An experiment at the Large Hadron Collider (LHC), the world's largest particle accelerator, has revealed the first evidence for a phenomenon known as charge-parity (CP) violation in the decays of subatomic particles called charm quarks. The results could help researchers get closer to explaining why matter dominates over antimatter in the universe, enabling galaxies, stars, planets, and—ultimately—living organisms to exist. A key theory in physics, CP symmetry holds that physical laws should stay the same if a particle's electric charge is switched—that is, if a particle is swapped for its antimatter counterpart—and if the particle's spatial coordinates are inverted, presenting a mirror image of itself. See also: CP symmetry and its violation; Electric charge; Parity (quantum mechanics); Symmetry breaking; Symmetry laws (physics)

Researchers searching for a hypothetical fourth type of neutrino have found no evidence for the particle, according to a new study. Very low-mass and highly abundant, neutrinos are elementary particles that barely interact with the rest of matter. In addition to the neutrino's three documented types, or flavors—known as electron, muon, and tau—a fourth flavor is allowed by various theories of particle physics. Sterile neutrinos are highly sought-after by scientists because the particles could help explain the fundamental mystery about neutrinos having mass in the first place, which goes against the predictions of the otherwise well-supported standard model of particle physics. In addition, sterile neutrinos could help account for matter's puzzling dominance over antimatter in our universe, while also serving as a plausible candidate for dark matter, a mysterious substance that theoretically outnumbers normal matter at least five to one. See also: Antimatter; Dark matter; Elementary particle; Fundamental interactions; Mass; Matter; Neutrino; Standard model

In a potential watershed moment for physics, the first results from the Muon g-2 (pronounced gee minus two) experiment have strongly indicated that particles called muons deviate from precise predictions of their behavior in the standard model of particle physics. The findings, announced on April 7, 2021, suggest that undiscovered particles or forces in the universe are responsible for the muon's unexpected variance. See also: Physics; Standard model

Researchers at the Hybrid Quantum Systems Group at the Institute of Technology in Zürich, Switzerland (ETH Zürich), have successfully induced a stable quantum superposition of vibrational states in a 16-microgram sapphire crystal—the largest system in which superposition has ever been observed. The research group hopes that this breakthrough can continue to push the boundaries of our understanding of quantum mechanics, as well as potentially contribute to applications in quantum computing. See also: Demonstration of quantum superposition using record-breaking large molecules

Physicists at the Thomas Jefferson National Accelerator Facility, commonly called Jefferson Lab or JLab, recently determined the strength of the strong nuclear interaction at the largest distances recorded so far. Also called the strong force, the strong nuclear interaction is responsible for binding fundamental particles known as quarks into composite particles, including the protons and neutrons that comprise the nuclei of atoms. The scientists sought to answer the unresolved theoretical question of whether coupling—the strength of bodies interacting via the strong force—increases, decreases, or remains constant with distance. Results showed that at large enough distances, the distance dependency of the strong force coupling dropped out and coupling became constant. These findings provide experimental support for multiple theories including those accounting for the origins of the overwhelming majority of the mass of ordinary matter in the universe. See also: Atomic nucleus; Elementary particle; Fundamental interaction; Mass; Matter; Neutron; Proton; Quark; Strong nuclear interactions; Theoretical physics

In prior experiments, scientists have reported two disparate measurements of the lifetime of the neutron with a statistically significant difference. The conflicting findings prompted scientists at Oak Ridge National Laboratory (ORNL) to test for “mirror neutrons,” one of the particles in a theoretical “mirror dark sector” of matter, as a potential explanation of the discrepancy in a series of new experiments. A paper published in Physical Review Letters by the research team at ORNL reported that no such particle was found in the experiments, casting doubt on the validity of the mirror dark sector theory and keeping the cause of the discrepancy a mystery. See also: Neutron

Physicists recently reported the first indication of asymmetry in the behaviors of neutrinos and antineutrinos—subatomic particles that have zero electric charge and interact with matter almost entirely through the weak nuclear force. This type of behavior is known as charge-parity (CP) violation. The finding could help solve a fundamental mystery about why matter and antimatter, which mutually annihilate upon contact, could not have formed in equal amounts in the big bang, as prevailing scientific theories have held. See also: Antimatter; Big bang theory; CP symmetry and its violation; Matter; Neutrino

For the first time, researchers have used the spin rates of black holes in the wide-ranging and ongoing search for dark matter. A recent study proposed that if a certain kind of dark matter particle, known as an ultralight boson, existed in a particular mass range, the particles would collectively slow the spin rates of black holes. The black hole spin rates measured, however, proved twice as fast as the theoretical framework allowed, thus ruling out ultralight bosons in the observed mass range. This negative result helpfully narrows down the possible properties dark matter might possess, while also demonstrating a new kind of probe for subsequent searches in new black hole mass regimes. See also: Black hole; Mass

For the first time, researchers have successfully accelerated electrons in a wave of plasma driven by protons. The breakthrough points to a way of constructing smaller, thriftier particle accelerators in the future. See also: Electron; Particle accelerator; Plasma (physics); Proton

Particle physicists have reported the likely first-ever sighting of a long-theorized subatomic process known as a triangle singularity. The process involves a pair of particles, called kaons, exchanging one of their two constituent particles before moving away from each other. When depicted in a Feynman diagram—a pictorial representation of particles and their interactions—the kaons and their paths form two corners and two legs of a triangle, respectively, while the two swapped particles comprise the third corner. Discovering this process could offer new insights into the strong force, one of the four fundamental forces of nature. The strong force governs interactions of the elementary particles, called quarks, that comprise kaons, as well as more familiar protons and neutron that constitute atoms. See also: Atom; Elementary particle; Feynman diagram; Fundamental interaction; Neutron; Physics; Proton; Quark; Strong nuclear interactions