BESIII Collaboration Achieves First Hyperon-Nucleon Cross Section Measurements at Collider

The BESIII collaboration turned a piece of accelerator infrastructure into a physics instrument, using the beryllium beam pipe of the BEPCII storage ring as a nuclear target to produce the first hyperon-nucleon cross section measurements ever made at an electron-positron collider. The results, published in Physical Review Letters (Phys. Rev. Lett. 130, 251902) on June 21, 2023, represent a clean break from the fixed-target paradigm that has defined hyperon scattering experiments for decades.

A novel method was used to study hyperon-nucleon interactions based on hyperons produced in the decays of 10 billion J/ψ events collected with the BESIII detector at the BEPCII storage ring, with the beam pipe serving as the target material. The approach solves a long-standing experimental bottleneck: experimental studies of hyperon-nucleon interactions have suffered from the difficulty of obtaining a stable hyperon beam, since the lifetime of ground-state hyperons is usually on the order of 10⁻¹⁰ seconds due to weak decay, too short to achieve a stable beam. By exploiting the enormous J/ψ dataset and treating the detector's own dense material as the scattering medium, BESIII effectively manufactured an intense hyperon beam from charmonium decay in situ.

The flagship result came from the reaction Ξ⁰n → Ξ⁻p, observed with approximately 20 signal events for the first time. The measured cross section for the nuclear-level process Ξ⁰ + ⁹Be → Ξ⁻ + p + ⁸Be was determined to be (22.1 ± 5.3 stat ± 4.5 syst) mb at a Ξ⁰ momentum of 0.818 GeV/c. Converting that result to a single-neutron cross section required an assumption about the ⁹Be nucleus: taking the effective number of reaction neutrons as 3, the collaboration derived a cross section for Ξ⁰n → Ξ⁻p of (7.4 ± 1.8 stat ± 1.5 syst) mb, a value consistent with theoretical predictions. The asymmetric treatment of the beryllium nucleus is a real systematic lever; a different assumption on effective neutron number shifts the result meaningfully, and readers working with these numbers should keep that conversion front of mind.

Alongside the Ξ⁰ analysis, BESIII delivered the first measurements of the Σ⁺n → Λp and Σ⁺n → Σ⁰p cross sections via Σ±-nucleus scattering. The extracted cross section for the Σ⁺n channel was determined to be (23.3 ± 3.7 ⁺¹¹·⁰/₋₅.₅) mb, where the first uncertainty term arises from the measured hyperon-nucleus cross section and the second, asymmetric term reflects the uncertainty in Z_eff, the effective number of participating nucleons used in the phenomenological conversion from nucleus-level to nucleon-level cross sections. That large asymmetric systematic on the second term is not an oversight; it quantifies exactly how much the result depends on the nuclear model assumption, and it will narrow as theoretical treatments of effective nucleon counting in light nuclei improve.

The collaboration also searched the Ξ⁻p final state for evidence of the H-dibaryon, the hypothetical six-quark state (uuddss) with strangeness -2 that has been hunted in various experiments for decades. No significant H-dibaryon signal was observed.

The novel method developed in these works extends the research field and opens a new era for experiments at e⁺e⁻ colliders, with results providing constraints for studies of the potential of strong interaction, the origin of color confinement, the unified model for baryon-baryon interactions, and the internal structure of neutron stars. The so-called "hyperon puzzle" of neutron stars, in which the appearance of hyperons in dense stellar matter affects the equation of state in ways that remain poorly constrained, is a direct beneficiary of precisely this type of data.

In the near future, the measurements will benefit from improved techniques and the larger datasets at the Super Tau-Charm Factory (STCF), a proposed e⁺e⁻ facility that would deliver luminosity roughly 100 times greater than BEPCII. The interactions between antihyperons and nucleus/nucleon, including scattering and annihilation, can also be studied using the method proposed in this work, opening a second major channel of investigation that has barely been touched experimentally. Prospects papers by Jianping Dai, Hai-Bo Li, Han Miao, and Jianyu Zhang have already mapped out which channels become accessible with larger datasets, and the list is substantial.

The core innovation here is architectural rather than purely statistical: BESIII did not build a new beamline or commission a dedicated fixed-target run. It recognized that the geometry and material composition of its own detector could double as the experimental apparatus for a class of measurements the community had essentially given up pursuing at colliders.