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Experiment Research

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.

E04-007 Technical Paper