A new physical mechanism is suggested to explain the universal depletion of high meson excitations. It takes into account the appearance of holes inside the string world sheet due to qq pair creation when the length of the string exceeds the critical value R 1 Ӎ1.4 fm. It is argued that a delicate balance between large N c loop suppression and a favorable gain in the action, produced by holes, creates a new metastable ͑predecay͒ stage with a renormalized string tension which now depends on the separation r. This results in smaller values of the slope of the radial Regge trajectories, in good agreement with the analysis of experimental data of Anisovich, Anisovich, and Sarantsev.

We present a study of the tensorial structure of the hadronic matrix elements of the angular momentum operators J. Well known results in the literature are shown to be incorrect, and we have taken pains to derive the correct expressions in three different ways, two involving explicit physical wave packets and the third, totally independent, based upon the rotational properties of the state vectors. Surprisingly it turns out that the results are very sensitive to the type of relativistic spin state used to describe the motion of the particle, i.e., whether a canonical (i.e., boost) state or a helicity state is utilized. We present results for the matrix elements of the angular momentum operators, valid in an arbitrary Lorentz frame, for both helicity states and canonical states. These results are relevant for the construction of angular momentum sum rules, relating the angular momentum of a nucleon to the spin and orbital angular momentum of its constituents. It turns out that it is necessary to distinguish carefully whether the motion of the partons is characterized via canonical or helicity spin states. Fortunately, for the simple parton model interpretation, when the proton moves along OZ, our results for the sum rule based upon the matrix elements of J z agree with the often used sum rule found in the literature. But for the components J x ; J y the results are different and lead to a new and very intuitive sum rule for transverse polarization.We shall return to this question in Secs. VA and V B.Of primary interest are the matrix elements of the angular momentum operators J k or, equivalently, the J ij . Consider the forward matrix element, at t 0, CRITIQUE OF THE ANGULAR MOMENTUM SUM RULES.

Meson Green's functions and decay constants f Γ in different channels Γ are calculated using the Field Correlator Method. Both, spectrum and f Γ , appear to be expressed only through universal constants: the string tension σ, α s , and the pole quark masses. For the S-wave states the calculated masses agree with the experimental numbers within ±5 MeV. For the D and D s mesons the values of f P (1S) are equal to 210(10) and 260(10) MeV, respectively, and their ratio f Ds /f D =1.24(3) agrees with recent CLEO experiment. The values f P (1S) = 182, 216, 438 MeV are obtained for the B, B s , and B c mesons with the ratio f Bs /f B =1.19(2) and f D /f B =1.14(2). The decay constants f P (2S) for the first radial excitations as well as the decay constants f V (1S) in the vector channel are also calculated. The difference of about 20% between f Ds and f D , f Bs and f B directly follows from our analytical formulas.

The di-electron widths of ψ(4040), ψ(4160), and ψ(4415), and their ratios are shown to be in good agreement with experiment, if in all cases the S − D mixing with a large mixing angle θ ≈ 34 • is taken. Arguments are presented why continuum states give small contributions to the wave functions at the origin. We find that the Y (4360) resonance, considered as a pure 3 3 D 1 state, would have very small di-electron width, Γ ee (Y (4360)) = 0.060 keV. On the contrary, for large mixing between the 4 3 S 1 and 3 3 D 1 states with the mixing angle θ = 34.8 • , Γ ee (ψ(4415)) = 0.57 keV coincides with the experimental number, while a second physical resonance, probably Y (4360), has also a rather large Γ ee (Y (∼ 4400)) = 0.61 keV. For the higher resonance Y (4660), considered as a pure 5 3 S 1 state, we predict the di-electron width Γ ee (Y (4660)) = 0.70 keV, but it becomes significantly smaller, namely 0.31 keV, if the mixing angle between the 5 3 S 1 and 4 3 D 1 states θ = 34 • . The mass and di-electron width of the 6 3 S 1 charmonium state are calculated.

The divergences appearing in the ͑3ϩ1͒-dimensional fermion-loop calculations are often regulated by smearing the vertices in a covariant manner. Performing a parallel light-front calculation, we corroborate the similarity between the vertex-smearing technique and the Pauli-Villars regularization. In the light-front calculation of the electromagnetic meson current, we find that the persistent end-point singularity that appears in the case of point vertices is removed even if the smeared vertex is taken to the limit of the point vertex. Recapitulating the current conservation, we substantiate the finiteness of both valence and nonvalence contributions in all components of the current with the regularized bound-state vertex. However, we stress that each contribution, valence or nonvalence, depends on the reference frame even though the sum is always frame independent. The numerical taxonomy of each contribution including the instantaneous contribution and the zero-mode contribution is presented in the , K, and D-meson form factors.

Velocity distributions for the Cl(2P3/2) and Cl(2P1/2) photofragments produced by photolysis of Cl2 in the region between 310 and 470 nm are measured using photofragment velocity mapping. Our results indicate that at short wavelengths the absorption spectrum is dominated by the 1u(1Πu) excited electronic state which produces two ground state chlorine atoms. The 0u+(B 3Πu) state which produces a spin-orbit excited and a ground state chlorine atom becomes significant at 350 nm and dominates the spectrum beyond 400 nm. Analysis of the photofragment angular distributions indicates that the Cl(2P3/2) photofragments are aligned and the magnitude of the alignment is quantitatively determined. Nonadiabatic curve crossing between the 1u(1Πu) and the 0u+(B 3Πu) electronic states is observed and quantified below 370 nm. The measured nonadiabatic transition probability is modeled using the Landau–Zener formula and the position of the curve crossing is estimated at ∼3 eV above the zero-point of ground electronic state of Cl2.

We study the form factors of vector mesons using a covariant fermion field theory model in 3ϩ1 dimensions. Performing a light-front ͑LF͒ calculation in the q ϩ ϭ0 frame in parallel with a manifestly covariant calculation, we note the existence of a nonvanishing zero-mode contribution to the light-front current J ϩ and find a way of avoiding the zero mode in the form factor calculations. Upon choosing the light-front gauge (⑀ hϭϮ ϩ ϭ0) with circular polarization and with spin projection hϭ↑↓ϭϮ, only the helicity zero-to-zero matrix element of the plus current receives zero-mode contributions. Therefore, one can obtain the exact light-front solution of the form factors using only the valence contribution if only the helicity components, (hЈh) ϭ(ϩϩ),(ϩϪ), and (ϩ0), are used. We also compare our results obtained from the light-front gauge in the light-front helicity basis ͑i.e. hϭϮ,0) with those obtained from the non-LF gauge in the instant form linear polarization basis ͑i.e. hϭx,y,z) where the zero-mode contributions to the form factors are unavoidable.

In this paper we discuss the relation between the standard covariant quantum field theory and light-front field theory. We define covariant theory by its Feynman diagrams, whereas light-front field theory is defined in terms of light-cone time-ordered diagrams. A general algorithm is proposed that produces the latter from any Feynman diagram. The procedure is illustrated in several cases. Technical problems that occur in the light-front formulation and have no counterpart in the covariant formulation are identified and solved. The problem of renormalization is not discussed in this paper.