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Using an 𝑒+𝑒− collision data sample of 2.93 fb−1 collected at a center-of-mass energy of 3.773 GeV by the BESIII detector at BEPCII, we report the observation of 𝐷0→𝑎0(980)−𝑒+𝜈𝑒 and evidence for 𝐷+→𝑎0(980)0𝑒+𝜈𝑒 with significances of 6.4𝜎 and 2.9𝜎, respectively. The absolute branching fractions are determined to be ℬ(𝐷0→𝑎0(980)−𝑒+𝜈𝑒)×ℬ(𝑎0(980)−→𝜂𝜋−) = [1.33+0.33−0.29(stat)±0.09(syst)]×10−4 and ℬ(𝐷+→𝑎0(980)0𝑒+𝜈𝑒)×ℬ(𝑎0(980)0→𝜂𝜋0)=[1.66+0.81
−0.66(stat)±0.11(syst)]×10−4. This is the first time the 𝑎0(980) meson has been measured in a 𝐷0 semileptonic decay, which would open one more interesting page in the investigation of the nature of the puzzling 𝑎0(980) states.
Using a data sample of 448.1×106 𝜓(3686) events collected at √𝑠=3.686 GeV with the BESIII detector at the Beijing Electron-Positron Collider II, we search for the rare decay 𝐽/𝜓→𝜙𝑒+𝑒− via 𝜓(3686)→𝜋+𝜋−𝐽/𝜓. No signal events are observed and the upper limit on the branching fraction is set to be ℬ(𝐽/𝜓→𝜙𝑒+𝑒−)<1.2×10−7 at the 90% confidence level, which is still about one order of magnitude higher than the Standard Model prediction.
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.
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.
A partial-wave analysis of the decay 𝐽/𝜓→𝐾+𝐾−𝜋0 has been made using (223.7±1.4)×106 𝐽/𝜓 events collected with the BESIII detector in 2009. The analysis, which is performed within the isobar-model approach, reveals contributions from 𝐾*2(1430)±, 𝐾*2(1980)± and 𝐾*4(2045)± decaying to 𝐾±𝜋0. The two latter states are observed in 𝐽/𝜓 decays for the first time. Two resonance signals decaying to 𝐾+𝐾− are also observed. These contributions cannot be reliably identified and their possible interpretations are discussed. The measured branching fraction 𝐵(𝐽/𝜓→𝐾+𝐾−𝜋0) of (2.88±0.01±0.12)×10−3 is more precise than previous results. Branching fractions for the reported contributions are presented as well. The results of the partial-wave analysis differ significantly from those previously obtained by BESII and BABAR.
We measure the Born cross sections of the process 𝑒+𝑒−→𝐾+𝐾−𝐾+𝐾− at center-of-mass (c.m.) energies, √𝑠, between 2.100 and 3.080 GeV. The data were collected using the BESIII detector at the BEPCII collider. An enhancement at √𝑠=2.232 GeV is observed, very close to the 𝑒+𝑒−→Λ¯Λ production threshold. A similar enhancement at the same c.m. energy is observed in the 𝑒+𝑒−→𝜙𝐾+𝐾− cross section. The energy dependence of the 𝐾+𝐾−𝐾+𝐾− and 𝜙𝐾+𝐾− cross sections differs significantly from that of 𝑒+𝑒−→𝜙𝜋+𝜋−.
Using 𝑒+𝑒−→Λ+𝑐¯Λ−𝑐 production from a 567 pb−1 data sample collected by BESIII at 4.6 GeV, a full angular analysis is carried out simultaneously on the four decay modes of Λ+𝑐→𝑝𝐾0𝑆, Λ𝜋+, Σ+𝜋0, and Σ0𝜋+. For the first time, the Λ+𝑐 transverse polarization is studied in unpolarized 𝑒+𝑒− collisions, where a nonzero effect is observed with a statistical significance of 2.1𝜎. The decay asymmetry parameters of the Λ+𝑐 weak hadronic decays into 𝑝𝐾0𝑆, Λ𝜋+, Σ+𝜋0 and Σ0𝜋+ are measured to be 0.18±0.43(stat)±0.14(syst), −0.80±0.11(stat)±0.02(syst), −0.57±0.10(stat)±0.07(syst), and −0.73±0.17(stat)±0.07(syst), respectively. In comparison with previous results, the measurements for the Λ𝜋+ and Σ+𝜋0 modes are consistent but with improved precision, while the parameters for the 𝑝𝐾0𝑆 and Σ0𝜋+ modes are measured for the first time.
Using a data sample of 448.1×106 ψ(3686) events collected at s√= 3.686 GeV with the BESIII detector at the BEPCII, we search for the rare decay J/ψ→ϕe+e− via ψ(3686)→π+π−J/ψ. No signal events are observed and the upper limit on the branching fraction is set to be B(J/ψ→ϕe+e−)<1.2×10−7 at the 90\% confidence level, which is still about one order of magnitude higher than the Standard Model prediction.
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.
We report a measurement of the observed cross sections of e+ e− → J/ψX based on 3.21 fb − 1 of data accumulated at energies from 3.645 to 3.891 GeV with the BESIII detector operated at the BEPCII collider. In analysis of the cross sections, we measured the decay branching fractions of B(ψ(3686) → J/ψX) = (64.4 ± 0.6 ± 1.6)% and B(ψ(3770) → J/ψX) = (0.5 ± 0.2 ± 0.1)% for the first time. The energy-dependent line shape of these cross sections cannot be well described by two Breit-Wigner (BW) amplitudes of the expected decays ψ (3686) → J/ψX and ψ(3770) → J/ψX. Instead, it can be better described with one more BW amplitude of the decay R(3760)→ J/ψX. Under this assumption, we extracted the R (3760) mass M R (3760 ) = 3766.2 ± 3.8 ± 0.4 MeV/c2, total width Γ tot R ( 3760 ) = 22.2 ± 5.9 ± 1.4 MeV, and product of leptonic width and decay branching fraction
ΓeeR(3760) B[R(3760) → J/ψX] = (79.4 ± 85.5 ± 11.7) eV. The significance of the R(3760) is 5.3σ. The first uncertainties of these measured quantities are from fits to the cross sections and second systematic.
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.
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.
We study the electromagnetic Dalitz decay 𝐽/𝜓→𝑒+𝑒−𝜂 and search for dielectron decays of a dark gauge boson (𝛾′) in 𝐽/𝜓→𝛾′𝜂 with the two 𝜂 decay modes 𝜂→𝛾𝛾 and 𝜂→𝜋+𝜋−𝜋0 using (1310.6±7.0)×106 𝐽/𝜓 events collected with the BESIII detector. The branching fraction of 𝐽/𝜓→𝑒+𝑒−𝜂 is measured to be (1.43±0.04(stat)±0.06(syst))×10−5, with a precision that is improved by a factor of 1.5 over the previous BESIII measurement. The corresponding dielectron invariant mass dependent modulus square of the transition form factor is explored for the first time, and the pole mass is determined to be Λ=2.84±0.11(stat)±0.08(syst) GeV/𝑐2. We find no evidence of 𝛾′ production and set 90% confidence level upper limits on the product branching fraction ℬ(𝐽/𝜓→𝛾′𝜂)×ℬ(𝛾′→𝑒+𝑒−) as well as the kinetic mixing strength between the standard model photon and 𝛾′ in the mass range of 0.01≤𝑚𝛾′≤2.4 GeV/𝑐2.
We report on new measurements of Cabibbo-suppressed semileptonic D+s decays using 3.19 fb−1 of e+e− annihilation data sample collected at a center-of-mass energy of 4.178~GeV with the BESIII detector at the BEPCII collider. Our results include branching fractions B(D+s→K0e+νe)=(3.25±0.38(stat.)±0.16(syst.))×10−3 and B(D+s→K∗0e+νe)=(2.37±0.26(stat.)±0.20(syst.))×10−3 which are much improved relative to previous measurements, and the first measurements of the hadronic form-factor parameters for these decays. For D+s→K0e+νe, we obtain f+(0)=0.720±0.084(stat.)±0.013(syst.), and for D+s→K∗0e+νe, we find form-factor ratios rV=V(0)/A1(0)=1.67±0.34(stat.)±0.16(syst.) and r2=A2(0)/A1(0)=0.77±0.28(stat.)±0.07(syst.).
We report new measurements of the cross sections for the production of Dbar D final states at the ψ(3770) resonance. Our data sample consists of an integrated luminosity of 2.93 fb−1 of e+e− annihilation data produced by the BEPCII collider and collected and analyzed with the BESIII detector. We exclusively reconstruct three D0 and six D+ hadronic decay modes and use the ratio of the yield of fully reconstructed Dbar D events ("double tags") to the yield of all reconstructed D or bar D mesons ("single tags") to determine the number of D0bar D0 and D+D− events, benefiting from the cancellation of many systematic uncertainties. Combining these yields with an independent determination of the integrated luminosity of the data sample, we find the cross sections to be σ(e+e− → D0bar D0) nb and σ(e+e− → D+D−) = (2.830 ± 0.011 ± 0.026) nb, where the uncertainties are statistical and systematic, respectively.
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.
By analyzing the large-angle Bhabha scattering events e+e− → (γ)e+e− and diphoton events e+e− → (γ)γγ for the data sets collected at center-of-mass (c.m.) energies between 2.2324 and 4.5900 GeV (131 energy points in total) with the upgraded Beijing Spectrometer (BESIII) at the Beijing Electron-Positron Collider (BEPCII), the integrated luminosities have been measured at the different c.m. energies, individually. The results are important inputs for the R value and J/ψ resonance parameter measurements.
Using a 3.19 fb−1 data sample collected at an 𝑒+𝑒− center-of-mass energy of 𝐸cm=4.178 GeV with the BESIII detector, we measure the branching fraction of the leptonic decay 𝐷+𝑠→𝜇+𝜈𝜇 to be ℬ𝐷+𝑠→𝜇+𝜈𝜇=(5.49±0.16stat±0.15syst)×10−3. Combining our branching fraction with the masses of the 𝐷+𝑠 and 𝜇+ and the lifetime of the 𝐷+𝑠, we determine 𝑓𝐷+𝑠|𝑉𝑐𝑠|=246.2±3.6stat±3.5syst MeV. Using the 𝑐→𝑠 quark mixing matrix element |𝑉𝑐𝑠| determined from a global standard model fit, we evaluate the 𝐷+𝑠 decay constant 𝑓𝐷+𝑠=252.9±3.7stat±3.6syst MeV. Alternatively, using the value of 𝑓𝐷+𝑠 calculated by lattice quantum chromodynamics, we find |𝑉𝑐𝑠|=0.985±0.014stat±0.014syst. These values of ℬ𝐷+𝑠→𝜇+𝜈𝜇, 𝑓𝐷+𝑠|𝑉𝑐𝑠|, 𝑓𝐷+𝑠 and |𝑉𝑐𝑠| are each the most precise results to date.
Measurement of cross sections for e⁺e⁻ → μ⁺μ⁻ at center-of-mass energies from 3.80 to 4.60 GeV
(2020)
The observed cross sections for 𝑒+𝑒−→𝜇+𝜇− at energies from 3.8 to 4.6 GeV are measured using data samples taken with the BESIII detector operated at the BEPCII collider. We measure the muonic widths and determine the branching fractions of the charmonium states 𝜓(4040), 𝜓(4160), and 𝜓(4415) decaying to 𝜇+𝜇−, as well as making a first determination of the phase of the amplitudes. In addition, we observe evidence for a structure in the dimuon cross section near 4.220 GeV/𝑐2, which we denote as 𝑆(4220). Analyzing a coherent sum of amplitudes yields eight solutions, one of which gives a mass of 𝑀𝑆(4220) = 4216.7±8.9±4.1 MeV/𝑐2, a total width of Γtot S(4220) = 47.2±22.8±10.5 MeV, and a muonic width of Γ𝜇𝜇 𝑆(4220) = 1.53±1.26±0.54 keV, where the first uncertainties are statistical and the second systematic. The eight solutions give the central values of the mass, total width, muonic width to be, respectively, in the range from 4212.8 to 4219.4 MeV/𝑐2, from 36.4 to 49.6 MeV, and from 1.09 to 1.53 keV. The statistical significance of the 𝑆(4220) signal is 3.9𝜎. Correcting the total dimuon cross section for radiative effects yields a statistical significance for this structure of 8.1𝜎.