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We present cross sections for the reaction e+e−→K0SK0L at center-of-mass energies ranging from 3.51 GeV to 4.95 GeV using data samples collected in the BESIII experiment, corresponding to a total integrated luminosity of 26.5 fb−1. The ratio of neutral-to-charged kaon form factors at large momentum transfers (12 GeV2<Q2<25 GeV2) is determined to be 0.21±0.01, which indicates a small but significant effect of flavor-SU(3) breaking in the kaon wave function, and consequently excludes the possibility that flavor-SU(3) breaking is the primary reason for the strong experimental violation of the pQCD prediction |F(π±)|/|F(K±)|=f2π/f2K, where F(π±) and F(K±) are the form factors, and fπ and fK are the decay constants of charged pions and kaons, respectively. We also observe a significant signal for the charmless decay ψ(3770)→K0SK0L for the first time. Within a 1σ contour of the likelihood value, the the branching fraction for ψ(3770)→K0SK0L is determined to be B=(2.63+1.40−1.59)×10−5, and the relative phase between the continuum and ψ(3770) amplitudes is ϕ=(−0.39+0.05−0.10)π. The branching fraction is in good agreement with the S- and D-wave charmonia mixing scheme proposed in the interpretation of the "ρπ puzzle" between J/ψ and ψ(3686) decays.
We study the process e+e−→Λ+cΛ¯−c at twelve center-of-mass energies from 4.6119 to 4.9509 GeV using data samples collected by the BESIII detector at the BEPCII collider. The Born cross sections and effective form factors (|Geff|) are determined with unprecedented precision after combining the single and double-tag methods based on the decay process Λ+c→pK−π+. Flat cross sections around 4.63 GeV are obtained and no indication of the resonant structure Y(4630), as reported by Belle, is found. In addition, no oscillatory behavior is discerned in the |Geff| energy-dependence of Λ+c, in contrast to what is seen for the proton and neutron cases. Analyzing the cross section together with the polar-angle distribution of the Λ+c baryon at each energy point, the moduli of electric and magnetic form factors (|GE| and |GM|) are extracted and separated. For the first time, the energy-dependence of the form factor ratio |GE/GM| is observed, which can be well described by an oscillatory function.
We study the process e+e−→Λ+cΛ¯−c at twelve center-of-mass energies from 4.6119 to 4.9509 GeV using data samples collected by the BESIII detector at the BEPCII collider. The Born cross sections and effective form factors (|Geff|) are determined with unprecedented precision after combining the single and double-tag methods based on the decay process Λ+c→pK−π+. Flat cross sections around 4.63 GeV are obtained and no indication of the resonant structure Y(4630), as reported by Belle, is found. In addition, no oscillatory behavior is discerned in the |Geff| energy-dependence of Λ+c, in contrast to what is seen for the proton and neutron cases. Analyzing the cross section together with the polar-angle distribution of the Λ+c baryon at each energy point, the moduli of electric and magnetic form factors (|GE| and |GM|) are extracted and separated. For the first time, the energy-dependence of the form factor ratio |GE/GM| is observed, which can be well described by an oscillatory function.
Based on (10087±44)×106 J/ψ events collected with the BESIII detector, a partial wave analysis of the decay J/ψ→γK0SK0Sη′ is performed. The mass and width of the X(2370) are measured to be 2395±11(stat)+26−94(syst) MeV/c2 and 188+18−17(stat)+124−33(syst) MeV, respectively. The corresponding product branching fraction is B[J/ψ→γX(2370)]×B[X(2370)→f0(980)η′]×B[f0(980)→K0SK0S]=(1.31±0.22(stat)+2.85−0.84(syst))×10−5. The statistical significance of the X(2370) is greater than 11.7σ and the spin-parity is determined to be 0−+ for the first time. The measured mass and spin-parity of the X(2370) are consistent with the predictions of the lightest pseudoscalar glueball.
The absolute branching fraction of the decay Λc(2625)+→Λ+cπ+π− is measured for the first time to be (50.7±5.0stat.±4.9syst.)% with 368.48 pb−1 of e+e− collision data collected by the BESIII detector at the center-of-mass energies of s√=4.918 and 4.950 GeV. This result is lower than the naive prediction of 67\%, obtained from isospin symmetry, by more than 2σ, thereby indicating that the novel mechanism referred to as the \textit{threshold effect}, proposed for the strong decays of Λc(2595)+, also applies to Λc(2625)+. This measurement is necessary to obtain the coupling constants for the transitions between s-wave and p-wave charmed baryons in heavy hadron chiral perturbation theory. In addition, we search for the decay Λc(2595)+→Λ+cπ+π−. No significant signal is observed, and the upper limit on its branching fraction is determined to be 80.8\% at the 90\% confidence level.
Evidence for the singly Cabibbo suppressed decay Λ+c→pπ0 is reported for the first time with a statistical significance of 3.7σ based on 6.0 fb−1 of e+e− collision data collected at center-of-mass energies between 4.600 and 4.843 GeV with the BESIII detector at the BEPCII collider. The absolute branching fraction of Λ+c→pπ0 is measured to be (1.56+0.72−0.58±0.20)×10−4. Combining with the branching fraction of Λ+c→nπ+, (6.6±1.3)×10−4, the ratio of the branching fractions of Λ+c→nπ+ and Λ+c→pπ0 is calculated to be 3.2+2.2−1.2. As an important input for the theoretical models describing the decay mechanisms of charmed baryons, our result indicates that the non-factorizable contributions play an essential role and their interference with the factorizable contributions should not be significant. In addition, the absolute branching fraction of Λ+c→pη is measured to be (1.63±0.31stat±0.11syst)×10−3.
By analyzing 7.33\,fb−1 of e+e− collision data collected at center-of-mass energies between 4.128 and 4.226 GeV with the BESIII detector, we search for the semileptonic decays D+s→K1(1270)0e+νe and D+s→b1(1235)0e+νe for the first time. No significant signals are observed for either decay mode. The upper limits on the (product) branching fractions are determined to be B[D+s→K1(1270)0e+νe]<4.1×10−4 and B[D+s→b1(1235)0e+νe]⋅B[b1(1235)0→ωπ0]<6.4×10−4 at 90\% confidence level.
Based on a data set of (27.12±0.10)×108 ψ(3686) events collected at the BESIII experiment, the absolute branching fractions of the three dominant Ω− decays are measured to be BΩ−→Ξ0π−=(25.03±0.44±0.53)%, BΩ−→Ξ−π0=(8.43±0.52±0.28)%, and BΩ−→ΛK−=(66.3±0.8±2.0)%, where the first and second uncertainties are statistical and systematic, respectively. The ratio between BΩ−→Ξ0π− and BΩ−→Ξ−π0 is determined to be 2.97±0.19±0.11, which is in good agreement with the PDG value of 2.74±0.15, but greater by more than four standard deviations than the theoretical prediction of 2 obtained from the ΔI=1/2 rule.
Based on a sample of (448.1±2.9)×106 𝜓(3686) events collected with the BESIII detector at BEPCII, the decays of 𝜓(3686)→𝐾−Λ¯Ξ++c.c. with ¯Ξ+→¯Λ𝜋+, ¯Λ→¯𝑝𝜋+ are studied. We investigate the two excited resonances, Ξ(1690)− and Ξ(1820)−, which are each observed with large significance (≫10𝜎) in the 𝐾−Λ invariant mass distributions. A partial wave analysis is performed, and the spin-parities of Ξ(1690)− and Ξ(1820)− are measured to be 12− and 32−, respectively. The masses, widths, and product branching fractions of Ξ(1690)− and Ξ(1820)− are also measured.
The singly Cabibbo-suppressed decay Λ+c→Σ−K+π+ is observed for the first time with a statistical significance of 6.4σ by using 4.5 fb−1 of e+e− collision data collected at center-of-mass energies between 4.600 and 4.699 GeV with the BESIII detector at BEPCII. The absolute branching fraction of Λ+c→Σ−K+π+ is measured to be (3.8±1.3stat±0.2syst)×10−4 in a model-independent approach. This is the first observation of a Cabibbo-suppressed Λ+c decay involving Σ− in the final state. The ratio of branching fractions between Λ+c→Σ−K+π+ and the Cabibbo-favored decay Λ+c→Σ−π+π+ is calculated to be (0.4±0.1)s2c, where sc≡sinθc=0.2248 with θc the Cabibbo mixing angle. This ratio significantly deviates from 1.0s2c and provides important information for the understanding of nonfactorization contributions in Λ+c decays.