Why do matter and the universe exist at all? Scientists get closer to the answer

• Nobel-Prize-winning physicist Eric A. Cornell and a team of scientists explored the problem of asymmetry.
• In a new physics study, they achieve a record-breaking measurement of electrons.
• The study, which measured electric dipole moments of electrons, could impact the future of particle physics.

The beginning of our universe was a violent and chaotic affair. Matter and antimatter particles came in and out of existence, disappearing in bursts of light. Created in pairs, if electrons, neutrons, protons and their antimatter equivalents (with opposite electric charges) touched, they annihilated each other, with only pure energy remaining, visible in the flashes of photons. And if this balance had kept up, we’d have nothing left. There’d be no universe — no us.

But clearly that’s not the case, and somehow matter prevailed, with leftover particles forming atoms, molecules and eventually all the matter we have in existence. And, puzzlingly, there’s not much antimatter we can find.

What accounts for this problem of asymmetry, even when the math seems to point the other way, calling for symmetry? A new paper from the group led by Nobel-Prize-winning physicist Eric A. Cornell of JILA /NIST at the University of Colorado Boulder , takes a step towards answering this question.

Dr. Eric Allin Cornell shared the 2001 Nobel Prize in Physics with Carl E. Wieman and Wolfgang Ketterle, for their work in synthesizing the first Bose–Einstein condensate in 1995. Now, Cornell's group of experimental physicists at JILA has been studying fundamental particles like electrons to spot asymmetry.

In their new study, published in Science , the group shares a record-breaking measurement of electrons, bringing us closer to figuring out the source of the asymmetry.

The team focused their attention on the so-called electric dipole moment of the electron (eEDM). The eEDM tells us how evenly the negative electric charge carried by an electron is distributed between its north and south poles. If there is an unevenness, with a measurement of eEDM above zero, that would indicate that the electron is not completely circular and is more egg-shaped. This, in turn, would be evidence of an asymmetry that could account for the existence of matter.

How they did it

Working with molecules of hafnium fluoride, Cornell’s team significantly advanced our ability to measure the eEDM. They managed to make a measurement that’s 2.4 times more precise than those made previously.

As the press release from the National Institute of Standards and Technology (NIST) explains, the process involved using an ultraviolet laser to strip electrons from molecules, creating a set of positively charged ions, which were then trapped.

An electromagnetic field alternated around the trap to cause the molecules to either align or not align. Lasers were then employed to measure the energy levels in the two groups that were created in this way. Any difference in the levels would point to the electrons not being symmetrical.

For the new experiment, the team was able to achieve longer measurement times than previously. This, in turn, led to better sensitivity and precision. They didn’t, however, notice any movement in the levels, concluding that least at this level of precision, electrons still appear to be circular.

Interesting Engineering spoke with Dr. Eric Cornell to get further insight into the group’s methods and the results.

The following has been edited slightly for clarity and flow.

Interview with Dr. Eric A. Cornell

Interesting Engineering : Why is it important to find evidence of asymmetry?

Dr. Cornell: We know from the beginning that there is asymmetry and in matter versus antimatter asymmetry, the universe is made up of one and not the other. And we can basically look back in time and see that after the Big Bang, there was a billion times more stuff in the universe than there is now. And for every 1 billion protons and 1 billion anti-protons, there was actually a billion and one protons, and so they all stuck together. And what was leftover was a tiny fraction of the matter and antimatter left after the Big Bang.

And it's really good that it wasn't exactly the same. Because had it been exactly the same, there'd be nothing left but light, right, so it is sort of mysterious, why this tiny little imperfection was leftover from the Big Bang. And especially because it had such an important significance — it's why we're all here.

So theory people — particle physicists who do theory, which is not me, I should emphasize — they come up with mathematical explanations. And, it turns out, it's very difficult to come up with a mathematical explanation that explains this origin of the Big Bang, and that doesn't also explain, doesn't also predict, [why] certain particles, ... like electrons, and protons and neutrons, should have this asymmetry.

So, there's good reason to believe that particles should have asymmetry. And beyond that, if we can see [asymmetry], it sort of helps illuminate what happened in the early seconds of the universe.

But beyond that, the electron by itself ought to be symmetric. And so the fact that it's asymmetric, it has to do with the fact that particles ... as they're charged -- they sort of polarized the world around them. And that polarization includes the polarization of vacuum, which includes at least sort of virtual presences of much heavier particles. And those are the particles that would give the electron its asymmetry.

So, it's a way of sort of looking for heavier particles. We don't see them directly, but we see evidence for them — particles which are too heavy to see in an accelerator, like the Large Hadron Collider.

So it's kind of interesting if we don't see anything, and so far, we haven't seen anything. It suggests that the particles that we haven't seen yet are more and more massive, as we as we put tighter and tighter limits on the electron’s dipole moment — that was our most recent result — a new, tighter limit.

Turns out every time we go a factor of two tighter in precision on the electron and we don't see anything, it is raising the sort of minimum threshold of what masses particles can have. The Large Hadron Collider did not see any particles outside of the Standard Model.  And that sort of sets the limit around one or two tera electron volts.

The results from the dipole moment experiments are pushing that up still higher. And as we push it up higher and higher, it gets to a point where you might sort of think there's no point in building a new accelerator unless we can build one bigger than "yay" big.

And so that has huge implications for the future of particle physics. So as I see it, I think I'm not just being vain —  It's an important measurement, and has implications for how particle physics should proceed, if you want to see new particles.

And the theorists who try and come up with explanations of physics beyond the standard model, including explaining that matter/antimatter asymmetry — for them, these measurements are constrained.

You know, it's like, if you play tennis, there is no point in playing unless you have a net — and there's no point in doing particle physics unless you have various constraints that sort of force you to have a theory which forms the reality, and reality is these dipole moments are really, really tiny.

Interesting Engineering : How did your measurement achieve greater precision than previous approaches?

Okay, so we're trying to measure an electric dipole moment. Which sounds sort of exotic, but measuring magnetic dipole moments is very, very standard. Like every time you hear about a magnetic resonance imaging machine (MRI), or in chemistry, they do something called ESR - electron spin resonance, these are just measuring the magnetic moment of protons.

So in the case of MRI, and electrons in the case of chemical ESR, basically what they do is they just put stuff in a big magnet, and they shine radio waves at it look to see if the electrons flip over or the protons flip over.  And in doing so, you measure the magnetic moment of the proton or the electron.

If instead you put a big magnetic field, a big electric field…   Now, if the electron has electric dipole moment, that will change the frequency of the photon that needs to flip them over. And so we'll see a small shift in in the resonance sign — we actually have a magnetic field.

And we add to it also an electric field, which either points in the same direction as the magnetic field or the other direction from the magnetic field. And ...and if we see a small change — that's the electric dipole moment.

How come we're able to do it better than everyone else? Well, you need a couple of things to do this experiment really well — you need a really big electric field... much bigger than if you just take two metal plates and charge them up.  After you get the field [to] right around 100 kilovolts per centimeter, you just start getting sparks, you can't make a field bigger than that in free space.

So where can you get a big electric field?  Well, nature's way of making a big electric field is a molecule. [With] sodium chloride [made up of atoms of sodium and negatively-charged chlorine], the sodium’s positive, the chlorine’s negative — you never think about it. But if you were a tiny little person that could live in between the sodium and the chlorine, you'd experience a really large electric field — much larger than we can have in the lab.

And we use in our case, a very, very heavy atom of hafnium on one side -it is very heavy and very positive. And on the other side, we use an atom [the ionic form of fluorine] which is extremely negative, even more negative than chlorine — so we use hafnium fluoride.

And the advantage it has over salt is that electrons are not all paired up. In salt, the electrons are paired up and it has no net electron spin. So you wouldn't be able to do this resonance experiment.

But in hafnium fluoride, or in our case hafnium fluoride plus, the molecule, the electrons have some spin. That spin points along the direction between the two atoms inside the molecule.  And so it appears that we're doing the experiment on the molecule.  Really, we're sort of doing it on the electrons inside the molecule.

You asked - how can we do it so well. So, one thing [is the] big electric field. The other thing is we want to measure a frequency.  And to measure a frequency really well, you need to measure it for a long time.  Basically, if you want to make a very precise measurement of an energy, which in physics is the same as the frequency, you want to measure a frequency really precisely -- so you want to look at it for a long time.  So there's a large uncertainty when it's doing the flipping over.

So we put the ions in a trap. We actually don't use hafnium fluoride — we use hafnium fluoride plus, so it's relatively easy to keep them in what amounts to a little box — an electrostatic box that keeps them around.

And we can look at them for several seconds. So this is called the coherence time — how long you can coherently probe a resonance. We probably have the longest coherence time anyone's ever seen in a molecule. So we have the narrowest lines, which allow us to see very small changes.

Those are our two big edges we have, compared to our competition — and we have a fair amount of competition. There's a lot of groups working [on this] now.

Interesting Engineering : Do you expect future studies to find a nonzero measurement of eEDM?

I sure hope so.  For me, it's very exciting to measure a more precise measurement, even if it's zero, just for all the reasons that I explained earlier. Zero with small error bars has a lot of meaning in this business. It would still be more exciting to measure a nonzero value, right? And we are trying to add about another factor of 10 — might be a factor of five — to the precision. We've already started building the new machine, which is going to take us several years. It's an even more complicated machine but it’s coming along.

Read the study “An improved bound on the electron’s electric dipole moment.”

Abstract

The imbalance of matter and antimatter in our Universe provides compelling motivation to search for undiscovered particles that violate charge-parity symmetry. Interactions with vacuum fluctuations of the fields associated with these new particles will induce an electric dipole moment of the electron (eEDM). We present the most precise measurement yet of the eEDM using electrons confined inside molecular ions, subjected to a huge intramolecular electric field, and evolving coherently for up to 3 seconds. Our result is consistent with zero and improves on the previous best upper bound by a factor of ~2.4. Our results provide constraints on broad classes of new physics above 10^13 electron volts, beyond the direct reach of the current particle colliders or those likely to be available in the coming decades.

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