The shape of a subatomic fireball

White outlines indicate the shapes of quark-gluon plasma droplets, derived from data measured with CMS.

Shapes in the subatomic realm are difficult to describe. Ask a physicist about the shape of an electron, and she might talk about the shape of the probability cloud of possible electron positions, or maybe say that the electron itself is a speck of zero size (to the limit of anyone’s ability to measure), or might just ask what you mean by “shape,” anyway. The macroscopic world that we know well is full of objects assembled from septillions of particles, and these particles are distributed in space as shapes. Fundamental particles are, by definition, not made of anything but themselves, so they can’t always be described in familiar terms like shape, color and taste (except metaphorically).

However, not all particles are fundamental. Mesons are pairs of quarks bound by the strong force, protons and neutrons are made of three quarks, and more exotic combinations may have recently been discovered. The quarks orbit one another in specific spatial patterns that could be called shapes, though the physicists who study them speak in terms of form factors and deep inelastic scattering.

Another spatially extended subatomic object is the fireball that forms when heavy nuclei collide. Unlike bound states of quarks, this substance is a kind of fluid — a rapidly expanding droplet in which quarks and gluons flow and mix. When it is first formed, the droplet is shaped like the overlap of the two nuclei: circular if the nuclei collided exactly face-on and almond-shaped if one was offset from the other (see figure in this related article). A few yoctoseconds later, the blob spreads out and changes shape according to pressure differences. The fact that this spreading is mostly elliptical is evidence that the quark-gluon fluid has almost zero (or exactly zero) viscosity.

But there’s more to that shape than a simple ellipse, as CMS scientists revealed in a recent paper. The complete picture looks like the figure above: peripheral collisions result in a mostly elliptical shape with pear-like offsets, while central collisions result in a droplet that is slightly triangular. The triangular component was at first surprising, since the overlap region of two circular nuclei would be symmetric. Nuclei are not rigidly circular, though, and asymmetries in the collision allow asymmetries in the droplet to develop.

Not only do these measurements improve our understanding of an extreme state of matter, but it is satisfying to finally see the shapes of liquid splashes a trillion times smaller than a drop of water.



The scientists pictured below performed this detailed measurement of the shapes of quark-gluon fireballs.

The CMS experiment has participated in the Frontier Science Result column since before the column’s name was invented. Over the last four years, articles describing measurements by US CMS scientists have benefited from editing like that being provided by current Fermilab Today editor Leah Hesla.