Complete Measurement of the Λ Electromagnetic Form Factors

BESIII Collaboration

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14 Citations (Scopus)


The exclusive process e+e-→ΛΛ̄, with Λ→pπ- and Λ̄→p̄π+, has been studied at s=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=|GE/GM| and, for the first time for any baryon, the relative phase Δφ=φE-φM. 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 αΛ=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
Article number122003
Pages (from-to)122003
Number of pages8
JournalPhysical Review Letters
Issue number12
Publication statusPublished - 20 Sep 2019




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