By Don Lincoln, Fermilab
In 1994, two big experiments were being run at a particle accelerator called the Tevatron, hosted by Fermi National Accelerator Laboratory, located just outside Chicago. As is very common, the experimental groups were simultaneously running the accelerator, analyzing data, and upgrading the equipment. In 2001, there was a final big upgrade and a serious effort was made to search for Higgs bosons.
Tevatron’s Era
At the time, the Tevatron was the most powerful particle accelerator in the world, slamming together a beam of protons and antimatter protons at energies over 2,000 times higher than the mass of a proton.
The Higgs boson gives particles their mass. The particles it interacts with more strongly get a higher mass. There’s another consequence of this stronger interaction with certain particles. If the boson interacts with particles more strongly, it will decay into them more often. Thus, what was done was to look for events in which very heavy particles were created.
Roadblocks for Tevatron
The heaviest particles known at the time (even still) are top quarks, which are about 180 times heavier than protons. The next lower are the Z bosons, then the W bosons—those are both in the ballpark range of 90 times heavier than protons—and then the bottom quarks, which are comparatively light, only just shy of five times heavier than the protons.
Top quarks are just super heavy, too heavy for the Tevatron to see in conjunction with Higgs bosons, but W and Z bosons are more reasonable. So, scientists looked for Higgs bosons decaying into those particles but didn’t see any. This meant they could rule out the possibility that Higgs bosons had a mass in the range of 156 that of a proton and about 190 times.
And that was sort of the end of the road for the Tevatron. That’s because a much more powerful accelerator called the Large Hadron Collider, or LHC, had begun operations.
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The Large Hadron Collider
The LHC is located at the CERN laboratory, just like the LEP accelerator. In fact, those two accelerators used the same tunnel. Technicians pulled out the LEP accelerator and put in the LHC one.
The LHC is designed to be seven times more powerful than the Tevatron and, although it wasn’t working at full potential back then, in 2011, it was still way more powerful. Once it turned on, the Tevatron was permanently outclassed.
In the LHC, scientists smashed together beams of protons, and they looked for the Higgs boson in much the same way as was done using the Tevatron. They saw even more definitively that the mass of the Higgs wasn’t super high. The LEP said the Higgs had to be heavier than 122 times the mass of a proton, and the Tevatron said that it was probably below 156 times the proton’s mass.
Result of LHC Data
Using the first hints of LHC data, scientists quickly ruled out the same range that was done using Tevatron data, which was between 166 and 187 times the mass of the proton. That was expected, but it was a nice independent confirmation.
Things really heated up in 2012, when there was a lot more data. And, on July 4, 2012, the two big experiments stopped saying what masses the Higgs boson didn’t have and said definitively that the Higgs boson had been found and that its mass was about 133 times that the mass of a proton.
Both experiments got the same answer, which was also nice.
In 2013, Peter Higgs and François Englert shared the Nobel Prize for their predictions of the Higgs field and Higgs boson. Englert had collaborated with Robert Brout, but he died in 2011 and didn’t live to see the discovery. And since the Nobel Prize can go to a maximum of three people, Guralnik, Hagen, and Kibble lost out.
Search Ends for Higgs Boson
So, where do we stand in the search for the Higgs? Technically, what the scientists announced in 2012 was that they found a particle that was consistent with being a Higgs boson. It wasn’t definitive.
But, in the ensuing years, they have done a lot of tests, verifying that the new particle had zero subatomic spin, which is what the Higgs boson has to have. They also have looked at its rate of decay into a ton of subatomic particles, for example the tau lepton, bottom quark, W and Z bosons, and even top quarks. It’s a tricky business talking about the last three particles, since they are too heavy to be daughters of the Higgs boson according to classical physics. But quantum mechanics allows for things to happen that are classically impossible and the measurements are in exact agreement with Higgs theory.
With these additional measurements, the world’s scientific community has concluded that we have certainly found the Higgs boson predicted back in 1964. It took half a century to accomplish that, but that’s simply the nature of research in the knowledge frontier.
Common Questions about the Discovery of Higgs Boson
Before the Large Hadron Collider, or LHC, began operations, the Tevatron was the most powerful particle accelerator in the world, slamming together a beam of protons and antimatter protons at energies over 2,000 times higher than the mass of a proton.
The heaviest particles known are top quarks, which are about 180 times heavier than protons. The next lower are the Z bosons, then the W bosons—those are both in the ballpark range of 90 times heavier than protons—and then the bottom quarks, which are comparatively light, only just shy of five times heavier than the protons.
The particle found in 2012 was consistent with being a Higgs boson, but it wasn’t definitive.
However, in the ensuing years, the scientists have done a lot of tests, verifying that the new particle had zero subatomic spin, which is what the Higgs boson has to have. They also have looked at its rate of decay into a ton of subatomic particles, for example the tau lepton, bottom quark, W and Z bosons, and even top quarks, and the measurements are in exact agreement with Higgs theory.
With these additional measurements, the world’s scientific community has concluded that we have certainly found the Higgs boson.
