A computational paradigm that can accurately simulate interactions between powerful laser beams and quantum fluctuations in a vacuum has been unveiled by researchers in the UK and Portugal. Led by Lily Zhang at the University of Oxford, the team hopes that their solver could lead to important new insights into the quantum nature of the vacuum.
Quantum electrodynamics (QED) provides a detailed picture of how light and matter interact, and has withstood decades of experimental scrutiny. So far, however, evidence for one of the theory’s key predictions about the nature of the vacuum has remained elusive.
Far from being empty, the vacuum contains a sea of virtual particles that are associated with quantum fluctuations. These particle–antiparticle pairs spontaneously pop into existence before annihilating almost instantly.
QED predicts that virtual particles create nonlinearities within the vacuum that can interact with powerful laser pulses. Underlying this effect is photon–photon scattering, something that particle physicists have tried to observe for several decades in accelerator experiments.
“So far, there has been no successful direct tests of photon–photon scattering,” Zhang explains. “However, the global emergence of multi-petawatt lasers has rekindled interest in testing the vacuum using just light itself.” For these experiments to succeed, robust analytical tools which can model the quantum vacuum’s responses to such immensely powerful lasers will be crucial.
So far, researchers have used computational tools that can only model simplified laser setups using 2D models. Zhang’s team addressed these limitations using a numerical technique called the Yee scheme. This is used to solve Maxwell’s equations of electromagnetism and is already widely used in plasma simulation. The method works by separately calculating electric and magnetic fields at staggered times and positions, ensuring greater stability and accuracy.
“The key challenge here is the nonlinear terms, which depend on the electromagnetic fields themselves,” Zhang explains. “We addressed this by combining the Yee scheme with an iterative loop that updates the nonlinear response at each time step until the solution converges.” Once integrated with a state-of-the-art plasma simulation code, the team was left with a fully 3D solver, capable of simulating arbitrary laser interactions within a vacuum.
To test their platform’s performance, they benchmarked it against existing theoretical predictions of vacuum birefringence, which is an effect triggered when a vacuum is distorted by and intense laser pulse that “pumps” the vacuum. In the context of photon–photon scattering, QED predicts that that these distortions will cause the light in a weaker probe beam to split into two separate rays, each with a different polarization and refractive index.
In addition, the team extended their solver to modelling four-wave mixing, which is a more complex effect whereby three input light beams interact in a vacuum to generate a fourth beam.
“The real-time simulation capability allowed us to track the evolving properties of this output signal, including its size, intensity, and duration, and link these to physical conditions at earlier stages of the interaction,” Zhang explains. “For instance, asymmetries in the signal beam were traced back to asymmetries in the interaction region, which is clearly observable from the simulation data.”
Just as the team hoped, their simulations of birefringence and four-wave mixing both closely matched their theoretical predictions – clearly showcasing their platform’s advanced capabilities.
For now, Zhang and colleagues hope that their solver could vastly reduce the computing resources required to develop robust 3D simulations of laser–vacuum interactions, making them far more efficient and accessible in turn. With its high flexibility in simulating arbitrary laser configurations, the team is now confident that their platform could soon be used to studying a diverse array of quantum vacuum effects – including birefringence, four-wave mixing, and many others.
“Looking ahead, we’re using the solver to explore new regimes, including novel beam profiles and exotic field interactions,” Zhang says. “Its structure also allows for easy extension to other nonlinear theories, such as Born–Infeld electrodynamics and axion-like particle fields. Ultimately, our goal is to create a versatile simulation platform for probing fundamental physics in the quantum vacuum.”
The research is described in Communications Physics.
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A new experiment has offered the clearest view yet of how gluons behave inside atomic nuclei. Conducted at the Thomas Jefferson National Accelerator Facility in the US, the study focused on a rare process called photoproduction. This involves high-energy photons interacting with protons confined in nuclei to produce J/psi mesons. The research sheds light on how gluons are distributed in nuclear matter and is a crucial step toward understanding the nature of protons within nuclei.
While gluons are responsible for generating most of the visible mass in the universe, their role inside nuclei remains poorly understood. These massless particles mediate the strong nuclear force, which binds quarks as well as protons and neutrons in nuclei. Gluons carry no electric charge and cannot be directly detected.
The theory that describes gluons is called quantum chromodynamics (QCD) and it is notoriously complex and difficult to test – especially in the dense, strongly interacting environment of a nucleus. That makes precision experiments essential for revealing how matter is held together at the deepest level.
The Jefferson Lab experiment focused on photoproduction, a process in which a high-energy photon strikes a particle and creates something new, in this case, a J/psi meson.
The J/psi comprises a charm quark and its antiquark and is especially useful for studying gluons. Charm quarks are much heavier than those found in ordinary matter and are not present in protons or neutrons. Therefore, they must be created entirely during the interaction, making the J/psi a particularly clean and sensitive probe of gluon behaviour inside nuclei.
Earlier studies had observed this process using free protons. This new experiment extends the approach to protons confined in nuclei to see how that environment affects gluon behaviour. The modification of quarks inside nuclei has been known since the 1980s and is called the EMC effect. However, much less is known about how gluons behave under the same conditions.
“Protons and neutrons do behave differently when they are bound inside nuclei than they do on their own,” says Jackson Pybus, now a postdoctoral fellow at Los Alamos National Laboratory and one of the experiment’s collaborators. “The nuclear physics community is still trying to work out the mechanisms behind the EMC effect. Until now, the distribution of high-momentum gluons in nuclei has remained an unexplored area.”
Pybus and colleagues used Jefferson Lab’s Experimental Hall D, which delivers an intense beam of high-energy photons. This setup had previously been used to study simpler systems, but this was the first time it was applied to heavier nuclei.
“This study looked for events where a photon strikes a proton inside the nucleus to knock it out while producing a J/psi,” Pybus explains. “By measuring the knocked-out proton, the produced J/psi, and the energy of the photon, we can reconstruct the reaction and learn how the gluons were behaving inside the nucleus.” This was done using the GlueX spectrometer.
Significantly, the experiment was accessing the “threshold” region – where the photon has just enough energy to produce a J/psi meson. Near-threshold interactions are particularly valuable because they are highly sensitive to the gluon structure of the target. Creating a heavy charm-anticharm pair requires a large energy transfer so interactions in this region reveal how gluons behave when little momentum is available. This is a regime where theoretical uncertainties in QCD are especially large.
Even more striking were the observations below this threshold. In so-called “sub-threshold” photoproduction, the incoming photon does not carry enough energy to produce the J/psi on its own, so it must draw additional energy from the internal motion of protons or from the nuclear medium itself. This is a well-understood mechanism in principle, but the rate at which it occurred in the experiment came as a surprise.
“Our study was the first to measure J/psi photoproduction from nuclei in the threshold region,” Pybus said. “The data indicate that the J/psi is produced more commonly than expected from protons that are moving with large momentum inside the nucleus, suggesting that these fast-moving protons could experience significant distortion to their internal gluons.”
The sub-threshold results were even harder to explain. “The number of subthreshold J/psi exceeded expectations,” Pybus added. “That raises questions about how the photon is able to pick up so much energy from the nucleus.”
The results suggest that gluons may be modified inside nuclei in ways that are not described by existing models – suggesting a new frontier in nuclear physics.
“This study has given us the first look at this sort of rare phenomenon that can teach us about the gluon inside the nucleus – just enough data to point to unexpected behaviours,” said Pybus. “Now that we know this measurement is possible, and that there are signs of interesting and unexplored phenomena, we’d like to perform a dedicated measurement focused on pinning down the sort of exotic effects we’re just now glimpsing.”
Follow-up experiments, including those planned at the future Electron-Ion Collider, are expected to build on these results. For now, this first glimpse at gluons in nuclei reveals that even decades after QCD’s development, the inner workings of nuclear matter remain only partially illuminated.
The research is described in Physical Review Letters.
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Independent measurements of the charge radius of the helium-3 nucleus using two different methods have yielded significantly different results – prompting a re-evaluation of underlying theory to reconcile them. The international CREMA Collaboration used muonic helium-3 ions to determine the radius, whereas a team in the Netherlands used a quantum-degenerate gas of helium-3 atoms.
The charge radius is a statistical measure of how far the electric charge of a particle extends into space. Both groups were mystified by the discrepancy in the values – which hints at physics beyond the Standard Model of particle physics. However, new theoretical calculations inspired by the results may have already resolved the discrepancy.
Both groups studied the difference between the charge radii of the helium-3 and helium-4 nuclei. CREMA used muonic helium ions, in which the remaining electrons replaced by muons. Muons are much more massive than electrons, so they spend more time near the nucleus – and are therefore more sensitive to the charge radius.
Muonic atoms have spectra at much shorter wavelengths than normal atoms. This affects values such as the Lamb shift. This is the energy difference in the 2S1/2 and 2P1/2 atomic states, which are split by interactions with virtual photons and vacuum polarization. This is most intense near the nucleus. More importantly, a muon in an S orbital becomes more sensitive to the finite size of the nucleus.
In 2010, CREMA used the charge radius of muonic hydrogen to conclude that the charge radius of the proton is significantly smaller than the current accepted value. The same procedure was then used with muonic helium-4 ions. Now, CREMA has used pulsed laser spectroscopy of muonic helium-3 ions to extract several key parameters including the Lamb shift and used them to calculate the charge radius of muonic helium-3 nuclei. They then calculated the difference with the charge radius in helium-4. The value they obtained was 15 times more accurate than any previously reported.
Meanwhile, at the Free University of Amsterdam in the Netherlands, researchers were taking a different approach, using conventional helium-3 atoms. This has significant challenges, because the effect of the nucleus on electrons is much smaller. However, it also means that an electron affects the nucleus it measures less than does a muon, which mitigates a source of theoretical uncertainty.
The Amsterdam team utilized the fact that the 2S triplet state in helium is extremely long-lived. ”If you manage to get the atom up there, it’s like a new ground state, and that means you can do laser cooling on it and it allows very efficient detection of the atoms,” explains Kjeld Eikema, one of the team’s leaders after its initial leader Wim Vassen died in 2019. In 2018, the Amsterdam group created an ultracold Bose–Einstein condensate (BEC) of helium-4 atoms in the 2S triplet state in an optical dipole trap before using laser spectroscopy to measure the ultra-narrow transition between the 2S triplet state and the higher 2S singlet state.
In the new work, the researchers turned to helium-3, which does not form a BEC but instead forms a degenerate Fermi gas. Interpreting the spectra of this required new discoveries itself. “Current theoretical models are insufficiently accurate to determine the charge radii from measurements on two-electron atoms,” Eikema explains. However, “the nice thing is that if you measure the transition directly in one isotope and then look at the difference with the other isotope, then most complications from the two electrons are common mode and drop out,” he says. This can be used to the determine the difference in the charge radii.
The researchers obtained a value that was even more precise than CREMA’s and larger by 3.6σ. The groups could find no obvious explanation for the discrepancy. “The scope of the physics involved in doing and interpreting these experiments is quite massive,” says Eikema; “a comparison is so interesting, because you can say ‘Well, is all this physics correct then? Are electrons and muons the same aside from their mass? Did we do the quantum electrodynamics correct for both normal atoms and muonic atoms? Did we do the nuclear polarization correctly?’” The results of both teams are described in Science (CREMA, Amsterdam).
While these papers were undergoing peer review, the work attracted the attention of two groups of theoretical physicists – one led by Xiao-Qiu Qi f the Wuhan Institute of Physics and Mathematics in China, and the other by Krzysztof Pachucki of the University of Warsaw in Poland. Both revised the calculation of the hyperfine structure of helium-3, finding that incorporating previously neglected higher orders into the calculation produced an unexpectedly large shift.
“Suddenly, by plugging this new value into our experiment – ping! – our determination comes within 1.2σ of theirs,” says Eikema; “which is a triumph for all the physics involved, and it shows how, by showing there’s a difference, other people think, ‘Maybe we should go and check our calculations,’ and it has improved the calculation of the hyperfine effect.” In this manner the ever improving experiments and theory calculations continue to seek the limits of the Standard Model.
Xiao-Qiu Qi and colleagues describe their calculations in Physical Review Research, while Pachucki’s team have published in Physical Review A.
Eikema adds “Personally I would have adjusted the value in our paper according to these new calculations, but Science preferred to keep the paper as it was at the time of submission and peer review, with an added final paragraph to explain the latest developments.”
Theoretical physicist Marko Horbatsch at Canada’s York University is impressed by the experimental results and bemused by the presentation. “I would say that their final answer is a great success,” he concludes. “There is validity in having the CREMA and Eikema work published side-by-side in a high-impact journal. It’s just that the fact that they agree should not be confined to a final sentence at the end of the paper.”
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By adapting mathematical techniques used in particle physics, researchers in Germany have developed an approach that could boost our understanding of the gravitational waves that are emitted when black holes collide. Led by Jan Plefka at The Humboldt University of Berlin, the team’s results could prove vital to the success of future gravitational-wave detectors.
Nearly a decade on from the first direct observations of gravitational waves, physicists are hopeful that the next generation of ground- and space-based observatories will soon allow us to study these ripples in space–time with unprecedented precision. But to ensure the success of upcoming projects like the LISA space mission, the increased sensitivity offered by these detectors will need to be accompanied with a deeper theoretical understanding of how gravitational waves are generated through the merging of two black holes.
In particular, they will need to predict more accurately the physical properties of gravitational waves produced by any given colliding pair and account for factors including their respective masses and orbital velocities. For this to happen, physicists will need to develop more precise solutions to the relativistic two-body problem. This problem is a key application of the Einstein field equations, which relate the geometry of space–time to the distribution of matter within it.
“Unlike its Newtonian counterpart, which is solved by Kepler’s Laws, the relativistic two-body problem cannot be solved exactly,” Plefka explains. “There is an ongoing international effort to apply quantum field theory (QFT) – the mathematical language of particle physics – to describe the classical two-body problem.”
In their study, Plefka’s team started from state-of-the-art techniques used in particle physics for modelling the scattering of colliding elementary particles, while accounting for their relativistic properties. When viewed from far away, each black hole can be approximated as a single point which, much like an elementary particle, carries a single mass, charge, and spin.
Taking advantage of this approximation, the researchers modified existing techniques in particle physics to create a framework called worldline quantum field theory (WQFT). “The advantage of WQFT is a clean separation between classical and quantum physics effects, allowing us to precisely target the classical physics effects relevant for the vast distances involved in astrophysical observables,” Plefka describes
Ordinarily, doing calculations with such an approach would involve solving millions of integrals that sum-up every single contribution to the black hole pair’s properties across all possible ways that the interaction between them could occur. To simplify the problem, Plefka’s team used a new algorithm that identified relationships between the integrals. This reduced the problem to just 250 “master integrals”, making the calculation vastly more manageable.
With these master integrals, the team could finally produce expressions for three key physical properties of black hole binaries within WQFT. These includes the changes in momentum during the gravity-mediated scattering of two black holes and the total energy radiated by both bodies over the course of the scattering.
Altogether, the team’s WQFT framework produced the most accurate solution to the Einstein field equations ever achieved to date. “In particular, the radiated energy we found contains a new class of mathematical functions known as ‘Calabi–Yau periods’,” Plefka explains. “While these functions are well-known in algebraic geometry and string theory, this marks the first time they have been shown to describe a genuine physical process.”
With its unprecedented insights into the structure of the relativistic two-body problem, the team’s approach could now be used to build more precise models of gravitational-wave formation, which could prove invaluable for the next generation of gravitational-wave detectors.
More broadly, however, Plefka predicts that the appearance of Calabi–Yau periods in their calculations could lead to an entirely new class of mathematical functions applicable to many areas beyond gravitational waves.
“We expect these periods to show up in other branches of physics, including collider physics, and the mathematical techniques we employed to calculate the relevant integrals will no doubt also apply there,” he says.
The research is described in Nature.
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Physicists at CERN have completed a “test run” for taking antimatter out of the laboratory and transporting it across the site of the European particle-physics facility. Although the test was carried out with ordinary protons, the team that performed it says that antiprotons could soon get the same treatment. The goal, they add, is to study antimatter in places other than the labs that create it, as this would enable more precise measurements of the differences between matter and antimatter. It could even help solve one of the biggest mysteries in physics: why does our universe appear to be made up almost entirely of matter, with only tiny amounts of antimatter?
According to the Standard Model of particle physics, each of the matter particles we see around us – from baryons like protons to leptons such as electrons – should have a corresponding antiparticle that is identical in every way apart from its charge and magnetic properties (which are reversed). This might sound straightforward, but it leads to a peculiar prediction. Under the Standard Model, the Big Bang that formed our universe nearly 14 billion years ago should have generated equal amounts of antimatter and matter. But if that were the case, there shouldn’t be any matter left, because whenever pairs of antimatter and matter particles collide, they annihilate each other in a burst of energy.
Physicists therefore suspect that there are other, more subtle differences between matter particles and their antimatter counterparts – differences that could explain why the former prevailed while the latter all but disappeared. By searching for these differences, they hope to shed more light on antimatter-matter asymmetry – and perhaps even reveal physics beyond the Standard Model.
At CERN’s Baryon-Antibaryon Symmetry Experiment (BASE) experiment, the search for matter-antimatter differences focuses on measuring the magnetic moment (or charge-to-mass ratio) of protons and antiprotons. These measurements need to be extremely precise, but this is difficult at CERN’s “Antimatter Factory” (AMF), which manufactures the necessary low-energy antiprotons in profusion. This is because essential nearby equipment – including the Antiproton Decelerator and ELENA, which reduce the energy of incoming antiprotons from GeV to MeV – produces magnetic field fluctuations that blur the signal.
To carry out more precise measurements, the team therefore needs a way of transporting the antiprotons to other, better-shielded, laboratories. This is easier said than done, because antimatter needs to be carefully isolated from its environment to prevent it from annihilating with the walls of its container or with ambient gas molecules.
The BASE team’s solution was to develop a device that can transport trapped antiprotons on a truck for substantial distances. It is this device, known as BASE-STEP (for Symmetry Tests in Experiments with Portable Antiprotons), that has now been field-tested for the first time.
During the test, the team successfully transported a cloud of about 105 trapped protons out of the AMF and across CERN’s Meyrin campus over a period of four hours. Although protons are not the same as antiprotons, BASE-STEP team leader Christian Smorra says they are just as sensitive to disturbances in their environment caused by, say, driving them around. “They are therefore ideal stand-ins for initial tests, because if we can transport protons, we should also be able to transport antiprotons,” he says.
The BASE-STEP device is mounted on an aluminium frame and measures 1.95 m x 0.85 m x 1.65 m. At 850‒900 kg, it is light enough to be transported using standard forklifts and cranes.
Like BASE, it traps particles in a Penning trap composed of gold-plated cylindrical electrode stacks made from oxygen-free copper. To further confine the protons and prevent them from colliding with the trap’s walls, this trap is surrounded by a superconducting magnet bore operated at cryogenic temperatures. The second electrode stack is also kept at ultralow pressures of 10-19 bar, which Smorra says is low enough to keep antiparticles from annihilating with residual gas molecules. To transport antiprotons instead of protons, Smorra adds, they would just need to switch the polarity of the electrodes.
The transportable trap system, which is detailed in Nature, is designed to remain operational on the road. It uses a carbon-steel vacuum chamber to shield the particles from stray magnetic fields, and its frame can handle accelerations of up to 1g (9.81 m/s2) in all directions over and above the usual (vertical) force of gravity. This means it can travel up and down slopes with a gradient of up to 10%, or approximately 6°.
Once the BASE-STEP device is re-configured to transport antiprotons, the first destination on the team’s list is a new Penning-trap system currently being constructed at the Heinrich Heine University in Düsseldorf, Germany. Here, physicists hope to search for charge-parity-time (CPT) violations in protons and antiprotons with a precision at least 100 times higher than is possible at CERN’s AMF.
“At BASE, we are currently performing measurements with a precision of 16 parts in a trillion,” explains BASE spokesperson Stefan Ulmer, an experimental physicist at Heinrich Heine and a researcher at CERN and Japan’s RIKEN laboratory. “These experiments are the most precise tests of matter/antimatter symmetry in the baryon sector to date, but to make these experiments better, we have no choice but to transport the particles out of CERN’s antimatter factory,” he tells Physics World.
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Researchers on the AEgIS collaboration at CERN have designed an experiment that could soon boost our understanding of how antimatter falls under gravity. Created by a team led by Francesco Guatieri at the Technical University of Munich, the scheme uses modified smartphone camera sensors to improve the spatial resolution of measurements of antimatter annihilations. This approach could be used in rigorous tests of the weak equivalence principle (WEP).
The WEP is a key concept of Albert Einstein’s general theory of relativity, which underpins our understanding of gravity. It suggests that within a gravitational field, all objects of should be accelerated at the same rate, regardless of their mass or whether they are matter or antimatter. Therefore, if matter and antimatter accelerate at different rates in freefall, it would reveal serious problems with the WEP.
In 2023 the ALPHA-g experiment at CERN was the first to observe how antimatter responds to gravity. They found that it falls down, with the tantalizing possibility that antimatter’s gravitational response is weaker than matter’s. Today, there are several experiments that are seeking to improve on this observation.
AEgIS’ approach is to create a horizontal beam of cold antihydrogen atoms and observe how the atoms fall under gravity. The drop will be measured by a moiré deflectometer in which a beam passes through two successive and aligned grids of horizontal slits before striking a position-sensitive detector. As the beam falls under gravity between the grids, the effect is similar to a slight horizontal misalignment of the grids. This creates a moiré pattern – or superlattice – that results in the particles making a distinctive pattern on the detector. By detecting a difference in the measured moiré pattern and that predicted by WEP, the AEgIS collaboration hopes to reveal a discrepancy with general relativity.
However, as Guatieri explains, a number of innovations are required for this to work. “For AEgIS to work, we need a detector with incredibly high spatial resolution. Previously, photographic plates were the only option, but they lacked real-time capabilities.”
AEgIS physicists are addressing this by developing a new vertexing detector. Instead of focussing on the antiparticles directly, their approach detects the secondary particles produced when the antimatter annihilates on contact with the detector. Tracing the trajectories of these particles back to their vertex gives the precise location of the annihilation.
Borrowing from industry, the team has created its vertexing detector using an array of modified mobile-phone camera sensors (see figure). Gautieri had already used this approach to measure the real-time positions of low-energy positrons (anti-electrons) with unprecedented precision.
“Mobile camera sensors have pixels smaller than 1 micron,” Guatieri describes. “We had to strip away the first layers of the sensors, which are made to deal with the advanced integrated electronics of mobile phones. This required high-level electronic design and micro-engineering.”
With these modifications in place, the team measured the positions of antiproton annihilations to within just 0.62 micron: making their detector some 35 times more precise than previous designs.
“Our solution, demonstrated for antiprotons and directly applicable to antihydrogen, combines photographic-plate-level resolution, real-time diagnostics, self-calibration and a good particle collection surface, all in one device,” Gautieri says.
With some further improvements, the AEgIS team is confident that their vertexing detector with boost the resolution of the freefall of horizontal antihydrogen beams – allowing rigorous tests of the WEC.
AEgIS team member Ruggero Caravita of Italy’s University of Trento adds, “This game-changing technology could also find broader applications in experiments where high position resolution is crucial, or to develop high-resolution trackers”. He says, “Its extraordinary resolution enables us to distinguish between different annihilation fragments, paving the way for new research on low-energy antiparticle annihilation in materials”.
The research is described in Science Advances.
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Physicists working on the ATLAS experiment on the Large Hadron Collider (LHC) are the first to report the production of top quark–antiquark pairs in collisions involving heavy nuclei. By colliding lead ions, CERN’s LHC creates a fleeting state of matter called the quark–gluon plasma. This is an extremely hot and dense soup of subatomic particles that includes deconfined quarks and gluons. This plasma is believed to have filled the early universe microseconds after the Big Bang.
“Heavy-ion collisions at the LHC recreate the quark–gluon plasma in a laboratory setting,” Anthony Badea, a postdoctoral researcher at the University of Chicago and one of the lead authors of a paper describing the research. As well as boosting our understanding of the early universe, studying the quark–gluon plasma at the LHC could also provide insights into quantum chromodynamics (QCD), which is the theory of how quarks and gluons interact.
Although the quark–gluon plasma at the LHC vanishes after about 10-23 s, scientists can study it by analysing how other particles produced in collisions move through it. The top quark is the heaviest known elementary particle and its short lifetime and distinct decay pattern offer a unique way to explore the quark–gluon plasma. This because the top quark decays before the quark–gluon plasma dissipates.
“The top quark decays into lighter particles that subsequently further decay,” explains Stefano Forte at the University of Milan, who was not involved in the research. “The time lag between these subsequent decays is modified if they happen within the quark–gluon plasma, and thus studying them has been suggested as a way to probe [quark–gluon plasma’s] structure. In order for this to be possible, the very first step is to know how many top quarks are produced in the first place, and determining this experimentally is what is done in this [ATLAS] study.”
The ATLAS team analysed data from lead–lead collisions and searched for events in which a top quark and its antimatter counterpart were produced. These particles can then decay in several different ways and the researchers focused on a less frequent but more easily identifiable mode known as the di-lepton channel. In this scenario, each top quark decays into a bottom quark and a W boson, which is a weak force-carrying particle that then transforms into a detectable lepton and an invisible neutrino.
The results not only confirmed that top quarks are created in this complex environment but also showed that their production rate matches predictions based on our current understanding of the strong nuclear force.
“This is a very important study,” says Juan Rojo, a theoretical physicist at the Free University of Amsterdam who did not take part in the research. “We have studied the production of top quarks, the heaviest known elementary particle, in the relatively simple proton–proton collisions for decades. This work represents the first time that we observe the production of these very heavy particles in a much more complex environment, with two lead nuclei colliding among them.”
As well as confirming QCD’s prediction of heavy-quark production in heavy-nuclei collisions, Rojo explains that “we have a novel probe to resolve the structure of the quark–gluon plasma”. He also says that future studies will enable us “to understand novel phenomena in the strong interactions such as how much gluons in a heavy nucleus differ from gluons within the proton”.
“This is a first step – a crucial one – but further studies will require larger samples of top quark events to explore more subtle effects,” adds Rojo.
The number of top quarks created in the ATLAS lead–lead collisions agrees with theoretical expectations. In the future, more detailed measurements could help refine our understanding of how quarks and gluons behave inside nuclei. Eventually, physicists hope to use top quarks not just to confirm existing models, but to reveal entirely new features of the quark–gluon plasma.
Rojo says we could, “learn about the time structure of the quark–gluon plasma, measurements which are ‘finer’ would be better, but for this we need to wait until more data is collected, in particular during the upcoming high-luminosity run of the LHC”.
Badea agrees that ATLAS’s observation opens the door to deeper explorations. “As we collect more nuclei collision data and improve our understanding of top-quark processes in proton collisions, the future will open up exciting prospects”.
The research is described in Physical Review Letters.
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