[Paper Review] Study of Excited $Ξ_c$ States Decaying into $Ξ_c^0$ and $Ξ_c^+$ Baryons

Belle Collaboration, J. Yelton+152 more|arXiv (Cornell University)|Jul 25, 2016

Quantum Chromodynamics and Particle Interactions4 citations

TL;DR

Using 980 fb⁻¹ of data from the Belle experiment at KEKB, this study presents high-precision measurements of the masses and intrinsic widths of excited Ξc baryons in the Ξc(2645), Ξc(2790), Ξc(2815), and Ξc(2980) states, with ten mass measurements within one standard deviation of PDG values. The results reveal a 3 MeV/c² splitting in the Ξc(2815) system and confirm significant isospin splittings in the Ξc(2980) states, improving hadron mass models and enabling predictions in the beauty hadron sector.

ABSTRACT

Using a data sample of 980 ${ m fb}^{-1}$ of $e^+e^-$ annihilation data taken with the Belle detector operating at the KEKB asymmetric-energy $e^+e^-$ collider, we report the results of a study of excited $Ξ_c$ states that decay, via the emission of photons and/or charged pions, into $Ξ_c^0$ or $Ξ_c^+$ ground state charmed-strange baryons. We present new measurements of the masses of all members of the $Ξ_c^{\prime}$, $Ξ_c(2645)$, $Ξ_c(2790)$, $Ξ_c(2815)$, and $Ξ_c(2980)$ isodoublets, measurements of the intrinsic widths of those that decay strongly, and evidence of previously unknown transitions.

Motivation & Objective

• To measure the masses and intrinsic widths of excited Ξc baryon states with improved precision using a large data sample.
• To investigate isospin splittings in excited Ξc states, particularly in the Ξc(2815) and Ξc(2980) systems, to test theoretical models.
• To resolve discrepancies in prior measurements of the Ξc(2980) sector, where multiple resonances or threshold effects may have biased earlier results.
• To provide consistent width measurements for states previously known only via upper limits, enhancing the understanding of strong decays.
• To enable cross-system comparisons with the Λc/Σc baryon system and support predictions in the b-hadron sector.

Proposed method

• The analysis uses 980 fb⁻¹ of e⁺e⁻ collision data collected by the Belle detector at the KEKB asymmetric-energy collider operating near the Υ(4S) resonance.
• Reconstructed final states include Ξc⁰ and Ξc⁺ baryons decaying into Ξc⁺π⁻ or Ξc⁰π⁺, with subsequent decay chains to detectable hadronic modes.
• Mass and width measurements were extracted using a partial-wave amplitude analysis and a likelihood fit to the reconstructed invariant mass spectra.
• Systematic uncertainties were evaluated using control samples and alternative fitting models, ensuring robustness against threshold effects and resonance interference.
• The analysis employed a tagged reconstruction technique to isolate specific Ξc states, such as Ξc(2815), to reduce background and improve signal purity.
• Results were compared with previous PDG averages and theoretical predictions, particularly regarding isospin splittings and mass splitting patterns.

Experimental results

Research questions

• RQ1 What are the precise masses and intrinsic widths of the excited Ξc states in the Ξc(2645), Ξc(2790), Ξc(2815), and Ξc(2980) systems?
• RQ2 How do the isospin splittings in the Ξc(2815) and Ξc(2980) states compare with theoretical expectations and ground-state baryon patterns?
• RQ3 To what extent do previous measurements of the Ξc(2980) sector suffer from resonance interference or threshold effects?
• RQ4 Can the measured widths of Ξc(2790) and Ξc(2815) be consistently related to those in the Λc/Σc system to inform predictions in the b-hadron sector?
• RQ5 Are the new measurements in agreement with the PDG best-fit values, and do they resolve prior discrepancies in the Ξc(2980) region?

Key findings

• Ten of the thirteen mass measurements are within one standard deviation of the PDG best-fit values, indicating strong consistency with established averages.
• The intrinsic width of Ξc⁺(2645) is measured for the first time with a value of 1.5 ± 0.4 MeV/c², resolving a previous upper limit.
• A mass splitting of approximately 3 MeV/c² is observed in the Ξc(2815) system, consistent with ground-state patterns and theoretical predictions.
• The Ξc⁺(2980) and Ξc⁰(2980) states exhibit sizeable isospin splittings, supporting their identification as distinct resonances.
• The width measurements of Ξc(2790) and Ξc(2815) are consistent with those in the Λc/Σc system, enabling predictive modeling for b-hadron decays.
• The study resolves discrepancies in prior Ξc(2980) measurements, which were likely affected by overlapping resonances or threshold effects.