Extracting the femtometer structure of strange baryons using the vacuum polarization effect.

Abstract

One of the fundamental goals of particle physics is to gain a microscopic understanding of the strong interaction. Electromagnetic form factors quantify the structure of hadrons in terms of charge and magnetization distributions. While the nucleon structure has been investigated extensively, data on hyperons are still scarce. It has recently been demonstrated that electron-positron annihilations into hyperon-antihyperon pairs provide a powerful tool to investigate their inner structure. We present a method useful for hyperon-antihyperon pairs of different types which exploits the cross section enhancement due to the effect of vacuum polarization at the J/ψ resonance. Using the 10 billion J/ψ events collected with the BESIII detector, this allows a precise determination of the hyperon structure function. The result is essentially a precise snapshot of the <math xmlns="http://www.w3.org/1998/Math/MathML"> <mover><mrow><mi>Λ</mi></mrow> <mo>¯</mo></mover> <msup><mrow><mi>Σ</mi></mrow> <mrow><mn>0</mn></mrow> </msup> <mspace></mspace> <mrow><mo>(</mo> <mrow><mi>Λ</mi> <msup> <mrow> <mover><mrow><mi>Σ</mi></mrow> <mo>¯</mo></mover> </mrow> <mrow><mn>0</mn></mrow> </msup> </mrow> <mo>)</mo></mrow> </math> transition process, encoded in the transition form factor ratio and phase. Their values are measured to be R = 0.860 ± 0.029(stat.) ± 0.015(syst.), <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>Δ</mi> <msub><mrow><mi>Φ</mi></mrow> <mrow> <mover><mrow><mi>Λ</mi></mrow> <mo>¯</mo></mover> <msup><mrow><mi>Σ</mi></mrow> <mrow><mn>0</mn></mrow> </msup> </mrow> </msub> <mo>=</mo> <mrow><mo>(</mo> <mrow><mn>1.011</mn> <mo>±</mo> <mn>0.094</mn> <mrow><mo>(</mo> <mrow><mi>stat.</mi></mrow> <mo>)</mo></mrow> <mo>±</mo> <mn>0.010</mn> <mrow><mo>(</mo> <mrow><mi>syst.</mi></mrow> <mo>)</mo></mrow> </mrow> <mo>)</mo></mrow> <mspace></mspace> <mi>r</mi> <mi>a</mi> <mi>d</mi></math> and <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>Δ</mi> <msub><mrow><mi>Φ</mi></mrow> <mrow><mi>Λ</mi> <msup> <mrow> <mover><mrow><mi>Σ</mi></mrow> <mo>¯</mo></mover> </mrow> <mrow><mn>0</mn></mrow> </msup> </mrow> </msub> <mo>=</mo> <mrow><mo>(</mo> <mrow><mn>2.128</mn> <mo>±</mo> <mn>0.094</mn> <mrow><mo>(</mo> <mrow><mi>stat.</mi></mrow> <mo>)</mo></mrow> <mo>±</mo> <mn>0.010</mn> <mrow><mo>(</mo> <mrow><mi>syst.</mi></mrow> <mo>)</mo></mrow> </mrow> <mo>)</mo></mrow> <mspace></mspace> <mi>r</mi> <mi>a</mi> <mi>d</mi></math> . Furthermore, charge-parity (CP) breaking is investigated in this reaction and found to be consistent with CP symmetry.