A new physics era at 12 GeV

A new physics era at 12 GeV
January 29, 2015

In several articles over the past years, we have written of progress with the CEBAF 12 GeV Upgrade Project. Since the beginning of 2014, commissioning of the accelerator has been a major feature of lab activities. And in the fall of 2014, we were able to talk of commissioning of the Hall D equipment and the GlueX experiment, the Deeply Virtual Compton Scattering experiment in Hall A and the Heavy Photon Search in Hall B, a pre-CLAS12 experiment. These are the harbingers of a new era, that of 12 GeV operations and physics.

We have been receiving proposals for experiments in the 12 GeV era for several years. These were first constrained to experiments that needed and used the initial suite of 12 GeV equipment. They included the part funded by the Department of Energy within the project per se, but those pieces of equipment funded by the National Science Foundation or other international agencies in conjunction with the project were also included. Over time, these constraints were relaxed.

The lab and Program Advisory Committee have developed a physics classification scheme for the 12 GeV era experiments as follows:

  • The hadron spectra as probes of QCD (GlueX and heavy baryon and meson spectroscopy)
  • The transverse structure of the hadrons (elastic and transition Form Factors)
  • The longitudinal structure of the hadrons (unpolarized and polarized parton distribution functions)
  • The 3D structure of the hadrons (Generalized Parton Distributions and Transverse Momentum Distributions)
  • Hadrons and cold nuclear matter (medium modification of the nucleons, quark hadronization, N-N correlations, hypernuclear spectroscopy, few-body experiments)
  • Low-energy tests of the Standard Model and Fundamental Symmetries

The first category is dominated by the GlueX experiment, which will search for exotic meson states predicted by quantum chromodynamics. This is widely recognized as one of the flagships of the new program, and the 12 GeV choice of machine energy optimizes the experiment for 2-3 GeV mesons. 

The second and third categories are recognizable as stalwarts of the Jefferson Lab program from the beginning. The increase in energy extends the reach in momentum transfer for this program, which benefits from about twenty individual experiments. In order fully to exploit the new machine, the Super BigBite Spectrometer is under construction for Hall A. This is an example of an apparatus that was not included in the 12 GeV project, but which exploits some innovative approaches pioneered during the 6 GeV era.

The above categories can be thought of as examining the internal structure of the nucleon one dimension at a time. However, advances in theory and phenomenology over the past two decades have opened a new experimental thrust. Under certain conditions, particular final states give access to a multi-dimensional picture of the parton distributions, captured in the fourth category. This technique is also supplemented by exploiting the observables associated with final states that include a single hadron as well as the final state electron.  

The experimental techniques developed at CEBAF exploiting the particular features of the machine, for example luminosity and polarization, have permitted the extension of the experimental program in fresh directions, including some of those found in the fifth and sixth categories. One is the nuclear structure realm, using parity violation measurements to explore the neutron distribution in nuclei. Another is to use that same technique to examine the fundamental symmetries at play in the standard model. It also turns out that the machine is ideal for searches for heavy photons, which appear in extensions of the standard model that could explain the apparent preponderance of dark matter in the universe.

This broadening physics potential at Jefferson Lab has been acknowledged and supported in several ways. First, came the approval of construction of the Super BigBite Spectrometer apparatus, encouragement for the Ring-Imaging CHerenkov counter initiative in CLAS12 led by our Italian collaborators, and of a RICH counter for GlueX based on special quartz bars once used by the BaBar experiment at SLAC. The DOE Office of High Energy Physics has also provided funding for the Heavy Photon Search in Hall B.

Most recently we enjoyed a very successful DOE physics review of the Moller Experiment, which would be a new, specialized, ultra-high precision probe of the electroweak mixing angle to be mounted in Hall A. The physics review is an important step towards obtaining the resources needed to construct the apparatus. This journey started with a Director’s Review of the experiment a couple of years ago. The next large initiative under consideration is a broader approach to a number of different measurements using a large solenoid. This experiment -- dubbed SoLID, Solenoid Large Intensity Device, which would use the recycled CLEO magnet from Cornell -- has such a review scheduled this month.

Overall, the program currently extends to about 70 approved experiments, which will certainly keep us occupied in all of the Experimental Halls for the next decade. We are excited to start this journey.