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The integrated luminosities of data samples collected in the BESIII experiment in 2016–2017 at center-of-mass energies between 4.19 and 4.28 GeV are measured with a precision better than 1% by analyzing large-angle Bhabha scattering events. The integrated luminosities of old datasets collected in 2010–2014 are updated by considering corrections related to detector performance, offsetting the effect of newly discovered readout errors in the electromagnetic calorimeter, which can haphazardly occur.
The Born cross section of the process e+e−→ηJ/ψ at a center-of-mass energy s√=3.773 GeV is measured to be (8.89±0.88±0.42) pb, using a data sample collected with the BESIII detector operating at the BEPCII storage ring. The decay ψ(3770)→ηJ/ψ is observed for the first time with a statistical significance of 7.4σ. From a fit to the dressed cross-section line-shape of e+e−→ηJ/ψ from s√=3.773 to 4.600 GeV we obtain the branching fraction of the decay ψ(3770)→ηJ/ψ to be (11.6±6.1±1.0)×10−4 when the ψ(3770) decay amplitude is added coherently to the other contributions, and (7.9±1.0±0.7)×10−4 when it is added incoherently. Here the first uncertainties are statistical and the second are systematic.
Using a data sample collected with the BESIII detector operating at the BEPCII storage ring, the Born cross section of the process 𝑒+𝑒−→𝜂𝐽/𝜓 at a center-of-mass energy √𝑠=3.773 GeV is measured to be (8.88±0.87±0.42) pb. We fit the cross section line shape before correcting for the initial state radiation from √𝑠=3.773 to 4.600 GeV to obtain the branching fraction ℬ(𝜓(3770)→𝜂𝐽/𝜓). We obtain ℬ(𝜓(3770)→𝜂𝐽/𝜓)=(11.3±5.9±1.1)×10−4 when the 𝜓(3770) decay amplitude is added coherently to the other contributions, and (8.7±1.0±0.8)×10−4 when it is added incoherently. Here the quoted uncertainties are statistical and systematic, respectively. In both cases, the statistical significance of 𝜓(3770) resonance is above 7𝜎. This is the first time the decay 𝜓(3770)→𝜂𝐽/𝜓 is observed with a statistical significance greater than 5𝜎.
By using 6.32 fb−1 of data collected with the BESIII detector at center-of-mass energies between 4.178 and 4.226 GeV, we perform an amplitude analysis of the decay D+s ! K0S + 0 and determine the relative fractions and phase differences of different intermediate processes, which include K0S (770)+, K0S (1450)+, K (892)0 +, K (892)+ 0, and K (1410)0 +. With the detection efficiency based on the amplitude analysis results, the absolute branching fraction is measured to be B(D+s ! K0S + 0) = (5.43 ± 0.30stat ± 0.15syst) × 10−3.
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.
Analyzing (448.1±2.9)×106 ψ(3686) events collected with the BESIII detector at the BEPCII collider, the ψ(3686)→ωK0SK0S decay is observed for the first time. The branching fraction for this decay is determined to be Bψ(3686)→ωK0SK0S=(7.04±0.39±0.36)×10−5, where the first uncertainty is statistical and the second is systematic.
Using a sample of (10.09±0.04)×109 J/ψ events collected with the BESIII detector, a partial wave analysis of J/ψ→γη′η′ is performed.The masses and widths of the observed resonances and their branching fractions are reported. The main contribution is from J/ψ→γf0(2020) with f0(2020)→η′η′, which is found with a significance of greater than 25σ. The product branching fraction B(J/ψ → γf0(2020))⋅B(f0(2020) → η′η′ is measured to be (2.63±0.06(stat.) + 0.31−0.46(syst.))×10−4.
We present the first experimental search for the rare charm decay D0→π0ν¯ν. It is based on an e+e− collision sample consisting of 10.6×10^6 pairs of D0¯D0 mesons collected by the BESIII detector at √s=3.773 GeV, corresponding to an integrated luminosity of 2.93 fb^−1. A data-driven method is used to ensure the reliability of the background modeling. No significant D0→π0ν¯ν signal is observed in data and an upper limit of the branching fraction is set to be 2.1×10^-4 at the 90% confidence level. This is the first experimental constraint on charmed-hadron decays into dineutrino final states.
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.
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±0.6stat±0.1syst)×10−4 with statistical significances of 8.0σ and 5.0σ, 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 (4.7+1.9−1.6stat±0.2syst)×10−4, with a statistical significance of 3.3σ. 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 4.5×10−4.
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.
Using J/ψ radiative decays from 9.0 billion J/ψ events collected by the BESIII detector, we search for di-muon decays of a CP-odd light Higgs boson (A0), predicted by many new physics models beyond the Standard Model, including the Next-to-Minimal Supersymmetric Standard Model. No evidence for the CP-odd light Higgs production is found, and we set 90% confidence level upper limits on the product branching fraction B(J/ψ→γA0)×B(A0→μ+μ−) in the range of (1.2−778.0)×10−9 for 0.212≤mA0≤3.0 GeV/c2. The new measurement is a 6-7 times improvement over our previous measurement, and is also slightly better than the BaBar measurement in the low-mass region for tanβ=1.
Using J/ψ radiative decays from 9.0 billion J/ψ events collected by the BESIII detector, we search for di-muon decays of a CP-odd light Higgs boson (A0), predicted by many new physics models beyond the Standard Model, including the Next-to-Minimal Supersymmetric Standard Model. No evidence for the CP-odd light Higgs production is found, and we set 90% confidence level upper limits on the product branching fraction B(J/ψ→γA0)×B(A0→μ+μ−) in the range of (1.2−778.0)×10−9 for 0.212≤mA0≤3.0 GeV/c2. The new measurement is a 6-7 times improvement over our previous measurement, and is also slightly better than the BaBar measurement in the low-mass region for tanβ=1.
Using 10.1 × 109 J/ψ events produced by the Beijing Electron Positron Collider (BEPCII) at a center-of-mass energy √s = 3.097 GeV and collected with the BESIII detector, we present a search for the rare semi-leptonic decay J/ψ → D−e+νe + c.c. No excess of signal above background is observed, and an upper limit on the branching fraction B(J/ψ → D−e +νe + c.c.) < 7.1 × 10−8 is obtained at 90% confidence level. This is an improvement of more than two orders of magnitude over the previous best limit.
Using (10.087±0.044)×109 𝐽/𝜓 events collected by the Beijing Spectrum III (BESIII) detector at the Beijing Electron Positron Collider II (BEPCII) collider, we search for the hyperon semileptonic decay Ξ−→Ξ0𝑒−¯𝜈𝑒. No significant signal is observed and the upper limit on the branching fraction ℬ(Ξ−→Ξ0𝑒−¯𝜈𝑒) is set to be 2.59×10−4 at 90% confidence level. This result is one order of magnitude more strict than the previous best limit.
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.
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.
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.