Probing CP symmetry and weak phases with entangled double-strange baryons

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.

K̅N interaction, p-wave terms

The inclusion of new ingredients that are expected to be specially relevant at higher energies could reveal more information about the physics behind the NLO terms of the chiral Lagrangian. In the present work, we explore the relevance of including partial waves higher than the L = 0, which is usually the only component considered in the literature to study the K̅N scattering phenomenology. In particular, we focus on the p-wave contribution, the effect of which is expected to be non-negligible, as we aim at obtaining the K−p scattering amplitudes at higher energies, necessary to describe the ηΛ, ηΣ0, K0Ξ0 and K+Ξ− production reactions. Extending the K̅N interaction to p-wave components is also relevant for studies of bound K− mesons in nuclei since their local momentum can acquire sizable values.

The K¯N Interaction in Higher Partial Waves

We present a chiral K¯N interaction model that has been developed and optimized in order to account for the experimental data of inelastic K¯N reaction channels that open at higher energies. In particular, we study the effect of the higher partial waves, which originate directly from the chiral Lagrangian, as they could supersede the role of high-spin resonances employed in earlier phenomenological models to describe meson-baryon cross sections in the 2 GeV region. We present a detailed derivation of the partial wave amplitudes that emerge from the chiral SU(3) meson-baryon Lagrangian up to the d-waves and next-to-leading order in the chiral expansion. We implement a nonperturbative unitarization in coupled channels and optimize the model parameters to a large pool of experimental data in the relevant energy range where these new contributions are expected to be important. The obtained results are encouraging. They indicate the ability of the chiral higher partial waves to extend the description of the scattering data to higher energies and to account for structures in the reaction cross-sections that cannot be accommodated by theoretical models limited to the s-waves.

Kaon Photoproduction and the Λ Decay Parameter α_{-}

The weak decay parameter α_{-} of the Λ is an important quantity for the extraction of polarization observables in various experiments. Moreover, in combination with α_{+} from Λ[over ¯] decay it provides a measure for matter-antimatter asymmetry. The weak decay parameter also affects the decay parameters of the Ξ and Ω baryons and, in general, any quantity in which the polarization of the Λ is relevant. The recently reported value by the BESIII Collaboration of 0.750(9)(4) is significantly larger than the previous PDG value of 0.642(13) that had been accepted and used for over 40 years. In this work we make an independent estimate of α_{-}, using an extensive set of polarization data measured in kaon photoproduction in the baryon resonance region and constraints set by spin algebra. The obtained value is 0.721(6)(5). The result is corroborated by multiple statistical tests as well as a modern phenomenological model, showing that our new value yields the best description of the data in question. Our analysis supports the new BESIII finding that α_{-} is significantly larger than the previous PDG value. Any experimental quantity relying on the value of α_{-} should therefore be reconsidered.

The role of isospin filtering reactions in the S = −1 sector

The present study reveals interesting constraining effects of isospin filtering reactions on the low energy constants present in meson-baryon chiral effective Lagrangian, particularly, on the next-to-leading order constants. Our model has been developed within the framework of Unitarized Chiral Perturbation Theory and has been fitted to two-body scattering data in the sector of S = −1. In addition, the model was further elaborated by means of the inclusion of high-spin hyperonic resonances.