Enlarge / The red arcs at the right of center are background galaxies distorted by gravitational lensing. The number, location, and degree of distortion of these images depends on the distribution of dark matter in the foreground. (credit: ESA/Hubble & NASA, A. Newman, M. Akhshik, K. Whitaker )

Decades after it became clear that the visible Universe is built on a framework of dark matter, we still don't know what dark matter actually is. On large scales, a variety of evidence points toward what are called WIMPs: weakly interacting massive particles. But there are a variety of details that are difficult to explain using WIMPs, and decades of searching for the particles have turned up nothing, leaving people open to the idea that something other than a WIMP comprises dark matter.

One of the many candidates is something called an axion , a force-carrying particle that was proposed to solve a problem in an unrelated area of physics. They're much lighter than WIMPs but have other properties that are consistent with dark matter, which has sustained low-level interest in them. Now, a new paper argues that there are features in a gravitational lens (largely the product of dark matter) that are best explained by axion-like properties.

Particle or wave?

So, what's an axion? On the simplest level, it's an extremely light particle with no spin that acts as a force carrier. They were originally proposed to ensure that quantum chromodynamics, which describes the behavior of the strong force that holds protons and neutrons together, doesn't break the conservation of charge parity. Enough work was done to make sure axions were compatible with other theoretical frameworks, and a few searches were done to try to detect them. But axions have mostly languished as one of a number of potential solutions to a problem that we haven't figured out how to resolve.

Enlarge / The Jefferson Lab particle accelerator, where the work took place. Electrons in the oval at top center are sent to different underground target rooms (circles at lower right). (credit: Jefferson Lab )

If you ask how much an object like a bicycle weighs, there's a simple answer. But if you ask where the mass of a bicycle is, things get more complex. The bike has a lot of parts—some of which move—that all have different volumes, shapes, and densities, so its mass is distributed irregularly around its form.

To an extent, this is similar to the question of where the mass of a proton is. The proton is a collection of quarks and gluons moving at relativistic speeds around a central point. Figuring out where its mass lives would be difficult even without the fact that the analogy with bicycles completely falls apart due to one awkward fact: A proton weighs much more than its component quarks, and the gluons that hold the quarks together are massless. In fact, the mass of the particles involved is somewhat irrelevant. "If you do calculations where you set the quark mass to zero, the proton is pretty much the same thing," physicist Sylvester Johannes Joosten told Ars.

Instead, much of the proton's mass comes from the incredibly high energy density created by the gluons' strong force interactions. So, to understand the mass of a proton, we have to understand what its gluons are up to. Which, given that they're massless and have no charge, is extremely difficult to do. But some experimental work has created a value for the proton's mass radius, which describes the distribution of mass within the particle. And it turns out the value is significantly different from the proton's charge radius.

Enlarge / Event display of a W-boson candidate decaying into a muon and a muon neutrino inside the ATLAS experiment. The blue line shows the reconstructed track of the muon, and the red arrow denotes the energy of the undetected muon neutrino. (credit: ATLAS Collaboration/CERN)

It's often said in science that extraordinary claims require extraordinary evidence. Recent measurements of the mass of the elementary particle known as the W boson provide a useful case study as to why. Last year , Fermilab physicists caused a stir when they reported a W boson mass measurement that deviated rather significantly from theoretical predictions of the so-called Standard Model of Particle Physics —a tantalizing hint of new physics. Others advised caution, since the measurement contradicted prior measurements.

That caution appears to have been warranted. The ATLAS collaboration at CERN's Large Hadron Collider (LHC) has announced a new, improved analysis of their own W boson data and found the measured value for its mass was still consistent with Standard Model. Caveat: It's a preliminary result. But it lessens the likelihood of Fermilab's 2022 measurement being correct.

"The W mass measurement is among the most challenging precision measurements performed at hadron colliders," said ATLAS spokesperson Andreas Hoecker . "It requires extremely accurate calibration of the measured particle energies and momenta, and a careful assessment and excellent control of modeling uncertainties. This updated result from ATLAS provides a stringent test, and confirms the consistency of our theoretical understanding of electroweak interactions.”

Enlarge / A diagram of the array of detectors in STEREO (left) and its location near a nuclear reactor (right). (credit: Loris Scola - CEA)

Neutrinos are probably the strangest particles we know about. They're far, far lighter than any other particle with mass and only interact with other matter via the weak force—which means they barely ever interact with anything. Three types (or flavors) of neutrinos have been identified, and any individual particle doesn't have a fixed identity. Instead, it can be viewed as a quantum superposition of all three flavors and will oscillate among these identities.

As if all that weren't enough, a set of strange measurements has suggested that there could be a fourth type of neutrino that doesn't even interact via the weak force, making it impossible to detect. These "sterile neutrinos" could potentially explain the tiny masses of the other neutrinos, as well as the existence of dark matter, but the whole "impossible to detect" thing makes it difficult to address their existence directly.

The strongest hints of their presence come from odd measurement results in experiments with other flavors of neutrinos. But a new study today rules out sterile neutrinos as an explanation for one of these oddities—even while confirming that the anomalous results are real.

Enlarge / Where the action happens inside the National Ignition Facility. (credit: Damien Jemison/LLNL )

On Monday, a paper was released that describes some confusing results from the National Ignition Facility, which uses a lot of very energetic lasers focused on a small target to begin a fusion reaction. Over the past few years, the facility has passed some key milestones, including ignition of fusion and creating what's termed a burning plasma.

Now, researchers have analyzed the properties of the plasma as it experiences these high-energy states. And to their surprise, they found that burning plasmas appear to behave differently from those that have experienced ignition. At the moment, there's no obvious explanation for the difference.

Ignition vs. burning

In the experiments at issue here, the material being used for fusion is a mix of tritium and deuterium, two heavier isotopes of hydrogen. These combine to produce a helium atom, leaving a spare neutron that's emitted; the energy of the fusion reaction is released in the form of a gamma ray.

Enlarge / Artist’s representation of a cosmic neutrino source shining above the IceCube Observatory at the South Pole. Beneath the ice are photodetectors that pick up the neutrino signals. (credit: IceCube/NSF)

Ever since French physicist Pierre Auger proposed in 1939 that cosmic rays must carry incredible amounts of energy, scientists have puzzled over what could be producing these powerful clusters of protons and neutrons raining down onto Earth's atmosphere. One possible means for identifying such sources is to backtrack the paths that high-energy cosmic neutrinos traveled on their way to Earth, since they are created by cosmic rays colliding with matter or radiation, producing particles that then decay into neutrinos and gamma rays.

Scientists with the IceCube neutrino observatory at the South Pole have now analyzed a decade's worth of such neutrino detections and discovered evidence that an active galaxy called Messier 77 (aka the Squid Galaxy) is a strong candidate for one such high-energy neutrino emitter, according to a new paper published in the journal Science. It brings astrophysicists one step closer to resolving the mystery of the origin of high-energy cosmic rays.

"This observation marks the dawn of being able to really do neutrino astronomy," IceCube member Janet Conrad of MIT told APS Physics . "We've struggled for so long to see potential cosmic neutrino sources at very high significance and now we've seen one. We've broken a barrier."

Enlarge / First spotted by astronomers in May 1999, Ryugu is essentially a large collection of loose rubble. (credit: JAXA)

The Japanese spacecraft Hayabusa2 returned to Earth in December 2020 bearing soil samples collected from a nearby asteroid, 162173 Ryugu . Those samples were divided between six scientific teams around the world for cutting-edge analysis to determine their composition in hopes of learning more about how such bodies form. The results of the first year of analysis of those samples appeared in a new paper published in the journal Science and included the detection of a precious drop of water embedded in a crystal.

These findings suggest that Ryugu was once part of a much larger asteroid that formed out of various materials some two million years after our Solar System (some 4.5 billion years ago). Over the next three million years, the parent body's carbon dioxide ice melted, resulting in a water-rich interior and a drier surface. When another space rock hit the parent body about a billion years ago, it broke apart, and some of the resulting debris formed Ryugu. An accompanying computer simulation supports this formation history, backed by the results of the sample analyses.

First spotted by astronomers in May 1999, Ryugu is essentially a large collection of loose rubble. As much as 50 percent of its volume could be empty space. Like the asteroid Bennu, Ryugu is shaped a bit like a spinning top: a round shape with a sharp equatorial ridge. Its name derives from a Japanese folktale in which a fisherman travels to an underwater palace called Ryūgū-jō ("Dragon Palace") on the back of a turtle.