Physicists
debate creation
of mini-Big Bang

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Did they or didn't they?
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 won’t matter if one thing goes wrong," Miller said.

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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.

HBT puzzle
To measure the duration of the plasma’s phase transition, physicists use an astronomy tool, called Hanbury Brown-Twiss interferometry. The HBT technique, named after researchers Robert Hanbury Brown and Richard Twiss, can find the diameter of stars using the radio signals from two separate telescopes.

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.

Brookhaven National Laboratory
As big as a house, the STAR detector at the Relativistic Heavy-Ion Collider searches for signatures of the form of matter that RHIC aims to create: the quark-gluon plasma.
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 planet’s 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.

Is the puzzle really solved?
Platt is unsure that Cramer and Miller’s 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 claim victory?

"It is not for me to say that we have found the quark-gluon plasma," Cramer said. "But we have made an important step."

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