Particles Behaving Badly

9:59 minutes

Colliding protons in a LHCb experiment. CredIt: CERN

The Standard Model is a rulebook for how subatomic particles like quarks and leptons and bosons and other bits of matter should interact. But every so often, physicists slam these particles together, and find that the shattered leftovers don’t stack up with the model’s predictions.

That situation is playing out now, as outlined in a recent review paper in Nature. The conundrum in question is the decay of a particle called a B-meson, and how many electrons, muons, and tau particles pop out after a collision. Physicists in a handful of experiments around the world have all observed a preponderance of tau particles—not what current theories would predict.

[The hunt for new particles continues.]

And while the strange observations have not been definitively proven just yet, theorists are already postulating what the culprit behind the unexpected effects could be. Perhaps it’s a new particle, they suggest: a “leptoquark,” which “talks” to leptons and quarks, or maybe a new kind of charged Higgs boson. Pearl Sandick, a theoretical physicist at the University of Utah, says the anomalies might point to something like supersymmetry, and could even help to explain the existence of dark matter.

But for now, physicists still need more proof that this odd observation is worthy of being called a discovery, says particle physicist Pauline Gagnon. “Until we have stronger evidence—much stronger than we have now—nobody will believe it.”

Segment Guests

Pearl Sandick

Pearl Sandick is an Associate Professor of Physics and Astronomy at the University of Utah. She’s based in Salt Lake City, Utah.

Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow. They are the smallest collisions on Earth, but create some of the biggest mysteries. I’m talking, of course, about collisions between tiny, subatomic particles, like the particle smashing they do over at the Large Hadron Collider in Switzerland.

As for the mysteries, experiments at the LHC have come up with a pretty puzzling result. The shattered remains leftover after a particle collision, well, they just don’t quite add up to what the prevailing theories predict. And those strange observations have occurred at a handful of other particle colliders too, as detailed last month in the journal Nature.

And all of this opens the door to some new, exciting physics, maybe even new particles! If, that is, these experiments can be replicated, because particle physicists are notoriously picky about their proof when it comes to new discoveries.

Joining me to talk about what we know, and what we don’t know, is Pearl Sandick, Associate Professor of Physics and Astronomy at the University of Utah in Salt Lake. She joins us from a theoretical cosmology conference in Stockholm. Welcome to Science Friday, Dr. Sandick.

PEARL SANDICK: Thank you for having me on.

IRA FLATOW: You’re welcome. Can you walk us through what this experiment was doing, and what have physicists been smashing together, and why what’s coming out is so weird?

PEARL SANDICK: Yeah. So there are actually three experiments. One, as you mentioned, is just the Large Hadron Collider. So it’s the LHCb experiment. The LHC is the highest energy man-made collider in the world. The other two experiments, we’re actually looking at collisions of electrons and positrons, and collision energies that are much lower. So about 1/1,000 of the LHC beam energies.

Those other experiments are Belle, in Japan, and BaBar, in California. But all of them– what they’re all looking for is you smash these particles together, you see what comes out at the end of the day, and if there are any particles that are left over, if the data doesn’t match the standard expectation, then you might have found some new physics.

IRA FLATOW: And so what was weird? What was the expectation that was not matched?

PEARL SANDICK: Yeah, OK. So with the experiments, we’re looking for whether the Standard Model lepton– so you’ve probably heard of the electron. So Standard Model includes two other charged leptons. So one that’s about 200 times heavier than the electron, that’s called the muon, and another that’s even heavier called the tau. And these three particles– the electron, the muon, and the tau– the only differences between them should really be their masses.

So according to the Standard Model, they should both act the same way under the weak nuclear force. They should have the same types of interactions. And what they’re doing is, in order to determine whether the electrons, muons, and taus feel the weak nuclear force in the same way, they smash these particles together. And then, what they’re looking for are some particles called B meson.

So mesons are objects made of a quark and an antiquark. And if one happens to be created in a collision, it’s not going to stick around for very long. But at the end of the day, you’ll have a final state that has maybe electrons, muons, or taus in it. And they look at the rates of those different productions of electrons, muons, and taus to see if they match the standard expectation.

So what they found is that it turns out that all three of these experiments see more events with taus than you would expect from the Standard Model alone.

IRA FLATOW: Wow. Does that mean–

PEARL SANDICK: Yeah, so it–

IRA FLATOW: Are people wringing their hands now about the Standard Model? Maybe it’s–

PEARL SANDICK: Yeah. So that could mean that there’s something else besides just the weak force, so something else besides just the Standard Model. But it’s a little bit confusing because it’s tough to reconcile these results with other results and find one model that fits everything.

IRA FLATOW: Yeah, that is the problem. That’s been the problem for 100 years, hasn’t it?


IRA FLATOW: But scientists, as I said, they’re very picky and they’re not all believing these results yet, are they? They’re not quite convinced.

PEARL SANDICK: Yeah, not completely. So these results have a statistical significance. And when you combine all three– so individually, they have some deviation from the Standard Model expectation. When you combine all three, because they all three agree, you actually get a stronger result.

But it hasn’t quite risen to that gold standard level that we expect in particle physics in order to be able to say that we have a new physics result. So we’re still waiting to see. But this is definitely an interesting indication.

IRA FLATOW: Could this mean that there are other particles yet that are causing this that we have not discovered?

PEARL SANDICK: Yeah. So there are a couple possible explanations, and they all have to do with new particles basically. So one of them is that you could have something that’s like the weak nuclear force, but the taus would feel differently than electrons or muons. So there’s a new particle that carries that new force, and we would call it a W-prime, an analogy with the W boson that carries the normal weak force. So that’s one possibility.

A second possibility is that there’s actually a new type of Higgs boson. So the Higgs that was discovered at the LHC, in 2012, doesn’t have any electric charge. But the particle that would be needed to explain these results actually does need to have an electric charge. So there could be a new charged Higgs boson that’s maybe heavier, so heavy enough that we haven’t seen it yet at the LHC. So that’s a possibility.

And the third possibility has to do with the fact that these events at all these experiments involve both leptons– so electrons, muons, and taus– and quarks. And so there could be some type of new particle that actually talks to both leptons and quarks. And we would call that a leptoquark. And there are models of leptoquarks that might explain this as well.

IRA FLATOW: Wow. Talk about a particle zoo all over again.

PEARL SANDICK: Yeah, for sure.

IRA FLATOW: What about dark matter? Could it be showing up here somehow? Because we don’t know what it is.

PEARL SANDICK: Yeah, that’s right. OK. So dark matter actually is related to this story, but in a kind of interesting way. So at this point, there’s not an obvious connection, but it’s certainly possible. And one of the things that might connect to this anomalous result with the need for dark matter is something called supersymmetry.

So there are a lot of symmetries in nature. The laws of physics don’t change with time, so we would say that they are symmetric under time translation. And it turns out that if you think about the mathematics that describes the particle physics that we understand, there’s only one more really different type of symmetry that you can add to the mathematics of the Standard Model.

And that symmetry is a symmetry between particles with integer spin, that we would call bosons, and particles with half-integer spin, that we would call fermions. So if this symmetry, the supersymmetry, exists, then what that means is that, at a minimum, there are twice as many particles as there are in the Standard Model alone.

So if you add that many particles, it sounds like you’re making things a lot more complicated. But at the end of the day, you actually get out some really nice consequences. So one is that one of those new particles could very likely be the dark matter. And it just happens to work out that you get roughly the right amount of dark matter left over in the universe.

Another interesting consequence is that the Higgs boson that was discovered is actually predicted within these models. But not only that, they predict extra charged Higgs bosons. And so those extra charged Higgses could be the things that are contributing to these anomalies from LHCb, Belle, and BaBar.

So it could be connected. But we really don’t know yet.

IRA FLATOW: I’m just sitting here with all these super particles swimming through my head. But back to where I was 40 years ago, when I first started covering this, is we’re back to more particles, super particles, other kinds of particles. There’s still no giant theory yet, is there, that will explain everything?

PEARL SANDICK: Not necessarily. I mean, we have to wait for the data to tell us what direction to look in. But I think we’re getting there. And also, these types of experiments that are being done, that look for anomalies– not necessarily just new particles, but that look for other indirect signals of new physics are really important.

IRA FLATOW: And when you say new physics, what does that mean to a layperson?

PEARL SANDICK: It means any kind of new thing that is different from the Standard Model or beyond the Standard Model. So the Standard Model works really well, but there are some reasons why we know there’s something beyond it. So the Standard Model only includes three of the four fundamental forces. Gravity is not included. Dark matter– no explanation for dark matter is anywhere in the Standard Model.

And so there are reasons why we think there’s something else. And we just categorize– I would call all of that new physics.

IRA FLATOW: Do we have the right tools? I mean, do we need a bigger atom smasher?

PEARL SANDICK: Yeah. So, yeah. We have good tools now. Checking these results is going to be really key to see if this particular anomaly is the thing that’s pointing us in the direction of new physics. And so there are a couple of things that are needed, actually, to check this thing.

One is that we need to make sure that the Standard Model calculation is actually really correct. Because if you have the Standard Model calculation wrong, then a deviation from the Standard Model might not really be a deviation from the Standard Model. But the other thing is we need more data to see if that statistical significance does rise to that gold standard level, where we’re willing to say, yeah, this is a discovery of new physics.

So BaBar is done operating. But Belle and LHCb are both upgraded and they’re going to give us a lot more collisions. And so that’ll give us the data that we need to be able to determine whether this is a real signal. And actually, it’ll give us some more information besides that, too, but that’s a good start.

IRA FLATOW: OK. So we’ll have to watch this space, as they say.

PEARL SANDICK: Yeah, absolutely.

IRA FLATOW: OK. Thank you, Dr. Sandick. Fascinating.

PEARL SANDICK: Yeah. Thank you.

IRA FLATOW: Pearl Sandick is associate professor of physics and astronomy at University of Utah in Salt Lake.

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