04/02/2021

Signs The Standard Model Of Physics May Be Incomplete

10:45 minutes

a 3d computer generated image of multiple layers of blue rectangles with thin yellow lines penetrating them
The decay of a B0 meson into a K*0 and an electron–positron pair in the LHCb detector, which is used for a sensitive test of lepton universality in the Standard Model. Credit: CERN

The pandemic has slowed many projects around the world, but scientists and engineers are nearing completion of a long-planned upgrade and maintenance period at CERN’s massive Large Hadron Collider project in Switzerland. The collider is currently cooling down and testing components, and aiming to start up for its third major run late this year. In the meantime, researchers have had time to sift through the data from previous experiments—and last week, they announced a finding that might indicate new physics at work.

The Standard Model of physics describes three of the universe’s fundamental forces, and how subatomic particles interact. One of the things it predicts is how particles decay into other components. Researchers at CERN analyzing particles called b-mesons found signs that their decay may not produce equal quantities of electrons and muons—as would be predicted by the Standard Model. While that discrepancy might not seem like a big deal, it could mean that there’s a previously undetected particle or force at play. However, the researchers don’t yet have enough data to say with confidence that their finding is real. They’ll need to collect several more years of data once the LHC restarts, as well as hope for confirmation from another major experiment in Japan.

Sheldon Stone, a distinguished professor of physics at Syracuse University and a member of the management committee of the LHCb Collaboration at CERN, joins Ira to talk about the anomaly in the data—and what it might mean if it’s proven to be real.


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Segment Guests

Sheldon Stone

Sheldon Stone is a distinguished professor of Physics at Syracuse University and a member of the management committee for the LHCb Collaboration.

Segment Transcript

IRA FLATOW: As scientists prepare to reopen the Large Hadron Collider following its scheduled maintenance period, they’ve had time at CERN to sift through the data from previous experiments. And last week, they announced a finding that might be significant– or might not be. Here to talk about it Sheldon Stone. He’s a distinguished Professor of Physics at Syracuse University and he’s on the Management Committee of the LHCB Collaboration. Welcome to Science Friday.

SHELDON STONE: Hello, Ira. Nice to be here.

IRA FLATOW: Nice to have you. Would it be correct to say that your experiments show that the behavior of elementary particles is different from what the accepted theories would suggest, what scientists would expect? Describe it to us.

SHELDON STONE: Yes. We’re seeing things that aren’t expected by what we call the standard model of particle physics. And as such, could be very interesting and give us insights to physics that we don’t know about, new forces, or new particles.

What we see is that, in certain processes involving B meson decays, the decay into two electrons plus something is different than the decay into two muons plus something. They should be the same. The only difference in the standard model between the forces for electrons, muons, and taus is their mass.

But we’re seeing things that have much larger differences in the very small difference between the electron and the muon mass. So we’re very excited about this. But yet the data are not statistically good enough to claim an actual effect. But they’re getting there.

IRA FLATOW: So let’s start out with explaining what leptons, muons, all those things are.

SHELDON STONE: OK. Particles have different classifications. But the electrons go around the nuclear material of the atom. Now it was found that there was another particle called a muon, which was exactly like the electron, except it had a mass that was about 210 times larger. Then in the 1970s, we discovered a third particle which is exactly like the electron and muon, except it was even heavier.

Now we don’t know exactly why there are three copies of these things. And only the electron is stable, which is good for our bodies. We’re seeing in these decays that maybe, just maybe, the decays into electrons and muons are different, which would tell us, basically, that there’s another force in nature that we didn’t know about and that we’re going to see something spectacular if we can confirm this effect.

IRA FLATOW: So let me see if I understand just briefly. So electrons, muons, and taus should behave the same in this kind of decay. But they don’t seem to be. Would that be correct?

SHELDON STONE: It’s correct for the electrons and muons. The taus are so much heavier that you really can’t see anything in this decay directly. So we’re just concentrating on electrons and muons. And they should behave the same. And lo and behold, there’s indications that they don’t.

IRA FLATOW: How much of an earthquake would that cause if they don’t?

SHELDON STONE: It’s already caused an earthquake, even though we haven’t proven anything yet. There was an updated presentation of our data that you mentioned last week. And already there is eight theoretical papers explaining what the new force is.

IRA FLATOW: So there should be a new force if this is true, is what you’re saying. What do you mean a new force?

SHELDON STONE: The new explanations are all related to new particles which are carriers of forces. The way we view physics is that particles carry force. For example, the photon carries the electromagnetic force. W and Z bosons carry the weak force. The gluon carries the strong force.

So if this was real, then one of the explanations is that there’s a new kind of particle called the leptoquark, which couples leptons, either electrons or muons, to quarks. This leptoquark field would be a new force of nature that we didn’t know about.

IRA FLATOW: But the new particle would mean a new force, then, is what you’re saying?

SHELDON STONE: Yes. But in this case, there are two ways of finding a new particle. One is to find it directly, like you produce it, which we’ve done. Remember, we talked about pentaquarks once? That’s what’s finding directly the new particle.

But this is finding a new particle virtually. That is to say, what we see in the data, we don’t see evidence of the decay of a new particle. But we can explain what we see if there was a new particle with these properties, or maybe another new particle with different properties.

So we’re seeing the effects of these new particles by using quantum mechanics. These new particles affect decay rates of the B mesons via virtual interactions. So we’re seeing it indirectly. By the standard of the statistics we have in the field, we say this is evidence for a new force.

But we don’t say that we’ve established the effect. We’re still not statistically there to say the effect is established. Which means it could go away when we get a lot more data.

IRA FLATOW: So you have to do more experiments, or do you need to just sift more through the data that you have?

SHELDON STONE: No. We’ve used all the data that we have. We’re at a threshold now where experiment’s been off for over a year. The LHC has been shut down. And we’re preparing an upgrade of our old experiment that lets us take data at 5 to 10 times the rate that the old one did.

So when we get this installed, which has been not so easy during a COVID year, as you might imagine, but we’re supposed to install it and start running next February. We can then, in a year or two– well, probably two– take as much data as we already took over the last 10 years. And then we would have a pretty good idea of whether what we’re seeing is real or not.

IRA FLATOW: This is Science Friday from WNYC Studios. Talking with physicist Sheldon Stone about some intriguing findings from the Large Hadron Collider in Switzerland.

Would this new force answer any questions in physics? And there are a lot of questions in physics we have right now. Would it answer any of those questions about how the world works, or our understanding of the building blocks of nature?

SHELDON STONE: Yes, it would. It would show us that there are more building blocks of nature than we understood. And it could point to insights for things that we don’t understand, like why there are three kinds of leptons that, if they’re not the same, that gives us answers.

We used to say before that there are three kinds of leptons that behave the same. Why is that? Well, now we may know that they don’t behave the same. And then there would be further questions as to why not.

So you keep probing in these different ways. I mean, we see three generations in other kinds of particles, like the quarks. And we have no idea why they’re there. So this could give us some insight as to why there are generations, some people call them families, where there is repetitions in nature that we don’t understand.

IRA FLATOW: Is it just your team seeing this effect? Or does this match up with other results from other teams?

SHELDON STONE: There are some other experiments that have started to measure this. But they don’t have the precision to see this effect. Their microscope is not focused tightly enough to see the difference that we see. Now hopefully within several years– there’s an experiment in Japan called Belle II. They will increase their data rate sufficiently so it can be able to confirm or deny this measurement.

IRA FLATOW: How do you get the public to care anything about this? Well, finding a new force in nature would actually be somewhat of a game changer. Now we don’t know exactly how it becomes a game changer, right? But when we discovered quantum mechanics, we then learned how to make transistors. And then we learned how to make electronics.

And who knows what insights it will give us into further technologies? And who knows how much it will tell us about the origin of the universe? Our theories on the origins of the universe are limited by the standard model of particle physics, what we know about particles.

And if we find out there were more particles and more forces around at the beginning of the universe, we’re going to learn a lot more. So maybe some people don’t care about it. But I care about it.

IRA FLATOW: Do physicists believe that the standard model is an incomplete model? That it’s good enough for now, but we’re going to have to do away with some of our thinking about it as we discover these new things we did not suspect were there?

SHELDON STONE: There is no physicist that I know that thinks the standard model is a complete theory. Everybody believes it’s an incomplete theory. It doesn’t explain dark matter. And maybe this is related to dark matter.

It doesn’t explain dark energy, which most people don’t understand. It has 20, 30 parameters that are put in by hand that we just assume are there because we can measure them. But we don’t know the relationship among these parameters or where they come from.

So everybody believes the standard model is incomplete. And that’s why people are building accelerators and trying to go beyond it. Maybe this is a hint beyond it. And that would be really exciting if it turned out to be true.

IRA FLATOW: You know, I’ve talked to a lot of physicists over the years. And some of them have told me, you know what? I get disappointed when we actually find the new particle. Because I enjoy the hunt more. Are you one of those people?

SHELDON STONE: No. Finding a new particle is really exciting. That’s when the champagne comes out. Finding a new force would even be more exciting. I would love if this was real. This would be great. And the hunt is fine, too. But having spent the last five years building this detector for this upgrade, the hunt can end for a while. And we could find something that would be much easier.

IRA FLATOW: Sheldon Stone, Distinguished Professor of Physics at Syracuse University. Thank you for taking time to be with us today.

SHELDON STONE: My pleasure. My pleasure.

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