent transition form factor of 𝐽/𝜓→𝑒+𝑒−𝑋(1835) is presented for the first time. The intermediate states 𝑋(2120) and 𝑋(2370) are also observed in the 𝜋+𝜋−𝜂′ invariant-mass spectrum with significances of 5.3𝜎 and 7.3𝜎. The corresponding product branching fractions for 𝐽/𝜓→𝑒+𝑒−𝑋, 𝑋→𝜋+𝜋−𝜂′ [𝑋=𝑋(1835), 𝑋(2120), and 𝑋(2370)] are reported.
Based on electron-positron collision data collected with the BESIII detector operating at the Beijing Electron Positron Collider II storage rings, the value of R≡σ(e+e−→hadrons)/σ(e+e−→μ+μ−) is measured at 14 center-of-mass energies from 2.2324 to 3.6710 GeV. The resulting uncertainties are less than 3.0%, and are dominated by systematic uncertainties.
Using a data sample of 4.481×108 𝜓(3686) events collected with the BESIII detector, we report the first observation of the four-lepton-decays 𝐽/𝜓→𝑒+𝑒−𝑒+𝑒− and 𝐽/𝜓→𝑒+𝑒−𝜇+𝜇− utilizing the process 𝜓(3686)→𝜋+𝜋−𝐽/𝜓. The branching fractions are determined to be [5.48±0.31(stat)±0.45(syst)]×10−5 and [3.53±0.22(stat)±0.13(syst)]×10−5, respectively. The results are consistent with theoretical predictions. No significant signal is observed for 𝐽/𝜓→𝜇+𝜇−𝜇+𝜇−, and an upper limit on the branching fraction is set at 1.6×10−6 at the 90% confidence level. A 𝐶𝑃 asymmetry observable is constructed for the first two channels, which is measured to be (−0.012±0.054±0.010) and (0.062±0.059±0.006), respectively. No evidence for 𝐶𝑃 violation is observed in this process.
Using inclusive decays of the J/ψ, a precise determination of the number of J/ψ events collected with the BESIII detector is performed. For the two data sets taken in 2009 and 2012, the numbers of J/ψ events are recalculated to be (224.0±1.3)×106 and (1088.5±4.4)×106 respectively, which are in good agreement with the previous measurements. For the J/ψ sample taken in 2017--2019, the number of events is determined to be (8774.0±39.4)×106. The total number of J/ψ events collected with the BESIII detector is determined to be (10087±44)×106, where the uncertainty is dominated by systematic effects and the statistical uncertainty is negligible.
Using inclusive decays of the J/ψ, a precise determination of the number of J/ψ events collected with the BESIII detector is performed. For the two data sets taken in 2009 and 2012, the numbers of J/ψ events are recalculated to be (224.0±1.3)×106 and (1088.5±4.4)×106 respectively, which are in good agreement with the previous measurements. For the J/ψ sample taken in 2017--2019, the number of events is determined to be (8774.0±39.4)×106. The total number of J/ψ events collected with the BESIII detector is determined to be (10087±44)×106, where the uncertainty is dominated by systematic effects and the statistical uncertainty is negligible.
Based on 3.19 fb−1 of e+e− collision data accumulated at the center-of-mass energy 4.178 GeV with the BESIII detector operating at the BEPCII collider, the electromagnetic Dalitz decay D∗0→D0e+e− is observed for the first time with a statistical significance of 13.2σ. The ratio of the branching fraction of D∗0→D0e+e− to that of D∗0→D0γ is measured to be (11.08±0.76±0.49)×10−3. By using the world average value of the branching fraction of D∗0→D0γ, the branching fraction of D∗0→D0e+e− is determined to be (3.91±0.27±0.17±0.10)×10−3, where the first uncertainty is statistical, the second systematic and the third external branching fractions.
Using 6.32~fb−1 of e+e− collision data recorded by the BESIII detector at center-of-mass energies between 4.178 to 4.226 GeV, we present the first measurement of the decay D+s→f0(980)e+νe,f0(980)→π0π0. The product branching fraction of D+s→f0(980)e+νe,f0(980)→π0π0 is measured to be (7.9±1.4stat±0.3syst)×10−4, with a statistical significance of 7.8σ. Furthermore, the upper limits on the product branching fractions of D+s→f0(500)e+νe with f0(500)→π0π0 and the branching fraction of D+s→K0SK0Se+νe are set to be 7.3×10−4 and 3.8×10−4 at 90\% confidence level, respectively. Our results provide valuable inputs to the understanding of the structures of light scalar mesons.
By analyzing e+e− annihilation data corresponding to an integrated luminosity of 2.93fb−1 collected at the center-of-mass energy of 3.773\,GeV with the BESIII detector, we report the first observations of the doubly Cabibbo-suppressed decays D+→K+π0π0 and D+→K+π0η. The branching fractions of D+→K+π0π0 and D+→K+π0η are measured to be (2.1±0.4stat±0.1syst)×10−4 and (2.1±0.5stat±0.1syst)×10−4 with statistical significances of 8.8σ and 5.5σ, respectively. In addition, we search for the subprocesses D+→K∗(892)+π0 and D+→K∗(892)+η with K∗(892)+→K+π0. The branching fraction of D+→K∗(892)+η is determined to be (4.4+1.8−1.5stat±0.2syst)×10−4, with a statistical significance of 3.2σ. No significant signal for D+→K∗(892)+π0 is found and we set an upper limit on the branching fraction of this decay at the 90\% confidence level to be 5.4×10−4.