T. Lappi

Some features of the glasma

Abstract We discuss high energy hadronic collisions within the theory of the color glass condensate. We point out that the initial electric and magnetic fields produced in such collisions are longitudinal. This leads to a novel string like description of the collisions, and a large Chern–Simons charge density made immediately after the collision. The presence of the longitudinal magnetic field suggests that essential to the description of these collisions is the decay of Chern–Simons charge.

The ridge in proton-proton collisions at the LHC

We show that the key features of the CMS result on the ridge correlation seen for high multiplicity events in p s = 7 TeV proton-proton collisions at the LHC can be understood in the Color Glass Condensate framework of high energy QCD. The same formalism underlies the explanation of the ridge events seen in A+A collisions at RHIC, albeit it is likely that flow effects may enhance the magnitude of the signal in the latter.

Long range two-particle rapidity correlations in A+A collisions from high energy QCD evolution

Long range rapidity correlations in A+A collisions are sensitive to strong color field dynamics at early times after the collision. These can be computed in a factorization formalism (Gelis, Lappi and Venugopalan (2009) [1]) which expresses the n-gluon inclusive spectrum at arbitrary rapidity separations in terms of the multi-parton correlations in the nuclear wavefunctions. This formalism includes all radiative and rescattering contributions, to leading accuracy in αsΔY, where ΔY is the rapidity separation between either one of the measured gluons and a projectile, or between the measured gluons themselves. In this paper, we use a mean field approximation for the evolution of the nuclear wavefunctions to obtain a compact result for inclusive two gluon correlations in terms of the unintegrated gluon distributions in the nuclear projectiles. The unintegrated gluon distributions satisfy the Balitsky–Kovchegov equation, which we solve with running coupling and with initial conditions constrained by existing data on electron–nucleus collisions. Our results are valid for arbitrary rapidity separations between measured gluons having transverse momenta p⊥,q⊥≳Qs, where Qs is the saturation scale in the nuclear wavefunctions. We compare our results to data on long range rapidity correlations observed in the near-side ridge at RHIC and make predictions for similar long range rapidity correlations at the LHC.

Renormalization group evolution of multi-gluon correlators in high energy QCD

Abstract Many-body QCD in leading high energy Regge asymptotics is described by the Balitsky–JIMWLK hierarchy of renormalization group equations for the x evolution of multi-point Wilson line correlators. These correlators are universal and ubiquitous in final states in deeply inelastic scattering and hadronic collisions. For instance, recently measured di-hadron correlations at forward rapidity in deuteron–gold collisions at the Relativistic Heavy Ion Collider (RHIC) are sensitive to four and six point correlators of Wilson lines in the small x color fields of the dense nuclear target. We evaluate these correlators numerically by solving the functional Langevin equation that describes the Balitsky–JIMWLK hierarchy. We compare the results to mean-field Gaussian and large N c approximations used in previous phenomenological studies. We comment on the implications of our results for quantitative studies of multi-gluon final states in high energy QCD.

Nuclear enhancement of universal dynamics of high parton densities.

We show that the enhancement of the saturation scale in large nuclei relative to the proton is significantly influenced by the effects of quantum evolution and the impact parameter dependence of dipole cross sections in high energy QCD. We demonstrate that there is a strong A dependence in diffractive deeply inelastic scattering and discuss its sensitivity to the measurement of the recoil nucleus.

Wilson line correlator in the MV model : relating the glasma to deep inelastic scattering

In the color glass condensate framework the saturation scale measured in deep inelastic scattering of high energy hadrons and nuclei can be determined from the correlator of Wilson lines in the hadron wavefunction. These same Wilson lines give the initial condition of the classical field computation of the initial gluon multiplicity and energy density in a heavy ion collision. In this paper the Wilson line correlator in both adjoint and fundamental representations is computed using exactly the same numerical procedure as has been used to calculate gluon production in a heavy ion collision. In particular the discretization of the longitudinal coordinate has a large numerical effect on the relation between the color charge density parameter g2μ and the saturation scale Qs. Our result for this relation is Qs≈0.6g2μ, which results in the classical Yang–Mills value for the “gluon liberation coefficient” c≈1.1.

HIGH ENERGY SCATTERING IN QUANTUM CHROMODYNAMICS

In this series of three lectures, we discuss several aspects of high energy scattering among hadrons in Quantum Chromodynamics. The first lecture is devoted to a description of the parton model, Bjorken scaling and the scaling violations due to the evolution of parton distributions with the transverse resolution scale. The second lecture describes parton evolution at small momentum fraction x, the phenomenon of gluon saturation and the Color Glass Condensate (CGC). In the third lecture, we present the application of the CGC to the study of high energy hadronic collisions, with emphasis on nucleus-nucleus collisions. In particular, we provide the outline of a proof of high energy factorization for inclusive gluon production.

Energy density of the glasma

The initial energy density produced in an ultrarelativistic heavy ion collision can, in the color glass condensate framework, be factorized into a product of the integrated gluon distributions of the nuclei. Although this energy density is well defined without any infrared cutoff besides the saturation scale, it is apparently logarithmically ultraviolet divergent. We argue that this divergence is not physically meaningful and does not affect the behavior of the system at any finite proper time.

Nuclear enhancement and suppression of diffractive structure functions at high energies

We compute diffractive structure functions for both protons and nuclei in the framework of color glass condensate models with impact parameter dependence. These models have previously been shown to provide good agreement with inclusive

Chemical thermalization in relativistic heavy ion collisions.

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