An experiment that scattered high-energy electrons from helium-3 and tritium nuclei has provided the first evidence for three-nucleon short-range correlations. The data were taken in 2018 at Jefferson Lab in the US and further studies of these correlations could improve our understanding of both atomic nuclei and neutron stars.

Atomic nuclei contain nucleons (protons and neutrons) that are bound together by the strong force. These nucleons are not static and they can move rapidly about the nucleus. While nucleons can move independently, they can also move as correlated pairs, trios and larger groupings. Studying this correlated motion can provide important insights into interactions between nucleons – interactions that define the structures of tiny nuclei and huge neutron stars.

The momenta of nucleons can be measured by scattering a beam of high-energy electrons from nuclei. This is because the de Broglie wavelength of these electrons is smaller that the size of the nucleons – allowing individual nucleons to be isolated. During the scattering process, momentum is exchanged between a nucleon and an electron, and how this occurs provides important insights into the correlations between nucleons.

Electron scattering has already revealed that most of the momentum in nuclei is associated with single nucleons, with some also assigned to correlated pairs. These experiments also suggested that nuclei have additional momenta that had not been accounted for.

Small but important

“We know that the three-nucleon interaction is important in the description of nuclear properties, even though it’s a very small contribution,” explains John Arrington at the Lawrence Berkeley National Laboratory in the US. “Until now, there’s never really been any indication that we’d observed them at all. This work provides a first glimpse at them.”

In 2018, Arrington and others did a series of electron-scattering experiments at Jefferson Lab with helium-3 and tritium targets. Now Arrington and an international team of physicists has scoured this scattering data for evidence of short-range, three-nucleon correlations.

Studying these correlations in nuclei with just three nucleons is advantageous because there are no correlations between four or more nucleons. These correlations would make it more difficult to isolate three-nucleon effects in the scattering data.

A further benefit of looking at tritium and helium-3 is that they are “mirror nuclei”. Tritium comprises one proton and two neutrons, while helium-3 comprises two protons and a neutron. The strong force that binds nucleons together acts equally on protons and neutrons. However, there are subtle differences in how protons and neutrons interact with each other – and these differences can be studied by comparing tritium and helium-3 electron scattering experiments.

A clean picture

“We’re trying to show that it’s possible to study three-nucleon correlations at Jefferson Lab even though we can’t get the energies necessary to do these studies in heavy nuclei,” says principle investigator Shujie Li, at Lawrence Berkeley. “These light systems give us a clean picture — that’s the reason we put in the effort of getting a radioactive target material.”

Both helium-3 and tritium are rare isotopes of their respective elements. Helium-3 is produced from the radioactive decay of tritium, which itself is produced in nuclear reactors. Tritium is a difficult isotope to work with because it is used to make nuclear weapons; has a half–life of about 12 years; and is toxic when ingested or inhaled. To succeed, the team had to create a special cryogenic chamber to contain their target of tritium gas.

Analysis of the scattering experiments revealed tantalizing hints of three-nucleon short-range correlations. Further investigation is need to determine exactly how the correlations occur. Three nucleons could become correlated simultaneously, for example, or an existing correlated pair could become correlated to a third nucleon.

Three-nucleon interactions are believed to play an important role in the properties of neutron stars, so further investigation into some of the smallest of nuclei could shed light on the inner workings of much more massive objects. “It’s much easier to study a three-nucleon correlation in the lab than in a neutron star,” says Arrington.

The research is described in Physics Letters B.

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Physicists have taken a major step toward unlocking the mysteries of antimatter by being the first to perform coherent spin spectroscopy on a single antiproton. Done by researchers on CERN’s BASE collaboration, the experiment measures the magnetic properties of antimatter with record-breaking precision. As a result, it could help us understand why there is much more matter than antimatter in the universe,

“The level of control the authors have achieved over an individual antimatter particle is unprecedented,” says Dmitry Budker, a physicist at the University of California, Berkeley, who was not involved in the study. “This opens the path to much more precise tests of fundamental symmetries of nature.”

In theory, the universe should have been born with equal amounts of matter and antimatter. Yet all the visible structures we see today – including stars, galaxies, planets and people – are made almost entirely of matter. This cosmic imbalance remains one of the biggest open questions in physics and is known as the baryon asymmetry problem.

“The general motivation for studying antiprotons is to test fundamental symmetries and our understanding of them,” says Stefan Ulmer, a senior member of BASE and head of the Ulmer Fundamental Symmetries Laboratory at RIKEN in Japan. “What we know about antimatter is that it appears as a symmetric solution to quantum mechanical equations – there’s no obvious reason why the universe should not contain equal amounts of matter and antimatter.”

This mystery can be probed by doing very precise comparisons of properties of matter and antimatter particles – in this case, the proton and the antiproton. For example, the Standard Model says that protons and antiprotons should have identical masses but equal and opposite electrical charges. Any deviations from the Standard Model description could shed light on baryon asymmetry.

Leap in precision

Now, the BASE (Baryon Antibaryon Symmetry Experiment) team has focused on coherent spectroscopy, which is a quantum technique that uses microwave pulses to manipulate the spin states of a single antiproton.

“We were doing spectroscopy on the spin of a single trapped antiproton, stored in a cryogenic Penning trap system,” Ulmer explains. “It is significant because this is of highest importance in studying the fundamental properties of the particle.”

By applying microwave radiation at just the right frequency, the team induced Rabi oscillations –periodic flipping of the antiproton’s spin – and observed the resulting resonances. The key result was a resonance peak 16 times narrower than in any previous antiproton measurements, meaning the team could pinpoint the transition frequency with much greater accuracy. Combined with a 1.5-fold improvement in signal-to-noise ratio, the measurement paves the way for at least a tenfold increase in the precision of antiproton magnetic moment measurements.“In principle, we could reduce the linewidth by another factor of ten if additional technology is developed,” says Ulmer.

Budker described the measurement as unprecedented, adding, “This is a key to future precise tests of CPT invariance and other fundamental-physics experiments”.

Deeply held principle

CPT symmetry – the idea that the laws of physics remain unchanged if charge, parity, and time are simultaneously reversed – is one of the most deeply held principles in physics. Testing it to higher and higher precision is essential for identifying any cracks in the Standard Model.

Ulmer says the team observed antiproton spin coherence times of up to 50 s. Coherence here refers to the ability of the antiproton’s quantum spin state to remain stable and unperturbed over time, which is essential for achieving high-precision measurements.

Measuring magnetic moments of nuclear particles is already notoriously difficult, but doing so for antimatter pushes the limits of experimental physics.

“These measurements require the development of experiments that are about three orders of magnitude more sensitive than any other apparatus developed before,” says Ulmer. “You need to build the world’s most sensitive detectors for single particles, the smallest Penning traps, and superimpose ultra-extreme magnetic gradients.”

The BASE team started development in 2005 and had early successes in proton measurements by 2011. Antiproton studies began in earnest in 2017, but achieving coherent spin control – as in the current work – required further innovations including ultra-homogeneous magnetic fields, cryogenic temperatures, and the exquisite control of noise.

Toward a deeper understanding

These improvements could also make other experiments possible. “This will also allow more precise measurements of other nuclear magnetic moments, and paves a path to better measurements in proton–antiproton mass comparisons,” Ulmer notes.

There may even be distant connections to quantum computing. “If coherence times for matter and antimatter are identical – something we aim to test – then the antimatter qubit might have applications in quantum information,” he says. “But honestly, operating an antimatter quantum computer, if you could do the same with matter, would be inefficient.”

More realistically, the team hopes to use their transportable trap system, BASE STEP, to bring antiprotons to a dedicated offline laboratory for even higher-resolution studies.

“The BASE collaboration keeps a steady course on increasing the precision of fundamental symmetry tests,” says Budker. “This is an important step in that direction.”

The research is described in Nature.

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In an era where video games often prioritize fast-paced action and instant gratification, Exographer offers a refreshing change. With a contemplative journey that intertwines the realms of particle physics and interactive storytelling, this beautifully pixelated game invites players to explore a decaying alien civilization through the lens of scientific discovery while challenging them with dexterity and intellect.​

Exographer was developed by particle physicist and science-fiction author Raphaël Granier de Cassagnac and his video-game studio SciFunGames. At its core, it is a puzzle-platformer – where the player’s character has to move around an environment using platforms while solving puzzles. The character in question is Ini, an alien explorer who discovers a multifunctional camera in the opening scenes of the game’s narrative. Stranded on a seemingly deserted planet, Ini is tasked with unlocking the mystery of the world’s fallen civilization.

The camera quickly becomes central to gameplay, allowing for environmental analysis, teleportation to previously visited locations and, most intriguingly, the discovery of subatomic particles through puzzles inspired by Feynman diagrams. These challenges require players to match particle trajectories using various analytical tools, mirroring the investigative processes of real-world physicists. ​

It is in these games where the particle physics really shines through. Beamlines have to be tracked and redirected to unveil greater understanding of the particles that make up this strange land and, with that, Ini’s abilities to understand the world.

As you crack one puzzle, a door opens and off you pootle to another blockage or locked door. Players will doubtless, as I did, find themselves wandering around areas pondering how to unlock it. A tip for those a little stuck: use the camera wherever a background seems a little different. In most circumstances, clues and cues will be waiting there.

Pixels and particles

The game’s environments are meticulously crafted, drawing inspiration from actual laboratories and observatories. I played the game on Nintendo Switch, but it is also available on several other platforms – including PS5, Xbox and Steam – and it looks pretty much identical on each. The pixel art style is not merely a visual choice but a thematic one, symbolizing the fundamental “pixels” of the universe of elementary particles. As players delve deeper, they encounter representations of particles including electrons, gluons and muons, each unlocking new abilities that alter gameplay and exploration. ​

Meanwhile, the character of Ini moves in a smooth and – for those gamers among us with a love of physics – realistic way. There is even a hint of lighter gravity as you hold down the button to activate a longer jump.

What sets Exographer apart is its ability to educate without compromising entertainment. The integration of scientific concepts is seamless, offering players a glimpse into the world of particle physics without overwhelming them. However, it’s worth noting that some puzzles may present a steep learning curve, potentially posing challenges for those less familiar with scientific reasoning.

Complementing the game’s visual and intellectual appeal is its atmospheric soundtrack, composed by Yann Van Der Cruyssen, known for his work on the game Stray. As with Stray – where you take the role of a stray cat with a backpack – the music enhances the sense of wonder and discovery, underscoring the game’s themes of exploration and scientific inquiry. ​

Exographer is more than just a game; it’s an experience that bridges the gap between science and (pixelated) art. It challenges players to think critically, to explore patiently, and to appreciate the intricate beauty of the universe’s building blocks. For those willing to engage with its depth, Exographer offers a rewarding journey that lingers after the console is turned off.

• 2024 SciFunGames and Abylight Studios Nintendo Switch £17.99; PS5 £15.99; Xbox £16.74; PC £16.75

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The first experimental evidence of the breaking of charge–parity (CP) symmetry in baryons has been obtained by CERN’s LHCb Collaboration. The result is consistent with the Standard Model of particle physics and could lead to constraints on theoretical attempts to extend the Standard Model to explain the excess of matter over antimatter in the universe.

Current models of cosmology say that the Big Bang produced a giant burst of matter and antimatter, the vast majority of which recombined and annihilated shortly afterwards. Today however, the universe appears to be made almost exclusively of matter with very little antimatter in evidence. This excess of matter is not explained by the Standard Model and it existence is an important mystery in physics.

In 1964, James Cronin, Valentine Fitch and colleagues at Princeton University in the US conducted an experiment on the decay of neutral K mesons. This showed that the weak interaction violated CP symmetry, indicating that matter and antimatter could behave differently. Fitch and Cronin bagged the 1980 Nobel Prize for Physics and the Soviet physicist Andrei Sakharov subsequently suggested that, if amplified at very high mass scales in the early universe, CP violation could have induced the matter–antimatter asymmetry shortly after the Big Bang.

Numerous observations of CP violation have subsequently been made in other mesonic systems. The phenomenon is now an accepted part of the Standard Model is parametrized by the Cabibbo–Kobayashi–Maskawa (CKM) matrix. This describes the various probabilities of quarks of different generations changing into each other through the weak interaction – a process called mixing.

Tiny effect

However, the CP violation produced through the CKM mechanism is much smaller effect than would have been required to create the matter left over by the Big Bang, as Xueting Yang of China’s Peking University explains.

“The number of baryons remaining divided by the number of photons produced when the baryons and antibaryons met and produced two photons is required to be about 10^-10 in Big Bang theory…whereas this kind of quantity is only 10^-18 in the Standard Model prediction.”

What is more, CP violation had never been observed in baryons. “Theoretically the prediction for baryon decay is very imprecise,” says Yang, who is a member of the LHCb collaboration. “It’s much more difficult to calculate it than the meson decays because there’s some interaction with the strong force.” Baryons (mostly protons and neutrons) make up almost all the hadronic matter in the universe, so this left open the slight possibility that the explanation might lie in some inconsistency between baryonic CP violation and the Standard Model prediction.

In the new work, Yang and colleagues at LHCb looked at the decays of beauty (or bottom) baryons and antibaryons. These heavy cousins of neutrons contain an up quark, a down quark and a beauty quark and were produced in proton–proton collisions at the Large Hadron Collider in 2011–2018. These baryons and antibaryons can decay via multiple channels. In one, a baryon decays to a proton, a positive K-meson and a pair of pions – or, conversely, an antibaryon decays to an antiproton, a negative K-meson and a pair of pions. CP violation should create an asymmetry between these processes, and the researchers looked for evidence of this asymmetry in the numbers of particles detected at different energies from all the collisions.

Standard Model prevails

The team found that the CP violation seen was consistent with the Standard Model and inconsistent with zero by 5.2σ. “The experimental result is more precise than what we can get from theory,” says Yang. Other LHCb researchers scrutinized alternative decay channels of the beauty baryon: “Their measurement results are still consistent with CP symmetry…There should be CP violation also in their decay channels, but we don’t have enough statistics to claim that the deviation is more than 5σ.”

The current data do not rule any extensions to the current Standard Model out, says Yang, simply because none of those extensions make precise predictions about the overall degree of CP violation expected in baryons. However, the LHC is now in its third run, and the researchers hope to acquire information on, for example, the intermediate particles involved in the decay: “We may be able to provide some measurements that are more comparable for theories and which can provide some constraints on the Standard Model predictions for CP violation,” says Yang.

The research is described in a paper in Nature.

“It’s an important paper – an old type of CP violation in a new system,” says Tom Browder of  the University of Hawaii. “Theorists will try to interpret this within the context of the Standard Model, and there have already been some attempts, but there are some uncertainties due to the strong interaction that preclude making a precise test.” He says the results could nevertheless potentially help to constrain extensions of the Standard Model, such as CP violating decays involving dark matter proposed by the late Ann Nelson at the University of Washington in Seattle and her colleagues.

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In 2018, the Muon g-2 Experiment at Fermilab near Chicago, set out to measure the muon’s anomalous magnetic moment to a precision of 140 parts per billion (ppb). This component of the muon’s magnetic moment is the result of several subtle quantum effects and is also known as the muon g-2 – which reflects how the gyromagnetic ratio of the muon deviates from the simple value of two.

After six years of producing, storing, and measuring more than a trillion muons, the collaboration released its long-anticipated final result in June, achieving an unprecedented precision of 127 ppb. This landmark measurement not only solidifies confidence in the experimental value of muon g-2 but also sets a new benchmark as the most precise accelerator-based measurement of a fundamental particle to date.

Studies of the muon g-2 have served as a rigorous test of the Standard Model – physicist’s leading theory describing known particles and forces – for much of the last century. Theoretically, the muon’s anomalous magnetic moment can be predicted from the Standard Model to a similar precision as the experiment. For decades, a persistent discrepancy between prediction and measurement hinted at the possibility of new physics, with experimental results favouring a higher value than the theory. Such a difference, if confirmed, could point to phenomena not accounted for in the Standard Model – potentially explaining unresolved mysteries like the existence of dark matter.

However, extraordinary claims require extraordinary scrutiny. To address the experimental side, Fermilab launched the Muon g-2 Experiment. On the theoretical side, the Muon g-2 Theory Initiative was established as a global collaboration of theorists working to refine the Standard Model prediction using state-of-the-art methods, techniques, and input data.

Problematic contribution

One of the most problematic contributions to the theoretical value is the hadronic vacuum polarization (HVP), historically determined using experimental data as input to complex calculations. While the Theory Initiative has improved these methods, progress has remained limited due to discrepancies in the available experimental data. Crucially, a recent input from the CMD-3 Experiment diverged significantly from previous results, suggesting a larger HVP contribution (see figure below). This, in turn, yields a Standard Model prediction that aligns with the new Fermilab measurement – apparently eliminating the discrepancy and, with it, any evidence of new physics.

Despite years of investigation, the origin of the CMD-3 tension remains unknown. Its result stands in contrast to a vast catalogue of earlier data from multiple experiments over decades. As a result, the traditional, data-driven approach to estimating the HVP is deemed currently unable to produce a reliable estimate .

Thanks to the efforts of the Theory Initiative, however, the HVP can now also be calculated using lattice QCD (quantum chromodynamics) simulations on supercomputers, reaching a precision comparable to that of the data-driven methods. Multiple independent lattice QCD groups have arrived at consistent values, which also agree with the Fermilab measurement, indicating no discrepancy and thus no sign of new physics. This computational feat, once considered out of reach, marks a major breakthrough. Yet, the tension remains unresolved: Why do lattice QCD and CMD-3 agree, while both conflict with decades of experimental data?

No physics beyond the Standard Model

Given the improved control in lattice QCD, the Theory Initiative has also recently updated its recommended Standard Model prediction with the HVP fully based on lattice results. The resulting value agrees with the Fermilab measurement and currently implies no evidence for physics beyond the Standard Model. However, the Initiative has emphasized that this is far from conclusive. Future predictions are intended to incorporate data-driven estimates again – once the inconsistencies in the experimental input are resolved.

The field now faces two possibilities. One is that the CMD-3 result and lattice QCD are correct. In this case, there is no new physics – but an impressive validation of the Standard Model. The other scenario is that new experimental HVP input data align with the older results, supporting a smaller HVP contribution. This would reintroduce the discrepancy with the Fermilab result, reviving the exciting possibility of new physics. In either case, the inconsistencies between CMD-3, lattice QCD, and the existing data must be explained.

So, is there new physics or not? We know there must be. The Standard Model cannot not explain dark matter, the accelerating expansion of the universe, the absence of antimatter, or the quantum nature of gravity. Precision tests like muon g-2 offer a window into this unknown. That window has not closed – for now it’s propped open.

Where we’ll be in five years is uncertain. The Muon g-2 Theory Initiative will continue to refine predictions and resolve open questions. For now, one thing is clear: the Muon g-2 Experiment at Fermilab has delivered an historic achievement and its legacy will continue to contribute to our understanding of fundamental physics for decades to come.

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This podcast features an interview with Sara Alderweireldt, who is a physicist working on the ATLAS experiment at CERN – the world-famous physics lab that straddles the Swiss-French border and is home to the Large Hadron Collider (LHC).

Based at the UK’s University of Edinburgh, Alderweireldt is in conversation with Physics World’s Margaret Harris and explains how physicists sift through the vast amount of information produced by ATLAS’ myriad detectors in search of new physics.

They also chat about the ongoing high-luminosity upgrade to the LHC and its experiments – which will be finished in 2030 – and the challenges and rewards of working a very long term project.

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