Complete Measurement of the Λ Electromagnetic Form Factors

(BESIII Collaboration)

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The exclusive process e^{+}e^{-}→ΛΛ[over ¯], with Λ→pπ^{-} and Λ[over ¯]→p[over ¯]π^{+}, has been studied at sqrt[s]=2.396  GeV for measurement of the timelike Λ electric and magnetic form factors, G_{E} and G_{M}. 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=|G_{E}/G_{M}| and, for the first time for any baryon, the relative phase ΔΦ=Φ_{E}-Φ_{M}. The resulting values are R=0.96±0.14(stat)±0.02(syst) and ΔΦ=37°±12°(stat)±6°(syst), respectively. These are obtained using the recently established and most precise value of the asymmetry parameter α_{Λ}=0.750±0.010 measured by BESIII. In addition, the cross section is measured with unprecedented precision to be σ=118.7±5.3(stat)±5.1(syst)  pb, which corresponds to an effective form factor of |G|=0.123±0.003(stat)±0.003(syst). The contribution from two-photon exchange is found to be negligible. Our result enables the first complete determination of baryon timelike electromagnetic form factors.

Original languageEnglish (US)
Article number122003
JournalPhysical review letters
Issue number12
StatePublished - Sep 20 2019

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© 2019 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Funded by SCOAP.

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