For every particle in the universe, physicists believe that there should exist an antiparticle with the same mass, but the opposite charge. When a particle and an antiparticle meet, they annihilate one another and are transformed into pure energy. Looking around, though, it's obvious that most of the antimatter has disappeared and the universe has not been annihilated into pure energy. Although antimatter has been observed in nature, it occurs in far smaller quantities than its twin, which begs one of the most perplexing questions in physics: where did all the antiparticles go? Or to put it another way: why do we exist?
Physicists have been puzzling over this matter-antimatter asymmetry for decades, but new data coming from the Large Hadron Collider beauty (LHCb) experiment may help shed some light on the problem. As reported last week in Nature, physicists at the LHC have observed CP violation in the decay of particles known as baryons and antibaryons for the first time. Although a little more data is needed before it can officially be declared a discovery, these observations may blow open the door for new experiments that will ultimately explain what happened to all the antimatter, and beyond that, why there is something in the universe rather than nothing.
In 1967, Russian physicist Andrei Sakharov proposed a solution to the puzzle of matter-antimatter asymmetry, but it required violating one of the fundamental properties of nature known as Charge-Parity (CP) symmetry. According to an MIT explainer, CP symmetry describes the correspondence between matter and antimatter through two operations known as charge conjugation and parity. Charge conjugation describes the process of turning a particle, such as an electron, into its antiparticle, such as a positron, whereas parity describes the inversion of a particle in space, such that if an electron was moving left to right, parity would make it move right to left.
Taken together, this means that when CP is applied to matter, it should result in a mirror image of antimatter that is equal, but opposite. Yet in the late 1950s and early 1960s, a handful of physicists began producing irrefutable evidence of CP symmetry violation, which would imply that the laws of physics were different for matter and antimatter. This resulted in a Nobel prize for the physicists behind it, and kickstarted a decades-long hunt in particle physics for miniscule differences between matter and antimatter in an effort to explain the universe's preference for the former.
In the last 50 years, CP violation has only been observed in a class of subatomic particles known as mesons, which are composed of two quarks. When two protons are fired at one another in a particle accelerator such as the LHC, they produce mesons and antimesons. It's the idiosyncratic ways these particles and antiparticles decay that reveal whether CP violation has occurred.
"The problem is that "the amount of Charge-Parity violation that exists in the Standard Model is not enough to explain the the matter-antimatter asymmetry in the universe," Makoto Fujiwara, a senior scientist at the ALPHA antimatter project, told me. "Charge-Parity violation exists, we know it exists and its been measured, but there's just not enough. It's too small."
In other words, Sakharov's theory of matter-antimatter asymmetry requires particles other than mesons to exhibit CP violation in order to make up the difference—and the LHCb observed the CP violation in a non-meson particle for the first time.
Using data compiled over the first three years of the LHC's life, researchers at the LHC beauty experiment compared the decay of baryon and antibaryon particles (baryons are in the same family of particles as mesons, but are composed of three quarks instead of two), which resulted from proton-proton collisions. It was the first time that CP violation in the process of baryon decay had ever been observed, partly due to the difficulty in producing the specific type of baryons in large enough quantities. After three years, the LHCb researchers had collected 6,000 examples of this type of decay.
When baryons and antibaryons decay, they leave behind a proton or antiproton and three charged particles called pions. When they observed the way the baryons and antibaryons would decay into these particles, the researchers found a significant level of asymmetry in how the baryons decayed into matter or antimatter. Put another way, this is strong evidence of CP violation in baryons, an observation which could eventually shed light on the problem of matter-antimatter asymmetry in the universe.
"This is the first time asymmetry in baryon decays have been measured," Nicola Neri, an Italian physicist working on the LHC beauty experiment, told me. "This study will not answer the question of why we ended up with a matter-dominated universe, but this is one piece of that puzzle."
Although the LHCb researchers had 6000 examples of this type of decay, more precise data is needed before these observations can be classed as a discovery of CP violation in baryons. The researchers at the LHCb are confident that, after a series of upgrades on the LHC that will allow them to collect up to ten times the amount of data being collected during the first three years of the experiment, they will have enough evidence to claim the discovery.
"This is very exciting time for the whole community," said Neri. "With these measurements and more data, we can do a systematic study to investigate the underlying physics that regulates baryon decays and the differences between matter and antimatter."
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