Current Experiments
Hall A
E06-010, Measurement of Single-Spin Asymmetry in a Semi-Inclusive Reaction on a Transversely Polarized Helium-3 Target
and
E07-013, Target Normal Single-Spin Asymmetry in Inclusive Deep-Inelastic Scattering with a Polarized Target
Since the late 1970s, an interesting phenomenon called "target single-spin asymmetry" has been observed. In this phenomenon, pions or kaons produced from a proton or neutron polarized perpendicular (transverse) to the incident beam demonstrate a clear left-right preference with respect to the direction of the incident beam. For example, a recent experiment at DESY showed that positively charged pions favor the left side in the reaction, while negatively charged pions favor the right side.
How does this happen? How do the quarks in a nucleon contribute to this left-right difference? Could it be that within the transversely polarized nucleon, the quarks have their spins aligned in the transverse direction already (i.e. a quark transversity distribution)? Do different types of quarks behave the same way in contributing to the left-right difference?
In E06-010, researchers will scatter high-energy electrons from a vertically polarized neutron target (helium-3) and observe the charged hadrons produced in the reaction (pions and kaons). Researchers will observe if a different number of particles is produced when the target neutron's spin direction is flipped from up to down.
E07-013 will also take data at the same time to address a slightly different left-right asymmetry that is two orders of magnitudes smaller. The experiment concerns inclusive deep-inelastic scattering, where a high-energy electron probe exchanges either one or two virtual photons with a quark in a nucleon. Only the scattered electron probe is observed.
In the case of one virtual photon exchange, the process is time-reversal invariant. In other words, the process is exactly the same when we exchange past with future: the initial state and final state look alike. Therefore, the number of electrons scattered to the left side should be exactly the same as those scattered to the right side (no left-right asymmetry or single-spin asymmetry is allowed).
However, when two virtual photons are exchanged, the situation becomes different; the final state and the initial state are not the same anymore, and the process is asymmetrical. Therefore, more electrons will scatter to one preferred side, and the interference between the two processes (one- vs. two-photon exchange) could generate a target single-spin asymmetry at the order of 0.01% An experiment conducted in 1969 at Stanford University set an upper limit on this type of asymmetry to less than 3%. After 40 years, E07-013 hopes to improve upon this upper limit by two orders of magnitude, or perhaps, for the first time, clearly demonstrate a non-zero asymmetry.
Hall B
E01-113: Deeply Virtual Compton Scattering with CLAS at 6 GeV (e1-DVCS)
An important goal of JLab is to provide a detailed three-dimensional picture of the nucleon in terms of its quark and gluon constituents and to understand how this complex structure leads to its well known properties such as mass, spin and magnetic moment. The "e1-DVCS" experiment is the first comprehensive study in Hall B specifically focusing on this task.
The experiment required the construction of special large hardware devices, which were implemented in Hall B's CEBAF Large Acceptance Spectrometer (CLAS) to optimize the detector's performance for these measurements. In the experiment, the electron beam was alternately polarized parallel and anti-parallel to the beam direction. Copious amounts of data were obtained for deeply virtual Compton scattering (DVCS), which is the "flagship" of the program, and meson production (DVMP).
The first part of the experiment was run in 2005, and initial results have begun to appear. The second part of the DVCS experiment will run from October 2008 to January 2009. The goal of the second part is to more than double the statistics of the DVCS events in the same kinematical region with emphasis on extraction of helicity-dependent cross sections. There will be a new scintillation hodoscope installed in front of the DVCS Inner Calorimeter (IC). This new device will allow separation of photon and electron showers in the IC. Identification of electrons in the IC opens new physics opportunities with small angle electron scattering.
Hall C
E07-003: Spin Asymmetries of the Nucleon Experiment
Protons and neutrons, collectively known as nucleons, are the building blocks of all atomic nuclei. Nucleons have a complicated internal structure, which gradually is being unveiled through the use of powerful electron "microscopes" that illuminate the interior of the nucleons with very short wavelength "light." The microscopes" are the particle accelerators like CEBAF (the Continuous Electron Beam Accelerator Facility) at Jefferson Lab, or the LHC (the Large Hadron Collider) in Europe. The "light" is composed of powerful real or virtual photons that, when polarized, can resolve not only the small components inside the nucleons, called partons, but can also tell us about their intrinsic motions.
SANE, the Spin Asymmetries of the Nucleon Experiment, will use the highest-energy polarized electrons and photons produced by CEBAF to illuminate a target of polarized hydrogen nuclei, or protons, in frozen ammonia. The electrons recoil in different numbers after interacting with the target, depending on the orientation of their intrinsic sense of rotation, or spin, relative to the spin of the target protons. The difference in numbers for parallel versus anti-parallel spins can be directly connected to the state of rotation of the proton's partons.
There are two kinds of partons: quarks and gluons. The exchange of gluons is what "glues" the constituents of the proton together. In SANE, we'll explore the likelihood of interactions between two quarks and a gluon, which can be observed by counting the differential rates of recoiling electrons when the electron spins are perpendicular or anti-perpendicular to the proton spins. Powerful theoretical tools, such as computer calculations of the nucleon structure from first principles, called Lattice QCD, and others, are available to compare the measurements to be made in SANE to models of the distribution of spinning quarks and gluons inside the proton. We'll try to see the insides of the proton in motion.
About 90 physicists from 22 U.S. and overseas institutions are working on SANE, including six Ph.D. and M.S. thesis students.
JLab's Accelerator and Experiment Schedule
Current Experiments Archive