Guillaume Beuf

Next-to-leading order corrections for the dipole factorization of deep inelastic scattering structure functions at low x

QCD at low Bjorken x has become a very active topic of research in particular thanks to deep inelastic scattering (DIS) experiments at HERA. A large part of the DIS phenomenology at low x since the start of HERA has been based on the dipole factorization derived by Nikolaev and Zakharov in Ref.[1]. Contrary to earlier works in low x QCD, which have been performed in momentum space like the derivation of the BFKL evolution [2–4] resummming the high-energy leading logs (LL), the dipole factorization is formulated in mixed space, specifying the transverse position and the light-cone momentum of the involved particles. The dipole factorization has been proposed not only for DIS structure functions [1] but also for other DIS observables at low x, like diffractive structure functions [5], deeply virtual Compton scattering and exclusive vector meson production [6]. The dipole factorization provides a very intuitive picture of DIS observables: the virtual photon radiated from the lepton first fluctuates into a quarkantiquark dipole, which then interacts with the target via gluon exchange(s). In the leading order (LO) version of the dipole factorization, the splitting probability for the photon into a dipole is known from perturbative QED. By contrast, the other factor, which is the dipole-target elastic scattering amplitude, contains all the QCD dynamics, both perturbative and non-perturbative. Numerous phenomenological studies based on the dipole factorization have been performed. In most of them, phenomenological models for the dipole-target amplitude encoding various effects have been used. However, it has been soon realized that the BFKL evolution can be rederived as the low x evolution of a dipole cascade in mixed space [7, 8]. Hence it is very natural to combine the two results, and pick the dipole-target amplitude among the solutions of the BFKL equation in mixed space. By adding such constraint from perturbative QCD, one reduces a priori the needed amount of modeling, down to the choice of the initial condition for the BFKL evolution. However, the BFKL evolution has some severe shortcomings like the violation of unitarity at high energy and the sensitivity to the non-perturbative infrared physics, especially if the coupling is running. The phenomenon of gluon saturation at high energy [9, 10] is both a consequence of those issues and a quasi-perfect solution to them. When taking gluon saturation into account, the BFKL equation is generalized into the B-JIMWLK equations, derived both from the high-energy operator product expansion of Wilson line operators [14] and from the Color Glass Condensate (CGC) effective theory [15–22] based on earlier works [23–25]. In a mean-field approximation, the B-JIMWLK equations reduce to the Balitsky-Kovchegov (BK) equation, also derived [26, 27] independently in the framework of Mueller’s dipole cascade [7, 8]. It has been soon realized that when using the LO dipole factorization together with the LL BK or B-JIMWLK equations, one describes the DIS data only qualitatively, because the obtained low x evolution is faster than in the data. One is thus led to consider higher order corrections. As a first step towards the NLO/NLL accuracy, the first contributions to the running of the coupling αs in the BK or B-JIMWLK equations have been calculated [28–30], leading to appropriate prescriptions to set the scale of the running coupling in the BK and B-JIMWLK equations. By simply promoting the coupling the BK and B-JIMWLK

Single inclusive particle production in proton-nucleus collisions at next-to-leading order in the hybrid formalism

We reconsider the perturbative next-to-leading calculation of the single inclusive hadron production in the framework of the hybrid formalism, applied to hadron production in proton-nucleus collisions. Our analysis, performed in the wave function approach, differs from the previous works in three points. First, we are careful to specify unambiguously the rapidity interval that has to be included in the evolution of the leading-order eikonal scattering amplitude. This is important, since varying this interval by a number of order unity changes the next-to-leading order correction that the calculation is meant to determine. Second, we introduce the explicit requirement that fast fluctuations in the projectile wave function which only exist for a short time are not resolved by the target. This Ioffe time cutoff also strongly affects the next-to-leading order terms. Third, our result does not employ the approximation of a large number of colors. Our final expressions are unambiguous and do not coincide at next-to-leading order with the results available in the literature.

Entropy flow of a perfect fluid in (1+1) hydrodynamics

Using the formalism of the Khalatnikov potential, we derive exact general formulae for the entropy flow dS/dy, where y is the rapidity, as a function of temperature for the (1+1) relativistic hydrodynamics of a perfect fluid. We study in particular flows dominated by a sufficiently long hydrodynamic evolution, and provide an explicit analytical solution for dS/dy. We discuss the theoretical implications of our general formulae and some phenomenological applications for heavy-ion collisions.

Hanbury–Brown–Twiss measurements at large rapidity separations, or can we measure the proton radius in p-A collisions?

Abstract We point out that current calculations of inclusive two-particle correlations in p-A collisions based on the Color Glass Condensate approach exhibit a contribution from Hanbury–Brown–Twiss correlations. These HBT correlations are quite distinct from the standard ones, in that they are apparent for particles widely separated in rapidity. The transverse size of the emitter which is reflected in these correlations is the gluonic size of the proton. This raises an interesting possibility of measuring the proton size directly by the HBT effect of particle pairs produced in p-A collisions.

Dipole factorization for DIS at NLO: Combining the qq¯ and qq¯g contributions

The NLO corrections to the DIS structure functions

Diffractive dijet production in deep inelastic scattering and photon-hadron collisions in the color glass condensate

F_2

Quark correlations in the color glass condensate: Pauli blocking and the ridge

and

Dipole factorization for DIS at NLO I: Loop correction to the photon to quark-antiquark light-front wave-functions

F_L

Heavy quarks in proton-nucleus collisions - the hybrid formalism

(or equivalently the photon-target cross sections

Universality of QCD traveling waves with running coupling

\sigma^{\gamma^*}_{T}