Why Didn’t Antimatter Destroy The Universe? (Script)

At one-one-thousandth of a second after the Big Bang, the great annihilation event should have wiped out all matter, leaving a universe of only radiation. Why still don't know why any matter survived. Well, a new finding from the LHC brings us one step closer to understanding why there's something rather than nothing.

—-

The job of physics is to understand the behavior of matter and energy and space and time—to understand the how and why of  “stuff”. But some would argue that the deeper philosophical question is why stuff even exists in the first place. Why is there something rather than nothing? It’s been said that this is a question for philosophy, not for physics. But physicists, who have a habit of thinking that no question is beyond their powers, tend to disagree. Look, it’s simple math right? Nothing is zero anything. Something is any non-zero amount of any type of stuff. So if you start with nothing you should be able to turn it into something by adding equal parts positive stuff and negative stuff. That’s still technically zero stuff. Nothing and something can be mathematically equivalent. 

At least, that’s a somewhat facetious interpretation of the argument that some physicists have given for why antimatter solves the something-from-nothing conundrum. Quantum fluctuations in an empty vacuum can produce matter and antimatter.

This doesn’t tell us where we got the vacuum or the laws of physics from in the first place, but that’s for another time. But antimatter does at least explain something about how a universe containing only bland, featureless potential became one full of stuff. 

There’s another challenge with this nothing-to-something story—matter and antimatter should destroy each other as soon as they form. Think of antimatter as the evil twin, or bizarro-world reflection of matter. Every particle of matter has an antimatter counterpart with precisely the same mass and precisely opposite in quantum properties like spin, electric charge. These quantum properties are strictly conserved—for example, positive electric charge can only appear from nothing if an equal negative electric charge also appears to balance the ledger.

Like Physicsgirl’s famous vortex-antivortex experiment, where twin whirlpools are created with a sweep of a dinner plate in a pool. It’s like angular momentum—the spin of each vortex—comes from nothing. But the vortices have opposite spin, so the total angular momentum remains zero. If these vortices find each other again, they cancel each other’s rotation and vanish.

So antimatter is like the antivortex—it carries away the opposite “twist” in the otherwise twist-free quantum fields, allowing matter to exist. And we see the appearance of matter-antimatter pairs all the time. Raw radiation like a photon can spontaneously form matter-antimatter pairs like an electron and positron in a process called pair production. It’s like the photon is the plate dragging through the pool of the electron field.

We think that the very early universe was one of pure and very intense radiation, and that all matter appeared in the form of matter-antimatter pairs. We also expect that these pairs would immediately annihilate each other. That means that after the universe cooled below the pair-production temperature, all the stuff in the universe would quickly vanish away leaving a light-filled universe that was empty of matter. That’s clearly not the case. What actually seems to have happened is that all of the antimatter was annihilated, presumably taking with it an equal amount of matter. But for some unknown reason there was just a little bit more matter than antimatter—1 particle per billion to be precise-ish—and those lucky particular winners of the musical chairs of annihilation would go on to become the stars, the galaxies, and you.

The miniscule asymmetry between the amount of matter versus antimatter at that early time is why there’s something rather than nothing—or at least nothing interesting. It’s a major outstanding problem in physics. According to the Standard Model of particle physics, matter and antimatter should behave exactly the same. That means if you replace every particle in the universe with its antimatter counterpart then nothing would change—positrons would orbit anti-nucleons, the anti-earth would revolve around the anti-sun, and anti-physicists shouldn’t notice anything awry. 

The idea that the laws of physics are the same for matter and antimatter is called charge-parity or CP symmetry. We can describe antimatter as regular matter that has had its quantum properties flipped. In particular, the precise antimatter counterpart of a matter particle has opposite charge—-whether electric charge, color charge, or whatever, and opposite parity—which is equivalent to a mirror-reflection, which notably flips its quantum spin. If the laws of physics are unchanged under this charge-parity or CP inversion then we say the universe is CP symmetric. 

But a CP-symmetric universe would have no way to generate more matter than antimatter at the beginning of time. For that reason, CP symmetry must be violated in subtle ways. And we’ve actually observed that violation. First in the oscillations and decay products of the Kaon—that’s a particular type of two quark particle or meson made of up and down quarks. We’ve also seen it in a number of other mesons that contain the bottom or beauty quark. When these unstable particles fall apart, they can leave behind a range of decay products, and the decay outcomes have different probabilities that depend on whether it was the matter or antimatter version of the meson.

But one thing we’d never seen until this new study is CP violation in baryons. These are 3-quark particles like the familiar protons and neutrons, but also more exotic types. And if we want to understand the matter-antimatter asymmetry, we probably should understand it in the context of the type of matter that makes up all of the visible stuff in the universe.

So without further ado, let’s look at the new result from the Large Hadron Collider. The LHC at CERN in Switzerland is most famous for finding the Higgs boson back in 2011. As the largest particle collider in the world the hope is it can do a lot more. Many thought that the next great achievement would be to discover supersymmetry—and its so-far failure to do so has confused and disappointed many physicists. But LHC is no one-trick pony. It supports many big experiments exploring big questions. 

One of those is the LHCb experiment. The b isn’t for backup or plan-b. It’s sole purpose is to explore the subtle asymmetries between matter and antimatter through one little particle—the bottom quark, or beauty quark as LHCb prefers to call it. All of the familiar matter in the universe is rooted in the up and down quarks that make up protons and neutrons, along with their electron companions. But there are 6 quark types in total, including the top, bottom, strange and charm quarks. Bottom-slash-beauty are especially interesting because they are unusually susceptible to CP violation. 

Let me try to give you a quick overview of how this symmetry violation comes about. Quarks transform or decay via the strong and weak interactions. Quark colour charge can change, and quark flavour or quark type can change.

These decays are often depicted with Feynman diagrams. Each diagram depicts a possible sequence of intermediate interactions that could lead from given input to output states—we call each diagram a channel for the decay. But because quantum mechanics is mind-bendingly weird, the situation between our known states—before and after the decay—is fundamentally uncertain. To calculate the probability of going from a given input to output, you have to add together the probabilities of all possible intermediate states, because in a sense all channels happen in a quantum superposition.

A more familiar example of quantum superposition is the double slit experiment. A particle travels through a pair of slits to a detector screen. It does so as a probability wave, and the final landing point of the particle is determined by how these waves interfere with each other after passing through the slits. It’s more probable that the particle will land where the waves stack up via constructive interference, and less likely where they cancel in destructive interference.

Well something like this also applies for the alternate intermediate states in a particle interaction. There’s interference between the possible spatial paths within each channel—each Feynman diagram. But there’s also interference between the different channels—a sort of cross-talk between the “alternate realities” of the different ways a quark can decay.

https://drive.google.com/file/d/1R3Bj_293M6L_7e_b4mpAhjMjgXl4CubI/view?usp=sharing

This only makes a difference if there’s an actual phase difference between channels—just like the phase offset in the double-slit waves can lead to cancelation. So the raw probabilities for each decay channel are identical for matter versus antimatter, but one thing that gets inverted in antimatter is the wavefunction phase that feeds into the decay probabilities. That leads to different interference profiles and so to different decay products. 

There are particular conditions required for this difference to be observable, but colour and flavour transformations mediated by the weak interaction do the trick, and this is particularly strong in the decay of the bottom quark. For this reason, CP violation experiments have focused on particles containing these particles—the LHCb that we’re discussing today, and also Belle II at the SuperKEKB accelerator in Japan and the BaBar experiment at the Stanford Linear Accelerator. All have seen CP violating decays in B-mesons. But just recently, LHCb managed to distinguish a significant difference in the decay of bottom-quark-containing baryons for the first time. LHCb studies these decays by taking the near-light-speed protons from the LHC ring and smashing them together in front of a series of detectors tuned to the expected energies of these decay products. They can pick out the bottom decays by sorting the many, many detections from each collision by the expected arrival time at the plates, and making use of the fact that these bottom-containing particles will have traveled a little from the original collision site before decaying. 

LHCb collected these decay products between 2011 and 2018, and spent the following years performing a careful analysis of the data. That included sifting through the enormous number of particle detections to identify the products of a very particular event. They wanted to isolate the result of a b-baryon or its antimatter counterpart breaking up into a proton or anti-proton respectively, along with a kaon and a pair of pions—this would be their best chance to detect CP violation in baryons. And, after all that particle-sorting and number crunching … there was indeed a very real difference between the rates of this decay between the matter and antimatter b-baryons.

The asymmetry is only around 2.5%, but with a confidence of 5.2 sigma. That means CP violation is formally detected in baryons—with only a one in several hundred thousand chance of occurring by random chance. And if b-baryons violate CP symmetry, it’s not a stretch to think that this is probably true for baryons in general, including the protons and neutrons that make up the familiar matter of the universe. The stuff of the anti-verse is subtly different to our universe.

So, have we explained why there’s something rather than nothing? Not yet. Even with this observation, the degree of CP violation isn’t enough to fully explain the amount of matter left over from the great annihilation event of the early universe. New sources of CP violation beyond the quarks are needed for that. There are high hopes that we’ll find the necessary asymmetry in the leptons—so, the electron and its sibling muon and tau, and the neutrinos. Fermilab’s Nova and upcoming DUNE, as well as the Japanese T2K and upcoming Hyper-Kamiokande hope to find lepton CP violation by observing how neutrinos oscillate between types. But if we don’t find the needed degree of asymmetry in experiments like these, it may mean we have to look beyond the standard model for new ways to explain the matter-antimatter imbalance. But one step at a time. For now, finding that matter and antimatter baryons are indeed gently different is a step towards answering a much bigger question: why does the universe have matter at all, rather than being a featureless, radiation-filled spacetime.