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Unraveling the structure of $Λ$ hyperons with polarized $Λ\barΛ$ pairs

The BESIII Collaboration

Abstract

With data collected in a dedicated energy scan from 2.3864 up to 3.0800 GeV, the BESIII collaboration provides the first complete energy-dependent measurements of the $Λ$ electromagnetic form factors in the time-like region. By combining double-tag and single-tag events from the $e^+e^- \to Λ\overlineΛ\to pπ^-\bar{p}π^+$ reaction, we achieve a complete decomposition of the spin structure of the reaction at five energy points, with high statistical and systematic precision. Our data reveal that while the modulus of the ratio between the electric and magnetic form factor, $R(q^2) = |G_E(q^2)/G_M(q^2)|$, remains fairly constant across the considered energy range, the relative phase $ΔΦ(q^2) = Φ_E(q^2) - Φ_M(q^2)$ changes by more than 90$^{\mathrm{o}}$ between 2.396 and 2.6544 GeV. Using a fit to our data based on dispersion relations, the complex form factor ratio is determined as a function of $q^2$ and the preferred solution has multiple zero-crossings in the complex plane. From the derivative of the ratio at $q^2 = 0$, the root-mean-squared charge radius of the $Λ$ is obtained. The two most probable solutions yield a negative root-mean-squared charge radius, indicating an asymmetric charge distribution where the $ds$ quark pair lies close to the center of the $Λ$ hyperon.

Unraveling the structure of $Λ$ hyperons with polarized $Λ\barΛ$ pairs

Abstract

With data collected in a dedicated energy scan from 2.3864 up to 3.0800 GeV, the BESIII collaboration provides the first complete energy-dependent measurements of the electromagnetic form factors in the time-like region. By combining double-tag and single-tag events from the reaction, we achieve a complete decomposition of the spin structure of the reaction at five energy points, with high statistical and systematic precision. Our data reveal that while the modulus of the ratio between the electric and magnetic form factor, , remains fairly constant across the considered energy range, the relative phase changes by more than 90 between 2.396 and 2.6544 GeV. Using a fit to our data based on dispersion relations, the complex form factor ratio is determined as a function of and the preferred solution has multiple zero-crossings in the complex plane. From the derivative of the ratio at , the root-mean-squared charge radius of the is obtained. The two most probable solutions yield a negative root-mean-squared charge radius, indicating an asymmetric charge distribution where the quark pair lies close to the center of the hyperon.

Paper Structure

This paper contains 6 equations, 2 figures, 4 tables.

Figures (2)

  • Figure 1: The effective form factor $G_{\text{eff}}$ from this work (black dots), BESIII ISR BESIII:2023ioy, BESIII energy scan BESIII:2017hywBESIII:2019nep and BaBar ISR BaBar:2007fsu. The horizontal error bars denote the bin size.
  • Figure 2: Top: The modulus $R=|G_E/G_M|$ from this work (black dots), from a previous BESIII study (white dot) BESIII:2019nep and from BaBar (gray dots) BaBar:2007fsu. The band represents a theoretical analysis outlined in Ref. Mangoni:2021qmd, showing the most probable scenario $[N_{\text{th}},N_{\text{asy}}] = [0,3]$. The BESIII 2019 data point is superseded by the point at the same CMS energy in this work and is therefore not included in the fit. The red dashed line marks the theoretical threshold $q^2_{th}$ and the red dashed-dotted line the physical threshold $q^2_{\text{phys}}$. Bottom: The corresponding measurements of the relative phase $\Delta\Phi$.