Paradox and Periphery: What's the Matter With Matter?

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Issue 16, Volume 110

By Rania Zaki 

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Identity is solid—well, it should be. Science tells us that just as a tree cannot become a rock, a human cannot become a sock. But inside the core of your atoms, there are particles that do exactly just that: their identities change.

Fundamental particles, unlike protons, cannot be composed of other particles. One type of fundamental particle, neutrinos, which astrophysicist Neil deGrasse Tyson ardently proclaims as the “ghost particle,” undergoes an identity reversal known as an oscillation. Oscillations produce electrons, muons, or taus. By studying the reversal, scientists discerned a close violation of physics’s symmetry, providing evidence for the explanation of matter.

But understanding matter, especially distinctions in the early universe, requires knowledge of its particles. From atoms to protons to our current understanding of subatomic particles, the field of particle physics is summarized in a surprisingly accurate theory: the Standard Model. Imagine the equivalence of matter and everything we see as white paint. By isolating the colors in the paint, one can observe the primary colors (red, blue, and green) in it. After a decade, the paint crusts and produces markings. The Standard Model is the same; the primary colors are fundamental particles known as quarks and neutrinos, which are the lepton particles made as a result of the decay of matter. Despite what you learned in chemistry, physics observes matter as color-neutral. When three quarks with each color amalgamate, or mix, into a particle, not only do they become color-neutral, but they also form atoms, leptons, and thus matter.

For centuries, humans formulated exegeses to explain the distinct beginning of the universe: the six days of creation in Genesis or the “Cosmic Egg” of the ancient Indian text, the Rigveda. Theorists and physicists sought part in this studious task, developing a generally agreed upon theory: during the universe’s creation, all energy was packed in a small space, a dot about one-trillionth the size of the period that ends this sentence. In a fraction of a second, when the temperature became about 20 million times hotter than our Sun, energy split into one matter particle for each antimatter particle––a replica of matter in all aspects except for charge and spin, notated by the terms "left-handed" or "right-handed." Then, the temperature cooled, and the high energy needed to produce these particles ceased. But something happened in between these two temperature changes, because if equal amounts of matter and antimatter had been formed in the beginning, matter should not have prevailed. This is because antimatter and matter are strongly attracted to each other, and when they collide, they annihilate one another. Matter and antimatter should be invariant to the law of physics referred to as CP-symmetry: C for charge conjugation, which transforms a particle into its antiparticle and P for parity, which creates the mirror image of the particle, producing an inverted left-handed particle (also known as a right-handed particle). In other words, a reaction replaced with antimatter acts the same as that of matter, meaning that matter and antimatter were equally created and destroyed. This symmetry should be universal—so why does matter exist, especially at an asymmetrical ratio to antimatter?

Physics said matter shouldn't have survived, until James Cronin and his coworkers provided conclusive evidence for the first violation of this symmetry. In simple words, Cronin claimed that the two types of particles acted differently. Two predominant theories, electroweak baryogenesis and leptogenesis, might explain how.

Electroweak baryogenesis is an elegant proposal pioneered in the late 1980s that explains the conundrum of the disproportionate presence of baryons compared to that of antibaryons. The theory posits that there are variations in the Higgs boson, a particle that signals a field known as the Higgs field, theorized to give particles mass. These variations would then initiate a phase transition that would favor baryons and matter over antibaryons and antimatter.

Leptogenesis, however, focuses on the “ghost particle” and leptons. Data demonstrate a compelling argument for a heavy right-handed neutrino, which spins toward the right (neutrons are generally left-handed, meaning their spins orient toward the left). At the beginning of the universe, these massive particles would have decayed rapidly when the universe was expanding rapidly, causing a disturbance to antilepton and lepton decays. This asymmetrical decay might explain the increased presence of matter compared to antimatter.

The team behind the T2K (Tokai to Kamioka) experiment in Japan acquired strong evidence collected over a decade for leptogenesis in the matter-antimatter disparity. Using a neutrino beam generated at the Japan Proton Accelerator Research Complex (J-PARC) in Tokai, beams of neutrinos or antineutrinos were ignited 295 kilometers to Kamioka in a tank with over 50,000 gallons of pure water and 6,000 sensors. When a neutrino collides with a neutron in the tank, a muon or an electron particle is produced. Neutrinos were analyzed based on these particles they released. Using sensitive equipment, lead physicist Federico Sanchez and his colleagues documented the change in the neutrinos’ states in both beams.

Combining this input with past experiments, the team debunked any account of complete CP symmetry on multiple parameters. Peculiarly, the team reported that 90 electron neutrinos and just 15 electron antineutrinos were observed. Instead of equal oscillations of electrons to muon neutrinos, neutrino oscillations occurred more frequently than antineutrinos. Despite any conflicting biases, the experiment conferred a maximum favor for neutrinos and came close to excluding any favor toward antineutrinos. The experiment repudiated with 95 percent certainty that neutrinos do not uphold CP symmetry, representing an indication of significant CP violation that favors the propagation of matter g during the early stages of the universe to that of antimatter.

However, the mystery is still unresolved. “We don’t call it a discovery yet,” said T2K team member Chang Kee Jung.

As journalists of “Nature,” Silvia Pascoli of Durham University in the U.K. and Jessica Turner of Fermilab in the U.S. said these results are “undeniably exciting.” However, remarkable claims require remarkable evidence—specifically, a 99.99994 percent credence to claim it a “certain discovery.” The team at T2K is meticulously prepared to deliver their findings with confidence.

The collaboration plan is to reduce systematic uncertainties by upgrading the magnetized detectors—up to 99,000 sensory detectors—which would allow for the precise collection of more data while J-PARC intends to upgrade the accelerator and increase the intensity of the beam. T2K spokesperson Atsuko Ichikawa said that they aspire to upgrade within the next two years, though she added that this depends on the outcome of the pandemic.

Larger experiments are in the works. Among them is the Deep Underground Neutrino Experiment (DUNE), a collaboration between the U.S. and the European Organization for Nuclear Research. In it, neutrinos will be beamed 800 miles from Fermilab in Illinois to a giant underground detector at the Sanford Underground Research Facility located in an old gold mine in Lead, South Dakota. This will be done to investigate how the neutrinos oscillate. Deputy director of Fermilab Dr. Lykken said, “Now we have a good hint that the DUNE experiment will be able to make a definitive discovery of CP violation relatively soon after it turns on later in this decade.”

Before the invention of massive particle accelerators, Albert Einstein had feverishly worked on a visionary goal for 30 years until death: the creation of a unified theory that describes all the forces of the universe. Though he left it incomplete, over the years, scientists reformed it. The Grand Unification Theory of 1979 described the combination of forces in the Standard Model under high energies. A part of this theory is the generic model of the “seesaw” mechanism. In it, a ratio is described: the higher the mass of the right-handed neutrinos, the lower the mass of left-handed neutrinos. The neutrino’s mass is minuscule, about one-millionth of that of an electron. The possible CP-violation indicated by the T2K experiment alludes to the asymmetrical possibility of these right-handed particles.

Without the contributions of Cronin or Einstein, the necessary foundation of knowledge that T2K built would have been impossible. As the understanding of these particles and their interactions become more clear, scientific data become ever more precise, and the innovation of technologies explores all parameters of study; the paradox of matter may be resolved. Before long, science may explain why we exist by understanding the identity switch in the millions of particles that pass through us before we blink. Ahoy!