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Institute
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) 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 baryon–antibaryon pairs provide a separation between these phases. Our method, exploiting spin entanglement between the double-strange Ξ− baryon and its antiparticle2 Ξ¯+
, has enabled a direct determination of the weak-phase difference, (ξP − ξS) = (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 methods3. Finally, we provide an independent measurement of the recently debated Λ decay parameter αΛ (refs. 4,5). The ΛΛ¯
asymmetry is in agreement with and compatible in precision to the most precise previous measurement.
We report the first measurements of the absolute branching fractions of D0 → K0 Lϕ, D0 → K0Lη, D0 → K0Lω, and D0 → K0Lη0, by analyzing 2.93 fb−1 of eþe− collision data taken at a center-of-mass energy of 3.773 GeV with the BESIII detector. Taking the world averages of the branching fractions of D0 → K0Sϕ, D0 → K0Sη, D0 → K0Sω, and D0 → K0Sη0, the K0S − K0L asymmetries RðD0; XÞ in these decay modes are obtained. The CP asymmetries in these decays are also determined. No significant CP violation is observed
Using a sample of (10.09±0.04)×109 J/ψ events collected with the BESIII detector, a partial wave analysis of J/ψ→γη′η′ is performed.The masses and widths of the observed resonances and their branching fractions are reported. The main contribution is from J/ψ→γf0(2020) with f0(2020)→η′η′, which is found with a significance of greater than 25σ. The product branching fraction B(J/ψ → γf0(2020))⋅B(f0(2020) → η′η′ is measured to be (2.63±0.06(stat.) + 0.31−0.46(syst.))×10−4.
Based on a sample of 4.4 fb−1 of e+e− annihilation data collected in the energy region between 4.6 GeV and 4.7 GeV with the BESIII detector at BEPCII, two singly Cabibbo-suppressed decays Λ+c→Σ0K+ and Λ+c→Σ+K0S are studied. The ratio of the branching fraction B(Λ+c→Σ0K+) relative to B(Λ+c→Σ0π+) is measured to be 0.0361±0.0073(stat.)±0.0005(syst.), and the ratio of B(Λ+c→Σ+K0S) relative to B(Λ+c→Σ+π+π−) is measured to be 0.0106±0.0031(stat.)±0.0004(syst.). After taking the world-average branching fractions of the reference decay channels, the branching fractions B(Λ+c→Σ0K+) and B(Λ+c→Σ+K0S) are determined to be (4.7±0.9(stat.)±0.1(syst.)±0.3(ref.))×10−4 and (4.8±1.4(stat.)±0.2(syst.)±0.3(ref.))×10−4, respectively. The branching fraction of the Λ+c→Σ+K0S decay is measured for the first time.
The study of the Cabibbo-favored semileptonic decay Λ+𝑐→Λ𝑒+𝜈𝑒 is reported using a 4.5 fb−1 data sample of 𝑒+𝑒− annihilations collected at center-of-mass energies ranging from 4.600 GeV to 4.699 GeV with the BESIII detector at the BEPCII collider. The branching fraction of the decay is measured to be ℬ(Λ+𝑐→Λ𝑒+𝜈𝑒)=(3.56±0.11stat±0.07syst)%, which is the most precise measurement to date. Furthermore, we perform an investigation of the internal dynamics in Λ+𝑐→Λ𝑒+𝜈𝑒. We provide the first direct comparisons of the differential decay rate and form factors with those predicted from lattice quantum chromodynamics (LQCD) calculations. Combining the measured branching fraction with a 𝑞2-integrated rate predicted by LQCD, we determine 𝑉𝑐𝑠|=0.936±0.017ℬ±0.024LQCD±0.007𝜏Λ𝑐.
Using 4.5 fb−1 of e+e− annihilation data samples collected at the center-of-mass energies ranging from 4.600~GeV to 4.699~GeV with the BESIII detector at the BEPCII collider, a first study of the semileptonic decays Λ+c→pK−e+νe, Λ+c→Λ(1520)e+νe and Λ+c→Λ(1405)e+νe is performed. The Λ+c→pK−e+νe decay is observed with a significance of 8.2σ and the branching fraction is measured to be B(Λ+c→pK−e+νe)=(0.88±0.17stat.±0.07syst.)×10−3. We also report evidence of Λ+c→Λ(1520)e+νe and Λ+c→Λ(1405)e+νe with significances of 3.3σ and 3.2σ, respectively, and measure B(Λ+c→Λ(1520)e+νe)=(1.02±0.52stat.±0.11syst.)×10−3 and B(Λ+c→Λ(1405)[→pK−]e+νe)=(0.42±0.19stat.±0.04syst.)×10−3. Combining these with the inclusive semileptonic Λ+c branching fraction measured by BESIII, the relative fraction is determined to be [B(Λ+c→pK−e+νe)/B(Λ+c→Xe+νe)]=(2.1±0.4stat.±0.2syst.)%, which provides a clear confirmation that semileptonic Λ+c decays are not saturated by the Λℓ+νℓ final state.
In high-energy heavy-ion collisions, partonic collectivity is evidenced by the constituent quark number scaling of elliptic flow anisotropy for identified hadrons. A breaking of this scaling and dominance of baryonic interactions is found for identified hadron collective flow measurements in sNN−−−√ = 3 GeV Au+Au collisions. In this paper, we report measurements of the first- and second-order azimuthal anisotropic parameters, v1 and v2, of light nuclei (d, t, 3He, 4He) produced in sNN−−−√ = 3 GeV Au+Au collisions at the STAR experiment. An atomic mass number scaling is found in the measured v1 slopes of light nuclei at mid-rapidity. For the measured v2 magnitude, a strong rapidity dependence is observed. Unlike v2 at higher collision energies, the v2 values at mid-rapidity for all light nuclei are negative and no scaling is observed with the atomic mass number. Calculations by the Jet AA Microscopic Transport Model (JAM), with baryonic mean-field plus nucleon coalescence, are in good agreement with our observations, implying baryonic interactions dominate the collective dynamics in 3 GeV Au+Au collisions at RHIC.
Using a data set corresponding to an integrated luminosity of 6.32 fb−1 recorded by the BESIII detector at center-of-mass energies between 4.178 and 4.226 GeV, an amplitude analysis of the decay D+s → π+π0π0 is performed, and the relative fractions and phases of different intermediate processes are determined. The absolute branching fraction of the decay D+s → π+π0π0 is measured to be (0.50 ± 0.04stat ± 0.02syst)%. Theabsolute branching fraction of the intermediate process D+s → f0(980)π+, f0(980) → π0π0 is determined to be (0.28 ± 0.04stat ± 0.04syst)%.
The Cabibbo-allowed weak radiative decay Λ+c→Σ+γ has been searched for in a sample of Λ+cΛ¯−c pairs produced in e+e− annihilations, corresponding to an integrated luminosity of 4.5fb−1 collected with the BESIII detector at center-of-mass energies between 4.60 and 4.70 GeV. No excess of signal above background is observed, and we set an upper limit on the branching fraction of this decay to be B(Λ+c→Σ+γ)<4.4×10−4 at a confidence level of 90\%, which is in agreement with Standard Model expectations.
Cross sections for the process e+e−→K0SK0SJ/ψ at center-of-mass energies from 4.128 to 4.950 GeV are measured using data samples with a total integrated luminosity of 21.2 fb−1 collected by the BESIII detector operating at the BEPCII storage ring. The Y(4230) state is observed in the energy dependence of the e+e−→K0SK0SJ/ψ cross section for the first time with a statistical significance of 26.0σ. In addition, an enhancement around 4.710 GeV, called the Y(4710), is seen with a statistical significance of 4.2σ. There is no clear structure around 4.484 GeV. Using a fit with a coherent sum of three Breit-Wigner functions, we determine the mass and width of the Y(4230) state to be 4226.9±6.6±21.9 MeV/c2 and 71.7±16.2±31.4 MeV, respectively, and the mass and width of the Y(4710) state to be 4704.0±52.3±69.5 MeV/c2 and 183.2±114.0±90.8 MeV, respectively, where the first uncertainties are statistical and the second are systematic. In addition, the average Born cross section ratio of e+e−→K0SK0SJ/ψ to e+e−→K+K−J/ψ is measured to be 0.388+0.035−0.028±0.016, or 0.426+0.038−0.031±0.018 if three-body phase space is considered.