Sometimes the original shape of an object can be hidden by folding. It is the same way in particle physics, in which a type of measurement can hide the underlying physics quantity you want to measure.
Today’s article involves an unconventional metaphor for physics analysis – origami. In origami, an artist takes a piece of paper and folds it into some sort of shape that hides the nature of the original piece of paper. If you want to see what the original paper looks like, you need to unfold the origami sculpture.
This is also true of particle physics analyses. To show the connection, I’d like to highlight a particularly cool analysis done by the CMS experiment.
In 2012, the CMS and ATLAS experiments jointly announced the discovery of a particle that was thought to be the Higgs boson. In the ensuing years, measurements on the discovered particle’s properties has solidified that claim.
However, the Higgs boson doesn’t exist in a vacuum. (Well, it does, but only smart aleck physicists would correct me on that.) What I mean is that the Higgs boson isn’t just created sitting there, waiting to be discovered. In order to make the Higgs boson, quarks and gluons must come together and interact, and the boson must decay into daughter particles. The Standard Model makes firm predictions on these processes, and, in order for the Standard Model to be validated, these predictions must be compared to measurements.
CMS scientists decided to look at the momentum of Higgs bosons after they were made. Momentum is related to the motion of particles and connects the Higgs sector with the theory of quantum chromodynamics (QCD), which is the theory of strong force interactions. QCD tells us what range of momenta should be expected. So far, this is pretty straightforward.
In order to get lots of Higgs bosons to study, scientists looked at instances in which the Higgs boson decayed into two W bosons, which subsequently decayed into an electron, a muon and their associated neutrinos. This particular decay chain was selected because it occurs pretty often and is relatively easy to identify.
These physicists contributed to this analysis.
The down side to this choice are the presence of neutrinos. First, they can’t be seen, and second, there are two of them. Thus scientists must make their measurements using the two observed particles (electron and muon) and a measurement of the combined two invisible particles (neutrinos). Because some information is missing, what they observe is not exactly what they wanted to. They needed some way convert what they measured to what they needed. Bringing back our metaphor, it’s like they saw a perfectly folded little paper origami bird, but they needed to see the original paper. So, what did they have to do? They unfolded their measurement.
By taking the known detector response, they were able to remove the detector effects and limitations to see what they needed. The result wasn’t quite perfect, much like an unfolded piece of origami still has creases and bends, but the basic shape of the paper was visible. And, in this measurement, the result was that the spectrum of momenta of Higgs boson was in good agreement with predictions. Subatomic origami made it possible to better understand the final missing piece of the Standard Model.
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