New Model Predicts How Heavy Quark Baryons Fall Apart

Inside the debris of particle collisions at the Large Hadron Collider, physicists have catalogued a remarkable zoo of subatomic particles built around heavy quarks. Since 2012 alone, experiments have reported 85 new hadrons, including 36 baryons, the three-quark cousins of protons and neutrons. Now, a new theoretical study published in The European Physical Journal C offers one of the most comprehensive calculations yet of how these exotic particles decay, providing a roadmap that experimenters can use to identify the many heavy baryons predicted to exist but never yet observed.

The work, carried out by Emmanuel Ortiz-Pacheco of ELTE Eötvös Loránd University in Budapest and the Universidad Nacional Autónoma de México, together with Roelof Bijker of the Universidad Nacional Autónoma de México, tackles a deceptively simple question: when a heavy baryon decays into a lighter baryon and a meson, how fast does it happen? The answer, expressed as a decay width, is a fingerprint of the particle’s internal structure. Because the strong force that binds quarks is intrinsically non-perturbative, no exact solution of quantum chromodynamics exists for these processes, and theorists must rely on models built around the symmetries of the quark content itself.

Ortiz-Pacheco and Bijker combined a non-relativistic quark model, in which baryons are described as three quarks moving in a harmonic oscillator potential, with the elementary emission model for the strong couplings. In this picture, a baryon emits a fully formed meson through one of its constituent quarks, much as an excited atom emits a photon. The total number of quarks is conserved before and after the emission, and the interaction takes the form of a three-line vertex between a quark and the elementary meson, mirroring the structure of fundamental gauge interactions. The emitted particles considered are the light pseudoscalar mesons: the pion, kaon, eta and eta-prime.

The study covers baryons containing one or two heavy quarks, either charm or bottom, in both their ground-state S-wave configurations and their first orbitally excited P-wave states. Singly heavy baryons fall into a flavor sextet, containing the Sigma, Xi-prime and Omega families, and an anti-triplet, containing the Lambda and Xi families. Doubly heavy baryons form flavor triplets, the Xi-QQ and Omega-QQ families. The relative strengths of the allowed decays are fixed by isoscalar factors of the SU(3) flavor symmetry, evaluated in the Baird-Biedenharn phase convention, while the two coupling constants governing the meson emission were determined in a global fit to 52 measured baryon decay widths, 32 in the charm sector and 20 in the bottom sector. The fit yielded values of g equal to 3.32 per GeV and h equal to minus 0.16 per GeV.

A central result of the analysis is a set of selection rules that forbid entire classes of decays. These rules emerge not from conservation laws alone but from the detailed structure of the spin-flavor wave functions of the baryons. For example, the decay of the rho-configuration of a sextet baryon into an anti-triplet baryon plus an octet meson vanishes because the flavor part of the transition involves only the two light quarks while the spin part involves only the heavy quark, and the two contributions cannot combine. Analogous cancellations forbid anti-triplet to anti-triplet transitions and the rho-mode decays of doubly heavy baryons. Such forbidden channels are marked explicitly in the calculated tables, distinguishing them from decays that are merely kinematically closed.

The comparison with experiment is encouraging for the singly heavy sector. The calculated widths of the ground-state Sigma-c baryons, which decay almost entirely through the Lambda-c pion channel, agree with the observation that these channels carry roughly one hundred percent of the branching fraction. The P-wave Sigma-c(2800) resonance, assigned to a lambda-mode excitation, is reproduced within the large experimental uncertainties reported by Belle and BaBar. In the bottom sector, the model reproduces the widths of the Sigma-b states observed by LHCb and supports the interpretation of the Sigma-b(6097) resonances as P-wave baryons. For the Lambda, Xi and Omega families, the calculated widths, generally below about 28 MeV, are in reasonable agreement with the available data, with the notable exception of the poorly established Lambda-c(2765), whose measured width of roughly 50 to 73 MeV far exceeds the calculated 5.8 MeV and whose very status remains uncertain.

The doubly heavy baryons tell a strikingly different story. Following recent discoveries by the LHCb Collaboration, including the doubly charmed Xi-cc(3621) states and the newly announced Omega-cc at a mass of 3727 MeV, interest in these particles has surged. In the elementary emission model, their strong decay widths are either forbidden outright or suppressed to less than about 1 MeV. The suppression arises from quark-mass-dependent factors in the radial integrals: because the meson is emitted by a light quark, the orbital contribution scales with ratios of light to heavy quark masses, which become very small when two heavy quarks are present. This contrasts sharply with chiral quark model and constituent quark model predictions, which often give widths of hundreds of MeV, a discrepancy that future experiments can decisively test.

The study also highlights how much work remains. In the charm sector, candidates exist for roughly half of the fourteen predicted P-wave Xi baryons, while in the bottom sector only three have been seen. Recent LHCb measurements of spin and parity for states such as Xi-c(3055) have overturned some quark model assignments, pointing instead to D-wave interpretations. The authors note that the puzzling Lambda-c(2765) might even be a Roper-like excitation, a radially excited 2S state lying some 500 MeV above the ground state, an interpretation that lies beyond the present 1S and 1P analysis but could be addressed in future work. Meanwhile, states such as Lambda-c(2880) with spin-parity 5/2-positive would require two quanta of excitation.

For the physics community, the value of this calculation lies in its completeness and its testability. By treating all singly and doubly heavy charm and bottom baryons within a single framework, with one set of fitted couplings and masses taken from the authors’ previous spectroscopic analysis, the model produces internally consistent predictions across the entire spectrum. Where the elementary emission model, the 3P0 model and the chiral quark model agree qualitatively for known decays, they diverge wildly, sometimes by an order of magnitude, for states not yet observed. That divergence is an opportunity: the next round of LHCb and Belle measurements of strong and radiative decay widths, together with spin-parity determinations, will sort the competing pictures and may reveal the missing P-wave baryons that the quark model insists must be there.

Subject of Research: Theoretical calculation of strong two-body decay widths of S- and P-wave singly and doubly heavy charm and bottom baryons using a non-relativistic quark model with the elementary emission model.

Article Title: Strong decay widths of S- and P-wave singly- and doubly-heavy charm and bottom baryons

Article References: Strong decay widths of S- and P-wave singly- and doubly-heavy charm and bottom baryons. (n.d.). https://doi.org/10.1140/epjc/s10052-026-16344-1

Image Credits: AI Generated

DOI: 10.1140/epjc/s10052-026-16344-1

Keywords: heavy baryons, strong decay widths, quark model, charm baryons, bottom baryons, doubly heavy baryons, elementary emission model, SU(3) flavor symmetry, LHCb, hadron spectroscopy, pseudoscalar meson emission, selection rules