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Institute
Precision measurements of the semileptonic decays 𝐷+𝑠→𝜂𝑒+𝜈𝑒 and 𝐷+𝑠→𝜂′𝑒+𝜈𝑒 are performed with 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. The branching fractions obtained are ℬ(𝐷+𝑠→𝜂𝑒+𝜈𝑒) = (2.255±0.039stat±0.051syst)% and ℬ(𝐷+𝑠→𝜂′𝑒+𝜈𝑒)=(0.810±0.038stat±0.024syst)%. Combining these results with the ℬ(𝐷+→𝜂𝑒+𝜈𝑒) and ℬ(𝐷+→𝜂′𝑒+𝜈𝑒) obtained from previous BESIII measurements, the 𝜂−𝜂′ mixing angle in the quark flavor basis is determined to be 𝜙P=(40.0±2.0stat±0.6syst)°. Moreover, from the fits to the partial decay rates of 𝐷+𝑠→𝜂𝑒+𝜈𝑒 and 𝐷+𝑠→𝜂′𝑒+𝜈𝑒, the products of the hadronic transition form factors 𝑓𝜂(′)+(0) and the modulus of the 𝑐→𝑠 Cabibbo-Kobayashi-Maskawa matrix element |𝑉𝑐𝑠| are determined by using different hadronic transition form factor parametrizations. Based on the two-parameter series expansion, the products 𝑓𝜂+(0)|𝑉𝑐𝑠| = 0.4519±0.0071stat±0.0065syst and 𝑓𝜂′+(0)|𝑉𝑐𝑠| = 0.525±0.024stat±0.009syst are extracted. All results determined in this work supersede those measured in the previous BESIII analyses based on the 3.19 fb−1 subsample of data at 4.178 GeV.
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.
Search for the lepton number violating decay Σ⁻ → pe⁻e⁻ and the rare inclusive decay Σ⁻ → Σ⁺X
(2021)
Using a data sample of (1310.6±7.0)×106 𝐽/𝜓 events taken with the BESIII detector at the center-of-mass energy of 3.097 GeV, we search for the first time for the lepton number violating decay Σ−→𝑝𝑒−𝑒− and the rare inclusive decay Σ−→Σ+𝑋, where 𝑋 denotes any possible particle combination. The Σ− candidates are tagged in 𝐽/𝜓→¯Σ(1385)+Σ− decays. No signal candidates are found, and the upper limits on the branching fractions at the 90% confidence level are determined to be ℬ(Σ−→𝑝𝑒−𝑒−)<6.7×10−5 and ℬ(Σ−→Σ+𝑋)<1.2×10−4.
The integrated luminosities of the 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 the old data sets collected in 2010--2014 are updated by considering correction related to the detector performance, offsettting the effect of newly discovered readout errors in the electromagnetic calorimeter that happen haphazardly.
Using 2.93 fb−1 of e+e− collision data taken with the BESIII detector at a center-of-mass energy of 3.773 GeV, the observation of the D0→K1(1270)−e+νe semileptonic decay is presented. The statistical significance of the decay D0→K1(1270)−e+νe is greater than 10σ. The branching fraction of D0→K1(1270)−e+νe is measured to be (1.09±0.13+0.09−0.13±0.12)×10−3. Here, the first uncertainty is statistical, the second is systematic, and the third originates from the assumed branching fraction of K1(1270)−→K−π+π−.
Using 2.93 fb−1 of e+e− collision data taken with the BESIII detector at a center-of-mass energy of 3.773 GeV, the observation of the D0→K1(1270)−e+νe semileptonic decay is presented. The statistical significance of the decay D0→K1(1270)−e+νe is greater than 10σ. The branching fraction of D0→K1(1270)−e+νe is measured to be (1.09±0.13+0.09−0.16±0.12)×10−3. Here, the first uncertainty is statistical, the second is systematic, and the third originates from the assumed branching fraction of K1(1270)−→K−π+π−. The fraction of longitudinal polarization in D0→K1(1270)−e+νe is determined for the first time to be 0.50±0.19stat±0.08syst.
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.
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.
Using data samples collected with the BESIII detector operating at the BEPCII storage ring at center-of-mass energies from 4.178 to 4.600 GeV, we study the process eþe− → π0Xð3872Þγ and search for Zcð4020Þ0 → Xð3872Þγ. We find no significant signal and set upper limits on σðeþe− → π0Xð3872ÞγÞ · BðXð3872Þ → πþπ−J=ψÞ and σðeþe− → π0Zcð4020Þ0Þ · BðZcð4020Þ0 → Xð3872ÞγÞ · BðXð3872Þ → πþπ−J=ψÞ for each energy point at 90% confidence level, which is of the order of several tenths pb.