Excited Baryons Research Articles

First Results from the BGO–OD Experiment at ELSA

BGO–OD is a new experiment at the ELSA accelerator at the University of Bonn. It combines a large-aperture forward magnetic spectrometer with a central BGO calorimeter, thus providing almost 4 $$\pi $$ charged particle tracking and high energy-resolution photon detection. This makes the setup ideally suited to investigate the photoproduction of forward going mesons with residual ground or excited state baryons remaining with little momentum transfer. These kinematics are especially interesting for associated strangeness photoproduction, and first results are presented.

Role of newly discovered Ξb(6227)− for constructing excited bottom baryon family

Selecting the newly observed $\Xi_b(6227)^-$ by LHCb as a study example, we decode its inner structure by giving the mass spectrum analysis and the investigation of its two-body strong decay behaviors. Our result indicates that the $\Xi_b(6227)^-$ is a good candidate of the $P$-wave $\Xi_b^{\prime}$ state with $J^P=3/2^-$ or $5/2^-$. In addition, we further provide the information of the properties of the partners of the $\Xi_b(6227)^-$. These predicted states include three $2S$ states and the remaining $1P$ states in the bottom-strange baryon family. The calculated sizable Okubo-Zweig-Iizuka$-$allowed decay widths of these partners show that the experimental search for them becomes possible via LHCb. We have a reason to believe that the present study can be treated as a start point for constructing the highly excited bottom baryon spectroscopy.

Quarks, Hadrons, and Emergent Spacetime

It is argued that important information on the emergence of space is hidden at the quark/hadron level. The arguments follow from the acceptance of the conception that space is an attribute of matter. They involve in particular the discussion of possibly relevant mass and distance scales, the generalization of the concept of mass as suggested by the phase-space-based explanation of the rishon model, and the phenomenological conclusions on the structure of excited baryons that are implied by baryon spectroscopy. A counterpart of the Eddington–Weinberg relation concerning Regge towers of hadronic resonances is noted.

N $$^*$$ ∗ Experiments and Their Impact on Strong QCD Physics

I give a brief overview of experimental studies of the spectrum and the structure of the excited states of the nucleon and what we learn about their internal structure. The focus is on the effort to obtain a more complete picture of the light-quark baryon excitation spectrum employing electromagnetic beams, and on the study of the transition form factors and helicity amplitudes and their dependence on the magnitude of the photon virtuality $Q^2$, especially for some of the most prominent resonances. The results were obtained in pion and eta electroproduction experiments off proton targets. They strengthen the connection of experiment and new results from modeling sQCD in DSE and Light Cone SR approaches. They also point to the nature of these states as 3-quark excitations at the core.

Mass and residue of $\Lambda (1405)$ Λ ( 1405 ) as hybrid and excited ordinary baryon

The nature of the $\Lambda (1405)$ has been a puzzle for decades, whether it is a standard three-quark baryon, a hybrid baryon or a baryon-meson molecule. More information on the decay channels of this particle and its strong, weak and electromagnetic interactions with other hadrons is needed to clarify its internal organization. The residue of this particle is one of the main inputs in investigation of its decay properties in many approaches. We calculate the mass and residue of the $\Lambda (1405)$ state in the context of QCD sum rules considering it as a hybrid baryon with three-quark-one-gluon content as well as an excited ordinary baryon with quantum numbers $I(J^{P})=0(1/2^{-})$ . The comparison of the obtained results on the mass with the average experimental value presented in PDG allows us to interpret this state as a hybrid baryon.

Prediction of exotic doubly charmed baryons within chiral effective field theory

In this work we study the possible QCD exotic states in the doubly charmed baryon sector. Within chiral effective theory, it is predicted that several excited baryons result from the $S$-wave scattering of ground-state doubly charmed baryons ($\Xi_{cc}^{++}, \Xi_{cc}^{+}, \Omega_{cc}^{+}$) and light pseudoscalar mesons ($\pi, K, \eta$). The excited doubly charmed baryons can be classified by the strangeness ($S$) and isospin ($I$) quantum numbers. Two of the excited states are clearly exotic, in the sense that they cannot be explained by the conventional baryons with three quarks, since their quantum numbers are $(S,I)=(1,0)$ and $(-1,1)$. Similar to the charmed scalar meson $D_{s0}^{*}(2317)$ in $DK$ scattering and the hyperon $\Lambda(1405)$ in $\bar{K}N$ scattering, one bound state below the $\Xi_{cc} \bar{K}$ threshold is predicted in the $(S,I)=(-1,0)$ channel. In addition, two resonant structures are found in the $\Xi_{cc}\pi, \Xi_{cc}\eta$ and $\Omega_{cc} K$ coupled-channel scattering with $(S,I)=(0,1/2)$. The corresponding pole positions and coupling strengths of the excited doubly charmed baryons are given. The scattering lengths of the ground-state doubly charmed baryons and light pseudoscalar mesons are also predicted. The current study may provide useful guides for future experimental measurements and lattice simulations.

Chiral symmetry and the properties of hadrons in the generalized Nambu – Jona-Lasinio model

Various aspects of the generalized Nambu – Jona-Lasinio model of QCD in four dimensions are reviewed. The properties of mesonic excitations are discussed in detail, with special attention on the chiral pion. Spontaneous chiral symmetry breaking in a vacuum and effective chiral symmetry restoration in the spectrum of highly excited mesons and baryons are described microscopically.

The decay of Hopf solitons in the Skyrme model

It is understood that the Skyrme model has a topologically interesting baryonic excitation which can model nuclei. So far no stable knotted solutions, of the Skyrme model, have been found. Here we investigate the dynamics of Hopf solitons decaying to the vacuum solution in the Skyrme model. In doing this we develop a matrix-free numerical method to identify the minimum eigenvalue of the Hessian of the corresponding energy functional. We also show that as isospinning Hopf solitons decay, they emit a cloud of isospinning radiation.

Baryon properties in the relativistic quark model

Properties of heavy and strange baryons are investigated in the framework of the relativistic quark-diquark picture. It is based on the relativistic quark model of hadrons, which was previously successfully applied for the calculation of meson properties. It is assumed that two quarks in a baryon form a diquark and baryon is considered as the bound quark-diquark system. The relativistic effects and diquark internal structure are consistently taken into account. Calculations are performed up to rather high orbital and radial excitations of heavy and strange baryons. On this basis the Regge trajectories are constructed. The rates of semileptonic decays of heavy baryons are calculated. The obtained results agree well with available experimental data.

Possibility of the existence of charmed exotica

We employ the chiral quark-soliton model to describe excited baryons with one heavy quark. Identifying known charmed baryons with multiplets allowed by the model, we argue that apart from regular excitations of the ground state multiplets, some of recently reported by the LHCb collaboration narrow $\Omega^{0}_{c}$ states, may correspond to the exotic pentaquarks. This interpretation can be easily verified experimentally, since exotic $\Omega^{0}_{c}$ states -- contrary to the regular excitations -- form isospin triplets, rather than singlets.

Interpretation of the new Omega _c^{0} states via their mass and width

The masses and pole residues of the ground and first radially excited Omega _c^{0} states with spin–parities J^{P}=1/2^{+},,3/2^{+}, as well as P-wave Omega _c^{0} with J^{P}=1/2^{-},,3/2^{-} are calculated by means of the two-point QCD sum rules. The strong decays of Omega _c^{0} baryons are also studied and the widths of these decay channels are computed. The relevant computations are performed in the context of the full QCD sum rules on the light cone. The results obtained for the masses and widths are confronted with recent experimental data of the LHCb Collaboration, which allow us to interpret Omega _c(3000)^{0}, Omega _c(3050)^{0}, and Omega _c(3119)^{0} as the excited css baryons with the quantum numbers (1P,,1/2^{-}), (1P,,3/2^{-}), and (2S,,3/2^{+}), respectively. The (2S,,1/2^{+}) state can be assigned either to the Omega _c(3066)^{0} state or the Omega _c(3090)^{0} excited baryon.

On the nature of the newly discovered Ω states

The mass and residue of the ground-state, as well as the first orbital and radial excitations of the heavy baryons with Q being b or c quark, for both and are calculated by means of the QCD two-point sum rule method using the general forms for the interpolating currents. In the calculations the quark, gluon and mixed vacuum condensates up to ten dimensions are taken into account. We compare our results for the masses of and baryons with the existing predictions of other theoretical works. Our results for the charmed baryons are confronted with the experimental data of the LHCb Collaboration to understand the nature of the recently observed narrow resonances. The predictions for the masses of the baryons with the same quantum numbers may shed light on future experimental searches for the corresponding bottom baryons.

Towers of positive parity excited baryons and their mixing in the large Nc limit

We consider configuration mixing for the nonstrange positive parity excited baryons in the $[\mathbf{56'},0^+],[\mathbf{56},2^+], [\mathbf{70},0^+] $ and $[\mathbf{70},2^+]$ quark model $SU(6) \times O(3)$ multiplets contained in the $N=2$ band. Starting from the effective mass operator for these states we show by an explicit calculation that in the large $N_c$ limit they fall into six towers of degenerate states labeled by $K=0,1,1',2,2',3$. We find that the mixing of the quark model states is much simpler than what is naively expected. To leading order in $N_c$ only states carrying the same $K$ label can mix, which implies that for the spin-flavor states we started with configuration mixing can be parameterized by just two constants, $\mu_{K=1}$ and $\mu_{K=2}$.

Quarksonic matter at high isospin density**Supported by Thousand Young Talents Program of China, Shanghai Natural Science Foundation, (14ZR1403000) and NSFC (11535012), China Postdoctoral Science Foundation (KLH1512072)

Analogous to the quarkyonic matter at high baryon density in which the quark Fermi seas and the baryonic excitations coexist, it is argued that a “quarksonic matter” phase appears at high isospin density where the quark (antiquark) Fermi seas and the mesonic excitations coexist. We explore this phase in detail in both large Nc and asymptotically free limits. In the large Nc limit, we sketch a phase diagram for the quarksonic matter. In the asymptotically free limit, we study the pion superfluidity and thermodynamics of the quarksonic matter by using both perturbative calculations and an effective model.

Structure of charmed baryons studied by pionic decays

We investigate the decays of the charmed baryons aiming at the systematic understanding of hadron internal structures based on the quark model by paying attention to heavy quark symmetry. We evaluate the decay widths from the one-pion emission for the known excited states, ${\mathrm{\ensuremath{\Lambda}}}_{c}^{*}(2595)$, ${\mathrm{\ensuremath{\Lambda}}}_{c}^{*}(2625)$, ${\mathrm{\ensuremath{\Lambda}}}_{c}^{*}(2765)$, ${\mathrm{\ensuremath{\Lambda}}}_{c}^{*}(2880)$, and ${\mathrm{\ensuremath{\Lambda}}}_{c}^{*}(2940)$, as well as for the ground states ${\mathrm{\ensuremath{\Sigma}}}_{c}(2455)$ and ${\mathrm{\ensuremath{\Sigma}}}_{c}^{*}(2520)$. The decay properties of the lower excited charmed baryons are well explained, and several important predictions for higher excited baryons are given. We find that the axial-vector-type coupling of the pion to the light quarks is essential, which is expected from chiral symmetry, to reproduce the decay widths especially of the low-lying ${\mathrm{\ensuremath{\Lambda}}}_{c}^{*}$ baryons. We emphasize the importance of the branching ratios of $\mathrm{\ensuremath{\Gamma}}({\mathrm{\ensuremath{\Sigma}}}_{c}^{*}\ensuremath{\pi})/\mathrm{\ensuremath{\Gamma}}({\mathrm{\ensuremath{\Sigma}}}_{c}\ensuremath{\pi})$ for the study of the nature of higher excited ${\mathrm{\ensuremath{\Lambda}}}_{c}^{*}$ baryons.

γ*N→N*(1520) form factors in the timelike regime

The covariant spectator quark model, tested before in a variety of electromagnetic baryon excitations, is applied here to the $\gamma^\ast N \to N^\ast(1520)$ reaction in the timelike regime. The transition form factors are first parametrized in the spacelike region in terms of a valence quark core model together with a parametrization of the meson cloud contribution. The form factor behavior in the timelike region is then predicted, as well as the $N^\ast(1520) \to \gamma N$ decay width and the $N^\ast (1520)$ Dalitz decay, $N^\ast (1520) \to e^+ e^- N$. Our results may help in the interpretation of dielectron production from elementary $pp$ collisions and from the new generation of HADES results using a pion beam. In the $q^2=0$--1 GeV$^2$ range we conclude that the {\it QED approximation} (a $q^2$ independent form factor model) underestimates the electromagnetic coupling of the $N^\ast(1520)$ from 1 up to 2 orders of magnitude. We conclude also that the $N^\ast (1520)$ and the $\Delta(1232)$ Dalitz decay widths are comparable.

Analysis of Radial Excitations of Octet Baryons in QCD Sum Rules

Using the QCD sum rules method, we estimate the mass and residues of the first radial excitations of octet baryons. The contributions coming from the ground state baryons are eliminated by constructing the linear combinations of the sum rules corresponding to different Lorentz structures. Our predictions of the masses of the first radial excitations of octet baryons are in good agreement with the data.

Partial wave analysis

The project A.2 of the SFB/TR16 “Subnuclear Structure of Matter” aimed at a combined multichannel partial wave analysis of all or of most data on pion and photo-induced reactions for invariant masses from the Δ(1232) region to 2300MeV. In this review the most important results are presented. It is shown that the data and their partial wave analysis have improved our understanding of the dynamics of excited light-quark baryons significantly.

Towards Precision Spectroscopy of Baryonic Resonances

Recent progress in baryon spectroscopy is reviewed. In a common effort, various groups have analyzed a set of new high-precision polarization observables from ELSA. The Julich-Bonn group has finalized the analysis of pion-induced meson-baryon production, the potoproduction of pions and eta mesons, and (almost) the K Λ final state. As data become preciser, statistical aspects in the analysis of excited baryons become increasingly relevant and several advances in this direction are proposed.

SU(3) flavor symmetry breaking in large Nc excited hyperons

The $1/N_c$ expansion method for studying the mass spectrum of excited baryons is shortly reviewed together with applications to mixed symmetric states. The $[{\bf 70, \ell^+}]$ multiplet, belonging to the $N$ = 2 band, is reanalyzed, with emphasis on hyperons and the SU(3) symmetry breaking operators entering the mass formula to first order. An important result is that the hierarchy of masses as a function of strangeness is correctly reproduced for all multiplets. Predictions for unknown excited hyperons to SU(6) $\times$ O(3) multiplets are made.