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CMS measures the diffusion wakes jets leave in quark-gluon plasma, 20 years after the prediction
CMS physicists have measured, for the first time, the diffusion wakes jets leave behind in quark-gluon plasma, 20 years after they were first predicted. The result adds a second test of the fluid picture of the matter that filled the universe for its first microseconds.
The Scientist · Science desk
What happened
- CMS made its plasma by colliding lead nuclei head-on at CERN, producing hot, dense droplets that last about 10^-22 seconds.
- The plasma's low viscosity was first inferred in 2005, from heavy-ion collisions at Brookhaven National Laboratory's Relativistic Heavy Ion Collider in New York.
- According to phys.org, the plasma's shear-viscosity-to-entropy-density ratio is under a tenth of room-temperature water's and about a millionth of dry air's.
- The CMS Collaboration published the wake measurement in Physical Review Letters.
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Why it matters
- capability Fluid models of the plasma now face a test beyond its viscosity: they have to reproduce the wake a jet leaves as it moves through the liquid.
- exposure The dip only appears after much larger flow structures are subtracted, so the subtraction method is the part of the result most open to challenge.
- precedent Other heavy-ion groups have spent the same two decades learning that subtraction, putting an independent measurement of the wake within their reach.
A boat on deep, smooth water leaves turbulence along the line directly behind it and two diffusion wakes at a theoretical 19.5 degrees to that line [4]. In the plasma, the boat is a jet. A quark or gluon occasionally gets enough energy to shoot off in one direction while an equal momentum streams the opposite way [11]. Interactions with the plasma usually knock the pair away from a clean 180 degrees, and the two streams become jets [11]. The prediction was that jets crossing a liquid plasma would leave diffusion wakes of their own [3].
The plasma is far from the smooth water of the analogy. "The QGP droplet explodes and expands in a complicated way with a typical expansion velocity of order half the speed of light," Olga Evdokimov of the University of Illinois Chicago, a CMS collaborator, told Phys.org [12]. Each collision leaves thousands of particles in a region a few to ten proton diameters across [13]. As the jets move out through the expanding droplet, large-scale changes appear in the particle distribution [17]. "Ridges, bumps and valleys that are much bigger than the tiny dip we expect from a single wake" occur, Evdokimov said [14]. Those structures have to be subtracted before a wake can show, a technique CMS and other groups developed over the past two decades [15].
Even after subtraction, one collision is not enough. The wakes are too thin to see event by event, so CMS examined millions of collisions and picked out a subset to combine [16]. I think averaging is the right design for a signal this small. Fluctuations that differ from one collision to the next tend to cancel, and a feature that recurs builds up. The news account of the result does not give the depth of the dip, its statistical significance or the full selection criteria.
The wake is not the first evidence that the plasma is a liquid. That conclusion dates to the 2005 viscosity inference at Brookhaven [7], and the wake prediction was built on it [3]. Evdokimov said the plasma is the "hottest, densest and most perfect fluid in the universe" [9]. In my view the value of the new result is narrower and sturdier. A liquid should respond to something moving through it, and the form of that response was on record for 20 years before CMS measured it [3].
The link to the early universe comes with conditions. The plasma filled the universe from about a trillionth of a second after the Big Bang until a few microseconds after [5], and it is about 200,000 times hotter than the center of the sun [6]. CMS observes lab droplets that last about 10^-22 seconds [10]. Carrying the result back to the first microseconds rests on the two being the same state of matter [5]. Taking one microsecond as a floor, the primordial phase lasted at least 10^16 times longer than a droplet [1].
What to watch
- A quantitative comparison of the measured dip's depth and angle with hydrodynamic predictions, to show how closely the liquid picture fits.
- Whether the dip holds up when the collision selection and background subtraction in the full Physical Review Letters paper are varied.