In pursuit of the post-inflation axion

**High-mass haloscope** Graduate student Heather Jackson at work on a prototype metamaterial plasma resonator for the ALPHA collaboration at UC Berkeley. Novel designs such as ALPHA’s plasma haloscopes and the MADMAX collaboration’s dielectric haloscopes are pushing axion searches into parameter space inaccessible to traditional designs. Credit: A J Gubser

One hundred µeV. 25 GHz. 10 m. This is the mass, frequency and de Broglie wavelength of a typical post-inflation axion. Though well motivated as a potential explanation for both the nature of dark matter and the absence of CP violation in the strong interaction, such axions subvert the “particle gas” picture of dark matter familiar to many high-energy physicists, and pose distinct challenges for experimentalists.

Axions could occupy countless orders of magnitude in mass, but those that result from symmetry breaking after cosmic inflation are a particularly interesting target, as their mass is predicted to lie within a narrow window of just one or two orders of magnitude, up to and around 100 µeV (see “Introducing the axion”). Assuming a mass of 100 µeV and a local dark-matter density of 0.4 GeV/cm³ in the Milky Way’s dark-matter halo, a back-of-the-envelope calculation indicates that every cubic de Broglie wavelength should contain more than 10²¹ axions. Such a high occupation number means that axion dark matter would act like a classical field. Moving through the Earth at several hundreds of kilometres per second, the Milky Way’s axion halo would be nonrelativistic and phase coherent over domains metres in width and tens of microseconds in duration.

Axion haloscopes seek to detect this halo via faint electric-field oscillations. The same couplings that should allow axions to decay to pairs of photons on timescales many orders of magnitude longer than the age of the universe should allow them to “mix” with photons in a strong magnetic field. The magnetic field provides a virtual photon, and the axion oscillates into a real photon. For several decades, the primary detection strategy has been to seek to detect their resonant conversion into an RF signal in a microwave cavity permeated by a magnetic field. The experiment is like a car radio. The cavity is tuned very slowly. At the frequency corresponding to the cosmic axion’s mass, a faint signal would be amplified.

The ADMX, CAPP and HAYSTAC experiments have led the search below 25 μeV. These searches are dauntingly difficult, requiring the whole experiment to be cooled down to around 100 mK. Quantum amplifiers must be able to read out signals as weak as 10^–24 W. The current generation of experiments can tune over about 10% of the resonant frequency, remaining stable at each small frequency step for 15 minutes before moving onto the next frequency. The steps are determined by the expected lineshape of the axion signal. Axion velocities in the Milky Way’s dark-matter halo should follow a thermal distribution set by the galaxy’s gravitational potential. This produces a spread of kinetic energies that broadens the corresponding photon frequency spectrum into a boosted-Maxwellian shape with a width about 10^–6 of the frequency. For a mass around 100 μeV, the expected width is about 25 kHz.

The trouble is that the resonance frequency of a cavity is set by its diameter: the larger the cavity, the smaller the accessible frequency. Because the signal power scales with the cavity volume, it is increasingly difficult to achieve a good sensitivity at higher masses. For a 100 µeV axion with frequency 25 GHz that oscillates into a 25 GHz photon, the cavity would have to be of order only a centimetre wide.

Probing this parameter space calls for novel detector concepts that decouple the mass of the axion from the volume where axions convert into radio photons. This realisation has motivated a new generation of haloscopes built around electromagnetic structures that no longer rely on the resonant frequency of a closed cavity, but instead engineer large effective volumes matched to high axion masses.

Two complementary approaches – dielectric haloscopes and plasma haloscopes – exploit this idea in different ways. Each offers the possibility of discovering a post-inflation axion in the coming decade.

The MADMAX dielectric haloscope

**A work in progress** A MADMAX prototype setup in the shielded laboratory at DESY/University of Hamburg. Microwaves from the booster (above, top right and enhanced image far right) are reflected from the focusing mirror (left) into a horn antenna (foreground right). Credit: J Egge/MADMAX Collab. 2025 *PRL* **134** 151004

Thanks to their electromagnetic coupling, a galactic halo of axions would drive a spatially uniform electric field oscillation parallel to an external magnetic field. For 100 µeV axions, it would oscillate at about 25 GHz. In such a field, a dielectric disc will emit photons perpendicular to its surfaces due to an electromagnetic boundary effect: the discontinuity in permittivity forces the axion-induced field to readjust, producing outgoing microwaves.

The Magnetized Disc and Mirror Axion (MADMAX) collaboration seeks to boost this signal through constructive interference. The trick is multiple discs, with tuneable spacing and a mirror to reflect the photons. As the axion halo would be a classical field, each disc should continuously emit radiation in both directions. For multiple dielectric discs, coherent radiation from all disc surfaces leads to constructive interference when the distance between the discs is about half the electromagnetic wavelength, potentially boosting axion-to-photon conversion in a broad frequency range. The experiment can be tuned for a given axion mass by controlling the spacing between the discs with micron-level precision. Arbitrarily many discs can be incorporated, thereby decoupling the volume where axions can convert into photons from the axion’s mass.

The MADMAX collaboration has developed two indirect techniques to measure the “boost factor” of its dielectric haloscopes. In the first method, scanning a bead along the volume maps the three-dimensional induced electric field, from which the boost factor is then computed as the integral of the electric field over the sensitive volume. This method yielded 15% uncertainty for a prototype booster with a mirror and three 30 cm-diameter sapphire discs (see “A work in progress” figure). By studying the response of the prototype in the absence of an external magnetic field, the collaboration set the world’s best limits on dark-photon dark matter in the mass range from 78.62 to 83.95 μeV.

The MADMAX collaboration has developed two indirect techniques to measure the “boost factor” of its dielectric haloscopes

The boost factor can alternatively be obtained by modelling the booster’s response using physical properties extracted from reflectivity measurements and the behaviour of the power spectrum in the given frequency range. This method was applied to MADMAX prototypes inside the world’s largest warm-bore superconducting dipole magnet. Named after the Italian physicist who designed it in the 1970s, the Morpurgo magnet is normally used to test subdetectors of the ATLAS experiment using beams from CERN’s North Area. Since MADMAX requires no beam, a first axion search using the diameter aperture took place during the 2024 winter shutdown of the LHC. The prototype booster included a 20 cm-diameter mirror and three sapphire discs separated by aluminium rings. Frequencies around 19 GHz were explored by adjusting the mirror position. No significant excess consistent with an axion signal was observed. Despite coming from a small prototype, these results surpass astrophysical bounds and constraints from the CERN Axion Solar Telescope (CAST), demonstrating the detection power of dielectric haloscopes.

As a next step, a prototype booster with a mirror and up to twenty 30 cm-diameter discs is expected to deliver a factor 10 to 100 improvement over the 2024 tests. The positions of its discs will be adjusted inside its stainless-steel cryostat using cryogenic piezo motors. The setup is currently being commissioned and is set for installation in the Morpurgo magnet during the third long shutdown of the LHC from mid-2026 to 2029. An important goal is to prove the broad-band scanning capacity of dielectric haloscopes at cryogenic temperatures and conditions close to those of the final MADMAX design. Operating at 4 K will enhance MADMAX’s sensitivity by reducing noise from thermal radiation. A prototype has already been successfully tested inside a custom-made glass fibre cryostat in the Morpurgo magnet in cooperation with CERN’s cryogenic laboratory.

The final baseline detector foresees a 9 T superconducting dipole magnet with a warm bore of about 1.3 m. A first design has been developed and important aspects of its technological feasibility have already been tested, such as quench protection and conductor performance. As a first step, an intermediate 4 T warm-bore magnet is being purchased. It should be available around 2030. Once constructed, the magnet will be installed at DESY’s axion platform inside the former HERA H1 iron yoke, where preparations for the required cryogenic infrastructure are underway.

With MADMAX’s prototype booster scaling towards its final size, and quantum detection techniques such as travelling-wave parametric amplifiers and single-photon detectors being developed, significant improvements in sensitivity are on the horizon for dielectric haloscopes. MADMAX is on a promising path to probing axion dark matter in the 40 to 400 µeV mass range at sensitivities sufficient to discover axion dark matter at the classic Dine–Fischler–Srednicki–Zhitnitsky (DFSZ) and Kim–Shifman–Vainshtein–Zakharov (KSVZ) theory benchmarks.

The ALPHA plasma haloscope

**Plasma tuning** A resonator with a spiral design at Stockholm University. Credit: J W Jeoung

In a plasma, photons acquire an effective mass determined by the plasma frequency, which depends on the density of charge carriers. If the plasma frequency is close to the axion’s Compton frequency, axion–photon mixing is resonantly enhanced. As the plasma could in principle be of any volume, the volume in which the axion field converts into photons has been decoupled from the axion mass – but tuning the plasma frequency is not feasible, preventing a detector based on this effect from scanning a wide range of masses.

In 2019, Matthew Lawson, Alexander Millar, Matteo Pancaldi, Edoardo Vitagliano and Frank Wilczek proposed performing this experiment using a metamaterial plasma with a tunable electromagnetic dispersion which mimics that of a real plasma. In a plasma haloscope, this metamaterial is a lattice of thin metallic wires embedded in vacuum. By adjusting the wire spacing, the diameter of the wires and their arrangement, the resonant plasma frequency can be tuned over a wide range.

The ALPHA collaboration was formed in 2021 to build a full-scale plasma haloscope capable of probing axion masses from 40 to 400 μeV, corresponding to axion frequencies from 10 to 100 GHz. While challenges related to detecting an extremely feeble signal remain, the simplicity of the cavity design, particularly in the magnet geometry and the tuning mechanism, offers flexibility.

ALPHA’s design can be pictured as a large-bore superconducting solenoid magnet, and a resonator housing an array of thin copper or superconducting wires stretched along the field direction. Photons are extracted through waveguides and fed into an ultra-low-noise microwave receiver chain, cooled by a dilution refrigerator to below 100 mK, developing quantum-sensing techniques developed in close collaboration with the HAYSTAC collaboration. Photons are amplified with Josephson parametric amplifiers – the same technique used for qubits used in quantum computers, and the topic of the 2025 Nobel Prize in Physics awarded to John Clarke, Michel Devoret and John Martinis. Tests at room temperature in 2022 and 2023 demonstrated that the response of the meta-plasma can be tuned across the 10 to 20 GHz range with a modest number of configuration changes, and that the quality factors exceed 10⁴ even before cooling down to cryogenic temperatures.

Two designs are being pursued to design a tuning mechanism that allows precise adjustment of the plasma frequency with minimal mechanical intervention: a spiral design where a single rotating rod tunes a set of three spiral arms relative to another set of fixed spiral arms (see “Plasma tuning” figure); and a design with multiple spinners rotating groups of wires relative to a fixed grid of wires.

It is an exciting time for axion searches

ALPHA’s development plan proceeds in two main stages. Phase I is currently being constructed at Yale University’s Wright Laboratory, and focuses on employing established technology to demonstrate the technique and search for axions with masses from 40 to 80 μeV. Phase I’s cavity, consisting of copper plasma resonators, will be immersed in a 9 T magnet, 17.5 cm in diameter and 50 cm tall. The expected conversion power in ALPHA’s frequency range is of order 10^–24 W – comparable to the thermal noise in a 50 Ω resistor cooled to 50 mK. The read-out chain therefore employs Josephson parametric amplifiers whose noise temperatures approach the standard quantum limit. The system is designed to scan continuously while maintaining sensitivity close to the KSVZ axion-photon coupling, a benchmark for well-motivated axion models. The data-acquisition strategy builds on techniques developed in ADMX and HAYSTAC: fast Fourier transforms of the time-stream, coherent stacking across overlapping frequency bins and real-time evaluation of excess-power statistics.

Several improvements are being developed in parallel for Phase II. Quantum sensing techniques have the potential to boost the signal while reducing noise. Such techniques include HAYSTAC-style noise squeezing, using cavity entanglement and state swapping to enhance the signal, and single-photon detection. Dramatically increasing the quality factor of superconducting plasma resonators will also significantly boost the signal. Last but not least, magnets with a larger bore and higher field, such as the ones being deployed at the neutron scattering facilities at Oak Ridge National Laboratory, are expected to expand the experimental reach up to 200 μeV and push the sensitivity to below the axion–photon coupling of the DFSZ model, another classic theoretical benchmark.

Beginning in 2026, ALPHA Phase I will start taking its first physics data, initially searching for dark photons – a dark-matter candidate that interacts with plasma without requiring the presence of a magnetic field. After commissioning ALPHA’s magnet, a full axion search will commence during 2027 and 2028.

It is an exciting time for axion searches. New experiments are coming online, implementing new ideas to expand the accessible mass ranges. Groups in Italy, Japan and Korea are exploring alternative metamaterial geometries, including superconducting wire meshes and photonic crystals that replicate plasma behaviour at higher frequencies. European teams linked to the IAXO collaboration are considering hybrid systems that couple plasma-like resonators to strong dipole magnets. ALPHA will search for axions in the well-motivated region, first focusing between 40 and 80 μeV, and then between 80 and 200 μeV.

Intense efforts are underway. Discoveries may be just around the corner.

Chen-Ning Yang 1922–2025

Chen-Ning Yang, a towering figure in science whose numerous insights shaped contemporary theoretical physics, passed away in Beijing on 18 October 2025 at the age of 103. Yang was one of the greatest physicists of the 20th century, whose profound contributions, often based on principles of symmetry, are central to our contemporary understanding of nature.

Yang was born in 1922 in China’s Anhui province, moving as a child to Tsinghua University in Beijing, when his father was appointed professor of mathematics. Displaced by war, in 1938 he enrolled at the National Southwest Associated University in Kunming, where he earned his Master of Science in 1944, not fully removed from ongoing hostilities in the Second Sino–Japanese War. Yang wrote that his taste in physics was already formed from his education in Kunming.

He was awarded a fellowship for further graduate study in the US and enrolled in 1945 at the University of Chicago. He studied with Enrico Fermi and wrote his thesis on applications of group theory to nuclear physics in 1948 with Edward Teller as his advisor. In 1949, Yang joined the Institute for Advanced Study in Princeton, New Jersey, where he emerged as one of the world’s leading scientists. He wrote that he would probably have taken Fermi’s advice and returned to Chicago, but remained in Princeton to be nearer to Chih Li Tu, whom he married in 1951.

Landmark papers

His years in Princeton were extraordinarily productive, with many landmark papers in particle physics, including a famous analysis of particle decays into two photons, and statistical mechanics, including the celebrated Ising model Lee–Yang circle theorem. Most significantly of all, Yang developed non-abelian gauge theories with Robert Mills in 1954. These have the property that once the gauge groups are identified, new gauge particles and their interactions are determined. Over the subsequent 30 years, a combination of theoretical advances and experimental discoveries identified the gauge particles of our world, establishing Yang–Mills theories as a cornerstone of modern physics, alongside Maxwell’s equations and Einstein’s theory of general relativity. A spontaneously broken Yang–Mills theory, incorporating the Higgs boson, and combined with a Maxwell field, describes the electromagnetic and weak interactions, while a fully unbroken theory, quantum chromodynamics, describes the strong interactions. None of this could have been foreseen in 1954, but as Yang later wrote, “we thought it was beautiful and should be published”.

Yang’s collaboration with Tsung-Dao Lee in 1956 on the groundbreaking possibility of parity non-conservation in weak interactions earned them the 1957 Nobel Prize in Physics, making them the first Nobel laureates of Chinese origin. The confirmation of non-conservation in the experiments of Chien-Shiung Wu and other groups led to further work, with Lee and Rudolf Oehme, on the possibility of charge conjugation and time reversal non-invariance, which were subsequently observed and are now recognised as relevant to the predominance of matter over antimatter in the universe. Around the time of the Nobel Prize, Yang, now famous, reunited with his father from China at CERN. This was their first time together since he left for his doctoral studies in Chicago.

In 1966, Yang accepted the position of Albert Einstein Professor at the new State University of New York at Stony Brook, to which he relocated with his family. In the same year, the Institute for Theoretical Physics, now the C.N. Yang Institute for Theoretical Physics, was founded, and he led it until his retirement from Stony Brook in 1999. At Stony Brook, he continued work in particle physics, and broke new ground in the quantum structure of integrable models and the geometry of gauge field theories. He also profoundly shaped statistical physics, in 1967, discovering the pivotal relation for one-dimensional quantum many-body problems, the Yang–Baxter equation, which opened new directions for research in statistical physics, integrable models, quantum groups and related fields of physics and mathematics.

Building bridges

In 1971, his visit to China sparked a wave of visits there by other well-known scholars, earning him recognition as a pioneer in building bridges of academic exchange between China and the US. As a prominent public figure, he went on to support the restoration and strengthening of basic scientific research in China. He also helped inspire a renaissance of fruitful interplay between physics and mathematics, through his work on the geometry of gauge fields, relating gauge theories to the mathematical concept of fibre bundles, a realisation that grew out of conversations in the 1970s with the mathematician James Simons.

Starting in 1997, he served as honorary director of the newly established Center for Advanced Study at Tsinghua University, now the Institute for Advanced Study, and became a professor at Tsinghua University in 1999. In 2003, he returned as a widower to his childhood home, the campus of Tsinghua University, also spending time at the Chinese University in Hong Kong. In his words, his “life can be said to form a circle”, including a second marriage, with Fan Weng. He took on developing the Institute for Advanced Study as his new mission. Yang poured immense effort into advancing fundamental disciplines and cultivating talents at Tsinghua, making contributions that greatly impacted the reform and development of Chinese higher education.

Yang was elected member or foreign member of more than 10 national and regional academies of sciences, received honorary doctorates from more than 20 prestigious universities worldwide, and was honoured with numerous awards.

In his collected papers, Yang wrote that “taste and style are so important in scientific research, as they are in literature, art and music.” With his own taste having served as his guide, Chen-Ning Yang leaves an opus of exceptional creativity and breadth, providing tools that have enabled generations of physicists to make new discoveries of their own.

European Strategy Group recommends FCC-ee

The European Strategy Group (ESG) has finalised its recommendations for the 2026 update to the European Strategy for Particle Physics. As required by the CERN Council, the recommendations include a preferred option for the next large-scale collider at CERN and a prioritised alternative option to be pursued if the preferred plan turns out not to be feasible or competitive.

“The electron–positron Future Circular Collider (FCC-ee) is recommended as the preferred option for the next flagship collider at CERN,” explains strategy secretary Karl Jakobs of the University of Freiburg. “A descoped FCC-ee is the preferred alternative option. Descoping scenarios include removing the top-quark run, constructing two rather than four interaction regions and experiments, and decreasing the RF-system power.”

The ESG drafted its recommendations in a dedicated meeting at Monte Verità in Ascona, Switzerland. From 1 to 5 December, 62 delegates from across the field built on community inputs and the work of the Physics Preparatory Group to elaborate a proposal for the update to the European Strategy for Particle Physics. The recommendations address a broad range of topics and goals related to research in high-energy physics in Europe and beyond (CERN Courier November/December 2025 p23).

Seven large-scale collider projects have been the subject of a comparative assessment: CLIC, FCC-ee, FCC-hh, LCF, LEP3, LHeC and a muon collider (see “Seven colliders for CERN”). Following community submissions to the strategy process in March 2025 and at the open symposium in Venice in June 2025, a consensus emerged that an electron–positron Higgs and electroweak factory is the optimal collider to follow the High-Luminosity LHC (HL-LHC), with FCC-ee the favoured machine of a strong majority of the community (CERN Courier September/October 2025 p24). The identification of a descoped FCC-ee as the preferred alternative option was a new development in Ascona.

“Descoping would reduce the construction cost of FCC-ee by approximately 15%,” says Jakobs. “Although this would have a significant impact on the breadth of the physics programme and the precision achieved, the descoped FCC-ee would still provide a very strong physics programme and a viable path towards high energies, compared to the alternative collider options. Should additional resources become available, these descoping scenarios would be reversible.”

“The other electron-positron collider options offer substantially reduced precision physics programmes and would not be competitive with a collider like the FCC-ee,” continues Jakobs. “Moreover, in themselves, they currently lack a viable path towards energies of 10 TeV.”

The FCC-ee would maintain European leadership in high-energy particle physics

In preparation for the Ascona meeting, working groups were set up to study national inputs, the physics and technology of the large-scale flagship collider projects, the implementation of the strategy, relations with other fields of physics, sustainability and environmental impact, public engagement, education and communication, as well as social and career aspects, and knowledge and technology transfer.

According to the ESG, the FCC-ee would deliver the world’s broadest high-precision particle-physics programme, with an outstanding discovery potential through the Higgs, electroweak, flavour and top-quark sectors, as well as advances in QCD. Its technical feasibility, scope and cost are defined by the FCC Feasibility Study (CERN Courier May/June 2025 p9). The FCC-ee would maintain European leadership in high-energy particle physics, says the ESG, as well as advancing technology and providing significant societal benefits.

“The FCC-ee or the descoped version would also pave the way towards a hadron collider reusing the tunnel and much of the infrastructure, providing direct discovery reach well beyond the 10 TeV parton energy scale, in line with the community’s ambition for exploration at the highest achievable energy,” concludes Jakobs. “The overwhelming endorsement of the FCC-ee by the particle-physics communities of CERN’s Member and Associate Member States further reinforces it as the preferred path.”

The recommendations of the ESG advise but do not constrain the CERN Council, which is expected to formally deliberate on the official update to the European Strategy for Particle Physics at a dedicated Council Session in Budapest in May 2026.

Two strikes for the light sterile neutrino

In the 1990s, the GALLEX and SAGE experiments studied solar electron neutrinos using large tanks of gallium. Every few days a neutrino would transform a neutron into a proton, and every few weeks the experimenters would count the resulting germanium atoms using radiochemical techniques. To control systematic uncertainties in these difficult experiments, they also exposed the detectors to well-understood radioactive sources of electron neutrinos. But both experiments reported 20% fewer electron neutrinos from radioactive decay than expected.

Thus was born the gallium anomaly, which was carefully checked and confirmed by SAGE’s successor, the BEST experiment, as recently as 2022. The most tempting explanation is the existence of a new particle: a “sterile” neutrino flavour that doesn’t interact via any Standard Model interaction. Neutrino oscillations would transform the missing 20% of electron neutrinos into undetectable sterile neutrinos. It would nevertheless have remained invisible to LEP’s famous measurement of the number of neutrino flavours as it would not couple to the Z boson.

Out the window

This interpretation has been in tension with neutrino-oscillation fits for some time, but a new measurement at the KATRIN experiment likely excludes a sterile-neutrino explanation of the gallium anomaly, says Patrick Huber (Virginia Tech). “There was a strong hint of that from solar neutrinos, but the KATRIN result really nails this window shut. That is not to say the gallium anomaly went away; the experimental evidence here is firm and stands at more than five sigma significance, even under the most conservative assumptions about nuclear cross sections and systematics. So this still requires an explanation, but due to KATRIN we now know for sure it can’t be a vanilla sterile neutrino.”

KATRIN’s main objective is to measure the mass of the electron neutrino (CERN Courier January/February 2020 p28). Though neutrino oscillations imply that the particle is massive, its mass has thus far proved to be below the sensitivity of experiments. The KATRIN experiment, based at the Karlsruhe Institute of Technology in Germany, seeks to remedy this with precise observations of the beta decay of tritium. The heavier the electron neutrino, the lower the maximum energy of the beta-decay electrons. Though KATRIN has not yet been able to uncover evidence for the tiny mass of the electron neutrino, the much larger mass of any sterile neutrino able to explain the gallium anomaly would have made itself felt in precise observations of the endpoint of the energy spectrum of beta-decay electrons thanks to mixing between the neutrino flavours.

After the new KATRIN analysis, the best fit of the sterile neutrino from the gallium anomaly is excluded at 96.6% confidence

“A sterile neutrino would manifest itself as a model-independent kink-like distortion in the beta-decay spectrum, rather than as a deficit in the event rate,” explains lead analyst Thierry Lasserre of the Max-Planck-Institut für Kernphysik, in Heidelberg, Germany. “After the new KATRIN analysis, including 36 million electrons in the last 40 electron volts below the endpoint, the best fit of the sterile neutrino from the gallium anomaly is excluded at 96.6% confidence.”

Though heavy sterile neutrinos remain a well motivated completion of the Standard Model of particle physics with the potential to solve problems in cosmology, light sterile neutrinos struck out a second time in the same volume of Nature last month, thanks to a new measurement at the MicroBooNE experiment at Fermilab, near Chicago.

The MicroBooNE collaboration was following up on a persistent anomaly uncovered by their sister experiment, MiniBooNE, which was itself following up on the infamous LSND anomaly of 2001 (CERN Courier July/August 2020 p32). Both experiments had reported an excess of electron neutrinos in a beam of muon neutrinos generated using a particle accelerator. Here, the sterile-neutrino explanation would be more subtle: muon neutrinos would have to oscillate twice, once into sterile neutrinos and then into electron neutrinos. Using a bespoke liquid-argon time projection chamber, the MicroBooNE collaboration excludes the single-light-sterile-neutrino interpretation of the LSND and MiniBooNE anomalies at 95% confidence.

“The MicroBooNE result is just confirming what we knew from global fits for a long time,” clarifies Huber. “We cannot treat the appearance of electron neutrinos in a muon neutrino beam as a two-flavour problem if a sterile neutrino is involved – if we accept this simple fact of quantum mechanics then LSND and MiniBooNE’s excess of electron neutrinos cannot be due to mixing with a sterile neutrino since the corresponding disappearance of electron and muon neutrinos has not been observed.”

One sterile-neutrino anomaly remains unmentioned, the reactor anomaly, but it has already evaporated into statistical insignificance thanks to new experiments and careful modelling of the flux of electron antineutrinos from nuclear reactors. The promise of experiments with reactor neutrinos is now exemplified by the rapid progress of the Jiangmen Underground Neutrino Observatory (JUNO) in China, which started data taking on 26 August last year (CERN Courier November/December 2025 p9).

Back to the standard paradigm

While the recent KATRIN and MicroBooNE analyses sought evidence for a hypothetical sterile neutrino beyond the standard scenario, JUNO operates within the standard three-flavour framework. Using just 59 days of data, the experiment independently exceeded the precision of previous global fits on two out of six of the parameters governing neutrino oscillations. These are the same mixing angle and mass splitting that govern the oscillations of solar electron neutrinos into other flavours – the very effect that GALLEX and SAGE were initially designed to study in the 1990s. As JUNO gathers data, it will resolve a fine-toothed comb that modulates this oscillation spectrum – the effect of a smaller mass splitting between the three neutrinos. JUNO is designed to resolve these tiny oscillations, revealing a fundamental aspect of nature’s design: the hierarchy of the small and large mass splittings.

“The JUNO result is very exciting,” says Huber, “not so much because of its immediate impact, but because it marks the very successful start of an experiment that will deeply change neutrino physics.”

The JUNO result is exciting because it marks the successful start of an experiment that will deeply change neutrino physics

JUNO is the first of a trio of a new generation of large-scale neutrino-oscillation experiments using controlled sources. Concluding a busy two-month period for neutrinos since the previous edition of CERN Courier was published, the launch of the nuSCOPE collaboration now dangles the promise of a valuable boost to the other two. One hundred physicists attended its kick-off workshop at CERN from 13 to 15 October 2025. The collaboration seeks to implement a concept first proposed 50 years ago by Bruno Pontecorvo: nuSCOPE will eliminate systematic uncertainties related to neutrino flux by measuring the energy and flavour of neutrinos as they are created as well as when they interact with a target.

If approved, nuSCOPE will study neutrino–nucleus interactions with a level of accuracy comparable to that in electron–nucleus scattering, and control the sources of uncertainty projected to be dominant in the DUNE experiment under construction in the US and at the Hyper-Kamiokande experiment under construction in Japan. DUNE and Hyper-Kamiokande both plan to study the oscillations of accelerator-produced beams of muon neutrinos. Their most specialised design goal is to observe another fundamental aspect of physics: whether the weak interaction treats neutrinos and antineutrinos symmetrically.

With three ambitious and sharply divergent experimental concepts, DUNE, Hyper-Kamiokande and JUNO promise substantial progress in neutrino physics in the coming decade. But KATRIN and MicroBooNE now leave precious little merit for the once compelling phenomenology of the single light sterile neutrino.

Two strikes, and you’re out.

Private donors pledge support for FCC

For the first time in CERN’s history, private donors (individuals and philanthropic foundations) have agreed to support a CERN flagship research project. Recently, a group of friends of CERN, including the Breakthrough Prize Foundation, The Eric and Wendy Schmidt Fund for Strategic Innovation, and the entrepreneurs John Elkann and Xavier Niel, have pledged significant funds towards the construction of the Future Circular Collider (FCC), the potential successor of the Large Hadron Collider. These potential contributions, totalling some 860 million euros and corresponding to 1 billion US dollars, would represent a major private-sector investment in the advancement of research in fundamental physics.

“It’s the first time in history that private donors wish to partner with CERN to build an extraordinary research instrument that will allow humanity to take major steps forward in our understanding of fundamental physics and the universe. I am profoundly grateful to them for their generosity, vision and unwavering commitment to knowledge and exploration. Their support is essential to the prospective realisation of the FCC and to enabling future generations of scientists to push the frontiers of scientific discovery and technology,” said CERN Director-General Fabiola Gianotti.

Understanding the fundamental nature of our universe is the mission that unites humanity

“Understanding the fundamental nature of our universe is the mission that unites humanity,” said Pete Worden, chairman of the Breakthrough Prize Foundation. “We’re proud to support the creation of the most powerful scientific instrument in history, that can shed new light on the deepest questions humanity can ask.”

“The Future Circular Collider is an instrument that could push the boundaries of human knowledge and deepen our understanding of the fundamental laws of the universe,” said Eric Schmidt. “Beyond the science, the technologies emerging from this project could benefit society in profound ways, from medicine to computing to sustainable energy, while training a new generation of innovators and problem-solvers. Wendy and I are inspired by the ambition of this project and by what it could mean for the future of humanity.”

“CERN’s Member States are extremely grateful for the interest expressed by our donors in contributing to the funding of the Laboratory’s next flagship project. This once again demonstrates CERN’s relevance and positive impact on society, and the strong interest in CERN’s future that exists well beyond our own particle-physics community,” said the president of the CERN Council Costas Fountas.

The FCC has also been included among 11 proposed “Moonshot” projects in the draft Multiannual Financial Framework for the years 2028–2034, released by the European Commission in July.

Based on strong input from the international particle-physics community, the FCC has been recommended as the preferred option for the next flagship collider at CERN in the ongoing process to update the European Strategy for Particle Physics, which will be concluded by the CERN Council in May 2026 (see “European Strategy Group recommends FCC-ee“). A decision by the CERN Council on the construction of the FCC is expected around 2028.

First indirect evidence for primordial monsters

A monster star giving birth to a quasar — **Collapsing monster** Simulation of a primordial monster star giving birth to a quasar – an extremely luminous active galactic nucleus powered by a supermassive black hole that is actively accreting matter. Credit: D Nandal

Cosmology has long predicted that the first generation of stars should differ strongly from those forming today. Born out of pristine gas of only hydrogen and helium, they could have reached masses between a thousand and ten thousand times that of the Sun, before collapsing after only a few million years. Such “primordial monsters” have been proposed as the seeds of the first quasars (see “Collapsing monster” image), but clear observations had until now been lacking.

An analysis of the galaxy GS 3073 using the James Webb Space Telescope (JWST) now carries an unexpectedly loud message from the first generation of stars: there is far too much nitrogen to be explained by known stellar populations. This mismatch suggests a different kind of stellar ancestor, one no longer present in our universe. It is the first indirect evidence for the long-sought primordial monsters, first proposed in the early 1960s by Fred Hoyle and William Fowler in the US, and independently by Yakov Zel’dovich and Igor Novikov in the Soviet Union, in attempts to explain the newly discovered quasars.

Black-hole powered

JWST’s near-infrared spectroscopy of GS 3073 reveals the highest nitrogen-to-oxygen ratio yet measured while surveying the universe’s first billion years. Its dense central gas contains almost as many nitrogen atoms as oxygen, while carbon and neon are comparatively modest. In addition, the galaxy has an active nucleus powered by a black hole that is already millions to hundreds of millions of times the mass of the Sun, despite the galaxy’s low metallicity.

Could a primordial monster explain GS 3073? The answer lies in how these huge stars mix and burn their fuel.

GS 3073 could offer the first chemical evidence for the largest stars the universe ever formed and to the early production of massive black holes

Simulations reveal that after an initial phase of hydrogen burning in the core, these stars ignite helium, producing large amounts of carbon and oxygen. Because the stars are so luminous and extended, their interiors are strongly convective. Hot material rises, cool material sinks and chemical elements are constantly stirred. Freshly made carbon from the helium-burning core leaks outward into a surrounding shell where hydrogen is still burning. There, a sequence of reactions known as the CNO cycle converts hydrogen into helium while steadily turning carbon into nitrogen. Over time, this process loads the outer parts of the star with nitrogen, while also moderately enhancing oxygen and neon. The heaviest elements produced in the final burning stages remain trapped in the core and never reach the surface before the star collapses.

Mass loss from such primordial stars is uncertain. Without metals, they cannot generate the strong line-driven winds familiar from massive stars today. Instead, mass may be lost through pulsations, eruptions or interactions in dense environments. But simulations allow a robust conclusion: supermassive primordial stars between roughly one thousand and ten thousand solar masses naturally produce gas with nitrogen-to-oxygen, carbon-to-oxygen and neon-to-oxygen ratios that match those measured in the dense regions of GS 3073. Stars significantly lighter or heavier than this range cannot reproduce the extreme nitrogen-to-oxygen ratio, even before carbon and neon are taken into account.

Under pressure

Radiation pressure could have supported these primordial monsters for no more than a few million years. As their cores contract and heat, photons become energetic enough to convert into electron–positron pairs, reducing the radiation pressure. For classical massive stars with masses in the range of nine to 120 times the mass of the sun, this instability leads to a thermonuclear explosion that we refer to as a supernova. By contrast, supermassive stars are so dominated by gravity due to their much larger mass that they collapse directly into black holes, without undergoing a supernova explosion.

This provides a natural path from supermassive primordial stars to the over-massive black hole now seen in GS 3073’s nucleus. In this scenario, one or a few such giants enrich the surrounding gas with nitrogen-rich material through mass loss during their lives, and leave behind black-hole seeds that later grow by accretion. If this picture is correct, GS 3073 offers the first chemical evidence for the largest stars the universe ever formed and ties them directly to the early production of massive black holes. Future JWST observations, together with next-generation ground-based telescopes, will search for more nitrogen-loud galaxies and map their chemical structures in greater detail.

Longest gamma-ray burst confounds astrophysicists

On 2 July 2025, NASA’s Fermi Gamma-ray Space Telescope observed a gamma-ray burst (GRB 250702B) of a record seven hours in duration. Intriguingly, high-resolution images from the Hubble Space Telescope (HST) and the James Webb Space Telescope (JWST) revealed that the burst emerged nearly 1900 light-years from the centre of its host galaxy, near the edge of its disc. But its most unusual feature is that it was seen in X-rays a full day before any gamma rays arrived.

The high-energy transient sky is filled with a cacophony of exotic explosions produced by stellar death. Short GRBs of less than two seconds are produced by the merging of compact objects such as black holes and neutron stars. Longer GRBs are produced by the death of massive stars, with “ultralong” GRBs most often hypothesised to originate in the collapse of massive blue supergiants, as they would allow for accretion onto their central black-hole engines over a period from tens of minutes to hours.

Peculiar observations

GRB 250702B lasted for at least 25,000 seconds (7 hours), superseding the previous longest GRB 111209A by over 10,000 seconds. However, the duration alone was not enough to identify this event as a different class of GRB or as an extreme outlier. Two other observations immediately marked GRB 250702B as peculiar: the multiple gamma-ray episodes seen by Fermi and other high-energy satellites; and the soft X-rays from 0.5 to 4 keV seen by China’s Einstein Probe over a period extending a full day before gamma rays were detected.

No previous GRB is known to have been preceded by X-ray emission over such a period. Nor is it an expectation of standard GRB models, even those invoking a blue supergiant. Instead, these X-rays suggest a relativistic tidal disruption event (TDE) – the shredding of a star by a massive black hole, launching a jet that moves near the speed of light. All known relativistic TDE systems are produced by supermassive black holes weighing a million times the mass of our Sun, or more. Such black holes are found at the centre of their host galaxies, but the HST and JWST observations revealed that the transient had occurred near the edge of its host galaxy’s disc (see “Not from the nucleus” image).

This peripheral origin opens the door to a more exotic scenario involving an intermediate-mass black hole (IMBH) weighing hundreds to thousands of solar masses. IMBHs are a missing link in black-hole evolution between the stellar-mass black holes that gravitational-wave detectors frequently see merging and the supermassive black holes found at the centre of most galaxies. Alternative scenarios reduce the black-hole mass even further, and include a micro-TDE, where a star is shredded by a stellar-mass black hole, or a helium star being eaten by a stellar-mass black hole.

There is little consensus on the origin of GRB 250702B, beyond that it involved an accreting black hole

The rapid gamma-ray variability observed by Fermi and other high-energy satellites is an important clue. The time variability of relativistic jets is thought to be orders of magnitude slower than the characteristic scale set by a black hole’s Schwarzschild radius. While an intermediate-mass black hole of a few hundred solar masses is not incompatible, the observed variability is nearly 100 times faster than that seen in relativistic TDEs. By contrast, with characteristic physical scales smaller in proportion to the smaller masses of their black holes, micro-TDEs and helium-star black-hole mergers have no difficulty accommodating such short-timescale variability.

The environment of the transient also provides crucial clues into its origin. JWST spectroscopy revealed that the light from the transient and its host galaxy was emitted 8 billion years ago, when the universe was just a teenager. The galaxy is among the largest and most massive at that age in the universe, and – unusually for galaxies hosting GRBs – a massive dust lane splits its disc in half. Ongoing star formation at the transient’s location suggests a stellar-mass progenitor, as opposed to an IMBH.

Despite numerous studies, there is little consensus on the origin of GRB 250702B, beyond that it involved an accreting black hole. Its exceptional duration and early X-ray emission initially suggested a supermassive black hole, but its rapid variability and location in its host galaxy instead point to a stellar-mass black hole, with a far rarer IMBH potentially splitting the difference. Given that it is a notably rare once-every-50-years event, the wait for the next ultralong GRB may be long, but astrophysicists are optimistic that theoretical advances will disentangle the different progenitor scenarios and reveal the origin of this extraordinary transient.

George Smoot 1945–2025

George Smoot, who led the team that first measured tiny fluctuations in the cosmic microwave background (CMB) and began a revolution in cosmology, passed away in Paris on 18 September 2025.

George earned his undergraduate and doctoral degrees at the Massachusetts Institute of Technology (MIT), and then moved to Berkeley, where he held positions at Lawrence Berkeley National Laboratory (Berkeley Lab) and the Space Sciences Laboratory at the University of California, Berkeley (UC Berkeley). Though trained as a particle physicist, he switched to cosmology and developed research projects, including using differential microwave radiometers (DMRs) on U-2 spy planes to detect the dipole anisotropy of the CMB, a consequence of the motion of the Earth relative to the universe as a whole. He then devoted himself to the measurement of the CMB in detail, and this undertaking occupied him from his proposal of a satellite experiment using DMRs in 1974 to the results of the Cosmic Background Explorer (COBE) satellite in 1992. George subsequently continued research and teaching as a member of the faculty of the UC Berkeley physics department.

In 2006, the Nobel Prize committee recognised John Mather for leading a team that determined the CMB spectrum was a blackbody (arising from thermal equilibrium) to exquisite precision, and George for leading a team that detected temperature variations across the sky in the CMB at the level of one part in a hundred thousand. Those variations were signatures of the primordial density fluctuations that gave rise to galaxies, and so eventually to us. They have been called the DNA of cosmic structure and provide a remarkable window on the early universe and high-energy physics beyond our particle accelerators. The excitement caused by the COBE CMB results was dramatically expressed by Stephen Hawking, who declared them to be “the discovery of the century, if not all time.”

After the Nobel Prize, George intensified his efforts in science education and training young scientists. Indeed, on the day of the prize, George continued to teach his undergraduate introductory physics class.

George created new research institutes internationally to support young scientists. He used his prize money to found the Berkeley Center for Cosmological Physics, a joint effort between UC Berkeley and Berkeley Lab. He also started an annual Berkeley Lab summer workshop for high-school students and teachers, now in its 19th year. Later, he founded the Instituto Avanzado de Cosmología and the international Essential Cosmology for the Next Generation winter schools in Mexico, the Paris Centre for Cosmological Physics, the Institute for the Early Universe in South Korea at the world’s largest women’s university, and more. Many of the scientists trained at those institutes went on to become faculty in their home countries and internationally, and formed their own research groups.

His open online course “Gravity! From the Big Bang to Black Holes” taught nearly 100,000 students

George took special pride in the Oersted Medal awarded to him by the American Association of Physics Teachers in 2009 for “outstanding, widespread, and lasting impact” on the teaching of physics. His massive open online course “Gravity! From the Big Bang to Black Holes” with Pierre Binétruy taught nearly 100,000 students.

In his later years, George’s scientific interests spanned not only the CMB (in particular the Planck satellite), but new sensor technologies such as kinetic inductance detectors and ultrafast detectors that could open up new windows on astrophysical phenomena, gravitational waves and gravitational lensing, features in the inflationary primordial fluctuation spectrum, and dark-matter properties.

The primordial density fluctuations for which George was awarded the Nobel Prize lie at the heart of almost every aspect of cosmology. The revolution started by the COBE results led to the convergence of cosmology and particle physics, exemplified by the centrality of dark matter as a primary issue for both disciplines. George will be remembered for this, for the many students whose lives he touched and whose research he inspired, and for his advocacy of international science.

From theories to signals

Over the past decade, many theoretical and experimental landscapes have shifted substantially. Traditional paradigms such as supersymmetry and extra dimensions – once the dominant drivers of LHC search strategies – have gradually given way to a more flexible, signature-oriented approach. The modern search programme is increasingly motivated by signals rather than full theories, providing an interesting backdrop for the return of the SEARCH conference series, which last took place in 2016. The larger and more ambitious 2025 edition attracted hundreds of participants to CERN from 20 to 24 October.

The workshop highlighted how much progress ATLAS and CMS have made in searches for long-lived particles, hidden-valley scenarios (see “Soft cloud” figure) and a host of other unconventional possibilities that now occupy centre stage. Although these ideas were once considered exotic, they have become natural extensions of models connected to cosmology, dark matter and electroweak symmetry breaking. Their experimental signatures are equally rich: displaced vertices, delayed showers, emerging jets or unusual track topologies that demand a rethinking of reconstruction strategies from the ground up.

Deep learning

The most transformative change since previous editions of SEARCH is the integration of AI-based algorithms into every layer of analysis. Deep-learning-driven b-tagging has dramatically increased sensitivity to final states involving heavy flavour, while machine learning is being embedded directly into hardware trigger systems to identify complex event features in real time. This is not technological novelty for its own sake: these tools directly expand the discovery reach of the experiments.

Novel ideas in reconstruction also stood out. Talks showcased how muon detectors can be repurposed as calorimeters to detect late-developing showers, and how tracking frameworks can be adapted to capture extremely displaced tracks that were once discarded as outliers. Such techniques illustrate a broader cultural shift: expanding the search frontier now often comes from reinterpreting detector capabilities in creative ways.

The most transformative change since previous editions of SEARCH is the integration of AI-based algorithms into every layer of analysis

Anomaly detection – the use of unsupervised or semi-supervised deep-learning models to identify data that deviate from learned patterns – was another major focus. These methods, used both offline and in level-one triggers, enable model-agnostic searches that do not rely on an explicit beyond-the-Standard-Model target. Participants noted that this is especially valuable for scenarios like quirks in dark-sector models, where realistic event-generation tools still do not exist. In these cases, anomaly detection may be the only feasible path to discovery.

The rising importance of precision was another theme threading through the discussions. The detailed understanding of detector performance achieved in recent years is unprecedented for a hadron collider. CMS’s muon calibration, which is crucial for its W-mass analysis, and ATLAS’s record-breaking jet-calibration accuracy exemplify the progress. This maturity opens the possibility that new physics could first appear as subtle deviations rather than as striking anomalies. As the era of the High-Luminosity LHC approaches, the upcoming additions of precision timing layers and advanced early-tracking capabilities will further strengthen this dimension of the search programme.

The workshop also provided a platform to explore connections between collider searches and other experimental efforts across particle physics. Strong first-order phase transitions, relevant to electroweak baryogenesis, motivated renewed interest in an additional scalar that would modify the Higgs potential. Such a particle could lie anywhere from the MeV scale up to hundreds of GeV – often below the mass ranges targeted by standard resonance searches. Alternative data-taking strategies such as data scouting and data parking offer new opportunities to probe this wide mass window systematically.

Complementarity with flavour physics at LHCb, long-lived particle searches at FASER, and precision experiments seeking electric dipole moments, axion-like particles and other ultralight states, was also highlighted. In a moment without an obvious theoretical favourite, this diversification of experimental approaches is a key strategic strength.

New directions in science are launched by new tools much more often than by new concepts

A recurring sentiment was that the LHC remains a formidable discovery machine, but the community must continue pushing its tools beyond their traditional boundaries. Many discussions at SEARCH 2025 echoed a famous remark by Freeman Dyson: “New directions in science are launched by new tools much more often than by new concepts.” The upcoming upgrades to ATLAS and CMS – precision timing, enhanced tracking earlier in the trigger chain and high-granularity readout – exemplify the kinds of new tools that can reshape the search landscape.

If SEARCH 2025 underscored the need to explore new signatures, technologies and experimental ideas, it also highlighted an equally important message: we must not lose sight of the physics questions that originally motivated the LHC programme. The hierarchy problem, the apparent fine tuning of quantum corrections to the Higgs mass that prevent it rising to the Planck scale, remains unresolved, and supersymmetry continues to offer its most compelling and robust solution by stabilising it through partner particles. With the dramatic advances in reconstruction, triggering and analysis techniques, and with the enormous increase in recorded data from Run 1 through Run 3, the time is ripe to revitalise the inclusive SUSY search programme. A comprehensive, modernised SUSY effort should be a defining element of the combined ATLAS and CMS legacy physics programme, ensuring that the field fully exploits the discovery potential of the LHC dataset accumulated so far.

Trigger-level search for dijet resonances

ATLAS figure 1 — **Fig. 1.** Observed (solid lines) and expected (dashed lines) 95% confidence upper limits on the coupling between quarks and an axial-vector mediator to the dark sector, as a function of the mass of the mediator. The TLA (thick blue line) places the strictest limits across a wide mass range. (Analysis references can be found in ATL-PHYS-PUB-2025-041.) Credit: ATLAS Collab 2025 ATL-PHYS-PUB-2025-041

The LHC’s increased collision energies have opened new territory for TeV-scale searches, but its vast datasets also provide unparalleled opportunities to thoroughly explore the electroweak scale. A new ATLAS result uses an unconventional trigger-level analysis (TLA) of the full Run 2 dataset to achieve record sensitivity to low-mass particles decaying into quarks or gluons. ATLAS employs a two-stage trigger system, with a fast hardware-based first-level trigger selecting about 100 kHz of events from the 40 MHz bunch-crossing rate, followed by a software high-level trigger (HLT) that performs detailed event reconstruction and further reduces the accepted event rate by about two orders of magnitude. By recording a much reduced event format at the trigger level, TLA preserves a substantially larger fraction of events than would normally be output by the HLT.

New particles that decay with a two-jet final state feature in many Standard Model (SM) extensions. For example, the properties of “dark mediators” that couple to both quarks and dark matter could explain the present abundance of dark matter by controlling how much of it remains after falling out of equilibrium with normal matter in the early universe. At the LHC, the coupling of dark mediators to quarks would enable both production and decay into quark–antiquark pairs. This should appear as resonances in the dijet mass distribution.

Searching for dijet resonances at low mass is challenging. Dijet production from strong interactions is one of the LHC’s most abundant signatures. Beyond requiring a precise understanding of these enormous backgrounds and the detector response, the low-mass dijet rate far exceeds what ATLAS can record. Only the most energetic dijet events can be kept, limiting conventional dijet searches to masses above approximately 1 TeV.

To access the low-mass region, ATLAS used TLA to record multi-jet events throughout Run 2. By dropping the raw detector data from the readout, these TLA events were ~200 times smaller than standard events while retaining all high-level jet and calorimeter-based variables reconstructed in real-time by the HLT.

The size reduction allowed ATLAS to record TLA events at rates of up to 27 kHz – compared to an average 1.2 kHz for the full detector readout. This rate was achieved in conjunction with the additional trigger bandwidth allocated to TLA at the end of LHC fills and a more efficient use of this bandwidth for dijet events. In Run 2, this was aided by ATLAS’s L1Topo trigger processor, which applies simple topological selections – such as angular correlations between jets – already at first level. The new result uses 1 billion dijet events, or up to 75 times the data sample available to the equivalent conventional search, achieving unprecedented statistical precision.

The new result achieves record sensitivity to low-mass particles decaying into quarks or gluons

This enormous dataset demands excellent control of systematic uncertainties. ATLAS developed a dedicated multi-step calibration for trigger-level jets, achieving a jet energy scale precision of 1 to 4%, comparable to calibrations using full detector readout. The overwhelming SM background was modelled using a data-driven fitting technique, reaching a relative precision better than 1 part in 10⁴.

The search has found the dijet invariant-mass distribution to be consistent with the background expectation. The analysis provides numerical results that can be used to constrain any of the numerous models of dijet resonances, as well as explicit constraints on a specific dark mediator model used as a common benchmark for many ATLAS and CMS searches. The result sets ATLAS’s most stringent exclusion limits to date on the potential coupling of such a mediator to quarks, across a broad range of mediator masses reaching as low as 375 GeV (see figure 1).

The dijet TLA during Run 2 has established a foundation for an expanded trigger-level physics programme. In Run 3, trigger-level jets incorporate tracking information, allowing flavour tagging and improving jet energy resolution and robustness against pile-up. ATLAS also records trigger-level photons and uses them in combination with partial detector readout at full granularity. These and other advances in TLA should enable future ATLAS searches to probe a wider variety of signatures at the electroweak scale.

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