Research Program:

1. The Central Scientific Question

The Standard Model of particle physics now lies at the center of much nuclear physics research. One of the basic elements of the Standard Model, the existence of quarks and gluons, was first inferred from the spectrum of elementary particles and from electron-scattering experiments. Susequently, Quantum Chromo-Dynamics (QCD) was developed to describe them. Just as the formulation of Maxwell's equations led to a quantitative understanding of electromagnetic phenomena in the late 19th century, so the development of QCD a century later has provided the theoretical foundation for the understanding of many strong interaction phenomena.

While a very good theoretical framework (called Quantum Chromo-Dynammics, or QCD) is able to accurately describe how quarks and gluons interact at extremely high energies (or, equivalently, when the quarks are very close together), it has been very difficult to apply QCD to lower energy (larger distance) phenomena. The paradox is that the increasing complexity of the quark-gluon interaction as they get further apart is critically important to their observed confinement within nucleons and mesons (the nuclear building-blocks), but we cannot perform the QCD calculations necessary to confirm our understanding. This is because in the ``confinement regime'' the quark-gluon coupling strength is too large to allow perturbative theoretic methods to be reliably used.

One of the central problems of modern physics remains the connection of the observed properties of the nuclear building blocks (protons, neutrons, mesons) to the underlying theoretical framework provided by QCD. I have a leadership role in a number of experiments designed to make detailed comparisons with QCD predictions, and so advance our understanding of Quantum Chromo-Dynamics in the ``confinement regime''.

Jefferson Lab (JLab) is the world's premier nuclear physics facility, with intense, high quality, polarized electron beams of energy up to 6 GeV, optimized to perform a wide variety of hadron structure measurements, as alluded to above. In September, 2008, construction approval was given to double the beam energy available at Jefferson Lab to 12 GeV and to upgrade the detector systems. This will allow a new generation of very promising and very interesting experiments to study the structure of hadronic matter. This work is now ongoing, and it is anticipated that the first higher energy electron beams will be available in 2014. We are involved in both the scientific and detector preparations of the upgraded facility, as explained below.

In the meantime, we have a window of opportunity for a new research program to be directed toward the recently-upgraded Institute of Nuclear Physics in Mainz, Germany. Recent upgrades to the Mainz microtron (high-quality, high-flux cw 1.5 GeV electron beam providing a beam of polarized photons), refurbished near- 4 π CB-TAPS detector system, and frozen-spin polarized proton target, will allow unique access to high-precision measurements of proton structure. These are also explained below.

For a non-technical description of my research, please visit:

2. Jefferson Lab Measurements

I have been involved in the JLab scientific program for approximately 20 years, and the majority of my efforts are now centered in Hall C. I am co-spokesperson of four 12 GeV experiments that will take data after the completion of the accelerator and experimental facility upgrade. One of the obstacles to our improved comprehension of non-perturbative QCD has been a lack of quality data, particularly for the Deep Exclusive electron scattering reactions where the system responds coherently to the incoming probe. The measurements are technically challenging and only now are made feasible with recent advances in accelerator and detector technology, as enabled by the ongoing upgrade of JLab Hall C. By making precision measurements of this nature, we hope to better understand the short-long range transition of QCD, and better make the elusive connection between the underlying quark/gluon dynamics and the observed structures of mesons and nucleons. The evaluation of these experiments by the JLab PAC has now been completed and is given in the table below. The goals of these experiments will then be briefly described.

Experiment Number Spokespersons (contact person underlined) Experiment Title Scientific Priority Rating PAC Days Awarded Anticipated Running Time (assuming 50% data taking efficiency)
E12-06-101D. Gaskell, G.M. Huber Measurement of the Charged Pion Form Factor to High Q2 A52 days104 days
E12-07-105T. Horn, G.M. Huber Scaling Study of the L-T Separated Pion Electroproduction Cross-Section at 11 GeV A-36 days72 days
E12-09-011T. Horn, G.M. Huber, P. Markowitz Studies of L-T Separated Kaon Electroproduction Cross-Section from 5-11 GeV B+40 days80 days
E11-002E.J. Brash, G.M. Huber, R. Ransome, S. Strauch Proton Recoil Polarization in the 4He(e,e'p)3H, 2H(e,e'p)n, and 1H(e,e'p) Reactions B+37 days74 days

2a. Pion Electric Form Factor

The flagship experiment of our research program is the measurement of the pion electric form factor (Fπ). The long-term interest in the π+ electric form factor is due to Farrar and Jackson [PRL 43(1979)246], who showed that the Fπ is rigorously calculable in pQCD. Their answer is exact and model-independent at infinite Q2, dependent only on the assumption of quark asymptotic freedom. Such a reliable result is rare in QCD. However, it is not known at what region of Q2 the value of Fπ will reflect the onset of pQCD and this value, as well as the actual behavior of Fπ as a function of Q2 in the non-perturbative transition region, are important tests of our understanding of QCD in bound hadron systems. This is especially important, because the smaller number of valence quarks in the pion means that the asymptotic regime will be reached at lower values of Q2 than for the nucleon form factors.

At low Q2<0.3 GeV2, the pion form factor is dominated by ``soft'' contributions, which are parameterized in terms of the pion rms charge radius. Our experiment at JLab is intended to address the description of Fπ in the gap between these ``soft'' and ``hard'' (pQCD) regions. This is where considerable theoretical effort has been expended in recent years. Some examples include next-to-leading order (NLO) QCD, QCD Sum Rules, Constituent Quark Models, and Bethe-Salpeter Equation calculations. Some of these approaches are more model-independent than others, but it is fair to say that all benefit from comparison to high quality Fπ data, to delineate the role of hard versus soft contributions at intermediate Q2. Lattice QCD simulations are not yet at the point where their uncertainties are smaller than those of the experimental data, but we anticipate this to occur in the next generation of lattice simulations. Since all QCD-based calculations use the pion as a first test case (the hydrogen atom, or more accurately, positronium atom of QCD), it is clear that Fπ will continue to be a topic of attention for some time.

The high quality, continuous electron beam of JLab makes these measurements possible. The measurement of the pion form factor is quite challenging. Since pion targets do not exist experimentally, the pion form factor cannot be measured directly above Q2>0.3 GeV2. For practical purposes it must be deduced from a careful study of the longitudinal and transverse (L/T) cross sections for p(e,e'π+)n kinematics selected to enhance the sensitive t-channel process and to minimize background contributions. The high quality electron beam and well-understood magnetic spectrometers at JLab offer the possibility to measure this form factor to good precision. There is no other existing or planned facility worldwide at which this program could be pursued.

Our collaboration has acquired data in Hall C in two experiments to date, and these results have been published (see the press releases above), generating over 400 citations to date. The JLab 12 GeV upgrade will allow Fπ measurements to be obtained at higher Q2, challenging QCD calculations in a more rigorous manner. We have used our experience to design the best Fπ experiment possible, and our proposal has been recognized as one of the cornerstones of the Hall C program since the 12 GeV upgrade was first proposed. For example, the U.S. Nuclear Science Advisory Committee (NSAC), profiled our experiment in its April 2002 report, writing: ``Another important issue in the physics of confinement is understanding the transition of the behavior of QCD from long distance scales (low Q2) to short distance scales (high Q2). The pion is one of the simplest QCD systems available for study, and the measurement of its elastic form factor is the best hope for seeing this transition experimentally.'' As listed in the table above, our experiment has received the highest `A' scientific priority rating, which is given to only the top ~15% of JLab experiments

2b. QCD Factorization Tests using Exclusive Pion and Kaon Production

Our two related 12 GeV experiments [E12-07-105, E12-09-011] will allow a direct comparison of the scaling properties of Deep Exclusive K+ and π+ electroproduction to better understand the onset of QCD factorization in the transition from hadronic to partonic degrees of freedom. In particular, if the QCD factorization regime has been reached in a given measurement, the longitudinal cross section will dominate over the transverse, and the separated cross sections will scale according to the 1/Qn expectations of pQCD. For Deep Exclusive Meson production, quark counting rules predict σL proportional to 1/Q6, and σT falling at least as fast as 1/Q8. In 2007, we published an initial scaling study of the 1/Qn dependences of charged pion electroproduction at fixed x. The study indicated only partial agreement with the factorization expectations, but the error bars on the fit were compromised by the limited kinematic range available at 6 GeV. Since the extraction of GPDs from Deep Exclusive electron scattering data relies on the factorization of the scattering amplitude into hard and soft processes, we have proposed to extend these studies to a broader kinematic range. We will also perform measurements of the longitudinal cross section at low -t to determine whether the K+ elastic form factor can be inferred from these data. If the studies are favorable, we would perform the first-ever extraction of the K+ form factor above Q2=0.35 GeV2. The K+ experiment is anticipated to be one of the initial suite of 12 GeV experiments planned for Hall C.

2c. Deep Exclusive ω Electroproduction

Recently, with the development of the collinear-factorization approach of QCD, theorists have drawn attention to Transition Distribution Amplitidues (TDAs) as a new probe of hadronic structure. This is particularly interesting because TDAs directly probe the three-quark plus sea q-qbar component in the nucleon's wave function. The kinematic domain demanded by the collinear factorization framework is unique. The meson must be emitted antiparallel to the virtual γ*, at extreme backward angle. The recoiling proton is detected at forward angle, along the axis of the momentum transfer vector, and the &omega<; meson is reconstructed in missing mass as a sharp peak atop a broad background.

We have a large quantity of p(e,e'p)ω data, extending to -t=4.8, -u=0.2 GeV2, i.e. extreme backward angle meson production. Our data are likely the only meson production experiment able to access this kinematic regime so cleanly. Preliminary indications are that the reaction is dominantly transverse, with significant LT and TT interference term contributions, but the extraction of absolutely normalized separated cross sections will take much more work. The interpretation of our data will also be guided by JLab Hall B data at somewhat lower -t<2.7 GeV2, whose cross section was observed to be nearly independent of Q2, pointing toward the coupling of the virtual photon to point-like objects in the high -t limit. We will acquire further backward-angle ω and φ data in the 12 GeV pion form factor experiment, including the Q2-dependence up to 6 GeV2.

2d. Super HMS Spectrometer

Our experiments will use the new SHMS spectrometer in coincidence with the existing HMS spectrometer in Hall C. The requirements upon this 11 GeV/c spectrometer are 5.5 degree forward angle capability, and good systematic error control in the L/T separation at high luminosity. Regina responsibilities include the design work for the heavy gas (C4F8O) Cerenkov detector required for good π identification in the SHMS. This work has been funded by NSERC and is now underway. The present schedule of the JLab upgrade anticipates that first commissioning of the higher energy beam and spectrometer to occur in 2015-16.

3. Interim Experimental Program at the Mainz Microtron

In order to maintain a constant level of scientific productivity until the completion of the JLab 12 GeV upgrade, I have taken advantage of the opportunity to collaborate with fellow Canadians at the CB-TAPS facility in Mainz, where we have taken substantial responsibilities in several experiments.

3a. Proton Spin Polarizabilities

Nucleon polarizabilities are fundamental structure observables like the charge and mass, but are related to the nucleon's internal dynamics, making them ideally suited for constraining QCD-based models of nucleon structure. Although the two scalar (spin-independent) polarizabilities α_E1 and β_M1, are well understood, very few experiments have attempted to extract the spin polarizabilities, which can be written as γ_E1E1, γ_M1M1, γ_M1E2, and γ_E1M2, and none have managed to separate all four. Unlike the scalar polarizabilities, these higher order polarizabilities unfortunately have no classical analog, but can be thought of as parameterizing the ``stiffness'' of the nucleon spin against electromagnetically induced deformations relative to the nucleon spin axis.

In order to extract all four spin polarizabilities, it will be necessary to conduct double-polarization (beam and target) asymmetry measurements under different conditions. We will use the known static polarizabilities along with the values of γ_0 and γ_π to eliminate two of the spin polarizabilities, and then vary the remaining two and fit to the data. Specifically, we will need to use:
  1. Circularly polarized photons near threshold and in the Δ_1232 resonance region, in concert with a longitudinally (transversely) polarized target, to determine γ_M1M1 (γ_E1E1).
  2. Linearly polarized photons (φ=0 vs.90deg) in both energy regions, but in particular near the resonance, to allow for the extraction of γ_M1M1 with an unpolarized target, and of γ_E1E1 with the target polarized perpendicular to the scattering plane.
  3. Linearly polarized (φ=45deg vs.135deg) photons in the resonance region with both longitudinally and transversely polarized targets in order to access γ_E1E1 and γ_M1M1. In this case, we expect the asymmetries themselves to be relatively small, i.e. on the 5-10% level, but the effects of the spin polarizabilities to be very large. Due to the excellent experimental capabilities, it should be possible to measure asymmetries to the few percent level.
Using these different experimental conditions should result in the extraction of γ_E1E1 and γ_M1M1 with only a small systematic error due to the dependence on the theoretical models needed to extract the polarizabilities from the observed asymmetries.

Given the theoretical interest and new opportunities presented by the new polarized target, we have been able to take data for the first part of the experiment in 2010 and we are optimistic to complete the remaining polarization measurements in 2014.

3b. Polarized Target and Beam Asymmetries in Exclusive π0 Production

These measurements will utilize polarized beams and targets to provide a stringent test of our current understanding that the pion is a Nambu-Goldstone boson due to spontaneous chiral symmetry breaking in QCD. Our experiment will test the detailed predictions of chiral perturbation theory and its energy region of convergence, in order to probe the spontaneous chiral symmetry breaking due to the mass difference of up and down quarks. The experiment will also acquire data on the time reversal odd transversely polarized target asymmetry, which is sensitive to the π-N phase shifts, and provide information on neutral charge states in an energy region not accessible to conventional π-N scattering experiments.