arXiv subjects

Guillaume Beuf

Publications and source records attributed to Guillaume Beuf.

At least 37 records · Page 2Linked to original sources

arXiv2023

Probing quark transverse momentum distributions in the Color Glass Condensate: quark-gluon dijets in Deep Inelastic Scattering at next-to-eikonal accuracy

We study the production, in Deep Inelastic Scattering at high energy, of a quark-gluon dijet induced by $t$-channel quark exchange with the target, which goes beyond the eikonal approximation. Throughout this study we follow the Color Glass Condensate approach, keep full dependence on the quark mass and consider the target to be unpolarized. We focus on the correlation limit in which the produced jets fly almost back-to-back and find a factorized expression for the cross section, for both longitudinal and transverse photons, involving the unpolarized quark transverse momentum dependent distribution. Quark-gluon dijets can be distinguished from other types of dijets at least in the heavy quark case, thanks to heavy flavor tagging. In that case, the dominant background is expected to come from the eikonal production of a quark-antiquark-gluon system. We propose experimental cuts to suppress that background contribution, making the process of quark-gluon dijet production in Deep Inelastic Scattering a new method to probe the quark transverse momentum dependent distribution at the Electron Ion Collider.

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arXiv2023

DIS dijet production at next-to-eikonal accuracy in the CGC

We compute dijet production in Deep Inelastic Scattering at low $x$ in the dipole formalism at next-to-eikonal accuracy. We calculate the contributions induced by single photon exchange of either longitudinal or transverse polarization. We include all types of corrections to the eikonal approximation in the gluon background field: (i) finite longitudinal width of the target, (ii) interaction of the quark-antiquark pair with the subleading (transverse) component of the background field and (iii) dynamics of the target which is encoded in the $z^-$-coordinate dependence of the background field. The final expressions for the dijet cross section are written as sum of a "generalized eikonal" contribution (where longitudinal momentum $p^+$ exchange between the target and the incoming quark-antiquark pair is allowed since the average $z^-$ dependence of the background field is kept) and explicit next-to-eikonal corrections that involve decorated dipole and quadrupole operators.

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arXiv2023

DIS dijet production at next-to-eikonal accuracy in the CGC

In this work, we derive the cross-section for inclusive DIS dijet production at full next-to-eikonal order. We include the corrections that stem from taking a finite width of the target, the interaction of the quark with the transverse component of the background field and also the dynamics of the target.

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arXiv2023

The case for an EIC Theory Alliance: Theoretical Challenges of the EIC

We outline the physics opportunities provided by the Electron Ion Collider (EIC). These include the study of the parton structure of the nucleon and nuclei, the onset of gluon saturation, the production of jets and heavy flavor, hadron spectroscopy and tests of fundamental symmetries. We review the present status and future challenges in EIC theory that have to be addressed in order to realize this ambitious and impactful physics program, including how to engage a diverse and inclusive workforce. In order to address these many-fold challenges, we propose a coordinated effort involving theory groups with differing expertise is needed. We discuss the scientific goals and scope of such an EIC Theory Alliance.

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arXiv2022

Dijet photoproduction at low $x$ at next-to-leading order and its back-to-back limit

We compute the cross section for the inclusive photoproduction of a pair of jets at next-to-leading order accuracy in the Color Glass Condensate (CGC) effective theory. The aim is to study the back-to-back limit, to investigate whether transverse momentum dependent (TMD) factorization can be recovered at this order. In particular, we focus on the large Sudakov double logarithms, which are major ingredients of the TMD evolution. Interestingly, the kinematical improvement of the low-$x$ resummation scheme turns out to be a key ingredient in the analysis.

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arXiv2022

Quark and scalar propagators at next-to-eikonal accuracy in the CGC through a dynamical background gluon field

We revisit the calculation of quark and scalar propagators in a gluon background field at Next-to-Eikonal accuracy, which will provide the first power correction to scattering processes in the high-energy limit, including gluon saturation effects. The two main results of our study are as follows. First, we derive a very general Next-to-Eikonal expansion of the quark propagator in terms of the scalar propagator, which allows us to isolate spinor-specific Next-to-Eikonal corrections from the universal ones. Second, we present a new method allowing, for the first time, a systematic calculation of the Next-to-Eikonal corrections coming from the $x^-$ dependence of the background field. This opens the possibility to study high-energy scattering processes on a target with non-trivial dynamics instead of on a static one, by going beyond infinite Lorentz time dilation of the target.

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arXiv2021

Quarks at next-to-eikonal accuracy in the CGC I: Forward quark-nucleus scattering

We derive Next-to-Eikonal (NEik) corrections to the background quark propagator, which stem from (i) considering a finite longitudinal width target instead of an infinitely thin shockwave and (ii) including the interaction of the quark with the transverse components of the background field. These two different corrections to the eikonal approximation combine together and provides a gauge covariant expression for the quark propagator at NEik accuracy. We then apply our results to quark (or antiquark) scattering on a nucleus at NEik accuracy, considering both unpolarized cross section and helicity asymmetry.

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arXiv2017

Dipole factorization for DIS at NLO: Combining the $q\bar{q}$ and $q\bar{q}g$ contributions

The NLO corrections to the DIS structure functions $F_2$ and $F_L$ (or equivalently the photon-target cross sections $σ^{γ^*}_{T}$ and $σ^{γ^*}_{L}$) at low $x_{Bj}$ are obtained, as a generalization of the dipole factorization formula. For the first time, the contributions of both the $q\bar{q}$ and the $q\bar{q}g$ Fock states in the photon are directly calculated, using earlier results for the $q\bar{q}$ light-front wave-functions at one loop inside a dressed virtual photon. Both the $q\bar{q}$ and the $q\bar{q}g$ contributions have UV divergences, which are shown to cancel each other, using conventional dimensional regularization as UV regulator. Finally, the resummation of high-energy logarithms on top of the NLO results for $σ^{γ^*}_{T}$ and $σ^{γ^*}_{L}$ is discussed.

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arXiv2017

Quark correlations in the Color Glass Condensate: Pauli blocking and the ridge

We consider, for the first time, correlations between produced quarks in p-A collisions in the framework of the Color Glass Condensate. We find a quark-quark ridge that shows a dip at $Δη\sim 2$ relative to the gluon-gluon ridge. The origin of this dip is the short range (in rapidity) Pauli blocking experienced by quarks in the wave function of the incoming projectile. We observe that these correlations, present in the initial state, survive the scattering process. We suggest that this effect may be observable in open charm-open charm correlations at the Large Hadron Collider.

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arXiv2016

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

The one-loop QCD corrections to the light-front wave-function for the quark-antiquark Fock state inside a transverse or longitudinal off-shell photon are explicitly calculated, both in full momentum space and in mixed space (a.k.a. dipole space). These results provide one of the main contributions to virtual NLO corrections to many DIS observables (inclusive or not) in the dipole factorization formalism at low Bjorken x. In a follow-up article, these one-loop corrections are combined with earlier results on the wave-function for the quark-antiquark-gluon Fock state, in order to get the full set of NLO corrections to the DIS structure functions $F_2$ and $F_L$ in the dipole factorization formalism, valid at low Bjorken x.

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arXiv2016

Diffractive Dijet Production in Deep Inelastic Scattering and Photon-Hadron Collisions in the Color Glass Condensate

We study exclusive dijet production in coherent diffractive processes in deep inelastic scattering and real (and virtual) photon-hadron ($γ^{(*)}$-h) collisions in the Color Glass Condensate formalism at leading order. We show that the diffractive dijet cross section is sensitive to the color-dipole orientation in the transverse plane, and is a good probe of possible correlations between the $q\bar{q}$-dipole transverse separation vector $\r$ and the dipole impact parameter $\b$. We also investigate the diffractive dijet azimuthal angle correlations and $t$-distributions in $γ^{(*)}$-h collisions and show that they are sensitive to gluon saturation effects in the small-$x$ region. In particular, we show that the $t$-distribution of diffractive dijet photo-production off a proton target exhibits a dip-type structure in the saturation region. This effect is similar to diffractive vector meson production. Besides, at variance with the inclusive case, the effect of saturation leads to stronger azimuthal correlations between the jets.

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arXiv2015

Heavy quarks in proton-nucleus collisions - the hybrid formalism

We explore the quark mass effects on inclusive hadron production in proton-nucleus collisions at high energies. We consider two processes. First, we compute the single inclusive cross-section for production of hadrons with open heavy flavour in the proton forward direction at leading order. Next, in the same kinematics, we calculate the heavy-quark contribution to single inclusive production of light or unidentified hadrons at next-to-leading-order. For both studies we exploit the hybrid formalism, that is the collinear factorisation on the proton side while high-density and high-energy effects are resummed on the side of the nucleus.

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arXiv2015

Bose enhancement and the ridge

We point out that Bose enhancement in a hadronic wave function generically leads to correlations between produced particles. We show explicitly, by calculating the projectile density matrix in the Color Glass Condensate approach to high-energy hadronic collisions, that the Bose enhancement of gluons in the projectile leads to azimuthal collimation of long range rapidity correlations of the produced particles, the so-called ridge correlations.

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arXiv2015

Next-to-next-to-eikonal corrections in the CGC

We extend the study of corrections to the eikonal approximation that was initiated in Ref. \cite{Altinoluk:2014oxa} to higher orders. These corrections associated with the finite width of the target are investigated and the gluon propagator in background field is calculated at next-to-next-to-eikonal accuracy. The result is then applied to the single inclusive gluon production cross section at central rapidities and the light-front helicity asymmetry, in pA collisions, in order to analyse these observables beyond the eikonal limit. The next-to-next-to-eikonal corrections to the unpolarized cross section are non-zero and provide the first corrections to the usual $k_\perp$-factorized expression. In contrast, the eikonal and next-to-next-to-eikonal contributions to the helicity asymmetry vanish, while the next-to-eikonal ones are non-zero.

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arXiv2014

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.

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arXiv2014

Next-to-eikonal corrections in the CGC: gluon production and spin asymmetries in pA collisions

We present a new method to systematically include corrections to the eikonal approximation in the background field formalism. Specifically, we calculate the subleading, power-suppressed corrections due to the finite width of the target or the finite energy of the projectile. Such power-suppressed corrections involve Wilson lines decorated by gradients of the background field - thus related to the density - of the target. The method is of generic applicability. As a first example, we study single inclusive gluon production in pA collisions, and various related spin asymmetries, beyond the eikonal accuracy.

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arXiv2014

Improving the kinematics for low-x QCD evolution equations in coordinate space

High-energy evolution equations, such as the BFKL, BK or JIMWLK equations, aim at resumming the high-energy (next-to-)leading logarithms appearing in QCD perturbative series. However, the standard derivations of those equations are performed in a strict high-energy limit, whereas such equations are then applied to scattering processes at large but finite energies. For that reason, there is typically a slight mismatch between the leading logs resummed by those evolution equations without finite-energy corrections and the leading logs actually present in the perturbative expansion of any observable. That mismatch is one of the sources of large corrections at NLO and NLL accuracy. In the case of the BFKL equation in momentum space, that problem is solved by including a kinematical constraint in the kernel, which is the most important finite-energy correction. In this paper, such an improvement of kinematics is performed in mixed-space (transverse positions and $k^+$) and with a factorization scheme in the light-cone momentum $k^+$ (in a frame in which the projectile is right-moving and the target left-moving). This is the usual choice of variables and factorization scheme for the the BK equation. A kinematically improved version of the BK equation is provided, consistent at finite energies. The results presented here are also a necessary step towards having the high energy limit of QCD (including gluon saturation) quantitatively under control beyond strict leading logarithmic accuracy.

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arXiv2013

Gluon saturation at higher orders and improvement of kinematics

Standard perturbative calculations lead to pathologically large NLO corrections to low-$x_{Bj}$ evolution equations like BFKL and BK. Using a more refined treatment of kinematics in mixed-space, relevant when gluon saturation sets on, one obtains an improved version of the BK equation, resumming to all orders the most severe of those large higher order corrections.

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