SLAC Experiment Make First Observations of Key Traits in
By Heather Rock Woods
The E158 experiment at SLAC has made vital
new observations that illuminate the nature of the weak force.
The weak force is tremendously important. Without it,
there would be no life on Earth. The weak force causes radioactive
decays, which are essential in making sunlight. Radioactive decays also
warm the inner earth, enabling liquid magma to move continents and
generate earthquakes. Radioactive decays can be used to measure the age
of the Earth and archeological samples, and to diagnose and treat
disease. The strength of the weak force affects how long the sun and
stars last, and the mix of basic elements in our universe.
illustration of the experimental setup in End Station A shows
the beam hitting the target and going through spectrometer
magnets to the detector region.
(Image courtesy of Mike
Using extraordinary precision, E158
made the landmark observation that the strength of the weak force acting
on two electrons lessens when the electrons are far apart. The results
have been accepted for publication in the field’s primary journal,
Physical Review Letters.
“Physicists have long expected that the weak force interactions would be
weaker at longer distances, but proving it wasn’t easy,” said experiment
co-spokesman Krishna Kumar, professor of physics at the University of
Massachusetts-Amherst. SLAC physicists made up one third of the
60-person collaboration and led experimental operations.
“The experiment could only be done at SLAC, using the
highest beam power SLAC has seen in 30 years,” Kumar said. “We measured
this minute effect with a massive beam. It was a huge technical
achievement— most people were skeptical about our chances for success.”
The precision measurements required enormous numbers of
electrons. The SLAC linear accelerator sent 500 billion electrons in a
single bunch to a target, and repeated this 700 million times. In half
of these electron bunches, the electrons were polarized to spin
right-handed. The electrons in the remaining bunches were polarized to
spin left-handed. Some electrons entering the target scattered off
target electrons by exchanging a mediator particle. The mediator is
almost always a photon, which transmits the electromagnetic force (think
visible light, radio waves, x-rays). The collaboration’s challenge was
to find the rate of rare events, those one in a million
electron-electron scatters that took place by exchanging a Z particle,
which mediates the weak force.
Because there is an asymmetry in how the weak force acts,
there is a slight difference in how often left-handed electrons scatter
using a Z particle compared to the rate for right-handed electrons. On
average, a bunch of right-handed electrons generates 20 million
scattering events, including several dozen Z scatters. Left-handed
bunches yield about five more Z-mediated scatters.
E158 made the first observation of this slight left-right
asymmetry, called parity violation, in electron-electron interactions in
2003. The asymmetry is so tiny—131 parts per billion—that if you did the
experiment with clocks, a left-handed clock would be only one hour
faster after 1,000 years.
Researchers used their precision asymmetry measurement to calculate the
long-distance strength of the electron’s weak charge, which determines
the strength of the weak force between two electrons. Precision
measurements have a history of enabling scientific discovery, as in the
case of inferring the existence of Neptune by observing Uranus’ wayward
Previous experiments at
SLAC and CERN in Geneva had measured the electron’s weak charge at short
distances. E158 has now demonstrated that, as predicted, at “long”
distances approximately the width of a proton, the electrons are far
apart and their weak charge is only half the size of the charge at short
The evidence that
the electron’s weak charge varies with distance—called running—is the
first demonstration of running in weak force interactions and confirms
for the first time an important aspect of Standard Model theory, which
describes the actions of the weak force, electromagnetism and the strong
The weak charge gets weaker because
of quantum fluctuations. The vacuum surrounding every particle randomly
spits out and reabsorbs virtual particles, making an ephemeral cloud
that effectively forms a screen between distant interacting electrons.
“E158 is sensitive to this rich
structure that exists in the microscopic world, the structure of
nothing, of the vacuum,” said Yury Kolomensky, leader of the analysis
and assistant professor of physics at the University of
sensitive to (but did not find) indirect signals from Z’ (Z prime)
particles, hypothetical fat cousins of Z particles that would carry a
yet-to-be discovered new force.
“E158 has been a real tour de force by a talented group
of experimentalists,” said Bill Marciano, Senior Theoretical Physicist
at Brookhaven National Laboratory. Although not on the experiment, he
has worked extensively on precision weak interaction calculations.
“Their high precision measurement of a tiny parity violating asymmetry
provides one of the best tests of the Standard Model and confirms the
expected running of the weak charge,” he said.
The collaboration clearly demonstrated that its
experimental technique can be used at current and future accelerators,
making the experiment’s technical contributions as significant as its
scientific success in drawing a more complete picture of one of nature’s
fundamental and profound forces.
The collaboration involved 60 physicists from SLAC,
University of Massachusetts, UC Berkeley, Syracuse, CalTech, Jefferson
Lab, Princeton, Smith College, University of Virginia, and Saclay in
France. SLAC is funded by the Department of Energy’s Office of Science.
For an animation, illustrations and details, see