Search

Using the helicity amplitudes formalism, we study deeply virtual exclusive electron photoproduction off an unpolarized nucleon target, ep→e′p′γ, through a range of kinematics both in the fixed target setting with initial electron energies of 6, 11, and 24 GeV and for an electron ion collider. We reformulate the cross section bringing to the forefront the defining features of the ep→e′p′γ process, where the observables are expressed as bilinear products of the independent helicity amplitudes which completely describe it in terms of the electric, magnetic, and axial currents of the nucleon. These contributions are checked against the Fourier harmonics-based formalism which has provided so far the underlying mathematical framework to study deeply virtual Compton scattering (DVCS) and related experiments. Using theoretical model calculations of the twist-two generalized parton distributions, H, E, ˜H, and ˜E, we uncover large discrepancies between the harmonic series and our proposed framework. Most importantly, these numerical differences appear in the intermediate Q² range which represents a sweet spot for extracting generalized parton distributions from data. We provide a framework that is ideal, on one side, to study and compare the different conventions that can be used to describe the leading order contribution to DVCS in QCD, while on the other it facilitates a quantitative extraction of physically meaningful information from experiment through traceable and controllable approximations in the intermediate Q² region. [ABSTRACT FROM AUTHOR]

The exclusive production of vector mesons and photons in ep collisions is investigated considering three phenomenological saturation models based on distinct assumptions for the treatment of the dipole-hadron scattering amplitude. The latest high precision HERA data for the reduced and vector meson cross sections are used to update the saturation model proposed by Marquet Peschanski, and Soyez, which predicts that the saturation scale is dependent of the squared momentum transfer t. The Marquet-Peschanski-Soyez predictions for the photon virtuality, energy and t dependencies of the exclusive ρ, J/Ψ, and deeply virtual Compton scattering cross sections are presented and a detailed comparison with the results derived using the impact parameter saturation models is performed. Our results indicate that a future experimental analysis of the t distribution dσ/dt for exclusive processes in the kinematical range that will covered by the EIC and LHeC, considering the distinct photon polarizations and large values of t, will be able to discriminate between the distinct approaches for the QCD dynamics at high energies. [ABSTRACT FROM AUTHOR]

Using the approach based on conformal symmetry we calculate the two-loop coefficient function for the axial-vector contributions to two-photon processes in the ̅MS scheme. This is the last missing element for the complete next-to-next-to-leading order (NNLO) calculation of the pion transition form factor γ*γ→π in perturbative QCD. The corresponding high-statistics measurement is planned by the Belle II collaboration and will allow one to put strong constraints on the pion light-cone distribution amplitude. The calculated NNLO corrections prove to be rather large and have to be taken into account. The same coefficient function determines the contribution of the axial-vector generalized parton distributions to deeply virtual Compton scattering (DVCS) which is investigated at the JLAB 12 GeV accelerator, by COMPASS at CERN, and in the future will be studied at the Electron Ion Collider EIC. [ABSTRACT FROM AUTHOR]

We calculate the leading-order QED radiative corrections to the process e-p → e-pl-l+ in the soft-photon approximation, in two different energy regimes which are of relevance to extract nucleon structure information. In the low-energy region, this process is studied to better constrain the hadronic corrections to precision muonic hydrogen spectroscopy. In the high-energy region, the beam-spin asymmetry for double-virtual Compton scattering allows us to directly access the generalized parton distributions. We find that the soft-photon radiative corrections have a large impact on the cross sections and are therefore of paramount importance to extract the nucleon structure information from this process. For the forward-backward asymmetry, the radiative corrections are found to affect the asymmetry only around or below the 1% level, whereas the beam-spin asymmetry is not affected at all in the soft-photon approximation, which makes them gold-plated observables to extract nucleon structure information in both the low- and high-energy regimes. [ABSTRACT FROM AUTHOR]

We investigate a simple toy model of particle scattering in the flat spacetime limit of an analogue-gravity model. The analogue-gravity medium is treated as a scalar field of phonons that obeys the Klein-Gordon equation and thus admits a Lorentz symmetry with respect to cs, the speed of sound in the medium. The particle from which the phonons are scattered is external to the system and does not obey the sonic Lorentz symmetry that the phonon field obeys. In-universe observers who use the exchange of sound to operationally measure distance and duration find that the external particle appears to be a sonically Lorentz-violating particle. By performing a sonic analogue to Compton scattering, in-universe observers can determine if they are in motion with respect to their medium. If in-universe observers were then to correctly postulate the dispersion relation of the external particle, their velocity with respect to the medium could be found. [ABSTRACT FROM AUTHOR]

The gamma factory is a proposal to back-scatter laser photons off a beam of partially-stripped ions at the LHC, producing a beam of ∼10 MeV to 1 GeV photons with intensities of 1016 to 1018 s-1. This implies ∼1023 to 1025 photons on target per year, many orders of magnitude greater than existing accelerator light sources and also far greater than all current and planned electron and proton fixed target experiments. We determine the gamma factory's discovery potential through "dark Compton scattering," γe→eX, where X is a new, weakly-interacting particle. For dark photons and other new gauge bosons with masses in the 1 to 100 MeV range, the gamma factory has the potential to discover extremely weakly-interacting particles with just a few hours of data and will probe couplings as low as ∼10-9 with a year of running. The gamma factory therefore may probe couplings lower than all other terrestrial experiments and is highly complementary to astrophysical probes. We outline the requirements of an experiment to realize this potential and determine the sensitivity reach for various experimental configurations. [ABSTRACT FROM AUTHOR]

Volkov states are exact solutions of the Dirac equation in the presence of an arbitrary plane wave. Accounting for the interaction between the Dirac field and the electromagnetic field, however, Volkov states, as well as free photon states, are not stable in the presence of the background plane-wave field but "decay" as electrons/positrons can emit photons and photons can transform into electron-positron pairs. By using the solutions of the corresponding Schwinger-Dyson equations within the locally constant field approximation, we compute the probabilities of nonlinear single Compton scattering and nonlinear Breit-Wheeler pair production by including the effects of the decay of electron, positron, and photon states. As a result, we find that the probabilities of these processes can be expressed as the integral over the light-cone time of the known probabilities valid for stable states per unit of light-cone time times a light-cone time-dependent exponential damping function for each interacting particle. The exponential function for an incoming (outgoing) either electron/positron or photon at each light-cone time corresponds to the total probability that either the electron/positron emits a photon via nonlinear Compton scattering or the photon transforms into an electron-positron pair via nonlinear Breit-Wheeler pair production until that light-cone time (from that light-cone time on). It is interesting that the exponential damping terms depend not only on the particles momentum but also on their spin (for electrons/positrons) and polarization (for photons). This additional dependence on the discrete quantum numbers prevents the application of the standard electron/positron spin and photon polarization sum rules in computing, for example, total probabilities. [ABSTRACT FROM AUTHOR]

We present a machine learning based approach to the cross section and asymmetries for deeply virtual Compton scattering from an unpolarized proton target using both an unpolarized and polarized electron beam. Machine learning methods are needed to study and eventually interpret the outcome of deeply virtual exclusive experiments since these reactions are characterized by a complex final state with a larger number of kinematic variables and observables, exponentially increasing the difficulty of quantitative analyses. Our deep neural network (FemtoNet) uncovers emergent features in the data and learns an accurate approximation of the cross section that outperforms standard baselines. FemtoNet reveals that the predictions in the unpolarized case systematically show a smaller relative median error than the polarized that can be ascribed to the presence of the Bethe-Heitler process. It also suggests that the t dependence can be more easily extrapolated than for the other variables, namely the skewness, ξ and four-momentum transfer, Q². Our approach is fully scalable and will be capable of handling larger datasets as they are released from future experiments. [ABSTRACT FROM AUTHOR]

We calculate the rate of collisional decay of the axial charge in an ultrarelativistic electron-positron plasma, also known as the chirality flipping rate. We find that contrary to the existing estimates, the chirality flipping rate appears already in the first order in the fine-structure constant α and is therefore orders of magnitude greater than previously believed. The main channels for the rapid relaxation of the axial charge are the collinear emission of a weakly damped photon and the Compton scattering. The latter contributes to the O(α) result because of the infrared divergence in its cross section, which is regularized on the soft scale ∼eT due to the thermal corrections. Our results are important for the description of the early Universe processes (such as leptogenesis or magnetogenesis) that affect differently left- and right-chiral fermions of the Standard Model, as discussed in more details in the companion Letter. [ABSTRACT FROM AUTHOR]