What holds the world together at the most fundamental level? How is 99% of the visible mass generated? The answer to these questions lies in the complex, strong interactions of quarks and gluons inside the protons and neutrons that make up the world around us.
The exploration of the three-dimensional partonic structure of the nucleon is one of the central aspects of the US medium energy nuclear physics program. One of the historically most successful experimental techniques to probe the quark-gluon substructure of the nucleon is to scatter relativistic electrons off it at high enough energies that they probe the individual, point-like, partons. The identity, distribution and polarization of hadrons detected in the final state can then be used to extract the quantum numbers of the initial quark. We call this process semi-inclusive deep inelastic scattering (SIDIS) and the experimental focus of this program over the next decade will be the new high luminosity 12 GeV electron beamline at Jefferson Lab in the US and the planned Electron Ion Collider (EIC) after that. A description of the CLAS12 experiment on which we focus our research can be found here CLAS12
To establish the connection between the final state hadrons and the initial quarks, so-called fragmentation functions (FFs) have to be measured, which describe the hadronization of the quarks. For this, additional experimental input from e+e– annihilation, where quark-antiquark pairs are produced with well-defined quantum numbers, is crucial.
A new tool in SIDIS, which has been pioneered by our group in several experiments, is the use of hadron pairs instead of single hadrons. Due to the additional degree of freedom in the relative hadron momentum, this allows a much more targeted access to the wave function of the nucleon. An example of a recent result of this effort is the currently most precise extraction of the so-called transversity Parton Distribution Function (PDF) , which is one of the three functions needed to describe the collinear lightcone wave function of the proton. This extraction also uses our measurements of FFs in e+e– at the Belle experiment . This is a truly unique capability of our group, the only one in the US actively measuring FFs in e+e– and, to our knowledge, the only one in the world with a FF program for the next generation B-factory – Belle II at SuperKEKB. We note that this is not for lack of interest, as this program is essential for the success of the physics programs at JLab and EIC as pointed out by the recent long range plan