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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) symmetry 1 . 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 antiparticle 2 Ξ ¯ + , has enabled a direct determination of the weak-phase difference, ( ξ P  −  ξ S ) = (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 methods 3 . Finally, we provide an independent measurement of the recently debated Λ decay parameter α Λ (refs.  4 , 5 ). The Λ Λ ¯ asymmetry is in agreement with and compatible in precision to the most precise previous measurement 4 . Using spin-entangled baryon–antibaryon pairs, the BESIII Collaboration reports on high-precision measurements of potential charge conjugation and parity (CP)-symmetry-violating effects in hadrons.

A bstract With data samples collected with the BESIII detector at seven energy points at s = 3 . 68 − 3 . 71 GeV, corresponding to an integrated luminosity of 333 pb − 1 , we present a study of the Λ transverse polarization in the e + e − → Λ Λ ¯ reaction. The significance of polarization by combining the seven energy points is found to be 2.6 σ including the systematic uncertainty, which implies a non-zero phase between the transition amplitudes of the Λ Λ ¯ helicity states. The modulus ratio and the relative phase of EM- psionic form factors combined with all energy points are measured to be R Ψ = 0.71 − 0.10 + 0.10 ± 0.03 and ∆Φ Ψ = 23 − 8.0 + 8.8 ± 1.6 ° , where the first uncertainties are statistical and the second systematic.

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-π0 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.

A bstract Using a sample of (10 . 09 ± 0 . 04) × 10 9 J/ψ decays collected with the BESIII detector, partial wave analyses of the decay J / ψ → γ K S 0 K S 0 π 0 are performed within the K S 0 K S 0 π 0 invariant mass region below 1.6 GeV/ c 2 . 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 K S 0 K S 0 π 0 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/ c 2 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/ c 2 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 K S 0 K S 0 π 0 invariant mass spectrum deteriorates significantly.

A bstract Using 6 . 32 fb − 1 of e + e − collision data collected at the center-of-mass energies between 4.178 and 4.226 GeV with the BESIII detector, we perform an amplitude analysis of the decay D s + → K + π + π − and determine the amplitudes of the various intermediate states. The absolute branching fraction of D s + → K + π + π − is measured to be (6 . 11 ± 0 . 18 stat . ± 0 . 11 syst . ) × 10 − 3 . The branching fractions of the dominant intermediate processes D s + → K + ρ 0 , ρ 0 → π + π − and D s + → K * (892) 0 π + , K * (892) 0 → K + π − are determined to be (1 . 96 ± 0 . 19 stat . ± 0 . 23 syst . ) × 10 − 3 and (1 . 85 ± 0 . 12 stat . ± 0 . 13 syst . ) × 10 − 3 , respectively. The intermediate resonances f 0 (500), f 0 (980), and f 0 (1370) are observed for the first time in this channel.

A bstract A partial wave analysis on the process e + e − → ωπ + π − is performed using 647 pb − 1 of data sample collected by using the BESIII detector operating at the BEPCII storage ring at center-of-mass (c.m.) energies from 2.000 GeV to 3.080 GeV. The Born cross section of the e + e − → ωπ + π − process is measured, with precision improved by a factor of 3 compared to that of previous studies. A structure near 2.25 GeV is observed in the energy-dependent cross sections of e + e − → ωπ + π − and ωπ 0 π 0 with a statistical significance of 7.6 σ , and its determined mass and width are 2232 ± 19 ± 27 MeV /c 2 and 93 ± 53 ± 20 MeV, respectively, where the first and second uncertainties are statistical and systematic, respectively. By analyzing the cross sections of subprocesses e + e − → ωf 0 (500), ωf 0 (980), ωf 0 (1370), ωf 2 (1270), and b 1 (1235) π , a structure, with mass M = 2200 ± 11 ± 17 MeV/ c 2 and width Γ = 74 ± 20 ± 24 MeV, is observed with a combined statistical significance of 7.9 σ . The measured resonance parameters will help to reveal the nature of vector states around 2.25 GeV.

A bstract Based on e + e − collision samples corresponding to an integrated luminosity of 4.4 fb − 1 collected with the BESIII detector at center-of-mass energies between 4 . 6 GeV and 4 . 7 GeV, a partial wave analysis of the charmed baryon hadronic decay Λ c + → Λ π + π 0 is performed, and the decays Λ c + → Λ ρ (770) + and Λ c + → Σ(1385) π are studied for the first time. Making use of the world-average branching fraction B Λ c + → Λ π + π 0 , their branching fractions are determined to be B Λ c + → Λ ρ 770 + = 4.06 ± 0.30 ± 0.35 ± 0.23 × 10 − 2 , B Λ c + → Σ 1385 + π 0 = 5.86 ± 0.49 ± 0.52 ± 0.35 × 10 − 3 , B Λ c + → Σ 1385 0 π + = 6.47 ± 0.59 ± 0.66 ± 0.38 × 10 − 3 , where the first uncertainties are statistical, the second are systematic, and the third are from the uncertainties of the branching fractions B Λ c + → Λ π + π 0 and B Σ 1385 → Λ π . In addition, the decay asymmetry parameters are measured to be α Λ ρ (770)+  =  − 0.763 ± 0.053 ± 0.045, α Σ 1385 + π 0 = − 0.917 ± 0.069 ± 0.056 , and α Σ 1385 0 π + = − 0.789 ± 0.098 ± 0.056 .

A bstract Based on 4.5 fb − 1 e + e − collision data collected with BESIII detector at seven energy points between 4.600 and 4.699 GeV, the branching fractions for Λ c + → pη and Λ c + → pω were measured by means of single-tag method. The branching fractions of Λ c + → pη and Λ c + → pω are determined to be (1.57 ± 0.11 stat ± 0.04 syst ) × 10 − 3 and (1.11 ± 0.20 stat ± 0.07 syst ) × 10 − 3 , with a statistical significance of greater than 10 σ and 5.7 σ , respectively. These results are consistent with the previous measurements by BESIII, LHCb and Belle, and the result of Λ c + → pη is the most precise to date.

In these proceedings two preliminary results from the BESIII collaboration on polarized Λ and$\\bar \\Lambda$are reported. Both results are first observations and concern the resonant and non-resonant processes${e^ - }{e^ + } \\to J/\\psi \\to \\Lambda \\bar \\Lambda$and${e^ - }{e^ + } \\to {\\gamma ^\\ast} \\to \\Lambda \\bar \\Lambda$at$\\sqrt s = 2.396\\,{\\rm{GeV}}$ , respectively. For$J/\\psi \\to \\Lambda \\bar \\Lambda$the relative phase between the electric and magnetic form factors, ΔΦ, has for the first time been determined,$\\Delta \\Phi = 42.4{(6)_{{\\rm{stat}}}(5)_{_{{\\rm{syst}}}^^\\circ$ . That$\\Delta \\Phi \\ne 0$allows for a simultaneous measurement also of the asymmetry decay parameters$\\Lambda \\to p{\\pi ^ - }\\,({\\alpha _ - })$ ,$\\bar \\Lambda \\to \\bar p{\\pi ^ + }\\,({\\alpha _ + })$and$\\bar \\Lambda \\to \\bar n{\\pi ^0}\\,({\\bar \\alpha _0})$ . The measured value of${\\alpha _ - } = 0.750 \\pm 0.009 \\pm 0.004$differs by 17(3)% from the PDG value. The simultaneous measurement for the asymmetry parameters allows for the most precise test of CP violation conducted for Λ decays, A CP = −0.006(12) stat (7) syst . The reaction${e^ - }{e^ + } \\to \\Lambda \\bar \\Lambda$at$\\sqrt s = 2.396\\,{\\rm{GeV}}$is the first complete measurement of the time-like electric ( G E ) and magnetic ( G M ) form factor of any baryon as also the ratio R = |( G E /( G M )| and ΔΦ have been determined: R = 0.96(14)(12) and$\\Delta \\Phi = 37{(12)_{{\\rm{stat}}}(6)_{{\\rm{stat}}}^\\circ$ . The obtained cross-section and effective form factor are σ = 119.0(53) stat (51) syst pb and | G | = 0.123(3) stat (3) syst , respectively.

We report the first measurement of the absolute branching fraction of the inclusive decay [Formula omitted]. The analysis is performed using an [Formula omitted] collision data sample corresponding to an integrated luminosity of 567 [Formula omitted] taken at [Formula omitted] = 4.6 GeV with the BESIII detector. Using eleven Cabibbo-favored [Formula omitted] decay modes and the double-tag technique, this absolute branching fraction is measured to be [Formula omitted], where the first uncertainty is statistical and the second systematic. The relative deviation between the branching fractions for the inclusive decay and the observed exclusive decays is [Formula omitted], which indicates that there may be some unobserved decay modes with a neutron or excited baryons in the final state.