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Category: particle physics – Page 16

First Signs of Quark–Gluon Plasma in Oxygen–Oxygen Collisions

When two heavy nuclei collide at relativistic speeds, the quarks and gluons that are usually bound inside them are briefly liberated, forming an exotic state of matter called quark–gluon plasma. As the quarks and gluons traverse this plasma, they lose energy through scattering, which limits the number of high-momentum particles that reach the detectors. This signature of quark–gluon plasma, called jet quenching, has been definitively observed only in collisions of heavy nuclei such as lead, leaving open the question of how large a nucleus must be to produce quark–gluon plasma. Now the CMS Collaboration at the Large Hadron Collider (LHC) at CERN has observed the first clear evidence of jet quenching in oxygen–oxygen collisions [1].

The LHC collided oxygen nuclei for the first time in 2025. Scientists in the CMS Collaboration measured the rate at which those collisions produced high-momentum daughter particles and compared it to the rate measured for proton–proton collisions at the same collision energy. In the absence of a quark–gluon plasma, the two rates—after accounting for the number of protons and neutrons in the oxygen nuclei—would be approximately equal. The researchers found that, in the oxygen–oxygen collisions, this ratio dipped significantly for daughter particles with energies of around 6 giga-electron-volts (GeV)—a clear indication of the jet-quenching phenomenon.

The oxygen–oxygen collision data recorded by the CMS team are qualitatively similar to those obtained from collisions of larger nuclei such as lead. They are also in better agreement with theoretical models that include quark–gluon energy loss than they are with models that omit it. The result provides the strongest evidence yet that a quark–gluon-plasma-like medium capable of jet quenching can form in collisions of nuclei as light as oxygen.

Classical physics can explain quantum weirdness, study shows

When you throw a ball in the air, the equations of classical physics will tell you exactly what path the ball will take as it falls, and when and where it will land. But if you were to squeeze that same ball down to the size of an atom or smaller, it would behave in ways beyond anything that classical physics can predict.

Or so we’ve thought.

MIT scientists have now shown that certain mathematical ideas from everyday classical physics can be used to describe the often weird and nonintuitive behavior that occurs at the quantum, subatomic scale.

Particle thought to break physics followed rules all along, research reveals

A tiny discrepancy in particle physics has loomed for decades as an exciting possible crack in one of science’s most successful theories, hinting at unknown forces or quantum objects. Now, an international team led by a Penn State physicist has published the most precise study yet to reveal the discrepancy was a fluke in calculation, not nature.

More than half a century of measurements of a fundamental property of the muon—the more massive, short-lived cousin of the electron—did not line up with theoretical predictions, raising hopes that new physics might be behind the unexplained inconsistency.

In a paper published in the journal Nature, a team led by a Penn State researcher describes one of the most precise calculations ever performed in particle physics, showing that the Standard Model—the theory describing the known building blocks of matter—still holds.

A new route for plasma-based particle accelerators

Plasma, the fourth state of matter, consists of a gas in which electrons are no longer bound to atoms, which allows electricity to flow freely. When beams of particles moving close to the speed of light travel through plasma, they disturb electrons and drive so-called plasma waves.

Researchers at the ELI Beamlines Facility and Czech Technical University in Prague recently explored the possibility of leveraging plasma waves driven by fast-moving beams of charged particles, such as protons or electrons, to create a relativistic mirror, a concept rooted in Einstein’s theory of special relativity.

Their theoretical analyses and the results of simulations testing their predictions were published in Physical Review E and Physical Review Research.

Soundwaves settle debate about elusive quantum particle

It was a head-spinning discovery. In 2018, researchers in Japan claimed to find concrete evidence of an elusive particle, a Majorana fermion, in a quantum spin liquid called ruthenium trichloride. Majoranas are highly sought-after by quantum materials scientists because when a pair are localized, or trapped, they can securely encode information and form a stable qubit—the building block of quantum computing.

Some researchers heralded the finding and used it to launch their own studies, while others believed the breakthrough—which was made by measuring what’s called the thermal Hall effect—was actually a mirage caused by defects in the material sample.

Cornell researchers have now waded into the debate and their findings, published in Nature, show both camps were wrong. By measuring the movement of sound waves rather than the flow of heat, the team discovered the thermal Hall effect was caused by rotating lattice vibrations called chiral phonons.

ATLAS sets record limits on Higgs boson’s self-interaction

One of the biggest open questions in particle physics today is how the Higgs boson interacts with itself. This “self-coupling” could help explain the evolution of the early universe and the mechanism that gives mass to elementary particles. To try to shed light on this fundamental interaction, the ATLAS Collaboration has recently studied one of the “golden” decay channels of a pair of Higgs bosons, where one Higgs boson decays into two photons and the other into a pair of bottom quarks.

Put a nanodiamond under intense pressure and it becomes flexible

Diamond is among the hardest naturally occurring substances on Earth, but if you shrink it down to the nanoscale, it is surprisingly elastic. And that could be useful for a host of applications such as quantum computing. In a paper published in the journal Physical Review X, Chongxin Shan at Zhengzhou University in China and colleagues studied diamonds as small as four nanometers across to see how they respond to pressure.

Scientists already know that nanodiamonds, which are thousands of times smaller than a grain of sand, can survive being stretched or squeezed in ways that destroy a regular diamond. But nobody knew how.

So the team placed individual nanodiamonds (ranging from 4 to 13 nanometers across) inside a transmission electron microscope between two diamond indenters and compressed them. These were connected to a sensor that measured how strongly each nanodiamond resisted being squeezed while a high-resolution camera imaged diamond atoms as they moved. The researchers backed up their observations with computer simulations.

Laser bursts flip nanoscale magnetic vortices at blistering speeds, opening a path to brain-like spintronics

Spintronics are devices that operate leveraging the spin, an intrinsic form of angular momentum, of electrons. The ability to switch magnetic states is central to the functioning of these devices, as it ultimately allows them to represent binary digits (i.e., “0” and “1”) when processing or storing information.

Some of these devices rely on magnetic vortices, nanoscale whirlpool-like patterns of magnetization that influence the alignment of spins. These vortices possess a property known as helicity, which is essentially the direction in which they rotate.

Reliably switching the helicity of magnetic vortices could open new possibilities for both neuromorphic computing systems, devices that mimic the brain’s neural organization, and multi-state memories. So far, however, this has proved challenging, mainly because it requires a synchronized wave-like rotation of spins without disrupting the geometric structure of vortices.

ATLAS acts as a cosmic-ray laboratory with first measurement of proton–oxygen collisions

Tens of kilometers above Earth’s surface, high-energy particles from outer space constantly strike the atmosphere, creating showers of energetic secondary particles that rain down from the sky. Approximately one of these particles passes through your head every second, but the “cosmic rays” that produce them are still not fully understood. In a recent paper posted to the arXiv preprint server, the ATLAS Collaboration describes how its first measurement of proton–oxygen collisions at the LHC could help us learn more about them.

Cosmic rays were discovered over a century ago by physicist Victor Hess in experiments conducted aboard hot-air balloons. Today, astrophysicists use detectors on the ground to image cosmic-ray showers and computer simulations of the showers to understand that data.

However, these simulations depend on properties of the strong force—one of the fundamental forces of the universe—which is difficult to accurately model. Current simulations disagree with one another, making it difficult for astrophysicists to interpret their measurements of cosmic rays.

What a Neutron Star Is Really Made Of

What happens to matter when it’s crushed beyond the point where atoms can exist? Inside a neutron star, the densest visible object in the universe, matter is compressed into states so extreme that physicists still don’t fully understand what’s there.

In this calm long-form space documentary, we take a journey layer by layer through the interior of a neutron star — from the crystalline crust where exotic nuclei form structures unlike anything on Earth, through the bizarre \.

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