Anna M. Stasto

Fully unintegrated parton correlation functions and factorization in lowest-order hard scattering

Motivated by the need to correct the potentially large kinematic errors in approximations used in the standard formulation of perturbative QCD, we reformulate deeply inelastic lepton-proton scattering in terms of gauge-invariant, universal parton correlation functions which depend on all components of parton four-momentum. Currently, different hard QCD processes are described by very different perturbative formalisms, each relying on its own set of kinematical approximations. In this paper we show how to set up formalism that avoids approximations on final-state momenta, and thus has a very general domain of applicability. The use of exact kinematics introduces a number of significant conceptual shifts already at leading order, and tightly constrains the formalism. We show how to define parton correlation functions that generalize the concepts of parton density, fragmentation function, and soft factor. After setting up a general subtraction formalism, we obtain a factorization theorem. To avoid complications with Ward identities, the full derivation is restricted to Abelian gauge theories; even so, the resulting structure is highly suggestive of a similar treatment for non-Abelian gauge theories.

Perturbative charm production and the prompt atmospheric neutrino flux in light of RHIC and LHC

A bstractWe re-evaluate the prompt atmospheric neutrino flux, using the measured charm cross sections at RHIC and the Large Hadron Collider to constrain perturbative QCD parameters such as the factorization and renormalization scales, as well as modern parton distribution functions and recent estimates of the cosmic-ray spectra. We find that our result for the prompt neutrino flux is lower than previous perturbative QCD estimates and, consequently, alters the signal-to-background statistics of the recent IceCube measurements at high energies.

Back-to-Back Correlations of Di-hadrons in dAu Collisions at RHIC

Abstract We perform a theoretical analysis of the azimuthal angular correlation of two-hadron productions in the forward dAu collisions at RHIC in the saturation formalism, and obtain a very good agreement with the experimental data. It is demonstrated that the suppression and broadening of the away side peak provide a unique signal for the onset of the saturation mechanism at small- x in a large nucleus. We emphasize that future experiments of di-hadron correlations in pA collisions at both RHIC and LHC, and in eA collisions at the planned electron–ion collider, shall provide us with a thorough study and understanding of the strong interaction dynamics in the saturation regime.

Universality of multiparticle production in QCD at high energies

By studying the color structure of multiparticle production processes in p+A-type (dilute-dense) collisions, we find that higher-point functions beyond typical dipoles and quadrupoles, e.g., sextupoles, octupoles, etc., naturally appear in the cross sections, but are explicitly suppressed in the large-Nc limit. We evaluate the sextupole in the McLerran-Venugopalan model and find that, in general, its analytical form cannot be written as combination of dipoles and quadrupoles. Within the color glass condensate framework, we present a proof that in the large-Nc limit, all multiparticle production processes in the collision of a dilute system off a dense system can, up to all orders in αs, be described in terms of only dipoles and quadrupoles.

Prompt atmospheric neutrino fluxes: perturbative QCD models and nuclear effects

A bstractWe evaluate the prompt atmospheric neutrino flux at high energies using three different frameworks for calculating the heavy quark production cross section in QCD: NLO perturbative QCD, kT factorization including low-x resummation, and the dipole model including parton saturation. We use QCD parameters, the value for the charm quark mass and the range for the factorization and renormalization scales that provide the best description of the total charm cross section measured at fixed target experiments, at RHIC and at LHC. Using these parameters we calculate differential cross sections for charm and bottom production and compare with the latest data on forward charm meson production from LHCb at 7 TeV and at 13 TeV, finding good agreement with the data. In addition, we investigate the role of nuclear shadowing by including nuclear parton distribution functions (PDF) for the target air nucleus using two different nuclear PDF schemes. Depending on the scheme used, we find the reduction of the flux due to nuclear effects varies from 10% to 50% at the highest energies. Finally, we compare our results with the IceCube limit on the prompt neutrino flux, which is already providing valuable information about some of the QCD models.

Numerical solution of the nonlinear evolution equation at small x with impact parameter and beyond the LL approximation

The nonlinear evolution equation at small x with impact parameter dependence is analyzed numerically. The saturation scales and the radius of expansion in the impact parameter are extracted as functions of rapidity. Running coupling is included in this evolution, and it is found that the solution is sensitive to the infrared regularization. Kinematical effects beyond the leading logarithmic approximation are taken partially into account by modifying the kernel which includes the rapidity-dependent cuts. These effects are important for the nonlinear evolution with the impact parameter dependence.

Unitarity constraints on semihard jet production in impact parameter space

The perturbative QCD formula for minijet production constitutes an important ingredient in models describing the total cross section and multiparticle production in hadron-hadron scattering at high energies. Using arguments based on s-channel unitarity we set bounds on the minimum value of pt for which the leading-twist minijet formula can be used. For large impact parameters where correlations between partons appear to be small we find that the minimum value of p{sub t}{sup c} should be greater than 2.5 GeV for LHC energies and greater than 3.5 GeV for cosmic ray energies of about 50 TeV. We also argue that for collisions with values of impact parameters typical for heavy particle production the values of minimum p{sub t} are likely to be considerably larger. We also analyze and quantify the potential role of saturation effects in the gluon density. We find that although saturation effects alone are not sufficient to restore unitarity, they are likely to play an important role at LHC energies.

Small x nonlinear evolution with impact parameter and the structure function data

The nonlinear Balitsky-Kovchegov equation at small x is solved numerically, incorporating impact parameter dependence. Confinement is modeled by including effective gluon mass in the dipole evolution kernel, which regulates the splitting of dipoles with large sizes. It is shown that the solution is sensitive to different implementations of the mass in the kernel. In addition, running coupling effects are taken into account in this analysis. Finally, a comparison of the calculations using the dipole framework with the inclusive data from HERA on the structure functions F{sub 2} and F{sub L} is performed.

Non-linear evolution in CCFM: the interplay between coherence and saturation

We solve the CCFM equation numerically in the presence of a boundary condition which effectively incorporates the non-linear dynamics. We retain the full dependence of the unintegrated gluon distribution on the coherence scale, and extract the saturation momentum. The resulting saturation scale is a function of both rapidity and the coherence momentum. In Deep Inelastic Scattering this will lead to a dependence of the saturation scale on the photon virtuality in addition to the usual xBj dependence. At asymptotic energies the interplay between the perturbative non-linear physics, and that of the QCD coherence, leads to an interesting and novel dynamics where the saturation momentum itself eventually saturates. We also investigate various implementations of the “non-Sudakov” form factor. It is shown that the non-linear dynamics leads to almost identical results for different form factors. Finally, different choices of the scale of the running coupling are analyzed and implications for the phenomenology are discussed.

F(L) proton structure function from the unified DGLAP/BFKL approach

We compute the longitudinal proton structure function F L from the k T factorization scheme, using the unified DGLAP/BFKL resummation approach at small x for the unintegrated gluon density. The differences between the k T factorization, collinear factorization, and dipole approaches are analyzed and discussed. The comparisons with the DESY collider HERA data are made and predictions for the proposed large hadron-electron collider are also provided.