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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.
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.
We report the measurement of the cross sections for e+e−→{nOCH} (nOCH stands for non-open charm hadrons) with improved precision at center-of-mass energies from 3.645 to 3.871 GeV. We observe for the first time a three-resonance structure in the energy-dependent lineshape of the cross sections, which are R(3760), R(3780) and R(3810) with significances of 9.4σ, 15.7σ, and 9.8σ, respectively. The R(3810) is observed for the first time. We found two solutions in analysis of the cross sections. For solution I [solution II], we measure the mass, the total width and the product of electronic width and nOCH decay branching fraction to be (3805.8±1.1±2.7) [(3805.8±1.1±2.7)] MeV/c2, (11.6±2.6±1.9) [(11.5±2.5±1.8)] MeV, and (10.8±3.2±2.3) [(11.0±2.9±2.4)] eV for the R(3810), respectively. In addition, we measure the branching fractions B(R(3760)→{nOCH})=(24.5±13.4±27.4)%[(6.8±5.4±7.6)%] for the first time, and B(R(3780)→{nOCH})=(11.6±5.8±7.8)%[(10.3±4.5±6.9)%]. Moreover, we determine the open-charm (OC) branching fraction B(R(3760)→{OC})=(75.5±13.4±27.4)%[(93.2±5.4±7.6)%], which supports the interpretation of R(3760) as an OC pair molecular state, but contained a simple four-quark state component. The first uncertainties are from fits to the cross sections, and the second are systematic.
We report the measurement of the inclusive cross sections for e+e−→nOCH (where nOCH denotes non-open charm hadrons) with improved precision at center-of-mass (c.m.) energies from 3.645 to 3.871 GeV. We observe three resonances: R(3760), R(3780), and R(3810) with significances of 8.1σ, 13.7σ, and 8.8σ, respectively. The R(3810) state is observed for the first time, while the R(3760) and R(3780) states are observed for the first time in the nOCH cross sections. Two sets of resonance parameters describe the energy-dependent line shape of the cross sections well. In set I [set II], the R(3810) state has mass (3805.7±1.1±2.7) [(3805.7±1.1±2.7)] MeV/c2, total width (11.6±2.9±1.9) [(11.5±2.8±1.9)] MeV, and an electronic width multiplied by the nOCH decay branching fraction of (10.9±3.8±2.5) [(11.0±3.4±2.5)] eV. In addition, we measure the branching fractions B[R(3760)→nOCH]=(25.2±16.1±30.4)%[(6.4±4.8±7.7)%] and B[R(3780)→nOCH]=(12.3±6.6±8.3)%[(10.4±4.8±7.0)%] for the first time. The R(3760) state can be interpreted as an open-charm (OC) molecular state, but containing a simple four-quark state component. The R(3810) state can be interpreted as a hadrocharmonium state.
A search has been performed for the semileptonic decays D0→K0SK−e+νe, D+→K0SK0Se+νe and D+→K+K−e+νe, using 7.9 fb−1 of e+e− annihilation data collected at the center-of-mass energy s√=3.773 GeV by the BESIII detector operating at the BEPCII collider. No significant signals are observed, and upper limits are set at the 90\% confidence level of 2.13×10−5, 1.54×10−5 and 2.10×10−5 for the branching fractions of D0→K0SK−e+νe, D+→K0SK0Se+νe and D+→K+K−e+νe, respectively.
A light scalar X0 or vector X1 particles have been introduced as a possible explanation for the (g−2)μ anomaly and dark matter phenomena.
Using (8.998±0.039)×109 $\jpsi$ events collected by the BESIII detector, we search for a light muon philic scalar X0 or vector X1 in the processes J/ψ→μ+μ−X0,1 with X0,1 invisible decays. No obvious signal is found, and the upper limits on the coupling g′0,1 between the muon and the X0,1 particles are set to be between 1.1×10−3 and 1.0×10−2 for the X0,1 mass in the range of 1<M(X0,1)<1000 MeV/c2 at 90% confidence level.
Using data samples collected at center-of-mass energies between 2.000 and 3.080 GeV with the BESIII detector operating at the BEPCII collider, a partial-wave analysis is performed on the process e+e−→ηπ+π−. In addition to the dominant e+e−→ρη component, the e+e−→a2(1320)π process is also sizeable, contributing up to 24% of the total reaction. The measured cross sections of the process e+e−→ηπ+π− are systematically higher than those of BaBar by more than 3σ at center-of-mass energies between 2.000 and 2.300 GeV. In the cross section lineshape for e+e−→a2(1320)π, a resonant structure is observed with a significance of 5.5σ, with M=(2044±31±4) MeV/c2, Γ=(163±69±24) MeV and BR⋅ΓRe+e−=(34.6±17.1±6.0) eV or (137.1±73.3±2.1) eV. In the cross section lineshape for e+e−→ρη, an evidence of a dip structure around 2180 MeV/c2 is observed with statistical significance of 3.0σ.
The measurement of the Cabibbo-favored semileptonic decay Λ+c→Λμ+νμ is reported using 4.5 fb−1 of e+e− annihilation data collected at center-of-mass energies ranging from 4.600~GeV to 4.699~GeV. The branching fraction of the decay is measured to be B(Λ+c→Λμ+νμ)=(3.48±0.14stat.±0.10syst.)%, three times more precise than the prior world average result. Tests of lepton flavor universality using Λ+c→Λℓ+νℓ (ℓ=e,μ) decays are reported for the first time, based on measurements of the differential decay rates and the forward-backward asymmetries in separate four-momentum transfer regions. The results are compatible with Standard Model predictions. Furthermore, we improve the determination of the form-factor parameters in Λ+c→Λℓ+νℓ decays, which provide stringent tests and calibration for lattice quantum chromodynamics (LQCD) calculations.
The Cabbibo-favored decay Λ+c→Ξ0K+π0 is studied for the first time using 6.1 fb−1 of e+e− collision data at center-of-mass energies between 4.600 and 4.840 GeV, collected with the BESIII detector at the BEPCII collider. With a double-tag method, the branching fraction of the three-body decay Λ+c→Ξ0K+π0 is measured to be (7.79±1.46±0.71)×10−3, where the first and second uncertainties are statistical and systematic, respectively. The branching fraction of the two-body decay Λ+c→Ξ(1530)0K+ is (5.99±1.04±0.29)×10−3, which is consistent with the previous result of (5.02±0.99±0.31)×10−3. In addition, the upper limit on the branching fraction of the doubly Cabbibo-suppressed decay Λ+c→nK+π0 is 7.1×10−4 at the 90% confidence level. The upper limits on the branching fractions of Λ+c→Σ0K+π0 and ΛK+π0 are also determined to be 1.8×10−3 and 2.0×10−3, respectively.