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Institut
A search for a massless dark photon γ′ is conducted 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. No significant signal is observed, and the upper limit on the branching fraction B(Λ+c→pγ′) is determined to be 8.0×10−5 at 90% confidence level.
A search for a massless dark photon γ′ is conducted 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. No significant signal is observed, and the upper limit on the branching fraction B(Λ+c→pγ′) is determined to be 8.0×10−5 at 90% confidence level.
Luminosities and energies of e⁺e⁻ collision data taken between √s=4.61 GeV and 4.95 GeV at BESIII
(2022)
From December 2019 to June 2021, the BESIII experiment collected about 5.85 fb−1 of data at center-of-mass energies between 4.61 GeV and 4.95 GeV. This is the highest collision energy BEPCII has reached so far. The accumulated e+e− annihilation data samples are useful for studying charmonium(-like) states and charmed-hadron decays. By adopting a novel method of analyzing the production of Λ+cΛ¯−c pairs in e+e− annihilation, the center-of-mass energies are measured with a precision of ∼0.6 MeV. Integrated luminosities are measured with a precision of better than 1\% by analyzing the events of large-angle Bhabha scattering. These measurements provide important inputs to the analyses based on these data samples.
The radiative hyperon decay Λ→nγ is studied using (10087±44)×106 J/ψ events collected with the BESIII detector operating at BEPCII. The absolute branching fraction of the decay Λ→nγ is determined with a significance of 5.6σ to be [0.832±0.038(stat.)±0.054(syst.)]×10−3, which lies significantly below the current PDG value. By analyzing the joint angular distribution of the decay products, the first determination of the decay asymmetry αγ is reported with a value of −0.16±0.10(stat.)±0.05(syst.).
Observation of ηc(2S) → 3(π⁺π⁻) and measurements of χcJ → 3(π⁺π⁻) in ψ(3686) radiative transitions
(2022)
The hadronic decay 𝜂𝑐(2𝑆)→3(𝜋+𝜋−) is observed with a statistical significance of 9.3 standard deviations using (448.1±2.9)×106 𝜓(3686) events collected by the BESIII detector at the BEPCII collider. The measured mass and width of 𝜂𝑐(2𝑆) are (3643.4±2.3 (stat)±4.4 (syst)) MeV/𝑐2 and (19.8±3.9 (stat)±3.1 (syst)) MeV, respectively, which are consistent with the world average values within two standard deviations. The product branching fraction ℬ[𝜓(3686)→𝛾𝜂𝑐(2𝑆)]×ℬ[𝜂𝑐(2𝑆)→3(𝜋+𝜋−)] is measured to be (9.2±1.0 (stat)±1.2 (syst))×10−6. Using ℬ[𝜓(3686)→𝛾𝜂𝑐(2𝑆)]=(7.0+3.4−2.5)×10−4, we obtain ℬ[𝜂𝑐(2𝑆)→3(𝜋+𝜋−)]=(1.31±0.15 (stat)±0.17 (syst) (+0.64−0.47) (extr))×10−2, where the third uncertainty is from ℬ[𝜓(3686)→𝛾𝜂𝑐(2𝑆)]. We also measure the 𝜒𝑐𝐽→3(𝜋+𝜋−) (𝐽=0, 1, 2) decays via 𝜓′→𝛾𝜒𝑐𝐽 transitions. The branching fractions are ℬ[𝜒𝑐0→3(𝜋+𝜋−)]=(2.080±0.006 (stat)±0.068 (syst))×10−2, ℬ[𝜒𝑐1→3(𝜋+𝜋−)]=(1.092±0.004 (stat)±0.035 (syst))×10−2, and ℬ[𝜒𝑐2→3(𝜋+𝜋−)]=(1.565±0.005 (stat)±0.048 (syst))×10−2.
Observation of ηc(2S) → 3(π⁺π⁻) and measurements of χcJ → 3(π⁺π⁻) in ψ(3686) radiative transitions
(2022)
The hadronic decay ηc(2S)→3(π+π−) is observed with a statistical significance of 9.3 standard deviations using (448.1±2.9)×106 ψ(3686) events collected by the BESIII detector at the BEPCII collider. The measured mass and width of ηc(2S) are (3643.4±2.3(stat.)±4.4(syst.)) MeV/c2 and (19.8±3.9(stat.)±3.1(syst.)) MeV, respectively, which are consistent with the world average values within two standard deviations. The product branching fraction B[ψ(3686) → γηc(2S)]×B[ηc(2S)→3(π+π−)] is measured to be (9.2±1.0(stat.)±0.9(syst.))×10−6. Using B[ψ(3686)→γηc(2S)]=(7.0+3.4−2.5)×10−4, we obtain B[ηc(2S)→3(π+π−)]=(1.31±0.15(stat.)±0.13(syst.)(+0.64−0.47)(extr))×10−2, where the third uncertainty is from B[ψ(3686)→γηc(2S)]. We also measure the χcJ→3(π+π−) (J=0,1,2) decays via ψ(3686)→γχcJ transitions. The branching fractions are B[χc0→3(π+π−)]=(2.080±0.006(stat.)±0.068(syst.))×10−2, B[χc1→3(π+π−)]=(1.092±0.004(stat.)±0.035(syst.))×10−2, and B[χc2→3(π+π−)]=(1.565±0.005(stat.)±0.048(syst.))×10−2.
Using a sample of 448.1×106 ψ(2S) events collected with the BESIII detector, we perform a study of the decay J/ψ→K+K− via ψ(2S)→π+π−J/ψ.
The branching fraction of J/ψ→K+K− is determined to be BK+K−=(3.072±0.023(stat.)±0.050(syst.))×10−4, which is consistent with previous measurements but with significantly improved precision.
Based on (10087±44)×106 𝐽/𝜓 events collected with the BESIII detector at BEPCII, the double Dalitz decay 𝜂′→𝑒+𝑒−𝑒+𝑒− is observed for the first time via the 𝐽/𝜓→𝛾𝜂′ decay process. The significance is found to be 5.7𝜎 with systematic uncertainties taken into consideration. Its branching fraction is determined to be ℬ(𝜂′→𝑒+𝑒−𝑒+𝑒−)=(4.5±1.0(stat)±0.5(sys))×10−6.
We report a search for a dark photon using 14.9~fb−1 of e+e− annihilation data taken at center-of-mass energies from 4.13 to 4.60~GeV with the BESIII detector operated at the BEPCII storage ring. The dark photon is assumed to be produced in the radiative annihilation process of e+e− and to predominantly decay into light dark matter particles, which escape from the detector undetected. The mass range from 1.5 to 2.9~GeV is scanned for the dark photon candidate, and no significant signal is observed. The mass dependent upper limits at the 90% confidence level on the coupling strength parameter ϵ for a dark photon coupling with an ordinary photon vary between 1.6×10−3 and 5.7×10−3.
The singly Cabibbo-suppressed decay D+s → K+π+π−π0 is observed by using a data set corresponding to an integrated luminosity of 6.32 fb−1 recorded by the BESIII detector at the centre-of-mass energies between 4.178 and 4.226 GeV. The first amplitude analysis of D+s → K+π+π−π0 reveals the sub-structures in this decay and determines the fractions and relative phases of different intermediate processes. The dominant intermediate process is D+s → K∗0ρ+, with a fit fraction of (40.5 ± 2.8stat. ± 1.5syst.)%. With the detection efficiency based on our amplitude analysis, the absolute branching fraction forD+s → K+π+π−π0 is measured to be (9.75 ± 0.54stat. ± 0.17syst.) × 10−3.