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

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$${\bar{{\Xi }}}^{+}$$ 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 $${\Lambda }\bar{{\Lambda }}$$ Λ Λ ¯ asymmetry is in agreement with and compatible in precision to the most precise previous measurement4.

First Simultaneous Measurement of $Ξ^0$ and $\barΞ^0$ Asymmetry Parameters in $ψ(3686)$ Decay

“…α ψ(3686) is measured to be 0.665 ± 0.086 ± 0.081, which is consistent with the previous BESIII measurement [38], and ∆Φ = −0.050 ± 0.150 ± 0.020 rad is measured for the first time and is significantly different from the one for Ξ − reported in Refs. [16,17]. The relative phase that is approximately zero implies an insignificant transverse polarization, which differs from the polarization observed in Λ from J/ψ, Σ + from both J/ψ and ψ(3686), and Ξ − from both J/ψ and ψ(3686) decays [12,[15][16][17].…”

“…Therefore, a test of CPV in hyperon decays is sensitive to the sources of CPV from physics beyond the SM [10,11]. At present, BESIII has performed CPV tests in the decays of Λ [12][13][14], Σ + [15], and Ξ − [16,17] hyperons, where for Ξ − hyperons, the most precise asymmetry parameter measurement was reported in J/ψ decay. BESIII has also performed the first determination of the weak phase in the Ξ − hyperon using entangled Ξ − Ξ+ pairs [16].…”

“…At present, BESIII has performed CPV tests in the decays of Λ [12][13][14], Σ + [15], and Ξ − [16,17] hyperons, where for Ξ − hyperons, the most precise asymmetry parameter measurement was reported in J/ψ decay. BESIII has also performed the first determination of the weak phase in the Ξ − hyperon using entangled Ξ − Ξ+ pairs [16]. However, CPV in Ξ 0 hyperon decays has not so far been searched for, the asymmetry parameter α Ξ 0 in the decay Ξ 0 → Λπ 0 has not been measured directly, only the product α Λ • α Ξ 0 has been reported [18,19], and the weak decay phase φ Ξ 0 was measured with large uncertainty [20][21][22].…”

“…1 as the same definition as Ref. [16] and [17]. The cascade Ξ and Λ polarization, P Ξ and P Λ , are related as…”

Light hyperon physics at BESIII

“…5.4 e + e − → J/ψ → Ξ − Ξ+ Sequential decays of double strange hyperons allow for measurements of both the α and φ parameters and a direct extraction of the weak phase difference. A DT analysis of the reaction J/ψ → Ξ − Ξ+ → Λπ − Λπ + → pπ − π − pπ + π + based on 1.3 × 10 9 J/ψ events was recently performed by BESIII [25], finding a total of 73,244 signal events with an estimated background of 187 ± 16 events. In order to completely describe the polarization and entanglement from production through the two-step decay, nine helicity angles must be determined for each event.…”