CERN’s July 4 declaration of victory in the quest to find the Higgs particle (or something very much like it) is a many-splendored triumph. It confirms, as it completes, the Standard Model of fundamental physics. It hints at the splendid new prospect of supersymmetry while debunking rival speculations. Most fundamentally, it reaffirms our scientific faith that nature works according to precise yet humanly comprehensible laws—and, importantly, rewards our moral commitment to testing that faith rigorously.
Inside the tunnel of the Large Hadron Collider, particles speed through a 27-kilometer ring of superconducting magnets. Credit: David Parker/Photo Researchers, Inc.
A few months ago, when the evidence was suggestive but not yet conclusive, I discussed here
the nature of the Higgs particle, and what its discovery would mean for the enterprise of physics. Now I will supplement that discussion, focusing on what it took
to win the victory.
Physicists had to overcome three challenges to discover the Higgs particle: producing it, detecting it, and proving that they really had produced and detected it.
To put these challenges in context, let me introduce another perspective on what the Higgs particle is: The Higgs particle is The Quantum of Ubiquitous Resistance. I’m referring here to a universe-filling medium that offers resistance to the motion of many elementary particles, thus producing what we commonly think of as their mass.
The Standard Model of physics—our best-yet model of the matter and forces that make our universe—requires, for consistency of its equations, that many of its ingredients are particles with zero mass. These particles should travel at the speed of light in empty space, but in reality, some of them—like quarks, leptons, and W and Z bosons—travel more slowly. What is slowing them down?
Our Standard Model comes equipped with a Standard Reconciliation: Space is never empty! Space is filled with a material that resists the motion of those particles. Over the past decades, physicists have deduced many of the properties of the Ubiquitous Resistance by observing its effects on the forms of matter we can see. They even gave it a name: the Higgs field. But none of the known particles had the right properties to build up the Ubiquitous Resistance. So theorists drew up the specifications for a particle that would do the job. They called it the Higgs particle.
But wishing doesn’t make it so. Only experiments can grant (or deny) theorists’ wishes. With that in mind, let us consider the three challenges facing experimental observation of the Higgs particle.
Any physical material, hit hard enough, is bound to break. The smallest possible shard reveals the most basic unit of the material: its “quantum.” For the Ubiquitous Resistance, that quantum is the Higgs particle.
To break off a piece of the Ubiquitous Resistance, though, requires producing disturbances of unprecedented intensity, albeit confined to tiny volumes of space for tiny intervals of time. That is what the Large Hadron Collider (LHC) is all about. By accelerating beams of protons to extremely high energy, and bringing them into collision, the LHC creates “Little Bangs” systematically.
Once you’ve produced a Higgs particle, the next challenge is to detect it. This isn’t as easy as it sounds, as the Higgs rapidly decays into other particles. We can look for those secondary particles, but most of them are useless for detection because they are produced more abundantly by other processes. The Higgs’ tiny signal competes with a cacophony of noise. The most likely mode of Higgs decay, into a bottom quarks and its antiparticle, in particular, is diluted by garden-variety strong interaction processes which produce those particles in droves.
So detection requires cunning.
Some decay processes that we might be able to detect are sketched below. Each has its own advantages and limitations, and each adds information, so experimenters have pursued them all. (For more information on the characters you’ll encounter below—W bosons, Z bosons, and the rest of the particle zoo, this is a good starting point.)
#1: Photon pairs
After a Higgs particle is created, quantum fluctuations convert it into a particle-antiparticle pair, which recombines into two photons.
The observable signal, in this case, is the pair of photons emerging from the decay. From the energy and momentum of the two photons, one can reconstruct the mass of the Higgs particle. This is significant because there are many other ways to make photons in collisions at the LHC that don’t require the production and decay of Higgs particles. The Higgs signal would be swamped, if not for the redeeming feature that randomly produced photons will “add up” to indicate random masses for their hypothetical progenitors, and only by rare accident land on the Higgs particle mass, whatever it happens to be. The signature of the Higgs, then, is an excess of photon pairs in a very narrow mass range. The mass where there’s an excess is fingered as the Higgs particle mass. Since the energy and momentum of photons can be measured accurately, this method gives an excellent measurement of the Higgs particle mass.
The main limitation of this technique, besides the unavoidable background “noise,” is the fact that this decay process is quite rare compared to other possibilities.
#2: W boson+ (Higgs -> bottom-antibottom)
Here is one of those other possibilities: In this case, the Higgs particle is produced as a byproduct of the creation of a W boson. The W boson itself decays, but in ways that experimentalists are thoroughly familiar with, and can often identify with confidence. The presence of the W boson, itself a relatively rare occurrence, helps this class of event to stand out above the strong interaction background. Thus the most common Higgs decay, into bottom-antibottom pairs, becomes discernable when you demand an accompanying W.
There are two more possibilities:
#3: Higgs -> WW -> lepton + antilepton + neutrino + antineutrino
#4: H -> ZZ -> 2 leptons + 2 antileptons
In Processes 3 and 4, the observed particles are leptons (l), which is just another way of saying that they might be either electrons or muons, and their antiparticles; the ghostly neutrinos escape detection. The Higgs boson barely interacts with those light particles, but it can communicate with them indirectly, through fluctuations in the W and Z boson fields (a.k.a. “virtual particles”). Process 4 is special, in that it is the only case where the background is so small that individual events, as opposed to enhanced probabilities, can be ascribed with confidence to Higgs particles.
By measuring the rates of all of these processes, one can determine how powerfully the Higgs communicates with many different things: two gluons, two photons, two Z bosons, two W bosons, and bottom-antibottom pairs. Their different rates are logically independent, of course, but theory connects them.
This is the final challenge. Finding the Higgs boson depends on assuming that the Standard Model is reliable, so we can work around the “background noise”. Here years of hard bread-and-butter work at earlier accelerators—especially the Large Electron-Positron Collider (LEP), which previously occupied the same CERN tunnel in which the LHC resides today, and the Tevatron at Fermilab, as well as at the LHC itself—pays off big. Over the years, many thousands of quantitative predictions of the Standard Model have been tested and verified. Its record is impeccable; it has earned our trust.
The next step is to search for data that the Standard Model can’t explain, like excesses of the decay products discussed earlier, and compare them against our predictions for yields from a hypothetical Higgs boson. Insofar as these quantitative predictions match the observations, which they do, one can speak of proof.
Future observations may reveal new effects, or small quantitative discrepancies in the effects already observed. (I’ll be surprised if they don’t!) But the original, simplest sketch of what The Quantum of Ubiquitous Resistance could possibly be resembles reality enough to pass muster, at least as its first draft.
Finally, I’d like to reprise the conclusion of my earlier piece, in which I considered what might happen if the hints of the Higgs did not pan out:
And if not?
I’ll be heartbroken. Mother Nature will have shown that Her taste is very different from mine. I don’t doubt that it’s superior, but I’ll have to struggle to understand it.
This week, we’ve come one step closer to understanding the rules that govern the universe. Two days ago, my colleagues at Fermilab announced our final results in a search for the answer to a mystery nearly 50 years old. In an intellectual tour de force, the CDF and my own DZero experiments analyzed a decade of data, combining dozens of hints that together tell an interesting tale. This announcement was an aperitif for an even more dramatic statement made today.
The construction of the CMS detector at the LHC. CMS is one of the detectors involved in the hunt for the Higgs. Credit: Mark Thiessen/National Geographic Society/Corbis
As physicists gathered in Melbourne, Australia, for the International Conference on High Energy Physics, one of the most anticipated conferences of the year, the two large collaborations at CERN made an extraordinary announcement. In back-to-back seminars held at CERN and simulcast to the conference (and the world), the leaders of two different experiments, CMS and ATLAS, gave strong evidence that we found something that can’t be explained by well-understood physics—something which could (and it’s worth emphasizing the “could”) be the Higgs boson.
The Higgs boson is the missing piece in the current best model of the universe, the Standard Model. In the Standard Model, building blocks called quarks and leptons are held together by the four known forces: gravity, electromagnetism and the strong and weak nuclear forces. Using these basic ideas, physicists can explain most of the measurements we have made. But one thing we have not been able to explain is one of the most fundamental and vexing questions in physics: Why do those building blocks have mass?
In 1964, Peter Higgs took some ideas that were floating around at the time, added a few of his own, and proposed a solution to this conundrum, which included a new particle that we now call the Higgs boson. The search for the Higgs boson is an energetic activity, directly involving as many as six thousand physicists—myself included—and the most powerful particle collider on Earth, the Large Hadron Collider (LHC) at CERN.
One of the fantastic benefits of being a physicist doing research at CERN and Fermilab is that I have been privileged to see this discovery evolve with an insider’s perspective in more than one world-class experiment and in collaboration with some of the finest minds on the planet. Over the past few years, we have searched through the data at both laboratories. Our measurements so far have shown where the Higgs boson isn’t. The results released today may finally show where it is.
The first tantalizing suggestions of the Higgs came in December of 2011, when scientists working with CMS and ATLAS announced that their data contained hints that the Higgs boson might be starting to show its face, and that it could have a mass about 125 times heavier than a proton. However, neither experiment had enough data to claim a discovery—or even to be certain that they were seeing anything at all.
In March, the search picked up again. This time, though, the LHC’s energy level and beam intensity were dialed up. If the LHC had been making Higgs bosons before, it would be making even more of them now—about 25% more, depending on the boson’s mass. The CERN management made their plans for 2012 so that both CMS and ATLAS would have enough “beam time” to independently discover the Higgs boson—if, that is, our hypotheses about its mass and other properties were correct. However, given the intellect and work ethic of the scientists involved, nobody really thought it would take the whole year to see a signal that “looked like” a Higgs boson, although proving anything we found was the actual Higgs boson predicted by the Standard Model could well take the entire years’ worth of data.
By June of this year, both LHC experiments had already recorded as much data in 2012 as in all of 2011. The accelerator and its detectors were performing superbly. Now the race was on to be the first to finish the job and find—or rule out—the Higgs boson.
ATLAS and CMS won’t find the Higgs itself, though; it disappears too quickly, decaying into other subatomic particles. It’s those particles that we’re looking for in the ATLAS and CMS data. Depending on the true mass of the Higgs boson, it could decay in several different ways. Seeing an excess of these decay products is an indication that we might have discovered the Higgs.
And that’s what we found! In the shrapnel of the LHC’s powerful collisions, the CMS experiment detected more pairs of photons and Z bosons than we can explain without some new kind of physics appearing. CMS also looked for supporting evidence in predicted decays to bottom quarks, W bosons and tau leptons. The ATLAS experiment also found an excess of events decaying into two photons and two Z bosons, but the ATLAS did not announce the results of their investigations into other decay modes.
To be certain that we didn’t adjust our analysis techniques to produce a preconceived result, we did the searches “blind,” meaning that we designed the analysis before we looked at the relevant data. This was especially important given that we saw hints in December 2011. We didn’t want that information to bias our searches in any way. That way, if the 2012 data told the same story as that of 2011, it would tell us something about the universe and not ourselves.
When all of our results are combined, CMS claims to have found a new boson with a mass of 125 GeV (or about 133 times heavier than a proton) and a statistical significance of about five sigma (which means that this result could happen 1 time in 3.5 million by accident), while ATLAS’ measurement indicates the existence of a particle with about the same mass (126 GeV) and the same statistical significance. While both experiments’ results are significant individually, the fact that both experiments are announcing similar observations and the 2011 and 2012 measurements are compatible lends tremendous credence to today’s announcement.
It is very important to stress that neither experiment team has claimed to have observed the Higgs boson. They have observed something without a doubt, but the Standard Model Higgs boson is a very specific thing. To be sure we’re seeing the Higgs boson and not a lookalike, we need to see it in all of the predicted decay modes.
For instance, the Higgs theory makes specific predictions about the relative probabilities of the Higgs decaying into pairs of bottom quarks, tau leptons and a whole myriad of possibilities. If all of the predicted possibilities aren’t seen, or aren’t seen in the right ratio, it might be that what we’re observing isn’t the Higgs boson after all. Furthermore, the Higgs boson is predicted to have exactly zero quantum mechanical spin. Until those and other properties are confirmed, it is possible that the experiments might be picking up traces of something entirely different. So, although what has been observed is consistent with being a Higgs boson, these measurements cannot rule out some other possibilities. In fact, this announcement is not the end of the story but rather the very beginning.
Higgs week is here!
This week, the search for the Higgs boson—the elusive subatomic particle that is a critical piece of the Standard Model of physics—may reach its climax when, on Wednesday, two research teams announce the results of their work at the Large Hadron Collider (LHC) at CERN.
But before there was the LHC, there was the Tevatron, a particle accelerator at Fermilab. And before the LHC’s big announcement, there was a not-quite-so-big announcement from the Tevatron teams as they gathered with colleagues this morning to announce the results of the most detailed analysis so far of ten years'-worth of their Higgs search data.
The Tevatron at Fermilab. Image courtesy of Fermilab.
The Tevatron shut down last year, passing the baton to the newer, more powerful LHC. But the scientists working on two of the Tevatron’s detectors, CDF and DZero, haven’t given up searching for traces of the Higgs in their own data. Using ever-smarter computer algorithms, they aim to wring as much information as they can out of the data they’ve accumulated. As Wade Fisher, the Michigan State University scientist representing DZero at this morning’s conference, put it: “We’re still working, we’re not stopping….There’s still gas in the tank.”
What they’ve found so far is suggestive of the Higgs, but doesn’t rise to the level of discovery. Combining data from both CDF and DZero, they’ve eked out a signal that might be due to the Higgs, but there is also a one-in-550 chance that it is down to random fluctuations.
To claim a discovery, the physicists need to whittle that random-chance number down to one in three and a half million—“five sigma,” in stat-speak.
That’s what the physics world will be holding its breath for on Wednesday, when two LHC collaborations release their results.
Will they confirm the hints that the Tevatron has seen? Or will these inklings—and our hopes of completing the Standard Model of physics--evaporate into the mist of random fluctuations?
As Fermilab’s Eric James put it this morning: “We’re likely, after all this time, to find something out one way or the other.”