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 12 GeV, optimized to perform a wide variety of hadron structure measurements, as alluded to above. The newly upgraded facilities will allow a new generation of very promising and very interesting experiments to study the structure of hadronic matter. These are described in further detail in Sec 2 below.

In parallel, we participate in an exciting program of measurements at 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 target, will allow unique access to high-precision measurements of nucleon structure. These are also explained in Sec 3 below.

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2. Jefferson Lab Measurements

I have been involved in the JLab scientific program for approximately 25 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.

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
E12-10-006BZ. Ahmed, G.M. Huber, Z. Ye Measurement of Deep Exclusive π- Production using a Transversely polarized 3He Target and the SoLID Spectrometer Run Group has A priority48 days96 days

Most of these 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. Our responsibilities included the SHMS heavy gas (C4F10) Cerenkov detector and assistance with the spectrometer commissioning.


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 2015 report, writing: ''The pion is seen as key to confirm the mechanisms that dynamically generate nearly all of the mass of hadrons and central to the effort to understand hadron structure... With such theoretical motifation, the study of the pion form factor is one of the flagship goals of the JLab 12 GeV Upgrade.'' As listed in the table above, our experiment has received the highest 'A' scientific priority rating from the JLab PAC, and was identified by them in 2014 as one of the ''high impact'' experiments in the initial JLab 12 GeV program.

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, starting in 2018.

2c. u-channel Deep Exclusive Vector Meson Electroproduction

Backward-angle (-u~ 0) meson electroproduction has been previously ignored, but is anticipated to offer complementary information to forward-angle (-t~ 0) meson electroproduction. This is particularly interesting with the development of the collinear-factorization approach of QCD, which has drawn attention to Transition Distribution Amplitidues (TDAs) as a new probe of hadronic structure. 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.


This regime is now open for study, following our pioneering E01-004 study to obtain low -u data on exclusive ω electroproduction, p(e,e'p)ω, simultaneously with the p(e,e'π+)n data. The recoiling proton is detected at forward angle, along the axis of the momentum transfer vector, and the meson is reconstructed in missing mass. The ω is reconstructed as a sharp peak atop a broad background (primarily multi-pion phase space and ρ production). Since the missing mass reconstruction method does not require the detection of the produced ω, it allows the acquisition of data in a kinematic regime that was considered to be inaccessible through the standard direct detection method. Our studies, including a full L/T/LT/TT separation at Q2=1.6, 2.45 GeV2, demonstrated that the technique is reliable. The L/T ratio demonstrated the dominance of σT as -u increases, particularly at the higher Q2=2.45 GeV2 setting. These conclusions are broadly in line with a theoretical model based on the transition distribution amplitude (TDA) framework. 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. We are also preparating to propose a new dedicated experiment to measure exclusive π0 electroproduction in u-channel kinematics.

2d. Transverse Single Spin Asymmetry with SoLID


Beyond 2020, we intend to pursue measurements of the single-spin asymmetry in deep exclusive π- electroproduction from the neutron in transversely polarized 3He, using the Solenoidal Large Intensity Detector (SoLID). Our primarily goal is the measurement of the AUTsin(φ-φS) polarization observable, which has been noted as being particularly sensitive to the polarized helicity-flip generalized parton distribution (GPD) E-tilde. Factorization studies have indicated that precocious scaling is likely to set in at moderate Q2~2-4 GeV2, as opposed to the absolute cross section, where scaling is not expected until Q2 >10 GeV2. Furthermore, this observable has been noted as being important for the reliable extraction of the charged pion form factor from pion electroproduction. Our secondary goal is the measurement of the AUTsin(φS) asymmetry, which is sensitive to the higher twist transversity GPDs, and provides valuable information on transverse photon contributions at small -t. We are pursuing these objectives with the SoLID Collaboration at Jefferson Lab, who approved our addition to the E12-10-006 Run Group as ''a flagship experiment for the SoLID GPD program''. We have also received equipment funds from CFI and the Fedoruk Centre for HGC prototyping studies.

2e. Simulations of Deep Exclusive Meson Production at EIC

A major future initiative by the international nuclear physics community is the proposal to construct the world's first electron-nucleus collider in the coming decade, with the flexibility to change the nuclear ion species as well as the beam energies. For electron-proton collisions, the Electron-Ion Collider (EIC) would be the world's first collider where both beams are polarized. The EIC luminosity is proposed to be 100-1000 times that of the former HERA accelerator at DESY. The 2015 U.S. long range plan for nuclear physics strongly recommended construction of the EIC. The project has not yet been approved by the U.S. Department of Energy (US-DOE), but most projects receiving this level of endorsement have eventually been constructed.

We are one of the Canadian groups involved in the early planning of the proposed EIC experimental program. The polarization degrees of freedom at EIC will enable unprecedented access to GPD distributions over a wide kinematic range, offering details of the internal proton wave function that will not be available from any other source. Furthermore, Deep Exclusive Meson Production will allow greatly improved access to the parton momentum distributions of the pion and kaon. The measurement involves very significant technical challenges, but if they can be overcome, Fπ could be measured to possibly up to Q2=25 GeV2. We are continuing our studies with more detailed Monte Carlo simulations.


3. Experimental Program at the Mainz Microtron

A unique opportunity to collaborate with fellow Canadians at the CB-TAPS facility in Mainz opened in 2008, where we have taken substantial responsibilities in several experiments. This is expected to remain an important part of our program for at least the next several years. Note this description is a bit dated, and does not yet include our plans to measure the neutron polarizabilties.

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.

Data on all three asymmetries were taken 2010-15, and the data analysis has reached a successful conclusion. A global analysis including all three sets of our asymmetry data, in addition to other measurements taken elsewhere, have reduced the uncertainties in the spin asymmetries by factors of 2-4. Several papers are now under preparation.


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.