[Tpe] alternate TPE public text

[Larry Weinstein]

Dear Folks,

Here is an alternative text for the JLab web site on the TPE 
experiment.  We can then send out the first text as a press release.

    In this experiment, we will measure the difference between how an
    electron and its anti-particle (a positron) interact with the proton
    in order to better understand the structure of the proton.  In order
    to measure this difference as precisely as possible, we will create
    a simultaneous mixed identical beam of electrons and positrons, pass
    it through a proton target, and measure the scattered electrons (or
    positrons) together with the knocked-out protons.

    In order to make this matter/antimatter beam, we will pass a high
    energy (5.5 GeV) electron beam through a thin tungsten sheet.  A few
    percent of the electrons will emit high energy photons.  The
    electrons will then be steered by a magnet into a beam dump.  The
    photons will then pass through another thin tungsten sheet.  A few
    percent of the photons will convert into a high energy electron (e-)
    and a high energy positron (e+).  This mixed
    electron/positron/photon beam will pass through a series of magnets
    that will separate the e+ and e- beams, block the photons, and
    recombine the e+ and e- beams.  The simultaneous mixed identical
    beam of electrons and positrons (our matter/antimatter beam) will
    then pass through a liquid hydrogen target at the center of the CLAS
    detector.  We will use the CLAS detector to measure and identify the
    scattered electrons (or positrons) together with the knocked-out
    protons.

    Surprisingly, there is only a very small difference between how
    electrons and positrons interact with protons.  This is because an
    electron passing to the right of a proton and a positron passing to
    the left of a proton will both be deflected to the left.

    Other experiments have measured the charge distribution of the
    proton using two methods that should give the same answer.  However,
    these methods disagree by a factor of three at large momentum.  The
    preferred explanation of this discrepancy predicts a several percent
    difference between the cross sections (probability) of electron and
    positron scattering from the proton.  Therefore we need to measure
    this difference to make sure we fully understand the charge
    distribution of the proton, one of the fundamental quantities in
    nuclear physics.


-- 
				Sincerely,
				Larry

-----------------------------------------------------------
Lawrence Weinstein
University Professor
Physics Department
Old Dominion University
Norfolk, VA 23529
757 683 5803
757 683 3038 (fax)
weinstein at odu.edu
http://www.lions.odu.edu/~lweinste/

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