Backscatter microscopes

Scattering reveals structure: the path of a bowling ball would hardly be deflected by a pin, but the ball could even bounce backward if it hit something heavy and hard, like a lamppost.

One hundred years ago, Hans Geiger and Ernest Marsden puzzled over the apparatus on their workbench, arguably the first particle physics experiment in history. Geiger had earned his doctorate only three years earlier and Marsden was an undergraduate. In this experiment, a vial of radon gas emitted a beam of charged particles at a tissue-thin leaf of gold. The particles hit the gold atoms and bounced off at random angles. The physicists counted the number of recoiling particles at each angle through a swiveling microscope. They were astounded to see them scattering everywhere, even backward.

The beam of particles was one of the most energetic known at the time, and it was expected to glance off of the gold atoms like a curling stone, not fly off like a hockey puck. Geiger, Marsden and their mentor Ernest Rutherford had discovered substructure within the atom: particles rebounded at such wild angles because they were hitting tiny, massive nuclei of protons and neutrons.

A century later, physicists around the world pore over data from the LHC. Beams of particles a million times more energetic than Geiger and Marsden’s collide head-on in the middle of a 40-foot detection system known as CMS. Some of these physicists, including students and postdocs at Fermilab, are looking for signs of substructure within quarks, just as quarks are within protons and neutrons, which are within nuclei, within atoms.

The majority of LHC collisions result in debris that scatters at small angles from the beams. But if quarks contain small, hard structures like the nucleus within the atom, then the highest-energy collisions would result in wide-angle scattering. In a recent paper, these physicists showed that there is no sign of substructure down to almost 10−20 meters, which is 370,000 times smaller than a gold nucleus. If Geiger and Marsden had discovered a nucleus the size of a city block, these scientists are looking for amoebas crawling on the sidewalk.

Are quarks the smallest parts of the nucleus, or is there a deeper layer? Though the instruments of discovery have become more sophisticated, the curiosity remains the same.

Jim Pivarski

The physicists pictured above performed key aspects of the search for substructure within quarks.
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