Scientists trying to recreate conditions that existed just a few millionths
of a second after the big bang that started the universe have run into a
mysterious problem – some of the reactions they are getting don’t mesh with
what they thought they were supposed to see.

Now, two University of Washington physicists have dusted off a quantum
mechanics technique usually associated with low-energy physics and applied
it to results from experiments at Brookhaven National Laboratory on New York’s
Long Island that produce high-energy collisions between gold nuclei. The
result is data much more in line with what theorists expected from the
experiments, said John Cramer, a UW physics professor. That means physicists
at Brookhaven probably have actually succeeded in creating quark-gluon
plasma, a state of matter that has not existed since a microsecond after the
big bang that began the universe.

As it turns out, the model the physicists were using was missing some
pieces, say Cramer and Gerald Miller, also a UW physics professor, whose
findings will be published this month in Physical Review Letters, a journal
of the American Physical Society.

“We think we’ve solved the puzzle by identifying important phenomena that
were left out of the model,” Cramer said.

Since 2000, scientists have been using the Relativistic Heavy Ion Collider
at Brookhaven to collide gold nuclei with each other at nearly the speed of
light. They are trying to get subatomic particles called quarks and gluons
to separate from the nuclei and form a superheated quark-gluon plasma, 40
billion times hotter than room temperature.

Physicists used a technique called Hanbury Brown-Twiss Interferometry,
originally used by astronomers to measure the size of stars, to learn the
size and duration of a fireball produced in the collision of two gold
nuclei. The technique focuses on momentum differences between pairs of
pions, the particles produced in the fireball.

Before the collider experiment began, scientists expected a quark-gluon
plasma to fuel a large and long-lasting fireball. Instead, the
interferometry data showed a fireball similar in size and duration to those
seen at much lower energies. Researchers also expected to see pions pushed
out of the plasma gradually, but instead they seem to explode out all at

“We expected to bring the nuclear liquid to a boil and produce a steam of
quark-gluon plasma,” Cramer said. “Instead, the boiler seems to be blowing
up in our faces.”

While other evidence suggested that the collider experiment had created a
quark-gluon plasma, the interferometry data pointed away from that
possibility. To solve the puzzle, Cramer and Miller used a phenomenon called
chiral symmetry restoration, which predicts that subatomic particles will
change in mass and size depending on their environment – in a hot, dense
plasma as opposed to a vacuum, for instance.

By adding that process to the model, they found that pions in the plasma
have to expend a large amount of energy to escape, as if they were stuck in
a deep hole and had to climb out. That is because chiral symmetry gives
pions a low mass when they are inside the plasma but a much higher mass once
outside. The scientists also allowed for some pions to disappear completely,
to transform into some other type of particle as they emerge from the

The result reconciles all the evidence from the collider experiments,
supporting the possibility that a quark-gluon plasma actually has been

“We have taken a quantum mechanics technique, called the nuclear optical
model, from an old and dusty shelf and applied it to puzzling new physics
results,” Miller said. “It’s really a scientific detective story.”

The work, supported by U.S. Department of Energy grants, adds to the general
understanding of what happened in the first microseconds after the big bang,
he said, “and what we bring to bear is a better microscope, the microscope
of quantum mechanics.”

Cramer noted that adding chiral symmetry restoration to the picture achieved
results very close to what computer models told scientists to expect, and
did so without forcing the experimental data to fit preconceived standards.

“A microsecond after the big bang, there was a state of matter that no one
was able to investigate until very recently,” he said. “We are still
learning, but our understanding is growing.”