Observation of new excited $\itΣ_{c}^{0}$ states in the $B^-\rightarrow \itΛ^+_c\overline{p}π^{-}$ decay

Abstract: An amplitude analysis of the $$B^-\rightarrow \itΛ<sup>+_c\overline{p}π<sup>{-}$$ decay is performed using proton-proton collision data, corresponding to an integrated luminosity of $9\text{ fb}<sup>{-1}$, collected with the \mbox{LHCb} detector at center-of-mass energies of 7, 8, and 13$\text{ TeV}$. In the $\itΛ<sup>+_cπ<sup>{-}$ invariant-mass spectrum, the known $\itΣ_c(2455)<sup>0$, $\itΣ_c(2520)<sup>0$, $\itΣ_c(2800)<sup>0$ baryons are observed, along with two new states: $\itΣ_c(2900)<sup>0$ and $\itΣ_c(3200)<sup>0$. The masses and widths of the three high-mass states, modeled with Breit--Wigner line shapes, are determined. This result expands the landscape of charm baryon spectroscopy and provides crucial data for understanding charm baryon structure and dynamics.

• The paper identifies two new excited Σc⁰ resonances at ~2.9 GeV and 3.2 GeV using amplitude analysis of B⁻ → Λc⁺ p̄ π⁻ decays.
• It employs an unbinned likelihood fit on 9 fb⁻¹ of LHCb data to extract precise resonance parameters and resolve overlapping signals.
• The findings clarify the Σc⁰(2800) structure and provide stringent tests for quark model predictions and lattice QCD calculations.

Observation of New Excited $\Sigma_c^0$ States in $B^- \rightarrow \Lambda_c^+ \overline{p} \pi^-$ Decays

Introduction

The spectroscopy of hadrons containing heavy quarks serves as a critical probe of nonperturbative QCD. In particular, the study of singly charmed baryons, such as those in the $\Sigma_c^0$ family, allows for stringent testing of constituent quark models, potential models, and QCD sum-rule predictions. Although several low-lying charmed baryons have been firmly established, the spectrum of excited $\Sigma_c^0$ baryons remains incomplete, with only a handful of states confirmed prior to this work. Here, the LHCb Collaboration presents an amplitude analysis of the $B^- \rightarrow \Lambda_c^+ \overline{p} \pi^-$ decay channel, leading to the observation of two previously unobserved excited $\Sigma_c^0$ resonant structures, in addition to the established $\Sigma_c^0(2455)$, $\Sigma_c^0(2520)$, and $\Sigma_c^0(2800)$.

Experimental Data Selection and Amplitude Analysis

The analysis is based on 9 fb$^{-1}$ of $B^- \rightarrow \Lambda_c^+ \overline{p} \pi^-$0 collision data collected by the LHCb detector at $B^- \rightarrow \Lambda_c^+ \overline{p} \pi^-$1 7, 8, and 13 TeV. Candidate events are reconstructed with the $B^- \rightarrow \Lambda_c^+ \overline{p} \pi^-$2 identified via its dominant hadronic decay $B^- \rightarrow \Lambda_c^+ \overline{p} \pi^-$3. Advanced particle identification, kinematic, and vertex quality criteria, complemented by BDT-based background suppression, result in a high-purity sample with $B^- \rightarrow \Lambda_c^+ \overline{p} \pi^-$4 $B^- \rightarrow \Lambda_c^+ \overline{p} \pi^-$5 signal events and a background fraction of $B^- \rightarrow \Lambda_c^+ \overline{p} \pi^-$6.

To investigate resonant substructure, an unbinned likelihood fit is performed to the Dalitz distribution of the $B^- \rightarrow \Lambda_c^+ \overline{p} \pi^-$7 final state. The analysis treats the $B^- \rightarrow \Lambda_c^+ \overline{p} \pi^-$8 as stable. To avoid complications from a pronounced $B^- \rightarrow \Lambda_c^+ \overline{p} \pi^-$9 threshold enhancement and the narrow $\Sigma_c^0$0, regions with $\Sigma_c^0$1 and $\Sigma_c^0$2 are excluded or handled separately.

Resonant Structure in the Invariant Mass Distributions

The $\Sigma_c^0$3 distribution reveals the expected $\Sigma_c^0$4, $\Sigma_c^0$5, and $\Sigma_c^0$6 features, as well as clear excesses indicative of two additional excited $\Sigma_c^0$7 states at approximately 2.9 and 3.2 GeV/$\Sigma_c^0$8. The amplitude fit incorporates all known resonant and nonresonant contributions, as well as S-wave and potential P-wave nonresonant amplitudes.

The key projections of the amplitude fit for the relevant two-body mass combinations are illustrated below.

Figure 2: Distributions of (left) $\Sigma_c^0$9, (middle) $\Sigma_c^0$0, and (right) $\Sigma_c^0$1 invariant masses, with the fit result overlaid for both solution groups.

Determination of Resonance Parameters and Ambiguities

The amplitude analysis accommodates interference effects and alternative quantum number assignments for the observed resonances. Multiple local minima are discovered, with two groups (A, B) of near-degenerate solutions distinguished by the preferred spin-parity hypotheses for the states around 2.8 and 2.9 GeV. The masses and widths extracted for the three high-mass $\Sigma_c^0$2 states differ significantly between these solution groups, particularly for the $\Sigma_c^0$3 and $\Sigma_c^0$4, though the $\Sigma_c^0$5 parameters remain stable. The relative contributions to the $\Sigma_c^0$6 decay also show sizable variations between solutions, with the $\Sigma_c^0$7 exhibiting a dominant yield in group A.

Strong statistical evidence supports the necessity of all three high-mass $\Sigma_c^0$8 amplitudes:

• A two-component model for the 2.8 GeV region ($\Sigma_c^0$9 and $B^- \rightarrow \Lambda_c^+ \overline{p} \pi^-$0) is favored over a single-resonance hypothesis at $B^- \rightarrow \Lambda_c^+ \overline{p} \pi^-$1 significance (systematics included).
• The $B^- \rightarrow \Lambda_c^+ \overline{p} \pi^-$2 resonance is required at $B^- \rightarrow \Lambda_c^+ \overline{p} \pi^-$3 significance.

The assignment $B^- \rightarrow \Lambda_c^+ \overline{p} \pi^-$4 for the $B^- \rightarrow \Lambda_c^+ \overline{p} \pi^-$5 is overwhelmingly favored, and the $B^- \rightarrow \Lambda_c^+ \overline{p} \pi^-$6 is confirmed as $B^- \rightarrow \Lambda_c^+ \overline{p} \pi^-$7 at $B^- \rightarrow \Lambda_c^+ \overline{p} \pi^-$8.

Systematic Uncertainties

Uncertainties in mass, width, and yield fractions arise from backgrounds, efficiency corrections, model dependence (fixed parameters, alternative S- and P-wave parametrizations, resonance radius, Dalitz acceptance), and the ambiguity in spin-parity assignments. For the $B^- \rightarrow \Lambda_c^+ \overline{p} \pi^-$9, results are stable across both solution groups, while for the $\Sigma_c^0$0 and $\Sigma_c^0$1, systematic uncertainties from the solution spread are dominant.

Discussion

The detailed amplitude analysis provides unambiguous evidence for two new excited $\Sigma_c^0$2 states:

• $\Sigma_c^0$3 with $\Sigma_c^0$4 GeV, $\Sigma_c^0$5 GeV (group A)
• $\Sigma_c^0$6 with $\Sigma_c^0$7 GeV, $\Sigma_c^0$8 GeV

The previously observed $\Sigma_c^0$9 is disambiguated into two separate resonances, resolving inconsistencies between prior Belle and Babar measurements. The results are compatible with quark-model expectations for higher-lying $\Sigma_c^0(2455)$0 excitations, including possible 2S- and 2P-wave assignments. Fit fractions show that the excited $\Sigma_c^0(2455)$1 yields exceed or rival those of the ground-state $\Sigma_c^0(2455)$2 in some solution scenarios—an unanticipated dynamical feature.

These findings both test and constrain theoretical models of charmed baryon spectroscopy. The observation of high-mass $\Sigma_c^0(2455)$3 states in $\Sigma_c^0(2455)$4 decays provides a new laboratory for understanding $\Sigma_c^0(2455)$5- and $\Sigma_c^0(2455)$6-mode excitations, heavy-quark symmetry, and the interplay of strong decay mechanisms.

Theoretical and Practical Implications

This analysis substantially expands the established charmed baryon spectrum, adding two high-mass, broad structures to the excitation pattern of $\Sigma_c^0(2455)$7. Precise mass and width measurements sharply constrain relativized quark models and Lattice QCD predictions. The separation of the $\Sigma_c^0(2455)$8 into two components sets a precedent for detailed amplitude analyses in other heavy flavor baryon sectors and underscores the need for careful treatment of broad, overlapping resonances in multi-body final states.

Furthermore, the significant fit fractions for the excited $\Sigma_c^0(2455)$9 baryons suggest dynamical features of the decay $\Sigma_c^0(2520)$0 that are not captured by simple phase-space or naive factorization-based models. These results motivate further theoretical work in the context of heavy-quark effective theory, as well as the exploration of exotic interpretations (e.g., molecular or multiquark components) for the observed states.

Conclusion

The LHCb amplitude analysis of $\Sigma_c^0(2520)$1 has resulted in the observation of two new excited $\Sigma_c^0(2520)$2 baryons at 2.9 and 3.2 GeV, providing high-precision measurements of their properties and clarifying the resonance structure first observed more than a decade prior. The findings directly impact the interpretation of charmed baryon spectroscopy, serving as benchmarks for nonperturbative QCD calculations and offering new directions for both experimental and theoretical research in heavy flavor baryon physics.