Bo-Wen Xiao

Universality of Unintegrated Gluon Distributions at small x

We systematically study dijet production in various processes in the small-x limit and establish an effective kt-factorization for hard processes in a system with dilute probes scattering on a dense target. In the large-Nc limit, the unintegrated gluon distributions involved in different processes are shown to be related to two widely proposed ones: the Weizsacker-Williams gluon distribution and the dipole gluon distribution.

Comparing energy loss and p⊥-broadening in perturbative QCD with strong coupling N=4 SYM theory☆

Abstract We compare medium induced energy loss and p ⊥ -broadening in perturbative QCD with that of the trailing string picture of SYM theory. We consider finite and infinite extent matter as well as relativistic heavy quarks which correspond to those being produced in the medium or external to it. When expressed in terms of the appropriate saturation momentum, we find identical parametric forms for energy loss in perturbative QCD and SYM theory. We find simple correspondences between p ⊥ -broadening in QCD and in SYM theory although p ⊥ -broadening is radiation dominated in SYM theory and multiple scattering dominated in perturbative QCD.

Inclusive hadron productions in p A collisions

We calculate inclusive hadron productions in pA collisions in the small-x saturation formalism at one-loop order. The differential cross section is written into a factorization form in the coordinate space at the next-to-leading order, while the naive form of the convolution in the transverse momentum space does not hold. The rapidity divergence with small-x dipole gluon distribution of the nucleus is factorized into the energy evolution of the dipole gluon distribution function, which is known as the Balitsky-Kovchegov equation. Furthermore, the collinear divergences associated with the incoming parton distribution of the nucleon and the outgoing fragmentation function of the final state hadron are factorized into the splittings of the associated parton distribution and fragmentation functions, which allows us to reproduce the well-known DGLAP equation. The hard coefficient function, which is finite and free of divergence of any kind, is evaluated at one-loop order.

kt-factorization for Hard Processes in Nuclei

Two widely proposed k(t)-dependent gluon distributions in the small-x saturation regime are investigated using two-particle back-to-back correlations in high energy scattering processes. The Weizsäcker-Williams gluon distribution, interpreted as the number density of gluon inside nucleus, is studied in the quark-antiquark jet correlation in deep inelastic scattering. On the other hand, the unintegrated gluon distribution, defined as the Fourier transform of the color-dipole cross section, is probed in the direct photon-jet correlation in pA collisions.

On the exact solution of the accelerating string in AdS5 space

Abstract In this Letter, the exact accelerating string solution of a heavy quark–antiquark pair is found in AdS 5 space. On the accelerating string, there is a particular scale which separates the radiation and the heavy quark. This scale is explicitly shown to be an event horizon in the proper frame of the heavy quark. Furthermore, we find a new correspondence, which relates the horizon in AdS 5 space on the gravity theory side to the Unruh temperature in Minkowski space on the field theory side of the AdS/CFT correspondence. p ⊥ -broadening and p L -broadening of the heavy quark due to radiation are computed using the AdS/CFT correspondence.

One-loop factorization for inclusive hadron production in p-A collisions in the saturation formalism.

We demonstrate the QCD factorization for inclusive hadron production in p-A collisions in the saturation formalism at one-loop order, with explicit calculation of both real and virtual gluon radiation diagrams. In particular, we find that the cross section can be written into a factorization form in the coordinate space at the next-to-leading order, while the naive form of the convolution in the transverse momentum space does not hold. The collinear divergences associated with the incoming parton distribution of the nucleon and the outgoing fragmentation function of the final-state hadron, as well as the rapidity divergence with small-x dipole gluon distribution of the nucleus are factorized into the splittings of the associated parton distribution and fragmentation functions and the energy evolution of the dipole gluon distribution function. The hard coefficient function is evaluated at one-loop order, and contains no divergence.

On the small-x evolution of the color quadrupole and the Weizsäcker–Williams gluon distribution

Abstract Color quadrupoles have been found to be important in the proper description of observables sensitive to the small- x regime in nuclei as well as in the operator definition of the Weizsacker–Williams gluon distribution. In this Letter, we derive the small- x evolution equation of the quadrupole and the Weizsacker–Williams gluon distribution without taking the large N c limit and study the properties of the equation in both dilute and saturation regime. We find that the quadrupole evolution follows the BFKL evolution in the dilute regime and then saturates in the dense region due to nonlinear terms. This leads us to conclude that the Weizsacker–Williams gluon distribution should obey the same geometrical behavior as the dipole gluon distribution as found in the inclusive DIS measurement.

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.

Sudakov double logarithms resummation in hard processes in the small-x saturation formalism

In this manuscript, we present a complete study of the Sudakov double logarithms resummation for various hard processes in eA and pA collisions in the small-x saturation formalism. We first employ a couple of slightly different formalisms to perform the one-loop analysis of the Higgs boson production process in pA collisions, and demonstrate that Sudakov-type logarithms arise as the leading correction and that they can be systematically resummed in addition to the usual small-x resummation. We further study the Sudakov double logarithms for other processes such as heavy quark pair production and back-to-back dijet production in eA and pA collisions through detailed calculation of the corresponding one-loop diagrams. As the most important contribution from the one-loop correction, the Sudakov factor should play an important role in the phenomenological study of saturation physics in the pA programs at RHIC and the LHC.

QCD resummation for single spin asymmetries.

We study the transverse momentum dependent factorization for single spin asymmetries in Drell-Yan and semi-inclusive deep inelastic scattering processes at one-loop order. The next-to-leading order hard factors are calculated in the Ji-Ma-Yuan factorization scheme. We further derive the QCD resummation formalisms for these observables following the Collins-Soper-Sterman method. The results are expressed in terms of the collinear correlation functions from initial and/or final state hadrons coupled with the Sudakov form factor containing all order soft-gluon resummation effects. The scheme-independent coefficients are calculated up to one-loop order.