[Paleopsych] Space.com: Mini Big Bang Created, Puzzling Results Too Explosive
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Fri Apr 15 20:42:42 UTC 2005
Mini Big Bang Created, Puzzling Results Too Explosive
By Michael Schirber
posted: 21 March 2005
06:27 am ET
What do you get when you turn the temperature up to a trillion
Quite a heating bill.
Actually physicists claim that at this temperature nuclear material
melts into an exotic form of matter called a quark-gluon plasma
thought to have been the state of the universe a microsecond after the
Recreating this primordial soup is the primary purpose of the
Relativistic Heavy-Ion Collider (RHIC) at Brookhaven National
Laboratory. After five years of data, it appears as if RHIC may have
But a big mystery looms over the detection: the putative plasma
explodes more violently than predicted.
"We expected to bring the nuclear liquid to a boil and produce a steam
of quark-gluon plasma," said John Cramer from the University of
Washington. "Instead, the boiler seems to be blowing up in our faces."
The explosive result, which goes by the name of the HBT puzzle, may
call into question what RHIC is making in its high-speed collisions,
or it might mean the theory needs retuning.
Cramer and his colleagues have another alternative explanation, too:
perhaps the explosion is not as explosive as the data suggests. The
scientists use 50-year old physics to reinterpret the measurements at
"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," said Gerald Miller, a coauthor also at the
University of Washington. "It's really a scientific detective story."
The main suspect in this detective story is the quark-gluon plasma.
But how do you know when youve seen it? The plasma cannot be observed
directly it disappears in less than a hundredth of a billionth of a
trillionth of a second. All that researchers can hope to do is detect
the particles that fly out when the plasma freezes back into normal
"You cant go in there and directly measure the quarks and gluons,"
Miller told SPACE.com. "You have to work back from what you measure to
what you believe was there."
Scott Platt from Michigan State University, who didn't participate in
the new research, compares detecting the quark-gluon plasma to what
astronomers have to do when studying an exploding star.
"They only see the light coming from the stars surface and then try to
infer what happened inside. We [physicists] have the same problem," he
Instead of light, RHIC researchers see thousands of particles mostly
pions, which are tiny things weighing about one-seventh as much as a
proton, itself subatomic. The pions show up in detectors set up around
collision points, where gold nuclei traveling at 99.995 percent of the
speed of light hit each other head-on.
To see a movie of a gold-on-gold collision click here (note that
the gold nuclei look like pancakes because they are traveling so
"We cant stick a barometer or thermometer into the collision center,"
Platt explained, but by a careful reconstruction of the flight paths
of all the debris coming out, scientists can extract information about
the brief, but intense, furnace created when gold nuclei smash into
From the RHIC data, research teams have identified three smoking guns
for the quark-gluon plasma:
* the collision center is under high pressure
* the collision center behaves a lot like a fluid
* very high energy particles do not escape
Although this evidence appears solid, physicists are hesitant to
say they have created the melted nuclear goop. "That debate is going
on as we speak," Platt said.
One of the reasons for this conservative approach has to do with how
fast the supposed plasma appears to freeze back into ordinary matter.
Theory assumed this phase transition would take almost twice as long
as was measured.
"In science, if you have a bunch of things that are right, it wont
matter if one thing goes wrong," Miller said.
The apparent explosion of pions and other particles coming from the
phase transition is the so-called HBT puzzle.
"It is the one RHIC observation that deserves the word puzzle or
surprise," Platt said.
To measure the duration of the plasmas phase transition, physicists
use an astronomy tool, called Hanbury Brown-Twiss (HBT)
interferometry, which can find the diameter of stars using the radio
signals from two separate telescopes.
Insert: Tiny Terms
Quark: subatomic particle that is the building block of protons,
neutrons and short-lived particles like pions.
Gluon: a particle that transmits the strong nuclear force literally
gluing quarks together into protons and neutrons and such.
Plasma: a separate form of matter often referring to a gas of freed
electrons and ions. In the case of the quark-gluon plasma, the quarks
and gluons are liberated from their usual bonds, and can interact with
one another freely.
Pion: Unlike protons and neutrons, which are made of three quarks, the
pion is made of just two quarks. Pions eventually decay into photons,
electrons and neutrinos.
Phase transition: A change between two forms of matter, like when
water freezes or boils. There is a phase transition between the
quark-gluon plasma and ordinary matter.
Michael Schirber, SPACE.com
End of insert
Instead of comparing radio waves, physicists compare two pions flying
out from the collision center. But these measurements require a lot of
modeling and approximations, Platt explained.
Cramer and Miller and their collaborators have redone the
calculations, incorporating something called the nuclear optical
model. This dates back to 1950s, when scientists were beginning to
understand the strong interactions inside the nucleus.
Effectively, this old-school physics accounts for the fact that, as
pions form out of the cooling plasma, they will have to climb their
way out of an attractive field similar to the gravitational field that
a rocket has to overcome to escape a planets clutches.
"This is not surprising, since it has already been shown that the
medium is very dense," Miller said. "It is as if the pions are trying
to leave a crowded room."
According to Cramer, this crowded room "distorts" the data, making the
transition look more explosive than it really is. In a sense, the HBT
puzzle could be a simple misinterpretation of what the data shows.
Platt is unsure that Cramer and Millers work, published this month in
Physical Review Letters, indeed clears up the HBT puzzle entirely.
"They pointed out one of the ways that the calculations can be
improved," he said. "But the analysis is ongoing."
If the puzzle does end up being solved, will physicists be ready to
"It is not for me to say that we have found the quark-gluon plasma,"
Cramer said. "But we have made an important step."
This article is part of SPACE.com's weekly Mystery Monday series.
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