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Recently, via calculation of spatial correlators of $J=0,1$ isovector operators using a chirally symmetric Dirac operator within $N_F=2$ QCD, it has been found that QCD at temperatures $T_c - 3 T_c$ is approximately $SU(2)_{CS}$ and $SU(4)$ symmetric. The latter symmetry suggests that the physical degrees of freedom are chirally symmetric quarks bound by the chromoelectric field into color singlet objects without chromomagnetic effects. This regime of QCD has been referred to as a Stringy Fluid. Here we calculate correlators for propagation in time direction at a temperature slightly above $T_c$ and find the same approximate symmetries. This means that the meson spectral function is chiral-spin and $SU(4)$ symmetric., 10 pages, 3 figures. Version published by PLB

Using interpolators with different SU(2)_L \times SU(2)_R transformation properties we study the chiral symmetry and spin contents of the rho- and rho'-mesons in lattice simulations with dynamical quarks. A ratio of couplings of the $\qbar\gamma^i{\tau}q$ and $\qbar\sigma^{0i}{\tau}q$ interpolators to a given meson state at different resolution scales tells one about the degree of chiral symmetry breaking in the meson wave function at these scales. Using a Gaussian gauge invariant smearing of the quark fields in the interpolators, we are able to extract the chiral content of mesons up to the infrared resolution of ~1 fm. In the ground state rho meson the chiral symmetry is strongly broken with comparable contributions of both the (0,1) + (1,0) and (1/2,1/2)_b chiral representations with the former being the leading contribution. In contrast, in the rho' meson the degree of chiral symmetry breaking is manifestly smaller and the leading representation is (1/2,1/2)_b. Using a unitary transformation from the chiral basis to the {2S +1}L_J basis, we are able to define and measure the angular momentum content of mesons in the rest frame. This definition is different from the traditional one which uses parton distributions in the infinite momentum frame. The rho meson is practically a 3S_1 state with no obvious trace of a "spin crisis". The rho' meson has a sizeable contribution of the 3D_1 wave, which implies that the rho' meson cannot be considered as a pure radial excitation of the rho meson., Comment: 10 pp

It is shown that experimental meson states with spins J=0,1,2,3 in the energy range 1.9 - 2.4 GeV obtained in recent partial wave analysis of proton-antiproton annihilation at LEAR remarkably confirm all predictions of chiral symmetry restoration. Classification of excited $\bar q q$ mesons according to the representations of chiral $U(2)_L \times U(2)_R$ group is performed. There are two important predictions of chiral symmetry restoration in highly excited mesons: (i) physical states must fill out approximately degenerate parity-chiral multiplets; (ii) some of the physical states with the given $I,J^{PC}$ are members of one parity-chiral multiplet, while the other states with the same $I,J^{PC}$ are members of the other parity-chiral multiplet. For example, while some of the excited $\rho(1,1^{--})$ states are systematically degenerate with $a_1(1,1^{++})$ states forming (0,1)+(1,0) chiral multiplets, the other excited $\rho(1,1^{--})$ states are degenerate with $h_1(0,1^{+-})$ states ((1/2,1/2) chiral multiplets). Hence, one of the predictions of chiral symmetry restoration is that the combined amount of $a_1(1,1^{++})$ and $h_1(0,1^{+-})$ states must coincide with the amount of $\rho(1,1^{--})$ states in the chirally restored regime. It is shown that the same rule applies (and experimentally confirmed) to many other meson states., Comment: 14 pages, discussion and conclusion section is largely extended

The recently discovered pentaquark $\Theta^+$ 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 $\Theta^+$ are constructed, which have a maximal overlap with this baryon if it is indeed a quark excitation in the 5Q system., Comment: 9 pp

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., Comment: 7 pages, LaTeX, 2 figs. The paper has been further expanded in order to make the point and physics more clear. To appear in Phys. Lett. B

It is argued that an appearance of the near parity doublets in the upper part of the light baryon spectrum is an evidence for the chiral symmetry restoration in the regime where a typical momentum of quarks is around the chiral symmetry restoration scale. At high enough baryon excitation energy the nontrivial gap solution, which signals the chiral symmetry breaking regime, disappears and the chiral symmetry should be restored. Thus one observes a phase transition in the upper part of the light baryon spectrum. The average kinetic energy of the constituent quarks in this region is just around the critical one $3T_c$., LaTeX, 6 pp. The final version of the paper to appear in Phys. Lett. B

Pion loop fluctuations of the constituent u and d quarks are shown to give only a minute contribution to the intrinsic charge radius of the neutron, under the assumption that the pion-quark coupling constant has its conventional value, with a cut-off scale of the order of 1.0 GeV. The contribution from the pion loops to the anomalous magnetic moment of the neutron represents a small ($\sim$ 12-14%) increase over of the static quark model value.

We demonstrate that the meson-exchange contribution to d(e,e′p) Δ ( E p ∼ 0.7 − 1 GeV and Δ being at rest) is small compared to the six-quark contribution. The cross-section is large enough to be measured at CEBAF. This means that through this reaction we could observe and study the quark degrees of freedom in the deuteron.

The 6q deuteron wave function s4p2 configuration are shown to lead directly to the occurence of excited nucleons (resonances) N 1 2 + ∗ (1570), N 1 2 − (1535) , etc. as subsystems. It is prodpsed to examine the 6q-structure directly using the d(e, e'p)N∗ process. The spectroscopic factors and momentum distributions for the process are calculated.

The main points of a consistent semi-microscopic model of orthogonal coupled channels |NN〉 and |S 6 〉 are given for the description of NN interaction and electromagnetic processes. The nucleon overlap region is correctly taken into account. The process of deuteron electrodisintegration near threshold is described. The meson exchange currents are taken into account too.