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In a recent video, I talked about the history of the search for the Higgs boson, specifically how the various experiments were able to narrow down the possible mass ranges in which the Higgs boson could exist. However, many of you asked for more details- how each experiment approached the problem. It's an interesting question, and therefore, due to popular demand, that's what I'm going to talk about today. (intro music) Over the past couple decades, there have been three distinct approaches to search for the Higgs. Each of these approaches are related to the accelerator being used. For each accelerator,
there were multiple experimental groups, but those groups used similar techniques. Thus, I will use the accelerators to define the three eras, specifically the LEP period, the Tevatron period, and the LHC period. The LEP accelerator was located at CERN in Europe. It operated from 1989 to late 2000 and it collided electrons and antimatter electrons. The beam energy was increased over the years. From 1989 to 1995, the collision energy was 91.2 GeV. Then, each subsequent year, the collision energy was raised. In 2000, the year the accelerator stopped
operating, the maximum collision energy was 208 GeV. By the way, to give a sense of scale, the mass of the proton is about 1 GeV. The Tevatron operated at Fermilab, my research home. It operated from 1986 to 2011 and collided protons and antimatter protons. From 1986 to 2001, the collision energy was 1,800 GeV. From 2001 to 2011, the collision energy was 1,960 GeV. The Large Hadron Collider, or LHC, also operates at CERN. It began operations in 2010 and continues today. The accelerator collides pairs of protons together, with the collision energy increasing over the
years. In 2010 and 2011, the collision energy was 7,000 GeV. During 2012, this was increased to 8,000 GeV. From 2015 to 2018, the collision energy was 13,000 GeV. And since 2022, the collision energy is 13,600 GeV. The accelerator was designed to run at 14,000 GeV, so future increases may still happen. However, remember that the announcement of the discovery of the Higgs boson was in 2012, so only the early running at seven to eight thousand GeV was relevant for the discovery. Each of these three accelerators searched for the Higgs boson in different ways. That's because
there are a handful of important considerations in the entire process. The first is just exactly how the accelerator will produce Higgs bosons. Since Higgs bosons don't normally exist in nature, we have to create them. This uses Einstein's equation E equals M C squared. We convert the beam energy into Higgs bosons. And, since each accelerator uses different beams, the way in which we convert beam energy into matter depends on the beam type and the beam energy. The second consideration is how Higgs bosons decay. The Higgs boson, being connected to the Higgs field,
which gives particles mass, tends to decay into the heaviest particles allowed by energy conservation. We now know the mass of the Higgs boson- it's about 125 GeV- which means that the Higgs boson can decay into pairs of bottom quarks. However, due to the vagaries of quantum mechanics, it's also possible for the Higgs boson to decay into pairs of W bosons and Z bosons, even though the energy isn't enough. It's weird, but, well, that's quantum mechanics for you. And, before the Higgs boson was discovered, other decay modes might have been possible. A third consideration is
whether the manner in which the Higgs boson decays is easy to identify or whether it looks like more pedestrian collisions. After all, in particle physics collisions, all sorts of things happen. If the particular type of decay you're looking for is produced far more often via other collisions, the Higgs signal could be hidden, like a whisper at a rock concert. Okay- remember that while we now know the mass of the Higgs boson, prior to the discovery, we didn't. The combination of theory and earlier precise measurements suggested that the mass of the Higgs boson was somewhere between
maybe 90 and 185 GeV. Or, of course, it might have not existed at all. Experimenters using each accelerator considered every possible way in which a Higgs boson could be created. They then combined that information with the amount of energy they had to work with, and the range of masses they could investigate. Finally, they considered the capabilities of their detectors, the various ways in which the Higgs boson could decay, and the rate at which other interactions could mimic the Higgs boson production. For each accelerator, specific combinations were determined to be the
most promising. Mind you, the researchers looked everywhere- it's just that I'm gonna tell you about the best ones. Let's start with the LEP accelerator. In the year 2000, LEP scientists raised the beam energy in hopes of creating a Z boson and a Higgs boson at the same time. Both of those particles are very unstable and they quickly decay into known particles - electrons, muons, quarks, and so on. One example would be when the Z boson decayed into a muon/antimatter muon pair, while the Higgs would decay into a bottom quark matter/antimatter pair. That's just one example.
The researchers looked at all possible combinations. Despite a few tantalizing hints in their data, the researchers ultimately concluded that they saw nothing, and they ruled out a Higgs boson with a mass below 114.4 GeV. The Tevatron had more energy and could, in principle, make isolated Higgs bosons. If that happened and the mass of the Higgs boson was under 160 GeV, the Higgs bosons would decay preferentially into bottom quark/antiquark pairs. Above 160 GeV, the preferred decay was into pairs of W bosons. The problem at the Tevatron is that making pairs
of bottom quarks via other processes is super easy. You see them all the time, which would swamp any Higgs signal. To get around that, Tevatron scientists looked for collisions in which Higgs bosons were created at the same time as either a W or Z boson. The basic idea is that collisions in which two rare particles are made don't usually occur. So, for different reasons, both LEP and Tevatron scientists looked for collisions in which Higgs bosons were produced at the same time as either W or Z bosons. And, of course, they didn't see the bosons themselves,
but rather, the decay products of those particles. Using this approach, Tevatron scientists were able to rule out Higgs bosons with masses close to 160 GeV and some lower masses, but there just wasn't enough data to rule out all masses. Mind you, if the Tevatron ran longer and collected data, the Higgs boson would have eventually been observed at Fermilab. But the LHC began operations, and the new machine's higher energy gave it capabilities that both the LEP and Tevatron accelerator just didn't have. Unlike the Tevatron, which collided protons and antiprotons,
the LHC collides pairs of protons. Both of these types of collisions are super messy, and are very prone to making events that look like Higgs bosons but aren't. However, the LHC detectors were built two decades after the Tevatron ones, and three decades after the LEP ones. The new detectors are simply better. In addition, the higher energy and higher collision rate in LHC beams makes more Higgs bosons per second than at the Tevatron. With more Higgs bosons, scientists could look at rare decays. These rare decays are hard to fake and easy to measure precisely, which makes
it easier to identify Higgs bosons. This gave LHC scientists opportunities that earlier researchers didn't have. For example, one super-clean example is an event in which a Higgs boson decays into two Z bosons and those Z bosons decay either into electrons or muons. This configuration is very rare, occurring only 0.013 percent of the time. That's super rare, but it also doesn't happen very often by more common physics. Furthermore, it's easy to measure accurately. Then there's the situation in which the Higgs boson decays into two photons. That's also very rare, occurring
only 0.229 percent of the time. But, again, this doesn't happen very often in more ordinary collisions. So that's what the LHC experimenters did. They looked for super-clean, but super-rare, examples of Higgs bosons and decays. One of the LHC experiments, called ATLAS, looked for Higgs bosons decaying into pairs of Z bosons, pairs of W bosons, or pairs of photons. They looked for other decays too, but those were the biggies. The other LHC experiment, called CMS, did basically the same thing. For example, here is an event in which the Higgs boson decayed into two photons.
And then they collected lots of data. This plot you see on the screen shows what the data looked like as it was being collected. The blue and green shaded areas is what we predicted to see if the Higgs boson didn't exist. The black dots are the data, and the red curve is the Higgs boson. We start out seeing no signal, but when enough data was collected, a little bump of data at a mass of 125 GeV is apparent. So that's basically how scientists searched for the Higgs boson using three accelerators over the course of ten or fifteen years. And, on July 4, 2012,
both of the big LHC experiments jointly announced discovery. Even better, the two experiments agreed with one another. The announcement of the discovery of the Higgs boson in 2012 wasn't the final word. Truthfully, all they knew for sure was that a new particle had been observed. It took years of more data to verify that the particle predicted back in the 1960s had been discovered. Scientists did that in a couple of ways, but one of the more persuasive measurements was to check to see if the particle decayed to other particles in the exact proportions predicted
by Higgs theory, and we can see that both LHC experiments validate the prediction. Mind you, this isn't the only cross-check, but it's a good one. And all of the other cross-checks tell the same story. So, I hope this answers the some of the questions that some viewers had. Searching for a rare and unknown particle is a tricky business, requiring researchers leverage their knowledge and the capabilities of both their accelerator and detectors. And, when they do that, Mother Nature eventually reveals her secrets.
(phasing sound) Okay- I hope this video gives a sense of the various ways in which a series of groups of scientists looked for the Higgs boson. The discovery was the culmination of half a century of anticipation. If you liked getting a peek behind the curtain, please like the video and especially subscribe to the channel. There will be lots more videos, each giving a glimpse into the world of physics. And those glimpses add up to a much better understanding of the laws of physics, which is definitely worthwhile. After all, physics is everything.
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