First Measurement of Timelike Compton Scattering

Area of Science:

• Nuclear Physics
• Particle Physics
• Quantum Chromodynamics

Background:

• Generalized Parton Distributions (GPDs) are crucial for understanding the structure of the nucleon.
• Timelike Compton Scattering (TCS) provides a unique probe of GPDs, complementing spacelike experiments.
• Previous experimental data on TCS were limited, hindering detailed studies of GPDs.

Purpose of the Study:

• To present the first measurement of the timelike Compton scattering (TCS) process: γp→p^{'}γ^{*}(γ^{*}→e^{+}e^{-}).
• To measure the photon beam polarization asymmetry and decay lepton angular asymmetries in TCS.
• To probe the imaginary and real parts of Compton form factors and test GPD universality.

Main Methods:

• Utilized the CLAS12 detector at Jefferson Lab for data acquisition.
• Employed a high-intensity photon beam incident on a proton target.
• Analyzed the decay products of the virtual photon (e^{+}e^{-}) to reconstruct the TCS process.

Main Results:

• Reported measurements of TCS in the range of timelike photon virtualities 2.25
• Presented the photon beam polarization asymmetry, sensitive to the imaginary part of Compton form factors.
• Measured the decay lepton angular asymmetry, accessing the real part of Compton form factors.

Conclusions:

• The results provide crucial data for constraining Generalized Parton Distributions (GPDs).
• The measurements offer a new avenue to test the universality of GPDs.
• This study advances our understanding of the nucleon's internal structure through the TCS process.