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Building on success, planning for the future

From 29 January to 1 February, the Chamonix Workshop 2024 upheld its long tradition of fostering open and collaborative discussions within CERN’s accelerator and physics communities. This year marked a significant shift with more explicit inclusion of the injector complex, acknowledging its crucial role in shaping future research endeavours. Chamonix discussions focused on three main areas: maximising the remaining years of Run 3; the High-Luminosity LHC (HL-LHC), preparations for Long Shutdown 3 and operations in Run 4; and a look to the further future and the proposed Future Circular Collider (FCC).

Immense effort

Analysing the performance of CERN’s accelerator complex, speakers noted the impressive progress to date, examined limitations in the LHC and injectors and discussed improvements for optimal performance in upcoming runs. It’s difficult to do justice to the immense technical effort made by all systems, operations and technical infrastructure teams that underpins the exploitation of the complex. Machine availability emerged as a crucial theme, recognised as critical for both maximising the potential of existing facilities and ensuring the success of the HL-LHC. Fault tracking, dedicated maintenance efforts and targeted infrastructure improvements across the complex were highlighted as key contributors to achieving and maintaining optimal uptime.

As the HL-LHC project moves into full series production, the technical challenges associated with magnets, cold powering and crab cavities are being addressed (CERN Courier January/February 2024 p37). Looking beyond Long Shutdown 3 (LS3), potential limitations are already being targeted now, with, for example, electron-cloud mitigation measures planned to be deployed in LS3. The transition to the high-luminosity era will involve a huge programme of work that requires meticulous preparation and a well-coordinated effort across the complex during LS3, which will see the deployment of the HL-LHC, a widespread consolidation effort, and other upgrades such as that planned for the ECN3 cavern at CERN’s North Area.

The vision for the next decades of these facilities is diverse, imaginative and well-motivated from a physics perspective

The breadth and depth of the physics being performed at CERN facilities is quite remarkable, and the Chamonix workshop reconfirmed the high demand from experimentalists across the board. The unique capabilities of ISOLDE, n_TOF, AD-ELENA, and the East and North Areas were recognised. The North Area, for example, provides protons, hadrons, electrons and ion beams for detector R&D, experiments, the CERN neutrino platform, irradiation facilities and counts more than 2000 users. The vision for the next decades of these facilities is diverse, imaginative and well-motivated from a physics perspective. The potential for long-term exploitation and leveraging fully the capabilities of the LHC and other facilities is considerable, demanding continued support and development.

In the longer term, CERN is exploring the potential construction of the FCC via a dedicated feasibility study that has just delivered a mid-term report – a summary of which was presented at Chamonix. The initiative is accompanied by R&D on key accelerator technologies. The physics case for FCC-ee was well made for an audience of mostly non-particle physicists, concluding that the FCC is the only proposed collider that covers each key area in the field – electroweak, QCD, flavour, Higgs and searches for phenomena beyond the Standard Model – in paradigm-shifting depth.

Environmental consciousness

Sustainability was another focus of the Chamonix workshop. Building and operating future facilities with environmental consciousness is a top priority, and full life-cycle analyses will be performed for any options to help ensure a low-carbon future.

Interesting times, lots to do. To quote former CERN Director-General Herwig Schopper from 1983: “It is therefore clear that, for some time to come, there will be interesting work to do and I doubt whether accelerator experts will find themselves without a job.”

Strange correlations benchmark hadronisation

ALICE figure 1 — **Fig. 1.** Pearson correlation coefficient between the net-Ξ number and net-kaon number, as a function of the charged-particle multiplicity of the collision. Model calculations are shown as coloured bands. Credit: arxiv.2405.19890

In high-energy hadronic and heavy-ion collisions, strange quarks are dominantly produced from gluon fusion. In contrast to u and d quarks, they are not present in the colliding particles. Since strangeness is a conserved quantity in QCD, the net number of strange and anti-strange particles must equal zero, making them prime observable to study the dynamics of these collisions. Various experimental results from high-multiplicity pp collisions at the LHC demonstrate striking similarities to Pb–Pb collision results. Notably, the fraction of hadrons carrying one or more strange quarks smoothly increases as a function of particle multiplicity in pp and p–Pb collisions to values consistent with those measured in peripheral Pb–Pb collisions. Multi-particle correlations in pp collisions also closely resemble those in Pb–Pb collisions.

Explaining such observations requires understanding the hadronisation mechanism, which governs how quarks and gluons rearrange into bound states (hadrons). Since there are no first-principle calculations of the hadronisation process available, phenomenological models are used, based on either the Lund string fragmentation (Pythia 8, HIJING) or a statistical approach assuming a system of hadrons and their resonances (HRG) at thermal and chemical equilibrium. Despite having vastly different approaches, both models successfully describe the enhanced production of strange hadrons. This similarity calls for new observables to decisively discriminate between these two approaches.

The data indicate a weaker opposite-sign strangeness correlation than that predicted by string fragmentation

In a recently published study, the ALICE collaboration measured correlations between particles arising from the conservation of quantum numbers to further distinguish the two models. In the string fragmentation model, the quantum numbers are conserved locally through the creation of quark–antiquark pairs from the breaking of colour strings. This leads to a short-range rapidity correlation between strange and anti-strange hadrons. On the other hand, in the statistical hadronisation approach, quantum numbers are conserved globally over a finite volume, leading to long-range correlations between both strange–strange and strange–anti-strange hadron pairs. Quantum-number conservation leads to correlated particle production that is probed by measuring the yields of charged kaons (with one strange quark) and multistrange baryons (Ξ^– and Ξ⁺) on an event-by-event basis. In ALICE, charged kaons are directly tracked in the detectors, while Ξ baryons are reconstructed via their weak decay to a charged pion and a Λ-baryon, which is itself identified via its weak decay into a proton and a charged pion.

Figure 1 shows the first measurement of the correlation between the “net number” of Ξ baryons and kaons, as a function of the charged-particle multiplicity at midrapidity in pp, p–Pb and Pb–Pb collisions, where the net number is the difference between particle and antiparticle multiplicities. The experimental results deviate from the uncorrelated baseline (dashed line), and string fragmentation models that mainly correlate strange hadrons with opposite strange quark content over a small rapidity range fail to describe both observables. At the same time, the measurements agree with the statistical hadronisation model description that includes opposite-sign and same-sign strangeness correlations over large rapidity intervals. The data indicate a weaker opposite-sign strangeness correlation than that predicted by string fragmentation, suggesting that the correlation volume for strangeness conservation extends to about three units of rapidity.

The present study will be extended using the recently collected data during LHC Run 3. The larger data samples will enable similar measurements for the triply strange Ω baryon, as well as the study of higher cumulants.

German community discusses future collider at CERN

German particle-physics community in Bonn — **Bonn workshop** The German particle-physics community gathered at Bonn to discuss the opportunities of a future collider at CERN. Credit: M Schott

More than 150 German particle physicists gathered at Bonn University for a community event on a future collider at CERN. More precisely, the focus set for this meeting was to discuss the opportunities that the FCC-ee would offer should this collider be built at CERN. The event was organised by the German committee for particle physics, KET, and took place from 22 to 24 May. Representatives from almost all German institutes and groups active in particle physics were present, an attendance that shows the large interest in the collider to be built at CERN after the successful completion of the HL-LHC programme.

The main workshop was preceded by a dedicated session with more than 80 early-career scientists, organised by the Young High Energy Physicists Association, yHEP, to bring the generation that will benefit most from a future collider at CERN up to speed on the workshop topics. It included a presentation by former ECFA chair Karl Jakobs (Freiburg University) “From Strategy Discussions to Decision-Taking for Large Projects”, explaining the mechanisms and bodies involved in setting a project like the FCC-ee on track.

The opening session of the main workshop featured a fresh view on “The physics case for an e⁺e^– collider at CERN” by Margarete Mühlleitner (KIT Karlsruhe), who spread excitement about the strong and comprehensive physics case from super-precise measurements of the properties of the Z boson, the W boson and the top quark to what most people associate with a future e⁺e^– collider: precision measurements of the Higgs boson and insights about its connection to many of the still open questions of particle physics like dark matter or the matter–antimatter asymmetry. Markus Klute (KIT Karlsruhe) gave an in-depth review of the FCC-ee project. The midterm results of the FCC feasibility study indicate that no showstoppers were found in all the aspects studied so far and that the integrated FCC programme offers unparalleled exploration potential through precision measurements and direct searches. The picture was rounded off by a presentation from Jenny List (DESY, Hamburg) who talked about alternative options to realise an e⁺e^– Higgs factory at CERN, and the perspective of the early-career researchers was highlighted by Michael Lupberger (Bonn University). While all these presentations concentrated on the science and technology of the FCC-ee or alternatives, Eckart Lilienthal, representing the German Ministry of Education and Research, BMBF, reminded the audience that a future collider project at CERN needs an affordable financial plan and that – given the large uncertainties at present – this requires the community to prepare for different scenarios including one without the FCC-ee. Lilienthal confirmed that the future of CERN remains of the highest priority to BMBF.

The event was an important step in building consensus in the German community for a future collider project at CERN

The workshop went on to review many aspects of the FCC-ee and possible alternatives in more detail: accelerator R&D, detector concepts and technologies, computing and software, theory challenges as well as sustainability. The workshop witnessed the first meetings of the newly established German detector R&D consortia on silicon detectors, gaseous detectors and calorimetry. They will receive BMBF funding for the next three years and will allow German groups to strongly participate in the recently formed international DRD consortia in the context of the ECFA detector roadmap.

The path ahead

The workshop concluded with discussion sessions on the future collider scenarios for CERN, the engagement of the German community and a path to prepare the German input to the update of the European Strategy for Particle Physics. A series of three additional community workshops will be held in Germany before this input is due in March 2025.

The Bonn event was an important step in building consensus in the German community for a future collider project at CERN. The FCC-ee project generated a lot of interest and many groups plan to embark more strongly on this project. Contributions concerning the physics case, theory challenges, detector design and development, software, computing, and accelerator development were discussed. Alternative options for a future collider project at CERN need to be kept open to address the unanswered fundamental questions of particle physics in case the FCC-ee is not built at CERN. This event was clear evidence that a bright future for CERN remains of highest priority for the German particle-physics community and funding agency.

Exploring the Higgs potential at ATLAS

ATLAS figure 1 — **Fig. 1.** 95% confidence upper limits on the signal strength for inclusive gluon–gluon and vector–boson fusion HH production from each of the five analyses and their statistical combination. Credit: ATLAS Collab. 2024 arXiv:2406.09971

Immediately after the Big Bang, all the particles we know about today were massless and moving at the speed of light. About 10^–12 seconds later, the scalar Higgs field spontaneously broke the symmetry of the electroweak force, separating it into the electromagnetic and weak forces, and giving mass to fundamental particles. Without this process, the universe as we know it would not exist.

Since its discovery in 2012, measurements of the Higgs boson – the particle associated with the new field – have refined our understanding of its properties, but it remains unknown how closely the field’s energy potential resembles the predicted Mexican hat shape. Studying the Higgs potential can provide insights into the dynamics of the early universe, and the stability of the vacuum with respect to potential future changes.

The Higgs boson’s self-coupling strength λ governs the cubic and quartic terms in the equation describing the potential. It can be probed using the pair production of Higgs bosons (HH), though this is experimentally challenging as this process is more than 1000 times less likely than the production of a single Higgs boson. This is partly due to destructive interference between the two leading order diagrams in the dominant gluon–gluon fusion production mode.

The ATLAS collaboration recently compiled a series of results targeting HH decays to bbγγ, bbττ, bbbb, bbll plus missing transverse energy (E_T^miss), and multilepton final states. Each analysis uses the full LHC Run 2 data set. A key parameter is the HH signal strength, μ_HH, which divides the measured HH production rate by the Standard Model (SM) prediction. This combination yields the strongest expected constraints to date on μ_HH, and an observed upper limit of 2.9 times the SM prediction (figure 1). The combination also sets the most stringent constraints to date on the strength of the Higgs boson’s self-coupling of –1.2 < κ_λ < 7.2, where κ_λ = λ/λ_SM, its value relative to the SM prediction.

Each analysis contributes in a complementary way to the global picture of HH interactions and faces its own set of unique challenges.

Despite its tiny branching fraction of just 0.26% of all HH decays, HH → bbγγ provides very good sensitivity to μ_HHthanks to the ATLAS detector’s excellent di-photon mass resolution. It also sets the best constraints on λ due to its sensitivity to HH events with low invariant mass.

The HH → bbττ analysis (7.3% of HH decays) exploits state-of-the-art hadronic–tau identification to control the complex mix of electroweak, multijet and top-quark backgrounds. It yields the strongest limits on μ_HH and the second tightest constraints on λ.

HH → bbbb (34%) has good sensitivity to μ_HH thanks to ATLAS’s excellent b-jet identification, but controlling the multijet background presents a formidable challenge, which is tackled in a fully data-driven fashion.

Studying the Higgs potential can provide insights into the dynamics of the early universe

The decays HH → bbWW and HH → bbττ in fully leptonic final states have very similar characteristics and are thus targeted in a single HH → bbll+E_T^miss analysis. Contributions from the bbZZ decay mode, where one Z decays to charged light leptons and the other to neutrinos, are also considered.

Finally, the HH → multilepton analysis is designed to catch decay modes where the HH system cannot be fully reconstructed due to ambiguity in how the decay products should be assigned to the two Higgs bosons. The analysis uses nine signal regions with different multiplicities of light charged leptons, hadronic taus and photons. It is complementary to all the exclusive channels discussed above.

For the ongoing LHC Run 3, ATLAS designed new triggers to enhance sensitivity to the hadronic HH → bbττ and HH → bbbb channels. Improved b-jet identification algorithms will increase the efficiency in selecting HH signals and distinguishing them from background processes. With these and other improvements, our prospects have never looked brighter for homing in on the Higgs self-coupling.

Estonia becomes 24th Member State

On 30 August CERN welcomed Estonia as its 24th Member State, marking the end of a formal application process that started in 2018 and crowning a period of cooperation that stretches back three decades.

“Estonia is delighted to join CERN as a full member because CERN accelerates more than tiny particles, it also accelerates international scientific collaboration and our economies,” said Estonia president Alar Karis. “We have seen this potential during our time as Associate Member State and are keen to begin our full contribution.”

The bilateral relationship formally began in 1996, when Estonia and CERN signed a first cooperation agreement. Estonia has been part of the CMS collaboration since 1997, participating in data analysis and the Worldwide LHC Computing Grid, for which Estonia operates a Tier 2 centre in Tallinn. Researchers from Estonia also contribute to other experiments including CLOUD, COMPASS, NA66 and TOTEM, and to studies for future colliders, while Estonian theorists are highly involved in collaborations with CERN.

“Estonia and CERN have been collaborating closely for some 30 years, and I am very pleased to welcome Estonia to the ever-growing group of CERN Member States,” said Director-General Fabiola Gianotti. “I am sure the country and its scientific community will benefit from increased opportunities in fundamental research, technology development, and education and training.”

Estonia has held Associate Member State status in the pre-stage to membership of CERN since February 2021. As a full Member State, Estonia will now have voting rights in the CERN Council, enhanced opportunities for Estonian nationals to be recruited by CERN and for Estonian industry to bid for CERN contracts.

“On behalf of the CERN Council, I warmly welcome Estonia as the newest Member State of CERN,” said Council president Eliezer Rabinovici. “I am happy to see the community of CERN Member States enlarging, and I am looking forward to the enhanced participation of Estonia in the CERN Council and to its additional scientific contributions to CERN.”

LHCb measures the weak mixing angle

LHCb figure 1 — **Fig. 1.** The LHCb measurement of the forward–backward asymmetry as a function of the absolute difference between the pseudorapidities of the two muons produced in the Z boson decay, |Δη|. Credit: LHCb Collab.

At the International Conference on High-Energy Physics in Prague in July, the LHCb collaboration presented an updated measurement of the weak mixing angle using the data collected at the experiment between 2016 and 2018. The measurement benefits from the unique forward coverage of the LHCb detector.

The success of electroweak theory in describing a wide range of measurements at different experiments is one of the crowning achievements of the Standard Model (SM) of particle physics. It explains electroweak phenomena using a small number of free parameters, allowing precise measurements of different quantities to be compared to each other. This facilitates powerful indirect searches for beyond-the-SM physics. Discrepancies between measurements might imply that new physics influences one process but not another, and global analyses of high-precision electroweak measurements are sensitive to the presence of new particles at multi-TeV scales. In 2022 the entire field was excited by a measurement of the W-boson mass that is significantly larger than the value predicted within these global analyses by the CDF collaboration, heightening interest in electroweak measurements.

The weak mixing angle is at the centre of electroweak physics. It describes the mixing of the U(1) and SU(2) fields, determines couplings of the Z boson, and can also be directly related to the ratio of the W and Z boson masses. Excitingly, the two most precise measurements to date, from LEP and SLD, are in significant tension. This raises the prospect of non-SM particles potentially influencing one of these measurements, since the weak mixing angle, as a fundamental parameter of nature, should otherwise be the same no matter how it is measured. There is therefore a major programme measuring the weak mixing angle at hadron colliders, with important contributions from CDF, D0, ATLAS, CMS and LHCb.

Since the weak mixing angle controls Z-boson couplings, it can be determined from measurements of the angular distributions of Z-boson decays. The LHCb collaboration measured around 860,000 Z-boson decays to two oppositely charged muons, determining the relative rate at which negatively charged muons are produced closer to the LHC beamline than positively charged muons as a function of the angular separation of the two muons. Corrections are then applied for detector effects. Comparison to theoretical predictions based on different values of the weak mixing angle allows the value best describing the data to be determined (figure 1).

The unique angular coverage of the LHCb detector is well-suited for this measurement for two key reasons. First, the statistical sensitivity to the weak mixing angle is largest in the forward region close to the beamline that the LHCb detector covers. Second, the leading systematic uncertainties in measurements of the weak mixing angle at hadron colliders typically arise from existing knowledge of the proton’s internal structure. These uncertainties are also smallest in the forward region.

The value of the weak mixing angle measured by LHCb is consistent with previous measurements and with SM expectations (see “Weak mixing angle” figure). Notably, the precision of the LHCb measurement remains limited by the size of the data sample collected, such that further improvements are expected with the data currently being collected using the upgraded LHCb detector. In addition, while other experiments profile effects associated with the proton’s internal structure to reduce uncertainties, the unique forward acceptance means that this is not yet necessary at LHCb. This advantage will also be important for future measurements: the small theoretical uncertainty means that the forthcoming Upgrade 2 of the LHCb experiment is expected to achieve a precision more than a factor of two better than the most precise measurements to date.

Become a Particle Physicist in Eight Simple Moves

Simone Ragoni is passionate about outreach. His Instagram page, quarktastic, has more than 10 thousand followers, and is one of the very few that successfully makes particle physics and academia relatable. He wrote Become a Particle Physicist in Eight Simple Moves while completing his PhD on the ALICE experiment. The first move is to sip his favourite beverage: coffee.

As a social-media manager and communicator, I’ve been following Ragoni for years. His main tool is humour. And I’m proof it works. I will always remember the basic structure of a proton, because life is indeed full of “ups” and “downs”.

Did I say gentle humour? Nah. Ragoni goes all the way. But he confesses that his humour can only be understood by a handful of people. Particle physics is esoteric – and readers will want to join the club. His book invites you into the world of a young particle physicist. Being a nerd is the new cool.

A highlight is when Ragoni describes how to keep those distributions fit. If you know, you know. There is a pun here and the author explains it very well. He next turns to the tedious work that goes into publishing a paper. “Monte Carlo simulations are our real playground,” he writes, “where we unleash all our fantasy, the perfect world where everything is nice.” But particle physicists are cautious. Five sigma is needed to claim a discovery – a one in 3.5 million chance of being wrong. The author concludes with encouragement to make your own measurements using CERN’s open data.

Ragoni’s book is a delightful gift for anyone whom you want to inspire to become a particle physicist of tomorrow or simply to convey the excitement of what you do, with a quirky bonus of being presented bilingually in English and Italian, should you be keen on improving your physics vocab in one of those languages. It is a gateway to the captivating world of particle physics, skilfully blending humour with profound insights, and inspiring readers to explore further and consider joining the ranks of future particle physicists.

I built a physics museum in my classroom

Teaching modern physics to high-school students presents many challenges: overpacked curricula focusing on classical physics; the depth of knowledge needed by students (and teachers) to understand these topics; and students being over-focused on grades and university admissions. By exposing my students to the work being done at major research laboratories around the world, I have managed to find a way to overcome many of those obstacles.

Some time ago, British Columbia removed provincial examinations, giving teachers a bit more freedom to make additions to their curricula. I chose to insert small one- or two-day units throughout the year, which give my students multiple exposure to modern physics topics. These short introductions over a two-year period mean that physics students don’t need to know all the fine details, which decreases their stress and concerns.

Knowledge sharing

Physics teachers are lucky to have access to high-quality professional development via workshops run by CERN, LIGO, the Perimeter Institute (which produces excellent resources for use in physics classes) and others. These often-week-long events give teachers an overview of how a given research facility works, in the hope that they will bring that knowledge back to their students. Along the way, the teachers attend lectures from leading researchers and see first-hand careers in the field that they can bring back to share with their class.

I have been fortunate enough to attend workshops at these facilities. I have also taken part in a research experience at SNOLAB, brought students on tours of TRIUMF and mentored my students as they conducted research at the Canadian Light Source. All these experiences have given me the knowledge and confidence to introduce the facilities and the work done at them to my students in a way that hopefully piques their curiosity.

The pieces provide a starting point for conversations around what these decommissioned parts were used for and the kind of science they supported

While at CERN for the 2019 international teacher programme, I had the opportunity to visit both the CMS and ALICE detectors and to attend lectures from renowned particle physicists. We spent time in S’Cool LAB and visited many of the behind-the-scenes parts of CERN. While all of these experiences left an imprint on my teaching, it was during quiet visits to what was then called the Microcosm garden – which hosts decommissioned pieces of accelerators and detectors as a form of art – that helped transform the physical space in my classroom.

In 2022 my school in British Columbia renovated a large, old classroom to become our new physics lab. Knowing that I had more space to work with than before, I was inspired to start building my own version of the Microcosm garden on my classroom walls. I soon connected with the outreach team at TRIUMF who were excited to help get my project started with a photomultiplier tube, a control panel from a xenon-gas handling system, a paddle scintillator and a light guide. Since then, I have added a Lucas cell from SNOLAB, a piece of the electron gun from the Canadian Light Source and, most recently, a small-strip half-gap prototype from the New Small Wheel upgrade of the ATLAS detector. The pieces provide a starting point for conversations with students around what these decommissioned parts were used for, and the kind of science they supported.

Equipped with some knowledge of what modern research in the field looks like, I have successfully built a system where I am able to inspire students to want to study physics. Since attending my first major workshop in 2018, I have seen an increase in the number of students entering physics majors. Some of them have already gone on to internships at CERN and TRIUMF, after getting their first exposure to these organisations in my classes. My hope is that by having pieces of the facilities I talk about displayed on my classroom walls, this will further inspire more of my students to want to learn about them, possibly setting them on paths to careers in physics.

A new generation, a new vision

The 2024 Aspen Winter Conference, The Future of High Energy Physics: A New Generation, A New Vision, attracted 50 early-career researchers (ECRs) from across the world to the Aspen Center for Physics, 8000 feet above sea level in the Colorado Rockies, from 24 to 29 March. The conference built on the many new ideas that arose from the recent Snowmass process of the US particle physics community (CERN Courier January/February 2024 p7). The conference sought to highlight the role of ECRs in realising bold long-term visions for the field, covering theoretical questions, the experimental vision for the next 50 years and the technologies required to make it a reality. Students, postdocs and junior faculty are often the drivers of new ideas in science. Helping them transition new ideas to the mainstream requires enthusiasm, community support and time.

Crossing frontiers

85% of the matter in the universe at most minimally interacts with the electromagnetic force but provided the gravitational seed for large-scale structure formation in the early universe. Hugh Lippincott (University of California, Santa Barbara) summarised cross-frontier searches. Pursuing all possible scenarios via direct detection will require scaling up existing technology and developing new technologies such as quantum sensors to probe lighter dark-matter candidates. On the one hand, the 60 to 80 tonne “XLZD” liquid xenon detector will merge the expertise of the XENONnT, LUX-ZEPLIN and DARWIN collaborations; on the low-mass side, Reina Maruyama (Yale) discussed the ALPHA and HAYSTAC haloscopes, which seek to convert axions into photons in highly tuned resonant cavities. Indirect detection and collider experiments will also play an important role in closing in on minimal dark-matter models.

Delegates expressed a sense of urgency to probe higher energies. Cari Cesarotti (MIT) advocated R&D towards a future muon collider, arguing that muons offer a clean and power-efficient route to the 10 TeV scale and above. Recently, experts have estimated that challenges due to the finite muon lifetime could be overcome on a 20-year technically limited timeline. Both CERN and China have proposed building 100 km-circumference tunnels, initially hosting an electron–positron collider followed by a 100 TeV hadron machine, however, the timeline suggests that almost all of the conference attendees would be retired before hadron collisions come online. Elliot Lipeles (Pennsylvania) proposed skipping the electron-positron stage and immediately pursuing an intermediate-energy hadron collider: existing magnets in a 100 km tunnel could produce 37 TeV collisions, advancing measurements of the Higgs self-coupling and electroweak phase transition, dark matter and its mediators, and naturalness.

The energy, intensity and cosmic frontiers of particle physics target deeply connected questions

Neutrinos were discussed at length. Georgia Karagiorgi (Columbia University) argued that three short-baseline anomalies remain, potentially hinting at additional sterile neutrinos or dark-sector portals. Julieta Gruszko (North Carolina at Chapel Hill) presented an exciting future for experiments that seek to discern the fundamental nature of neutrinos. A new tonne-scale generation of detectors comprising LEGEND1000, nEXO and CUPID may succeed in confirming the Majorana nature of the neutrino if they observe neutrinoless double beta decay.

Talks on the importance of science communication and education provoked a great deal of discussion. Ethan Siegal, host of popular podcast “Starts with a Bang” spoke on public outreach, Kevin Pedro (Fermilab) on advocacy with policymakers in Washington, DC, and Roger Freedman (University of California Santa Barbara) on educating the next generation of physicists. In public programming, Nausheen Shah (Wayne State) was the guest speaker at a screening of Hidden Figures, the inspiring true story of the black women who helped the US win the space race, and Philip Chang (University of California San Diego) lectured on “An Invitation to Imagine Something from Nothing”.

The energy, intensity and cosmic frontiers of particle physics target deeply connected questions. Dark matter, dark energy, cosmic inflation and baryogenesis have remained unexplained for decades, and the structure of the Standard Model itself provokes questions, not least in relation to the Higgs boson and neutrinos. Innovative and complementary experiments are needed across all areas of particle physics. Judging from the 2024 Aspen Winter Conference, the future of the field is in good hands.

Threshold moment for medical photon counting

7th Workshop on Medical Applications of Spectroscopic X-ray Detectors participants — **Industry engagement** The participants of the 7th Workshop on Medical Applications of Spectroscopic X-ray Detectors at CERN. Credit: M Cavazza/CERN-PHOTO-202404-083-4

The seventh workshop on Medical Applications of Spectroscopic X-ray Detectors was held at CERN from 15 to 18 April. This year’s workshop brought together more than 100 experts in medical imaging, radiology, physics and engineering. The workshop focused on the latest advancements in spectroscopic X-ray detectors and their applications in medical diagnostics and treatment. Such detectors, whose origins are found in detector R&D for high-energy physics, are now experiencing a breakthrough moment in medical practice.

Spectroscopic X-ray detectors represent a significant advancement in medical imaging. Unlike traditional X-ray detectors that measure only the intensity of X-rays, these advanced detectors can differentiate the energies of X-ray photons. This enables enhanced tissue differentiation, improved tumour detection and advanced material characterisation, which may lead in certain cases to functional imaging without the need for radioactive tracers.

The technology has its roots in the 1980s and 1990s when the high-energy-physics community centred around CERN developed a combination of segmented silicon sensors and very large-scale integration (VLSI) readout circuits to enable precision measurements at unprecedented event rates, leading to the development of hybrid pixel detectors (see p37). In the context of the Medipix Collaborations, CERN has coordinated research on spectroscopic X-ray detectors including the development of photon-counting detectors and new semiconductor materials that offer higher sensitivity and energy resolution. By the late 1990s, several groups had proofs of concept, and by 2008, pre-clinical spectral photon-counting computed-tomography (CT) systems were under investigation.

Spectroscopic X-ray detectors offer unparalleled diagnostic capabilities, enabling more detailed imaging and earlier and precise disease detection

In 2011, leading researchers in the field decided to bring together engineers, physicists and clinicians to help address the scientific, medical and engineering challenges associated with guiding the technology toward clinical adoption. In 2021, the FDA approval of Siemens Healthineers’ photon-counting CT scanner marked a significant milestone in the field of medical imaging, validating the clinical benefits of spectroscopic X-ray detectors. The mobile CT scanner, OmniTom Elite from NeuroLogica, approved in March 2022, also integrates photon counting detector (PCD) technology. The 3D colour X-ray scanner developed by MARS Bioimaging, in collaboration with CERN based on Medipix3 technology, has already shown significant promise in pre-clinical and clinical trials. Clinical trials of MARS scanners demonstrated its applications for detecting acute fractures, evaluation of fracture healing and assessment of osseous integration at the bone–metal interface for fracture fixations and joint replacements. With more than 300 million CT scans being performed annually around the world, the potential impact for spectroscopic X-ray imaging is enormous, but technical and medical challenges remain, and the need for this highly specialised workshop continues.

The scientific presentations in the 2024 workshop covered the integration of spectroscopic CT in clinical workflows, addressed technical challenges in photon counting detector technology and explored new semiconductor materials for X-ray detectors. The technical sessions on detector physics and technology discussed new methodologies for manufacturing high-purity cadmium–zinc–tellurium semiconductor crystals and techniques to enhance the quantum efficiency of current detectors. Sessions on clinical applications and imaging techniques included case studies demonstrating the benefits of multi-energy CT in cardiology and neurology, and advances in using spectroscopic detectors for enhanced contrast agent differentiation. The sessions on computational methods and data processing covered the implementation of AI algorithms to improve image reconstruction and analysis, and efficient storage and retrieval systems for large-scale spectral imaging datasets. The sessions on regulatory and safety aspects focused on the regulatory pathway for new spectroscopic X-ray detectors, ensuring patient and operator safety with high-energy X-ray systems.

Enhancing patient outcomes

The field of spectroscopic X-ray detectors is rapidly evolving. Continued research, collaboration and innovation to enhance medical diagnostics and treatment outcomes will be essential. Spectroscopic X-ray detectors offer unparalleled diagnostic capabilities, enabling more detailed imaging and earlier and precise disease detection, which improves patient outcomes. To stay competitive and meet the demand for precision medicine, medical institutions are increasingly adopting advanced imaging technologies. Continued collaboration among researchers, physicists and industry leaders will drive innovation, benefiting patients, healthcare providers and research institutions.

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