Timelike Compton Scattering Research Articles

Investigating the timelike Compton scattering in photon–proton interactions at the EIC

Information about the three-dimensional description of the quark and gluon content of hadrons, described by the generalized parton distributions (GPDs), can be probed by exclusive processes in electron–proton (ep) collisions. In this letter, we investigate the timelike Compton scattering (TCS) in ep collisions at the future electron-ion collider (EIC). Such process is characterized by the exclusive dilepton production through the subprocess γp→γ∗p→l+l-p\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\gamma p \\rightarrow \\gamma ^*p \\rightarrow l^+ l^- p$$\\end{document}, with the real photon in the initial state being emitted by the incoming electron. Assuming a given model for the GPDs, the TCS differential cross-section is estimated, as well the contribution associated to the interference between the TCS and Bethe–Heitler (BH) amplitudes. Predictions for the TCS, BH and interference contributions are presented considering the kinematical range expected to be covered by the EIC detectors. Moreover, the polarized photon asymmetry is also studied. Our results indicated that a future experimental analysis, considering photon circular polarizations, can be useful to probe the interference contribution and constrain the description of the GPDs for the proton.

Exclusive dilepton production in photon-induced interactions at RHIC

The exclusive dilepton photoproduction in pp and pAu collisions at RHIC is investigated considering the contribution of the Bethe–Heitler (BH) and the timelike Compton scattering (TCS) processes. The TCS amplitude is estimated at next-to-leading-order of the strong running coupling constant alpha _s and assuming the Goloskokov–Kroll model for the quark and gluon generalized parton distributions. The TCS differential cross section is estimated as well the contribution associated to the interference between the TCS and BH amplitudes. Predictions for the TCS, BH and interference contributions are presented considering the kinematical range covered by the RHIC detectors.

Timelike Compton scattering in ultraperipheral pPb collisions

The study of exclusive processes in photon-induced interactions allows us to probe the generalized parton distributions (GPDs), which provide information about the 3-dimensional description of the quark and gluon content of hadrons. In this paper we investigate the timelike Compton scattering (TCS) in ultraperipheral pPb collisions at the LHC. Such process is characterized by the exclusive dilepton production through the subprocess γp→γ⁎p→l+l−p, with the real photon in the initial state being emitted by the nucleus. Assuming a given model for the GPDs, the TCS differential cross section is estimated as well the contribution associated to the interference between the TCS and Bethe–Heitler (BH) amplitudes. Predictions for the TCS, BH and interference contributions are presented considering the kinematical range covered by the LHC detectors.

What do DVCS data tell us about TCS observables?

Deeply virtual Compton scattering (DVCS) and timelike Compton scattering (TCS) leading twist amplitudes are intimately related thanks to their analytic properties as a function of Q^2Q2. We exploit this feature to use Compton form factors previously extracted from available DVCS data and derive data-driven predictions for TCS observables to be measured in near future experiments. Our results quantitatively illustrate the complementarity of DVCS and TCS experiments.

First Measurement of Timelike Compton Scattering.

We present the first measurement of the timelike Compton scattering process, γp→p^{'}γ^{*}(γ^{*}→e^{+}e^{-}), obtained with the CLAS12 detector at Jefferson Lab. The photon beam polarization and the decay lepton angular asymmetries are reported in the range of timelike photon virtualities 2.25<Q^{'2}<9 GeV^{2}, squared momentum transferred 0.1<-t<0.8 GeV^{2}, and average total center-of-mass energy squared s=14.5 GeV^{2}. The photon beam polarization asymmetry, similar to the beam-spin asymmetry in deep virtual Compton scattering, is sensitive to the imaginary part of the Compton form factors and provides a way to test the universality of the generalized parton distributions. The angular asymmetry of the decay leptons accesses the real part of the Compton form factors and thus the D-term in the parametrization of the generalized parton distributions.

Exclusive dilepton production via timelike Compton scattering in heavy ion collisions

In the present work, $k_T$-factorization formalism is applied to compute the exclusive dilepton production by timelike Compton scattering (TCS) in $eA$, $pA$ and $AA$ collisions. The nuclear effects are investigated considering heavy and light ions. The production cross section in terms of invariant mass and rapidity distribution of the lepton pair is shown. The analysis is done for electron-ion collisions at the Large Hadron-Electron Collider (LHeC), its high-energy upgrade (HE-LHeC) and at the Future Circular Collider (FCC) in lepton-hadron mode. Additionally, ultraperipheral heavy ion collisions at future runs of the Large Hadron Collider (LHC) and at the FCC (hadron-hadron mode) are also considered.

Leading-order QED radiative corrections to timelike Compton scattering on the proton

We evaluate the leading-order QED radiative corrections to the timelike Compton scattering (TCS) process $\gamma p \to l^- l^+ p$. We study these corrections in two energy regimes using different models for the TCS amplitude. In the low-energy regime we calculate the contribution due to the proton and its lowest-energy excitation, the $\Delta(1232)$ resonance. In the high-energy near-forward kinematical regime we calculate the TCS amplitude in a handbag approach in terms of Generalized Parton Distributions (GPDs). On the level of cross sections we find the QED radiative corrections to be in the $5 -10\%$ range in the low-energy regime and around $20\%$ in the high-energy regime. We show that in both the di-lepton forward-backward asymmetry as well as in the photon beam helicity asymmetry these corrections nearly cancel out, making them gold-plated observables to extract the real and imaginary parts of the TCS amplitude. We demonstrate in particular the sensitivity of these asymmetries on GPD parameterizations for a recent CLAS12@JLab TCS experiment.

Timelike Compton Scattering with CLAS12 at Jefferson Lab

Generalized Parton Distributions (GPDs) describe the correlations between the longitudinal momentum and the transverse position of the partons inside the nucleon. They are nowadays the subject of an intense effort of research, in the perspective of understanding nucleon structure. GPDs have been studied mainly using Deeply Virtual Compton Scattering (DVCS, ep → e’p’γ). Here we highlight the measurement of the time-reversal conjugate process of DVCS, Timelike Compton Scattering (TCS) using data taken by CLAS12. The experimental measurement of the TCS angular asymmetry will provide new information on the real part of GPDs. This proceeding assesses the current status of the TCS analysis and presents preliminary results based on CLAS12 data.

Dilepton production through timelike Compton scattering within the kT -factorization approach

In this work we consider the dilepton production via timelike Compton scattering (TCS) in electron-proton and proton-proton collisions. In particular, the differential cross section in terms of the dilepton invariant mass and rapidity is computed within the $k_T$-factorization approach. Besides, we utilize distinct unintegrated gluon distributions (UGD) in order to compare their impact on the differential cross section of TCS in $pp$ ($ep$) collisions evaluated at the LHC (LHeC), HL-LHC (LHeC), HE-LHC (LHeC) and FCC-hh (eh) center-of-mass energies.

Data-driven study of timelike Compton scattering

In the framework of collinear QCD factorization, the leading twist scattering amplitudes for deeply virtual Compton scattering (DVCS) and timelike Compton scattering (TCS) are intimately related thanks to analytic properties of leading and next-to-leading order amplitudes. We exploit this welcome feature to make data-driven predictions for TCS observables to be measured in near future experiments. Using a recent extraction of DVCS Compton form factors from most of the existing experimental data for that process, we derive TCS amplitudes and calculate TCS observables only assuming leading-twist dominance. Artificial neural network techniques are used for an essential reduction of model dependency, while a careful propagation of experimental uncertainties is achieved with replica methods. Our analysis allows for stringent tests of the leading twist dominance of DVCS and TCS amplitudes. Moreover, this study helps to understand quantitatively the complementarity of DVCS and TCS measurements to test the universality of generalized parton distributions, which is crucial e.g. to perform the nucleon tomography.

Timelike Compton Scattering off the nucleon: observables and experimental perspectives for JLab at 12 GeV

Hard exclusive processes such as photoproduction or electroproduction of photon or meson off the nucleon provide access to the Generalized Parton Distributions (GPDs), in the regime where the scattering amplitude is factorized into a hard and a soft part. GPDs contain the correlation between the longitudinal momentum fraction and the transverse spatial densities of quarks and gluons in the nucleon. Timelike Compton Scattering (TCS) correspond to the reaction γN → γ*N → e + e − N , where the photon is scattered off a quark. It is measured through its interference with the associated Bethe-Heitler process, which has the same final state. TCS allows to access the GPDs and test their universality by comparison to the results obtained with the DVCS process ( eN → eγN ). Also, results obtained with TCS provide additional independent constrains to the GPDs parameterization.We will present the physical motivations for TCS, with our theoretical predictions for TCS observables and their dependencies. We calculated for JLab 12 GeV energies all the single and double beam and/or target polarization observables off the proton and off the neutron. We will also present the experimental perspectives for the next years at JLab. Two proposals were already accepted at JLab: in Hall B, with the CLAS12 spectrometer, in order to measure the unpolarized cross section and in Hall A, with the SoLID spectrometer, in order to measure the unpolarized cross section and the beam spin asymmetry at high intensity. A Letter Of Intent was also submitted in order to measure the transverse target spin asymmetries in Hall C. We will discuss the merits of this different experiments and present some of the expected results.

Timelike Compton scattering off the proton and generalized parton distributions

We study the exclusive photoproduction of an electron-positron pair on a neutron target in the Jefferson Lab energy domain. The reaction consists of two processes: the Bethe-Heitler and the Timelike Compton Scattering. The latter process provides potentially access to the Generalized Parton Distributions (GPDs) of the nucleon. We calculate all the unpolarized, single- and double-spin observables of the reaction and study their sensitivities to GPDs.

Timelike Compton Scattering at JLab

It is demonstrated, that with exclusive final state, data from electron scattering experiments that are recorded with loose trigger requirements can be used to analyze photoproduction reactions. A preliminary results on Timelike Compton Scattering using the electroproduction data from the CLAS detector at Jefferson Lab are presented. In particular, using final state (pe-e+) photoproduction of vector mesons and timelike photon is studied. Angular asymmetries in Timelike Compton Scattering region is compared with model predictions in the framework of Generalized Parton Distribution.

On Timelike Compton Scattering at Medium and High Energies

We emphasize the complementarity of timelike and spacelike studies of deep exclusive processes, taking as an example the case of timelike Compton Scattering (TCS) i.e. the exclusive photoproduction of a lepton pair with large invariant mass, vs deeply virtual Compton scattering (DVCS) i.e. the exclusive leptoproduction of a real photon. Both amplitudes factorize with the same generalized parton distributions (GPDs) as their soft parts and coefficient functions which differ significantly at next to leading order in alpha_s. We also stress that data on TCS at very high energy should be available soon thanks to the study of ultraperipheral collisions at the LHC, opening a window on quark and gluon GPDs at very small skewness.