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During the 2016-17 and 2018-19 running periods, the BESIII experiment collected 7.5~fb−1 of e+e− collision data at center-of-mass energies ranging from 4.13 to 4.44 GeV. These data samples are primarily used for the study of excited charmonium and charmoniumlike states. By analyzing the di-muon process e+e−→(γISR/FSR)μ+μ−, we measure the center-of-mass energies of the data samples with a precision of 0.6 MeV. Through a run-by-run study, we find that the center-of-mass energies were stable throughout most of the data-taking period.
A search for the charged lepton flavor violating decay 𝐽/𝜓→𝑒±𝜏∓ with 𝜏∓→𝜋∓𝜋0𝜈𝜏 is performed with about 10×109 𝐽/𝜓 events collected with the BESIII detector at the BEPCII. No significant signal is observed, and an upper limit is set on the branching fraction ℬ(𝐽/𝜓→𝑒±𝜏∓)<7.5×10−8 at the 90% confidence level. This improves the previously published limit by two orders of magnitude.
Using 2.93 fb−1 of 𝑒+𝑒− collision data taken at a center-of-mass energy of 3.773 GeV with the BESIII detector, we report the first measurements of the absolute branching fractions of 14 hadronic 𝐷0(+) decays to exclusive final states with an 𝜂, e.g., 𝐷0→𝐾−𝜋+𝜂, 𝐾0𝑆𝜋0𝜂, 𝐾+𝐾−𝜂, 𝐾0𝑆𝐾0𝑆𝜂, 𝐾−𝜋+𝜋0𝜂, 𝐾0𝑆𝜋+𝜋−𝜂, 𝐾0𝑆𝜋0𝜋0𝜂, and 𝜋+𝜋−𝜋0𝜂; 𝐷+→𝐾0𝑆𝜋+𝜂, 𝐾0𝑆𝐾+𝜂, 𝐾−𝜋+𝜋+𝜂, 𝐾0𝑆𝜋+𝜋0𝜂, 𝜋+𝜋+𝜋−𝜂, and 𝜋+𝜋0𝜋0𝜂. Among these decays, the 𝐷0→𝐾−𝜋+𝜂 and 𝐷+→𝐾0 𝑆𝜋+𝜂 decays have the largest branching fractions, which are ℬ(𝐷0→𝐾−𝜋+𝜂) = (1.853±0.025stat±0.031syst)% and ℬ(𝐷+→𝐾0𝑆𝜋+𝜂) = (1.309±0.037stat±0.031syst)%, respectively. The charge-parity asymmetries for the six decays with highest event yields are determined, and no statistically significant charge-parity violation is found.
During the 2016-17 and 2018-19 running periods, the BESIII experiment collected 7.5~fb−1 of e+e− collision data at center-of-mass energies ranging from 4.13 to 4.44 GeV. These data samples are primarily used for the study of excited charmonium and charmoniumlike states. By analyzing the di-muon process e+e−→(γISR/FSR)μ+μ−, we measure the center-of-mass energies of the data samples with a precision of 0.6 MeV. Through a run-by-run study, we find that the center-of-mass energies were stable throughout most of the data-taking period.
A search for invisible decays of the Λ baryon is carried out in the process 𝐽/𝜓→Λ¯Λ based on (1.0087±0.0044)×1010 𝐽/𝜓 events collected with the BESIII detector located at the BEPCII storage ring. No signals are found for the invisible decays of Λ baryon, and the upper limit of the branching fraction is determined to be 7.4×10−5 at the 90% confidence level. This is the first search for invisible decays of baryons; such searches will play an important role in constraining dark sector models related to the baryon asymmetry.
We measure the inclusive semielectronic decay branching fraction of the D+s meson. A double-tag technique is applied to e+e− annihilation data collected by the BESIII experiment at the BEPCII collider, operating in the center-of-mass energy range 4.178−4.230 GeV. We select positrons from D+s→Xe+νe with momenta greater than 200 MeV/c, and determine the laboratory momentum spectrum, accounting for the effects of detector efficiency and resolution. The total positron yield and semielectronic branching fraction are determined by extrapolating this spectrum below the momentum cutoff. We measure the D+s semielectronic branching fraction to be B(D+s→Xe+νe)=(6.30±0.13(stat.)±0.10(syst.))%, showing no evidence for unobserved exclusive semielectronic modes. We combine this result with external data taken from literature to determine the ratio of the D+s and D0 semielectronic widths, Γ(D+s→Xe+νe)Γ(D0→Xe+νe)=0.790±0.016(stat.)±0.020(syst.). Our results are consistent with and more precise than previous measurements.
Using 2.93 fb−1 of e+e− collision data collected with the BESIII detector at a center-of-mass energy of 3.773 GeV, we measure the absolute branching fractions of the decays D0→K−e+νe and D+→K¯0e+νe to be (3.567±0.031stat±0.025syst)% and (8.68±0.14stat±0.16syst)%, respectively. Starting with the process e+e−→DD¯, a new reconstruction method is employed to select events that contain candidates for both D→K¯e+νe and D¯→Ke−ν¯e decays. The branching fractions reported in this work are consistent within uncertainties with previous BESIII measurements that selected events containing D→K¯e+νe and hadronic D¯ decays. Combining our results with the lifetimes of the D0 and D+ mesons and the previous BESIII measurements leads to a ratio of the two decay partial widths of Γ¯D0→K−e+νeΓ¯D+→K¯0e+νe=1.039±0.021. This ratio supports isospin symmetry in the D0→K−e+νe and D+→K¯0e+νe decays within 1.9σ.
Utilizing the data set corresponding to an integrated luminosity of 3.19 fb−1 collected by the BESIII detector at a center-of-mass energy of 4.178 GeV, we perform an amplitude analysis of the 𝐷+
𝑠→𝜋+𝜋−𝜋+ decay. The sample contains 13,797 candidates with a signal purity of ∼80%. The amplitude and phase of the contributing 𝜋𝜋 𝒮 wave are measured based on a quasi-model-independent approach, along with the amplitudes and phases of the 𝒫 and 𝒟 waves parametrized by Breit-Wigner models. The fit fractions of different intermediate decay channels are also reported.
Using 10.1 × 109 J/ψ events produced by the Beijing Electron Positron Collider (BEPCII) at a center-of-mass energy √s = 3.097 GeV and collected with the BESIII detector, we present a search for the rare semi-leptonic decay J/ψ → D−e+νe + c.c. No excess of signal above background is observed, and an upper limit on the branching fraction B(J/ψ → D−e +νe + c.c.) < 7.1 × 10−8 is obtained at 90% confidence level. This is an improvement of more than two orders of magnitude over the previous best limit.
Using a data sample corresponding to an integrated luminosity of 2.93 fb−1 collected at a center-of-mass energy √𝑠=3.773 GeV by the BESIII detector, the decay 𝐷0→𝜔𝜙 is observed for the first time. The branching fraction is measured to be (6.48±0.96±0.40)×10−4 with a significance of 6.3𝜎, where the first and second uncertainties are statistical and systematic, respectively. An angular analysis reveals that the 𝜙 and 𝜔 mesons from the 𝐷0→𝜔𝜙 decay are transversely polarized. The 95% confidence level upper limit on longitudinal polarization fraction is set to be less than 0.24, which is inconsistent with current theoretical expectations and challenges our understanding of the underlying dynamics in charm meson decays.
We report the first observation of the semimuonic decay 𝐷+→𝜔𝜇+𝜈𝜇 using an 𝑒+𝑒− collision data sample corresponding to an integrated luminosity of 2.93 fb−1 collected with the BESIII detector at a center-of-mass energy of 3.773 GeV. The absolute branching fraction of the 𝐷+→𝜔𝜇+𝜈𝜇 decay is measured to be ℬ𝐷+→𝜔𝜇+𝜈𝜇=(17.7±1.8stat±1.1syst)×10−4. Its ratio with the world average value of the branching fraction of the 𝐷+→𝜔𝑒+𝜈𝑒 decay probes lepton flavor universality and it is determined to be ℬ𝐷+→𝜔𝜇+𝜈𝜇/ℬPDG 𝐷+→𝜔𝑒+𝜈𝑒=1.05±0.14, in agreement with the standard model expectation within one standard deviation.
Ten hadronic final states of the ℎ𝑐 decays are investigated via the process 𝜓(3686)→𝜋0ℎ𝑐, using a data sample of (448.1±2.9)×106 𝜓(3686) events collected with the BESIII detector. The decay channel ℎ𝑐→𝐾+𝐾−𝜋+𝜋−𝜋0 is observed for the first time and has a measured significance of 6.0𝜎. The corresponding branching fraction is determined to be ℬ(ℎ𝑐→𝐾+𝐾−𝜋+𝜋−𝜋0)=(3.3±0.6±0.6)×10−3 (where the uncertainties are statistical and systematic, respectively). Evidence for the decays ℎ𝑐→𝜋+𝜋−𝜋0𝜂 and ℎ𝑐→𝐾0𝑆𝐾±𝜋∓𝜋+𝜋− is found with a significance of 3.6𝜎 and 3.8𝜎, respectively. The corresponding branching fractions (and upper limits) are obtained to be ℬ(ℎ𝑐→𝜋+𝜋−𝜋0𝜂)=(7.2±1.8±1.3)×10−3 (<1.8×10−2) and ℬ(ℎ𝑐→𝐾0𝑆𝐾±𝜋∓𝜋+𝜋−)=(2.8±0.9±0.5)×10−3 (<4.7×10−3). Upper limits on the branching fractions for the final states ℎ𝑐→𝐾+𝐾−𝜋0, 𝐾+𝐾−𝜂, 𝐾+𝐾−𝜋+𝜋−𝜂, 2(𝐾+𝐾−)𝜋0, 𝐾+𝐾−𝜋0𝜂, 𝐾0𝑆𝐾±𝜋∓, and 𝑝¯𝑝𝜋0𝜋0 are determined at a confidence level of 90%.
Using data samples with a total integrated luminosity of 20.1 fb−1 collected by the BESIII detector operating at the BEPCII collider, the cross section of the process 𝑒+𝑒−→𝜋+𝜋−𝜓(3686) is measured at center-of-mass energies between 4.0076 and 4.6984 GeV. The measured cross section is consistent with previous results, and with much improved precision. A fit to the measured energy-dependent cross section, which includes three Breit-Wigner functions and a nonresonant contribution, confirms the existence of the charmonium-like states 𝑌(4220), 𝑌(4390), and 𝑌(4660). This is the first observation of the 𝑌(4660) at the BESIII experiment.
Using a total of 5.25 fb−1 of e+e− collision data with center-of-mass energies from 4.236 to 4.600 GeV, we report the first observation of the process e+e− → ηψ(2S) with a statistical significance of 4.9 standard deviations. The data sets were collected by the BESIII detector operating at the BEPCII storage ring. We measure the yield of events integrated over center-of-mass energies and also present the energy dependence of the measured cross section.
Using a sample of 106 million 𝜓(3686) decays, 𝜓(3686)→𝛾𝜒𝑐𝐽(𝐽=0,1,2) and 𝜓(3686)→𝛾𝜒𝑐𝐽,𝜒𝑐𝐽→𝛾𝐽/𝜓(𝐽=1,2) events are utilized to study inclusive 𝜒𝑐𝐽→anything, 𝜒𝑐𝐽→hadrons, and 𝐽/𝜓→anything distributions, including distributions of the number of charged tracks, electromagnetic calorimeter showers, and 𝜋0s, and to compare them with distributions obtained from the BESIII Monte Carlo simulation. Information from each Monte Carlo simulated decay event is used to construct matrices connecting the detected distributions to the input predetection “produced” distributions. Assuming these matrices also apply to data, they are used to predict the analogous produced distributions of the decay events. Using these, the charged particle multiplicities are compared with results from MARK I. Further, comparison of the distributions of the number of photons in data with those in Monte Carlo simulation indicates that G-parity conservation should be taken into consideration in the simulation.
By analyzing a data sample corresponding to an integrated luminosity of 2.93 fb−1 collected at a center-of-mass energy of 3.773 GeV with By analyzing a data sample corresponding to an integrated luminosity of 2.93 fb−1 collected at a center-of-mass energy of 3.773 GeV with the BESIII detector, we measure for the first time the absolute branching fraction of the 𝐷+→𝜂𝜇+𝜈𝜇 decay to be ℬ𝐷+→𝜂𝜇+𝜈𝜇=(10.4±1.0stat±0.5syst)×10−4. Using the world averaged value of ℬ𝐷+→𝜂𝑒+𝜈𝑒, the ratio of the two branching fractions is determined to be ℬ𝐷+→𝜂𝜇+𝜈𝜇/ℬ𝐷+→𝜂𝑒+𝜈𝑒=0.91±0.13(stat+syst), which agrees with the theoretical expectation of lepton flavor universality within uncertainty. By studying the differential decay rates in five four-momentum transfer intervals, we obtain the product of the hadronic form factor 𝑓𝜂+(0) and the 𝑐→𝑑 Cabibbo-Kobayashi-Maskawa matrix element |𝑉𝑐𝑑| to be 𝑓𝜂+(0)|𝑉𝑐𝑑|=0.087±0.008stat±0.002syst. Taking the input of |𝑉𝑐𝑑| from the global fit in the standard model, we determine 𝑓𝜂+(0)=0.39±0.04stat±0.01syst. On the other hand, using the value of 𝑓𝜂+(0) calculated in theory, we find |𝑉𝑐𝑑| = 0.242±0.022stat±0.006syst±0.033theory.
Using a data sample of 4.481×108 𝜓(3686) events collected with the BESIII detector, we report the first observation of the four-lepton-decays 𝐽/𝜓→𝑒+𝑒−𝑒+𝑒− and 𝐽/𝜓→𝑒+𝑒−𝜇+𝜇− utilizing the process 𝜓(3686)→𝜋+𝜋−𝐽/𝜓. The branching fractions are determined to be [5.48±0.31(stat)±0.45(syst)]×10−5 and [3.53±0.22(stat)±0.13(syst)]×10−5, respectively. The results are consistent with theoretical predictions. No significant signal is observed for 𝐽/𝜓→𝜇+𝜇−𝜇+𝜇−, and an upper limit on the branching fraction is set at 1.6×10−6 at the 90% confidence level. A 𝐶𝑃 asymmetry observable is constructed for the first two channels, which is measured to be (−0.012±0.054±0.010) and (0.062±0.059±0.006), respectively. No evidence for 𝐶𝑃 violation is observed in this process.
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 rare decay 𝜂′→𝜋+𝜋−𝑒+𝑒− is studied using a sample of 1.3×109 𝐽/𝜓 events collected with the BESIII detector at BEPCII in 2009 and 2012. The branching fraction is measured with improved precision to be (2.42±0.05stat±0.08syst)×10−3. Due to the inclusion of new data, this result supersedes the last BESIII result on this branching fraction. In addition, the 𝐶𝑃-violating asymmetry in the angle between the decay planes of the 𝜋+𝜋−-pair and the 𝑒+𝑒−-pair is investigated. A measurable value would indicate physics beyond the standard model; the result is 𝒜𝐶𝑃=(2.9±3.7stat±1.1syst)%, which is consistent with the standard model expectation of no 𝐶𝑃-violation. The precision is comparable to the asymmetry measurement in the 𝐾0𝐿→𝜋+𝜋−𝑒+𝑒− decay where the observed (14±2)% effect is driven by a standard model mechanism.
Using a sample of about 1010 𝐽/𝜓 events collected at a center-of-mass energy √𝑠=3.097 GeV with the BESIII detector, the electromagnetic Dalitz decays 𝐽/𝜓→𝑒+𝑒−𝜋+𝜋−𝜂′, with 𝜂′→𝛾𝜋+𝜋− and 𝜂′→𝜋+𝜋−𝜂, have been studied. The decay 𝐽/𝜓→𝑒+𝑒−𝑋(1835) is observed with a significance of 15𝜎, and also an 𝑒+𝑒− invariant-mass dependent transition form factor of 𝐽/𝜓→𝑒+𝑒−𝑋(1835) is presented for the first time. The intermediate states 𝑋(2120) and 𝑋(2370) are also observed in the 𝜋+𝜋−𝜂′ invariant-mass spectrum with significances of 5.3𝜎 and 7.3𝜎. The corresponding product branching fractions for 𝐽/𝜓→𝑒+𝑒−𝑋, 𝑋→𝜋+𝜋−𝜂′ [𝑋=𝑋(1835), 𝑋(2120), and 𝑋(2370)] are reported.