We present a study of the process \(e^+e^-\to\pi^{\pm}(D\bar{D}^*)^{\mp}\) using data samples of 1092~pb\(^{-1}\) at \(\sqrt{s}=4.23\)~GeV and 826~pb\(^{-1}\) at \(\sqrt{s}=4.26\)~GeV collected with ...the BESIII detector at the BEPCII storage ring. With full reconstruction of the \(D\) meson pair and the bachelor \(\pi^{\pm}\) in the final state, we confirm the existence of the charged structure \(Z_c(3885)^{\mp}\) in the \((D\bar{D}^*)^{\mp}\) system in the two isospin processes \(e^+e^-\to\pi^+D^0D^{*-}\) and \(e^+e^-\to\pi^+D^-D^{*0}\). By performing a simultaneous fit, the statistical significance of \(Zc(3885)^{\mp}\) signal is determined to be greater than 10\(\sigma\), and its pole mass and width are measured to be \(M_{\rm{pole}}\)=(3881.7\(\pm\)1.6(stat.)\(\pm\)1.6(syst.))~MeV/\(c^2\) and \(\Gamma_{\rm{pole}}\)=(26.6\(\pm\)2.0(stat.)\(\pm\)2.1(syst.))~MeV, respectively. The Born cross section times the \((D\bar{D}^*)^{\mp}\) branching fraction (\(\sigma(e^+e^-\to\pi^{\pm}Z_{c}(3885)^{\mp}) \times Br(Z_{c}(3885)^{\mp}\to(D\bar{D}^*)^{\mp})\)) is measured to be \((141.6\pm7.9(\text{stat.})\pm12.3(\text{syst.}))~\text{pb}\) at \(\sqrt{s}=4.23\)~GeV and \((108.4\pm6.9(\text{stat.})\pm8.8(\text{syst.}))~\text{pb}\) at \(\sqrt{s}=4.26\)~GeV. The polar angular distribution of the \(\pi^{\pm}\)-\(Z_c(3885)^{\mp}\) system is consistent with the expectation of a quantum number assignment of \(J^P=1^+\) for \(Z_c(3885)^{\mp}\).
An amplitude analysis of the \(\pi^{0}\pi^{0}\) system produced in radiative \(J/\psi\) decays is presented. In particular, a piecewise function that describes the dynamics of the \(\pi^{0}\pi^{0}\) ...system is determined as a function of \(M_{\pi^{0}\pi^{0}}\) from an analysis of the \((1.311\pm0.011)\times10^{9}\) \(J/\psi\) decays collected by the BESIII detector. The goal of this analysis is to provide a description of the scalar and tensor components of the \(\pi^0\pi^0\) system while making minimal assumptions about the properties or number of poles in the amplitude. Such a model-independent description allows one to integrate these results with other related results from complementary reactions in the development of phenomenological models, which can then be used to directly fit experimental data to obtain parameters of interest. The branching fraction of \(J/\psi \to \gamma \pi^{0}\pi^{0}\) is determined to be \((1.15\pm0.05)\times10^{-3}\), where the uncertainty is systematic only and the statistical uncertainty is negligible.
Using a sample of 1.31 billion \(J/\psi\) events collected with the BESIII detector at the BEPCII collider, the decays \(J/\psi \to \phi \pi^{+}\pi^{-}\pi^{0}\) and \(J/\psi \to \phi ...\pi^{0}\pi^{0}\pi^{0}\) are investigated. The isospin violating decay \(J/\psi \to \phi \pi^{0} f_{0}(980)\) with \(f_{0}(980) \to \pi\pi\), is observed for the first time. The width of the \(f_{0}(980)\) obtained from the dipion mass spectrum is found to be much smaller than the world average value. In the \(\pi^{0} f_{0}(980)\) mass spectrum, there is evidence of \(f_1(1285)\) production. By studying the decay \(J/\psi \to \phi\eta'\), the branching fractions of \(\eta' \to \pi^{+}\pi^{-}\pi^{0}\) and \(\eta' \to \pi^{0}\pi^{0}\pi^{0}\), as well as their ratio, are also measured.
The decay \(Z_c(3900)^\pm\to\omega\pi^\pm\) is searched for using data samples collected with the BESIII detector operating at the BEPCII storage ring at center-of-mass energies \(\sqrt{s}=4.23\) and ...\(4.26\)~GeV. No significant signal for the \(Z_c(3900)^\pm\) is found, and upper limits at the 90\% confidence level on the Born cross section for the process \(e^+e^-\to Z_c(3900)^\pm\pi^\mp\to\omega\pi^+\pi^-\) are determined to be \(0.26\) and \(0.18\) pb at \(\sqrt{s}=4.23\) and 4.26 GeV, respectively.
A cylindrical GEM detector is under development, to serve as an upgraded inner tracker at the BESIII spectrometer. It will consist of three layers of cylindrically-shaped triple GEMs surrounding the ...interaction point. The experiment is taking data at the e+e- collider BEPCII in Beijing (China) and the GEM tracker will be installed in 2018. Tests on the performances of triple GEMs in strong magnetic field have been run by means of the muon beam available in the H4 line of SPS (CERN) with both planar chambers and the first cylindrical prototype. Efficiencies and resolutions have been evaluated using different gains, gas mixtures, with and without magnetic field. The obtained efficiency is 97-98% on single coordinate view, in many operational arrangements. The spatial resolution for planar GEMs has been evaluated with two different algorithms for the position determination: the charge centroid and the micro time projection chamber (mu-TPC) methods. The two modes are complementary and are able to cope with the asymmetry of the electron avalanche when running in magnetic field, and with non-orthogonal incident tracks. With the charge centroid, a resolution lower than 100 micron has been reached without magnetic field and lower than 200 micron with a magnetic field up to 1 T. The mu-TPC mode showed to be able to improve those results. In the first beam test with the cylindrical prototype, the detector had a very good stability under different voltage configurations and particle intensities. The resolution evaluation is in progress.
Using \(2.92\) fb\(^{-1}\) of electron-positron annihilation data collected at \(\sqrt{s} = 3.773\)~GeV with the BESIII detector, we report the results of a search for the flavor-changing neutral ...current process \(D^0\to\gamma\gamma\) using a double-tag technique. We find no signal and set an upper limit at \(90\%\) confidence level for the branching fraction of \(B(D^0\to\gamma\gamma) < 3.8\times10^{-6}\). We also investigate \(D^0\)-meson decay into two neutral pions, obtaining a branching fraction of \(B(D^0\to\pi^0\pi^0) = (8.24\pm0.21(\text{stat.})\pm0.30(\text{syst.}))\times10^{-4}\), the most precise measurement to date and consistent with the current world average
For the upgrade of the inner tracker of the BESIII spectrometer, planned for 2018, a lightweight tracker based on an innovative Cylindrical Gas Electron Multiplier (CGEM) detector is now under ...development. The analogue readout of the CGEM enables the use of a charge centroid algorithm to improve the spatial resolution to better than 130 um while loosening the pitch strip to 650 um, which allows to reduce the total number of channels to about 10 000. The channels are readout by 160 dedicated integrated 64-channel front-end ASICs, providing a time and charge measurement and featuring a fully-digital output. The energy measurement is extracted either from the time-over-threshold (ToT) or the 10-bit digitisation of the peak amplitude of the signal. The time of the event is generated by quad-buffered low-power TDCs, allowing for rates in excess of 60 kHz per channel. The TDCs are based on analogue interpolation techniques and produce a time stamp (or two, if working in ToT mode) of the event with a time resolution better than 50 ps. The front-end noise, based on a CSA and CR-RC2 shapers, dominate the channel intrinsic time jitter, which is less than 5 ns r.m.s.. The time information of the hit can be used to reconstruct the track path, operating the detector as a small TPC and hence improving the position resolution when the distribution of the cloud, due to large incident angle or magnetic field, is very broad. Event data is collected by an off-detector motherboard, where each GEM-ROC readout card handles 4 ASIC carrier PCBs (512 channels). Configuration upload and data readout between the off-detector electronics and the VME-based data collector cards are managed by bi-directional fibre optical links.
Based on a sample of \(1.31 \times 10^9\) \(J/\psi\) events collected with the BESIII detector at the BEPCII collider, Dalitz plot analyses of selected 79,625 \(\eta\rightarrow\pi^{+}\pi^{-}\pi^0\) ...events, 33,908 \(\eta\rightarrow\pi^0\pi^0\pi^0\) events and 1,888 \(\eta^{\prime}\rightarrow\pi^0\pi^0\pi^0\) events are performed. The measured matrix elements of \(\eta\rightarrow\pi^+\pi^-\pi^0\) are in reasonable agreement with previous measurements. The Dalitz plot slope parameters of \(\eta\rightarrow\pi^0\pi^0\pi^0\) and \(\eta^{\prime}\rightarrow\pi^0\pi^0\pi^0\) are determined to be \(-0.055 \pm 0.014 \pm 0.004\) and \(-0.640 \pm 0.046 \pm 0.047\), respectively, where the first uncertainties are statistical and the second systematic. Both values are consistent with previous measurements, while the precision of the latter one is improved by a factor of three. Final state interactions are found to have an important role in those decays.
In an analysis of a 2.92~fb\(^{-1}\) data sample taken at 3.773~GeV with the BESIII detector operated at the BEPCII collider, we measure the absolute decay branching fractions to be \(\mathcal B(D^0 ...\to K^-e^+\nu_e)=(3.505\pm 0.014 \pm 0.033)\%\) and \(\mathcal B(D^0 \to \pi^-e^+\nu_e)=(0.295\pm 0.004\pm 0.003)\%\). From a study of the differential decay rates we obtain the products of hadronic form factor and the magnitude of the CKM matrix element \(f_{+}^K(0)|V_{cs}|=0.7172\pm0.0025\pm 0.0035\) and \(f_{+}^{\pi}(0)|V_{cd}|=0.1435\pm0.0018\pm 0.0009\). Combining these products with the values of \(|V_{cs(d)}|\) from the SM constraint fit, we extract the hadronic form factors \(f^K_+(0) = 0.7368\pm0.0026\pm 0.0036\) and \(f^\pi_+(0) = 0.6372\pm0.0080\pm 0.0044\), and their ratio \(f_+^{\pi}(0)/f_+^{K}(0)=0.8649\pm 0.0112\pm 0.0073\). These form factors and their ratio are used to test unquenched Lattice QCD calculations of the form factors and a light cone sum rule (LCSR) calculation of their ratio. The measured value of \(f_+^{K(\pi)}(0) |V_{cs(d)}|\) and the lattice QCD value for \(f^{K(\pi)}_+(0)\) are used to extract values of the CKM matrix elements of \(|V_{cs}|=0.9601 \pm 0.0033 \pm 0.0047 \pm 0.0239\) and \(|V_{cd}|=0.2155 \pm 0.0027 \pm 0.0014 \pm 0.0094\), where the third errors are due to the uncertainties in lattice QCD calculations of the form factors. Using the LCSR value for \(f_+^\pi(0)/f_+^K(0)\), we determine the ratio \(|V_{cd}|/|V_{cs}|=0.238\pm 0.004\pm 0.002\pm 0.011\), where the third error is from the uncertainty in the LCSR normalization. In addition, we measure form factor parameters for three different theoretical models that describe the weak hadronic charged currents for these two semileptonic decays. All of these measurements are the most precise to date.
Using a sample of \(1.31\) billion \(J/\psi\) events accumulated with the BESIII detector at the BEPCII collider, we report the observation of the decay \(J/\psi \rightarrow \phi\pi^{0}\), which is ...the first evidence for a doubly Okubo-Zweig-Iizuka suppressed electromagnetic \(J/\psi\) decay. A clear structure is observed in the \(K^{+} K^{-}\) mass spectrum around 1.02 GeV/\(c^2\), which can be attributed to interference between \(J/\psi \rightarrow \phi\pi^{0}\) and \(J/\psi \rightarrow K^{+}K^{-}\pi^{0}\) decays. Due to this interference, two possible solutions are found. The corresponding measured values of the branching fraction of \(J/\psi \to \phi\pi^{0}\) are \(2.94 \pm 0.16\text{(stat.)} \pm 0.16\text{(syst.)} \times 10^{-6}\) and \(1.24 \pm 0.33\text{(stat.)} \pm 0.30\text{(syst.)} \times 10^{-7}\).
