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Discoveries in physics and cosmology can occur in two ways. The first is that they can be completely unexpected, like the observation that the expansion of the universe is speeding up. The second is when a theory predicts some particular phenomenon, which is eventually validated by experiments. One such example was the observation of the Higgs boson in 2012. That discovery occurred nearly half a century after the particle was predicted. Let's take a look at how just how that happened. (intro music) The Higgs boson is a vibration of a thing called the Higgs field. The Higgs field gives mass to fundamental subatomic particles, like electrons and the quarks found inside protons and neutrons.
We can't see the Higgs field itself, but we can know it exists by seeing vibrations of the field. Using an imperfect analogy, it's like not being able to see water, because it's clear, but knowing that water exists because you can see waves. I've made a couple of videos where I explained the Higgs boson and field in simple ways and the links are in the description. But this video isn't about what the Higgs boson is, it's about how it was discovered. The story begins back in the 1960s. Physicists were trying to figure out a way to combine quantum mechanics and special
relativity in a way that would provide an accurate quantum treatment of fields. Before this work, fields like electric fields and magnetic fields were thought to be continuous. Physicists were trying to figure how to make the fields quantized. Essentially, they were trying to figure out how to include particles like photons in what are now called quantum field theories. The original work predicted that only massless particles would exist. This was done by Julian Schwinger back in 1961. The next year, Philip Anderson figured out a way to give particles mass, but his theory
didn't include relativity. In 1964, three groups of people worked out ways to add relativity into the theory. One group included Tom Kibble, Gerald Guralnik, and Carl Hagen. Another group included Francois Englert and Robert Brout. The third group was Peter Higgs, working alone. Of course, it was Peter Higgs from which the Higgs field got its name. However, these six guys didn't predict the Higgs boson itself. That occurred in 1967, when other researchers tried to apply the earlier work to the weak force. Steven Weinberg, Sheldon Glashow, and Abdus Salam were responsible
for that effort. When the efforts of all of these guys were combined (and others too, of course), the Higgs boson was predicted. The problem was that nobody really had the faintest idea what the particle's mass would be. The first real attempt to search for the Higgs boson was conducted using the LEP accelerator at the CERN laboratory in Europe. The LEP accelerator was a ring 27 kilometers in circumference and it was designed to study the Z boson, which is one of the particles that mediate the weak nuclear force. The LEP accelerator was eventually upgraded in energy to
be able to study the W boson, which is the other particle that mediates the weak nuclear force. The LEP accelerator collided electrons and antimatter electrons together, and it was a spectacularly successful accelerator. It ran from 1989 to 2000. Researchers using the LEP accelerator never observed the Higgs boson, but they were able to put some constraints on its mass. If the Higgs boson existed, it had to be heavier than about 114 times heavier than a proton. And, in 2000, the LEP accelerator was turned off for the last time so it could be dismantled to give space for the Large
Hadron Collider to be built in the same tunnel. While the LEP accelerator was searching for the Higgs boson, it wasn't the only high energy accelerator running at the time. Here at Fermilab, we were running the Tevatron, which back then was the highest energy particle accelerator in the world. We collided protons and antimatter protons together at energies about ten times higher than the highest ever achieved at LEP. With that much more energy, using the Tevatron to search for the Higgs boson was an obvious thing to do. The Tevatron ran from the mid-1980s through 2011,
but it was in 2000 that things got interesting. Remember that we didn't know if the Higgs boson really existed and, if it did exist, we didn't know what its mass was. The LEP measurements ruled out masses under 114 times as heavy as a proton. And some precision measurements ruled out a mass higher than about 185 times as heavy as a proton. In this picture and in all others, the yellow is the range of masses where the Higgs boson could still exist. Because the Fermilab Tevatron could in principle produce collisions with a mass as much as 2000 times heavier than a proton,
it meant we could look at all energies. That 2000 number was the absolute maximum and was very rare. It was far more common to create collisions with say three or four hundred times the mass of a proton. But the bottom line is that we could search for very high mass Higgs bosons. And we didn't find any. So, back in 2009, measurements from the Fermilab Tevatron ruled out the range of 160 to 170 as possible masses for the Higgs boson. Of course, we didn't stop there. We continued to collect data and do more analysis. There were some intermediate reports of measurements, but
by March of 2012, we'd ruled out some more space. Tevatron data hadn't discovered a Higgs boson, but it ruled out the mass range of about 100 to 106, which partially confirmed the earlier LEP range, and also from 147 to 180. Furthermore, the data was starting to show signs that the Higgs boson might exist in the range of 115 to 135. So that was exciting. Meanwhile, the Large Hadron Collider (or LHC) was coming on. By 2010, the LHC was up and running, with 3.5 times the energy of the Tevatron, plus more collisions per second. And in 2011, the LHC energy got boosted to four times
the energy. The writing was on the wall. It's like comparing a propeller-driven airplane to a jet. It's just hard to beat new technology. So, with the LHC in the race, the only advantage the Tevatron experiments had was that we had years of experience working with the detectors. There was always the hope that the Tevatron scientists would find the Higgs first. But the odds didn't look good. By the spring of 2012, the LHC experiments ruled out a Higgs boson mass between 130 and 474. And then July 2012 rolled around. It was time for sudden death. I should probably pause for
a minute and tell you a bit about the Tevatron/LHC competition. The competition was there, it's true. Both laboratories certainly wanted bragging rights. But it wasn't so easy to distinguish between the two efforts. Many scientists were working on both experiments. I was, for example. I was analyzing data both on the DZero experiment at the Tevatron and the CMS experiment at the LHC. And I wasn't unique. Lots of scientists were moving from the Tevatron to the LHC. As I said, the writing was on the wall. The truth is that the knowledge gained in Tevatron research helped LHC
research, just like Tevatron scientists benefited from earlier work. In any event, on July 2, 2012, the Tevatron experiments announced where we stood. The green region shows the range of masses the Tevatron ruled out. But you can also see the purple range. That's the range that the LHC had ruled out. And you see that the LHC ruled out more than the Tevatron. Furthermore, the yellow, which was the range where the Higgs boson could still exist, was getting very small. But, still, nobody had found the Higgs boson yet. That changed two days later when, on July 4, 2012,
the LHC experiments announced their results. There were two LHC experiments that could have found the Higgs and both of them announced the discovery that day. The ATLAS experiment said that they found the Higgs boson and it had a mass of 126.5, while the CMS experiment came up with a mass of 125.3. Given the uncertainties in their measurements, the two experiments basically confirmed one another, and the Higgs boson was found. So, what's the status now, over a decade after the Higgs boson was discovered? Basically, the journey continues. More data has confirmed
the discovery. When all data is combined, the current estimate for the mass of the Higgs boson is 125.25 plus or minus 0.17. Furthermore, we've confirmed lots of the properties that the Higgs boson was predicted to have back in the 1960s and 70s. Currently, scientists are making precise measurements of the properties of the Higgs boson, looking for anomalies. And that brings us back full circle to when I started this video and I mentioned serendipitous discoveries, like the accelerating expansion of the universe. Given that the Higgs boson and Higgs field
is intimately tied to the mass of fundamental subatomic particles, it's entirely possible that precise measurements will turn up a surprise or two. Now, if you'll excuse me, I gotta get back to the lab. A couple of students found an unexplained wiggle in the data. It might be nothing. Indeed, it's probably nothing. But we won't know unless we go take a look. (phasing sound) Okay- so that was a bit of a trip down memory lane. Being part of a big discovery is great fun. And we should give credit where credit is due. On behalf of my hundreds of Tevatron colleagues and thousands of LHC ones, I'd like
to thank the support staff at both Fermilab and CERN who keep the accelerators operating and make the discoveries possible. Research like this is a group effort and everyone does their part. If you liked the video, please subscribe to the channel and share on social media. I'm sure your friends will want to hear about the history of this momentous discovery - one of the biggest ones in physics. And that's saying a lot because, well, as you know- physics is everything. (outro music)
