Deep Inelastic Scattering in the Color Glass Formalism
I discuss dome phenomenological consequences of the Color Glass formalism for deep inelastic scattering.
hep-ph↗
arXiv subjects
Kazunori Itakura
Publications and source records attributed to Kazunori Itakura.
Baryon Stopping and the Valence Quark Distribution at Small x
We argue that the amount of baryon stopping observed in the central rapidity region of heavy ion collisions at RHIC is proportional to the nuclear valence quark distributions at small x. By generalizing Mueller's dipole model to describe Reggeons we construct a non-linear evolution equation for the valence quark distributions at small x in the leading double-logarithmic approximation. The equation includes the effects of gluon saturation in it. The solution of the evolution equation gives a valence quark distribution function $dn_{val}/dy \sim e^{-(0.4\div0.5) y}$. We show that this y-dependence as well as the predictions of Regge theory are consistent with the net-proton rapidity distribution reported by BRAHMS.
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A Gaussian effective theory for gluon saturation
We construct a Gaussian approximation to the effective theory for the Colour Glass Condensate which describes correctly the gluon distribution both in the low density regime at high transverse momenta (above the saturation scale $Q_s$), and in the high density regime below $Q_s$, and provides a simple interpolation between these two regimes. At high momenta, the effective theory reproduces the BFKL dynamics, while at low momenta, it exhibits gluon saturation and, related to it, colour neutrality over the short distance scale $1/Q_s \ll 1/Λ_{QCD}$. Gauge--invariant quantities computed within this approximation are automatically infrared finite.
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Structure change of Cooper pairs in color superconductivity -- Crossover from BCS to BEC ? --
We discuss a possibility of transition from color superconductivity of the standard BCS type at high density, to Bose-Einstein Condensation (BEC) of Cooper pairs at lower density. Examining two-flavor QCD over a wide range of baryon density, we found the size of a Cooper pair becomes small enough to be comparable to the averaged quark-quark distance at lower density. We also consider the same problem in two-color QCD.
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Structural Change of Cooper Pairs and Momentum-dependent Gap in Color Superconductivity
The two-flavor color superconductivity is studied over a wide range of baryon density with a single model. We pay a special attention to the spatial-momentum dependence of the gap and to the spatial-structure of Cooper pairs. At extremely high baryon density (O(10^{10} rho_0) with rho_0 being the normal nuclear matter density), our model becomes equivalent to the usual perturbative QCD treatment and the gap is shown to have a sharp peak near the Fermi surface due to the weak-coupling nature of QCD. On the other hand, the gap is a smooth function of the momentum at lower densities (O(10 rho_0)) due to strong color magnetic and electric interactions. To study the structural change of Cooper pairs from high density to lower density, quark correlation in the color superconductor is studied both in the momentum space and in the coordinate space. The size of the Cooper pair is shown to become comparable to the averaged inter-quark distance at low densities. Also, effects of the momentum-dependent running coupling and the antiquark pairing, which are both small at high density, are shown to be non-negligible at low densities. These features are highly contrasted to the standard BCS superconductivity in metals.
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Constraints on color dipole-nucleon cross section from diffractive heavy quarkonium production
We study the hard color dipole-nucleon cross section within perturbative QCD and discuss its relation to observables in diffractive leptoproduction of heavy quarkonium. The dipole cross section calculated with the unintegrated gluon density of the nucleon substantially differs from the well-known perturbative form $σ_{dip} \sim b^2$ for $b > 0.3$fm, where $b$ is the transverse separation of the dipole. We show the measured ratio of $ψ'$ to $J / ψ$ photoproduction cross sections constrains the dipole cross section at intermediate $b$, and in fact excludes the simple $σ_{dip} \sim b^2$ behavior. We also calculate the $t$-slopes of the diffractive $J / ψ, ψ'$ productions. We emphasize the difference of $t$-slopes, $B_{J/ψ} - B_{ψ'}$, is dominated by the dipole-nucleon dynamics. This difference is found to be about $0.3 {GeV}^{-2}$ with our dipole cross section.
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Gap Equations from Fermionic Constraints on the Light-Front
We investigate how the gap equations are obtained in the light-front formalism within the four-Fermi theories. Instead of the zero-mode constraint, we find that the "fermionic constraints" on nondynamical spinor components are responsible for symmetry breaking. Careful treatment of the infrared divergence is indispensable for obtaining the gap equation.
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Dynamical Symmetry Breaking in Light-Front Gross-Neveu Model
We investigate dynamical symmetry breaking of the Gross-Neveu model in the light-front formalism without introducing auxiliary fields. While this system cannot have zero-mode constraints, we find that a nontrivial solution to the constraint on nondynamical spinor fields is responsible for symmetry breaking. The fermionic constraint is solved by systematic 1/$N$ expansion using the boson expansion method as a technique. Carefully treating the infrared divergence, we obtain a nonzero vacuum expectation value for fermion condensate in the leading order. We derive the 't Hooft equation including the effect of condensation, and determine the Hamiltonian consistently with the equation of motion.
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Boson Expansion Methods in (1+1)-dimensional Light-Front QCD
We derive a bosonic Hamiltonian from two dimensional QCD on the light-front. To obtain the bosonic theory we find that it is useful to apply the boson expansion method which is the standard technique in quantum many-body physics. We introduce bilocal boson operators to represent the gauge-invariant quark bilinears and then local boson operators as the collective states of the bilocal bosons. If we adopt the Holstein-Primakoff type among various representations, we obtain a theory of infinitely many interacting bosons, whose masses are the eigenvalues of the 't Hooft equation. In the large $N$ limit, since the interaction disappears and the bosons are identified with mesons, we obtain a free Hamiltonian with infinite kinds of mesons.
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Quantum Hamiltonian Reduction of the Schwinger Model
We reexamine a unitary-transformation method of extracting a physical Hamiltonian from a gauge field theory after quantizing all degrees of freedom including redundant variables. We show that this {\it quantum Hamiltonian reduction} method suffers from crucial modifications arising from regularization of composite operators. We assess the effects of regularization in the simplest gauge field theory, the Schwinger model. Without regularization, the quantum reduction yields the identical Hamiltonian with the classically reduced one. On the other hand, with regularization incorporated, the resulting Hamiltonian of the quantum reduction disagrees with that of the classical reduction. However, we find that the discrepancy is resolved by redefinitions of fermion currents and that the results are again consistent with those of the classical reduction.
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