By Don Lincoln, Fermilab
In the 1960s, scientists found a way to unify electromagnetism and the weak nuclear force into a combined force called the electroweak force. According to the electroweak theory, along with a massless photon, there should also be a massive neutral particle called the Z boson, two massive electrically charged particles called the W bosons, and finally a massive and neutral particle called the Higgs boson.
S-p-pbar-S Accelerator
In 1981, a particle accelerator operating at the CERN laboratory in Switzerland began operations. It was called the S-p-pbar-S. The S-p-pbar-S accelerator only accelerated the particles. Researchers needed detectors to inspect the collisions for the signature that W and Z bosons were created. And they actually built two, called UA1 and UA2.
The two experiments started collecting data in 1981 and, for the first few months, the accelerator underwent teething pains and delivered small amounts of beam. But after a shakedown period, things picked up. More and more beam was being delivered and the two experiments were furiously analyzing the data.
Discovery of W and Z Bosons
In January of 1983, the UA1 experiment announced that they had unambiguously discovered the W boson. It had a mass of 85 times that of the proton; very nearly the same mass as a rubidium atom. In June of 1983, the two experiments announced that they had discovered the Z boson, with a mass 96 times that of a proton, or the mass of a molybdenum atom. Both rubidium and molybdenum are extremely heavy.
With the discovery, a good amount of electroweak theory had been confirmed. Scientists had the photon and the W and Z bosons under their belt. The final missing piece was the Higgs boson.
This article comes directly from content in the video series The Evidence for Modern Physics: How We Know What We Know. Watch it now, on Wondrium.
Search for Higgs Boson
The search for the Higgs boson was very hard. And the reason was that the Higgs theory didn’t really nail down the expected range for its mass. The theory predicted that the mass of the Higgs boson was somewhere between 10 and 1,000 times heavier than a proton.
So, in the early years, not much progress was made. There were some simple limits from experiments and measurements in the 1970s and 1980s, and they determined that if the Higgs boson existed (which wasn’t guaranteed in those days), its mass was over 20 times heavier than a proton.
The first real chance to look for the Higgs boson didn’t really begin until about 1991 when the CERN laboratory turned on a new and much bigger accelerator, called LEP. LEP stands for Large Electron Positron, which of course means that the accelerator was large and collided electrons and positrons. Positrons, of course, are antimatter electrons.
The LEP Accelerator
Initially, the LEP accelerator ran at exactly the right energy to make tons and tons of Z bosons. They studied the Z bosons closely, and it will be a long time before anyone surpasses their measurements. But the experiments also had something to say about the Higgs boson. Because there was no indication of the Z boson decaying into a Higgs boson, scientists knew that the Higgs boson—if it existed—had to have a mass of more than the Z boson, or about 96 times that of a proton. We knew this in the early 1990s.
The CERN accelerator scientists made a series of upgrades to the LEP accelerator, eventually more than doubling its operating energy. In the end, the accelerator ran at an energy equivalent to a smidge over 220 times more than the energy it would take to make a proton.
Since the mass of the Higgs boson was unknown, several different ways to look for it were attempted by LEP scientists. They looked for the electron and positron to simply make a single Higgs boson. They also looked for cases where the Higgs boson was made in conjunction with a W or Z boson. Depending on the final mass of the Higgs boson, any one of those processes could be the most common way to make one.
End of LEP Era
The LEP accelerator ran until the year 2000. There was a bit of excitement in the last few weeks, where researchers thought they saw hints of a Higgs boson with a mass of about 122 times the mass of a proton and they even got an extension of the time the accelerator would operate of a month or two. But that hint evaporated in light of more data, as so many hints do.
When the LEP accelerator finished running, the LEP accelerator scientists announced that they could have found the Higgs boson if it had a mass of 122 times that of a proton, but they found nothing. Accordingly, they concluded that if the Higgs boson existed, it would have to have a mass heavier than that. And that was the end of the LEP era.
Common Questions about the Search for Higgs Boson
In January of 1983, the UA1 experiment announced that they had unambiguously discovered the W boson. It had a mass of 85 times that of the proton; very nearly the same mass as a rubidium atom. In June of 1983, UA1 and UA2 experiments announced that they had discovered the Z boson, with a mass 96 times that of a proton, or the mass of a molybdenum atom.
The search for the Higgs boson was hard, very hard because the Higgs theory didn’t really nail down the expected range for its mass. The theory predicted that the mass of the Higgs boson was somewhere between 10 and 1,000 times heavier than a proton.
The LEP, which stood for Large Electron Positron, was large and collided electrons and positrons. Initially, the LEP accelerator ran at exactly the right energy to make tons and tons of Z bosons. Later, the CERN accelerator scientists made a series of upgrades, eventually more than doubling its operating energy. In the end, the accelerator ran at an energy equivalent to a smidge over 220 times more than the energy it would take to make a proton.
