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Emphasising the free circulation of scientists

Physics is a universal language that unites scientists worldwide. No event illustrates this more vividly than the general assembly of the International Union of Pure and Applied Physics (IUPAP). The 33rd assembly convened 100 delegates representing territories around the world in Haikou, China, from 10 to 14 October 2024. Amid today’s polarised global landscape, one clear commitment emerged: to uphold the universality of science and ensure the free movement of scientists.

IUPAP was established in 1922 in the aftermath of World War I to coordinate international efforts in physics. Its logo is recognisable from conferences and proceedings, but its mission is less widely understood. IUPAP is the only worldwide organisation dedicated to the advancement of all fields of physics. Its goals include promoting global development and cooperation in physics by sponsoring international meetings; strengthening physics education, especially in developing countries; increasing diversity and inclusion in physics; advancing the participation and recognition of women and of people from under-represented groups; enhancing the visibility of early-career talents; and promoting international agreements on symbols, units, nomenclature and standards. At the 33rd assembly, 300 physicists were elected to the executive council and specialised commissions for a period of three years.

Global scientific initiatives were highlighted, including the International Year of Quantum Science and Technology (IYQ2025) and the International Decade on Science for Sustainable Development (IDSSD) from 2024 to 2033, which was adopted by the United Nations General Assembly in August 2023. A key session addressed the importance of industry partnerships, with delegates exploring strategies to engage companies in IYQ2025 and IDSSD to further IUPAP’s mission of using physics to drive societal progress. Nobel laureate Giorgio Parisi discussed the role of physics in promoting a sustainable future, and public lectures by fellow laureates Barry Barish, Takaaki Kajita and Samuel Ting filled the 1820-seat Oriental Universal Theater with enthusiastic students.

A key focus of the meeting was visa-related issues affecting international conferences. Delegates reaffirmed the union’s commitment to scientists’ freedom of movement. IUPAP stands against any discrimination in physics and will continue to sponsor events only in locations that uphold this value – a stance that is orthogonal to the policy of countries imposing sanctions on scientists affiliated with specific institutions.

A joint session with the fall meeting of the Chinese Physical Society celebrated the 25th anniversary of the IUPAP working group “Women in Physics” and emphasised diversity, equity and inclusion in the field. Since 2002, IUPAP has established precise guidelines for the sponsorship of conferences to ensure that women are fairly represented among participants, speakers and committee members, and has actively monitored the data ever since. This has contributed to a significant change in the participation of women in IUPAP-sponsored conferences. IUPAP is now building on this still-necessary work on gender by focusing on discrimination on the grounds of disability and ethnicity.

The closing ceremony brought together the themes of continuity and change. Incoming president Silvina Ponce Dawson (University of Buenos Aires) and president-designate Sunil Gupta (Tata Institute) outlined their joint commitment to maintaining an open dialogue among all physicists in an increasingly fragmented world, and to promoting physics as an essential tool for development and sustainability. Outgoing leaders Michel Spiro (CNRS) and Bruce McKellar (University of Melbourne) were honoured for their contributions, and the ceremonial handover symbolised a smooth transition of leadership.

As the general assembly concluded, there was a palpable sense of momentum. From strategic modernisation to deeper engagement with global issues, IUPAP is well-positioned to make physics more relevant and accessible. The resounding message was one of unity and purpose: the physics community is dedicated to leveraging science for a brighter, more sustainable future.

The new hackerpreneur

The World Wide Web, AI and quantum computing – what do these technologies have in common? They all started out as “hacks”, says Jiannan Zhang, founder of the open-source community platform DoraHacks. “When the Web was invented at CERN, it demonstrated that in order to fundamentally change how people live and work, you have to think of new ways to use existing technology,” says Zhang. “Progress cannot be made if you always start from scratch. That’s what hackathons are for.”

Ten years ago, Zhang helped organise the first CERN Webfest, a hackathon that explores creative uses of technology for science and society. Webfest helped Zhang develop his coding skills and knowledge of physics by applying it to something beyond his own discipline. He also made long-lasting connections with teammates, who were from different academic backgrounds and all over the world. After participating in more hackathons, Zhang’s growing “hacker spirit” inspired him to start his own company. In 2024 Zhang returned to Webfest not as a participant, but as the CEO of DoraHacks.

Hackathons are social coding events often spanning multiple days. They are inclusive and open – no academic institution or corporate backing is required – making them accessible to a diverse range of talented individuals. Participants work in teams, pooling their skills to tackle technical problems through software, hardware or a business plan for a new product. Physicists, computer scientists, engineers and entrepreneurs all bring their strengths to the table. Young scientists can pursue work that may not fit within typical research structures, develop their skills, and build portfolios and professional networks.

“If you’re really passionate about something, you should be able to jump on a project and work on it,” says Zhang. “You shouldn’t need to be associated with a university or have a PhD to pursue it.”

For early-career researchers, hackathons offer more than just technical challenges. They provide an alternative entry point into research and industry, bridging the gap between academia and real-world applications. University-run hackathons often attract corporate sponsors, giving them the budget to rent out stadiums with hundreds, sometimes thousands, of attendees.

“These large-scale hackathons really capture the attention of headhunters and mentors from industry,” explains Zhang. “They see the events as a recruitment pool. It can be a really effective way to advance careers and speak to representatives of big companies, as well as enhancing your coding skills.”

In the 2010s, weekend hackathons served as Zhang’s stepping stone into entrepreneurship. “I used to sit in the computer-science common room and work on my hacks. That’s how I met most of my friends,” recalled Zhang. “But later I realised that to build something great, I had to effectively organise people and capital. So I started to skip my computer-science classes and sneak into the business classrooms.” Zhang would hide in the back row of the business lectures, plotting his plan towards entrepreneurship. He networked with peers to evaluate different business models each day. “It was fun to combine our knowledge of engineering and business theory,” he added. “It made the journey a lot less stressful.”

But the transition from science to entrepreneurship was hard. “At the start you must learn and do everything yourself. The good thing is you’re exposed to lots of new skills and new people, but you also have to force yourself to do things you’re not usually good at.”

This is a dilemma many entrepreneurs face: whether to learn new skills from scratch, or to find business partners and delegate tasks. But finding trustworthy business partners is not always easy, and making the wrong decision can hinder the start up’s progress. That’s why planning the company’s vision and mission from the start is so important.

“The solution is actually pretty straight forward,” says Zhang. “You need to spend more time completing the important milestones yourself, to ensure you have a feasible product. Once you make the business plan and vision clear, you get support from everywhere.”

Decentralised community governance

Rather than hackathon participants competing for a week before abandoning their code, Zhang started DoraHacks to give teams from all over the world a chance to turn their ideas into fully developed products. “I want hackathons to be more than a recruitment tool,” he explains. “They should foster open-source development and decentralised community governance. Today, a hacker from Tanzania can collaborate virtually with a team in the US, and teams gain support to develop real products. This helps make tech fields much more diverse and accessible.”

Zhang’s company enables this by reducing logistical costs for organisers and providing funding mechanisms for participants, making hackathons accessible to aspiring researchers beyond academic institutions. As the community expands, new doors open for young scientists at the start of their careers.

“The business model is changing,” says Zhang. Hackathons are becoming fundamental to emerging technologies, particularly in areas like quantum computing, blockchain and AI, which often start out open source. “There will be a major shift in the process of product creation. Instead of building products in isolation, new technologies rely on platforms and infrastructure where hackers can contribute.”

Today, hackathons aren’t just about coding or networking – they’re about pushing the boundaries of what’s possible, creating meaningful solutions and launching new career paths. They act as incubators for ideas with lasting impact. Zhang wants to help these ideas become reality. “The future of innovation is collaborative and open source,” he says. “The old world relies on corporations building moats around closed-source technology, which is inefficient and inaccessible. The new world is centred around open platform technology, where people can build on top of old projects. This collaborative spirit is what makes the hacker movement so important.”

The value of being messy

The line between science communication and public relations has become increasingly blurred. On one side, scientific press officers highlight institutional success, secure funding and showcase breakthrough discoveries. On the other, science communicators and journalists present scientific findings in a way that educates and entertains readers – acknowledging both the triumphs and the inherent uncertainties of the scientific process.

The core difference between these approaches lies in how they handle the inevitable messiness of science. Science isn’t a smooth, linear path of consistent triumphs; it’s an uncertain, trial-and-error journey. This uncertainty, and our willingness to discuss it openly, is what distinguishes authentic science communication from a polished public relations (PR) pitch. By necessity, PR often strives to present a neat narrative, free of controversy or doubt, but this risks creating a distorted perception of what science actually is.

Finding your voice

Take, for example, the situation in particle physics. Experiments probing the fundamental laws of physics are often critiqued in the press for their hefty price tags – particularly when people are eager to see resources directed towards solving global crises like climate change or preventing future pandemics. When researchers and science communicators are finding their voice, a pressing question is how much messiness to communicate in uncertain times.

After completing my PhD as part of the ATLAS collaboration, I became a science journalist and communicator, connecting audiences across Europe and America with the joy of learning about fundamental physics. After a recent talk at the Royal Institution in London, in which I explained how ATLAS measures fundamental particles, I received an email from a colleague. The only question the talk prompted him to ask was about the safety of colliding protons, aiming to create undiscovered particles. This reaction reflects how scientific misinformation – such as the idea that experiments at CERN could endanger the planet – can be persistent and difficult to eradicate.

In response to such criticisms and concerns, I have argued many times for the value of fundamental physics research, often highlighting the vast number of technological advancements it enables, from touch screens to healthcare advances. However, we must be wary not to only rely on this PR tactic of stressing the tangible benefits of research, as it can sometimes sidestep the uncertainties and iterative nature of scientific investigation, presenting an oversimplified version of scientific progress.

**Evolving understanding** From Democritus to the Standard Model, scientific progress has not been serene. Credit: A Epshtein

This PR-driven approach risks undermining public understanding and trust in science in the long run. When science is framed solely as a series of grand successes without any setbacks, people may become confused or disillusioned when they inevitably encounter controversies or failures. Instead, this is where honest science communication shines – admitting that our understanding evolves, that we make mistakes and that uncertainties are an integral part of the process.

Our evolving understanding of particle physics is a perfect illustration of this. From Democritus’ concept of “indivisible atoms” to the development of the Standard Model, every new discovery has refined or even overhauled our previous understanding. This is the essence of science – always refining, never perfect – and it’s exactly what we should be communicating to the public.

Embracing this messiness doesn’t necessarily reduce public trust. When presenting scientific results to the public, it’s important to remember that uncertainty can take many forms, and how we communicate these forms can significantly affect credibility. Technical uncertainty – expressing complexity or incomplete information – often increases audience trust, as it communicates the real intricacies of scientific research. Conversely, consensus uncertainty – spotlighting disagreements or controversies among experts – can have a negative impact on credibility. When it comes to genuine disagreements among scientists, effectively communicating uncertainty to the public requires a thoughtful balance. Transparency is key: acknowledging the existence of different scientific perspectives helps the public understand that science is a dynamic process. Providing context about why disagreements exist, whether due to limited data or competing theoretical frameworks, also helps in making the uncertainty comprehensible.

Embrace errors

In other words, the next time you present your latest results on social media, don’t shy away from including the error bars. And if you must have a public argument with a colleague about what the results mean, context is essential!

Acknowledging the existence of different scientific perspectives helps the public understand that science is a dynamic process

No one knows where the next breakthrough will come from or how it might solve the challenges we face. In an information ecosystem increasingly filled with misinformation, scientists and science communicators must help people understand the iterative, uncertain and evolving nature of science. As science communicators, we should be cautious not to stray too far into PR territory. Authentic communication doesn’t mean glossing over uncertainties but rather embracing them as an essential part of the story. This way, the public can appreciate science not just as a collection of established facts, but as an ongoing, dynamic process – messy, yet ultimately satisfying.

Cornering compressed SUSY

CMS figure 1 — **Fig. 1.** Limits on stop pair production with decays through top quarks and LSPs. The colour scale shows 95% CL upper limits on the product of the cross-section and branching fractions as a function of stop mass and stop–LSP mass difference. The expected (magenta line) and observed (black line) lower mass limits are indicated assuming 100% branching fractions. Credit: CMS-PAS-SUS-23-003

Since the LHC began operations in 2008, the CMS experiment has been searching for signs of supersymmetry (SUSY) – the only remaining spacetime symmetry not yet observed to have consequences for physics. It has explored higher and higher masses of supersymmetric particles (sparticles) with increasing collision energies and growing datasets. No evidence has been observed so far. A new CMS analysis using data recorded between 2016 and 2018 continues this search in an often overlooked, difficult corner of SUSY manifestations: compressed sparticle mass spectra.

The masses of SUSY sparticles have very important implications for both the physics of our universe and how they could be potentially produced and observed at experiments like CMS. The heavier the sparticle, the rarer its appearance. On the other hand, when heavy sparticles decay, their mass is converted to the masses and momenta of SM particles, like leptons and jets. These particles are detected by CMS, with large masses leaving potentially spectacular (and conspicuous) signatures. Each heavy sparticle is expected to continue to decay to lighter ones, ending with the lightest SUSY particles (LSPs). LSPs, though massive, are stable and do not decay in the detector. Instead, they appear as missing momentum. In cases of compressed sparticle mass spectra, the mass difference between the initially produced sparticles and LSPs is small. This means the low rates of production of massive sparticles are not accompanied by high-momentum decay products in the detector. Most of their mass ends up escaping in the form of invisible particles, significantly complicating observation.

This new CMS result turns this difficulty on its head, using a kinematic observable R_ISR, which is directly sensitive to the mass of LSPs as opposed to the mass difference between parent sparticles and LSPs. The result is even better discrimination between SUSY and SM backgrounds when sparticle spectra are more compressed.

This approach focuses on events where putative SUSY candidates receive a significant “kick” from initial-state radiation (ISR) – additional jets recoiling opposite the system of sparticles. When the sparticle masses are highly compressed, the invisible, massive LSPs receive most of the ISR momentum-kick, with this fraction telling us about the LSP masses through the R_ISR observable.

Given the generic applicability of the approach, the analysis is able to systematically probe a large class of possible scenarios. This includes events with various numbers of leptons (0, 1, 2 or 3) and jets (including those from heavy-flavour quarks), with a focus on objects with low momentum. These multiplicities, along with R_ISR and other selected discriminating variables, are used to categorise recorded events and a comprehensive fit is performed to all these regions. Compressed SUSY signals would appear at larger values of R_ISR, while bins at lower values are used to model and constrain SM backgrounds. With more than 2000 different bins in R_ISR, over several hundred object-based categories, a significant fraction of the experimental phase space in which compressed SUSY could hide is scrutinised.

In the absence of significant observed deviations in data yields from SM expectations, a large collection of SUSY scenarios can be excluded at high confidence level (CL), including those with the production of stop quarks, EWKinos and sleptons. As can be seen in the results for stop quarks (figure 1), the analysis is able to achieve excellent sensitivity to compressed SUSY. Here, as for many of the SUSY scenarios considered, the analysis provides the world’s most stringent constraints on compressed SUSY, further narrowing the space it could be hiding.

Chinese space station gears up for astrophysics

Completed in 2022, China’s Tiangong space station represents one of the biggest projects in space exploration in recent decades. Like the International Space Station, its ability to provide large amounts of power, support heavy payloads and access powerful communication and computing facilities give it many advantages over typical satellite platforms. As such, both Chinese and international collaborations have been developing a number of science missions ranging from optical astronomy to the detection of cosmic rays with PeV energies.

For optical astronomy, the space station will be accompanied by the Xuntian telescope, which can be translated to “survey the heavens”. Xuntian is currently planned to be launched in mid-2025 to fly alongside Tiangong, thereby allowing for regular maintenance. Although its spatial resolution will be similar to that of the Hubble Space Telescope, Xuntian’s field of view will be about 300 times larger, allowing the observation of many objects at the same time. In addition to producing impressive images similar to those sent by Hubble, the instrument will be important for cosmological studies where large statistics for astronomical objects are typically required to study their evolution.

Another instrument that will observe large portions of the sky is LyRIC (Lyman UV Radiation from Interstellar medium and Circum-galactic medium). After being placed on the space station in the coming years, LyRIC will probe the poorly studied far-ultraviolet regime that contains emission lines from neutral hydrogen and other elements. While difficult to measure, this allows studies of baryonic matter in the universe, which can be used to answer important questions such as why only about half of the total baryons in the standard “ΛCDM” cosmological model can be accounted for.

At slightly higher energies, the Diffuse X-ray Explorer (DIXE) aims to use a novel type of X-ray detector to reach an energy resolution better than 1% in the 0.1 to 10 keV energy range. It achieves this using cryogenic transition-edge sensors (TESs), which exploit the rapid change in resistance that occurs during a superconducting phase transition. In this regime, the resistivity of the material is highly dependent on its temperature, allowing the detection of minuscule temperature increases resulting from X-rays being absorbed by the material. Positioned to scan the sky above the Tiangong space station, DIXE will be able, among other things, to measure the velocity of material that appears to have been emitted by the Milky Way during an active stage of its central black hole. Its high-energy resolution will allow Doppler shifts of the order of several eV to be measured, requiring the TES detectors to operate at 50 mK. Achieving such temperatures demands a cooling system of 640 W – a power level that is difficult to achieve on a satellite, but relatively easy to acquire on a space station. As such, DIXE will be one of the first detectors using this new technology when it launches in 2025, leading the way for missions such as the European ATHENA mission that plans to use it starting in 2037.

Although not as large or mature as the International Space Station, Tiangong’s capacity to host cutting-edge astrophysics missions is catching up

POLAR-2 was accepted as an international payload on the China space station through the United Nations Office for Outer Space Affairs and has since become a CERN-recognised experiment. The mission started as a Swiss, German, Polish and Chinese collaboration building on the success of POLAR, which flew on the space station’s predecessor Tiangong-2. Like its earlier incarnation, POLAR-2 measures the polarisation of high-energy X rays or gamma rays to provide insights into, for example, the magnetic fields that produced the emission. As one of the most sensitive gamma-ray detectors in the sky, POLAR-2 can also play an important role in alerting other instruments when a bright gamma-ray transient, such as a gamma-ray burst, appears. The importance of such alerts has resulted in the expansion of POLAR-2 to include an accompanying imaging spectrometer, which will provide detailed spectral and location information on any gamma-ray transient. Also now foreseen for this second payload is an additional wide-field-of-view X-ray polarimeter. The international team developing the three instruments, which are scheduled to be launched in 2027, is led by the Institute of High Energy Physics in Beijing.

For studying the universe using even higher energy emissions, the space station will host the High Energy cosmic-Radiation Detection Facility (HERD). HERD is designed to study both cosmic rays and gamma rays at energies beyond those accessible to instruments like AMS-02, CALET (CERN Courier July/August 2024 p24) and DAMPE. It aims to achieve this, in part, by simply being larger, resulting in a mass that is currently only possible to support on a space station. The HERD calorimeter will be 55 radiation lengths long and consist of several tonnes of scintillating cubic LYSO crystals. The instrument will also use high-precision silicon trackers, which in combination with the deep calorimeter, will provide a better angular resolution and a geometrical acceptance 30 times larger than the present AMS-02 (which is due to be upgraded next year). This will allow HERD to probe the cosmic-ray spectrum up to PeV energies, filling in the energy gap between current space missions and ground-based detectors. HERD started out as an international mission with a large European contribution, however delays on the European side regarding participation, in combination with a launch requirement of 2027, mean that it is currently foreseen to be a fully Chinese mission.

Although not as large or mature as the International Space Station, Tiangong’s capacity to host cutting-edge astrophysics missions is catching up. As well as providing researchers with a pristine view of the electromagnetic universe, instruments such as HERD will enable vital cross-checks of data from AMS-02 and other unique experiments in space.

Taking the lead in the monopole hunt

ATLAS figure 1 — **Fig. 1.** Simulated monopole pair-production signature for PbPb ultraperipheral collisions in the ATLAS pixel detector. Credit: ATLAS Collab./CERN

Magnetic monopoles are hypothetical particles that would carry magnetic charge, a concept first proposed by Paul Dirac in 1931. He pointed out that if monopoles exist, electric charge must be quantised, meaning that particle charges must be integer multiples of a fundamental charge. Electric charge quantisation is indeed observed in nature, with no other known explanation for this striking phenomenon. The ATLAS collaboration performed a search for these elusive particles using lead–lead (PbPb) collisions at 5.36 TeV from Run 3 of the Large Hadron Collider.

The search targeted the production of monopole–antimonopole pairs via photon–photon interactions, a process enhanced in heavy-ion collisions due to the strong electromagnetic fields (∝Z²) generated by the Z = 82 lead nuclei. Ultraperipheral collisions are ideal for this search, as they feature electromagnetic interactions without direct nuclear contact, allowing rare processes like monopole production to dominate in visible signatures. The ATLAS study employed a novel detection technique exploiting the expected highly ionising nature of these particles, leaving a characteristic signal in the innermost silicon detectors of the ATLAS experiment (figure 1).

The analysis employed a non-perturbative semiclassical model to estimate monopole production. Traditional perturbative models, which rely on Feynman diagrams, are inadequate due to the large coupling constant of magnetic monopoles. Instead, the study used a model based on the Schwinger mechanism, adapted for magnetic fields, to predict monopole production in the ultraperipheral collisions’ strong magnetic fields. This approach offers a more robust
theoretical framework for the search.

ATLAS figure 2 — **Fig. 2.** 95% confidence upper limits on the monopole pair-production cross-section in PbPb ultraperipheral collisions at 5.36 TeV. The limits are compared with model predictions and limits from the MoEDAL experiment. Credit: ATLAS Collab./CERN

The experiment’s trigger system was critical to the search. Given the high ionisation signature of monopoles, traditional calorimeter-based triggers were unsuitable, as even high-momentum monopoles lose energy rapidly through ionisation and do not reach the calorimeter. Instead, the trigger, newly introduced for the 2023 PbPb data-taking campaign, focused on detecting the forward neutrons emitted during electromagnetic interactions. The level-1 trigger system identified neutrons using the Zero-Degree Calorimeter, while the high-level trigger required more than 100 clusters of pixel-detector hits in the inner detector – an approach sensitive to monopoles due to their high ionisation signatures.

Additionally, the analysis examined the topology of pixel clusters to further refine the search, as a more aligned azimuthal distribution in the data would indicate a signature consistent with monopoles (figure 1), while the uniform distribution typically associated with beam-induced backgrounds could be identified and suppressed.

No significant monopole signal is observed beyond the expected background, with the latter being estimated using a data-driven technique. Consequently, the analysis set new upper limits on the cross-section for magnetic monopole production (figure 2), significantly improving existing limits for low-mass monopoles in the 20–150 GeV range. Assuming a non-perturbative semiclassical model, the search excludes monopoles with a single Dirac magnetic charge and masses below 120 GeV. The techniques developed in this search will open new possibilities to study other highly ionising particles that may emerge from beyond-Standard Model physics.

Unprecedented progress in energy-efficient RF

Forty-five experts from industry and academia met in the magnificent city of Toledo, Spain from 23 to 25 September 2024 for the second workshop on efficient RF sources. Part of the I.FAST initiative on sustainable concepts and technologies (CERN Courier July/August 2024 p20), the event focused on recent advances in energy-efficient technology for RF sources essential to accelerators. Progress in the last two years has been unprecedented, with new initiatives and accomplishments around the world fuelled by the ambitious goals of new, high-energy particle-physics projects.

Out of more than 30 presentations, a significant number featured pulsed, high-peak-power RF sources working at frequencies above 3 GHz in the S, C and X bands. These involve high-efficiency klystrons that are being designed, built and tested for the KEK e^–/e⁺ Injector, the new EuPRAXIA@SPARC_LAB linac, the CLIC testing facilities, muon collider R&D, the CEPC injector linac and the C³ project. Reported increases in beam-to-RF power efficiency range from 15 percentage points for the retrofit prototype for CLIC to more than 25 points (expected) for a new greenfield klystron design that can be used across most new projects.

A very dynamic area for R&D is the search of efficient sources for the continuous wave (CW) and long-pulse RF needed for circular accelerators. Typically working in the L-band, existing devices deliver less than 3 MW in peak power. Solid-state amplifiers, inductive output tubes, klystrons, magnetrons, triodes and exotic newly rediscovered vacuum tubes called “tristrons” compete in this arena. Successful prototypes have been built for the High-Luminosity LHC and CEPC with power efficiency gains of 10 to 20 points. In the case of the LHC, this will allow 15% more power without an impact on the electricity bill; in the case of a circular Higgs factory, this will allow a 30% reduction. CERN and SLAC are also investigating very-high-efficiency vacuum tubes for the Future Circular Collider with a potential reduction of close to 50% on the final electricity bill. A collaboration between academia and industry would certainly be required to bring this exciting new technology to light.

Besides the astounding advances in vacuum-tube technology, solid-state amplifiers based on cheap transistors are undergoing a major transformation thanks to the adoption of gallium-nitride technology. Commercial amplifiers are now capable of delivering kilowatts of power at low duty cycles with a power efficiency of 80%, while Uppsala University and the European Spallation Source have demonstrated the same efficiency for combined systems working in CW.

The search for energy efficiency does not stop at designing and building more efficient RF sources. All aspects of operation, power combination and using permanent magnets and efficient modulators need to be folded in, as described by many concrete examples during the workshop. The field is thriving.

ICFA talks strategy and sustainability in Prague

ICFA, the International Committee for Future Accelerators, was formed in 1976 to promote international collaboration in all phases of the construction and exploitation of very-high-energy accelerators. Its 96th meeting took place on 20 and 21 July during the recent ICHEP conference in Prague. Almost all of the 16 members from across the world attended in person, making the assembly lively and constructive.

The committee heard extensive reports from the leading HEP laboratories and various world regions on their recent activities and plans, including a presentation by Paris Sphicas, the chair of the European Committee for Future Accelerators (ECFA), on the process for the update of the European strategy for particle physics (ESPP). Launched by CERN Council in March 2024, the ESPP update is charged with recommending the next collider project at CERN after HL-LHC operation.

A global task

The ESPP update is also of high interest to non-European institutions and projects. Consequently, in addition to the expected inputs to the strategy from European HEP communities, those from non-European HEP communities are also welcome. Moreover, the recent US P5 report and the Chinese plans for CEPC, with a potential positive decision in 2025/2026, and discussions about the ILC project in Japan, will be important elements of the work to be carried out in the context of the ESPP update. They also emphasise the global nature of high-energy physics.

An integral part of the work of ICFA is carried out within its panels, which have been very active. Presentations were given from the new panel on the Data Lifecycle (chair Kati Lassila-Perini, Helsinki), the Beam Dynamics panel (new chair Yuan He, IMPCAS) and the Advanced and Novel Accelerators panel (new chair Patric Muggli, Max Planck Munich, proxied at the meeting by Brigitte Cros, Paris-Saclay). The Instrumentation and Innovation Development panel (chair Ian Shipsey, Oxford) is setting an example with its numerous schools, the ICFA instrumentation awards and centrally sponsored instrumentation studentships for early-career researchers from underserved world regions. Finally, the chair of the ILC International Development Team panel (Tatsuya Nakada, EPFL) summarised the latest status of the ILC Technological Network, and the proposed ILC collider project in Japan.

ICFA noted interesting structural developments in the global organisation of HEP

A special session was devoted to the sustainability of HEP accelerator infrastructures, considering the need to invest efforts into guidelines that enable better comparison of the environmental reports of labs and infrastructures, in particular for future facilities. It was therefore natural for ICFA to also hear reports not only from the panel on Sustainable Accelerators and Colliders led by Thomas Roser (BNL), but also from the European Lab Directors Working Group on Sustainability. This group, chaired by Caterina Bloise (INFN) and Maxim Titov (CEA), is mandated to develop a set of key indicators and a methodology for the reporting on future HEP projects, to be delivered in time for the ESPP update.

Finally, ICFA noted some very interesting structural developments in the global organisation of HEP. In the Asia-Oceania region, ACFA-HEP was recently formed as a sub-panel under the Asian Committee for Future Accelerators (ACFA), aiming for a better coordination of HEP activities in this particular region of the world. Hopefully, this will encourage other world regions to organise themselves in a similar way in order to strengthen their voice in the global HEP community – for example in Latin America. Here, a meeting was organised in August by the Latin American Association for High Energy, Cosmology and Astroparticle Physics (LAA-HECAP) to bring together scientists, institutions and funding agencies from across Latin America to coordinate actions for jointly funding research projects across the continent.

The next in-person ICFA meeting will be held during the Lepton–Photon conference in Madison, Wisconsin (USA), in August 2025.

Isolating photons at low Bjorken x

ALICE figure 1 — **Fig. 1.** World compilation of the isolated-photon cross-section at midrapidity, scaled by (√s)ⁿ to approximately reveal the gluon PDF. n = 4.5. Credit: D D’Enterria/ALICE Collab.

In high-energy collisions at the LHC, prompt photons are those that do not originate from particle decays and are instead directly produced by the hard scattering of quarks and gluons (partons). Due to their early production, they provide a clean method to probe the partons inside the colliding nucleons, and in particular the fraction of the momentum of the nucleon carried by each parton (Bjorken x). The distribution of each parton in Bjorken x is known as its parton distribution function (PDF).

Theoretical models of particle production rely on the precise knowledge of PDFs, which are derived from vast amounts of experimental data. The high centre-of-mass energies (√s) at the LHC probe very small values of the momentum fraction, Bjorken x. At “midrapidity”, when a parton scatters with a large angle with respect to the beam axis, and a prompt photon is produced in the final state, a useful approximation to Bjorken x is provided by the dimensionless variable x_T = 2p_T/√s, where p_T is the transverse momentum of the prompt photon.

Prompt photons can also be produced by next-to-leading order processes such as parton fragmentation or bremsstrahlung. A clean separation of the different prompt photon sources is difficult experimentally, but fragmentation can be suppressed by selecting “isolated photons”. For a photon to be considered isolated, the sum of the transverse energies or transverse momenta of the particles produced in a cone around the photon must be smaller than some threshold – a selection that can be done both in the experimental measurement and theoretical calculations. An isolation requirement also helps to reduce the background of decay photons, since hadrons that can decay to photons are often produced in jet fragmentation.

The ALICE collaboration now reports the measurement of the differential cross-section for isolated photons in proton–proton collisions at √s = 13 TeV at midrapidity. The photon measurement is performed by the electromagnetic calorimeter, and the isolated photons are selected by combining with the data from the central inner tracking system and time-projection chamber, requiring that the summed p_T of the charged particles in a cone of angular radius 0.4 radians centred on the photon candidate be smaller than 1.5 GeV/c. The isolated photon cross-sections are obtained within the transverse momentum range from 7 to 200 GeV/c, corresponding to 1.1 × 10^–3 < x_T < 30.8 × 10^–3.

Figure 1 shows the new ALICE results alongside those from ATLAS, CMS and prior measurements in proton–proton and proton–antiproton collisions at lower values of √s. The figure spans more than 15 orders of magnitude on the y-axis, representing the cross-section, over a wide range of x_T. The present measurement probes the smallest Bjorken x with isolated photons at midrapidity to date. The experimental data points show an agreement between all the measurements when scaled with the collision energy to the power n = 4.5. Such a scaling is designed to cancel the predicted 1/(p_T)ⁿ dependence of partonic 2 → 2 scattering cross-sections in perturbative QCD and reveal insights into the gluon PDF (see “The other 99%“).

This measurement will help to constrain the gluon PDF and will play a crucial role in exploring medium-induced modifications of hard probes in nucleus–nucleus collisions.

R(D) ratios in line at LHCb

LHCb figure 1 — **Fig. 1.** Fit projection as a function of the energy of the muon in the rest frame of the B meson in the signal enriched region for all four decays used in the analysis. Credit: LHCb Collab. 2024 arXiv:2406.03387

The accidental symmetries observed between the three generations of leptons are poorly understood, with no compelling theoretical motivation in the framework of the Standard Model (SM). The b → cτ^–ν_τ transition has the potential to reveal new particles or forces that interact primarily with third-generation particles, which are subject to the less stringent experimental constraints at present. As a tree-level SM process mediated by W-boson exchange, its amplitude is large, resulting in large branching fractions and significant data samples to analyse.

The observable under scrutiny is the ratio of decay rates between the signal mode involving τ and ν_τ leptons from the third generation of fermions and the normalisation mode containing μ and ν_μ leptons from the second generation. Within the SM, this lepton flavour universality (LFU) ratio deviates from unity only due to the different mass of the charged leptons – but new contributions could change the value of the ratios. A longstanding tension exists between the SM prediction and the experimental measurements, requiring further input to clarify the source of the discrepancy.

The LHCb collaboration analysed four decay modes: B⁰ → D⁽^*⁾⁺ℓ^–ν_ℓ, with ℓ representing τ or μ. Each is selected using the same visible final state of one muon and light hadrons from the decay of the charm meson. In the normalisation mode, the muon originates directly from the B-hadron decay, while in the signal mode, it arises from the decay of the τ lepton. The four contributions are analysed simultaneously, yielding two LFU ratios between taus and muons – one using the ground state of the D⁺ meson and one the excited state D^*⁺.

The control of the background contributions is particularly complicated in this analysis as the final state is not fully reconstructible, limiting the resolution on some of the discriminating variables. Instead, a three-dimensional template fit separates the signal and the normalisation from the background versus: the momentum transferred to the lepton pair (q²); the energy of the muon in the rest frame of the B meson (E_μ^*); and the invariant mass missing from the visible system. Each contribution is modelled using a template histogram derived either from simulation or from selected control samples in data.

This constitutes the world’s second most precise measurement of R(D)

To prevent the simulated data sample size from becoming a limiting factor in the precision of the measurement, a fast tracker-only simulation technique was exploited for the first time in LHCb. Another novel aspect of this work is the use of the HAMMER software tool during the minimisation procedure of the likelihood fit, which enables a fast, but exact, variation of a template as a function of the decay-model parameters. This variation is important to allow the form factors of both the signal and normalisation channels to vary as the constraints derived from the predictions that use precise lattice calculations can have larger uncertainties than those obtained from the fit.

The fit projection over one of the discriminating variables is shown in figure 1, illustrating the complexity of the analysed data sample but nonetheless showcasing LHCb’s ability to distinguish the signal modes (red and orange) from the normalisation modes (two shades of blue) and background contributions.

The measured LFU ratios are in good agreement with the current world average and the predictions of the SM: R(D⁺) = 0.249 ± 0.043 (stat.) ± 0.047 (syst.) and R(D^*⁺) = 0.402 ± 0.081(stat.) ± 0.085 (syst.). Under isospin symmetry assumptions, this constitutes the world’s second most precise measurement of R(D), following a 2019 measurement by the Belle collaboration. This analysis complements other ongoing efforts at LHCb and other experiments to test LFU across different decay channels. The precision of the measurements reported here is primarily limited by the size of the signal and control samples, so more precise measurements are expected with future LHCb datasets.

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