Baryon Spectrum Research Articles

Isospin splittings in the decuplet baryon spectrum from dynamical QCD + QED

We report a new analysis of the isospin splittings within the decuplet baryon spectrum. Our numerical results are based upon five ensembles of dynamical QCD + QED lattices. The analysis is carried out within a flavour-breaking expansion which encodes the effects of breaking the quark masses and electromagnetic charges away from an approximate SU(3) symmetric point. The results display total isospin splittings within the approximate SU(2) multiplets that are compatible with phenomenological estimates. Further, new insight is gained into these splittings by separating the contributions arising from strong and electromagnetic effects. We also present an update of earlier results on the octet baryon spectrum.

Spectrum and structure of octet and decuplet baryons and their positive-parity excitations

A continuum approach to the three valence-quark bound-state problem in quantum field theory, employing parametrisations of the necessary kernel elements, is used to compute the spectrum and Poincar\'e-covariant wave functions for all flavour-$SU(3)$ octet and decuplet baryons and their first positive-parity excitations. Such analyses predict the existence of nonpointlike, dynamical quark-quark (diquark) correlations within all baryons; and a uniformly sound description of the systems studied is obtained by retaining flavour-antitriplet--scalar and flavour-sextet--pseudovector diquarks. Thus constituted, the rest-frame wave function of every system studied is primarily $S$-wave in character; and the first positive-parity excitation of each octet or decuplet baryon exhibits the characteristics of a radial excitation. Importantly, every ground-state octet and decuplet baryon possesses a radial excitation. Hence, the analysis predicts the existence of positive-parity excitations of the $\Xi$, $\Xi^\ast$, $\Omega$ baryons, with masses, respectively (in GeV): 1.84(08), 1.89(04), 2.05(02). These states have not yet been empirically identified. This body of analysis suggests that the expression of emergent mass generation is the same in all $u$, $d$, $s$ baryons and, notably, that dynamical quark-quark correlations play an essential role in the structure of each one. It also provides the basis for developing an array of predictions that can be tested in new generation experiments.

Transverse momentum spectra of heavy baryons and mesons in forward rapidity range in small collision system at the LHC

The transverse momentum spectra of $$\varLambda ^+_c$$ baryons and $$D^0$$ mesons produced in proton–lead (p–Pb) collisions at the center-of-mass energy per nucleon pair $$\sqrt{s_{NN}}=5\,{\text {TeV}}$$ (more exactly 5.02 TeV), $$D^0+{\bar{D}}^0$$ , $$D^++D^-$$ , $$D^+_s+D^-_s$$ , and $$D^{*+}+D^{*-}$$ (exactly $$D^*(2010)^++D^*(2010)^-$$ ) mesons produced in proton–proton (pp) collisions at the center-of-mass energy $$\sqrt{s}=5$$ and 13 TeV, $$\varLambda ^0_b$$ baryons and $${\bar{B}}^0$$ mesons produced in pp collisions at $$\sqrt{s}=7$$ and 8 TeV, as well as $$B^++B^-$$ mesons produced in pp collisions at $$\sqrt{s}=7$$ and 13 TeV measured by the LHCb collaboration in forward (backward) rapidity range are described by two models. The first model is based on the superposition of two Schwinger mechanisms, and the second one is based on the superposition of Tsallis statistics and inverse power law. The experimental data are fitted by the two two-component models. Then, the free parameters which depend on rapidity are extracted and analyzed.

On the spin-observables in pseudoscalar meson photoproduction

Spin-observables in pseudoscalar meson photoproduction are discussed. This work is complementary to the earlier works on this topic. Here, the reaction amplitude is expressed in Pauli-spin basis which allows to calculate all the observables straightforwardly. We make use of the fact that the underlying reflection symmetry about the reaction plane can be most conveniently and easily exploited in this basis to help find the non-vanishing and independent observables in this reaction. The important issue of complete experiments is reviewed. By expressing the reaction amplitude in Pauli-spin basis, many sets of eight observables—distinct from those found in earlier works from the amplitude in transversity basis—capable of determining the reaction amplitude up to an overall phase are found. It is also shown that some of the combinations of the spin observables are suited for studying certain aspects of the reaction dynamics. We, then, carry out a (strictly) model-independent partial-wave analysis, in particular, of the peculiar angular behavior of the beam asymmetry observed in photoproduction very close to threshold (Levi Sandri et al 2015 Eur. Phys. J. A 51 77). This work should be useful, especially, for newcomers in the field of baryon spectroscopy, where the photoproduction reactions are a major tool for probing the baryon spectra.

Masses of ground-state mesons and baryons, including those with heavy quarks

Using a confining, symmetry-preserving regularisation of a vector$\times$vector contact interaction, we compute the spectra of ground-state pseudoscalar and vector $(f\bar g)$ mesons, scalar and axial-vector $(fg)$ diquarks, and $J^P=1/2^+, 3/2^+$ $(fgh)$ baryons, where $f,g,h \in \{u,d,s,c,b\}$. The diquark correlations are essentially dynamical and play a key role in formulating and solving the three-valence-quark baryon problems. The baryon spectrum obtained from this largely-algebraic approach reproduces the 22 known experimental masses with an accuracy of $2.9(2.4)$%. It also possesses the richness of states typical of constituent-quark models, predicting many heavy-quark baryons not yet observed. This study indicates that diquark correlations are an important component of all baryons; and owing to the dynamical character of the diquarks, it is typically the lightest allowed diquark correlation which defines the most important component of a baryon's Faddeev amplitude.

Magnetized baryons and the QCD phase diagram: NJL model meets the lattice

We determine the baryon spectrum of 1 + 1 + 1-flavor QCD in the presence of strong background magnetic fields using lattice simulations at physical quark masses for the first time. Our results show a splitting within multiplets according to the electric charge of the baryons and reveal, in particular, a reduction of the nucleon masses for strong magnetic fields. This first-principles input is used to define constituent quark masses and is employed to set the free parameters of the Polyakov loop-extended Nambu-Jona-Lasinio (PNJL) model in a magnetic field-dependent manner. The so constructed model is shown to exhibit inverse magnetic catalysis at high temperatures and a reduction of the transition temperature as the magnetic field grows — in line with non-perturbative lattice results. This is contrary to the naive variant of this model, which gives incorrect results for this fundamental phase diagram. Our findings demonstrate that the magnetic field dependence of the PNJL model can be reconciled with the lattice findings in a systematic way, employing solely zero-temperature first-principles input.

Universal scaling of meson and baryon spectra in p-Pb collisions at 5.02TeV

We systematically investigate the scaling property of mesons (pions and kaons) and baryons (protons, [Formula: see text], [Formula: see text] and [Formula: see text]) transverse momentum ([Formula: see text]) spectra at different centrality classes (0–5[Formula: see text], 5–10[Formula: see text], 10–20[Formula: see text], 20–40[Formula: see text], 40–60[Formula: see text], 60–80[Formula: see text] and 80–100[Formula: see text]) in proton-lead collisions with center of mass energy per nucleon pair 5.02[Formula: see text]TeV. In the low [Formula: see text] region with [Formula: see text][Formula: see text]GeV/c, a universal scaling independent of the centrality is observed in the pion (kaon, proton, [Formula: see text], [Formula: see text] and [Formula: see text]) spectra when a dilatation, [Formula: see text], is applied. Here, [Formula: see text] is a scaling parameter depending on the centrality class. We find that the rates at which ln[Formula: see text] changes with the logarithmic value of the average value of the number of participating nucleons, ln[Formula: see text], are stronger for baryons than those for mesons. In the high [Formula: see text] region, there is a deviation from the scaling. The more peripheral the collisions are, the more obvious the violation of the scaling is. In the framework of the color string percolation (CSP) model, we show that mesons and baryons are generated from the decay of clusters formed by strings overlapping in the transverse plane with the same size dispersion but with different mean size. The mean size of clusters for baryons is smaller than that of mesons. For the same hadrons at different centrality classes, the mean size of clusters decreases with the increase of centrality. The fragmentation functions for cluster decay are different for different hadrons, while they are universal for the same hadrons at different centrality classes. The universal scaling of the meson and baryon spectra in the low [Formula: see text] region can be quantitatively understood with the CSP model at the same time.

Resummed Kinetic Field Theory: using Mesoscopic Particle Hydrodynamics to describe baryonic matter in a cosmological framework

We present a new analytical description of baryonic matter in a cosmological framework, using the formalism of “Kinetic Field Theory” (KFT)—a statistical field theory approach to structure formation based on the dynamics of classical particles. So far, only the purely gravitational dynamics of dark matter had been considered in KFT, but an accurate description of cosmic structure formation requires to also take into account the baryonic gas dynamics. In this paper, we propose to achieve this by incorporating an effective mesoscopic particle model of hydrodynamics into the recently developed framework of Resummed KFT. Our main result is the baryonic density contrast power spectrum computed to lowest perturbative order, assuming a simplified isothermal fluid model. Compared to the spectrum of dark matter, the baryonic spectrum shows a suppression of power as well as an oscillatory behaviour associated with sound waves on scales smaller than the Jeans length. We further compare our result to the linear spectrum of an isothermal fluid obtained from Eulerian perturbation theory (EPT), finding good quantitative agreement within the approximations we made in the EPT calculation. A subsequent paper will resolve the problem of coupling both dark and baryonic matter, to gain a full model of cosmic matter. Applying the mesoscopic particle approach to more general ideal or viscous fluids will also be subject of upcoming work.

Hidden charm pentaquarks: mass spectrum, magnetic moments and photocouplings

We develop an extension of the usual three-flavor quark model to four flavors (u, d, s and c), and discuss the classification of pentaquark states with hidden charm (). We fit our model to the known baryon spectrum and we predict the double and triple charm baryons, finding good agreement with the most recent lattice QCD calculations. We compute the ground state of hidden charm pentaquarks and their associated magnetic moments and electromagnetic couplings, of interest to pentaquark photoproduction experiments.

Baryonic susceptibilities, quark-diquark models, and quark-hadron duality at finite temperature

Fluctuations of conserved charges such as baryon number, electric charge and strangeness may provide a test for completeness of states in lattice QCD for three light flavors. We elaborate on the idea that the corresponding susceptibilities can be saturated with excited baryonic states with an underlying quark-diquark structure with a linearly confining interaction. Using Polyakov-loop correlators we show that in the static limit, the quark-diquark potential coincides with the quark-antiquark potential in marked agreement with recent lattice studies. We thus study in a quark-diquark model the baryonic fluctuations of electric charge, baryon number and strangeness: $\chi_{BQ}$, $\chi_{BB}$ and $\chi_{BS}$; by considering a realization of the hadron resonance gas model in the light flavor sector of QCD. These results have been obtained by using the baryon spectrum computed within a relativistic quark-diquark model, leading to an overall good agreement with the spectrum obtained with other quark models and with lattice data for the fluctuations.

Structure of Baryons in a Semi-Relativistic Quark Model

The spectroscopy of light and strange baryons and elastic electromagnetic form factors of nucleon have been studied through a simple semi-relativistic quark model. For SU(6)-invariant part of spectrum, we treated the baryons as a spin-independent three-body bound system and presented the exact analytical solution of three-body Klein–Gordon equation. Considering the SU(6)-invariant interaction as a combination of scalar linear and vector Coulombic-like potentials, we obtained analytical formulas for energy levels and the hyperradial wave functions which have been employed in the calculations of the mass spectrum of baryons, electromagnetic elastic form factors, and charge radii of nucleon. The evaluated observables have been compared with experimental data, and it has been shown that the present model leads to a fairly good description of the observed resonances.

Finite-Volume Spectrum of π^{+}π^{+} and π^{+}π^{+}π^{+} Systems.

The abinitio understanding of hadronic three-body systems above threshold, such as exotic resonances or the baryon spectrum, requires the mapping of the finite-volume eigenvalue spectrum, produced in lattice QCD calculations, to the infinite volume. We present the first application of such a formalism to a physical system in form of three interacting positively charged pions. The results for the ground state energies agree with the available lattice QCD results by the NPLQCD collaboration at unphysical pion masses. Extrapolations to physical pion masses are performed using input from effective field theory. The excited energy spectrum is predicted. This demonstrates the feasibility to determine three-body amplitudes above threshold from lattice QCD, including resonance properties of axial mesons, exotics, and excited baryons.

Towards the minimal spectrum of excited baryons

In the light baryon sector resonances can be broad and overlapping and are in most cases not directly visible in the cross section data. Automatized model selection techniques that introduce penalties for resonances can be used to determine the minimally needed set of resonances to describe the data. Several possible penalization schemes are compared. As an application we perform a blindfold identification of hyperon resonances in the $\bar{K} N \to K \Xi$ reaction based on the Least Absolute Shrinkage and Selection Operator (LASSO) in combination with the Bayesian Information Criterion (BIC). We find ten resonances --- out of the 21 above-threshold hyperon resonances with spin $J\le 7/2$ listed by the Particle Data Group. In traditional analyses, it is practically impossible to test the relevance of all resonances and their combinations that may potentially contribute to the reaction. By contrast, the present method proves capable of determining the relevant resonances among a large pool of candidates.

Jefferson Lab Report

Jefferson Laboratory is finishing a major upgrade and has already started operations with the 12 GeV continuous electron beam. The main research direction is the study of the structure of hadrons, including a search for gluon excitations in the spectra of light mesons and baryons, and studies of multidimensional images of the nucleon. Studied of certain properties of atomic nuclei are also ongoing. There is also an active program of searching for effects beyond the Standard Model in parity-violating electron scattering, as well as a search for new particles.

Lattice Evidence for Bound Heavy Tetraquarks

We investigate the possibility of qq'$ \bar {Q}\bar {Q}' $ tetraquark bound states using nf = 2 + 1 lattice QCD with pion masses ≃ 164, 299 and 415 MeV. Two types of lattice interpolating operator are chosen, reflecting first diquarkantidiquark and second meson-meson structure. Performing variational analysis using these operators and their mixings, we determine the ground and first excited states from the lattice correlators. Using non-relativistic QCD to simulate the bottom quarks and the Tsukuba formulation of relativistic heavy quarks for charm quarks, we study the ud$ \bar {b}\bar {b} $, ℓs$ \bar {b}\bar {b} $ as well as ud$ \bar {c}\bar {b} $, channels with ℓ= u, d. In the case of the ud$ \bar {b}\bar {b} $ and ℓs$ \bar {b}\bar {b} $ channels unambiguous signals for JP=1+ tetraquarks are found with binding energies 189(10) and 98(7) MeV below the corresponding free two-meson thresholds at the physical point. These tetraquarks are therefore strong-interaction stable, implying they are stable under strong as well as electromagnetic interactions while they can decay weakly. So far these are the first exotic hadrons predicted to have this feature. Further evidence for binding is found in the ud$ \bar {c}\bar {b} $ channel, whereby the binding energy broadly straddles the electromagnetic stability threshold. Studying further the quark mass dependence we vary the heavy quark mass in ud$ \bar {Q}\bar {Q} $, ℓs$ \bar {Q}\bar {Q} $ as well as ud$ \bar {Q}\bar {b} $, ℓs$ \bar {Q}\bar {b} $ between roughly 0.7 and 6.3 times the bottom quark mass. The observed mass dependence of these four flavor channels closely follows a behaviour argued from phenomenological considerations of the heavy baryon spectrum.

Baryon Structure and Reactions from Dyson–Schwinger Equations

We discuss the light and strange baryon spectrum obtained within the Dyson–Schwinger/Bethe–Salpeter approach. We compare results from the covariant three-body Faddeev equation and its quark–diquark simplification using the same elementary quark–gluon interaction. The quark–diquark picture allows us to make predictions for excited states in the hyperon sector which can be tested by experiments.

Mass spectra of singly heavy baryons in a self-consistent chiral quark-soliton model

We investigate the mass spectra of the lowest-lying singly heavy baryons, based on the self-consistent chiral quark-soliton model. We take into account the rotational $1/N_c$ and strange current quark mass ($m_{\mathrm{s}}$) corrections. Regarding $m_{\mathrm{s}}$ as a small perturbation, we expand the effective chiral action to the second order with respect to $m_{s}$. The mass spectra of heavy baryons are computed and compared with the experimental data. Fitting the classical masses of the heavy baryon to the center mass of each representation, we determine the masses of all the lowest-lying singly heavy baryons. We predict the mass of the $\Omega_b^*$ baryon to be 6081.9 MeV, when the second-order $m_{\mathrm{s}}$ corrections are included.

Fluctuations and correlations in thermal QCD

We study the equation of state, fluctuations and static correlators of electric charge, baryon number and strangeness, by considering a realization of the Hadron Resonance Gas model in the light flavor sector of QCD. We emphasize the importance of these observables to study, within this approach, the possible existence of exotic and missing states in the hadron spectrum. Some preliminary results for the baryon spectrum have been obtained within a relativistic quark-diquark model, leading to an overall good agreement with the spectrum obtained with other quark models. Finally, it is conjectured, within the Hadron Resonance Gas approach, the existence of a singularity in the correlators at zero temperature, which turns out to be analogous to the divergence of the partition function at the Hagedorn temperature.

Baryon spectrum of SU(4) composite Higgs theory with two distinct fermion representations

We use lattice simulations to compute the baryon spectrum of SU(4) lattice gauge theory coupled to dynamical fermions in the fundamental and two-index antisymmetric (sextet) representations simultaneously. This model is closely related to a composite Higgs model in which the chimera baryon made up of fermions from both representations plays the role of a composite top-quark partner. The dependence of the baryon masses on each underlying fermion mass is found to be generally consistent with a quark-model description and large-Nc scaling. We combine our numerical results with experimental bounds on the scale of the new strong sector to derive a lower bound on the mass of the top partner.

Hadronic superpartners from a superconformal and supersymmetric algebra

Through the embedding of superconformal quantum mechanics into AdS space, it is possible to construct an effective supersymmetric QCD light-front Hamiltonian for hadrons, which includes a spin-spin interaction between the hadronic constituents. A specific breaking of conformal symmetry determines a unique effective quark-confining potential for light hadrons, as well as remarkable connections between the meson, baryon, and tetraquark spectra. The pion is massless in the chiral limit and has no supersymmetric partner. The excitation spectra of relativistic light-quark meson, baryon and tetraquark bound states lie on linear Regge trajectories with identical slopes in the radial and orbital quantum numbers. Although conformal symmetry is strongly broken by the heavy quark mass, the basic underlying supersymmetric mechanism, which transforms mesons to baryons (and baryons to tetraquarks) into each other, still holds and gives remarkable connections across the entire spectrum of light, heavy-light and double-heavy hadrons. Here we show that all the observed hadrons can be related through this effective supersymmetric QCD, and that it can be used to identify the structure of the new charmonium states.