[Tpe] proposed public explanation

[Larry Weinstein]

Dear Folks,

Here is my attempt at a popularization/press-release from 2006.  
Comments are welcome.  We can easily add or subtract names from the list.

- Larry

*ODU-FIU-USM-Jefferson Lab team to produce antimatter beam*

* *

An intense antimatter beam will be produced in a new project at the 
Thomas Jefferson National Accelerator Facility (Jefferson Lab). The beam 
will contain both electrons and positrons, the antiparticle of the 
common electron. (Normal atoms are composed of electrons orbiting a 
nucleus composed of protons and neutrons.) The proposed beam will be the 
most intense high energy mixed matter-antimatter beam in the world. 
Scientists will use this high-energy anti-matter beam to study the 
structure of the proton.

An international team of scientists, led by Physics Professors Larry 
Weinstein (Old Dominion University, Norfolk, VA), Brian Raue (Florida 
International University, Miami, FL) and Will Brooks (Universidad Santa 
Maria, Valparaiso, Chile), is designing and building the apparatus to 
produce this beam in experimental Hall B at Jefferson Lab. They will 
start with the Jefferson Lab high energy 5.5 billion electron-Volt (5.5 
GeV) electron beam. When the electrons pass through a thin metal foil, 
about 1% of them will radiate a high energy gamma ray. The electrons 
will be separated from the gamma rays with a large magnet. When the 
gamma rays (which are virtually pure energy) pass through a thin metal 
foil, about 5% of them will transform into matter (using Einstein's 
/E/=/mc/^2 ) in the form of electron-positron pairs. A further set of 
magnets will be used to separate the electron (matter) and positron 
(antimatter) beams, block the gamma rays, and then recombine the matter 
and antimatter beams. The electrons and positrons in the mixed beam will 
not annihilate each other because they are spread over a distance of 
about two inches. The beams are kept in vacuum to minimize interaction 
with material. An October 2006 test run produced a beam with about 100 
million electrons and positrons per second.

The mixed matter/antimatter beam will be used to study the structure of 
the proton. By precisely measuring the difference between how high 
energy negatively-charged electrons and positively-charged positrons 
collide with protons, scientists will learn more about how electric 
charge is distributed within the proton. One of the peculiar aspects of 
this measurement is that the interactions of electrons and positrons 
with the proton are almost identical. Although the electron is attracted 
to the proton and the positron is repelled by it, both have an equal 
probability of scattering to the left or to the right. By using a mixed 
electron-positron beam and measuring their interactions simultaneously, 
scientists will be able to measure the difference between their 
interactions very precisely.


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