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Institute
Based on (448.1±2.9)×106 ψ(3686) events collected with the BESIII detector operating at the BEPCII collider, the decay ψ(3686)→ϕK0SK0S is observed for the first time. Taking the interference between ψ(3686) decay and continuum production into account, the branching fraction of this decay is measured to be B(ψ(3686)→ϕK0SK0S) = (3.53 ± 0.20 ± 0.21)×10−5, where the first uncertainty is statistical and the second is systematic. Combining with the world average value for B(J/ψ→ϕK0SK0S), the ratio B(ψ(3686)→ϕK0SK0S)/B(J/ψ→ϕK0SK0S) is determined to be (6.0±1.6)%, which is suppressed relative to the 12% rule.
The Born cross sections of the process e+e−→D∗0D∗−π+ at center-of-mass energies from 4.189 to 4.951 GeV are measured for the first time. The data samples used correspond to an integrated luminosity of 17.9fb−1 and were collected by the BESIII detector operating at the BEPCII storage ring. Three enhancements around 4.20, 4.47 and 4.67 GeV are visible. The resonances have masses of 4209.6±4.7±5.9MeV/c2, 4469.1±26.2±3.6MeV/c2 and 4675.3±29.5±3.5MeV/c2 and widths of 81.6±17.8±9.0MeV, 246.3±36.7±9.4MeV, and 218.3±72.9±9.3MeV, respectively, where the first uncertainties are statistical and the second systematic. The first and third resonances are consistent with the ψ(4230) and ψ(4660) states, respectively, while the second one is compatible with the ψ(4500) observed in the e+e−→K+K−J/ψ process. These three charmoniumlike ψ states are observed in e+e−→D∗0D∗−π+ process for the first time.
Measurements of the electric and magnetic form factors of the neutron for timelike momentum transfer
(2023)
We present the first measurements of the electric and magnetic form factors of the neutron in the time-like (positive q2) region as function of four-momentum transfer. We explored the differential cross sections of the reaction e+e−→n¯n with data collected with the BESIII detector at the BEPCII accelerator, corresponding to an integrated luminosity of 354.6 pb−1 in total at twelve center-of-mass energies between s√=2.0−2.95 GeV. A relative uncertainty of 18% and 12% for the electric and magnetic form factors, respectively, is achieved at s√=2.3935 GeV. Our results are comparable in accuracy to those from electron scattering in the comparable space-like (negative q2) region of four-momentum transfer. The electromagnetic form factor ratio Rem≡|GE|/|GM| is within the uncertainties close to unity. We compare our result on |GE| and |GM| to recent model predictions, and the measurements in the space-like region to test the analyticity of electromagnetic form factors.
Using a data sample of e+e− collision data corresponding to an integrated luminosity of 19 fb−1 collected with the BESIII detector at the BEPCII collider, we search for the production of deuterons and antideuterons via e+e−→ppπ−d¯+c.c. for the first time at center-of-mass energies between 4.13 and 4.70 GeV. No significant signal is observed and the upper limit of the e+e−→ppπ−d¯+c.c. cross section is determined to be from 9.0 to 145 fb depending on the center-of-mass energy at the 90% confidence level.
The first direct measurement of the absolute branching fraction of Σ+→Λe+νe is reported based on an e+e− annihilation sample of (10087±44)×106 J/ψ events collected with the BESIII detector at s√=3.097 GeV. The branching fraction is determined to be B(Σ+→Λe+νe)=[2.93±0.74(stat)±0.13(syst)]×10−5, which is the most precise measurement obtained in a single experiment to date and also the first result obtained at a collider experiment. Combining this result with the world average of B(Σ−→Λe−ν¯e) and the lifetimes of Σ±, the ratio, Γ(Σ−→Λe−ν¯e)Γ(Σ+→Λe+νe), is determined to be 1.06±0.28, which is within 1.8 standard deviations of the value expected in the absence of second-class currents that are forbidden in the Standard Model.
The Born cross sections of the process e+e−→D∗0D∗−π+ at center-of-mass energies from 4.189 to 4.951 GeV are measured for the first time. The data samples used correspond to an integrated luminosity of 17.9fb−1 and were collected by the BESIII detector operating at the BEPCII storage ring. Three enhancements around 4.20, 4.47 and 4.67 GeV are visible. The resonances have masses of 4209.6±4.7±5.9MeV/c2, 4469.1±26.2±3.6MeV/c2 and 4675.3±29.5±3.5MeV/c2 and widths of 81.6±17.8±9.0MeV, 246.3±36.7±9.4MeV, and 218.3±72.9±9.3MeV, respectively, where the first uncertainties are statistical and the second systematic. The first and third resonances are consistent with the ψ(4230) and ψ(4660) states, respectively, while the second one is compatible with the ψ(4500) observed in the e+e−→K+K−J/ψ process. These three charmoniumlike ψ states are observed in e+e−→D∗0D∗−π+ process for the first time.
Observation of three charmonium-like states with JPC = 1⁻⁻ in e⁺e⁻− → D*⁰D*⁻π⁺ + c.c. process
(2023)
The Born cross sections of the process e+e−→D∗0D∗−π++c.c. at center-of-mass energies from 4.189 to 4.951 GeV are measured for the first time. The data samples used correspond to an integrated luminosity of 17.9fb−1 and were collected by the BESIII detector operating at the BEPCII storage ring. Three enhancements around 4.20, 4.47 and 4.67 GeV are visible. The resonances have masses of 4209.6±4.7±5.9MeV/c2, 4469.1±26.2±3.6MeV/c2 and 4675.3±29.5±3.5MeV/c2 and widths of 81.6±17.8±9.0MeV, 246.3±36.7±9.4MeV and 218.3±72.9±9.3MeV, respectively, where the first uncertainties are statistical and the second systematic. The first and third resonances are consistent with the Y(4230) and Y(4660) states, respectively, while the second one is compatible with the Y(4500) observed in the e+e−→K+K−J/ψ process. These three Y states are observed in e+e−→D∗0D∗−π++c.c. process for the first time.
Using a data sample of (1.0087±0.0044)×1010 𝐽/𝜓 decay events collected with the BESIII detector at the center-of-mass energy of √𝑠=3.097 GeV, we present a search for the hyperon semileptonic decay Ξ0→Σ−𝑒+𝜈𝑒 which violates the Δ𝑆=Δ𝑄 rule. No significant signal is observed, and the upper limit on the branching fraction ℬ(Ξ0→Σ−𝑒+𝜈𝑒) is determined to be 1.6×10−4 at the 90% confidence level. This result improves the previous upper limit result by about one order of magnitude.
Based on e+e− collision samples corresponding to an integrated luminosity of 4.4 fb−1 collected with the BESIII detector at center-of-mass energies between 4.6GeV and 4.7GeV, a partial wave analysis of the charmed baryon hadronic decay Λ+c→Λπ+π0 is performed, and the decays Λ+c→Λρ(770)+ and Λ+c→Σ(1385)π are studied for the first time. Making use of the world-average branching fraction B(Λ+c→Λπ+π0), their branching fractions are determined to be B(Λ+c→Λρ(770)+)=B(Λ+c→Σ(1385)+π0)=B(Λ+c→Σ(1385)0π+)=(4.06±0.30±0.35±0.23)×10−2,(5.86±0.49±0.52±0.35)×10−3,(6.47±0.59±0.66±0.38)×10−3, where the first uncertainties are statistical, the second are systematic, and the third are from the uncertainties of the branching fractions B(Λ+c→Λπ+π0) and B(Σ(1385)→Λπ). In addition, %according to amplitudes determined from the partial wave analysis, the decay asymmetry parameters are measured to be αΛρ(770)+=−0.763±0.053±0.039, αΣ(1385)+π0=−0.917±0.069±0.046, and αΣ(1385)0π+=−0.789±0.098±0.056.
Based on 10 billion J/ψ events collected at the BESIII experiment, a search for CP violation in Λ decay is performed in the difference between CP-odd decay parameters α− for Λ→pπ− and α+ for Λ¯→p¯π+ by using the process e+e−→J/ψ→ΛΛ¯. With a five-dimensional fit to the full angular distributions of the daughter baryon, the most precise values for the decay parameters are determined to be α−=0.7519±0.0036±0.0024 and α+=−0.7559±0.0036±0.0030, respectively. The Λ and Λ¯ averaged value of the decay parameter is extracted to be αavg=0.7542±0.0010±0.0024 with unprecedented accuracy. The CP asymmetry ACP=(α−+α+)/(α−−α+) is determined to be −0.0025 ± 0.0046 ± 0.0012, which is one of the most precise measurements in the baryon sector. The reported results for the decay parameter will play an important role in the studies of the polarizations and CP violations for the strange, charmed and beauty baryons.
Using a data sample of (1.0087±0.0044)×1010 J/ψ decay events collected with the BESIII detector at the center-of-mass energy of s√=3.097 GeV, we present a search for the hyperon semileptonic decay Ξ0→Σ−e+νe which violates the ΔS=ΔQ rule. No significant signal is observed, and the upper limit on the branching fraction B(Ξ0→Σ−e+νe) is determined to be 1.6×10−4 at the 90% confidence level. This result improves the previous upper limit result by about one order of magnitude.
A measurement of the 𝐶𝑃-even fraction of the decay 𝐷0→𝜋+𝜋−𝜋+𝜋− is performed with a quantum-correlated 𝜓(3770)→𝐷¯𝐷 data sample collected by the BESIII experiment, corresponding to an integrated luminosity of 2.93 fb−1. Using a combination of 𝐶𝑃 eigenstates, 𝐷→𝜋+𝜋−𝜋0 and 𝐷→𝐾0𝑆,𝐿𝜋+𝜋− as tagging modes, the 𝐶𝑃-even fraction is measured to be 𝐹4𝜋+=0.735±0.015±0.005, where the first uncertainty is statistical and the second is systematic. This is the most precise determination of this quantity to date. It provides valuable model-independent input for the measurement of the angle 𝛾 of the Cabibbo-Kobayashi-Maskawa matrix with 𝐵±→𝐷𝐾± decays, and for time-dependent studies of 𝐶𝑃 violation and mixing in the 𝐷0−¯𝐷0 system.
Using 9.9 fb−1 of e+e− collision data collected by the BESIII detector at center-of-mass energies between 4.15 and 4.30 GeV, we search for the processes e+e−→γX(3872) with X(3872)→π0χc0 and X(3872)→ππχc0. Depending on the fitting model, the statistical significance for X(3872)→π0χc0 ranges from 1.3σ to 2.8σ. We set upper limits (at 90\% C.L.) of B(X(3872)→π0χc0)B(X(3872)→π+π−J/ψ)<3.6, B(X(3872)→π+π−χc0)B(X(3872)→π+π−J/ψ)<0.68, and B(X(3872)→π0π0χc0)B(X(3872)→π+π−J/ψ)<1.7. Combined with the BESIII measurement of X(3872)→π0χc1, we also set an upper limit of B(X(3872)→π0χc0)B(X(3872)→π0χc1)<4.4.
Using 9.9 fb−1 of e+e− collision data collected by the BESIII detector at center-of-mass energies between 4.15 and 4.30 GeV, we search for the processes e+e−→γX(3872) with X(3872)→π0χc0 and X(3872)→ππχc0. Depending on the fitting model, the statistical significance for X(3872)→π0χc0 ranges from 1.3σ to 2.8σ. We set upper limits (at 90\% C.L.) of B(X(3872)→π0χc0)B(X(3872)→π+π−J/ψ)<3.6, B(X(3872)→π+π−χc0)B(X(3872)→π+π−J/ψ)<0.68, and B(X(3872)→π0π0χc0)B(X(3872)→π+π−J/ψ)<1.7. Combined with the BESIII measurement of X(3872)→π0χc1, we also set an upper limit of B(X(3872)→π0χc0)B(X(3872)→π0χc1)<4.4.
Using 9.9 fb−1 of e+e− collision data collected by the BESIII detector at center-of-mass energies between 4.15 and 4.30 GeV, we search for the processes e+e−→γX(3872) with X(3872)→π0χc0 and X(3872)→ππχc0. Depending on the fitting model, the statistical significance for X(3872)→π0χc0 ranges from 1.3σ to 2.8σ. We set upper limits (at 90\% C.L.) of B(X(3872)→π0χc0)B(X(3872)→π+π−J/ψ)<3.6, B(X(3872)→π+π−χc0)B(X(3872)→π+π−J/ψ)<0.68, and B(X(3872)→π0π0χc0)B(X(3872)→π+π−J/ψ)<1.7. Combined with the BESIII measurement of X(3872)→π0χc1, we also set an upper limit of B(X(3872)→π0χc0)B(X(3872)→π0χc1)<4.4.
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.
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.
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.
Using 15.6 fb−1 of e+e− collision data collected at twenty-four center-of-mass energies from 4.0 to 4.6 GeV with the BESIII detector, the helicity amplitudes of the process e+e− → π+π−ω are analyzed for the first time. Born cross section measurements of two-body intermediate resonance states with statistical significance greater than 5σ are presented, such as f0(500), f0(980), f2(1270), f0(1370), b1(1235)±, and ρ(1450)±. In addition, evidence of a resonance state in e+e− → π+π−ω production is found. The mass of this state obtained by line shape fitting is about 4.2 GeV/c2, which is consistent with the production of ψ(4160) or Y(4220).
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.