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
. 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
Formula: see text, has enabled a direct determination of the weak-phase difference, (ξ
- ξ
) = (1.2 ± 3.4 ± 0.8) × 10
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
. Finally, we provide an independent measurement of the recently debated Λ decay parameter α
(refs.
). The Formula: see text asymmetry is in agreement with and compatible in precision to the most precise previous measurement
.
Based on 10 billion J/psi events collected at the BESIII experiment, a search for CP violation in Lambda decay is performed in the difference between CP-odd decay parameters alpha_ for Lambda -> p ...pi(-) and alpha(+) for (Lambda) over bar -> (p) over bar pi(+) by using the process e(+)e(-) -> J/psi -> Lambda(Lambda) over bar. With a five-dimensional fit to the full angular distributions of the daughter baryon, the most precise values for the decay parameters arc determined to be a_ = 0.7519 +/- 0.0036 +/- 0.0024 and alpha(+) = -0.7559 +/- 0.0036 +/- 0.0030, respectively. The Lambda and (Lambda) over bar averaged value of the decay parameter is extracted to be a(avg) = 0.7542 +/- 0.0010 +/- 0.0024 with unprecedented accuracy. The CP asymmetry A(CP) = (alpha_ + alpha(+))/(a_-alpha(+)) is determined to be -0.0025 +/- 0.0046 +/- 0.0012, which is one of the most precise measurements in the baryon sector. The reported results for the decay parameter will play an important role in the studies of the polarizations and CP violations for the strange, charmed and beauty baryons.
We report new measurements of the branching fraction B (Ds+ → ℓ+ν), where ℓ+ is either μ+ or τ+ ( → π+ ντ), based on 6.32 fb−1 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 Ds+ → μ+ νμ and Ds+ → τ+ ντ components yields B (Ds+ → τ+ ντ) = ( 5.21 ± 0.25 ± 0.17 ) × 10−2, B (Ds+ → μ+ νμ) = ( 5.35 ± 0.13 ± 0.16 ) × 10−3, and the ratio of decay widths ..., where the first uncertainties are statistical and the second systematic. No evidence of CP asymmetry is observed in the decay rates Ds± → μ± νμ and Ds± → τ± ντ : ACP (μ± ν) = (− 1.2 ± 2.5 ± 1.0) % and ACP (τ± ν) = ( + 2.9 ± 4.8 ± 1.0)%. Constraining our measurement to the Standard Model expectation of lepton universality (R = 9.75), we find the more precise results B (Ds+ → τ+ ντ) = ( 5.22 ± 0.10 ± 0.14 ) × 10−2 and ACP ( τ± ντ) = ( − 0.1 ± 1.9 ± 1.0 )%. Combining our results with inputs external to our analysis, we determine the c →s quark mixing matrix element, Ds+ decay constant, and ratio of the decay constants to be | Vcs | = 0.973 ± 0.009 ± 0.014 , fD+s = 249.9 ± 2.4 ± 3.5 MeV , and fD+s / fD+ = 1.232 ± 0.035, respectively.
PDF