No abstract

Hamiltonian light-front quantum field theory constitutes a framework for the non-perturbative solution of invariant masses and correlated parton amplitudes of self-bound systems. By choosing the light-front gauge and adopting a basis function representation, we obtain a large, sparse, Hamiltonian matrix for mass eigenstates of gauge theories that is solvable by adapting the ab initio no-core methods of nuclear many-body theory. Full covariance is recovered in the continuum limit, the infinite matrix limit. There is considerable freedom in the choice of the orthonormal and complete set of basis functions with convenience and convergence rates providing key considerations. Here, we use a two-dimensional harmonic oscillator basis for transverse modes that corresponds with eigensolutions of the soft-wall AdS/QCD model obtained from light-front holography. We outline our approach and present illustrative features of some non-interacting systems in a cavity. We illustrate the first steps towards solving QED by obtaining the mass eigenstates of an electron in a cavity in small basis spaces and discuss the computational challenges.Comment: 35 pages, 15 figures, Revised to correct Fig. 7 and add new Fig. 15 with spectral results for electron in a transverse cavit

We study the hadron spectra in nearly central A+A collisions at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC) in a broad transverse momentum range. We cover the low-p T spectra using longitudinally boost-invariant hydrodynamics with initial energy and net-baryon number densities from the perturbative QCD (pQCD)+saturation model. Buildup of the transverse flow and sensitivity of the spectra to a single decoupling temperature T dec are studied. Comparison with RHIC data at √ s NN = 130 and 200 GeV suggests a rather high value T dec = 150 MeV. The high-p T spectra are computed using factorized pQCD cross sections, nuclear parton distributions, fragmentation functions, and describing partonic energy loss in the quark-gluon plasma by quenching weights. Overall normalization is fixed on the basis of p+p(p) data and the strength of energy loss is determined from RHIC Au+Au data. Uncertainties are discussed. With constraints from RHIC data, we predict the p T spectra of hadrons in 5% most central Pb+Pb collisions at the LHC energy √ s NN = 5500 GeV. Because of the closed framework for primary production, we can also predict the net-baryon number at midrapidity, as well as the strength of partonic energy losses at the LHC. Both at the LHC and RHIC, we recognize a rather narrow crossover region in the p T spectra, where the hydrodynamic and pQCD fragmentation components become of equal size. We argue that in this crossover region the two contributions are to a good approximation mutually independent. In particular, our results suggest a wider p T region of applicability for hydrodynamical models at the LHC than at RHIC.

This writeup is a compilation of the predictions for the forthcoming Heavy Ion Program at the Large Hadron Collider, as presented at the CERN Theory Institute ‘Heavy Ion Collisions at the LHC—Last Call for Predictions’, held from 14th May to 10th June 2007.

We propose a physically motivated parametrization for the unpolarized generalized parton distributions. At zero value of the skewness variable, ζ, the parametrization is constrained by simultaneously fitting the experimental data on both the nucleon elastic form factors and the deep inelastic structure functions. A rich phenomenology can be addressed based on this parametrization. In particular, we track the behavior of the average: i) interparton distances as a function of the momentum fraction, X, ii) X as a function of the four-momentum transfer, t; iii) the intrinsic transverse momentum k ⊥ as a function of X. We discuss the extension of our parametrization to ζ = 0 where additional constraints are provided by higher moments of the generalized parton distributions obtained from ab initio lattice QCD calculations.

We determine the spatial (impact parameter) dependence of nuclear parton distribution functions (nPDFs) using the A-dependence of the spatially independent (averaged) global fits EPS09 and EKS98. We work under the assumption that the spatial dependence can be formulated as a power series of the nuclear thickness functions T A . To reproduce the A-dependence over the entire x range we need terms up to [T A ] 4 . As an outcome, we release two sets, EPS09s (LO, NLO, error sets) and EKS98s, of spatially dependent nPDFs for public use. We also discuss the implementation of these into the existing calculations. With our results, the centrality dependence of nuclear hard-process observables can be studied consistently with the globally fitted nPDFs for the first time.As an application, we first calculate the LO nuclear modification factor R 1jet AA for primary partonic-jet production in different centrality classes in Au+Au collisions at RHIC and Pb+Pb collisions at LHC. Also the corresponding central-to-peripheral ratios R 1jet CP are studied. We also calculate the LO and NLO nuclear modification factors for single inclusive neutral pion production, R π 0 dAu , at mid-and forward rapidities in different centrality classes in d+Au collisions at RHIC. In particular, we show that our results are compatible with the PHENIX mid-rapidity data within the overall normalization uncertainties given by the experiment. Finally, we show our predictions for the corresponding modifications R π 0 pPb in the forthcoming p+Pb collisions at LHC.

Basis Light-front Quantization has been proposed as a nonperturbative framework for solving quantum field theory. We apply this approach to Quantum Electrodynamics and explicitly solve for the light-front wave function of a physical electron. Based on the resulting light-front wave function, we evaluate the electron anomalous magnetic moment. Nonperturbative mass renormalization is performed. Upon extrapolation to the infinite basis limit our numerical results agree with the Schwinger result obtained in perturbation theory to an accuracy of 0.06%.

We compute the distributions of charged particles at large transverse momenta in pp(p), pA and AA collisions in the framework of perturbative QCD, by using collinear factorization and the modern PDFs and fragmentation functions. At the highest cmsenergies the shape of the spectra measured in pp(p) collisions at large q T can be well explained. The difference between the data and the lowest-order computation is quantified in terms of a constant K-factor for each energy. The K-factor is found to systematically decrease with growing √ s. Also a lower limit for the partonic transverse momentum, p 0 , is extracted for each √ s based on the comparison with the measurements. A systematic increase of p 0 as a function of √ s is found. Nuclear effects in the charged-particle spectra in pA and AA collisions at RHIC and LHC are studied in the framework of collinear factorization by applying the EKS98 nuclear corrections to the parton distributions. The nuclear effects are shown to mostly enhance the computed spectra. A comparison with the recent PHENIX data from central and peripheral Au+Au collisions at RHIC is done.