Unraveling the structure of $\Lambda$ hyperons with polarized $\Lambda \bar{\Lambda }$ 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 $\Lambda$ electromagnetic form factors in the time-like region. By combining double-tag and single-tag events from the $e^{+}{e}^{-}\to \Lambda \overline{\Lambda }\to p{\pi }^{-}\bar{p}{\pi }^{+}$ 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 $\Delta \Phi (q^2)={\Phi }_E(q^2)-{\Phi }_M(q^2)$ 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 $\Lambda$ 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 $\Lambda$ hyperon.

arXiv comment10 pages, 2 figures, 4 tables

References in corpus (33)

• Design and Construction of the BESIII Detector
• Polarization and Entanglement in Baryon-Antibaryon Pair Production in Electron-Positron Annihilation
• Flavor decomposition of the elastic nucleon electromagnetic form factors
• Study of e+e- --> Lambda anti-Lambda, Lambda anti-Sigma0, Sigma0 anti-Sigma0 using Initial State Radiation with BABAR
• Nucleon and $\Delta$ elastic and transition form factors
• Hadronic structure functions in the $e^{+}{e}^{-}\to \bar{\Lambda }\Lambda$ reaction
• Exclusive event generator for $e^{+}{e}^{-}$ scan experiments
• Weak phases and CP-symmetry tests in sequential decays of entangled double-strange baryons
• Measurement of proton electromagnetic form factors in $e^{+}{e}^{-}\to p\bar{p}$ in the energy region 2.00-3.08 GeV
• New Features in the Electromagnetic Structure of the Neutron
• Polarization Transfer Observables in Elastic Electron Proton Scattering at $Q^2=$2.5, 5.2, 6.8, and 8.5 GeV${}^2$
• Observation of a cross-section enhancement near mass threshold in $e^{+}{e}^{-}\to \Lambda \bar{\Lambda }$
• Empirical Consequences of Emergent Mass
• Precise Measurements of Decay Parameters and $CP$ Asymmetry with Entangled $\Lambda -\bar{\Lambda }$ Pairs
• Measurement of the Sigma^- Charge Radius by Sigma^- Electron Elastic Scattering
• Complete measurement of $\Lambda$ electromagnetic form factors
• Elastic electromagnetic form factors of vector mesons
• Measurements of Sigma+ and Sigma- Time-like Electromagnetic Form Factors for center-of-mass energies from 2.3864 to 3.0200 GeV
• Luminosity measurements for the R scan experiment at BESIII
• Measurement of the cross section for $e^{+}{e}^{-}\to {\Xi }^{-}{\bar{\Xi }}^{+}$ and observation of an excited $\Xi$ baryon
• The electromagnetic form factors of $\Lambda$ hyperon in the vector meson dominance model
• Tests of $CP$ symmetry in the entangled ${\Xi }^0-{\bar{\Xi }}^0$ Pairs
• Hyperon electromagnetic form factors in the timelike region
• Measurement of cross-section for $e^{+}{e}^{-}\to {\Xi }^{-}{\bar{\Xi }}^{+}$ near threshold at BESIII
• Measurement of the neutron charge radius and the role of its constituents
• Measurement of cross section for $e^{+}{e}^{-}\to {\Xi }^0{\bar{\Xi }}^0$ near threshold
• Kaon Photoproduction and the $\Lambda$ Decay Parameter ${\alpha }_{-}$
• Measurement of the cross section for $e^{+}{e}^{-}\to \Lambda \bar{\Lambda }$ and evidence of the decay $\psi (3770)\to \Lambda \bar{\Lambda }$
• Hyperon electromagnetic timelike elastic form factors at large $q^2$
• Tuning and validation of hadronic event generator for $R$ value measurements in the tau-charm region
• The potential of $\Lambda$ and ${\Xi }^{-}$ studies with PANDA at FAIR
• Determination of the ${\Sigma }^{+}$ Timelike Electromagnetic Form Factors
• Dispersion-theoretical analysis of the electromagnetic form factors of the $\Lambda$ hyperon