Measurement of the CP -even fraction of D0 →π+π-π+π-

A measurement of the CP-even fraction of the decay D0→π+π-π+π- is performed with a quantum-correlated ψ(3770)→DD¯ 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→KS,L0π+π- as tagging modes, the CP-even fraction is measured to be F+4π=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-D¯0 system

Study of η(1405)/η(1475) in J/ψ→γKS0KS0π0 decay

Using a sample of (10.09 ± 0.04) × 109J/ψ decays collected with the BESIII detector, partial wave analyses of the decay J/ψ→γKS0KS0π0 are performed within the KS0KS0π0 invariant mass region below 1.6 GeV/c2. The covariant tensor amplitude method is used in both mass independent and mass dependent approaches. Both analysis approaches exhibit dominant pseudoscalar and axial vector components, and show good consistency for the other individual components. Furthermore, the mass dependent analysis reveals that the KS0KS0π0 invariant mass spectrum for the pseudoscalar component can be well described with two isoscalar resonant states using relativistic Breit-Wigner model, i.e., the η(1405) with a mass of 1391.7±0.7−0.3+11.3 MeV/c2 and a width of 60.8±1.2−12.0+5.5 MeV, and the η(1475) with a mass of 1507.6±1.6−32.2+15.5 MeV/c2 and a width of 115.8±2.4−10.9+14.8 MeV. The first and second uncertainties are statistical and systematic, respectively. Alternate models for the pseudoscalar component are also tested, but the description of the KS0KS0π0 invariant mass spectrum deteriorates significantly. [Figure not available: see fulltext.

Measurement of the cross section of e + e − → ηπ + π − at center-of-mass energies from 3.872 GeV to 4.700 GeV

Using data samples with an integrated luminosity of 19 fb−1 at twenty-eight center-of-mass energies from 3.872 GeV to 4.700 GeV collected with the BESIII detector at the BEPCII electron-positron collider, the process e+e− → ηπ+π− and the intermediate process e+e− → ηρ0 are studied for the first time. The Born cross sections are measured. No significant resonance structure is observed in the cross section lineshape. [Figure not available: see fulltext.]

Search for a massless dark photon in Λc+ →pγ′ decay

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(Λc+→pγ′) is determined to be 8.0×10-5 at 90% confidence level

Observation of the doubly Cabibbo-suppressed decays D + → K + π 0 π 0 and D + → K + π 0 η

By analyzing e+e− annihilation data corresponding to an integrated luminosity of 2.93 fb−1 collected at the center-of-mass energy of 3.773 GeV with the BESIII detector, we report the first observations of the doubly Cabibbo-suppressed decays D+→ K+π0π0 and D+→ K+π0η. The branching fractions of D+→ K+π0π0 and D+→ K+π0η are measured to be (2.1 ± 0.4stat ± 0.1syst) × 10−4 and (2.1 ± 0.5stat ± 0.1syst) × 10−4 with statistical significances of 8.8σ and 5.5σ, respectively. In addition, we search for the subprocesses D+→ K*(892)+π0 and D+→ K*(892)+η with K*(892)+→ K+π0. The branching fraction of D+→ K*(892)+η is determined to be (4.4−1.5stat+1.8± 0.2syst) × 10−4, with a statistical significance of 3.2σ. No significant signal for D+→ K*(892)+π0 is found and we set an upper limit on the branching fraction of this decay at the 90% confidence level to be 5.4 × 10−4. [Figure not available: see fulltext.

Study of e+e- → ω- ω ¯ + at center-of-mass energies from 3.49 to 3.67 GeV

Using data samples of e+e- collisions collected with the BESIII detector at eight center-of-mass energy points between 3.49 and 3.67 GeV, corresponding to an integrated luminosity of 670 pb-1, we present the upper limits of Born cross sections and the effective form factor for the process e+e-→ω-ω¯+. A fit to the cross sections using a perturbative QCD-derived energy-dependent function shows no significant threshold effect. The upper limit on the measured effective form factor is consistent with a theoretical prediction within the uncertainty of 1σ. These results provide new experimental information on the production mechanism of ω

Improved measurement of the strong-phase difference δDKπ in quantum-correlated DD¯ decays

The decay D→ K-π+ is studied in a sample of quantum-correlated DD¯ pairs, based on a data set corresponding to an integrated luminosity of 2.93 fb- 1 collected at the ψ(3770) resonance by the BESIII experiment. The asymmetry between CP-odd and CP-even eigenstate decays into K-π+ is determined to be AKπ= 0.132 ± 0.011 ± 0.007 , where the first uncertainty is statistical and the second is systematic. This measurement is an update of an earlier study exploiting additional tagging modes, including several decay modes involving a KL0 meson. The branching fractions of the KL0 modes are determined as input to the analysis in a manner that is independent of any strong phase uncertainty. Using the predominantly CP-even tag D→ π+π-π and the ensemble of CP-odd eigenstate tags, the observable AKππππ0 is measured to be 0.130 ± 0.012 ± 0.008. The two asymmetries are sensitive to rDKπcosδDKπ, where rDKπ and δDKπ are the ratio of amplitudes and phase difference, respectively, between the doubly Cabibbo-suppressed and Cabibbo-favoured decays. In addition, events containing D→ K-π+ tagged by D→KS,L0π+π- are studied in bins of phase space of the three-body decays. This analysis has sensitivity to both rDKπcosδDKπ and rDKπsinδDKπ. A fit to AKπ, AKππππ0 and the phase-space distribution of the D→KS,L0π+π- tags yields δDKπ=(187.6-9.7+8.9-6.4+5.4)∘, where external constraints are applied for rDKπ and other relevant parameters. This is the most precise measurement of δDKπ in quantum-correlated DD¯ decays

Observation of the decay ψ(3686) → Σ −Σ ¯ + and measurement of its angular distribution

Using (448.1 ± 2.9) × 106ψ(3686) events collected with the BESIII detector at the BEPCII collider, the decay ψ(3686) →Σ −Σ ¯ + is observed for the first time with a branching fraction of (2.82 ± 0.04stat.± 0.08syst.) × 10−4, and the angular parameter αΣ− is measured to be 0.96 ± 0.09stat.± 0.03syst.. [Figure not available: see fulltext.

Determination of spin and parity of D(s)⁎ mesons

The spin and parity of the charmed mesons Ds⁎+, D⁎0 and D⁎+ are determined for the first time to be JP=1− with significances greater than 10σ over other hypotheses of 2+ and 3−, using an e+e− collision data sample with an integrated luminosity of 3.19 fb−1 collected by the BESIII detector at a center-of-mass energy of 4.178 GeV. Different spin-parity hypotheses are tested via a helicity amplitude analysis of the processes e+e−→Ds⁎+Ds−, D⁎0D0 and D⁎+D−, with Ds⁎+→Ds+γ, D⁎0→D0π0, and D⁎+→D+π0. The results confirm the quark model predictions.</p