Lambda hyperon spin polarization and CP tests at BESIII

• In these proceedings two preliminary results from the BESIII collaboration on polarized Lambda and (Lambda) over bar are reported. Both results are first observations and concern the resonant and non-resonant processes e(-)e(+) -> J/psi -> Lambda(Lambda) over bar and e(-)e(+) -> gamma* -> Lambda Lambda at root s = 2.396 GeV, respectively. For J/psi -> Lambda Lambda the relative phase between the electric and magnetic form factors, Delta Phi, has for the first time been determined, Delta Phi = 42.4(6)(stat) (5)(stat degrees). That Delta Phi not equal 0 allows for a simultaneous measurement also of the asymmetry decay parameters Lambda -> p pi(-) (alpha_), (Lambda) over bar -> (p) over bar pi(+) (alpha(+)) and (Lambda) over bar -> (n) over bar pi(0) ((alpha) over bar (0)). The measured value of alpha_ = 0.750 +/- 0.009 +/- 0.004 differs by 17(3)% from the PDG value. The simultaneous measurement for the asymmetry parameters allows for the most precise test of CP violation conducted for A decays, A(cp) = - 0.006(12)(stat) (7)(syst). The reaction e(-)e(+) -> Lambda(Lambda) over bar at root s = 2.396 GeV is the first complete measurement of the time-like electric (G(E)) and magnetic (G(M)) form factor of any baryon as also the ratio R = vertical bar(GE)/(G(M))vertical bar and Delta Phi have been determined: R = 0.96(14)(12) and Delta Phi = 37(12)(stat) (6)(stat)degrees. The obtained cross-section and effective form factor are sigma = 119.0(53)(stat)(51)(syst) pb and vertical bar G vertical bar = 0.123 (3)(stat) (3)(syst), respectively.

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NATURVETENSKAP  -- Fysik -- Subatomär fysik (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences -- Subatomic Physics (hsv//eng)

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