Chiral symmetry restoration in excited hadrons
The evidence, theoretical justification and implications of chiral symmetry restoration in excited hadrons are presented.
hep-ph↗
arXiv subjects
L. Ya. Glozman
Publications and source records attributed to L. Ya. Glozman.
Low-Lying Nucleons from Chirally Improved Fermions
We report on our preliminary results on the low-lying excited nucleon spectra which we obtain through a variational basis formed with three different interpolators.
hep-lat↗
$Θ^+$ in a chiral constituent quark model and its interpolating fields
The recently discovered pentaquark $Θ^+$ is described within the chiral constituent quark model. Within this picture the flavor-spin interaction between valence quarks inverts the $(1s)^4$ and $(1s)^3(1p)$ levels of the four-quark subsystem and consequently the lowest-lying pentaquark is a positive parity, I=0, J=1/2 state of the flavor antidecuplet, similar to the soliton model prediction. Contrary to the soliton model, however, the quark picture predicts its spin-orbit partner with $J=3/2$. Different interpolating fields intended for lattice calculations of $Θ^+$ are constructed, which have a maximal overlap with this baryon if it is indeed a quark excitation in the 5Q system.
hep-ph↗
Chiral and U(1)_A restorations high in the hadron spectrum and the semiclassical approximation
In quantum systems with large $n$ (radial quantum number) or large angular momentum the semiclassical (WKB) approximation is valid. A physical content of the semiclassical approximation is that the quantum fluctuations effects are suppressed and vanish asymptotically. The chiral as well as $U(1)_A$ breakings in QCD is a result of quantum fluctuations. Hence these breakings must be absent (suppressed) high in the spectrum and the spectrum of high-lying hadrons must exhibit symmetries of the classical QCD Lagrangian.
hep-ph↗
Nucleon electromagnetic and axial form factors in point-form relativistic quantum mechanics
Results for the proton and neutron electric and magnetic form factors as well as the nucleon axial form factor are presented for constituent quark models, based on either one-gluon-exchange and Goldstone-boson-exchange dynamics. The calculations are performed in a covariant framework using the point-form approach to relativistic quantum mechanics. The only input to the calculations is the nucleon wave function of the corresponding constituent quark model. A comparison is given to results of the instanton-induced constituent quark model treated with the Bethe-Salpeter equation.
hep-ph↗
Chiral symmetry restoration in hadron spectra
The evidence and the theoretical justification of chiral symmetry restoration in high-lying hadrons is presented.
hep-ph↗
Chiral symmetry restoration and the string picture of hadrons
QCD string picture of highly excited hadrons very naturally explains parity doubling once the chiral symmetry is restored high in the spectrum. In particular, the spin-orbit and tensor interactions of quarks at the ends of the string, related to dynamics of the string, vanish. High in the spectrum there appears higher degree of degeneracy, namely parity doublets with different angular momentum cluster around energy of the string in the given quantum state.
hep-ph↗
SU(2)_L * SU(2)_R and U(1)_A restorations high in the hadron spectrum and what it tells us about
Recent data for highly excited mesons suggest that not only the chiral $SU(2)_L\times SU(2)_R$ symmetry of QCD is restored high in the spectrum but also the $ U(1)_A$ symmetry. This means that it is not a confining interaction in QCD which triggers the spontaneous breaking of chiral symmetry. The restoration of the $U(2)_L\times U(2)_R$ symmetry of the QCD Lagrangian implies the appearance of multiplets of this group high in the hadron spectra. Such type of multiplets is naturally explained within the string picture of confinement. It also supports the scenario that the $U(1)_A$ breaking is related to instantons and not to the gluonic interaction responsible for confinement.
hep-ph↗
Does one observe chiral symmetry restoration in baryon spectrum?
It has recently been suggested that the parity doublet structure seen in the spectrum of highly excited baryons may be due to effective chiral symmetry restoration for these states. We review the recent developments in this field. We demonstrate with a simple quantum-mechanical example that it is a very natural property of quantum systems that a symmetry breaking effect which is important for the low-lying spectrum of the system, can become unimportant for the highly-lying states; the highly lying states reveal a multiplet structure of nearly degenerate states. Using the well established concepts of quark-hadron duality, asymptotic freedom in QCD and validity of the operator product expansion in QCD we show that the spectral densities obtained with the local currents that are connected to each other via chiral transformations, very high in the spectrum must coincide. Hence effects of spontaneous breaking of chiral symmetry in QCD vacuum that are crucially important for the low-lying spectra, become irrelevant for the highly-lying states. Then to the extent that identifiable hadronic resonances still exist in the continuum spectrum at high excitations this implies that the highly excited hadrons must fall into multiplets associated with the representations of the chiral group. We demonstrate that this is indeed the case for meson spectra in the large $N_c$ limit. All possible parity-chiral multiplets are classified for baryons and it is demonstrated that the existing data on highly excited $N$ and $Δ$ states at masses of 2 GeV and higher is consistent with approximate chiral symmetry restoration. However new experimental studies are needed to achieve any definitive conclusions.
hep-ph↗
Covariant electroweak nucleon form factors in a chiral constituent quark model
Results for the proton and neutron electric and magnetic form factors as well asthe nucleon axial and induced pseudoscalar form factors are presented for the chiral constituent quark model based on Goldstone-boson-exchange dynamics. The calculations are performed in a covariant framework using the point-form approach to relativistic quantum mechanics. The direct predictions of the model yield a remarkably consistent picture of the electroweak nucleon structure.
hep-ph↗
Chiral multiplets versus parity doublets in highly excited baryons
It has recently been suggested that the parity doublet structure seen in the spectrum of highly excited baryons may be due to effective chiral restoration for these states. We argue how the idea of chiral symmetry restoration high in the spectrum is consistent with the concept of quark-hadron duality. If chiral symmetry is effectively restored for highly-lying states, then the baryons should fall into representations of $SU(2)_L\times SU(2)_R$ that are compatible with the given parity of the states - the parity-chiral multiplets. We classify all possible parity-chiral multiplets: (i) $(1/2,0)\oplus(0, 1/2)$ that contain parity doublet for nucleon spectrum;(ii) $(3/2,0) \oplus (0, 3/2)$ consists of the parity doublet for delta spectrum; (iii) $(1/2,1) \oplus (1, 1/2)$ contains one parity doublet in the nucleon spectrum and one parity doublet in the delta spectrum of the same spin that are degenerate in mass. Here we show that the available spectroscopic data for nonstrange baryons in the $\sim$ 2 GeV range is consistent with all possibilities, but the approximate degeneracy of parity doublets in nucleon and delta spectra support the latter possibility with excited baryons approximately falling into $(1/2,1) \oplus (1, 1/2)$ representation of $SU(2)_L\timesSU(2)_R$ with approximate degeneracy between positive and negative parity $N$ and $Δ$ resonances of the same spin.
hep-ph↗
Covariant axial form factor of the nucleon in a chiral constituent quark model
The axial form factor G_A of the nucleon is investigated for the Goldstone-boson-exchange constituent quark model using the point-form approach to relativistic quantum mechanics. The results, being covariant, show large contributions from relativistic boost effects. The predictions are obtained directly from the quark-model wave functions, without any further input such as vertex or constituent-quark form factors, and fall remarkably close to the available experimental data.
nucl-th↗
Chiral symmetry restoration and the linear form of baryonic Regge trajectories
It has recently been suggested that the observed structure of parity doublets, seen in the spectrum of highly excited baryons, may be due to the effective restoration of chiral symmetry for these states. This chiral symmetry restoration high in the spectrum is consistent with the concept of quark-hadron duality. Moreover, if QCD dynamics implies the linear Regge trajectories for highly excited baryons, then the MacDowell symmetry requires the parity doubling to appear, which shows that linear Regge trajectories and chiral symmetry restoration are well compatible. On the contrary, in the low-energy part of baryon spectrum the parity doublets are absent because of spontaneous chiral symmetry breaking. Then the MacDowell symmetry implies that there should be no linear parallel Regge trajectories. Experimental data shows that this is indeed the case.
hep-ph↗
Point form relativistic quantum mechanics, antiparticles and exchange currents
It is shown that the dynamical observables calculated with the point form relativistic quantum mechanics incorporate effects of particle-antiparticle creation from the vacuum by interactions. The electromagnetic observables obtained with the point form impulse approximation include contributions from the ``pair'' exchange currents that are associated with the interactions between particles. This implies that the recently calculated nucleon electromagnetic formfactors with the chiral constituent quark model automatically take into account effects of ``pair'' exchange currents that are associated with the Goldstone boson exchange between the constituent quarks as well as with the confining interaction.
hep-ph↗
Pion loop fluctuations of constituent quarks and baryons
The implications of the pion (meson) degrees of freedom for baryon properties, taken at the constituent quark and baryon levels are compared. It is shown that there is a dramatic qualitative difference between two approaches.
nucl-th↗
On Isgur's "Critique of a Pion Exchange Model for Interquark Forces"
The conceptual issues of low-energy baryon physics are discussed. In particular, a comparison between the naive one gluon exchange model for the interaction between constituent quarks in hadrons and the Goldstone boson exchange picture is made. The "defects" of the Goldstone boson exchange model for baryons, indicated by Isgur are examined in detail. {\it All} of the purported ``defects'' are shown to lack a valid basis.
nucl-th↗
The chiral symmetry restoration phase transition in baryon spectrum
It is shown that in the large $N_c$ limit the light baryon spectrum exhibits the chiral restoration phase transition at high enough excitation energy. Such a phase transition is evidenced by the systematical parity doublets observed in the upper part of $N$ and $Δ$ spectra.
hep-ph↗
Chiral aspects of baryon structure in the quark model
The implications of chiral symmetry for the quark model are discussed. In particular its connection with the meson-baryon approach is outlined. It is stressed that in the closure approximation, where the intermediate baryon states are used instead of the original quark basis, it is imperative to take into account the whole infinite tower of the radially excited states in order that the crucially important effects related to the short-range part of the meson-exchange interaction be preserved. It is shown that the chiral constituent quark model is able to explain the nucleon axial vector coupling constant and automatically incorporates all the necessary loop corrections.
hep-ph↗