Antimatter Mystery Continues To Perplex Scientists

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One of the biggest mysteries of science is why there is something rather than nothing. According to our best scientific theories, the universe should consist of a featureless bath of energy. Yet that prediction is obviously wrong. While scientists have discovered a few hints on ways to solve the problem, it’s still an outstanding question. And a recent paper released by the LHCb collaboration, using the Large Hadron Collider, the world’s most powerful particle accelerator, has added to our confusion.

Probably the most famous scientific equation of all times is Einstein’s E = mc2. This simple equation says that energy (E) is equal to mass (m), times a constant (c2). More colloquially, it says that energy can be converted into matter and then back again. However, you need to be bit more cautious. Technically, it says that energy can be converted into equal amounts of matter and antimatter.

Antimatter is a cousin of matter. Actually, it’s really the opposite of matter – antimatter is yang to matter’s yin. If you combine matter and antimatter, it makes energy – and a lot of it. Combine a gram of matter and antimatter, and the result is an energy release comparable to the atomic explosion of Hiroshima.

Antimatter was discovered in 1931 and its existence is universally accepted by the scientific community. However, it isn’t something that exists in large quantities in the universe, and that deficit is a huge scientific mystery. That’s because, according to our best understanding of how the universe came into existence, there should be as much matter as antimatter.

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he currently accepted theory of the origins of the universe is called the Big Bang. Originally, the universe was smaller and hotter, and it has been expanding and cooling for nearly 14 billion years. During the earlier and hotter phase, the universe was full of energy. And, where there is a lot of energy, there should be matter and antimatter made in equal quantities. This means that shortly after the Big Bang, the universe should have contained matter and antimatter. Then, as the universe expanded, that matter and antimatter should have bumped into one another, leaving a vast and featureless bath of energy.

Yet this obviously isn’t true. Our universe consists of matter and energy, but no large amounts of antimatter are to be found. And, given that matter is all around us, it seems that there must have been something in the early universe that favored matter over antimatter.

In the 1960s, scientists found that when they made a form of matter called the K meson in particle accelerators that they slightly favored matter K mesons over antimatter ones.

Recently, scientists at the Large Hadron Collider were investigating another form of mesons, called B mesons. Mesons, like protons and neutrons, are made of smaller particles called quarks. However, while the proton contains three quarks, mesons all contain a quark and an antimatter quark. Different types of mesons contain different mixtures of quarks and antiquarks.

The electrically neutral B meson contains a d-type quark and an antimatter b-type quark.  The antimatter neutral B meson has the opposite quark content – a b-type quark and antimatter d-type quark.

Because the two types of quarks (b-type and d-type) have the same electrical charge, inside the B meson, the two types of quarks can exchange which is matter and which is antimatter via a complicated interaction. Thus, a neutral B meson can convert into an antimatter neutral B meson and back again. In fact, the particle oscillates its identity back and forth trillions of times per second.

Scientists have found that the oscillation back and forth isn’t completely even. By looking at a specific decay of neutral B mesons into two other (and lighter) mesons, called the K meson and pi mesons, scientists observed that the B meson  isn’t in both states equally. The neutral B meson has a slight preference (55% vs 45%) to be a d-type quark and b-type antiquark.

Now this asymmetry isn’t enough to explain why our universe is made of matter and not antimatter, but it is a powerful clue. It is also a clear demonstration that there are at least a few subatomic processes that favor matter over antimatter. 

However, another study investigated not neutral B mesons, but ones with electrical charge. When researchers again studied the decay of these particles into K and pi mesons, they found no preference in the decays. This was very mysterious. Scientists are quite perplexed why the universe should exhibit a preference for matter in for neutral B mesons, but not for charged ones. 

It is unclear exactly what message this disagreement is telling us. It could just mean that the existing theory needs a small tweak, but it could well be an important clue in the question of why the universe is made entirely of matter – indeed the question of why we exist at all.

It’s too soon to tell what the answer will be, but that’s how science is much of the time. A clue here, a peculiarity over there, or the occasional loose thread can be a small observation. Or, sometimes, when you tug that loose thread, the whole thing unravels, and you have to knit an entirely new sweater.  

Operations at the Large Hadron Collider have been on hold for two years now for refurbishments and upgrades. It was expected to resume in March 2021, but Covid caused a brief delay and it is now scheduled to resume in May instead. With new data, maybe researchers will be able to sort it all out.

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