This symmetry violation makes hydrogen possible, a requirement for life
BLOOMINGTON, Ind. — Scientists at the Indiana University Cyclotron Facility
in Bloomington have made the first unambiguous detection of a rare process,
the fusion of two nuclei of heavy hydrogen to form a nucleus of helium and an
uncharged pion. The pion is one of the subatomic particles responsible for the
strong force that holds every nucleus together. The achievement will be
announced Saturday (April 5) at the meeting of the American Physical Society
in Philadelphia.
“Scientists have searched for this rare fusion process since the 1950s,” said
IU physicist Edward Stephenson, the leader of the research team. “The process
would not happen at all if nature did not allow a small violation of what is
known as charge symmetry. If this symmetry violation had happened to be in the
other direction, hydrogen would not have survived after the Big Bang, and the
universe would not have the hydrogen fuel that keeps stars shining, including
our sun, making human life possible. Sometimes large consequences hang on
delicate balances in nature.”
One effect of this charge symmetry violation is that the neutron is slightly
heavier than its charged partner, the proton. As a result, isolated neutrons
decay into protons in about 10 minutes. “If the charge symmetry violation had
been in the other direction instead, and if the proton had been heavier than
the neutron by the same slight amount, protons would have decayed into
neutrons and hydrogen could not have survived,” Stephenson explained.
The rate at which the rare fusion process occurs is expected to be a key
piece of information in finding the cause for this violation of charge
symmetry, he said. Theorists have proposed that the violation originates
with quarks, the small particles that are found inside protons and neutrons.
“The rate of the process will tell scientists how much of the violation comes
from the fact that quarks carry small electrical charges, and how much comes
from the difference in mass between the two types of quarks found inside
neutrons and protons,” Stephenson said.
The IU team used the electron-cooled storage ring at the cyclotron laboratory
to focus a beam of heavy hydrogen onto a target of the same material. The
high precision of the beam allowed them to use just enough energy to make the
uncharged pion without producing unwanted heavier particles. Sensitive
detectors tracked the helium nuclei and captured the two photons or particles
of light that are produced when the pion decays.
The team worked around the clock for two months, seeing at most only five of
the rare events per day, Stephenson said. However, the several dozen events
that they collected will be enough to allow scientists to test their theories
about the violation of charge symmtery.
Their research was supported by a grant from the National Science Foundation.
For more information, contact Stephenson at 812-855-5469 or
stephens@iucf.indiana.edu [mailto:stephens@iucf.indiana.edu].
Related Links:
* IU Cyclotron Facility [http://www.iucf.indiana.edu/]
