The Large Hadron Collider (LHC) has recreated the world's tiniest droplets of a primordial state of matter that last existed moments after the Big Bang, some 13.82 billion years ago. This surprise result was achieved by firing proton "bullets" into lead ions, creating a subatomic blood spatter-like effect.
Using LHC data, physicists of Vanderbilt University in Nashville, Tenn., analyzed the results of colliding protons with lead ions inside the particle accelerator's detectors. Until recently, the LHC had only carried out proton-proton or lead-lead collisions at high energy, so colliding lead ions with protons is a whole new ballgame.
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Lead ions are 208-times more massive than single protons, so carry more energy. Therefore, lead-lead collisions are, by their nature, very energetic. The LHC has been carrying out lead-lead collisions since 2010, revealing fascinating insights into the conditions of the Universe immediately after the Big Bang. In an effort to access even more exotic states of matter, test runs of proton-lead collisions have been successful, but only now has this unexpected discovery been made.
In the process of sifting through the huge quantities of proton-lead collision data, the Vanderbilt researchers noticed that tiny droplets of a primordial state of matter were being created. And "tiny" is the operative word. The droplets measure only three to five protons wide - approximately 1/100,000th of the size of a hydrogen atom or 1/100,000,000th the width of a virus, the researchers say.
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"With this discovery, we seem to be seeing the very origin of collective behavior," said Julia Velkovska, professor of physics at Vanderbilt who investigates heavy ion collisions in the LHC's CMS detector. "Regardless of the material that we are using, collisions have to be violent enough to produce about 50 sub-atomic particles before we begin to see collective, flow-like behavior."
Moments after the Big Bang, all matter was a high-energy, high-temperature soup. All matter and forces were mixed together as an indistinguishable mess. As the Universe rapidly cooled and expanded, the matter began to separate from the fundamental forces of nature. Of huge interest to particle physicists is a state of matter called a quark-gluon plasma. By colliding ions at increasingly high energies, the conditions of the Big Bang can be replicated on very tiny time scales, giving us a glimpse at a state of matter that hasn't existed naturally for 13.82 billion years.
It was theorized that quark-gluon plasma - a state of matter where quarks and the strong force-carrying gluons have yet to condense into separate families of particles - would likely act like a gas. So, in the early 2000′s, physicists were surprised to find that quark-gluon plasma actually had a collective behavior. This discovery of tiny plasma blobs after proton-lead collisions supports the finding that primordial plasmas behave like liquids.
Interestingly, the researchers had no clue that proton-lead collisions would be powerful enough to create a quark-gluon plasma - such collisions, theoretically, shouldn't generate enough energy.
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"The proton-lead collisions are something like shooting a bullet through an apple while lead-lead collisions are more like smashing two apples together: A lot more energy is released in the latter," said Velkovska.
The data that Velkovska's team analyzed came from a calibration run in September 2012, but they found, by accident, that five percent of the more violent proton-lead collisions in that run exhibited this collective behavior. The only way to explain this behavior was that tiny droplets of quark-gluon plasma had formed around the "hole" made by the speeding protons (bullets) blasting through the more massive lead particles (apples). The quark-gluon liquid droplets formed by proton-lead collisions were 1/10th the size of those produced by lead-lead collisions, making them the smallest drops of quark-gluon debris to be measured from any particle collision event.
The Vanderbilt team's research has been published in the journal Physics Letters B.
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