In the US, the 4th of July means celebration: picnics and fireworks and, among the more historically minded, even a respectful reminder of the country’s first official step toward independence nearly two and a half centuries ago. For particle physicists, July 4 has a different meaning: it is the birthday of the Higgs boson†
Exactly ten years ago, researchers announced the discovery of the Higgs boson, popularly referred to as the “God particle,” after Leon Lederman’s book of the same name. The boson is the observable consequence of the Higgs field, an energy field that spans the universe and gives mass to nature’s smallest known building blocks. The theory describing those building blocks is called the Standard Model, and the Higgs boson was the last part of the theory to be observed.
The missing piece of the standard model
The Standard Model is the most successful theory proposed to describe the nature of the physical world. Using just twelve particles and a few forces, scientists can explain the outcome of all the experiments investigating the nature of matter and energy in the subatomic realm.
The familiar world of pizzas and puppies, tornadoes and sunsets doesn’t require the full power of the standard model; instead, only a subset is needed. Our world is made of atoms, each composed of even smaller protons, neutrons and electrons. Physicists know of even smaller subatomic particles called quarks, which are found in protons and neutrons. Combined with two different types of nuclear forces and electromagnetism, researchers understand why the sun burns and airplanes fly.
The standard model almost died
The standard model was developed in the 1960s and 1970s and looked promising. However, it had a seemingly fatal flaw. It only worked if all of nature’s building blocks had zero mass, and even when the model was being developed, this was known to be untrue. After all, the mass of the electron had been measured and known for decades and the electron is one of the twelve particles of the Standard Model.
This prediction could have killed the theory, but it was saved in 1964, when three different groups of physicists proposed what has come to be known as the Higgs field — named after Peter Higgs, one of the physicists involved in the development of the theory. The Higgs field is an addition to the Standard Model—actually a plaster—that preserved the model’s explanatory power, but also gave mass to nature’s tiniest building blocks. However, the entire building of the Standard Model relied on the discovery of the Higgs boson. Not a Higgs boson (or anything similar), and the whole theoretical concept was compromised.
A high stakes yacht
So the hunt was on. And it wasn’t easy. It took nearly half a century to develop the technology capable of producing Higgs bosons. Finally, in the summer of 2012, researchers succeeded. (Disclosure: I am one of the co-authors of the two papers describing the discovery of the boson.) How did we do it?
Scientists took two beams of highly energetic protons and accelerated them nearly to the speed of light using the Large Hadron Collider (LHC), the world’s most powerful particle accelerator. The beams collided in the middle of two huge detectors, each the size of an average-sized office building. Using the physics embodied in Einstein’s famous equation, E = mc2the energy of the collision was converted into Higgs bosons.
The Higgs boson was not directly detected because it decays too quickly, so researchers looked for its decay products. After much effort and cross-checking of data, the discovery of the Higgs boson was formally announced. The Standard Model of particle physics was validated and this grand achievement was recognized by the 2013 Nobel Prize in Physics to Peter Higgs and François Englert, two of the architects of the Higgs theory.
What have we learned since then?
However, the first data reported a decade ago was preliminary and sketchy, with many questions still unresolved, including the possibility that additional variants of Higgs bosons have yet to be discovered. So, what improvements have the intervening years brought?
For starters, researchers are now pretty sure that the Higgs boson predicted in 1964 and the particle discovered in 2012 are the same. Furthermore, there are probably no additional variants. Scientists have studied the decay properties of the boson and found that it decays in several ways, just as expected. In short, the Higgs theory seems to hold true.
On the other hand, the Higgs theory is simply an addition to the massless particle version of the Standard Model. It does not arise from underlying principles. This means that we don’t really understand it on a satisfactory level. To reach that level of understanding, more research is needed.
What we will continue to learn
For the next two decades, scientists will continue to use the LHC. The LHC began operations in 2011, with the discovery of Higgs in 2012. Since then, researchers have collected 20 times as much data, making them even more confident that they have found “the” Higgs boson. In the coming years, scientists will double the data again. Then, after a few years of upgrades, the LHC will resume operations, providing 20 times the data currently recorded. In other words, today’s data represents only five percent of what the final result is expected to be, with a corresponding improvement in our understanding.
However, to really dig into the underpinnings of the Higgs boson theory, scientists need to create another accelerator, one that collides electrons and antimatter electrons. Such an accelerator would be much more precise than the very powerful but clumsy LHC. If groundbreaking research were to open one door, the LHC would be a sledgehammer, while the next accelerator would be a wrench.
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Construction of this new accelerator has not yet started. (No final decision has been made to build it.) But if approved, it won’t be operational until the 2040s. This new accelerator could improve the measurements enough to reduce the uncertainties by a factor of ten and often more. It would be able to precisely measure the lifetime of the Higgs boson and even investigate how the Higgs field gives mass to the Higgs boson itself. Measurements like these will shed light on the fundamental origins of the Higgs field.
The past decade was the childhood of experimental Higgs physics. The next decade will bring our understanding of the Higgs boson into full maturity, followed by a lifetime of discoveries.
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