Current Experiments
Hall A
E04-007: Precision Measurement of Electroproduction of pi0 Near Threshold: A Test of Chiral QCD Dynamics
The focus of this experiment is to learn how energy is converted to produce particles called pions. The particle of interest is the neutral pion, which is considered one of the simplest systems since it is made up of a quark and antiquark pair with no net charge and no spin. Therefore, it has no electric or magnetic interactions, and this makes it very difficult to detect. The absence of electricity and magnetism also constrains energy transfer mechanisms on how it is produced.
A beam of electrons is directed onto a new, 2.5-inch long, one-inch diameter liquid hydrogen target. The Hall A High Resolution Magnetic Spectrometer is placed to detect the electrons that interacted with the protons in hydrogen nuclei in the target to produce neutral pions with very low energy and velocity. Since energy is related to mass through E = mc2, part of the electron energy that is transferred to the proton is ultimately converted to the mass of the pion.
When the pions barely make it out of the proton (called threshold pions), the protons recoil into a predictable narrow range of angles in the laboratory. Scientists will measure the energies and angles of all the protons as they recoil into the BigBite magnet and wire chamber detection system, which will provide information about the undetected pion.
Under these experimental conditions, the rate of pions produced and how this rate varies with energy and angle can be calculated using different models of how the pion is produced. Previous threshold pion data show a strong disagreement with the presently accepted model called Chiral Dynamics. The new high-precision measurements with smaller systematic errors and greater phase space coverage will provide more leverage to decipher the models and also provide a check on previous data.
Hall B
g12 Run Period
E04-005: Search for New Forms of Hadronic Matter in Photoproduction
E04-017: Study of Pentaquark States in Photoproduction off Protons
In the past 40 years, a picture has emerged in which quarks are the fundamental entities from which protons and neutrons -- and hence, the nucleus of the atom -- is constructed. The theory of Quantum Chromodynamics (QCD) has also emerged as the description of the interaction between quarks that forms this nuclear matter. QCD introduces the mediator of this interaction, the gluon, which itself can be an active participant in the interaction, and thus can be a constituent of nuclear matter. Particles in which the gluon is a constituent may exhibit properties that distinguish these particles from nuclear matter constructed solely from quarks. Searches for such "hybrid states" have yielded tantalizing results at Brookhaven National Laboratory and the Crystal Barrel at CERN.
There have been suggestions that the photon beam generated from high-energy electrons may provide a fertile environment to produce these states at Jefferson Lab. In fact, the principle motivation for the construction of the Hall D detector is to search for hybrids. However, Hall B's CEBAF Large Acceptance Spectrometer (CLAS), using the highest energy electrons currently available, near 6 GeV, will attempt to identify these particles from the observed final states to which they decay.
In addition, the high data rate available with CLAS will allow several other experiments to run simultaneously. Based on suggestive results from an earlier high-energy run, a search for pentaquark baryons, constructed of four quarks and one anti-quark, will be run concurrently. The same data will afford a plan to measure cascade baryon spectroscopy. Cascade resonances, due to their unique combination of quarks, make them an ideal laboratory for understanding the interaction of quarks within protons and neutrons, which also gives insight into the nature QCD in the predominant low-energy regime, where the theory is least understood.
Hall C
E04-108: Measurement of GEp/GMp to Q2=9 GeV2 via recoil polarization
and
E04-019: Measurement of the Two-Photon Exchange Contribution in ep Elastic Scattering Using Recoil Polarization
Experiment E04-108 in Hall C is the third of a series of investigations of the structure of the proton at smaller and smaller distances. These investigations probe the proton’s form factors, which can be thought of as being mathematically related to the actual distribution of electric charge and current inside the proton.
There are two ways to measure form factors. In both, high-energy electrons are “scattered” from targets like the proton; this scattering is equivalent to viewing an object invisible to the naked eye with a powerful microscope. The first of these methods, the polarization observables technique, will be used in the forthcoming experiment. The polarization technique measures the amount of spin polarization that the incoming electron transfers to the proton as a result of the collision between the electron and the proton.
Form factor measurements using the polarization technique are thought to be more accurate compared to a second method, cross section measurements. Cross section measurements determine the probability that the incoming electrons will be deflected by a given angle after scattering from the proton
Cross section measurements may not be as accurate, because they are affected by secondary processes; the polarization technique is only weakly affected by these secondary processes. That may explain a discrepancy between results from cross section vs. polarization measurements. A second experiment, E04-019, will run interlaced with E04-108 and is designed to help identify the source of the discrepancy. It is a polarization measurement of the form factors over a range of electron-scattering angles and energy at constant proton momentum.
JLab's Accelerator and Experiment Schedule
Current Experiments Archive