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The polarization of Ξ and ΞΒ― hyperons along the beam direction has been measured relative to the second and third harmonic event planes in isobar Ru+Ru and Zr+Zr collisions at βsNN = 200 GeV. This is the first experimental evidence of the hyperon polarization by the triangular flow originating from the initial density fluctuations. The amplitudes of the sine modulation for the second and third harmonic results are comparable in magnitude, increase from central to peripheral collisions, and show a mild pT dependence. The azimuthal angle dependence of the polarization follows the vorticity pattern expected due to elliptic and triangular anisotropic flow, and qualitatively disagree with most hydrodynamic model calculations based on thermal vorticity and shear induced contributions. The model results based on one of existing implementations of the shear contribution lead to a correct azimuthal angle dependence, but predict centrality and pT dependence that still disagree with experimental measurements. Thus, our results provide stringent constraints on the thermal vorticity and shear-induced contributions to hyperon polarization. Comparison to previous measurements at RHIC and the LHC for the second-order harmonic results shows little dependence on the collision system size and collision energy.
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 matterβantimatter asymmetry is the existence of processes that violate the combined charge conjugation and parity (CP) symmetry1. 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 baryonβantibaryon pairs provide a separation between these phases. Our method, exploiting spin entanglement between the double-strange Ξβ baryon and its antiparticle2 ΞΒ―+
, has enabled a direct determination of the weak-phase difference, (ΞΎPβββΞΎS)β= (1.2βΒ±β3.4βΒ±β0.8)βΓβ10β2βrad. 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 methods3. Finally, we provide an independent measurement of the recently debated Ξ decay parameter Ξ±Ξ (refs.β4,5). The ΞΞΒ―
asymmetry is in agreement with and compatible in precision to the most precise previous measurement.
A search for the charged lepton flavor violating decay π½/πβπΒ±β’πβ with πββπββ’π0β’ππ is performed with about 10Γ109ββπ½/π events collected with the BESIII detector at the BEPCII. No significant signal is observed, and an upper limit is set on the branching fraction β¬β‘(π½/πβπΒ±β’πβ)<7.5Γ10β8 at the 90% confidence level. This improves the previously published limit by two orders of magnitude.
Using a dedicated data sample taken in 2018 on the J/Ο peak, we perform a detailed study of the trigger efficiencies of the BESIII detector. The efficiencies are determined from three representative physics processes, namely Bhabha scattering, dimuon production and generic hadronic events with charged particles. The combined efficiency of all active triggers approaches 100% in most cases, with uncertainties small enough not to affect most physics analyses.
Ten hadronic final states of the βπ decays are investigated via the process πβ‘(3686)βπ0β’βπ, using a data sample of (448.1Β±2.9)Γ106ββπβ‘(3686) events collected with the BESIII detector. The decay channel βπβπΎ+β’πΎββ’π+β’πββ’π0 is observed for the first time and has a measured significance of 6.0β’π. The corresponding branching fraction is determined to be β¬β‘(βπβπΎ+β’πΎββ’π+β’πββ’π0)=(3.3Β±0.6Β±0.6)Γ10β3 (where the uncertainties are statistical and systematic, respectively). Evidence for the decays βπβπ+β’πββ’π0β’π and βπβπΎ0πβ’πΎΒ±β’πββ’π+β’πβ is found with a significance of 3.6β’π and 3.8β’π, respectively. The corresponding branching fractions (and upper limits) are obtained to be β¬β‘(βπβπ+β’πββ’π0β’π)=(7.2Β±1.8Β±1.3)Γ10β3 (<1.8Γ10β2) and β¬β‘(βπβπΎ0πβ’πΎΒ±β’πββ’π+β’πβ)=(2.8Β±0.9Β±0.5)Γ10β3 (<4.7Γ10β3). Upper limits on the branching fractions for the final states βπβπΎ+β’πΎββ’π0, πΎ+β’πΎββ’π, πΎ+β’πΎββ’π+β’πββ’π, 2β’(πΎ+β’πΎβ)β’π0, πΎ+β’πΎββ’π0β’π, πΎ0πβ’πΎΒ±β’πβ, and πβ’Β―πβ’π0β’π0 are determined at a confidence level of 90%.
Using a data sample of 4.481Γ108ββπβ‘(3686) events collected with the BESIII detector, we report the first observation of the four-lepton-decays π½/πβπ+β’πββ’π+β’πβ and π½/πβπ+β’πββ’π+β’πβ utilizing the process πβ‘(3686)βπ+β’πββ’π½/π. The branching fractions are determined to be [5.48Β±0.31β’(stat)Β±0.45β’(syst)]Γ10β5 and [3.53Β±0.22β’(stat)Β±0.13β’(syst)]Γ10β5, respectively. The results are consistent with theoretical predictions. No significant signal is observed for π½/πβπ+β’πββ’π+β’πβ, and an upper limit on the branching fraction is set at 1.6Γ10β6 at the 90% confidence level. A πΆβ’π asymmetry observable is constructed for the first two channels, which is measured to be (β0.012Β±0.054Β±0.010) and (0.062Β±0.059Β±0.006), respectively. No evidence for πΆβ’π violation is observed in this process.
Using a data sample corresponding to an integrated luminosity of 2.93 fbβ1 collected at a center-of-mass energy sβ=3.773 GeV by the BESIII detector, the decay D0βΟΟ is observed for the first time. The branching fraction is measured to be (6.48Β±0.96Β±0.38)Γ10β4 with a significance of 6.3Ο, where the first and second uncertainties are statistical and systematic, respectively. An angular analysis reveals that the Ο and Ο mesons from the D0βΟΟ decay are transversely polarized, which is inconsistent with current theoretical expectations and challenges our understanding of the underlying dynamics in charm meson decays.
We report new measurements of the branching fraction β¬β‘(π·+π ββ+β’π), where β+ is either π+ or π+β‘(βπ+β’Β―ππ), based on 6.32ββfbβ1 of electron-positron annihilation data collected by the BESIII experiment at six center-of-mass energy points between 4.178 and 4.226 GeV. Simultaneously floating the π·+π βπ+β’ππ and π·+π βπ+β’ππ components yields β¬β‘(π·+π βπ+β’ππ)=(5.21Β±0.25Β±0.17)Γ10β2, β¬β‘(π·+π βπ+β’ππ)=(5.35Β±0.13Β±0.16)Γ10β3, and the ratio of decay widths π
=Ξβ‘(π·+π βπ+β’ππ)Ξβ‘(π·+π βπ+β’ππ)=9.7β’3+0.61β0.58Β±0.36, where the first uncertainties are statistical and the second systematic. No evidence of πΆβ’π asymmetry is observed in the decay rates π·Β±π βπΒ±β’ππ and π·Β±π βπΒ±β’ππ: π΄πΆβ’πβ‘(πΒ±β’π)=(β1.2Β±2.5Β±1.0)% and π΄πΆβ’πβ‘(πΒ±β’π)=(+2.9Β±4.8Β±1.0)%. Constraining our measurement to the Standard Model expectation of lepton universality (π
=9.75), we find the more precise results β¬β‘(π·+π βπ+β’ππ)=(5.22Β±0.10Β±0.14)Γ10β2 and π΄πΆβ’πβ‘(πΒ±β’ππ)=(β0.1Β±1.9Β±1.0)%. Combining our results with inputs external to our analysis, we determine the πβΒ―π quark mixing matrix element, π·+π decay constant, and ratio of the decay constants to be |ππβ’π |=0.973Β±0.009Β±0.014, ππ·+π =249.9Β±2.4Β±3.5ββMeV, and ππ·+π /ππ·+=1.232Β±0.035, respectively.
Using a data sample corresponding to an integrated luminosity of 2.93 fbβ1 collected at a center-of-mass energy sβ=3.773 GeV by the BESIII detector, the decay D0βΟΟ is observed for the first time. The branching fraction is measured to be (6.48Β±0.96Β±0.38)Γ10β4 with a significance of 6.3Ο, where the first and second uncertainties are statistical and systematic, respectively. An angular analysis reveals that the Ο and Ο mesons from the D0βΟΟ decay are transversely polarized.