Why you should care about the Higgs boson
What the big discovery means to you.
Patrick Fox, Fermilab
Could you explain the July 4 announcement in your own words?
The teams behind two general-purpose experiments at CERN (ATLAS and CMS) both presented results from their search for the Higgs boson. They looked at several ways the Higgs can be produced and subsequently decay, and they both saw events consistent with the presence of a standard model Higgs boson.
They looked at two ways in which the Higgs can decay — at events with two photons in them and events with four leptons (e.g., four electrons) in them. And in both cases, they saw a “bump.” That is, when they added up the energies of the two photons, they found that a small subset of them always had a combined energy of 125 giga electron volts (GeV). That was more than would be expected from random events alone. This excess, and a similar one in four leptons, was an indication of the presence of a Higgs boson. Both experiments’ results are consistent with each other and point toward a Higgs mass of 125 GeV.
Each experiment has approximately “five sigma” significance, the gold standard in particle physics. This is a statistical measure of how likely this result is to happen by accident alone if there were no Higgs, and thus of how confident we are that it’s a real effect and not just some fluke. The probability of this excess to happen by accident is the same as the chance of tossing a coin and calling the outcome correctly 21 times in a row, so we are confident there is — and indeed both experiments announced the observation of — a new particle! We will now spend the coming months accumulating more data and analyzing it to see if this new particle has all of the right features to be a Higgs.
What does this mean for science?
First, it is very exciting and the first discovery of a new particle since the tau neutrino in 2000. But unlike the tau, which is a copy of previously seen particles, the Higgs is a unique type of particle: a fundamental scalar (meaning it has a spin of zero). We have never observed this type of particle before; it is the final particle in the standard model that we were waiting to see. This one observation also impacts how we think about the field. Before this, many logical possibilities remained for what completed the standard model. Now that we have seen a Higgs, many of these possibilities are ruled out. Furthermore, the Higgs has a special role to play in the standard model; it is the physical manifestation of the field responsible for the mass of all fundamental particles (electrons, quarks, Z bosons, etc.). Without the Higgs field, and the associated boson, everything would be massless and move at the speed of light — clearly not the world we live in.
What does it matter to the average non-scientist?
I have received several emails from old high school friends that I have not spoken to for many years congratulating me on this discovery and asking me questions about what it all means. So this discovery clearly has great resonance with the public. I think people are always interested to hear news about fundamental physics and to learn that we understand a little better how the universe works. It’s also a great story of man’s ingenuity and endeavor. Physicists originally predicted this particle (assuming it really is the Higgs boson) more than 40 years ago. It is extremely hard to make, and then observe, at colliders. Despite this, we used the most complicated machine ever built to sift through enormous amounts of data and discovered that this prediction made so long ago appears to be true. It’s a great detective story.
Is there a connection between this finding and astronomy?
The Higgs is heavy, 130 times the mass of a proton, and so has not existed (until we started making them again at colliders) since the early stages of the universe, right after the Big Bang. It is also possible that when dark matter annihilates in the center of our galaxy, the outcome of the energetic annihilation may include Higgs particles, which quickly decay. However, there is also somewhat of a philosophical connection to astronomy. In some sense, the Higgs field, which permeates all of space and time, is like the aether of yore — the theoretical medium through which light propagated. Unlike the aether (whose non-discovery by Albert Michelson and Edward Morley led to Albert Einstein developing the theory of relativity), the Higgs field does not break a symmetry of space and time but instead breaks an “internal” symmetry and results in gauge bosons (or force-carrying particles) like the W and Z becoming massive.
What was your reaction when you heard the news?
The timing of the talks at CERN was such that they started in Chicago at 2 a.m. on July 4. So, I went to bed for a few hours the night before and then got up very early and drove to Fermilab, which had set up a live feed in the main seminar room. I was surprised to see the room completely full, and in fact there had to be an overflow room to accommodate all the people (scientists and non-scientists) who had shown up. When the first plots that contained the evidence for a new particle were shown, people broke into spontaneous applause, and when both talks were finished, there was a standing ovation. Afterward, we had champagne and cake. People were very excited.