Measurement of Λ transverse polarization in e+e− collisions at $$ \sqrt{s} $$ = 3.68 − 3.71 GeV

Abstract

With data samples collected with the BESIII detector at seven energy points at $$ \sqrt{s} $$ s = 3.68 − 3.71 GeV, corresponding to an integrated luminosity of 333 pb−1, we present a study of the Λ transverse polarization in the e+e−→$$ \Lambda \overline{\Lambda} $$ Λ Λ ¯ reaction. The significance of polarization by combining the seven energy points is found to be 2.6σ including the systematic uncertainty, which implies a non-zero phase between the transition amplitudes of the $$ \Lambda \overline{\Lambda} $$ Λ Λ ¯ helicity states. The modulus ratio and the relative phase of EM-psionic form factors combined with all energy points are measured to be RΨ = $$ {0.71}_{-0.10}^{+0.10} $$ 0.71 − 0.10 + 0.10 ± 0.03 and ∆ΦΨ = $$ {23}_{-8.0}^{+8.8} $$ 23 − 8.0 + 8.8 ± 1.6°, where the first uncertainties are statistical and the second systematic.

Measurement of Born cross section of $$ {e}^{+}{e}^{-}\to {\Sigma}^{+}{\overline{\Sigma}}^{-} $$ at center-of-mass energies between 3.510 and 4.951 GeV

“…Since there are not enough events to determine the angular distribution for each energy point separately, a control sample with large statistics and a DIY model [44] with customizable angular distribution are used. With a large sample of ψ(3770) events as the control sample, the angular distribution is obtained by a maximum likelihood fit to the helicity amplitude [73][74][75].…”

Measurement of the cross section of $$ {e}^{+}{e}^{-}\to {\Xi}^{-}{\overline{\Xi}}^{+} $$ at center-of-mass energies between 3.510 and 4.843 GeV

“…The reconstruction of Λ and Ξ − baryons follows refs. [44][45][46][47]. Briefly, to reconstruct Λ candidates and suppress non-Λ background, a secondary vertex fit [48] is applied to all pπ − combinations, and their invariant mass is required to be within 5 MeV/c 2 from the known Λ mass.…”