It's an illustration of a heavy ion collision among two lead nuclei at a beam energy of 160 GeV per nucleon. I've seen this picture over and over again in talks, leaflets, and even printed in books, typically as a motivation for why heavy ion physics is the thing to do and the quark gluon plasma is cool. Followed by a praise of whatever model it is that the speaker/author is working on.
Now raise your hand if you know who made the picture and how it was made?
Nobody? Well, there's the rub. As often as I've seen the picture, as often the credits were missing. If it is credited to anybody, it's credited to a CERN press release of February 2000 that was summing up the results of the CERN-SPS heavy-ion program before the start of RHIC at Brookhaven. The picture was used as an illustration for that.
As it happens, Stefan and I know exactly who spent weeks on this figure and how its production came along, since at that time we were sharing an office with the unknown, uncredited physicist behind it. The picture was made by Henning Weber, then a PhD candidate at the Institute for Theoretical Physics in Frankfurt. Henning has meanwhile left academia, which is why you've probably never heard of him. The picture is a visualization of data generated with a numerical model called UrQMD (Ultra-relativistic Quantum Molecular Dynamics). From the code's numerical output that's usually just a looong list of numbers, Henning made a couple of videos showing how a heavy ion collision in this model looks like. He still has his website up, you can look at the videos here.
The grey balls are color-neutral hadrons which the initial nuclei consist of. Shown is a not-quite central collision, one in which the nuclei have a non-zero impact parameter. The red, blue and green balls represent temporarily unconfined quarks in the collision region. After the collision, the quarks hadronize again. Well, actually, the UrQMD code does not explicitly treat color degrees of freedom, so the colors are an artistic rendition of what technically are called "preformed hadrons". The below shows a screenshot of one of Henning's movies. Same picture, but with modest credits to the Frankfurt UrQMD group at the bottom:
So what happened? Well, Henning was asked by his supervisor to provide a visually appealing picture for an upcoming CERN press conference. Henning sat down and spent some days and nights on the picture that he would later refer to as the "frog spawn picture," because said supervisor insisted on making the balls semi-transparent giving them the appearance of fish eggs. This association was even stronger after the relativistic squeeze of the nuclei was removed. More accurately, the nuclei should be flattened to about 1/10 of the initial size in the direction of motion – the Lorentz gamma factor for the SPS fixed-target collisions at 160 GeV/nucleon is, in the center-of-mass frame, γ = 9.2.
The picture then was sent to CERN and used in the press release. And somewhere along the line the bottom with the credits vanished.