Evidence for quark-diquark structure of baryons from fluctuations of conserved charges

We study fluctuations and correlations of conserved charges in QCD using a string-based description of the hadronic mass spectrum. Mesons and baryons are modeled as open relativistic strings with quark-antiquark and quark-diquark endpoints, respectively, leading to an exponential Hagedorn growth of states with a limiting temperature fixed by the string tension. We find that continuous Hagedorn spectra constrained by experimentally established hadrons underestimate net-baryon number fluctuations obtained in lattice QCD calculations. By extracting the Hagedorn string spectrum directly from lattice QCD through a fit to the second-order net-baryon number susceptibility, we obtain a consistent description of a broad set of fluctuations of conserved charges from LQCD with the Hagedorn temperature $T_H \simeq 323~$MeV, without introducing additional free parameters. Our results provide thermodynamic evidence in support of a string quark-diquark picture of baryons in the confined phase of QCD.

💡 Research Summary

The paper investigates fluctuations and correlations of conserved charges (baryon number B, electric charge Q, and strangeness S) in Quantum Chromodynamics (QCD) by employing a string‑based description of the hadronic mass spectrum. In this framework mesons are modeled as open relativistic strings with a quark–antiquark pair at the ends, while baryons are represented as open strings with a single quark at one end and a diquark (a correlated quark pair in the attractive colour‑(\bar 3) channel) at the other. The universal asymptotic density of states for an open string in four dimensions is given by
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