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physicsworld_feedResearchers at the Large Hadron Collider (LHC) have discovered a new particle, the Ξcc⁺, (“Xi cc plus”), a heavier cousin of the proton. The particle’s fleeting existence had made it invisible for decades, but the upgraded LHCb detector captured it in just one year of data, opening a new window into the forces that hold quarks together.
Quarks are the fundamental building blocks of protons and neutrons, which in turn combine to form atomic nuclei. Protons themselves are made from two up quarks and one down quark, held together by the strong force. This is described by a sophisticated theory known as quantum chromodynamics (QCD). The Ξcc⁺ is unusual because it replaces the two up quarks with heavier charm quarks, keeping just one down quark.
“Up and down quarks are labels we give to distinguish the different types of quark,” Tim Gershon of the University of Warwick, told Physics World in an email. “In the Ξcc⁺, both up quarks are replaced by the heavier charm quark. Since the charm and up quarks differ only by their mass – in particular having the same charge – this provides an ideal way to test QCD,” explains Gershon who is spokesperson-elect for LHCb.
This quark content change makes the Ξcc⁺ roughly four times heavier than a proton. Its extremely short lifetime, less than a trillionth of a second, is why previous experiments could not detect it, despite the particle being produced frequently in LHC collisions.
“The key development that made the observation possible was the upgrade of the LHCb detector,” Gershon says. “We could observe the Ξcc⁺ in one year of data-taking, while we had not been able to do so in a decade of data collected with the original LHCb detector.”
The Ξcc⁺ appears briefly in proton–proton collisions before decaying into three lighter particles: a Λc⁺ baryon, a K⁻ meson, and a π⁺ meson. These decay further into five final particles, including a proton, two K⁻ mesons, and two π⁺ mesons. By reconstructing the trajectories of these particles, researchers saw a sharp signal corresponding to the existence of the Ξcc⁺ particle.
This observation also settles a long-standing question. Over twenty years ago, the SELEX experiment at Fermilab in the US reported hints of the particle. However, the signal could not be confirmed. The LHCb measurement provides a clear, unambiguous detection.
“Studies of particles containing two heavy quarks are very interesting for tests of the QCD binding mechanisms, and this observation provides important new data in that direction,” Gershon says.
The discovery relied on upgrades to the LHCb detector. A silicon pixel system called the Vertex Locator tracks particle paths with incredible precision, while a Ring Imaging Cherenkov system identifies particle types based on the light they emit. These improvements allow the detector to collect much larger amounts of data than before, making rare particle discoveries possible.
The discovery of the Ξcc⁺ is just the beginning. Physicists now aim to measure its properties in detail, including its lifetime and additional decay channels. Beyond this, they hope to find even heavier cousins, where one or both charm quarks are replaced by a beauty quark – called Ξbc and Ξbb respectively.
“These may be out of reach with the current LHCb detector – although we will try our best!” Gershon says. “But we do expect to be able to observe them with a future upgrade called LHCb Upgrade II. Unfortunately, the UK funding for this upgrade has recently been put in doubt due to decisions made at the UKRI funding agency. This latest result reiterates the uniqueness of LHCb – no other experiment can make these measurements – and the importance of finding a solution to be able to fund LHCb Upgrade II.”
The post Heavier cousin of the proton discovered at the LHC appeared first on Physics World.
physicsworld_feedA new technique for directly growing diamond layers in selected areas on technologically relevant substrates could help remove heat precisely where it is needed in electronic devices, improving their performance. The scalable technique, which relies on microwave plasma chemical vapour deposition, can create diamond patterns on silicon and gallium nitride across length scales ranging from microns to full 2-inch wafers.
Unwanted heat is a major problem in electronics, and the issue only gets worse as devices become smaller. Synthetic polycrystalline diamond could come into its own here, thanks to the material’s high thermal conductivity, which allows it to efficiently dissipate heat. The problem, however, is that diamond is very hard and chemically resistant. This makes it difficult to shape using the conventional “top-down” techniques employed to carve fully-grown diamond layers to the sizes required.
In the new work, a team of researchers led by materials scientists Xiang Zhang and Pulickel Ajayan and electrical and computer engineer Yuji Zhao of Rice University in the US turned to a bottom-up approach in which they build up diamond layer-by-layer using a plasma chemical vapour deposition technique. Their process, which is detailed in Applied Physics Letters, involves using microwave energy to ionize methane gas (CH4) so that it breaks down into its constituent carbon and hydrogen atoms. The carbon atoms then settle onto the substrate and assemble via a process that begins with nucleation. “Here, individual carbon atoms act as ‘seeds’ that other carbon atoms can latch on to,” explains Zhao.
Under these conditions, the researchers are able to control the thickness of the diamond by varying the growth time.
To control the precise location of the carbon seeds, the team employed two techniques. The first was photolithography – a routine method in microelectronics that involves passing a light beam through a transmission mask to project an image of the mask’s light-absorption pattern onto a (usually silicon) wafer. The wafer itself is covered with a photosensitive polymer called a resist. Changing the intensity of the light leads to different exposure levels in the resist-covered material, making it possible to create small, finely detailed structures.
The approach, explains Zhao, is akin to using light to create a precise stencil, with the resulting structure acting as a mould for the diamond seeds. “Once the substrate wafers have been prepped, we spread a liquid containing nanodiamonds over their surface. These tiny specks then act as the starters for the diamond growth.”
The particle size of the nanodiamond seeds was 5–10 nm, which ensured a high nucleation density (estimated to be around 1011–1012 cm-2) for subsequent diamond growth, Zhao adds. High-magnification scanning electron microscopy revealed that the diamond films consisted of densely packed grains that were smaller than a micron and that the patterned diamond films were around 2.5–3.5 µm thick. Raman spectroscopy confirmed that a diamond film had formed across the entire patterned region and that it was highly crystalline.
To prove how versatile this approach was, the team decided to selectively fabricate complex geometries – for example, a diamond structure in the shape of an owl, which is the mascot of Rice University – on a gallium nitride substrate.
This technique worked well for small-area patterns, but for larger wafers, a different approach was required, explains Zhao. Instead of conventional photoresist lithography, the team laminated a commercially available lapping film onto a silicon wafer that served as a removable masking layer. A standard laser cutter was then used to define the boundaries of the desired pattern by selectively cutting through the film.
Next, the engraved regions were peeled off, exposing the underlying substrate only in the predefined areas. “We then carried out nanodiamond seeding by spin-coating a nanodiamond suspension over the entire wafer,” says Zhao. “After solvent evaporation, we mechanically lifted off the remaining lapping film, removing the nanodiamond seeds from the masked regions to leave a patterned seed layer on the exposed substrate that diamond can then grow on.”
This approach allowed the researchers to scale up to a full 2-inch wafer.
“The key result is that we can grow diamond on selected, predefined areas on technologically relevant substrates,” Zhao tells Physics World. “This will allow diamond – the best bulk thermal conductor known – to be placed precisely where heat removal is needed in a device, making practical integration much more feasible. Indeed, we showed that our films when employed as heat spreaders on a silicon substrate can reduce the operating temperature by more than 23 °C compared to bare silicon.”
The team also discovered that smaller diamond islands were better at dissipating heat than a continuous diamond coating. “We found that the 50-micron diamond patterns achieved the most effective cooling because of their higher perimeter-to-area ratio,” Zhang explains. “These geometric features increase the density of the edge regions and help the heat dissipate more efficiently in three dimensions down into the silicon substrate.”
Thermal management is now a universal challenge – and is needed everywhere from AI GPUs and advanced logic (for example, FinFET technologies) to power electronics and photonics, Zhang adds. “As the global demand for AI accelerates, the associated power consumption and heat generation are becoming critical limits. Selective diamond integration offers a pathway to more efficient heat spreading across a broad range of technologies.”
Looking ahead, the researchers say they will now be working on direct device-level integration and making quantitative thermal measurements. They will also further optimize the material quality and interface engineering.
The post Diamond films cool down electronics precisely where needed appeared first on Physics World.
physicsworld_feedResearchers have experimentally observed a new kind of particle in transition‑metal dichalcogenide bilayers called doubly charged excitons, or quaternions. A single exciton is an electron bound to a hole, and combining an even number of fermions can create a boson with integer spin. In this system, one electron and three holes (or one hole and three electrons) bind together into a stable, doubly charged bosonic complex. Because bosons can occupy the same quantum state, these quaternions could in principle form a Bose-Einstein condensate, a collective phase in which all particles share a single macroscopic wavefunction. For charged bosons, such a condensate could carry electrical current with zero resistance, opening a pathway to a new kind of superconductivity.
The researchers confirmed the existence of quaternions through two key measurements. By continuously tuning the electron and hole densities, they observed the expected population behaviour of the bound state, and by applying magnetic fields, they identified the complex as a spin‑triplet. These signatures match theoretical predictions for a doubly charged exciton.
Unlike exciton or polariton condensates, a quaternion condensate is not expected to emit coherent light, and the experiments indeed show no signs of spectral narrowing or other coherence effects. Achieving condensation will require overcoming practical challenges, including heating from the optical pump and nonradiative Auger recombination at high densities, both of which raise the critical density for condensation. Better cooling and possible lateral confinement could help reach the required regime.
Although true Bose-Einstein condensation is not possible in an infinite two‑dimensional system, finite 2D systems can still undergo a transition that is effectively indistinguishable from condensation if the coherence length exceeds the system size. This makes it reasonable to search for superfluidity, and potentially superconductivity, in this platform. The strong long‑range Coulomb repulsion between quaternions also raises the possibility of entirely different quantum phases, such as a bosonic Wigner crystal or even a supersolid.
The establishment of these doubly charged exciton complexes in screened transition‑metal dichalcogenide bilayers opens a promising new direction in quantum materials research, with the real prospect of discovering a non‑BCS form of superconductivity (one that does not rely on the conventional Cooper‑pair mechanism) and other exotic states of matter.
Light-induced electron pairing in a bilayer structure
Qiaochu Wan et al 2026 Rep. Prog. Phys. 89 018003
Bose–Einstein condensation and indirect excitons: a review by Monique Combescot, Roland Combescot and François Dubin (2017)
The post Superconductivity’s new contender appeared first on Physics World.
physicsworld_feedOpen quantum systems appear in quantum computers, quantum magnets and spintronics, but their behaviour is extremely difficult to model. The environment introduces memory effects (non‑Markovian dynamics) and strong system-bath interactions (non‑perturbative regimes), where most existing methods fail or require switching between entirely different techniques depending on the parameters. This research presents a single unified framework that can handle all these regimes for interacting quantum spins coupled to bosonic environments.
The approach combines Schwinger-Keldysh field theory with the two‑particle‑irreducible (2PI) effective action and crucially uses a 1/N expansion of Schwinger bosons rather than a perturbative expansion in the system-bath coupling. This allows the method to remain accurate even in strongly non‑perturbative regimes. The framework can compute advanced quantities such as multitime spin correlations, which are essential for understanding quantum phase transitions and nonequilibrium transport in quantum materials.
The authors benchmark their method against quasi‑exact tensor‑network simulations of the spin‑boson model, showing excellent agreement in the regimes where tensor‑network methods are applicable, and then apply it to more complex spin‑chain models with multiple baths where no other method currently works. Because it supports arbitrary spin value, geometry, dimensionality, and bath spectral function, the framework offers a general and computationally tractable route to simulating many‑body open quantum systems.
Overall, this work provides a powerful field‑theoretic tool for studying driven‑dissipative quantum systems, with applications ranging from quantum computing to quantum magnonics and spintronics.
Felipe Reyes-Osorio et al 2026 Rep. Prog. Phys. 89 018002
Keldysh field theory for driven open quantum systems by L M Sieberer, M Buchhold and S Diehl (2016)
The post A single theory for complicated quantum systems appeared first on Physics World.
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physicsworld_feedIn April 1989, the Soviet Navy’s nuclear submarine Komsomolets caught fire while cruising 335 m beneath the surface of the Norwegian Sea. It was able to surface and 27 of 69 crew members survived the ordeal. The vessel then sank and now lies in 1680 m of water about 180 km off the coast of Norway’s Bear Island.
As well as being powered by a nuclear reactor, the Komsomolets is believed to contain two torpedo-mounted nuclear warheads. Not surprisingly, people are very concerned about the wreck and the possibility of radioactive materials leaking from the vessel.
Indeed, a Russian expedition in 1994 revealed that plutonium was leaking from one of the warheads. The following year, fractures in the hull and the torpedo tubes was sealed. Since then measurements taken near the Komsomolets suggest that any radioactive leakage is rapidly diluted by the surrounding water.
Now, scientists in Norway led by Justin Gwynn and Hilde Elise Heldal, have completed a comprehensive analysis of data taken by a 2019 survey of Komsomolets. The wreck’s marine environment was explored using Ægir 6000, which is a remote-controlled vehicle that is equipped with an array of cameras and other instruments and is capable of diving to 6000 m.
Writing in the Proceedings of the National Academy of Sciences, the team says analysis of seawater and sediment samples collected near the torpedo compartment reveals no evidence of plutonium being released from the warheads. However, analysis of samples from near a ventilation pipe show that radioactive material is being released intermittently from the nuclear reactor. By measuring the ratio of plutonium to uranium in the region, the team concluded that the fuel in the reactor is corroding.
Despite releases over the past three decades, Ægir 6000 found little evidence that radionuclides were accumulating in the region of the wreck – most likely because of the diluting effect of seawater.
The research is described in PNAS, where the team concludes, “Considering the global increase in military activities and geopolitical tensions, the fate of Komsomolets and the nuclear material within it can provide us with important insights as to impacts of any future accident involving nuclear powered vessels and nuclear weapons at sea”.
The post Sunken nuclear submarine is leaking radioactive material intermittently appeared first on Physics World.
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