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B. Pire

Publications and source records attributed to B. Pire.

At least 127 records · Page 7Linked to original sources
arXiv2009

QCD factorization of exclusive processes beyond leading twist: gamma*T -> rhoT impact factor with twist three accuracy

We describe a consistent approach to factorization of scattering amplitudes for exclusive processes beyond the leading twist approximation. The method involves the Taylor expansion of the scattering amplitude in the momentum space around the dominant light-cone direction and thus naturally introduces an appropriate set of non-perturbative correlators which encode effects not only of the lowest but also of the higher Fock states of the produced particle. The reduction of original set of correlators to a set of independent ones is achieved with the help of equations of motion and invariance of the scattering amplitude under rotation on the light-cone. We compare the proposed method with the covariant method formulated in the coordinate space, based on the operator product expansion. We prove the equivalence of two proposed parametrizations of the rhoT distribution amplitudes. As a concrete application, we compute the expressions of the impact factor for the transition of virtual photon to transversally polarised rho-meson up to the twist 3 accuracy within these two quite differents methods and show that they are identical.

hep-ph↗
arXiv2009

On the description of exclusive processes beyond the leading twist approximation

We describe hard exclusive processes beyond the leading twist approximation in a framework based on the Taylor expansion of the amplitude around the dominant light-cone directions. This naturally introduces an appropriate set of non-perturbative correlators whose number is minimalized after taking into account QCD equations of motion and the invariance under rotation on the light-cone. We examplify this method at the twist 3 level and show that the coordinate and momentum space descriptions are fully equivalent.

hep-ph↗
arXiv2009

Transversity Generalized Parton Distributions from gamma N -> pi rhot N' with a Large pi rhot Invariant Mass

The chiral-odd transversity generalized parton distributions of the nucleon can be accessed experimentally through the exclusive photoproduction process gamma N -> pi rhoT N', with or without beam and target polarization, provided the vector meson is produced in a transversely polarized state. The kinematical domain of factorization is defined through a large invariant mass of the meson pair and a small transverse momentum of the final nucleon. We calculate perturbatively the scattering amplitude at leading order in alpha_s and build a simple model for the dominant transversity GPD H_T(x, xi, t) based on the concept of double distribution. Counting rate estimates are in progress.

hep-ph↗
arXiv2009

Hard diffractive processes and non-perturbative matrix elements beyond leading twist: rhoT-meson production

Within the kt-factorization approach, we evaluate the impact factor of the transition gamma* -> rhoT taking into account the twist 3 contributions. We show that a gauge invariant expression is obtained with the help of QCD equations of motion. More generally, relying on these equations and on the invariance under rotation on the light-cone of the factorized amplitude, the non-perturbative Distribution Amplitudes can be reduced to a minimal set. This opens the way to a consistent treatment of factorization for exclusive processes with a transversally polarized vector meson. We prove the equivalence of two proposed parametrizations of the rhoT distribution amplitudes.

hep-ph↗
arXiv2009

Pomeron Odderon interference in production of two pi+ pi- pairs at LHC and ILC

We propose to look for the Odderon through the production of two pion pairs in photon collisions at high energies. We calculate the corresponding matrix elements in kT-factorization and discuss the possibility to reveal the existence of the perturbative Odderon by charge asymmetries, relying on models for the generalized distribution amplitudes of pi+ pi-. The application of this strategy to ultraperipheral collisions at the LHC suffers from the difficulty to trigger on interesting events and is plagued with severe background problems in p-p mode. Electron - positron colliders like ILC seem to better suit this physics.

hep-ph↗
arXiv2009

Probing non-perturbative QCD through hadronic matrix elements extracted from exclusive hard processes

QCD is the theory of strong interactions and non-perturbative methods have been developed to address the confinement property of QCD. Many experimental measurements probe the confining dynamics, and it is well-known that hard scattering processes allow the extraction of non perturbative hadronic matrix elements. To study exclusive hard processes, such as electromagnetic form factors and reactions like gamma* N -> gamma N', gamma* N -> pi N', gamma* gamma -> pi pi, antiproton proton ->gamma* pi in particular kinematics (named as generalized Bjorken regime), one introduces specific non-perturbative objects, namely generalized parton distributions (GPDs), distribution amplitudes (DA) and transition distribution amplitudes (TDA), which are Fourier transformed non-diagonal matrix elements of non-local operators on the light-cone. We review here a selected sample of exclusive amplitudes in which the quark and gluon content of hadrons is probed, and emphasize that much remains to be done to successfully compute their non-perturbative parts. We present some difficulties with respect to the application of the much publicized AdS-QCD approach to the calculation of these partonic quantities.

hep-ph↗
arXiv2009

Probing the nucleon's transversity and the photon's distribution amplitude in lepton pair photoproduction

We describe a new way to access the chiral odd transversity parton distribution in the proton through the photoproduction of lepton pairs. The basic ingredient is the interference of the usual Bethe Heitler or Drell-Yan amplitudes with the amplitude of a process, where the photon couples to quarks through its chiral-odd distribution amplitude, which is normalized to the magnetic susceptibility of the QCD vacuum. A promising phenomenology of single and double spin observables emerges from the unusual features of this amplitude.

hep-ph↗
arXiv2009

On the physical Relevance of the Study of gamma* gamma -> pi0 pi0 at small t and large Q2

We discuss the relevance of a dedicated measurement of exclusive production of a pair of neutral pions in a hard gamma* gamma scattering at small momentum transfer. In this case, the virtuality of one photon provides us with a hard scale in the process, enabling us to perform a QCD calculation of this reaction rate using the concept of Transition Distribution Amplitudes (TDA). Those are related by sum rules to the pion axial form factor F^pi_A and, as a direct consequence, a cross-section measurement of this process at intense beam electron-positron colliders such as CLEO, KEK-B and PEP-II, or Super-B would provide us with a unique measurement of the neutral pion axial form factor F^pi0_A at small scale.

hep-ph↗
arXiv2009

Pomeron Odderon interference in production of pi+ pi- pairs at LHC

We discuss the production of two pion pairs in photon collisions at high energies as it can take place in ultraperipheral collisions at hadron colliders such as the LHC. We calculate the according matrix elements in kT factorization and discuss the possibility to reveal the existence of the perturbative Odderon by charge asymmetries.

hep-ph↗
arXiv2009

Timelike Compton Scattering in Ultraperipheral Collisions

Exclusive photoproduction of dileptons, gamma N ->l+l- N, is and will be measured in ultraperipheral collisions at hadron colliders. We demonstrate that the timelike deeply virtual Compton scattering (TCS) mechanism gamma q -> l+l- q, where the lepton pair comes from the subprocess gamma q -> gamma* q, dominates in some accessible kinematical regions, thus opening a new way to study generalized parton distributions (GPD) in the nucleon at small skewedness.

hep-ph↗
arXiv2009

Timelike Compton Scattering at LHC

Exclusive photoproduction of dileptons, gamma N ->l+l- N, is and will be measured in ultraperipheralcollisions at hadron colliders. We demonstrate that the timelike deeply virtual Compton scattering (TCS) mechanism gamma q -> l+l- q, where the lepton pair comes from the subprocess gamma q -> gamma* q, dominates in some accessible kinematical regions, thus opening a new way to study generalized parton distributions (GPD) in the nucleon at small skewedness.

hep-ph↗
arXiv2009

Probing photon structure in DVCS on a photon target

The factorization of the amplitude for the deeply virtual Compton scattering (DVCS) process gamma*(Q) gamma -> gamma gamma at high Q2 is demonstrated in two distinct kinematical domains, allowing to define the photon generalized parton distributions and the diphoton generalized distribution amplitudes. Both these quantities exhibit an anomalous scaling behaviour and obey new inhomogeneous QCD evolution equations.

hep-ph↗
arXiv2008

gamma* -> rhoT impact factor with twist three accuracy

We evaluate the impact factor of the transition gamma* -> rhoT taking into account the twist 3 contributions. We show that a gauge invariant expression is obtained with the help of QCD equations of motion. Our results are free of end-point singularities. This opens the way to a consistent treatment of factorization for exclusive processes with a transversally polarized vector meson.

hep-ph↗
arXiv2008

On AdS/QCD correspondence and the partonic picture of deep inelastic scattering

We critically examine the question of scaling of the Deep Inelastic Scattering process in the medium Bjorken x region on a scalar boson in the framework of the AdS/QCD correspondence. To get the right polarization structure of the forward electroproduction amplitude, we show that one needs to add (at least) the scalar to scalar and scalar to vector hadronic amplitudes. This illustrates how the partonic picture may emerge from a simple scenario based on the AdS/QCD correspondence, provided one allows the conformal dimension of the hadronic field to equal 1 and use the concept of "hadron - parton duality" .

hep-ph↗
arXiv2008

Can one measure timelike Compton scattering at LHC ?

Exclusive photoproduction of dileptons, gamma N->e+e-N, is and will be measured in ultraperipheral collisions at hadron colliders, such as the Tevatron, RHIC and the LHC . We demonstrate that the timelike deeply virtual Compton scattering (TCS) mechanism gamma q -> e+e- q where the lepton pair comes from the subprocess gamma q -> gamma* q dominates in some accessible kinematical regions, thus opening a new way to study generalized parton distributions (GPD) in the nucleon. High energy kinematics enables to probe parton distributions at small skewedness. This subprocess interferes at the amplitude level with the pure QED subprocess gamma gamma* -> e+e- where the virtual photon is radiated from the nucleon.

hep-ph↗
arXiv2008

Hard Pomeron-Odderon interference effects in the production of pi^+pi^- pairs in high energy gamma-gamma collisions at the LHC

We estimate the production of two meson pairs in high energy photon photon collisions produced in ultraperipheral collisions at LHC. We show that the study of charge asymmetries may reveal the existence of the perturbative Odderon and discuss the concrete event rates expected at the LHC. Sizable rates and asymmetries are expected in the case of proton-proton collisions and medium values of gamma-gamma energies \sqrt{s_{γγ}} \approx 20GeV. Proton-proton collisions will benefit from a high rate due to a large effective gamma-gamma luminosity and ion-ion collisions with a somewhat lower rate from the possibility to trigger on ultraperipheral collisions and a reduced background from strong interactions.

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