Complete Measurement of the Λ Electromagnetic Form Factors
Kolcu, Onur Buğra
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The exclusive process eþe− → ΛΛ¯ , with Λ → pπ− and Λ¯ → p¯ πþ, has been studied at ffiffi s p ¼ 2.396 GeV for measurement of the timelike Λ electric and magnetic form factors, GE and GM. A data sample, corresponding to an integrated luminosity of 66.9 pb−1, was collected with the BESIII detector for this purpose. A multidimensional analysis with a complete decomposition of the spin structure of the reaction enables a determination of the modulus of the ratio R ¼ jGE=GMj and, for the first time for any baryon, the relative phase ΔΦ ¼ ΦE − ΦM. The resulting values are R ¼ 0.960.14ðstatÞ0.02ðsystÞ and ΔΦ ¼ 37° 12°ðstatÞ 6°ðsystÞ, respectively. These are obtained using the recently established and most precise value of the asymmetry parameter αΛ ¼ 0.750 0.010 measured by BESIII. In addition, the cross section is measured with unprecedented precision to be σ ¼ 118.75.3ðstatÞ5.1ðsystÞ pb, which corresponds to an effective form factor of jGj ¼ 0.1230.003ðstatÞ0.003ðsystÞ. The contribution from two-photon exchange is found to be negligible. Our result enables the first complete determination of baryon timelike electromagnetic form factors.
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