世界上最小的液滴

Physicists² may have created the smallest drops of liquid ever made in the lab. That possibility has been raised by the results of a recent experiment conducted by Vanderbilt physicist¹ Julia Velkovska and her colleagues at the Large Hadron Collider, the world's largest and most powerful particle collider located at the European Laboratory for Nuclear and Particle Physics (CERN) in Switzerland. Evidence of the minuscule³（极小的） droplets⁴ was extracted from the results of colliding protons with lead ions at velocities⁵ approaching the speed of light.   According to the scientists' calculations, these short-lived droplets are the size of three to five protons. To provide a sense of scale, that is about one-100,000th the size of a hydrogen atom or one-100,000,000th the size of a virus.   "With this discovery, we seem to be seeing the very origin of collective behavior," said Velkovska, professor of physics at Vanderbilt who serves as a co-convener of the heavy ion program of the CMS detector⁶, the LHC instrument that made the unexpected discovery. "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."   These tiny droplets "flow" in a manner similar to the behavior of the quark-gluon plasma⁷, a state of matter that is a mixture of the sub-atomic particles that makes up protons and neutrons⁸ and only exists at extreme temperatures and densities⁹. Cosmologists propose that the entire universe once consisted of this strongly interacting elixir¹⁰ for fractions of a second after the Big Bang when conditions were dramatically hotter and denser¹¹ than they are today. Now that the universe has spent billions of years expanding and cooling, the only way scientists can reproduce this primordial¹² plasma is to bang atomic nuclei¹³ together with tremendous energy.   The new observations are contained in a paper submitted by the CMS collaboration¹⁴ to the journal Physical Review D and posted on the arXiv preprint server. In addition, Vanderbilt doctoral student Shengquan Tuo recently presented the new results at a workshop held in the European Centre for Theoretical Studies in Nuclear Physics and Related Areas in Trento, Italy.