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Dear Folks,<br>
<br>
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.<br>
<br>
<blockquote>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.<br>
<br>
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.<br>
<br>
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. <br>
<br>
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. <br>
</blockquote>
<br>
<pre cols="72" class="moz-signature">--
Sincerely,
Larry
-----------------------------------------------------------
Lawrence Weinstein
University Professor
Physics Department
Old Dominion University
Norfolk, VA 23529
757 683 5803
757 683 3038 (fax)
<a href="mailto:weinstein@odu.edu">weinstein@odu.edu</a>
<a href="http://www.lions.odu.edu/~lweinste/">http://www.lions.odu.edu/~lweinste/</a></pre>
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