Since the mid-1970s, the Standard Model of particle physics has, as the name suggests, served as the standard theory for what fundamental particles exist in the universe, and how they interact with each other. While a mountain of experimental evidence from the last 50 years supports the model, there are a couple of major phenomena that it doesn’t account for.
Among them, one of the biggest unexplained issues is called the “matter-antimatter asymmetry problem,” or put simply, why we live in a world dominated by matter.
Chloé Malbrunot, an adjunct professor in McGill’s physics department and a research scientist at TRIUMF, recently co-authored a paper published in Philosophical Transactions of the Royal Society A, which outlines several of the newest experimental methods to push the boundaries of precision measurement and dig deeper into this puzzle.
Before looking at the experimental methods they discuss, it’ll help to define two key terms, “antimatter” and “CPT symmetry.”
To understand what antimatter is, we first need to look at what components make up “normal” matter. Inside each atom of matter, there are electrons, neutrons, and protons. Electrons are considered “fundamental particles,” meaning that they can’t be split up into any smaller parts. Neutrons and protons, on the other hand, are composites made up of fundamental particles called quarks.
In the Standard Model, each fundamental particle has a corresponding antiparticle, which is essentially the same particle but with the opposite charge. For example, electrons, which have a negative charge, have corresponding antiparticles called positrons, which are positively charged. The same goes for quarks and the other types of fundamental particles.
Here’s the catch: When antimatter and matter come into contact, they annihilate each other instantly, destroying themselves and producing a burst of energy.
As Malbrunot explained, the Standard Model predicts that during the Big Bang, roughly equal amounts of antimatter and matter should have been created. Under the Standard Model, matter and antimatter should have annihilated themselves, leaving a universe full of radiation, with no matter or antimatter. Instead, some of the matter survived, leaving the universe as we know it today, with none of the “primordial” antimatter—antimatter atoms remaining from the Big Bag—left.
“For me, this is almost the most intriguing question,” Malbrunot said in an interview with The Tribune. “I mean, there are a lot of big questions, like ‘what is dark energy?’ and ‘what is dark matter?’ but matter-antimatter asymmetry—if things had been going the way the Standard Model says, we would just not be here. We would just be a world of photons.”
Next, let’s take a look at CPT (charge, parity, and time) symmetry. The Standard Model says that if charge, parity, and time are all reversed, then the laws of physics should continue to function just the same as we predict they will now. Originally, physicists believed that P (parity) symmetry was true on its own.
“It’s a bit more complex than this, but pretty much if you look at the physics through a mirror, you would expect the same result,” Malbrunot explained. “And that was believed to be true for a very long time, until it was discovered that it’s actually not true at all and it’s maximally violated in weak decays.”
Subsequently, physicists added C, meaning that if both charge and parity were reversed, symmetry would hold. This principle is called “CP symmetry.” As it turns out, this principle doesn’t hold in all cases, and accordingly, the Standard Model incorporates a small amount of CP violation.
Finally, T was added, forming the CPT symmetry principle found in frameworks like Quantum Field Theory.
“If you add time-reversal symmetry, so basically, if you rewind the time, then Quantum Field Theory is very, very strong in saying that CPT in the Standard Model should be conserved,” Malbrunot said. “And to date, there’s been no measurements that contradict this.”
Understanding CP and CPT symmetry is key to the matter-antimatter asymmetry puzzle because if there are violations of CP and CPT symmetry, it essentially means that matter and antimatter obey slightly different laws of physics. These differences could help to explain the discrepancy between matter and antimatter, or, in the case of CPT symmetry, point the way toward entirely new models of particle physics.
In fact, as CP violation is observed, this is one of the most promising explanations for the matter-antimatter puzzle. However, the CP violation observed so far is miniscule compared to the apparent discrepancy between matter and antimatter.
“The level of CP violation that we measure in the Standard Model is just not enough to account for this by nine orders of magnitude,” Malbrunot explained. “So, we are looking for more CP violations. Maybe there are processes that we did not take into account that are violating CP and that could explain how the universe developed into a matter-dominated world.”
A promising place to find some CP violation would be in observing something called an “electric dipole moment” (EDM) of a fundamental particle like an electron. In simple terms, an EDM is the separation between a positive and negative electrical charge in an atom. While an occurrence of this in an elementary particle would violate CP symmetry, and thus be surprising under the Standard Model, there are several new physics scenarios that allow it. Given this, the observation of an EDM of a fundamental particle would provide evidence for new sources of CP violation, as predicted by some theories of physics that go beyond the Standard Model.
Experimentally, physicists have been searching for these EDMs using long-lived radioactive atoms like thorium and fhafnium. In the paper, the authors outline an emerging field using large, short-lived radioactive atoms instead. This is more difficult because the atoms have to be specially produced in radioactive ion beam facilities, such as the Isotope Separator and Accelerator facility at TRIUMF, but it offers several advantages. One of these advantages is that larger atoms are potentially useable, since the larger radioactive atoms tend to have shorter half-lives. These larger atoms produce more energy when undergoing an EDM, making it easier to measure.
Another new approach is to “freeze” the atoms into a solid matrix before attempting to measure an EDM. While the above experiments used atoms in a gas state, the denser solid would allow observation of more atoms at once, meaning it would be possible to get more sensitive measurements.
All of these experimental methods are targeted at proving the existence of more CP violation than the Standard Model incorporates, but the paper also outlines the latest approaches attempting to test CPT symmetry.
To test CPT symmetry, one method is to measure the fundamental properties of matter particles, and compare them with their corresponding antiparticles. As the CPT theorem implies that matter and antimatter should behave exactly the same, any discrepancy observed between their fundamental properties would be evidence against the theory, and indicates that we need a new theoretical framework.
An ideal candidate for this is hydrogen, as antihydrogen is the only anti-atom that physicists can produce so far, and even this is very difficult. Hydrogen is also ideal because researchers have measured its properties using spectral imaging with extreme precision.
“This is one of the most precisely known transitions in nature,” Malbrunot explained. “So if we could measure antihydrogen with the same precision, then we would have an extremely precise test of CPT.”
With that in mind, most experiments in this field are aimed at achieving the highest possible precision, which involves creating atoms that are extremely cold and slow, allowing us to measure them with higher accuracy.
Once we have produced antihydrogen atoms that are cold enough—for example, using laser cooling—scientists hope to build on the precision by developing something called a fountain.
“Basically, you launch your particles, and so through gravity, they will slow down until at one point, they even stop and then come back down,” Malbrunot explained.
This combines the precision gained from using cold and slow atoms with the fact that they travel through the interaction region multiple times, as they rise and fall.
While fountains have been made for other atomic substances, hydrogen and antihydrogen are very difficult to cool to the necessary temperatures. However, recent research on cooling these atoms has proved promising, leading to the possibility of using these techniques in the future. If this was achieved, it would allow for spectral measurements of extremely high precision in antihydrogen.
Using the best techniques currently available, all of the measurements that researchers have observed in antihydrogen match those of hydrogen. However, new experiments are being developed to get even higher precision, with the hope that somewhere deep in the decimal places, there is actually a miniscule difference between them.
“That’s currently the fate all the precision Standard Model tests,” Malbrunot said. “Basically, [our measurements] agree with the Standard Model, and so the only thing you can do is go to higher precision and hope that somewhere, something will crack and you will find where our new physics is.”