Today, scientists at CERN, the European Organization for Nuclear Research, have announced that for the first time they have observed how the Higgs boson is transformed into elementary particles known as bottom quarks when it decays. Physicists have predicted that this is the most common way in which most Higgs bosons should decay, but until now it has been extremely difficult to discern the subtle signals of decay. The discovery is an important step in understanding how the Higgs boson masses all the fundamental particles in the universe.
The scientists discovered their discovery using the ATLAS and CMS detectors, two large-scale experiments to analyze the high-energy particle collisions produced by the Large Hadron Collider (LHC) at CERN, the largest and most powerful particle accelerator in the world. 1
Both the ATLAS and CMS teams spent several years refining techniques and integrating more data in their search for the most common Higgs boson decay. Finally, both experiments confirmed that for the first time they found evidence of a Higgs boson breaking down into a bottom quark with a statistically high level of confidence.
MIT physicists at the Nuclear Sciences Laboratory were involved in the analysis and interpretation of data for this new discovery, including Philip Harris, Assistant Professor of Physics. MIT News talked to Harris, who is also a member of the CMS experiment, about finding a vanishing transformation and how the new Higgs discovery could help physicists understand why the universe has a mass.
Q: Put this discovery a bit in context for us. How important is it that your team has watched the Higgs boson decay to bottom quarks?
A: The Higgs boson has two distinct mechanisms: it confers particles of beta decay to the forces involved in the electroweak interaction; and it gives mass to the fundamental particles within the atom, the quarks, and the leptons (such as electrons and muons). Despite the fact that it accounts for both mechanisms, the Higgs discovery and subsequent Higgs property measurements were largely performed with the electroweak force particles. We have recently observed direct Higgs interactions with matter. This measurement, the Higgs boson decays into a bottom quark, is the first time we have directly observed Higgs-to-Quark interactions. This confirms that quarks actually get mass from the Higgs mechanism.
Q: How difficult was this discovery, and how was it finally observed?
A: About 60 percent of all Higgs decays belong to bottom quarks. This is the largest single decay channel of the Higgs boson. However, it is also the channel that has the largest background [noise from surrounding particles]. Depending on how you count it, it's about a million times larger than the channels we used to discover the Higgs boson.
People like to compare Higgs measurements with finding a needle in a haystack. Here I think a more appropriate analogy is a magic eye stereogram. They are looking for a wide distortion of the data, which is very difficult to recognize. The trick to seeing this distortion is like a magic eye: you need to figure out how to focus properly.
To calibrate our "focus," we considered the electroweak force particle, the Z boson, and its decay bottom quarks. Once we were able to see how the Z boson went in bottom quarks, we set our target on the Higgs boson, and there it was. I would like to emphasize that in order to see this distortion clear, we had to rely on a technology that was still in its infancy at the time of the discovery of the Higgs boson, including some of the recent advances in machine learning. A few years ago, it was taught in your Standard Particle Physics class that it is impossible to observe the Higgs decays in some of these channels.
Q: The original discovery of the Higgs boson was touted as a groundbreaking discovery that will eventually uncover the mystery of why atoms have mass. How will this new discovery of Higgs decay help to solve this mystery?
A: After the Higgs boson discovery, we learned a lot about how the Higgs mechanism gives mass to different particles. However, many would argue that high-energy physics has become even more interesting after the Higgs boson discovery because it looks like our conventional view of particle physics no longer fits exactly.
One of the Best Testing Methods Our view is to measure the properties of the Higgs boson. The Higgs-to-Bottom-Quark decay is essential for this understanding because it allows us to directly study the properties of Higgs and quark-matter interactions, and because of its large decay rate we can measure the Higgs boson in all possible scenarios other decay modes not possible.
This observation gives us a new and powerful tool to study the Higgs boson. In fact, as part of this measurement, we were able to measure Higgs bosons with energies twice those of the highest observed Higgs bosons.
Long sought decay of the Higgs boson observed