We extract the e+e−→π+π− cross section in the energy range between 600 and 900 MeV, exploiting the method of initial state radiation. A data set with an integrated luminosity of 2.93 fb−1 taken at a ...center-of-mass energy of 3.773 GeV with the BESIII detector at the BEPCII collider is used. The cross section is measured with a systematic uncertainty of 0.9%. We extract the pion form factor |Fπ|2 as well as the contribution of the measured cross section to the leading-order hadronic vacuum polarization contribution to (g−2)μ. We find this value to be aμππ,LO(600–900MeV)=(368.2±2.5stat±3.3sys)⋅10−10, which is between the corresponding values using the BaBar or KLOE data.
We report an amplitude analysis and branching fraction measurement of D+s → K+K−π+ decay using a data sample of 3.19 fb−1 recorded with BESIII detector at a center-of-mass energy of 4.178 GeV. We ...perform a model-independent partial wave analysis in the low K+K− mass region to determine the K+K− S-wave line shape, followed by an amplitude analysis of our very pure high-statistics sample. With the detection efficiency based on the amplitude analysis results, the absolute branching fraction is measured to be B(D+s → K+K− π+) = (5.47 ± 0.0 8stat ± 0.13sys)%.
The Born cross sections of the e+e−→Σ+Σ¯− and e+e−→Σ−Σ¯+ processes are determined for center-of-mass energy from 2.3864 to 3.0200 GeV with the BESIII detector. The cross section lineshapes can be ...described properly by a pQCD function and the resulting ratio of effective form factors for the Σ+ and Σ− is consistent with 3. In addition, ratios of the Σ+ electric and magnetic form factors, |GE/GM|, are obtained at three center-of-mass energies through an analysis of the angular distributions. These measurements, which are studied for the first time in the off-resonance region, provide precision experimental input for understanding baryonic structure. The observed new features of the Σ± form factors require more theoretical discussions for the hyperons.
The exclusive process e+e−→ΛΛ¯, with Λ→pπ− and Λ¯→p¯π+, has been studied at s=2.396 GeV for measurement of the timelike Λ electric and magnetic form factors, GE and GM. A data sample, corresponding ...to an integrated luminosity of 66.9 pb−1, was collected with the BESIII detector for this purpose. A multidimensional analysis with a complete decomposition of the spin structure of the reaction enables a determination of the modulus of the ratio R=|GE/GM| and, for the first time for any baryon, the relative phase ΔΦ=ΦE−ΦM. The resulting values are R=0.96±0.14(stat)±0.02(syst) and ΔΦ=37°±12°(stat)±6°(syst), respectively. These are obtained using the recently established and most precise value of the asymmetry parameter αΛ=0.750±0.010 measured by BESIII. In addition, the cross section is measured with unprecedented precision to be σ=118.7±5.3(stat)±5.1(syst) pb, which corresponds to an effective form factor of |G|=0.123±0.003(stat)±0.003(syst). The contribution from two-photon exchange is found to be negligible. Our result enables the first complete determination of baryon timelike electromagnetic form factors.
Cross sections of the process e+e− → π0 π0 J / ψ at center-of-mass energies between 3.808 and 4.600 GeV are measured with high precision by using 12.4 fb−1 of data samples collected with the BESIII ...detector operating at the BEPCII collider facility. A fit to the measured energy-dependent cross sections confirms the existence of the charmoniumlike state Y ( 4220 ) . The mass and width of the Y ( 4220 ) are determined to be ( 4220.4 ± 2.4 ± 2.3 ) MeV / c2 and ( 46.2 ± 4.7 ± 2.1 ) MeV , respectively, where the first uncertainties are statistical and the second systematic. The mass and width are consistent with those measured in the process e+e− → π+π− J / ψ. The neutral charmonium-like state Zc ( 3900 )0 is observed prominently in the π 0 J / ψ invariant-mass spectrum, and, for the first time, an amplitude analysis is performed to study its properties. The spin-parity of Zc ( 3900 )0 is determined to be JP = 1+, and the pole position is ( 3893.1 ± 2.2 ± 3.0 ) − i ( 22.2 ± 2.6 ± 7.0 ) MeV / c2, which is consistent with previous studies of electrically charged Zc ( 3900 )±. In addition, cross sections of e+e− → π0Zc ( 3900 )0 → π0π0 J / ψ are extracted, and the corresponding line shape is found to agree with that of the Y ( 4220 ).
Using a data sample corresponding to an integrated luminosity of 2.93 fb−1 taken at a center-of-mass energy of 3.773 GeV with the BESIII detector operated at the BEPCII collider, we perform an ...analysis of the semileptonic decays D0(+)→π−(0)μ+νμ. The branching fractions of D0→π−μ+νμ and D+→π0μ+νμ are measured to be (0.272±0.008stat±0.006syst)% and (0.350±0.011stat±0.010syst)%, respectively, where the former is of much improved precision compared to previous results and the latter is determined for the first time. Using these results along with previous BESIII measurements of D0(+)→π−(0)e+νe, we calculate the branching fraction ratios to be R0≡BD0→π−μ+νμ/BD0→π−e+νe=0.922±0.030stat±0.022syst and R+≡BD+→π0μ+νμ/BD+→π0e+νe=0.964±0.037stat±0.026syst, which are compatible with the theoretical expectation of lepton flavor universality within 1.7σ and 0.5σ, respectively. We also examine the branching fraction ratios in different four-momentum transfer square regions, and find no significant deviations from the standard model predictions.
Using a total of 9.0 fb−1 of e+e− collision data with center-of-mass energies between 4.15 and 4.30 GeV collected by the BESIII detector, we search for the processes e+e−→γX(3872) with X(3872)→π0χcJ ...for J=0, 1, 2. We report the first observation of X(3872)→π0χc1, a new decay mode of the X(3872), with a statistical significance of more than 5σ for all systematic fit variations. Normalizing to the previously established process e+e−→γX(3872) with X(3872)→π+π−J/ψ, we find B(X(3872)→π0χc1)/B(X(3872)→π+π−J/ψ)=0.88−0.27+0.33±0.10, where the first error is statistical and the second is systematic. We set 90% confidence level upper limits on the corresponding ratios for the decays to π0χc0 and π0χc2 of 19 and 1.1, respectively.
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