Probing CP Symmetry And Weak Phases With Entangled Double ...

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Abstract

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 [Formula: see text], 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 [Formula: see text] asymmetry is in agreement with and compatible in precision to the most precise previous measurement4.

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Conflict of interest statement

The authors declare no competing interests.

Figures

Fig. 1

Fig. 1. Illustration of the polarization vectors…

Fig. 1. Illustration of the polarization vectors of Ξ − and Λ in relation to…

Fig. 1. Illustration of the polarization vectors of Ξ− and Λ in relation to the decay parameters α, β and γ of the Ξ− → Λπ− decay. The Λ polarization PΛ− has a component in the longitudinal as well as the transverse direction, where the former (zˆ) is defined by the Λ momentum. The longitudinal component depends on the Λ emission angle and arises from the transferred Ξ− polarization PΞ− combined with the decay parameter α. The remaining Ξ− polarization is transferred to the transverse components according to βΞPΞ− (xˆ) and γΞPΞ (yˆ). Quarks: d, down; s, strange; u, up; u¯, antiup.
Fig. 2

Fig. 2. Polarization in and spin correlations…

Fig. 2. Polarization in and spin correlations of the e + e − → Ξ …

a ,…
Fig. 2. Polarization in and spin correlations of the e+e−→Ξ−Ξ¯+ reaction. a, Polarization in the e+e−→Ξ−Ξ¯+ reaction. b–d, Spin correlations of the e+e−→Ξ−Ξ¯+ reaction. The coordinate systems ℛΞ and ℛΞ¯ of the Ξ− and Ξ¯+, respectively, are described in the text. The data points are determined independently in each bin of the Ξ− cosine scattering angle in the e+e− centre-of-momentum system. The blue curves represent the expected angular dependence obtained with the production parameters αψ and ΔΦ from the global maximum log-likelihood fit. The error bars indicate the statistical uncertainties.
Extended Data Fig. 1

Extended Data Fig. 1. Final-state particle momenta…

Extended Data Fig. 1. Final-state particle momenta of pions and protons for the decay process …

Extended Data Fig. 1. Final-state particle momenta of pions and protons for the decay process J/ψ→Ξ−Ξ¯+→Λπ−Λ¯π+→pπ−π−p¯π+π+. The non-overlapping momentum ranges of the protons and pions allow for a straightforward assignment of particle species. The blue boxes and the red solid line denote the experimental and simulated data, respectively.
Extended Data Fig. 2

Extended Data Fig. 2. Invariant mass distributions…

Extended Data Fig. 2. Invariant mass distributions of the Ξ − and Ξ ¯ + …

Extended Data Fig. 2. Invariant mass distributions of the Ξ− and Ξ¯+ signal candidates. Distribution of the invariant masses mΛπ− versus mΛ¯π+. The Ξ−Ξ¯+ candidates appear as an enhancement around mΛπ−=mΛ¯π+=1.32GeV/c2. The structure at mΛπ−=mΛ¯π+=1.39GeV/c2 is from the reaction J/ψ→Σ(1385)−Σ¯(1385)+.
Extended Data Fig. 3

Extended Data Fig. 3. Invariant mass distribution…

Extended Data Fig. 3. Invariant mass distribution of the Ξ − signal candidates before the…

Extended Data Fig. 3. Invariant mass distribution of the Ξ− signal candidates before the final selection criterion. The mΛπ− distribution, in log scale, for the BESIII data sample before the Λπ− mass window has been applied. The final requirement selects the events between the two lines. The total number of events in the distribution is 76,523.
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References

    1. Sakharov AD. Violation of CP invariance, C asymmetry, and baryon asymmetry of the universe. Zh. Eksp. Teor. Fiz. Pisma. 1967;5:32–35.
    1. Adlarson P, Kupsc A. CP symmetry tests in the cascade–anticascade decay of charmonium. Phys. Rev. D. 2019;100:114005. doi: 10.1103/PhysRevD.100.114005. - DOI
    1. HyperCP Collaboration New measurement of Ξ− → Λπ− decay parameters. Phys. Rev. Lett. 2004;93:011802. doi: 10.1103/PhysRevLett.93.011802. - DOI
    1. The BESIII Collaboration Polarization and entanglement in baryon–antibaryon pair production in electron–positron annihilation. Nat. Phys. 2019;15:631–634. doi: 10.1038/s41567-019-0494-8. - DOI
    1. Ireland DG, et al. Kaon photoproduction and the Λ decay parameter α−. Phys. Rev. Lett. 2019;123:182301. doi: 10.1103/PhysRevLett.123.182301. - DOI - PubMed
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