All scientists are driven to explain how and why things are the way they are. The differences among the various scientific disciplines lie in the scale and location at which we choose to investigate these basic questions. Two of the most fundamental questions one could ask about matter – anything that takes up space and has mass – are: “What is matter made of?” and “Why does matter have mass?” It is very rare nowadays to see a scientific discovery that helps answer questions as fundamental as these, which is why the July 4th announcement that scientists at the European Council for Nuclear Research (CERN) had discovered a new particle that behaved like the Higgs boson was such a big deal – it represents a step toward a more complete understanding of the universe.
In thinking about mass, we might think of how much that large box of biology textbooks weighs and realize that weight has something to do with the gravitational force. However, how exactly that force affects matter was a mystery, so particle physicists turned to subatomic scales to study the building blocks of matter. It was at this subatomic level that University of Edinburgh Professor Peter Higgs had theorized that a particle was responsible for mass. Just like you can’t talk about chemistry without understanding atoms, or biology without cells, it is difficult to understand what the Higgs boson is, and why it is significant, without understanding some basics of particle physics.
All matter that we can see in the universe is comprised of 12 fundamental subatomic particles, at a level smaller than the more familiar protons and neutrons that comprise the nuclei of atoms. To go along with the twelve fundamental particles, there are four fundamental forces that govern these particles’ interactions. The gravitational force is probably the most familiar, but within an atom it is so much weaker than the other three forces that its effect is considered negligible at the particle level. The remaining three forces are the strong force, which binds protons and neutrons together within an atomic nucleus, the weak force, which is responsible for radioactivity and enables the hydrogen fusion that fuels the sun’s energy, and the electromagnetic force, which determines how electrons interact and explains most everyday chemical interactions.
But how are these forces transferred between particles? Associated with each of these forces are mass-less force-carriers, which are bundles of energy from each of these fields that can act on particles with mass. The photon is the force carrier for the electromagnetic force, the gluon for the strong force, and the W and Z bosons for the weak force. In 1964, Professor Higgs proposed the existence of a particle that acted as a force carrier for a field permeating the entire universe, now known as the Higgs field, to give mass to matter. This particle, if it existed, would be the final piece of evidence verifying the validity of the Standard Model of Physics.
So what are these force carriers and how do they work? To answer this question, let’s travel back in time to the formation of the universe. The immediate result of the Big Bang was a universe comprised of mass-less particles and energy. Somehow, some of these particles acquired mass only fractions of a second after the Big Bang. How is this possible? Scientists believe that they got this mass from traveling through the Higgs field, which, as we noted before, permeates the entire universe. As particles moved through the field they gained mass and slowed down.
To borrow an analogy from Physicist David Miller, these particles acted like a celebrity walking into a party, who is forced to slow down as more and more people surround her. This is in contrast to mass-less particles, such as photons, which move through the Higgs field without gaining mass – much in the same way that a non-celebrity would be able to walk through the party without being slowed by admirers. The force-carrying particle that represents the Higgs field is the Higgs boson. As the new, mass-less particles moved through the Higgs Field they interacted with Higgs bosons and gained mass, and the rest, as they say, is history. This theory has been the generally accepted explanation of why the universe has mass since its proposal by Dr. Higgs in the 1960s, but until recently it lacked one key component… evidence that this particle actually exists!
Force-carrying particles like gluons and bosons decay quickly, and are therefore very challenging to observe. So how have scientists been searching for the Higgs boson? CERN’s Large Hadron Collider (LHC) was built to recreate the conditions present one billionth of a second after the Big Bang.
Figure 1. One of the massive detectors found in the Large Hadron Collider, currently the largest particle accelerator in the world. (Image credit: CERN)
The immense release of energy caused by smashing protons together over and over again inside the Collider at nearly the speed of light causes new particles to form, which almost immediately decay into more stable particles. The majority of the time nothing particularly exciting happens, but by observing the repeated results of these decaying particles millions of times since the LCH began operation in 2008, scientists have been able to look for anomalies that would indicate something different, something predicted to happen only rarely – such as the creation of a Higgs boson. If scientists were unable to find such anomalies, or if they found something totally unexpected, it would mean that something was wrong with the Standard Model, and that we would need to rethink our fundamental understanding of the physical model of the universe.
As it turns out, scientists found a new particle with properties that match what would be expected of a Higg boson. The new particle is 133 times heavier than a proton and behaves the way physicists predicted a Higgs boson to behave, in terms of the particles that result from its formation and subsequent degradation. Furthermore, the LHC’s measurements are so good that statistically there is less than a one in a million chance that the conclusion is wrong. That being said, in physics, sometimes a less than one in a million chance is still not enough, so scientists are being cautious for the time being – and trying very hard not to over-state their findings. However, it is looking more and more certain that they have found the Higgs boson.
Filling in the remaining hole in the model that explains how the twelve particles and three of the four forces are related to each other is no small feat. But there is still plenty of work for physicists to do. For example, the Standard Model does not yet explain why gravity is so much weaker than the other forces, or the nature of the mysterious dark matter that accounts for 84% of all matter in the universe. Dr. Higgs never expected the discovery of his particle to happen in his lifetime – it is exciting to think about what other big discoveries about the nature of the universe will follow in the wake of the Higgs boson discovery!
Heather Craig Olins is a doctoral candidate in the department of Organismic and Evolutionary Biology at Harvard University.
Special thanks to Victoria Swisher and Ken Patton for providing expertise and commentary for this piece.
CERN primer on particle physics: http://public.web.cern.ch/public/en/science/Science-en.html
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