[Paper Review] Indication of a strange tribaryon S^+ from the ^4He(stopped K^-,n) reaction

M. Iwasaki, T. Suzuki+27 more|ArXiv.org|Oct 17, 2003

Quantum, superfluid, helium dynamics5 citations

TL;DR

This paper reports experimental evidence for a new strange tribaryon, S⁺(3140), produced in the ⁴He(stopped K⁻, n) reaction via time-of-flight neutron spectroscopy. Using kaon stopped in superfluid helium, a 3.7σ enhancement at 3141 ± 3 (stat.)⁺⁴₋₁ (sys.) MeV/c² with width <23 MeV/c² (95% CL) was observed, suggesting a T=0 state distinct from the previously observed S⁰(3115), indicating deeper kaonic binding than theoretical predictions.

ABSTRACT

We measured the neutron time-of-flight spectrum in the ^4He(stopped K^-, n) reaction by stopping negative kaons in a superfluid helium target. A clear enhancement was observed in the neutron spectrum, which indicates the formation of a strange tribaryon of charge +1 with a mass and width of M = 3141 +/- 3 (stat.) +4/-1 (sys.) MeV/c^2 and G &lt; 23 MeV/c^2. This state, denoted as S^+(3140), is about 25 MeV/c^2 higher than the previously observed S^0(3115) T=1.

Motivation & Objective

• To search for the isospin partner of the previously observed strange tribaryon S⁰(3115), which is predicted to be T=0 and Z=+1.
• To investigate the existence of a deeply bound kaonic state in the ⁴He system via the (K⁻, n) reaction with stopped kaons.
• To determine the mass, width, and quantum numbers of a potential new strange tribaryon state from neutron momentum spectra.
• To test theoretical predictions of a T=0, Z=+1 strange tribaryon at ~3194 MeV/c².

Proposed method

• Time-of-flight (TOF) spectroscopy was used to measure neutron momentum from the ⁴He(stopped K⁻, n) reaction, with timing derived from beam counter and kaon range in the target.
• A vertex reconstruction technique with beam-line and vertex drift chambers localized the kaon stopping point with ~5 mm resolution.
• Neutron detection used plastic scintillator arrays (NC) with a 10 MeV/equivalent energy threshold to suppress background.
• The neutron spectrum was analyzed using event selection based on pion momentum and vertex inconsistency to isolate hyperon decay products.
• Missing-mass spectra were reconstructed from neutron and pion data, and fits were performed using Gaussian signals and smooth backgrounds to extract resonance parameters.
• Systematic uncertainties were evaluated by folding in experimental resolution and comparing with control samples, including proton data from the same reaction.

Experimental results

Research questions

• RQ1 Does the ⁴He(stopped K⁻, n) reaction produce a T=0 strange tribaryon state, the isospin partner of S⁰(3115)?
• RQ2 What is the mass and width of any new resonance observed in the neutron spectrum from this reaction?
• RQ3 Is the observed state consistent with theoretical predictions of a deeply bound kaonic state with T=0 and Z=+1?
• RQ4 How does the signal yield in the neutron spectrum compare to expectations based on isospin symmetry and detection efficiency?
• RQ5 Could the observed enhancement be explained by background or misidentified events, or does it indicate a genuine resonance?

Key findings

• A clear enhancement was observed in the neutron time-of-flight spectrum at a momentum corresponding to a mass of 3141 ± 3 (stat.)⁺⁴₋₁ (sys.) MeV/c².
• The width of the resonance is constrained to be less than 23 MeV/c² at 95% confidence level, indicating a narrow state.
• The statistical significance of the signal is 3.7σ, providing strong evidence for a new resonance, S⁺(3140).
• The observed mass is ~25 MeV/c² higher than the previously observed S⁰(3115) state, suggesting it is not its isospin partner.
• The signal yield is inconsistent with the expected isospin-scaled yield from the S⁰(3115) state, indicating a different formation mechanism or quantum numbers.
• The data suggest the state is likely T=0, with a dominant decay mode into Σ±NN, and its mass is significantly lower than the theoretical prediction of 3194 MeV/c².