What first drew you to physics, and to accelerators in particular?
In school I liked and did well in science and math. I liked the feeling of certainty of math. There is an objective truth in math. And I was fascinated by the fact that I could use math to describe physical phenomena, to capture the complexity of the world in elegant mathematical equations. I also had an excellent, rigorous high-school physics teacher, whom I admired.
Accelerators offered the possibility of addressing fundamental challenges in (accelerator) physics and technology, and getting verifiable results in a reasonable amount of time to have a material impact. In addition, particle accelerators enable research and discovery in a vast range of scientific fields (such as particle and nuclear physics, X-ray and neutron science) and societal applications such as cancer treatment and radioisotope production.
What was your thesis topic?
My PhD thesis tackled experimentally, theoretically and via computer simulations the nonlinear dynamics of transverse particle oscillations in the former Tevatron collider at Fermilab, motivated by planning for the Superconducting Super Collider. Nonlinearities were introduced in the Tevatron by special sextupole magnets. In a series of experiments, we obtained accurate measurements of various phase–space features with sextupoles switched on. One of the features was the experimental demonstration of “nonlinear resonance islands” – protons captured on fixed points in phase space.
What have been the most rewarding and challenging aspects of your career so far?
There are many rewarding aspects of what I do. Seeing an audience, especially young people, who light up when I explain a fascinating concept. Pointing to something tangible that I contributed towards that will enable scientists to make discoveries in accelerator, particle or nuclear physics. Having conceived, worked on and advocated certain types of accelerators and seeing them realised. Predicting a behaviour of the particle beam, and verifying it in experiments. Also, troubleshooting a serious problem, and after days and nights of toil, finding the origin of or solution to the problem.
In terms of challenges, at Fermilab right now we are working on very complex and challenging projects like LBNF/DUNE. It involves more than 1400 international collaborators preparing and building a technically complex endeavour almost one mile underground. The mere scale of the operation is enormous but the pay-off is completing something unprecedented and enabling groundbreaking discoveries.
What are your goals as Fermilab director?
First and foremost, the completion of LBNF/DUNE to advance neutrino physics. Also, the completion of the remainder of the 2014 “P5” programmes, including the HL-LHC upgrades of the accelerator and CMS detector, and a new experiment at Fermilab called Mu2e. Looking to the future, when the next P5 report is completed, we will launch the next series of projects. Quantum technology is also a growing focus. Fermilab hosts one of five national quantum centres in addition to being a partner in a second one. We utilise our world-leading expertise in superconducting radiofrequency technology and instrumentation/control to advance quantum information technologies, as well as conducting unique dark-matter searches using this expertise.
Is being director different to what you imaged?
It takes a lot of hard work to build an excellent team, exceeding my initial projection. But equally our staff’s commitment, good will and dedication have also exceeded my expectations.
Which collider should follow the LHC, and what is the role of the US/Fermilab in realising such a project?
No matter which collider is chosen, there is still a lot of R&D required for any path concerning magnets, radiofrequency cavities and detectors. This R&D is crucial to multiple applications. I would advocate the development of these capabilities to push the state of the art for accelerators and detectors in the near future. Future colliders are an important component of the current Snowmass/P5 community planning exercise. Here, Fermilab is aligned with the previous P5 and is committed to following the next P5 recommendations.
How would you describe high-energy physics today compared to when you entered the field?
In the 1980s, the major building blocks of the Standard Model were largely in place, and the focus of the field was to experimentally verify many of its predictions. Today, the Standard Model is much more thoroughly tested, but there is evidence that it does not completely describe the whole picture.
The upcoming century promises a fascinating array of ground-breaking discoveries
A lot of present-day research is about physics beyond the Standard Model, including dark matter, dark energy and the question of matter–antimatter asymmetry in the universe. In parallel, technologies have advanced tremendously since the 1980s, enabling unprecedented precision, parameter reach and new discoveries. This applies to accelerators, telescopes, detectors and computing. The upcoming century promises a fascinating array of groundbreaking discoveries, all of which will fundamentally further our understanding of the universe.
What can be done to ensure that there are more female laboratory directors worldwide?
I think it is important to increase the pipeline, starting with efforts to attract young people in elementary and high school. We need to look at changing the cultural perspective that women can’t do STEM and get to the point where the entire culture is open-minded. We also need to change the make-up of the committees.
It is important to encourage females to take on leadership positions and then support and empower them with enlightened mentors. Once they develop their careers, we will have a much bigger pool for future lab directors. We must inspire and empower young girls and women to follow their dreams, and help them stay focused to succeed.
At a ceremony in the CERN Globe on 27 September, the winners of “Mining the Future” – a competition co-organised by CERN and the University of Leoben to identify the best way to handle excavated materials from the proposed Future Circular Collider (FCC) project – were announced. Launched in June 2021 in the frame of the European Union co-funded FCC Innovation Study, Mining the Future invited experts beyond the physics community to seek sustainable ways of reusing the heterogeneous sedimentary rock that would need to be excavated for the FCC infrastructure, which is centered on a 91 km-circumference tunnel in the Geneva basin. Twelve proposals, submitted by consortia of universities, major companies and start-ups, were reviewed based on their technological readiness, innovative potential and socioeconomic impact.
Following final pitches in the Globe by the four shortlisted entrants, a consortium led by Swiss firm BG Ingenieurs Conseils was awarded first prize – including support to the value of €40,000 to bring the technology to maturity – for their proposal “Molasse is the New Ore”. Using a near real-time flow analysis that has been demonstrated in cement plants, the proposal would see excavated materials immediately identified and separated for further processing on-site, treating them not as waste that needs to be managed and thereby serving environmental objectives and efficiency targets.
The runners-up were proposals led by: Amberg (to sort, characterise and redistribute the molasse into fractions of known compositions and recycle each material on a large scale locally); Briques Technique Concept (to produce bricks from the excavated material for the construction of nearby buildings); and Edaphos (to process the molasses into topsoil-like material in a process known as soil conditioning). Although only one winner was chosen, it emerged during the ceremony that an integrated approach of all four shortlisted scenarios would be a valid scenario for managing the estimated 7–8 million m3 of molasse materials required for the FCC construction project.
“This is a key ingredient for the FCC feasibility study while also creating business opportunities for applying these technologies in different markets,” said competition creator Johannes Gutleber of CERN. “The proposals submitted in the course of the contest show that designing a new research infrastructure acts as an amplifier of ideas for society at large.”
Physics-based industries are as important to the Swiss economy as production or trade, concludes a new report by the Swiss Physical Society (SPS). Seeking to determine the impact of physics on Swiss society, and motivated by a similar Europe-wide study completed in 2019 by the European Physical Society, the SPS team, with support from the Swiss Academy of Natural Sciences and Swiss service-provider IMSD, carried out a statistical analysis revealing key indicators of the national value of physics.
Currently, states the report, the turnover of physics-based industries (PBI) in Switzerland is estimated to exceed CHF 274 billion in revenue, and is expected to grow further. PBI, defined as those industries that are strongly reliant on modern technologies developed by physicists, were divided into 11 categories ranging from pharmaceuticals and medical instruments to electricity supply and general manufacturing. The share of PBI in Switzerland’s gross value added (GVA) was found to be CHF 91.5 billion, or 13% of the total for 2019, while the number of full-time equivalent jobs was 417,000 (9.8%). Furthermore, the specific GVA for PBI increased by 6.3% from 2015 to 2019 – almost three times higher than the average increase among all economic-activity sectors during the same period.
Innovative ideas that come out of fundamental, curiosity-driven research are at the source of what leads to success in society
Hans Peter Beck
Not included in these figures are the contributions of physicists who are employed in other industries, nor additional economic impact due to downstream effects such as household spending associated with economic activity in PBI. Estimating the GVA multiplier associated with the impact of PBI to be between 2.31 and 2.49, the report concludes that every CHF 1.00 of direct physics-related output contributes CHF 2.31 to 2.49 to the economy-wide output. Beyond economic impact, the report also evaluated the contribution of education and innovation to Swiss society, and highlighted ways in which to address the shortage of skilled workers and the gender gap.
“The impact physics has on society has been studied multiple times in a variety of countries and all arrive at the same conclusion: economic success in a modern, technology-driven society is the fruit of long-term support for physics in education and research,” says former SPS president Hans Peter Beck. “Innovative ideas that come out of fundamental, curiosity-driven research are at the source of what leads to success in society.”
In September, CERN approved a new policy for open science, with immediate effect. Developed by the Open Science Strategy Working Group (OSWG), which includes members from CERN departments and experiments, the policy aims to make all CERN research fully accessible, reproducible, inclusive, democratic and transparent for both researchers and wider society.
Open science has always been one of CERN’s key values, dating back to the signing of the CERN Convention at UNESCO in 1952. The new policy follows the 2020 update of the European Strategy for Particle Physics, which highlighted the importance of open science, and UNESCO’s Recommendation on Open Science, published in 2021. It encompasses the existing policies for open access and open data, which make all research papers and experimental data publicly available. It also brings together other existing elements of open science – open-source software and hardware, research integrity, open infrastructure and research assessment (which make research reliable and reproducible) and training, outreach and citizen science, which aim to educate and create dialogue with the next generation of researchers and the public.
“The publication of the Open Science Policy gives a solid framework in which the popular suite of open-source tools and services provided by CERN, including Zenodo, Invenio and REANA, can continue to grow and support the adoption of open-science practices, not only within physics but also across the globe’s research communities,” said Enrica Porcari, head of CERN’s IT department.
The OSWG will continue to assess how open science evolves at CERN, developing the policy in accordance with new best practices. Alongside this, a new open-science report will be published each year, showing CERN’s continued commitment to the initiative.
Alain Magnon, a well-known French nuclear physicist and long-serving spokesperson of the COMPASS collaboration at CERN (2003–2010), passed away on 18 March 2022. Retired from IRFU CEA Saclay for more than 10 years, he remained an enthusiastic COMPASS member, valuably participating in the activities of the Illinois and Matrivani groups. In recent years he was an active contributor to the Physics Beyond Colliders working group and to the MUonE project at CERN.
After graduating as an engineer from the École centrale des arts et manufactures in Paris in the late 1960s, Alain joined the nuclear physics division at Saclay where he worked on the first prototypes of multi-wire proportional chambers. Interested in continuing his career as a nuclear physicist, he later moved to the University of Chicago to carry out his PhD thesis work on the hyperfine structure of muonium, under the supervision of Val Telegdi.
Returning to Saclay, Alain played a leading role in measurements of the muon lifetime and capture rates, resulting in one of the most precise determinations of the Fermi weak-coupling constant. These measurements were later extended to both positive and negatively charged muons using an ultra-pure liquid hydrogen target. Mastering advanced cryogenic and vacuum technologies, Alain worked hard to reduce the impurity level of the target to negligible values. He also participated in one of the earliest measurements of the pion electromagnetic radius in coincidence (e, e′π) experiments. Later, Alain contributed to one of the first experiments on parity-violation at the MIT–Bates accelerator under the direction of Vernon Hughes. As a member of the (e, e′p) group at Saclay, he devoted great efforts to measuring the proton form factor within the 40Ca nucleus, providing evidence that the bound nucleon form factor has the same Q2 dependence as that of the free nucleon.
At the beginning of the 1990s, Alain switched to high-energy muon scattering. He made important contributions to the SMC polarised target and served as the contact for the collaboration. Later, he became one of the founding members of the COMPASS experiment. As head of the Saclay group, he proposed and led the project for the construction of large-sized drift chambers. He also coordinated the crucial Saclay–CERN work to repair and test the COMPASS large-acceptance superconducting magnet. Project and group leader, accomplished detector expert and tenacious spokesperson, Alain Magnon played an essential role in the success of COMPASS as a unique experiment and as a renowned international collaboration.
All of his colleagues and friends will miss Alain and his rigorous and resolute approach to instrumentation and scientific research.
European Laboratories for Accelerator Based Sciences (EURO-LABS) aims to provide unified transnational access to leading research infrastructures across Europe. Taking over from previously running independent programmes, it brings together the nuclear physics, the high-energy accelerator, and the high-energy detector R&D communities. With 33 partners from European countries, EURO-LABS forms a large network of laboratories and institutes ranging from modest sized test infrastructures to large-scale ESFRI facilities such as SPIRAL2. Its goal is to enable research at the technological frontiers in accelerator and detector development and to open wider avenues in both basic and applied research in diverse topics, from optimal running of reactors to mimicking reactions in the stars. Within this large network, EURO-LABS will ensure diversity and actively support researchers from different nationalities, gender, age, grade, and variety of professional expertise.
Sharing information to support users at test facilities is pivotal. Targeted improvements such as new isotope-enriched targets for high-quality standard medical radioisotope production, improved beam- profile monitors, or magnetic-field measurement instruments in cryogenic conditions will further enhance the capabilities of facilities to address the challenges of the coming decades. Through an active and open data management plan following the FAIR principle, EURO-LABS will act as a gateway for information to facilitate research across disciplines and provide training for young researchers.
Funded by the European Commission, EURO-LABS started on 1 September and will run until August 2026. At the kick off meeting, held in Bologna from 3 to 5 October, presentations offered a detailed overview of the research infrastructures and facilities providing particle and ion beams at energies from meV to GeV. Exchanges during the meeting gave participants a view of the strengths and synergies on offer, planting the seeds for fruitful collaborations.
Prospects for testing and developing techniques for present and future accelerators were among the highlights of the meeting. In the high-energy accelerator sector, this requires state of the art test benches for cryogenic equipment such as magnets, superconducting cavities and associated novel materials, electron and plasma beams, as well as specialised test-beam facilities. Facilities at CERN, DESY and PSI, for example, allow the study of performances and radiation effects on detectors for the HL-LHC and beyond while also enabling nuclei to be explored under extreme conditions. Benefiting from past experiences, a streamlined procedure for handling transnational-access applications to all research infrastructures across the different fields of EURO-LABS was defined.
On the last day of the meeting, the consortium’s governing board, chaired by Edda Gschwendtner (CERN), met for the first time. The governing board further appointed Navin Alahari (GANIL, France) as EURO-LABS scienfitic coordinator, Paolo Giacomelli (INFN-BO, Italy) as project corodinator, Maria Colonna (INFN-LNS, Italy), Ilias Efhymiopoulos (CERN) and Marko Mikuz (Univ.Lubljana, Slovenia) as deputy scientific coordinator and work-package and Maria J G Borge (CSIC, Spain) and Adam Maj (IFJ, Poland) as work-package leaders.
With all facilities declaring their readiness to receive the first transnational users, the next annual meeting will be hosted by IFJ-PAN in Krakow, Poland.
Announced on 4 October, the 2022 Nobel Prize in Physics has been awarded to Alain Aspect, John Clauser and Anton Zeilinger for groundbreaking experiments with entangled photons that open a path to advanced quantum technologies. Working independently in the 1970s and 1980s, their work established the violation of Bell inequalities – as formulated by the late CERN theorist John Bell – and pioneered the field of quantum information science.
First elucidated by Schrödinger in 1935, entanglement sparked a long debate about the physical interpretation of quantum mechanics. Was it a complete theory, or was the paradoxical correlation between entangled particles due to hidden variables that dictate in which state an experiment will find them? In 1964 John Bell proposed a theorem, known as Bell’s inequalities, that allowed this question to be put to the test. It states that if hidden variables are in play, the correlation between the results of a large number of measurements will never exceed a certain value; conversely, if quantum mechanics is complete, this value can be exceeded, as measured experimentally.
John Clauser (J F Clauser & Associates, US) was the first to investigate Bell’s theorem experimentally, obtaining measurements that clearly violated a Bell inequality and thus supported quantum mechanics. Alain Aspect (Université Paris-Saclay and École Polytechnique, France) put the findings on even more solid ground by devising ways to perform measurements of entangled pairs of photons after they had left their source, thus ruling out the effects of the setting in which they were emitted. Using refined tools and a long series of experiments, Anton Zeilinger (University of Vienna, Austria) used entangled states to demonstrate, among other things, quantum teleportation.
These delicate, pioneering experiments not only confirmed quantum theory, but established the basis for a new field of science and technology that has applications in computing, communication, sensing and simulation. In 2020 CERN joined this rapidly growing global endeavour with the launch of the CERN Quantum Technology Initiative.
Foundational work in quantum-information science was also the subject of the 2023 Breakthrough Prize in Fundamental Physics, announced in September, for which Charles H Bennett (IBM), Gilles Brassard (Montréal), David Deutsch (Oxford) and Peter Shor (MIT) will receive $3 million each.
Collaboration, applied research services and innovation networks: these are the reference points of an evolving business development strategy that’s building bridges between DESY’s large-scale research infrastructure and end-users across European industry. The goal: to open up the laboratory’s mission in basic science to support technology innovation and, by extension, deliver at-scale economic and societal impact.
As a German national laboratory rooted in physics, and one of the world’s leading accelerator research centres, DESY’s scientific endeavours are organised along four main coordinates: particle physics, photon science, astroparticle physics and the accelerator physics division. Those parallel lines of enquiry, pursued jointly with an established network of regional and international partners, make DESY a magnet for more than 3000 guest scientists from over 40 countries every year.
In the same way, the laboratory is a coveted research partner for industry and business, its leading-edge experimental facilities offering a unique addition to the R&D pipeline of Europe’s small and medium-sized enterprises as well as established technology companies.
Technology transfer pathways
Industry collaboration with DESY is nothing if not diverse, spanning applied R&D and innovation initiatives across topics such as compact next-generation accelerator technologies, advanced laser systems for quality control in semiconductor-chip production, and the 3D printing of custom resins to create parts for use in ultrahigh-vacuum environments. While such cooperative efforts often involve established companies from many different industries, partners from academic science play an equally significant role – and typically with start-up or technology transfer ambitions as part of the mix.
This is the case for an envisaged spin-off project in which scientists from the University of Hamburg and DESY are working together on a portable liquid biopsy device for medical diagnostics applications (for example, in cancer screening and treatment evaluation). Reciprocity is the key to success here: the university researchers bring their background in nanoanalytics to the project, while DESY physicists contribute deep domain knowledge and expertise on the development of advanced detector technologies for particle physics experiments. As a result, a prototype test station for high-sensitivity in-situ analysis is now in the works, with the interaction of the nanochannels and the detector in the test device representing a significant R&D challenge in terms of precision mechanics (while the DESY team also provides expertise in pattern recognition to accelerate the readout of test results).
Elsewhere, DESY’s MicroTCA Technology Lab (TechLab) represents a prominent case study of direct industry collaboration, fostering the uptake of the MicroTCA.4 open electronics standard for applications in research and industry. Originally developed for the telecommunications market, the standard was subsequently adapted by DESY and its network of industrial partners – among them NAT (Germany), Struck (Germany) and CAENels (Italy) – for deployment within particle accelerator control systems (enabling precision measurements of many analogue signals with simultaneous high-performance digital processing in a single controller).
As such, MicroTCA.4 provides a core enabling technology in the control systems of the European X-ray Free Electron Laser (European XFEL), which runs over a 3.4 km span from DESY’s Hamburg location to the main European XFEL campus in the town of Schenefeld. Another bespoke application of the standard is to be found in the ground-based control centre of the Laser Interferometer Space Antenna (LISA), a space-based gravitational wave detector jointly developed by NASA and the European Space Agency.
Underpinning this technology transfer success is a parallel emphasis on knowledge transfer and education. This is the reason why TechLab, which sits as part of the business development office at DESY, offers a programme of annual workshops and seminars for current and prospective users of MicroTCA.4. The long-term vision, moreover, is to develop a commercially self-sustaining TechLab spin-off based on the development and dissemination of MicroTCA into new applications and markets.
Industry services to fast-track innovation
One of the principal drivers of industrial engagement with DESY is the laboratory’s PETRA III synchrotron light source (comprising a 2.3 km-circumference storage ring and 25 experimental beamlines). This high-brilliance X-ray facility is exploited by academic and industrial scientists to shed light on the structure and behaviour of matter at the atomic and molecular level across a range of disciplines – from clean-energy technologies to drug development and healthcare, from structural biology and nanotech to food and agricultural sciences.
DESY photon scientists and engineers ensure that industrial users are in a position to maximise the return on their PETRA III beam time
Operationally, DESY photon scientists and engineers ensure that industrial users are in a position to maximise the return on their PETRA III beam time, offering a portfolio of services that includes feasibility studies, sample preparation, execution of measurements, as well as downstream analysis of experimental data. Industry customers can request proprietary access to PETRA III via the business development office (under a non-disclosure agreement if necessary), while clients do not even need to come to DESY themselves, with options for a mail-in sample service or even remote access studies in certain circumstances. Publication is not required under this access model, though discounted fees are available to industry users willing to publish a “success story” or scientific paper in partnership with DESY.
Alongside the proprietary route, companies are able to access PETRA III beam time through academic partners. This pathway is free of charge and based on research proposals with strong scientific or socioeconomic impact (established via a competitive review process), with a requirement that results are subsequently published in the formal scientific literature. Notwithstanding the possibilities offered by DESY itself, the laboratory’s on-campus partners – namely the Helmholtz Centre Hereon and the European Molecular Biology Laboratory (EMBL) – use PETRA III to deliver a suite of dedicated services that help industry customers address diverse problems in the applied R&D pipeline.
The German biotech company BioNTech is a case in point. Best known for its successful mRNA vaccine against SARS-Cov-2 infections, BioNTech conducted a programme of X-ray scattering experiments at EMBL’s PETRA III beamline P12. The results of these investigations are now helping the company’s scientists to better package mRNA within nanoparticles for experimental vaccines and drugs. Along an altogether different R&D track, PETRA III has helped industry users gain novel insights into the inner life of conventional AAA batteries – studies that could ultimately lead to products with significantly extended lifetimes. Using non-invasive X-ray diffraction computer tomography, applied under time-resolved conditions, the studies revealed aspects of the battery ageing process by examining phase transformations in the electrodes and electrolyte during charging and discharge.
Building an innovation ecosystem
While access to DESY’s front-line experimental facilities represents a game-changer for many industry customers, the realisation of new commercial products and technologies does not happen in a vacuum. Innovators, for their part, need specialist resources and expert networks to bring their ideas to life – whether that’s in the form of direct investment, strategic consultancy, business and entrepreneurship education, or access to state-of-the-art laboratories and workshops for prototyping, testing, metrology and early-stage product qualification.
DESY is single-minded in its support for this wider “innovation ecosystem”, with a range of complementary initiatives to encourage knowledge exchange and collaboration among early-career scientists, entrepreneurs and senior managers and engineers in established technology companies. The DESY Start-up Office, for example, offers new technology businesses access to a range of services, including management consultancy, business plan development and networking opportunities with potential suppliers and customers. There’s also the Start-up Labs Bahrenfeld, an innovation centre and technology incubator on the DESY Hamburg campus that provides laboratory and office space to young technology companies. The incubator’s current portfolio of 16 start-ups reflects DESY’s pre-eminence in lasers, detectors and enabling photonic technologies, with seven of the companies also targeting applications in the life sciences.
A more focused initiative is the CAROTS 2.0 Startup School, which provides scientists with the core competencies for running their own scientific service companies (intermediary providers of analytical research services to help industry make greater use of large-scale science facilities like DESY). Longer term, the DESY Innovation Factory is set to open in 2025, creating an ambitious vehicle for the commercial development of novel ideas in advanced materials and the life sciences, while fostering cooperation between the research community and technology companies in various growth phases. There will be two locations, one on the DESY campus and one in the nearby innovation and technology park Vorhornweg.
Basic science, applied opportunities
If the network effects of DESY’s innovation ecosystem are a key enabler of technology transfer and industry engagement, so too is the relentless evolution of the laboratory’s accelerator R&D programme. Consider the rapid advances in compact plasma-based accelerators, offering field strengths in the GV/m regime and the prospect of a paradigm shift to a new generation of user-friendly particle accelerators – even potentially “bringing the accelerator to the problem” for specific applications. With a dedicated team working on the miniaturisation of particle accelerators, DESY is intent on maturing plasma technologies for its core areas of expertise in particle physics and photon science while simultaneously targeting medical and industrial use-cases from the outset.
Meanwhile, plans are taking shape for PETRA IV and conversion of the PETRA storage ring into an ultralow-emittance synchrotron source. By generating beams of hard X-rays with unprecedented coherence properties that can be focused down to the nm regime, PETRA IV will provide scientists and engineers with the ultimate 3D process microscope for all manner of industry-relevant problems – whether that’s addressing individual organelles in living cells, following metabolic pathways with elemental and molecular specificity, or observing correlations in functional materials over mm length scales and under working conditions.
Fundamental science never stops at DESY. Neither, it seems, do the downstream opportunities for industrial collaboration and technology innovation.
One hundred years ago, Otto Stern and Walther Gerlach performed their ground-breaking experiment shooting silver atoms through an inhomogeneous magnetic field, separating them according to their spatially quantised angular momentum. It was a clear victory of quantum theory over the still widely used classical picture of the atom. The results also paved the way to the introduction of the concept of spin, an intrinsic angular momentum, as an inherent property of subatomic particles.
The idea of spin was met with plenty of scepticism. Abraham Pais noted in his book George Uhlenbeck and the Discovery of Electron Spin that Ralph Kronig finishing his PhD at Columbia University in 1925 and travelling through Europe, introduced the idea to Heisenberg and Pauli, who dryly commented that “it is indeed very clever but of course has nothing to do with reality”. Feeling ridiculed, Kronig dropped the idea. A few months later, still against strong resistance by established experts but this time with sufficient backing by their mentor Paul Ehrenfest, Leiden graduate-students George Uhlenbeck and Samuel Goudsmit published their seminal Nature paper on the “spinning” electron. “In the future I shall trust my own judgement more and that of others less,” wrote Kronig in a letter to Hendrik Kramers in March 1926.
Spin crisis
Spin quickly became a cornerstone of 20th-century physics. Related works of paramount importance were Pauli’s exclusion principle and Dirac’s description of relativistic spin-1/2 particles, as well as the spin-statistics theorems (namely the Fermi–Dirac and Bose–Einstein distributions for identical half-integer–spin and integer–spin particles, respectively). But more than half a century after its introduction, spin re-emerged as a puzzle. By then, a rather robust theoretical framework, the Standard Model, had been established within which many precision calculations became a comfortable standard. It could have been all that simple: since the proton consists of two valence-up and one valence-down quarks, with spin up and down (i.e. parallel and antiparallel to the proton’s spin, respectively), the origin of its spin is easily explained. The problem dubbed “spin crisis” arose in the late 1980s, when the European Muon Collaboration at CERN found that the contribution of quarks to the proton spin was consistent with zero, within the then still-large uncertainties, and that the so-called Ellis–Jaffe sum rule – ultimately not fundamental but model-dependent – was badly violated. What had been missed?
Today, after decades of intense experimental and theoretical work, our picture of the proton and its spin emerging from high-energy interactions has changed substantially. The role of gluons both in unpolarised and polarised protons is non-trivial. More importantly, transverse degrees of freedom, both in position and momentum space, and the corresponding role of orbital angular momentum, have become essential ingredients in the modern description of the proton structure. This description goes beyond the picture of collinearly moving partons encapsulated by the fraction of the parent proton’s momentum and the scale at which they are probed; numerous effects, unexplainable in the simple picture, have now become theoretically accessible.
Understanding the mysteries
The HERMES experiment at DESY, which operated between 1995 and 2007, has been a pioneer in unravelling the mysteries of the proton spin, and the experiment is the protagonist in a new book by Richard Milner and Erhard Steffens, two veterans in this field as well as the driving forces behind HERMES. The subtitle and preface clarify that this is a personal account and recollection of the history of HERMES, from an emergent idea on both sides of the Atlantic to a nascent collaboration and experiment, and finally as an extremely successful addition to the physics programme of HERA (the world’s only lepton–proton collider, which started running at DESY 30 years ago for one and a half decades).
Milner and Steffens are both experts on polarised gas targets, with complementary backgrounds leading to rather different perspectives. Indeed, HERMES was independently developed within a North American initiative, in which Milner was the driving force, and a European initiative around the Heidelberg MPI-K led by Klaus Rith, with Erhard Steffens as a long-time senior group member. In 1988 two independent letters of intent submitted to DESY triggered sufficient interest in the idea of a fixed-target experiment with a polarised gas target internal to the HERA lepton ring; the proponents were subsequently urged to collaborate in submitting a common proposal. In the meantime, HERMES’ feasibility needed to be demonstrated. A sufficiently high lepton-polarisation had to be established, as well as smooth running of a polarised gas target in the harsh HERA environment without disturbing the machine and the main HERA experiments H1 and Zeus.
By summer 1993, HERMES was fully approved, and in 1995 the data taking started with polarised 3He. The subsequently used target of polarised hydrogen or deuterium employed the same concepts that Stern and Gerlach had already used in their famous experiment. The next decade saw several upgrades and additions to the physics programme, and data taking continued until summer 2007. In all those years, the backbone of HERMES was an intense and polarised lepton beam that traversed a target of pure gas in a storage cell, highly polarised or unpolarised, avoiding extensive and in parts model-dependent corrections. This constellation was combined with a detector that, from the very beginning, was designed to not only detect the scattered leptons but also the “spray” produced in coincidence. These features allowed a diverse set of processes to be studied, leading to numerous pioneering measurements and insights that motivated, and continue to motivate, new experimental programmes around the world, including some at CERN.
Richard Milner and Erhard Steffens provide extensive insights, in particular into the historic aspects of HERMES, which are difficult to obtain elsewhere. The book gives an insightful discussion of the installation of the experiment and of the outstanding efforts of a group of highly motivated and dedicated individuals who worked too often in complete ignorance of (or in defiance of) standard working hours. Their account enthrals the reader with vivid anecdotes, surprising twists and personal stories, all told in a colloquial style. While clearly not meant as a textbook – indeed, one might notice small mistakes and inconsistencies in a few places – this book makes for worthwhile and enjoyable reading, not only for people familiar with the subject but equally for outsiders. In particular, younger generations of physicists working in large-scale collaborations might be surprised to learn that it needs only a small group and little time to start an experiment that goes on to have a tremendous impact on our understanding of nature’s basic constituents.
Released in March 2022 on Disney+, Parallels merges two of the most popular concepts in science fiction: time travel and the multiverse. The series, in French, created by Quoc Dan Trang and directed by Benjamin Rocher and Jean-Baptiste Saurel, is set in a village in the mountains of the French–Swiss border where a particle-physics laboratory called “ERN” and a collider strongly resembling the LHC have a major role.
The story begins with a group of four friends who recently graduated from middle school celebrating one of their birthdays near an area where, 10 years earlier, a kid called Hugo disappeared. At the same time, ERN is performing an experiment with its particle accelerator. However, something goes wrong. The lights go out in the village, while a strange space–time phenomenon unfolds, transporting the teenagers to different timelines once the lights are restored. Does this have anything to do with the particle accelerator? Where, or rather “when” are they? Each of the teenagers tries to unravel their temporal confusion in an attempt to return to their original timeline.
Parallels offers a chance to go beyond fiction and explore the often even more incredible ideas explored for real in particle physics
Although the age of the main characters targets younger audiences, Parallels addresses topics such as depression, regret and family issues, which, combined with some humour, make it relevant to other age groups. The visual effects and music create a suspenseful atmosphere and the compact nature of the series (six episodes of around 35 minutes each) draws the viewer into watching it in a single session.
CERN’s experiments and locations are referenced several times throughout, ranging from visual details in the ERN buildings to mentions of ATLAS, CMS and the Antiproton Decelerator – going so far as to reference an “FCC scheduled for operations in October 2025”. The Globe of Science and Innovation and the CMS silicon tracker are also represented.
Many of the concepts introduced, especially those related to the LHC experiments, are not scientifically accurate. The clear depiction of CERN in all but name may also make some physicists feel uncomfortable, given that the plot plays on YouTube-based conspiracy theories about what CERN’s experiments are capable of. For young science-fiction lovers, however, and especially for those who love to unravel temporal paradoxes, as in the popular Netflix series Stranger Things, Parallels is worth a look. For the more inquisitive and open-minded viewer, it also offers a chance to go beyond fiction and explore the often even more incredible ideas explored for real in particle physics.
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