Searching for the color glass condensate with the STAR Forward Meson Spectrometer

Early results from the Solenoidal Tracker At RHIC (STAR) using prototype forward calorimeters (FPD/FPD++) have shown that in d+Au collisions at $ \sqrt {s_{NN} } $ \sqrt {s_{NN} } = 200 GeV a stronger suppression of forward π ⁰ yield is observed than would be expected from shadowing effects. Exploratory measurements and observations can be qualitatively interpreted using the Color Glass Condensate (CGC) model to describe the colliding nucleus. A new detector, the Forward Meson Spectrometer (FMS), has been built to measure forward pion and photon production to quantify gluon suppression at low x in heavy nuclei over a large range in x and p _T .     

Gluons, chirality and v 2 from the color glass condensate

I review recent applications of the classical Yang-Mills formalism to the dynamics in the central-rapidity region of high-energy heavy-ion collisions.     

Using dileptons to probe the color glass condensate

The rapidity and transverse momentum dependence of the nuclear modification ratio for dilepton production at RHIC and LHC is presented, calculated in the color glass condensate (CGC) framework. The transverse momentum ratio is compared for two distinct dilepton mass values and a suppression of the Cronin peak is verified even for large mass. The nuclear modification ratio suppression in the dilepton rapidity spectra, as obtained experimentally for hadrons at RHIC, is verified for LHC energies at large transverse momentum, although not present at RHIC energies. The ratio between LHC and RHIC nuclear modification ratios is evaluated in the CGC, showing the large saturation effects at LHC compared with the RHIC results. These results consolidate the dilepton as a most suitable observable to investigate the QCD high density approaches.     

Initial conditions and global event properties from color glass condensate

Perturbative unitarization from non-linear effects is thought to deplete the gluon density for transverse momenta below the saturation scale. Such effects also modify the distribution of gluons produced in heavy-ion collisions in transverse impact parameter space. I discuss some of the consequences for the initial conditions for hydrodynamic models of heavy-ion collisions and for hard “tomographic” probes. Also, I stress the importance of realistic modelling of the fluctuations of the valence sources for the small-x fields in the impact parameter plane. Such models can now be combined with solutions of running–coupling Balitsky–Kovchegov evolution to obtain controlled predictions for initial conditions at the LHC.     

The color glass condensate: an overview

The color glass condensate is a theory of the dynamical properties of partons in the Regge limit of QCD: , and the center of mass energy squared . We provide a brief introduction to the theoretical ideas underlying the color glass condensate and discuss the application of these ideas to high energy scattering in QCD.Arrival of the final proofs: 5 April 2005R. Venugopalan: Present address: Institute for Theoretical Physics II, Univ. of Hamburg, Luruper Chaussee 49, 22761, Hamburg, Germany     

Probing the color glass condensate in an electron-ion collider

Perturbative quantum chromodynamics (pQCD) predicts that the small-x gluons in a hadron wavefunction should form a color glass condensate (CGC), characterized by a saturation scale Q_s(x,A), which is energy and atomic number dependent. In this paper, we study the predictions of CGC physics for electron-ion collisions at high energies. We consider that the nucleus at high energies acts as an amplifier of the physics of high parton densities and estimate the nuclear structure function F₂^A(x,Q²), as well as the longitudinal and charm contributions, using a generalization for nuclear targets of the Iancu–Itakura–Munier model that describes the ep HERA data quite well. Moreover, we investigate the behavior of the logarithmic slopes of the total and longitudinal structure functions in the kinematical region of the future electron-ion collider eRHIC.     

Searching evidence for the color glass condensate at RHIC

This contribution discusses the phenomenon of parton saturation, the color glass picture of hadronic wavefunctions, and their relevance in the early stages of nucleus–nucleus collisions. Evidence for the color glass condensate in the presently available RHIC data is critically reviewed.     

The renormalization group equation for the color glass condensate

We present an explicit and simple form of the renormalization group equation which governs the quantum evolution of the effective theory for the Color Glass Condensate (CGC). This is a functional Fokker–Planck equation for the probability density of the color field which describes the CGC in the covariant gauge. It is equivalent to the Euclidean time evolution equation for a second quantized current–current Hamiltonian in two spatial dimensions. The quantum corrections are included in the leading log approximation, but the equation is fully non-linear with respect to the generally strong background field. In the weak field limit, it reduces to the BFKL equation, while in the general non-linear case it generates the evolution equations for Wilson-line operators previously derived by Balitsky and Kovchegov within perturbative QCD.     

Gluon fragmentation functions from quark jets

The analysis of events with a fixed, different from unity, fraction of energy flowing into a solid angle ΔΩ can be used to extract the probability functions related to the scaling violations of deep inelastic scattering. One can obtain from the study of a quark jet, both the gluon and the quark fragmentation functions into hadrons.     

Dilepton p T suppression in the color glass condensate

Dilepton production is investigated in proton-nucleus collisions in the forward region using the color glass condensate approach. The transverse momentum distribution (p_T), more precisely the ratio between proton-nucleus and proton-proton differential cross section for LHC energies is evaluated, showing the effects of saturation at small p_T, and presenting a suppression of the Cronin type peak at moderate p_T. These features indicate the dilepton as the most suitable probe to study the properties of the saturated regime and the Cronin effect in these systems.Arrival of the final proofs: 4 July 2005PACS: 11.15.Kc, 24.85. + p     

Quark chiral condensate from the overlap quark propagator

From the overlap lattice quark propagator calculated in the Landau gauge,we determine the quark chiral condensate by fitting operator product expansion formulas to the lattice data.The quark propagators are computed on domain wall fermion configurations generated by the RBC-UKQCD Collaborations with N_f = 2 + 1flavors.Three ensembles with different light sea quark masses are used at one lattice spacing 1/a = 1.75(4) Ge V.We obtain ψψ (2 GeV)MS =(-304(15)(20) MeV)~3in the SU(2) chiral limit.     

Collective non-Abelian instabilities in a melting color glass condensate

We present first results for (3 + 1)D simulations of SU(2) Yang-Mills equations for matter expanding into the vacuum after a heavy ion collision. Violations of boost invariance cause a non-Abelian Weibel instability leading soft modes to grow with proper time tau as exp(gamma square root(g2 mu tau)), where g2 mu is a scale arising from the saturation of gluons in the nuclear wave function. The scale for the growth rate gamma is set by a plasmon mass, defined as omega(pl) = kappa0 square root(g2 mu/tau)), generated dynamically in the collision. We compare the numerical ratio gamma/kappa0 to the corresponding value predicted by the hard thermal loop formalism for anisotropic plasmas.     

Signals of a Weibel instability in the melting color glass condensate

Based on our work hep-ph/0510121, we discuss further the numerical study of classical SU(2) 3+1-D Yang-Mills equations for matter produced in a high-energy heavy-ion collision. The growth of the amplitude of fluctuations as exp(Γ ) (where g ²μ is a scale arising from the saturation of gluons in the nuclear wave function) is shown to be robust over a wide range of initial amplitudes that violate boost invariance. We argue that this growth is due to a non-Abelian Weibel instability, the scale of which is set by a dynamically generated plasmon mass. We discuss the relation of Γ to the prediction from kinetic theory.     

Long range forward–backward correlations and the color glass condensate

We discuss forward–backward correlations in the multiplicity of produced particles in heavy ion collisions. We find the color glass condensate generates distinctive predictions for the long range component of this correlation. In particular, we predict the growth of the long range correlation with the centrality of the collision. We argue that the correlation for baryons is less strong than that for mesons.     

Effect of non-eikonal corrections on azimuthal asymmetries in the color glass condensate

The European Physical Journal C - A generalized theory of electroweak interaction is developed based on the underlying geometrical structure of the tangent bundle with symmetries arising from...     

Low momentum π-meson production from evolvable quark condensate

The pion production by sigma decay in hot and dense matter in the framework of the Nambu-Jona-Lasinio model [1] is investigated. The kinetic equation for joint evolution of pions and sigmas is constructed. The pion enhancement due to additional sigma creation via inertial mechanism is calculated. The text was submitted by the authors in English.