What’s Next for the Large Hadron Collider?
And what, exactly, is a Higgs boson, anyway?
Steven Cherry: Hi, this is Steven Cherry for IEEE Spectrum’s “Techwise Conversations.”
One of the big science stories this year—maybe the big science story—has been the maybe or definite discovery of the Higgs boson. Amid all the bewilderment about whether the Large Hadron Collider, at CERN, did or didn’t find the Higgs this summer, my own personal bewilderment is, like the particle itself, much more basic: What is the Higgs boson, anyway?
Luckily, the right person to ask works just down the hall from me. She’s Rachel Courtland, sort of our physics—as well as semiconductors—editor here at Spectrum. Even before the LHC explosion and accident in 2008, she’s been obsessively following its experiments. This summer, she interviewed a number of particle physicists to get to the bottom—pun intended—of its particle physics discoveries. She’s my guest today.
Rachel, welcome to the podcast.
Rachel Courtland: Thanks, Steven.
Steven Cherry: So, Rachel, before we talk about the LHC and significance of discovering the Higgs and whether or not it was discovered, I need to ask what metaphysicists call the ti este question, the what-is question: What is the Higgs boson?
Rachel Courtland: Ah, yes. I can answer easily enough, but the answer probably won’t be that satisfying. Are you ready?
Steven Cherry: I’m ready.
Rachel Courtland: Okay. The Higgs boson is just a ripple in the Higgs field.
Steven Cherry: I can see where this is going. Okay, what’s the Higgs field?
Rachel Courtland: So that’s a little trickier to explain. I suppose the simplest way to put it is that the Higgs field is a theoretical field that physicists think fills all of space. If the theory is right, the Higgs field is present all around us and inside us, and it’s responsible for giving all other particles the masses that they have. Exactly how massive a particle is depends on how strongly it interacts with the Higgs field. The top quark, for example, which was only just discovered in 1995, interacts very strongly with the field and is thus very massive. It’s actually the most massive particle we’ve found yet—it has about as much heft as an atom of tungsten. The electron interacts less strongly and so isn’t as massive. And the photon doesn’t interact with the Higgs field directly at all, so it has no mass.
Steven Cherry: So the Higgs field is what the ancient metaphysicists called the aether? Sort of a thin, molasses-like goo that the rest of matter sits in?
Rachel Courtland: Well, as it happens, that’s probably the most famous analogy for the Higgs field. The idea is that the Higgs field imparts mass much like molasses exerts drag on things that move through it. But I asked Matt Strassler, a theoretical physicist at Rutgers University about that, and he says that analogy really doesn’t work.
Matt Strassler: The problem is that even when an electron is standing still, it has a mass. It doesn’t have to be moving, it doesn’t actually have to be slowed down by molasses. The Higgs field will give a mass to the electron even when the electron is standing still. So it doesn’t have anything to do with motion or drag.
Steven Cherry: So that’s the most famous analogy—should we move to the second-best?
Rachel Courtland: Well, I’ve never been really satisfied with any of the analogies I’ve heard, so I asked Strassler if he has a favorite. And it turns out he’s just as stumped.
Matt Strassler: The problem is this Higgs field really is different from anything we know, so in the end all analogies will fail. And all I can say is, well, it is possible for a field to change the mass of another particle, and that’s want the Higgs field does. We’ve never experienced anything like this, and perhaps that’s in part why it took so long for people to invent the idea.
Steven Cherry: Okay, well I at least understand that the Higgs boson isn’t just another particle, it’s sort of the master particle, the particle di tutti particles. And we’re studying it in order to understand this really fundamental thing.
Rachel Courtland: Right, the Higgs boson is our best bet for studying the Higgs field. By colliding particles together, physicists can create particles with high enough energies to interact fairly strongly with the Higgs field. One of the best ways I’ve heard it described is that you’re essentially wiggling the Higgs field and creating a little quantized chunk in the form of a Higgs boson. Studying the properties of that particle will tell you a lot about the Higgs field, which is very very hard to study in any other way.
Steven Cherry: And that, I guess, brings us to the LHC and the news from this summer. Some people said they wanted more confirmation. So did they find it?
Rachel Courtland: : That’s the big question, right? So I called Joe Incandela, the spokesperson for the Compact Muon Solenoid or CMS experiment. It’s one of two detectors that found the new particle. I caught up with him not too long after the announcement and just asked him outright: Did you find it?
Joe Incandela: I tend to think it’s a Higgs.
Steven Cherry: Wait a minute—did he say a Higgs?
Rachel Courtland: Yep.
Steven Cherry: Not the Higgs.
Rachel Courtland: Well, it turns out there isn’t just one possible Higgs boson.
Joe Incandela: The thing is there are many different models with Higgs particles in them.
Rachel Courtland: Here’s the thing. The simplest model of particle physics, the one that we have the most evidence for, is called the standard model. And in the standard model, there is only one form of Higgs particle.
But particle physicists know the standard model is incomplete. For one thing, it doesn’t include gravity. It doesn’t contain any particles that are good candidates for the dark matter that astrophysicists speculate makes up about 22 percent of the universe. And it’s also really inelegant—without some very fine tuning of parameters, the standard model says the mass of the Higgs boson should be staggeringly high, basically out of reach of any particle collider we could hope to build.
So there are a range of other theories out there that have been proposed to fill in the blanks. Probably the most famous one is called “supersymmetry,” which proposes that every particle in nature has a supermassive partner. The electron has a supersymmetric partner, for example, called the selectron. And all the quarks that make up protons and neutrons in atoms have supersymmetric partners called squarks. There are different flavors of supersymmetry, but all supersymmertric theories, Incandela told me, propose a minimum of five Higgs particles.
The problem is that a lot of these theories have particles that look a lot like the simple, solitary Higgs particle proposed by the standard model, so figuring out exactly what kind of Higgs the new particle is will take some time. And it’s still not quite clear whether the new particle is a Higgs at all.
Joe Incandela: Technically speaking, we don’t even know if this is a Higgs—consistent with what you expect of a Higgs particle.
Rachel Courtland: Incandela told me there are three main things that the hundreds of physicists working on CMS and the LHC’s other Higgs-hunting experiment, Atlas, still need to measure in order to understand exactly what they found. The most important of those things is the spin.
Steven Cherry: You know, it was right about here as an undergraduate that I left physics and engineering for pure math. Remind me what spin is.
Rachel Courtland: Well “spin” is an intrinsic property of fundamental particles. It’s a measure of their intrinsic angular momentum. When it comes to quantum mechanics, particles don’t really physically rotate. But when they fragment into pieces, into other particles, the result is pretty much the same. If a particle is spin zero, and it breaks into two pieces, those pieces should go in any arbitrary direction. But if the particle does have spin, it changes the angular distribution of the decay products.
The Higgs should have a spin of zero, so physicists at CMS and Atlas will be looking at this.
Joe Incandela: That’s a really key thing. If we prove it’s spin zero, I guess that Peter Higgs and some others will win the Nobel Prize.
Rachel Courtland: Incandela says the team might be able to say something about the spin of the new particle with a decent amount of certainty by the end of this year, if CMS and Atlas combine their data.
Steven Cherry: But I take it that will only tell you if you’ve found a Higgs, not the Higgs.
Rachel Courtland: That’s right. So there are two other things the experiments will need to look for. One will be parity, which will basically tell physicists whether the particle is identical to its mirror image or reversed.
The other thing Incandela says they’ll be looking at is all of the different ways that the Higgs boson falls apart. The Higgs boson doesn’t last very long on its own; it very quickly decays into other particles.
Joe Incandela: ZZ, photons, WW, tau tau, and b b-bar.
Steven Cherry: Yeah, that was Physics III. I was busy taking topology by then. What are they?
Rachel Courtland: Incandela’s listing decay channels, which are basically pairs of other fundamental particles that can be created when a Higgs boson decays. ZZ is a pair of Z bosons. Then there are photons and W particles; tau leptons, which are like electrons, only more massive; and a pair that consists of a b meson and its antiparticle.
The standard model makes very precise predictions for the relative proportions of each of these decay channels. So if physicists see something different, it could indicate they’re seeing a particle that’s more complex than the standard-model Higgs.
When Incandela’s team presented their results in July, they reported they were seeing more photons than expected—they were about 1.5 times more abundant than the standard model predicted. They also couldn’t find any decay in some of the other channels. But all of those statistics had big uncertainties associated with them, so it’s not really clear whether they’re real. Still, if they hold up with more data, that might be the first hint physicists would have that they’re dealing with an unusual Higgs particle that hints at more physics to be discovered.
Joe Incandela: If we still see an excess in photons by the end of the year, then that’s very, very exciting: It means we’ve got something that’s not a standard-model Higgs.
Steven Cherry: Oh, well that sounds promising.
Yes it does. And we’ll have more information on all three of these things in December, or at March at the very latest. That’s when the LHC experiments will be presenting the results from the rest of this year’s run at the Moriond physics conference [in Italy]. But there is the possibility that there might not be any big news coming out of the LHC in the next six months.
Steven Cherry: Why is that?
Rachel Courtland: Well, it turns out that finding a new particle at the LHC might actually be a lot easier than pinning down its properties.
Joe Incandela: There’s something people may find shocking. The LHC is designed as really kind of a—well, it’s a discovery machine. It’s not a superhigh-precision machine.
Steven Cherry: Rachel, the LHC cost €7.5 billion. They spent that on a not-superhigh-precision machine?
Rachel Courtland: Yeah. This was a little strange to hear, so I asked Incandela to explain. It really comes down to one fact. Yes, the LHC is a really wonderful piece of engineering. But at the end of the day, it’s colliding protons, which aren’t fundamental particles. Each one has three quarks zooming around in there, held together by gluons, and because they’re quantum-mechanical creatures, they’re probabilistic.
Exactly what pops out of each proton-proton collision varies depending on how much the proton’s total momentum is distributed among those quarks and gluons. So the collider ends up producing collisions with a really wide range of energies.
Joe Incandela: We don’t have a constraint on the total energy that’s going into the collision at a hadron collider. You’re producing all kinds of collisions at all kinds of energies all the time, so you study everything from really low energy, like we recently discovered some rare b baryon, and that’s at 5, 10 gigaelectron volts. We’re doing searches at the same time for particles that are up at 3, 5 teraelectron volts, a factor of 1000 in energy. We can do that simultaneously because the collisions range over such a large energy.
Rachel Courtland: So the LHC will help you find new particles, but if you want to study one particular particle in detail, you’ll have trouble. To pin down a particle’s properties, you really want to be able to create it in very large numbers. And to do that you have to be able to fine-tune the collision energy so you can create particles of exactly the right mass—or energy, since they’re equivalent.
So the LHC might have a little bit of trouble pinning down exactly what kind of particle has been found. If the new boson differs dramatically from what the standard model predicts, the LHC will be able to see that. But a lot of models predict particles that look a lot like the standard-model Higgs, and if the experiments only see properties that are consistent with the standard-model Higgs, they won’t be able to say whether it is in fact the standard-model Higgs or something else.
Joe Incandela: If it’s a standard-model Higgs, we’ll never be able to prove it with the LHC. That message I don’t think has really gotten out.
Steven Cherry: So, the LHC—what’s it good for?
Well, even if the LHC can’t prove it’s found a standard model Higgs with certainty, Incandela says the collider will still be able to do very high precision measurements.
And the LHC has already shown that it’s really good at finding new things. The Higgs – assuming it is some form of Higgs boson – is the poster child here. Physicists didn’t know how massive it would be, so they needed a powerful collider that could sweep over a wide range of energies to find it.
One new thing physicists are looking for now is evidence of supersymmetry. Incandela says the CMS team is especially interested in the supersymmetric partner to the top quark, which could be low enough in energy to be created at the LHC. That hunt is likely to continue through much of 2013 while the collider is shut down for repairs.
Joe Incandela: We’re looking now; we’re actually gathering extra data, I don’t know if anyone mentioned this to you, but we’re taking a certain amount of data we’re analyzing as we go, including what we’re showing on the Higgs, but we’re sort of storing on the side just as much data on the side that we can’t process right now, that we’ll process during the shutdown. And that data is targeting this kind of a search. So we’re going to look for supersymmetry in many different guises.
Rachel Courtland: If something turns up, Incandela says it will be huge.
Joe Incandela: Discovering supersymmetry would be a revolution in particle physics. It would keep us occupied for the next century.
Rachel Courtland: At the same time, physicists are also thinking about what might come after the LHC. If you want to study the Higgs in more detail, it would help to have a Higgs factory. And for that, you want to be able to collide fundamental particles that don’t have an internal structure. Electrons and their antiparticles are really good candidates for that kind of work: You can fine-tune their speed and produce collisions of very consistent energy.
Steven Cherry: When would we see a detector like that?
Rachel Courtland: Oh, these things are still very much on the drawing board, and I think a lot of physicists are waiting to see what the LHC will turn up.
Steven Cherry: So the LHC shuts down in February. For how long?
Rachel Courtland: Well, it will go through about 20 months of repairs. After that big accident in 2008, the LHC managers decided it was safe to run the collider not a lot more than half of its design energy. But to get it to full power, the collider needs to be shut down and repaired. Basically, they must build in better connections in the parts of the machine that carry current when the superconducting magnets in the machine aren’t working to prevent parts from overheating.
Steven Cherry: So that takes it almost to the end of 2014, and then they can run it at full power?
Rachel Courtland: Right. If all goes well, the collider will be able to create collisions with energies of 13 trillion electron volts, up from 8. That should mean more Higgs—or Higgs-like—particles and more of whatever else there is to find.
Steven Cherry: Very good. Thanks for joining us today.
Rachel Courtland: Thanks for having me.
Steven Cherry: We’ve been speaking with IEEE Spectrum associate editor Rachel Courtland about the Higgs boson, the Higgs field, and what’s next for high-energy particle physics.
For IEEE Spectrum’s “Techwise Conversations,” I’m Steven Cherry.
Announcer: “Techwise Conversations” is sponsored by National Instruments.
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