This three-dimensional view shows the proton-lead collision that produced collective flow behavior. The green lines are the trajectories of the subatomic particles produced by the collision, reconstructed by the CMS tracking system. The red and blue bars represent the energy measured by the instrument's two sets of calorimeters.
Scientists think they've created the smallest drops of liquid ever — the size of only three to five protons.
The droplets were made inside the world's largest particle accelerator, the Large Hadron Collider in Switzerland, where particles are sped up to near light speed and then smashed together. When researchers collided protons with lead nuclei, they were surprised to find that the result was teeny, tiny droplets of liquid.
These liquid drops are minuscule, measuring about one-100,000th the size of a hydrogen atom or one-100,000,000th the size of a typical virus. [Dazzling Droplets: Photos Reveal Mini Worlds]
The researchers consider the droplets liquid because they flow more like a liquid than like any other state of matter.
"With this discovery, we seem to be seeing the very origin of collective behavior," Vanderbilt University physicist Julia Velkovska said in a statement. "Regardless of the material that we are using, collisions have to be violent enough to produce about 50 subatomic particles before we begin to see collective, flowlike behavior," added Velkovska, who is a co-convener of the heavy-ion program of the Compact Muon Solenoid, the LHC detector where the droplets were made.
In fact, the droplets appear to be tiny bits of one of the hottest liquids known, called quark-gluon plasma. This plasma, essentially a soup of quarks and gluons (the subatomic ingredients of the protons and neutrons that make up atomic nuclei), has been made at LHC and other particle accelerators before.
When quark-gluon plasma was first discovered in the early 2000s inside the Relativistic Heavy Ion Collider at Brookhaven National Laboratory on Long Island, physicists initially thought it would behave as a gas does. Instead, they found it had liquid properties. Scientists think this plasma represents the state of the whole universe just moments after it was born in the Big Bang, when the universe was extremely hot and dense.
The first artificial quark-gluon plasma was produced by smashing two gold nuclei together, and was later re-created with collisions of two lead nuclei. The CMS researchers wanted to test whether quark-gluon plasma could also be made by colliding a lead nucleus with a proton, which is 208 times less massive than lead; they expected these impacts would not be energetic enough to produce the plasma.
"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," Velkovska said.
The results of the experiment were unexpected. In about 5 percent of collisions — those that were most violent — enough energy was released around the "bullet hole" where the proton smashed through the lead that some of the protons and neutrons there melted. This material seemed to form droplets of liquid about one-tenth the size of the quark-gluon plasma batches created by lead-lead and gold-gold impacts.
Quark-gluon plasma is still a mysterious form of matter, and the scientists can't be absolutely sure yet that what they saw were liquid droplets. Further tests should help differentiate between that interpretation and other possible explanations of the results, the researchers said.
Velkovska and her colleagues detailed their findings in a paper submitted to the journal Physics Letters B.
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