A particle accelerator just 0.2 millimetres long is the smallest device of its kind ever built. It is the first tiny accelerator that can produce fast and well-focused bunches of electrons, and could have medical applications. Eventually it could be made small enough to fit on the tip of a pen.

Particle accelerators, such as the Large Hadron Collider or those in medical facilities to treat cancer, speed up particles like electrons using electric fields and magnets. These electric fields are typically generated using radio waves, which have wavelengths measured in metres or centimetres. Peter Hommelhoff at the University of Erlangen-Nuremberg in Germany and his collaborators chose to accelerate particles using a different kind of electromagnetic wave – light – which has a much shorter wavelength measured in hundreds of nanometres. This allowed them to shrink the size of their accelerator from kilometres-wide to under a millimetre.

To make it, they used silicon shaped into thousands of 2-micrometre-tall pillars arranged into two parallel lines, each 0.2 millimetres long. To run the accelerator, the researchers shone laser light on this pillar-lined “runway” from above while injecting electrons into it from the side. The light waves from the laser interacted with the pillars to create an electromagnetic field that made the electrons clump together in narrow bunches. These particle clusters accelerated through the structure at speeds of over a hundred thousand kilometres per second.

The team experimented with adding more pillars to the runway. When they built a 0.5-millimetre-long version, they found that they could accelerate the electrons at even greater rates, increasing the energy they carried by 43 per cent. This indicates the accelerator is scalable, and can be made more powerful while remaining small enough to be integrated on chips, or even directly on the end of an optical fibre, says Hommelhoff.

Pietro Musumeci at the University of California, Los Angeles says some tiny accelerators have been built before, but this is the first such device that not only accelerates electrons, but also keeps them constrained in a relatively narrow beam that can be used in scientific experiments. “An accelerator is not just a scheme that gives energy to a particle; you also need to be able to transversely confine the particles,” he says.

At the moment, the new device only gives electrons about a millionth of the energy that they gain in larger accelerators. But Hommelhoff says there may be ways to boost each electron’s energy. He thinks making the pillars from a glass material called fused silica, which can withstand more intense laser light, could help.

Hommelhoff says scientists first proposed using light to shrink accelerators in the 1960s, but engineering challenges made it difficult to execute at the time.

“We think we can eventually shrink accelerators so that they will fit into the tip of a pen. Then, you can really think of new treatment tools for doctors or small-scale sterilisation tools for biology labs,” says Hommelhoff. “We think that we haven’t even yet conceived of all the possible [applications].”

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The standard model of particle physics is often illustrated as a simple grid showing the 17 basic particles (shown above). But an alternative way of visualising it reveals the complex rules that govern how the particles and forces interact.

The conventional grid shows three generations of quarks (which feel the strong force) and leptons (which don’t). Then there are the bosons that mediate three of the fundamental forces of nature – the strong and weak nuclear forces and electromagnetism. But it doesn’t give us the full picture.

For one, there are parts missing, like the fact that most particles can occur in two forms of a property called handedness: right-handed and left-handed. It also tells us nothing about which particles feel which forces. There are mysteries it glosses over, too, like the fact there are no right-handed neutrinos, at least that we know of. “The standard grid, as lovely as it is, looks finished,” says Chris Quigg, a theoretical physicist at the Fermi National Accelerator Laboratory in Illinois. “But the standard model is not finished.”

Quigg thought we needed a new way to visualise the theory that reflected its messiness. In 2005, he came up with his answer: the double simplex (shown above). Made of two pyramids, linked by the Higgs boson, one half represents left-handed particles and the other right-handed ones. Each pyramid vertex groups generations of …

The standard model of particle physics is beginning to show cracks. A fundamental particle called the muon has been caught behaving strangely, and new experimental results from Fermilab in Illinois have shown that it is definitely acting differently than the standard model would predict, which could mean that there are strange forces and particles out there beyond our best theoretical model.

What’s strange about the muons’ behaviour?
The discrepancies showed up in the rate at which muons spin when exposed to a magnetic field. This frequency, denoted by a number called the g-factor, is determined by interactions between muons and other particles. If the standard model is correct and accounts for all the particles and forces in existence, the g-factor should be precisely 2. But a series of measurements dating back to 2006 have shown that muons seem to rotate ever-so-slightly faster than expected.

How is the g-factor measured?
The spin rate of a muon is measured using a physical phenomenon called precession, in which the particle wobbles slightly as it spins. At Fermilab, muons are blasted around a magnetic storage ring at nearly the speed of light, and as they travel they interact with virtual particles that blink in and out of existence due to quantum effects. Then, physicists map the muons’ precession rates on what’s called a wiggle plot, which they use to calculate their g-factors.

How are these new measurements different from the ones taken since 2006?
The new Fermilab measurements are more precise than any that have been taken before, measuring the g-factor to a precision of 0.2 in a million. That is twice as precise as Fermilab’s previous set of measurements, announced in 2021. Crucially, it is precise enough to reach a statistical confidence level of 5 sigma, meaning that there is about a 1 in 3.5 million chance that a pattern of data like this would show up as a statistical fluke if the standard model were actually correct. In particle physics, a 5-sigma measurement is considered a secure discovery, rather than just a hint.

How did they achieve this precision?
For a start,  this new result involved analysing far more data than was possible in 2021. Then, only data collected in 2018 was available to analyse, whereas the new research added data from 2019 and 2020, more than quadrupling the total number of muons observed. The experimental protocol itself has also been improved in a campaign that included stabilising the muon beam and better characterising the magnetic field used to make the muons spin. The researchers are now working to incorporate data from 2021 to 2023 in their final, most precise report on the g-factor of muons, which is expected to be released in 2025.

What does this mean for particle physics?
The broader impact of these measurements is still up in the air, especially as theoretical efforts to understand muons’ g-factors are still ongoing. But if the discrepancy between measurements and observations stays in future calculations, that means that the standard model is most likely missing some sort of particle. That particle could be popping up as a virtual particle, interfering with muons through some as-yet-undetected force, and then disappearing again. But it will take even more precise measurements to tell anything about such a particle, if it exists.

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A mysterious particle has been discovered inside a superconducting crystal, more than 60 years after it was first predicted. The particle, called Pines’s demon, could explain why some materials superconduct, meaning they have zero electrical resistance, and help guide the search for new superconducting materials.

The particle is a type of plasmon, so-called because it arises from a collection of charged particles called a plasma. Such plasmas can form when electrons float freely from an atom, like in metallic materials. Plasmons are …

Researchers at the CERN particle physics laboratory’s Large Hadron Collider (LHC) in Switzerland have discovered a new type of particle called a strange pentaquark. Finding exotic particles like this could help us figure out how hadrons – subatomic particles such as protons and neutrons that are made up of quarks – are held together.

Pentaquarks, true to their name, are made up of four quarks and one antiquark, and they are not expected to form anywhere in nature, making them extraordinarily rare. “Usually conventional hadrons are made up …

DEEP in the heart of every atom lurk protons, tiny particles from which the chemical elements were forged, first in the searing heat of the big bang and then in the nuclear furnaces of stars. The number of protons in an atom determines whether it is hydrogen, carbon, oxygen or uranium. They make up more than 86 per cent of the visible matter in the universe by mass, and they are fundamental to our existence. Yet we still don’t really understand them.

It was just over a century ago that Ernest Rutherford demonstrated that protons are one of the basic building blocks of all atomic nuclei. Yet despite our best efforts in the intervening years, much about this ubiquitous particle remains shrouded in mystery.

Whether protons live forever, how big they are and what they are really made from are just some of the questions physicists continue to grapple with. Finding the answers won’t just change how we think about the particles themselves. It could alter our understanding of the universe and the fundamental laws that govern it. Here are five of the biggest unanswered questions about the proton.

1. What are protons made of?

The simple, oft-repeated story is that the proton is made of three quarks – two up quarks and one down quark – locked together by the vice-like grip of the strong nuclear force that binds atomic nuclei. However, when physicists started looking at higher and higher resolution, they discovered that around these three “valence” quarks in a proton is a churning, quantum-mechanical sea of other particles that pop in and out of existence.

Most of the time, …

Microsoft researchers have made a controversial claim that they have seen evidence of an elusive particle that could solve some of the biggest headaches in quantum computing, but some experts are questioning the discovery.

Quantum computers process information using quantum bits, or qubits, but current iterations can be prone to error.

“What the field needs is a new kind of qubit,” says Chetan Nayak at Microsoft Quantum.

He and his colleagues say they have taken a significant step towards building qubits from quasiparticles, which are not true particles but collective vibrations that can emerge when particles like electrons act together. The quasiparticles in question are called Majorana zero modes, which act as their own antiparticle and have a charge and energy that equate to zero. That makes them resilient to disturbances – so they could make unprecedentedly reliable qubits – but also makes them notoriously hard to find.

The Microsoft researchers say devices they built exhibited behaviours consistent with Majorana zero modes. The main components of each device were an extremely thin semiconducting wire made and a piece of superconducting aluminium.

This isn’t the first time Microsoft has claimed to have found Majorana zero modes. A 2018 paper by a different group of researchers at the company was retracted from the scientific journal Nature in 2021 after it didn’t hold up to scrutiny. At the time, Sergey Frolov at the University of Pittsburgh in Pennsylvania and his colleagues found that imperfections in the semiconductor wire could produce quantum effects easily mistaken for Majorana zero modes.

“To see Majorana zero modes, the wire must be like a very long, very even road with no bumps. If there is any disorder in the wire, electrons can get stuck on these imperfections and assume quantum states that mimic Majorana zero modes,” says Frolov.

In the new experiment, the team used a more complex test called the topological gap protocol. To pass the test, a device must simultaneously show signatures of Majorana zero modes at each end of the wire, and also show that the electrons are in an energy range where a special kind of superconductivity emerges.

“Rather than look for one particular simple signature of Majorana zero modes, we looked for a mosaic of signatures,” says Nayak.

The researchers tested this protocol on hundreds of computer simulations of devices, which considered any impurities in the wires, before using it on experimental data. Nayak says they calculated that for any device that passed the topological gap protocol, the probability of there not actually being a Majorana zero mode within it was less than 8 per cent.

Not all researchers in the field are convinced. Henry Legg at the University of Basel in Switzerland and his colleagues recently published a set of calculations showing that this test can be fooled by impurities in the wires. “The topological gap protocol as currently implemented is certainly not loophole free,” he says.

Frolov says that a few details imply that what seem to be Majorana zero modes would be revealed as an effect of disorder if the experiment were repeated with even more sensitive measurements. These include small differences between measurements for the left and right edges of the wire, as well as the measurements of electrons’ energies – the same energies can be indicative of emerging Majorana zero modes or of dirt trapping the electrons.

Anton Akhmerov at the Delft University of Technology in the Netherlands says that for him, the new experiment is not viable evidence that Majorana zero modes have been detected until another team of researchers reproduces it. But this may be difficult as some details of how Microsoft’s devices were manufactured have not been published on account of being trade secrets, he says.

Microsoft’s team already has its sights on making the device more complex and more like a quantum computer. “We are confident enough that we want our next milestone to be building an actual qubit. That will be the best way to make the doubters less doubtful,” Nayak says.

Matthias Troyer at Microsoft says the finding is a step towards building a quantum supercomputer that could execute billions of reliable operations per second.

Even if the finding holds, doubt remains about the usefulness of any such qubits. “Evidence for Majorana zero modes in quantum wires has been sought eagerly for over 10 years, and I’m glad to see this recent progress. However, imperfections in the materials continue to limit the performance of these devices,” says John Preskill at the California Institute of Technology.

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• quantum computing/
• quantum physics

A shock measurement of the mass of a fundamental particle called the W boson ignited the physics world in April, as it appeared to flout the standard model of particle physics.

Hundreds of papers have been written since then to try to explain the result, which seems to defy our best theory of how the universe works. Experiments have been proposed to confirm the findings, while the facts and figures have been pored over to look for possible …

A micrometre-sized device that produces light by firing a beam of electrons over a slab of crystal could be used to build tiny particle accelerators and X-ray machines. Such chip-sized devices could be manufactured  more quickly, cheaply and compactly than current particle accelerators.

Built by Yi Yang at the University of Hong Kong and his colleagues at the Massachusetts Institute of Technology, the new device consists of a special piece of silicon called a photonic crystal, a modified scanning electron microscope that fires a beam of electrons over it and a device that detects the emitted light. The set-up takes advantage of the electromagnetic fields that surround electrons as they move, which can make charged particles within a nearby material – in this case, the photonic crystal – become excited and emit light.

From mathematical models, the researchers knew that they could enhance the interactions between the crystal and the electrons by adding a pattern to the former, so they etched a grid of circular indentations, each about 100 nanometres wide, into it. Light and electrons don’t normally interact much, but engineering the energy and momentum of the light to match that of the electrons allows for unusually large interactions between the two. This matching method could eventually enhance light emissions up to a million times, says Yang.

That light has many potential uses, from spectroscopy, in which the light helps scientists learn about the internal structure of different materials, to light-based communication.

Notably, it can be used to make tiny particle accelerators, says Peter Hommelhoff at the University of Erlangen-Nuremberg in Germany. Researchers could use intense light pulses to accelerate particles instead of hitting them with microwaves, as is more common, he says.

Thomas Krauss at the University of York in the UK says the new device may not only be a step toward tiny particle accelerators but also toward smaller X-ray machines. X-rays are essentially light waves with wavelengths too short for us to see. By adapting the silicon pattern and the speed of the electrons in the device, it might be possible to change the wavelength of emitted light to X-rays.

“When you get an X-ray at your doctor, it’s a big beast of a machine. Now we can imagine doing it with a little light source, on a chip,” he says. That could make X-ray technology more accessible for small or remote medical facilities or make it portable for use by first responders in an accident.

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