The entire universe is filled with an invisible background that quietly gives fundamental particles their mass. Without it, everything would move at the speed of light, meaning no atoms, no chemistry, and no possibility for life. It’s called the Higgs field, and you can imagine it like a perfectly smooth pond. Most of the time, this “pond” is completely invisible. However, if we disturb it, we get a momentary excitation—a ripple, so to speak—that is visible. We call this ripple the Higgs boson, and by studying its properties we can learn a lot about the field that produces it. (And thus, the origin of mass.)
But we also have a lot of questions about the Higgs boson itself. For instance, is it one solid blob? Or does it have substructure? This is one of the questions my group at Johns Hopkins and our colleagues on the CMS experiment are currently exploring.
However, unlike a ripple in a pond, we cannot generate Higgs bosons so easily. We need to concentrate a huge amount of energy into a very tiny region of space, and even if the conditions are nearly perfect, we still only have a tiny chance of creating a Higgs boson. Luckily, at CERN we have the Large Hadron Collider, which has such a high collision rate that we can produce nearly one Higgs per second during favorable operating conditions.
To study whether the Higgs boson has substructure, we had to look for a very special class of events. Unlike stable particles such as electrons, which always have the same mass, the Higgs boson is very short lived and can momentarily appear as a heavier version of itself (What we call, “off shell.”) Occasionally, Nature can produce such virtual Higgs states with an effective mass several times larger than the Higgs boson’s usual mass. If the Higgs boson is fundamental, the rate for these events should follow the precise prediction.
But if the Higgs boson is not elementary and instead has internal structure at distances we can currently probe, then such heavy off-shell Higgs states would be much harder to produce.
To see if these off-shell Higgs bosons exist, we needed to tease out a tiny signal from the enormous backgrounds. To do this, our team had to develop a detailed computer simulation model, cutting‑edge data analysis techniques, and advanced statistical modeling.
When we unblinded the data, we saw that the rate of off-shell Higgs boson production matches our model’s prediction, indicating that the Higgs boson shows no evidence of hidden structure at the distances we can currently probe and is therefore most likely fundamental. If the Higgs boson is composite, then its substructure must exist at distances far smaller than what we can currently probe.
Our result suggests that if the Higgs boson has any finite size at all, it must be tiny: smaller than 0.2 attometers (2 × 10⁻¹⁹ meters.) By comparison, a proton has a radius of about 840 attometers and is the smallest object in ordinary matter known to have a finite size.
But as a scientist, I also know that history likes to prove us wrong. Once, the atom was thought to be indivisible. Then protons and neutrons were considered fundamental, until we discovered quarks and gluons inside. Today, quarks and gluons (and the Higgs boson) are treated as elementary, and our model describing them is self‑consistent and incredibly successful. But that doesn’t prove that the Higgs boson is truly fundamental; it only means that, with the model and tools we have, it looks that way.
This is what makes Higgs research so exciting and so important. The Higgs boson was only discovered 14 years ago, and it is unlike anything we’ve seen before. The Higgs boson and the field that generates it sit right at the edge of what we know, and what our current machines can test. By pushing Higgs research to the extremes, we are not only probing some of the deepest questions we have about the universe; we are pushing today’s best technology to its limit and driving the development of new computing tools and analysis techniques.
