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Using data samples with an integrated luminosity of 22.42 fb−1 collected by the BESIII detector operating at the BEPCII storage ring, we measure the cross sections of the $e^{+}e^{-}\rightarrow\etaJ/\psi$ process at center-of-mass energies from 3.808 to 4.951 GeV. Three structures are observed in the line shape of the measured cross sections. A maximum-likelihood fit with ψ(4040), two additional resonances, and a non-resonant component is performed. The mass and width of the first additional state are (4219.7±2.5±4.5)MeV/c2 and (80.7±4.4±1.4)MeV, respectively, consistent with the ψ(4230). For the second state, the mass and width are (4386±13±17)MeV/c2 and (177±32±13)MeV, respectively, consistent with the ψ(4360). The first uncertainties are statistical and the second ones are systematic. The statistical significance of ψ(4040) is 8.0σ and those for ψ(4230) and ψ(4360) are more than 10.0σ.
Ten hadronic final states of the ℎ𝑐 decays are investigated via the process 𝜓(3686)→𝜋0ℎ𝑐, using a data sample of (448.1±2.9)×106 𝜓(3686) events collected with the BESIII detector. The decay channel ℎ𝑐→𝐾+𝐾−𝜋+𝜋−𝜋0 is observed for the first time and has a measured significance of 6.0𝜎. The corresponding branching fraction is determined to be ℬ(ℎ𝑐→𝐾+𝐾−𝜋+𝜋−𝜋0)=(3.3±0.6±0.6)×10−3 (where the uncertainties are statistical and systematic, respectively). Evidence for the decays ℎ𝑐→𝜋+𝜋−𝜋0𝜂 and ℎ𝑐→𝐾0𝑆𝐾±𝜋∓𝜋+𝜋− is found with a significance of 3.6𝜎 and 3.8𝜎, respectively. The corresponding branching fractions (and upper limits) are obtained to be ℬ(ℎ𝑐→𝜋+𝜋−𝜋0𝜂)=(7.2±1.8±1.3)×10−3 (<1.8×10−2) and ℬ(ℎ𝑐→𝐾0𝑆𝐾±𝜋∓𝜋+𝜋−)=(2.8±0.9±0.5)×10−3 (<4.7×10−3). Upper limits on the branching fractions for the final states ℎ𝑐→𝐾+𝐾−𝜋0, 𝐾+𝐾−𝜂, 𝐾+𝐾−𝜋+𝜋−𝜂, 2(𝐾+𝐾−)𝜋0, 𝐾+𝐾−𝜋0𝜂, 𝐾0𝑆𝐾±𝜋∓, and 𝑝¯𝑝𝜋0𝜋0 are determined at a confidence level of 90%.
Using a sample of (10087±44)×106 𝐽/𝜓 events, which is about 45 times larger than that was previously analyzed, a further investigation on the 𝐽/𝜓→𝛾3(𝜋+𝜋−) decay is performed. A significant distortion at 1.84 GeV/𝑐2 in the line shape of the 3(𝜋+𝜋−) invariant mass spectrum is observed for the first time, which could be resolved by two overlapping resonant structures, 𝑋(1840) and 𝑋(1880). The new state 𝑋(1880) is observed with a statistical significance larger than 10𝜎. The mass and width of 𝑋(1880) are determined to be 1882.1±1.7±0.7 MeV/𝑐2 and 30.7±5.5±2.4 MeV, respectively, which indicates the existence of a 𝑝¯ 𝑝 bound state.
The Ξ0 asymmetry parameters are measured using entangled quantum Ξ0 − Ξ¯ 0 pairs from a sample of ð448.1 2.9Þ × 106 ψð3686Þ events collected with the BESIII detector at BEPCII. The relative phase between the transition amplitudes of the Ξ0Ξ¯ 0 helicity states is measured to be ΔΦ ¼ −0.050 0.150 0.020 rad, which implies that there is no obvious polarization at the current level of statistics. The decay parameters of the Ξ0 hyperon ðαΞ0 ; αΞ¯ 0 ; ϕΞ0 ; ϕΞ¯ 0 Þ and the angular distribution parameter ½αψð3686Þ and ΔΦ are measured simultaneously for the first time. In addition, the CP asymmetry observables are determined to be AΞ0 CP ¼ ðαΞ0 þ αΞ¯ 0 Þ=ðαΞ0 − αΞ¯ 0 Þ ¼ −0.007 0.082 0.025 and ΔϕΞ0 CP ¼ ðϕΞ0 þ ϕΞ¯ 0 Þ=2 ¼ −0.079 0.082 0.010 rad, which are consistent with CP conservation.
Based on (10.09±0.04)×109 J/ψ events collected with the BESIII detector operating at the BEPCII collider, a partial wave analysis of the decay J/ψ→ϕπ0η is performed. We observe for the first time two new structures on the ϕη invariant mass distribution, with statistical significances of 24.0σ and 16.9σ; the first with JPC = 1+−, mass M = (1911 ± 6 (stat.) ± 14 (sys.))~MeV/c2, and width Γ= (149 ± 12 (stat.) ± 23 (sys.))~MeV, the second with JPC = 1−−, mass M = (1996 ± 11 (stat.) ± 30 (sys.))~MeV/c2, and width Γ = (148 ± 16 (stat.) ± 66 (sys.))~MeV. These measurements provide important input for the strangeonium spectrum. In addition, the f0(980)−a0(980)0 mixing signal in J/ψ→ϕf0(980)→ϕa0(980)0 and the corresponding electromagnetic decay J/ψ→ϕa0(980)0 are measured with improved precision, providing crucial information to understand the nature of a0(980)0 and f0(980).
Using an 𝑒+𝑒− collision data sample of (27.08±0.14)×108 𝜓(3686) events collected by the BESIII detector, we report the first observation of 𝜒𝑐𝐽→Ω−¯Ω+ (𝐽=0, 1, 2) decays with significances of 5.6𝜎, 6.4𝜎, and 18𝜎, respectively, where the 𝜒𝑐𝐽 mesons are produced in the radiative 𝜓(3686) decays. The branching fractions are determined to be ℬ(𝜒𝑐0→Ω−¯Ω+) = (3.51±0.54±0.29)×10−5, ℬ(𝜒𝑐1→Ω−¯Ω+)=(1.49±0.23±0.10)×10−5, and ℬ(𝜒𝑐2→Ω−¯Ω+)=(4.52±0.24±0.18)×10−5, where the first and second uncertainties are statistical and systematic, respectively.
Search for the reaction channel e⁺e⁻ → ηcηπ⁺π⁻ at center-of-mass energies from 4.23 to 4.60 GeV
(2021)
Using data collected with the BESIII detector operating at the Beijing Electron Positron Collider, we search for the process 𝑒+𝑒−→𝜂𝑐𝜂𝜋+𝜋−. The search is performed using five large datasets recorded at center-of-mass energies of 4.23, 4.26, 4.36, 4.42, and 4.60 GeV. The 𝜂𝑐 meson is reconstructed in 16 exclusive decay modes. No signal is observed in the 𝜂𝑐 mass region at any center-of-mass energy. The upper limits on the reaction cross sections are determined to be 6.2, 10.8, 27.6, 22.6 and 23.7 pb at the 90% confidence level at the center-of-mass energies listed above.
Though immensely successful, the standard model of particle physics does not offer any explanation as to why our Universe contains so much more matter than antimatter. A key to a dynamically generated matter–antimatter asymmetry is the existence of processes that violate the combined charge conjugation and parity (CP) symmetry1. As such, precision tests of CP symmetry may be used to search for physics beyond the standard model. However, hadrons decay through an interplay of strong and weak processes, quantified in terms of relative phases between the amplitudes. Although previous experiments constructed CP observables that depend on both strong and weak phases, we present an approach where sequential two-body decays of entangled multi-strange baryon–antibaryon pairs provide a separation between these phases. Our method, exploiting spin entanglement between the double-strange Ξ− baryon and its antiparticle2 Ξ¯+
, has enabled a direct determination of the weak-phase difference, (ξP − ξS) = (1.2 ± 3.4 ± 0.8) × 10−2 rad. Furthermore, three independent CP observables can be constructed from our measured parameters. The precision in the estimated parameters for a given data sample size is several orders of magnitude greater than achieved with previous methods3. Finally, we provide an independent measurement of the recently debated Λ decay parameter αΛ (refs. 4,5). The ΛΛ¯
asymmetry is in agreement with and compatible in precision to the most precise previous measurement.
Using data samples collected with the BESIII detector operating at the BEPCII storage ring at center-of-mass energies from 4.178 to 4.600 GeV, we study the process eþe− → π0Xð3872Þγ and search for Zcð4020Þ0 → Xð3872Þγ. We find no significant signal and set upper limits on σðeþe− → π0Xð3872ÞγÞ · BðXð3872Þ → πþπ−J=ψÞ and σðeþe− → π0Zcð4020Þ0Þ · BðZcð4020Þ0 → Xð3872ÞγÞ · BðXð3872Þ → πþπ−J=ψÞ for each energy point at 90% confidence level, which is of the order of several tenths pb.
Based on an e+e− collision data sample corresponding to an integrated luminosity of 2.93 fb−1 collected with the BESIII detector at √s=3.773 GeV, the first amplitude analysis of the singly Cabibbo-suppressed decay D+→K+K0Sπ0 is performed. From the amplitude analysis, the K∗(892)+K0S component is found to be dominant with a fraction of (57.1±2.6±4.2)%, where the first uncertainty is statistical and the second systematic. In combination with the absolute branching fraction B(D+→K+K0Sπ0) measured by BESIII, we obtain B(D+→K∗(892)+K0S)=(8.69±0.40±0.64±0.51)×10−3, where the third uncertainty is due to the branching fraction B(D+→K+K0Sπ0). The precision of this result is significantly improved compared to the previous measurement. This result also differs from most of theoretical predictions by about 4σ, which may help to improve the understanding of the dynamics behind.