[Paper Review] Reconstructing Sigma0 decays in STAR

G. Van Buren|CERN Bulletin|Dec 15, 2004

High-Energy Particle Collisions Research4 citations

TL;DR

This paper presents a novel reconstruction technique for Σ⁰ baryons in STAR's heavy-ion collisions using photon conversions (γ → e⁺e⁻) to identify the decay photon, combined with PID to tag the associated Λ. The method yields a measured Σ⁰/Λ ratio of 0.6 ± 0.3 in minimum-bias 200 GeV d+Au collisions, providing a critical benchmark for understanding strangeness enhancement and resonance feeddown in quark-gluon plasma studies.

ABSTRACT

Typical experimental measurements do not separate the contributions of Sigma state decays from (anti)Lambda yields. An ongoing analysis in STAR is resolving the contribution from excited Sigma states, but the primary contribution comes from electromagnetic decays of the (anti)Sigma0. Photon conversions into e+e- pairs in detector material have been used by STAR to identify photons from pi0 decays. A similar technique has been used here to identify photons from (anti)Sigma0 decays in conjunction with STAR's excellent PID capabilities for finding the associated (anti)Lambda daughters. We report here on progress toward measuring the (anti)Sigma0 yields in various nuclear collisions at RHIC.

Motivation & Objective

• To isolate and measure the contribution of Σ⁰ baryons to the total Λ-like baryon yield in nuclear collisions, which is otherwise obscured by weak decays and feeddown.
• To resolve the ambiguity in Λ spectra and antihyperon yields (e.g., in ̄Λ/̄p ratios) by explicitly identifying Σ⁰ → Λγ decays.
• To test theoretical models of strangeness production, including thermal and coalescence models, by measuring the Σ⁰/Λ yield ratio in heavy-ion collisions.
• To develop and validate a photon reconstruction technique using electron dE/dx and low pT^AP variables to distinguish γ → e⁺e⁻ conversions from combinatorial backgrounds.
• To enable future measurements of Σ⁰ yields in Au+Au, Cu+Cu, and p+p systems by improving reconstruction efficiency and reducing background.

Proposed method

• Utilizes photon conversions (γ → e⁺e⁻) in detector material to reconstruct the decay photon from Σ⁰ → Λγ, leveraging the topological similarity to V0 decays.
• Applies stringent cuts on electron dE/dx and the Armenteros-Podolanski variable pT^AP to suppress combinatorial backgrounds and isolate true conversion vertices.
• Combines reconstructed photons with identified Λ candidates (from K⁰_S → π⁺π⁻ decay) to form invariant mass distributions for Σ⁰ reconstruction.
• Employs a background subtraction technique by rotating Σ⁰ daughter candidates about the beam axis to estimate the combinatorial background shape.
• Uses uncorrected event counts from four pT bins to derive preliminary yields, with systematic uncertainties dominated by statistical fluctuations.
• Plans to adopt a more efficient γ-finder algorithm from the π⁰ analysis to improve reconstruction efficiency and signal-to-noise in future data.

Experimental results

Research questions

• RQ1 What is the relative yield of Σ⁰ compared to Λ in minimum-bias 200 GeV d+Au collisions, and how does it compare to theoretical expectations?
• RQ2 To what extent do resonance feeddown contributions (e.g., from Σ(1385), Λ(1405), Λ(1520)) affect the observed Σ⁰ and Λ yields?
• RQ3 Can the Σ⁰/Λ yield ratio serve as a probe of quark coalescence or thermalization in heavy-ion collisions?
• RQ4 How do final-state interactions and medium effects differ between Σ⁰ and Λ baryons, given their similar cross sections?
• RQ5 Can the photon conversion technique be extended to measure Σ⁰ in Au+Au, Cu+Cu, and p+p collisions with sufficient signal-to-noise?

Key findings

• A clear signal for (anti)Σ⁰ is observed in 14.7 million minimum-bias d+Au events at √s = 200 GeV, with a reconstructed invariant mass distribution showing a peak consistent with Σ⁰ decay.
• The uncorrected Σ⁰ + ̄Σ⁰ yield in mid-rapidity (|y| < 0.75) yields a ratio of ̄Σ⁰/Σ⁰ = 0.6 ± 0.3, indicating a slight excess of anti-Σ⁰ over Σ⁰.
• Weak decay contributions to the measurable Σ⁰ yield are negligible, with Ξ⁰ and Ξ⁻ decays contributing less than 1% of their respective yields.
• Strong decay feeddown is significant: 12 ± 2% of Σ(1385) decays contribute to the Σ⁰ signal, while 100% and 40% of Λ(1405) and Λ(1520) decays, respectively, feed into the Σ⁰ sample.
• The current analysis is limited by combinatorial background, but future use of a higher-efficiency γ-finder from the π⁰ analysis is expected to improve signal detection.
• The method is extendable to other systems: improved statistics in 200 GeV Au+Au, reduced background in 62 GeV Au+Au, and sufficient p+p data are expected to allow future Σ⁰ measurements.