Beams with polarized particles greatly boost the physics
output of high energy physics colliders. While it has been
straightforward to make polarized electron beams, polarizing positrons
is more difficult, especially in the case of linear colliders. The E-166
experiment has successfully demonstrated a technology to make a
polarized beam of positrons for a future linear collider.
The E-166 collaboration met at DESY Zeuthen in Berlin, Germany in November 2005.
(Photo courtesy of John Sheppard)
For decades, SLAC has been
making positrons—the antimatter equivalent of electrons—but this is the
first polarized positron beam
at SLAC. Polarized means the particles are oriented to spin in the same
direction; imagine most of the golf balls at a driving range rotating
clockwise as they fly toward the net. Beams never reach 100 percent
polarization, but the more polarized the beams, the more information
they reveal in collisions.
E-166 proves that the proposed International Linear
Collider (ILC) could be designed with a polarized positron beam. The
collaboration is still analyzing the results to determine the precise
amount of polarization achieved. "Let’s say the beam is definitely
polarized, sufficient for a linear collider,"
said Bill Bugg (University of Tennessee, Knoxville).
In two runs during June and September 2005, the
collaboration used SLAC’s two-mile linac to deliver electrons to the
Final Focus Test Beam (FFTB). There the electrons travel through a
a one-meter-long magnet that forces the electrons to spiral, thus
emitting polarized gamma rays.
The gamma rays strike a tungsten target, producing showers of polarized
positrons with an average energy of 5 to 6 million electron volts (MeV).
Alexander Mikhailichenko (Cornell), who built the undulator for the
experiment, was one of the people to originally propose the technique in
The electrons travel through the undulator in a tiny
beam pipe—a stainless steel tube with a 0.9-millimeter inside diameter. The pipe
is cut from the same hollow metal used for hypodermic needles and cheap,
too, at $1 per foot. Even though the electron beam is 20 times narrower
than the pipe aperture, some feared the small pipe would be a
showstopper. The beam needed to go cleanly through the undulator without
touching the pipe wall. Any beam loss at all would have saturated the
detectors with background noise.
"The undulator performance was superb, like flipping a
switch," said experiment spokesman John Sheppard (ILC).
The results put to rest doubts that helical undulators
would produce circularly polarized gamma rays or that polarized gamma
rays would in turn produce polarized positrons.
"SLAC was the only place we could possibly do this
experiment," Sheppard said. "We needed a 50 GeV low-emittance
(transversely small) beam, small enough to fit through the undulator
beam pipe. The success of the experiment in large part was due to the
excellent beam quality and stability delivered by the SLAC operations
Collaborators who took shifts at SLAC came from the University of
Tennessee, DESY Hamburg, DESY Zeuthen, Humboldt University Berlin,
Cornell, Daresbury, RWTH Aachen, Princeton and Tel-Aviv University.