CERN’s ATLAS Collaboration has released a new measurement of the mass of the Higgs boson with an accuracy of 0.09 percent. Since the Higgs’ mass is one of the fundamental factors that describes many parts of the world, this sort of accuracy can improve our understanding of many other particle interactions.
For decades the Higgs boson was the great white whale of particle physics, the last particle in the Standard Model to be found. The Large Hadron Collider was built partly to find it. In 2012 it was found with sufficient certainty to please the physics community, leading to the giving of the 2013 Nobel Prize. Nevertheless, those results still involved great doubt regarding the Higgs’ mass, probably the most important thing we could learn about it other than its presence.
Since then, there have been several attempts to narrow down the possible mass range further, with this latest finding the most successful yet.
Masses for subatomic particles are measured in electron volts, which sounds confusing until you remember that by Einstein’s famous equation, energy and mass are equal, other than the factor of the speed of light squared. According to the new finding, the Higgs’ mass is 125.11 ± 0.11 gigaelectron volts (GeV).
This puts the Higgs’ mass near the bottom of the 2012 statement, which placed it between 125 and 126 GeV. Previous work had found that for the Higgs to be the particle we thought it was, it needed to have a mass between 114 and 143 GeV, so the true number turns out to be quite near the middle of that range.
Peter Higgs, after whom the particle was named, and colleagues decided the Higgs was necessary because there had to be a particle that carried the Higgs Field. This would put mass on many particles, especially the W and Z particles that transfer the weak nuclear force. Without such particles, the universe simply wouldn’t work.
That was in 1964, and the search for it took almost 50 years, followed by another decade and counting to narrow the mass further. The exact value of the mass shapes the way the Higgs interacts with other particles, shaping what we expect and where we look for many other parts of physics today, and in the universe’s first moments.
It might seem an easy matter to find and measure the mass of such an important particle, but a key feature of the Higgs is that it doesn’t last very long. Its estimated lifetime is around 10-22 seconds. Consequently, we never get a look at the Higgs itself, only reconstructing it from the products it decays into.
In this case, the measurement was made by first slamming protons together at great speed to make Higgs bosons, then watching their breakdown. The decay took place through two channels, either to high-energy gamma rays or to a real and virtual Z boson pair, which then decay to four leptons, each of which was measured.