by Daniel Ang

figures by Aparna Nathan

Particle physics says that the universe shouldn’t exist.

This is a radical claim! But if the current theories that underlie particle physics are correct and complete, then the Big Bang that birthed the universe would have simply resulted in a massive flash of light. Nothing else would remain – no stars, planets or galaxies. And neither you nor I would be around to read this text.

Why does particle physics conclude that the universe shouldn’t exist? In this article, we will explore the most widely accepted theory in particle physics – the Standard Model- our inability to find any significant new physics beyond it, and the implications for our overall understanding of the universe.

Why the Standard Model is important

Particle physics is the study of subatomic particles and their interactions with one another. These include protons, neutrons, and electrons, which are the basic constituents of most naturally occurring matter, as well as more obscure particles such as muons and neutrinos, which can only be produced in significant amounts artificially. Some subatomic particles like protons and neutrons are made of fundamental particles called quarks. Others such as electrons are considered fundamental particles in their own right. While we cannot see any of these particles with our naked eye, they give rise to the nature and properties of all the macro-level matter around us (Figure 1).

Throughout the 20th century, particle physics progressed rapidly, from Rutherford’s discovery of the structure of the atom (1911) all the way to the development of the Standard Model (SM), a theoretical framework that incorporates the 17 types of known fundamental particles and their interactions (Figure 2). As a theory, the SM has been very accurate, giving predictions that have been borne out by multiple experiments. Its latest major victory was the discovery of a new particle called the Higgs boson – the last fundamental particle proposed by the model that had not previously been directly observed. The SM currently represents our best understanding of the universe. In almost all of our experiments, we have not been able to find definite signs of new physics beyond the SM. But, as we shall see, this theory has some fundamental problems, as it fails to explain major natural phenomena in our universe.

The problem of dark matter

One of the limitations of the Standard Model is the existence of dark matter: mysterious masses of matter that do not reflect light and are thus invisible to the naked eye and telescopes. However, we can infer the presence of dark matter through its gravitational interaction with normal, light-reflecting matter, which results in anomalies like altered galaxy rotation curves. It is unlikely that dark matter consists of any of the particles in the SM. Rather, the most probable explanation for this evidence would be the existence of as-of-yet undiscovered particles that behave and interact differently than those outlined in the SM.

Many experiments have been designed to look for these new particles. For example, dark matter experiments LUX and XENON1T use a large tank of liquid xenon to find experimental evidence for the existence of a hypothetical dark matter particle called a WIMP, or a Weakly Interacting Massive Particle. If such a particle were to pass through the tank, it would interact with the xenon and produce a flash of light that is picked up by detectors. Unfortunately, despite over a decade of similar searches, no WIMPs have been detected by either experiment. Nor have any other dark matter experiments detected a clear, unambiguous signal of new particles. While this does not rule out all the range of possibilities of dark matter, null results like these have already constrained many simpler classes of theories.

In light of our failure to verify any of our proposed theories about dark matter with experimental evidence, we have to make do with the fact that there is a great hole in our understanding of matter in the universe. Still, can’t we at least be satisfied that the Standard Model describes ordinary, everyday matter – protons, neutrons, electrons – very well?

Unfortunately, the answer is: not quite.

The problem of baryon asymmetry

In the beginning of this article, I described how the Standard Model tells us that instead of stars and galaxies, there should only be pure light. To understand why this is the case, we have to talk about antimatter, a new type of matter different from ordinary matter and dark matter. Antimatter has certain properties that are opposite to ordinary matter (which we shall henceforth just call matter) but otherwise seems exactly the same. The first antimatter particle discovered was the positron (1932), whose properties such as mass and magnetism are identical to the more familiar electron. The only difference is that its charge is positive instead of negative. In addition, when matter and antimatter come into contact with one another, they annihilate, leaving only pure energy mostly in the form of photons.

As we know that antimatter is the perfect opposite of matter, if the Standard Model is a complete theory, the Big Bang should have produced equal amounts of matter and antimatter. These two different, opposite types of matter would mix with each other and annihilate, resulting in a bath of pure, formless energy, mostly light (Figure 3).

Of course, the reality is that we have more than just light in this universe. So there must be some missing piece in the puzzle. Specifically, there must be asymmetric processes happening in the universe that produce more matter than antimatter. After the Big Bang, matter would win out, leading to the abundance of galaxies and planets we see today. This is called the baryon asymmetry problem.

What are these asymmetric processes? The SM does contain some processes that would result in a tiny bit more matter being produced than antimatter. But it is far from enough to explain the amount of matter we see today. Thus, if there is no new physics beyond the SM, particle physics would have no good explanation for baryon asymmetry, utterly failing in its goal to explain the origin, nature, and behavior of matter.

In response to this quandary, particle physicists run experiments to look for new asymmetric processes beyond what is predicted by the SM. In the Large Hadron Collider, particles are smashed together at high speeds in hopes of observing these asymmetric processes. In 2017, some evidence was found of particle decays that result in more matter than antimatter. However, it remains to be seen whether these results are sufficient to completely solve the baryon asymmetry problem. Similarly, experiments like the ACME EDM experiment also investigate these asymmetric processes by looking at their effects on fundamental particles, such as the charge distribution of the electron. However, despite the incredible degree of precision achieved, the most recent ACME EDM result failed to find evidence that explains baryon asymmetry.

A never-ending search

As we have seen, the Standard Model presents us with a conundrum. Because it cannot explain the evidence of dark matter nor baryon asymmetry, we are almost sure that it has to be wrong at some level. Yet, every time we get into the laboratory to pinpoint exactly how it fails, we have come up empty handed. In other words, there seems to be a great tension at the heart of particle physics!

Will the answer to this conundrum be lying around in the corner, in the next magnitude of improvement in precision in these experiments, or is it impossibly distant, forever inaccessible without recreating the Big Bang itself? We can only hope that it is the former, as I think the human capacity to keep asking these great questions will never be exhausted.

Daniel Ang is a fourth-year graduate student in physics at Harvard University who works on the ACME EDM experiment.

Aparna Nathan is a second year Ph.D. student in the Bioinformatics and Integrative Genomics program at Harvard University.

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