arXiv subjects

Bernard Pire

Publications and source records attributed to Bernard Pire.

At least 37 records · Page 2Linked to original sources

arXiv2022

Accessing GPDs through the exclusive photoproduction of a $ γ$-meson pair

We consider the exclusive photo-production of a photon-meson pair with a large invariant mass, working in the QCD factorisation framework. Explicitly, we consider a $ ρ$-meson or a charged $ π$ in the final state. This process gives access both to chiral-even GPDs and chiral-odd GPDs, which are not well-known experimentally, especially the latter ones. The computation is performed at leading order and leading twist. We discuss the prospects of measuring them in experiments, focusing on the kinematics at the JLab 12-GeV experiment, and pPb ultra-peripheral collisions at LHC. In particular, the latter gives access to the small $ ξ$ regime of GPDs.

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arXiv2022

Kinematical higher-twist corrections in $γ^* γ\to ππ$

We apply the Braun-Manashov technique to improve the description of $γ^*(q_1) γ(q_2) \to M(p_1) M(p_2)$ amplitudes at large $Q^2=-q_1^2$ and small $s=(q_1+q_2)^2$. We derive the kinematical higher-twist contributions of order $s/Q^2$ and $m^2/Q^2$ to the helicity amplitudes and estimate their sizes in the kinematics accessible at Belle and Belle II.Since pion GPDs cannot be directly measured by experiment, $ππ$ GDAs are the best way to investigate the energy-momentum tensor form factors for pions.

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arXiv2022

Backward timelike Compton scattering to decipher the photon content of the nucleon

The exclusive photoproduction off nucleon of a large invariant mass lepton pair in the backward region specified by the small Mandelstam variable $-u$ is discussed in the framework of collinear QCD factorization. The amplitude is factorized in terms of photon-to-nucleon Transition Distribution Amplitudes (TDAs) which encode the photon content of the nucleon. Model estimates of these new non-perturbative objects are described and the corresponding cross sections calculated. The background due to the electromagnetic Bethe-Heitler process is shown to be negligible in the kinematical regime of interest.

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arXiv2022

Phenomenology of diphoton photoproduction at next-to-leading order

We develop the analysis of diphoton exclusive photoproduction in the kinematics where a collinear QCD factorization framework applies, namely nearly forward large invariant mass diphoton production. We work at the leading twist level and at the next-to-leading order (NLO) in the strong coupling constant $α_S$. We compare our predictions for cross-sections with Born order calculations for the experimental conditions accessible to JLab experiments and show the interesting sensitivity of our results to various models of generalized parton distributions (GPDs). The NLO corrections are rather large and negative but do not prevent the studied reaction from being a promising tool for the extraction of C-dd GPDs, which do not contribute to either spacelike or timelike deeply virtual Compton scattering amplitudes.

hep-ph↗

arXiv2021

Collinear factorization of diphoton photoproduction at next to leading order

We calculate in the framework of collinear factorization the amplitude for the photoproduction of a near forward large mass diphoton at leading twist and next to leading order (NLO) in $α_s$. We demonstrate the validity of factorization at this order, which was never achieved for such a reaction where the coefficient function describes a $2 \to 3$ hard process. While the Born order amplitude was purely imaginary and only probed the $x=\pm ξ$ cross-over line of generalized parton distributions (GPD) domain, the NLO result contains both a real and an imaginary part and probes the whole domain of definition of quark GPDs. The phenomenology of our results for medium (JLab) and higher energy (EIC) experiments will be developed in a future study.

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arXiv2021

NLO collinear factorization of large mass diphoton photoproduction amplitude

We calculate large mass diphoton exclusive photoproduction in the framework of collinear QCD factorization at next to leading order in αs and at leading twist. Collinear divergences of the coefficient function are absorbed by the evolution of the generalized parton distributions (GPDs). This result enlarges the existing factorization proofs to 2 -> 3 processes, opening new reactions to a trustable extraction of GPDs.

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arXiv2021

Diffractive deeply virtual Compton scattering

Diffractive deeply virtual Compton scattering (DiDVCS) is the process $γ^*(- Q^2) + N \rightarrow ρ^0 + γ^* (Q'^2)+ N'$, where N is a nucleon or light nucleus, in the kinematical regime of large rapidity gap between the $ρ^0$ and the final photon-nucleus system, and in the generalized Bjorken regime where both photon virtualities $Q^2$ and $ Q'^2$ are large. We show that this process has the unique virtue of combining the large diffractive cross sections at high energy with the tomographic ability of deeply virtual Compton scattering to scrutinize the quark and gluon content of nucleons and light nuclei. Its study at an electron-ion collider would enlighten the internal structure of hadrons.

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arXiv2020

New opportunities at the photon energy frontier

Ultra-peripheral collisions (UPCs) involving heavy ions and protons are the energy frontier for photon-mediated interactions. UPC photons can be used for many purposes, including probing low-$x$ gluons via photoproduction of dijets and vector mesons, probes of beyond-standard-model processes, such as those enabled by light-by-light scattering, and studies of two-photon production of the Higgs.

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arXiv2020

Probing Nucleons and Nuclei in High Energy Collisions

This volume is a collection of contributions for the 7-week program "Probing Nucleons and Nuclei in High Energy Collisions" that was held at the Institute for Nuclear Theory in Seattle, WA, USA, from October 1 until November 16, 2018. The program was dedicated to the physics of the Electron Ion Collider (EIC), the world's first polarized electron-nucleon (ep) and electron-nucleus (eA) collider to be constructed in the USA. These proceedings are organized by chapters, corresponding to the weeks of the program: Week I, Generalized parton distributions; Week II, Transverse spin and TMDs; Week III, Longitudinal spin; Week IV, Symposium week; Weeks V & VI, eA collisions; Week VII, pA and AA collisions. We hope these proceedings will be useful to readers as a compilation of EIC-related science at the end of the second decade of the XXI century.

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arXiv2019

Nucleon-to-meson transition distribution amplitudes in impact parameter space

Recent analyses of backward meson electroproduction support the validity of a collinear QCD factorization framework for these hard exclusive reactions. This opens a way to the extraction of nucleon-to-meson transition distribution amplitudes (TDAs) from the experimental data. Similarly to the generalized parton distributions, TDAs - after the Fourier transform in the transverse plane - carry valuable information on the transverse location of hadron constituents. We address the properties of integrated nucleon-to-meson TDAs in the impact parameter representation. We argue that the emerging picture provides an intuitive interpretation for the hadron structural information contained in nucleon-to-meson TDAs and allows to study diquark-quark contents of fast moving hadrons in the transverse plane.

hep-ph↗

arXiv2012

Timelike Compton Scattering - New Theoretical Results and Experimental Possibilities

We review recent progress in the study of timelike Compton scattering (TCS), the crossed process of deeply virtual Compton scattering. We emphasize the need to include NLO corrections to any phenomenological program to extract Generalized Parton Distributions (GPDs) from near future experimental data. We point out that TCS at high energy should be available through a study of ultraperipheral collisions at RHIC and LHC, opening a window on quark and gluon GPDs at very small skewness.

hep-ph↗

arXiv2011

πN transition distribution amplitudes: Their symmetries and constraints from chiral dynamics

Baryon to meson transition distribution amplitudes (TDAs) extend the concept of generalized parton distributions. Baryon to meson TDAs appear as building blocks in the collinear factorized description of amplitudes for a class of hard exclusive reactions, prominent examples of which being hard exclusive meson electroproduction off a nucleon in the backward region and baryon-antibaryon annihilation into a meson and a lepton pair. We study the general properties of these objects following from the underlying symmetries of QCD. In particular, the Lorentz symmetry results in the polynomiality property of the Mellin moments in longitudinal momentum fractions. We present a detailed account of the isotopic and permutation symmetry properties of nucleon to pion (πN) TDAs. This restricts the number of independent leading twist πN TDAs to eight functions, providing description of all isotopic channels. Using chiral symmetry and the crossing relation between πN TDAs and πN generalized distribution amplitudes we establish soft pion theorems for πN TDAs, which determine the magnitude of πN TDAs. Finally, we build a simple resonance exchange model for πN TDAs considering N and Δ(1232) exchange contributions into the isospin-1/2 and isospin-3/2 πN TDAs.

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arXiv2010

Baryon to meson transition distribution amplitudes and their spectral representation

We consider the problem of construction of a spectral representation for nucleon to meson transition distribution amplitudes (TDAs), non-diagonal matrix elements of nonlocal three quark light-cone operators between a nucleon and a meson states. We introduce the notion of quadruple distributions and generalize Radyshkin's factorized Ansatz for this issue. Modelling of baryon to meson TDAs in the complete domain of their definition opens the way to quantitative estimates of cross-sections for various hard exclusive reactions.

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arXiv2010

A spectral representation for baryon to meson transition distribution amplitudes

We construct a spectral representation for the baron to meson transition distribution amplitudes (TDAs), i.e. matrix elements involving three quark correlators which arise in the description of baryon to meson transitions within the factorization approach to hard exclusive reactions. We generalize for these quantities the notion of double distributions introduced in the context of generalized parton distributions. We propose the generalization of A.Radyushkin's factorized Ansatz for the case of baryon to meson TDAs. Our construction opens the way to modeling of baryon to meson TDAs in their complete domain of definition and quantitative estimates of cross-sections for various hard exclusive reactions.

hep-ph↗

arXiv2009

Looking for the Odderon in photon collisions

We discuss the production of two pion pairs in photon collisions at high energies. We calculate the according matrix elements in k_T-factorization and discuss the possibility to reveal the existence of the perturbative Odderon by charge asymmetries.

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arXiv2002

Femto-Photography of Protons to Nuclei with Deeply Virtual Compton Scattering

Developments in deeply virtual Compton scattering allow the direct measurements of scattering amplitudes for exchange of a highly virtual photon with fine spatial resolution. Real-space images of the target can be obtained from this information. Spatial resolution is determined by the momentum transfer rather than the wavelength of the detected photon. Quantum photographs of the proton, nuclei, and other elementary particles with resolution on the scale of a fraction of a femtometer is feasible with existing experimental technology.

hep-ph↗

arXiv2000

gamma* gamma -> pi pi at large Q^2

The QCD analysis of the process gamma* gamma -> pi pi at large Q^2 and small center-of-mass energy allows one to access a new hadronic observable describing the exclusive transition from a q qbar or g g state to a pair of mesons. A fruitful study may be envisaged at existing machines.

hep-ph↗

arXiv2000

Exclusive production of pion pairs in gamma^* gamma collisions at large Q^2

We perform a QCD analysis of the exclusive production of two mesons in gamma^* gamma collisions in the kinematical domain of large photon virtuality Q and small hadronic invariant mass W. This reaction is dominated by a scale invariant mechanism which factorizes into a perturbative subprocess, gamma^* gamma --> q qbar or gamma^* gamma --> g g, and a generalized two-meson distribution amplitude. We develop in detail the phenomenology of this process at e^+ e^- colliders. Using a simple model for the two-pion distribution amplitude, based on its general properties, we estimate the cross section for the kinematics accessible at BABAR, BELLE, CLEO and LEP.

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