The EPS High Energy and Particle Physics Prize is awarded for an outstanding contribution in a experimental, theoretical or technological achievement. This year, the recipients are Cecilia Jarlskog for the discovery of an invariant measure of CP violation in both quark and lepton sectors; and the Daya Bay and RENO collaborations for the observation of short-baseline reactor electron-antineutrino disappearance, providing the first determination of the neutrino mixing angle, which paves the way for the detection of CP violation in the lepton sector.
The 2023 Giuseppe and Vanna Cocconi Prize (honouring contributions in particle astrophysics and cosmology in the past 15 years) is awarded to the SDSS/BOSS/eBOSS collaborations for their outstanding contributions to observational cosmology, including the development of the baryon-acoustic oscillation measurement into a prime cosmological tool, using it to robustly probe the history of the expansion rate of the Universe back to one-fifth of its age providing crucial information on dark energy, the Hubble constant, and neutrino masses.
The 2023 Gribov Medal is awarded to Netta Engelhardt for her groundbreaking contributions to the understanding of quantum information in gravity and black-hole physics. This medal goes to early-career researchers working in theoretical physics or field theory.
The 2023 Young Experimental Physicist Prize of the High Energy and Particle Physics Division of the EPS – for early-career experimental physicists – is awarded to Valentina Cairo for her outstanding contributions to the ATLAS experiment: from the construction of the inner tracker, to the development of novel track and vertex reconstruction algorithms and to searches for di-Higgs boson production.
Honouring achievements in outreach, education, and the promotion of diversity, the 2023 Outreach Prize of the High Energy and Particle Physics Division of the EPS is awarded to Jácome (Jay) Armas. It recognizes his outstanding combination of activities on science communication, most notably for the “Science & Cocktails” event series, revolving around science lectures which incorporate elements of the nightlife such as music/art performances and cocktail craftsmanship and reaching out to hundreds of thousands in five different cities world-wide.
“Robust software solutions that deliver step-function advances in accelerator design, engineering and research productivity.” Writ large, that’s the unifying goal shaping the day-to-day work of physicists and software engineers at RadiaSoft, a Boulder, Colorado-based company that specialises in the provision of high-level research, design and scientific consulting services in beamline physics, accelerator science and associated machine-learning technologies. Theirs is a wide-ranging remit that spans electron linacs, free-electron lasers (FELs), synchrotron radiation generated in electron rings, high-intensity proton synchrotrons and accumulator rings, with the RadiaSoft team bringing creativity and novel solutions to the design and simulation of high-power particle beams and directed radiation.
“As an R&D company, we work across an expansive canvas encompassing accelerator physics, enabling platform technologies and engineering for multi-stage accelerator facilities,” Jonathan Edelen, an accelerator physicist and president of RadiaSoft, told CERN Courier. “We work collaboratively with our scientific customers – often funded under the US government’s Small Business Innovation Research (SBIR) programme – and help them tackle physics and engineering problems that they don’t have the expertise or time to solve on their own.”
According to Edelen, the secret of RadiaSoft’s success lies in combining “broad and deep technical domain knowledge” with a wealth of prior project experience at large-scale accelerator facilities plus the development of customised solutions for smaller operations. “That collective know-how,” he said, “allows us to tackle all manner of design and optimisation problems in accelerator science. We work on projects ranging from fundamental studies of esoteric beam dynamics to the simulation and design of RF control systems to the development of machine-learning algorithms for accelerator controls.”
Software innovation
All of this core expertise in accelerator physics and engineering doesn’t stand alone in the RadiaSoft corporate portfolio. Underpinning the consultancy’s value proposition is a software development team that majors on the realisation of intuitive, powerful graphical user interfaces for open-source, high-performance simulation codes. This same team – which aggregates many years of experience working with all sorts of simulation packages for accelerator physics and subsystems – is the driving force behind RadiaSoft’s flagship product Sirepo.
Sirepo is a browser-based, computer-aided engineering (CAE) gateway with graphical user interface. The platform supports over a dozen community-developed particle-accelerator simulation codes, a specialised machine-learning application, a controls-modelling application and a private JupyterHub instance. Users can build simulations with drag-and-drop inputs, collaborate in real-time with shareable URLs, or download their simulations to work from the command line.
“Sirepo makes it easier for users to learn and run these simulation codes, many of which can be difficult to work with,” explained Edelen. “Equally important, RadiaSoft handles all of the dependency packaging, installation and versioning of the codes within Sirepo, while our cloud-based computing architecture allows users to share their simulations by simply sending a URL to colleagues.”
At a granular level, Sirepo aggregates a portfolio of codes to facilitate the modelling of beam dynamics for a range of particle acceleration schemes. Users can employ elegant, for example, to study varied configurations such as linacs and rings, while OPAL can model electron guns, beamlines and linacs with space charge, wake fields and coherent synchrotron radiation. Meanwhile, MAD-X is used for large-scale lattice design, with options for optimisation and export for use in other tracking codes within Sirepo. Users can also delve into time-dependent, 3D modelling for FELs, with Genesis offering simulation for single-pass FELs and extensions to cover oscillators or multi-stage set-ups. Sirepo’s interoperability means that the user can easily coordinate these codes for an end-to-end simulation of the beamline.
Other dedicated applications within Sirepo act as a repository for more specific simulation software for use-cases with magnets, X-ray beamlines, control systems, plasmas, neutronics and machine learning. “The codes within Sirepo are used in the design and optimisation of accelerator facilities worldwide,” noted Edelen, “and are relevant through the full life-cycle of an accelerator – from new-build through commissioning, acceptance and regular operations.”
Next up in the evolution of the RadiaSoft simulation environment is Sirepo Omega. Due to go live over the summer, this unified workflow manager will enable scientific users to create series simulations within the Sirepo “sandbox” – the outputs from one simulation automatically flowing along the simulation chain to generate a final result (with users, if they wish, able to probe individual simulations on a localised basis throughout the workflow).
“Sirepo Omega is all about convergence, automation and enhanced workflow efficiency,” said Edelen. For example, all coordinate transformations between different simulations are handled automatically by Sirepo Omega, while users can track particles automatically through all the sub-simulations. “The user will also get plots and summarised outputs in one convenient location, so there’s no need to go digging into each application to retrieve the results,” Edelen added.
Optimisation is better by design
Another foundational building block of the RadiaSoft service offering is rsopt, a Python framework for testing and running black-box optimisation problems in accelerator science and engineering. As such, the rsopt library and workflow manager give scientists a high degree of modularity when it comes to the choice of optimisation algorithms and code execution methods (including easy-to-use functionality for logging and saving results). Integration with Sirepo also enables templating for a number of codes related to particle accelerator simulation, making it straightforward to parse existing input files and use them without any additional modification.
“As well as ease-of-use and maintainability, rsopt combines a lot of powerful utilities for typical accelerator optimisation problems,” explained Chris Hall, senior scientist at RadiaSoft. A case study in this regard might be an accelerator code representing the design for an FEL – a common requirement being to minimise the bunch length or maximise the peak current while minimising emittance growth.
A recent RadiaSoft collaboration involved the use of the rsopt library for the simulation and training of a non-destructive beam diagnostic capable of characterising the transverse and longitudinal profiles of ultrashort, high-brightness electron beams in FELs and next-generation colliders. “One of the attractions of rsopt,” added Hall, “is the ability to carry out various samplings to generate data for machine-learning models or to get a better understanding of your problem space.”
What’s more, interoperability is hard-wired into rsopt to enable platform-independent execution and scaling to massively parallel systems. “This means users can easily move their code execution between computational environments and utilise algorithms from multiple libraries without having to refactor their own code,” noted Hall.
The future’s bright for RadiaSoft. Having established a sustainable R&D business model in North America over the past 10 years, international expansion is the next step on the group’s commercial roadmap. “Watch this space,” Edelen concluded. “The task for us right now is finding the right path to grow our R&D activity by collaborating with scientists and engineers at large-scale accelerator facilities in Europe.”
This webinar will give a high-level overview of how scientists can model particle accelerators using Sirepo, an open-source scientific computing gateway.
The speaker, Jonathan Edelen, will work through examples using three of Sirepo’s applications that best highlight the different modelling regimes for simulating a free-electron laser.
Jonathan Edelen, president, earned a PhD in accelerator physics from Colorado State University, after which he was selected for the prestigious Bardeen Fellowship at Fermilab. While at Fermilab he worked on RF systems and thermionic cathode sources at the Advanced Photon Source. Currently, Jon is focused on building advanced control algorithms for particle accelerators including solutions involving machine learning.
The direct detection of gravitational waves in 2015 opened a new window to the universe, allowing researchers to study the cosmos by merging data from multiple sources. There are currently four gravitational wave telescopes (GWTs) in operation: LIGO at two sites in the US, Virgo in Italy, KAGRA in Japan, and GEO600 in Germany. Discussions are ongoing to establish an additional site in India. The detection of gravitational waves is based on Michelson laser interferometry with Fabry-Perot cavities, which reveals the expansion and contraction of space at the level of ten-thousandths of the size of an atomic nucleus, i.e. 10-19 m. Despite the extremely low strain that needs to be detected, an average of one gravitational wave is measured per week of measurement by studying and minimising all possible noise sources, including seismic vibration and residual gas scattering. The latter is reduced by placing the interferometer in a pipe where ultrahigh vacuum is generated. In the case of Virgo, the vacuum inside the two perpendicular 3 km-long arms of the interferometer is lower than 10-9 mbar.
While current facilities are being operated and upgraded, the gravitational-wave community is also focusing on a new generation of GWTs that will provide even better sensitivity. This would be achieved by longer interferometer arms, together with a drastic reduction of noise that might require cryogenic cooling of the mirrors. The two leading studies are the Einstein Telescope (ET) in Europe and the Cosmic Explorer (CE) in the US. The total length of the vacuum vessels envisaged for the ET and CE interferometers is 120 km and 160 km, respectively, with a tube diameter of 1 to 1.2 m. The required operational pressures are typical to those needed for modern accelerators (i.e. in the region of 10-10 mbar for hydrogen and even lower for other gas species). The next generation of GWTs would therefore represent the largest ultrahigh vacuum systems ever built.
The next generation of gravitational-wave telescopes would represent the largest ultrahigh vacuum systems ever built.
Producing these pressures is not difficult, as present vacuum systems of GWT interferometers have a comparable degree of vacuum. Instead, the challenge is cost. Indeed, if the previous generation solutions were adopted, the vacuum pipe system would amount to half of the estimated cost of CE and not far from one-third of ET, which is dominated by underground civil engineering. Reducing the cost of vacuum systems requires the development of different technical approaches with respect to previous-generation facilities. Developing cheaper technologies is also a key subject for future accelerators and a synergy in terms of manufacturing methods, surface treatments and installation procedures is already visible.
Within an official framework between CERN and the lead institutes of the ET study – Nikhef in the Netherlands and INFN in Italy – CERN’s TE-VSC and EN-MME groups are sharing their expertise in vacuum, materials, manufacturing and surface treatments with the gravitational-wave community. The activity started in September 2022 and is expected to conclude at the end of 2025 with a technical design report and a full test of a vacuum-vessel pilot sector. During the workshop “Beampipes for Gravitational Wave Telescopes 2023”, held at CERN from 27 to 29 March, 85 specialists from different communities encompassing accelerator and gravitational-wave technologies and from companies that focus on steel production, pipe manufacturing and vacuum equipment gathered to discuss the latest progress. The event followed a similar one hosted by LIGO Livingston in 2019, which gave important directions for research topics.
Plotting a course
In a series of introductory contributions, the basic theoretical elements regarding vacuum requirements and the status of CE and ET studies were presented, highlighting initiatives in vacuum and material technologies undertaken in Europe and the US. The detailed description of current GWT vacuum systems provided a starting point for the presentations of ongoing developments. To conduct an effective cost analysis and reduction, the entire process must be taken into account — including raw material production and treatment, manufacturing, surface treatment, logistics, installation, and commissioning in the tunnel. Additionally, the interfaces with the experimental areas and other services such as civil engineering, electrical distribution and ventilation are essential to assess the impact of technological choices for the vacuum pipes.
The selection criteria for the structural materials of the pipe were discussed, with steel currently being the material of choice. Ferritic steels would contribute to a significant cost reduction compared to austenitic steel, which is currently used in accelerators, because they do not contain nickel. Furthermore, thanks to their body-centred cubic crystallographic structure, ferritic steels have a much lower content of residual hydrogen – the first enemy for the attainment of ultrahigh vacuum – and thus do not require expensive solid-state degassing treatments. The cheapest ferritic steels are “mild steels” which are common materials in gas pipelines after treatment to fight corrosion. Ferritic stainless steels, which contain more than 12% in weight of dissolved chromium, are also being studied for GWT applications. While first results are encouraging, the magnetic properties of these materials must be considered to avoid anomalous transmission of electromagnetic signals and of the induced mechanical vibrations.
Four solutions regarding the design and manufacturing of the pipes and their support system were discussed at the March workshop. The baseline is a 3 to 4 mm-thick tube similar to the ones operational in Virgo and LIGO, with some modifications to cope with the new tunnel environment and stricter sensitivity requirements. Another option is a 1 to 1.5 mm-thick corrugated vessel that does not require reinforcement and expansion bellows. Additionally, designs based on double-wall pipes were discussed, with the inner wall being thin and easy to heat and the external wall performing the structural role. An insulation vacuum would be generated between the two walls without the cleanliness and pressure requirements imposed on the laser beam vacuum. The forces acting on the inner wall during pressure transients would be minimised by opening axial movement valves, which are not yet fully designed. Finally, a gas-pipeline solution was also considered, which would be produced by a half-inch thick wall made of mild steel. The main advantage of this solution is its relatively low cost, as it is a standard approach used in the oil and gas industry. However, corrosion protection and ultrahigh vacuum needs would require surface treatment on both sides of the pipe walls. These treatments are currently under consideration. For all types of design, the integration of optical baffles (which provide an intermittent reduction of the pipe aperture to block scattered photons) is a matter of intense study, with options for position, material, surface treatment, and installation reported. The transfer of vibrations from the tunnel structure to the baffle is also another hot topic.
The manufacturing of the pipes directly from metal coils and their surface treatment can be carried out at supplier facilities or directly at the installation site. The former approach would reduce the cost of infrastructure and manpower, while the latter would reduce transport costs and provide an additional degree of freedom to the global logistics as storage area would be minimized. The study of in-situ production was brought to its limit in a conceptual study of a process that from a coil could deliver pipes as long as desired directly in the underground areas: The metal coil arrives in the tunnel; then it is installed in a dedicated machine that unrolls the coil and welds the metallic sheet to form the pipe to any length.
These topics will undergo further development in the coming months, and the results will be incorporated into a comprehensive technical design report. This report will include a detailed cost optimization and will be validated in a pilot sector at CERN. With just under two and a half years of the project remaining, its success will demand a substantial effort and resolute motivation. The optimism instilled by the enthusiasm and collaborative approach demonstrated by all participants at the workshop is therefore highly encouraging.
Researchers at Jefferson lab in the US have gained a deeper understanding of the role of gluons in providing mass to visible matter. Based on measurements of the photoproduction of J/ψ particles, the findings suggest that the proton’s structure has three distinct regions, with an inner core driven by gluonic interactions making up most of its mass.
Although the charge and spin of the proton have been extensively studied for decades, relatively little is known about its mass distribution. This is because gluons, which despite being massless provide a sizeable contribution to the proton’s mass, are neutral and thus cannot be studied directly using electromagnetic probes. The Jefferson team instead used the gluonic gravitational form factors (GFFs). Similar to electromagnetic form factors, which provide information about a hadron’s charge and magnetisation distributions, the GFFs (technically the matrix elements of the proton’s energy–momentum tensor) encode mechanical properties of the proton such as its mass, density, pressure and shear distributions.
To access the GFFs, the team measured the threshold cross section of exclusive J/ψ photoproduction at different energies by forcing photons with energies between 9.1 and 10.6 GeV to interact with a liquid hydrogen target. Gluons dominate the production of J/ψ at small momentum transfer since J/ψ mesons share no valence quarks with the proton. Due to the J/ψ’s vector quantum numbers, this process can occur at certain energies by gluons in scalar (dilaton-like) and tensor (graviton-like) states. The researchers fed their cross-section results into QCD models describing the gluonic GFFs and extracted the parameters defining the GFFs, enabling them to deduce one mass radius and one scalar radius.
We need a new generation of high-precision J/ψ experiments to get a better picture
Zein-Eddine Meziani
The analysis revealed a scalar proton radius of 1 fm, which is substantially larger than both the charge radius (around 0.85 fm) and the proton mass radius (0.75 fm). This led the team to propose that the proton structure consists of three distinct regions: an inner core that makes up most of the mass radius and is dominated by the tensor gluonic field structure, followed by the charge radius resulting from the relativistic motion of quarks, all enveloped in a larger confining scalar gluon density.
“Given that the proton’s scalar gluon radius is the largest we need to understand how this converts to our understanding of the gluonic structure of nuclei. For example, what would be the scalar radius of 4He compared to its charge radius?” says study leader Zein-Eddine Meziani of Argonne. The team plans to extend its studies to include the J/ψ muon final state decay, doubling the statistics of the current measurement, and to extract the gluon pressure distribution. “It is hard to say much right now, but this is a field in its infancy and the direct role of gluons in nuclei is not well understood,” adds Meziani. “We need a new generation of high-precision J/ψ experiments to get a better picture.”
Giorgio Brianti, a pillar of CERN throughout his 40-year career, passed away on 6 April at the age of 92. He played a major role in the success of CERN and in particular the LEP project, and his legacy lives on across the whole of the accelerator complex.
Giorgio began his engineering studies at the University of Parma and continued them for three years in Bologna, where he obtained his laurea degree in May 1954. Driven by a taste for research, he learned, thanks to his thesis advisor, that Edoardo Amaldi was setting up an international organization in Geneva called CERN and was invited to meet him in Rome in June 1954. In his autobiography – written for his family and friends – Giorgio describes this meeting as follows: “Edoardo Amaldi received me very warmly and, after various discussions, he said to me: ‘you can go home: you will receive a letter of appointment from Geneva soon’. I thus had the privilege of participating in one of the most important intellectual adventures in Europe, and perhaps the world, which in half a century has made CERN ‘the’ world laboratory for particle physics.”
Giorgio had boundless admiration for John Adams, who had been recruited by Amaldi a year earlier, recounting: “John was only 34 years old, but had a very natural authority. To say that we had a conversation would be an exaggeration, due to my still very hesitant English, but I understood that I was assigned to the magnet group”. After participating in the design of the main bending magnets for the Proton Synchrotron, Giorgio was sent by Adams to Genoa for three years to supervise the construction of 100 magnets made by the leading Italian company in the sector, Ansaldo. Upon his return, he was entrusted with the control group and in 1964 he was appointed head of the synchro-cyclotron (SC) division. After only four years he was asked to create a new division to build a very innovative synchrotron – the Booster – capable of injecting protons into the PS and significantly increasing the intensity of the accelerated current. He described this period as perhaps his happiest from a technical point of view. Adams – who had been appointed Director General of the new CERN-Lab II to construct the 400 GeV Super Proton Synchrotron (SPS) – also entrusted Giorgio with designing and building the experimental areas and their beam lines. The 40th anniversary of their inauguration was celebrated with him in 2018 and the current fixed-target experimental programme profits to this day from his foresight.
Giorgio has left us not only an intellectual but also a spiritual legacy
In January 1979 Giorgio was made head of the SPS division, but only two years later he was called to a more important role, that of technical director, by the newly appointed Director General Herwig Schopper. As Giorgio writes: “The main objectives of the mandate were to build the LEP… which was to be installed in a 27 km circumference tunnel over 100 m deep, and to complete the SPS proton-antiproton program, a very risky enterprise, but whose success in 1982 and 1983 was decisive for the future of CERN”. The enormous technical work required to transform the SPS into a proton-antiproton collider that went on to discover the W and Z bosons took place in parallel with the construction of LEP and the launch of the Large Hadron Collider (LHC) project, which Giorgio personally devoted himself to starting in 1982.
The LHC occupied Giorgio for nearly 15 years, starting from almost nothing. As he writes: “It was initially a quasi-clandestine activity to avoid possible reactions from the delegates of the Member States, who would not have understood an initiative parallel to that of the LEP. The first public appearance of the potential project, which already bore the name Large Hadron Collider, took place at a workshop held in Lausanne and at CERN in the spring of 1984.”
The LHC project received a significant boost from Carlo Rubbia, who became Director General in 1989 and appointed Giorgio as director of future accelerators. While LEP was operating at full capacity during these years, under his leadership new technologies were developed and the first prototypes of high-field superconducting magnets were created. The construction programme for the LHC was preliminarily approved in 1994, under the leadership of Chris Llewellyn Smith. In 1996, one year after Giorgio’s retirement, the final approval was granted. Giorgio continued to work, of course! In particular, in 1996 he agreed to chair the advisory committee of the Proton Ion Medical Machine Study, a working group established within CERN aimed at designing and developing a new synchrotron for medical purposes for the treatment of radio-resistant tumours with carbon ion beams. The first centre was built in Italy, in Pavia, by the Italian Foundation National Centre for Oncological Hadrontherapy (CNAO). He was also an active member of the editorial board of the book “Technology meets Research,” which celebrated 60 years of interaction at CERN between technology and fundamental science.
Giorgio has left us not only an intellectual but also a spiritual legacy. He was a man of great moral rigour, with a strong and contemplative Christian faith, determined to achieve his goals but mindful not to hurt others. He was very attached to his family and friends. His intelligence, kindness, and generosity shone through his eyes and – despite his reserved character – touched the lives of everyone he met.
On 27 March, during the 30th edition of the Deep-Inelastic Scattering and Related Subjects workshop (DIS2023) held in Michigan, Adinda de Wit and Yong Zhao received the 2023 Guido Altarelli Awards for experiment and theory. The prizes, named after CERN’s Guido Altarelli, who made seminal contributions to QCD, recognise exceptional achievements from young scientists in deep-inelastic scattering and related subjects.
CMS collaborator Adinda de Wit (University of Zurich) was awarded the experimental prize for her achievements in understanding the nature of the Higgs boson, including precision studies of its couplings and decay channels. She received her PhD from Imperial College London, then took up a postdoc position at DESY followed by the University of Zurich and is presently at LLR. Co-convener of the CMS Higgs physics analysis group and past co-convener of the CMS Higgs combination and properties group, de Wit also received the Herta-Sponer-Prize by the German Physical Society.
Yong Zhao (Argonne National Laboratory) was awarded the theory prize for fundamental contributions to ab initio calculations of parton distributions in lattice QCD. He received his PhD from the University of Maryland, and then held postdoc positions at Brookhaven and MIT before joining Argonne laboratory as an assistant physicist. Yong also received the 2022 Kenneth G. Wilson Award for Excellence in Lattice Field Theory for fundamental contributions to calculations of parton physics on lattice.
During the award ceremony, Nobel laureate Giorgio Parisi joined in via Zoom to reminisce about his collaboration with Altarelli. Together they contributed to QCD evolution equations for parton densities, known as the Altarelli-Parisi or DGLAP equations.
The DIS series covers a large spectrum of topics in high-energy physics. One part of the conference is devoted to the most recent results from large experiments at Brookhaven, CERN, DESY, Fermilab, Jlab and KEK, as well as corresponding theoretical advances. The workshop demonstrated how DIS and related subjects permeate a broad range of physics topics from hadron colliders to spin physics, neutrino physics and more. The next workshop will be held in Grenoble, France from 8-12 April 2024.
On 13 and 14 March CERN hosted an international workshop on atom interferometry and the prospects for future large-scale experiments employing this quantum-sensing technique. The workshop had some 300 registered participants, of whom about half participated in person. As outlined in a keynote introductory colloquium by Mark Kasevich (Stanford), one of the pioneers of the field, this quantum sensing technology holds great promise for making ultra-sensitive measurements in fundamental physics. Like light interferometry, atom interferometry involves measuring interference patterns, but between atomic wave packets rather than light waves. Interactions between coherent waves of ultralight bosonic dark matter and Standard Model particles could induce an observable shift in the interference phase, as could the passage of gravitational waves.
Atom interferometry is a well-established concept that can provide exceptionally high sensitivity, e.g., to inertial/gravitational effects. Experimental designs take advantage of features used by state-of-the-art atomic clocks in combination with established techniques for building inertial sensors. This makes atom interferometry an ideal candidate to hunt for physics beyond the Standard Model such as waves of ultralight bosonic dark matter, or to measure gravitational waves in a frequency range around 1 Hz that is inaccessible to laser interference experiments on Earth, such as LIGO, Virgo and KAGRA, or the upcoming space-borne experiment LISA. As discussed during the workshop, measurements of gravitational waves in this frequency range could reveal mergers of black holes with masses intermediate between those accessible to laser interferometers, casting light on the formation of the supermassive black holes known to inhabit the centres of galaxies. Atom interferometer experiments can also explore the limits of quantum mechanics and its interface with gravity, for example by measuring a gravitational analogue of the Aharonov-Bohm effect.
A deep shaft at Point 4 of the LHC is a promising location for an atom interferometer with a vertical baseline of over 100 m
Although the potential of atom interferometers for fundamental scientific measurements was the principal focus of the meeting, it was also emphasised that technologies based on the same principles also have wide-ranging practical applications. These include gravimetry, geodesy, navigation, time-keeping and Earth observation from space, providing, for example, a novel and sensitive technique for monitoring the effects of climate change through measurements of the Earth’s gravitational field.
Several large atom interferometers with a length of 10m already exist, for example at Stanford University, or are planned, for example in Hanover (VLBAI), Wuhan and at Oxford University (AION). However, many of the proposed physics measurements require next-generation setups with a length of 100m, and such experiments are under construction at Fermilab (MAGIS), in France (MIGA) and in China (ZAIGA). The Atomic Interferometric Observatory and Network (AION) collaboration is evaluating possible sites in the UK and at CERN. In this context, a recent conceptual feasibility study supported by the CERN Physics Beyond Colliders study group concluded that a deep shaft at Point 4 of the LHC is a promising location for an atom interferometer with a vertical baseline of over 100 m. The March workshop provided a forum for discussing such projects, their current status, future plans and prospective sensitivities.
Looking further ahead, participants discussed the prospects for one or more km-scale atom interferometers, which would provide the maximal sensitivity possible with a terrestrial experiment to search for ultralight dark matter and gravitational waves. It was agreed that the global community interested in such experiments would work together towards establishing an informal proto-collaboration that could develop the science case for such facilities, provide a forum for exchanging ideas how to develop the necessary technological advances and develop a roadmap for their realisation.
A highlight of the workshop was a poster session that provided an opportunity for 30 early-career researchers to present their ideas and current work on projects exploiting the quantum properties of cold atoms and related topics. The liveliness of this session showed how this interdisciplinary field at the boundaries between atomic physics, particle physics, astrophysics and cosmology is inspiring the next generation of researchers. These researchers may form the core of the team that will lead atom interferometers to their full potential.
Energetic beams of particles are used to explore the fundamental forces of nature, produce known and unknown particles such as the Higgs boson at the LHC, and generate new forms of matter, for example at the future FAIR facility. Photon science also relies on particle beams: electron beams that emit pulses of intense synchrotron light, including soft and hard X-rays, in either circular or linear machines. Such light sources enable time-resolved measurements of biological, chemical and physical structures on the molecular down to the atomic scale, allowing a diverse global community of users to investigate systems ranging from viruses and bacteria to materials science, planetary science, environmental science, nanotechnology and archaeology. Last but not least, particle beams for industry and health support many societal applications ranging from the X-ray inspection of cargo containers to food sterilisation, and from chip manufacturing to cancer therapy.
This scientific success story has been made possible through a continuous cycle of innovation in the physics and technology of particle accelerators, driven for many decades by exploratory research in nuclear and particle physics. The invention of radio-frequency (RF) technology in the 1920s opened the path to an energy gain of several tens of MeV per metre. Very-high-energy accelerators were constructed with RF technology, entering the GeV and finally the TeV energy scales at the Tevatron and the LHC. New collision schemes were developed, for example the mini “beta squeeze” in the 1970s, advancing luminosity and collision rates by orders of magnitudes. The invention of stochastic cooling at CERN enabled the discovery of the W and Z bosons 40 years ago.
However, intrinsic technological and conceptual limits mean that the size and cost of RF-based particle accelerators are increasing as researchers seek higher beam energies. Colliders for particle physics have reached a circumference of 27 km at LEP/LHC and close to 100 km for next-generation facilities such as the proposed Future Circular Collider. Machines for photon science, operating in the GeV regime, occupy a footprint of up to several km and the approval of new facilities is becoming limited by physical and financial constraints. As a result, the exponential progress in maximum beam energy that has taken place during the past several decades has started to saturate (see “Levelling off” figure). For photon science, where beam-time on the most powerful facilities is heavily over-subscribed, progress in scientific research and capabilities threatens to become limited by access. It is therefore hoped that the development of innovative and compact accelerator technology will provide a practical path to more research facilities and ultimately to higher beam energies for the same investment.
At present the most successful new technology relies on the concept of plasma acceleration. Proposed in 1979, this technique promises energy gains up to 100 GeV per metre of acceleration and therefore up to 1000 times higher than is possible in RF accelerators. In essence, the metallic walls of an RF cavity, with their intrinsic field limitations, are replaced by a dynamic and robust plasma structure with very high fields. First, the free electrons in a neutral plasma are used to convert the transverse ponderomotive force of a laser, or the transverse space charge force of a charged particle beam, into a longitudinal accelerating field. While the “light” electrons in the plasma column are expelled from the path of the driving force, the “heavy” plasma ions remain in place. The ions therefore establish a restoring force and re-attract the oscillating plasma electrons. A plasma cavity forms behind the drive pulse in which the main electron beam is placed and accelerated with up to 100 GV per metre. Difficulties in the plasma-acceleration scheme arise from the small scales involved (sub-mm transverse diameter), the required micrometre tolerances and stability. Different concepts include laser-driven plasma wakefield acceleration (LWFA), electron-driven plasma wakefield acceleration (PWFA) and proton-beam-driven plasma wakefield acceleration. Gains in electron energy have reached 8 GeV (BELLA, Berkeley), 42 GeV (FFTB, SLAC) and 2 GeV (AWAKE, CERN) in these three schemes, respectively.
At the same time, the beam quality of plasma-acceleration schemes has advanced sufficiently to reach the quality required for free-electron lasers (FELs): linac-based facilities that produce extremely brilliant and short pulses of radiation for the study of ultrafast molecular and other processes. There have been several reports of free-electron lasing in plasma-based accelerators in recent years, one relying on LWFA by a team in China and one on PWFA by the EuPRAXIA team in Frascati, Italy. Another publication by a French and German team has recently demonstrated seeding of the FEL process in a LWFA plasma accelerator.
Scientific and technical progress in plasma accelerators is driven by several dozen groups and a number of major test facilities worldwide, including internationally leading programmes at CERN, STFC, CNRS, DESY, various centres and institutes in the Helmholtz Association, INFN, LBNL, RAL, Shanghai XFEL, SCAPA, SLAC, SPRING-8, Tsinghua University and others. In Europe, the 2020 update of the European strategy for particle physics included plasma accelerators as one of five major themes, and a strategic analysis towards a possible plasma-based collider was published in a 2022 CERN Yellow Report on future accelerator R&D.
Enter EuPRAXIA
In 2014 researchers in Europe agreed that a combined, coordinated R&D effort should be set up to realise a larger plasma-based accelerator facility that serves as a demonstrator. The project should aim to produce high-quality 5 GeV electron beams via innovative laser- and electron-driven plasma wakefield acceleration, achieving a significant reduction in size and possible savings in cost over state-of-the-art RF accelerators. This project was named the European Plasma Research Accelerator with Excellence in Applications (EuPRAXIA) and it was agreed that it should deliver pulses of X rays, photons, electrons and positrons to users from several disciplines. EuPRAXIA’s beams will mainly serve the fields of structural biology, chemistry, material science, medical imaging, particle-physics detectors and archaeology. It is not a dedicated particle-physics facility but will be an important stepping stone towards any plasma-based collider.
The EuPRAXIA project started in 2015 with a design study, which was funded under the European Union (EU) Horizon 2020 programme and culminated at the end of 2019 with the publication of the worldwide first conceptual design report for a plasma-accelerator facility. The targets set out in 2014 could all be achieved in the EuPRAXIA conceptual design. In particular, it was shown that sufficiently competitive performances could be reached and that an initial reduction in facility size by a factor of two-to-three is indeed achievable for a 5 GeV plasma-based FEL facility. The published design includes realistic constraints on transfer lines, facility infrastructure, laser-lab space, undulator technologies, user areas and radiation shielding. Several innovative solutions were developed, including the use of magnetic chicanes for high quality, multi-stage plasma accelerators. The EuPRAXIA conceptual design report was submitted to peer review and published in 2020.
The EuPRAXIA implementation plan proposes a distributed research infrastructure with two construction and user sites and several centres of excellence. The presently foreseen centres, in the Czech Republic, France, Germany, Hungary, Portugal and the UK, will support R&D, prototyping and the construction of machine components for the two user sites. This distributed concept will ensure international competitiveness and leverage existing investments in Europe in an optimal way. Having received official government support from Italy, Portugal, the Czech Republic, Hungary and UK, the consortium applied in 2020 to the European Strategy Forum on Research Infrastructures (ESFRI). The proposed facility for a free-electron laser was then included in the 2021 ESFRI roadmap, which identifies those research facilities of pan-European importance that correspond to the long-term needs of European research communities. EuPRAXIA is the first ever plasma-accelerator project on the ESFRI roadmap and the first accelerator project since the 2016 placement of the High-Luminosity LHC.
Stepping stones to a user facility
In 2023 the European plasma-accelerator community received a major impulse for the development of a user-ready plasma-accelerator facility with the funding of several multi-million euro initiatives under the umbrella of the EuPRAXIA project. These are the EuPRAXIA preparatory phase, EuPRAXIA doctoral network and EuPRAXIA advanced photon sources, as well as funding for the construction of one of the EuPRAXIA sites in Frascati, near Rome (see “Frascati future” image).
The EU, Switzerland and the UK have awarded €3.69 million to the EuPRAXIA preparatory phase, which comprises 34 participating institutes from Italy, the Czech Republic, France, Germany, Greece, Hungary, Israel, Portugal, Spain, Switzerland, the UK, the US and CERN as an international organisation. The new grant will give the consortium a unique chance to prepare the full implementation of EuPRAXIA over the next four years. The project will fund 548 person-months, including additional funding from the UK and Switzerland, and will be supported by an additional 1010 person-months in-kind. The preparatory-phase project will connect research institutions and industry from the above countries plus China, Japan, Poland and Sweden, which signed the EuPRAXIA ESFRI consortium agreement, and define the full implementation of the €569 million EuPRAXIA facility as a new, distributed research infrastructure for Europe.
Alongside the EuPRAXIA preparatory phase, a new Marie Skłodowska-Curie doctoral network, coordinated by INFN, has also been funded by the EU and the UK. The network, which started in January 2023 and benefits from more than €3.2 million over its four-year duration, will offer 12 high-level fellowships between 10 universities, six research centres and seven industry partners that will carry out an interdisciplinary and cross-sector plasma-accelerator research and training programme. The project’s focus is on scientific and technical innovations, and on boosting the career prospects of its fellows.
EuPRAXIA at Frascati
Italy is supporting the EuPRAXIA advanced photon sources project (EuAPS) with €22 million. This project has been promoted by INFN, CNR and Tor Vergata University of Rome. EuAPS will fulfil some of the scientific goals defined in the EuPRAXIA conceptual design report by building and commissioning a distributed user facility providing users with advanced photon sources; these consist of a plasma-based betatron source delivering soft X-rays, a mid-power, high-repetition-rate laser and a high-power laser. The funding comes in addition to about €120 million for construction of the beam-driven facility and the FEL facility of EuPRAXIA at Frascati. R&D activities for the beam-driven facility are currently being performed at the INFN SPARC_LAB laboratory.
EuPRAXIA is the first ever plasma-accelerator project on the ESFRI roadmap
EuPRAXIA will be the user facility of the future for the INFN Frascati National Laboratory. The European site for the second, laser-driven leg of EuPRAXIA will be decided in 2024 as part of the preparatory-phase project. Present candidate sites include ELI-Beamlines in the Czech Republic, the future EPAC facility in the UK and CNR in Italy. With its foreseen electron energy range of 1–5 GeV, the facility will enable applications in diverse domains, for instance, as a compact free-electron laser, compact sources for medical imaging and positron generation, tabletop test beams for particle detectors, and deeply penetrating X-ray and gamma-ray sources for materials testing. The first parts of EuPRAXIA are foreseen to enter into operation in 2028 at Frascati and are designed to be a stepping stone for possible future plasma-based facilities, such as linear colliders at the energy frontier. The project is driven by the excellence, ingenuity and hard work of several hundred physicists, engineers, students and support staff who have worked on EuPRAXIA since 2015, connecting, at present, 54 institutes and industries from 18 countries in Europe, Asia and the US.
Following its year-end technical stop (YETS) beginning on 28 November 2022, the LHC is springing back to life to continue Run 3 operations at the energy frontier. The restart process began on 13 February with the beam commissioning of Linac4, which was upgraded during the technical stop to allow a 30% increase in the peak current, to be taken advantage of in future runs. On 3 March the Proton Synchrotron Booster began beam commissioning, followed by the Proton Synchrotron (PS).
In the early hours of 17 March, the PS sent protons down the transfer lines to the door of the Super Proton Synchrotron (SPS), and the meticulous process of adjusting the thousands of machine parameters began. Following a rigorous beam-based realignment campaign, and a brief interruption to allow transport and metrology experts to move selected magnets, sometimes by only a fraction of a millimetre, SPS operators re-injected the beam and quantified and validated the orbit correction ready for injection into the LHC. Right on schedule, on 28 March the first beams successfully entered the LHC. Thanks to very fast threading, both beams were circulating the same day, even producing first “splash” events in the detectors. As the Courier went to press, the intensity ramp-up was under way. Collisions in the LHC are expected to commence by the end of April, heralding the start of a relatively short but intense physics run that is scheduled to end on 30 October.
Refinements
Among many improvements to the accelerator complex made during the YETS, four modules in the SPS kicker system were upgraded to reduce the amount of heat deposited by the beam, and new instruments were installed in the LHC tunnel. These include the beam gas curtain, which will provide 2D images of the alignment of the beams to make data-taking more precise. Ten years in the making, the device was designed for the high-luminosity upgrade of the LHC (HL-LHC) as part of a collaboration between CERN, Liverpool University, the Cockcroft Institute and GSI.
“It’s a challenging year ahead, with the 2023 run length reduced by 20% for energy cost reasons,” says Rende Steerenberg, head of the operations group. “But we maintain the integrated-luminosity goal of 75 fb–1 by enhancing the beam performance and maximising beam availability.”
To cope with the higher luminosities during Run 3, and to prepare for a further luminosity leap at the HL-LHC beginning in 2029, many upgrades to the four main LHC experiments took place during Long Shutdown 2 (LS2) from 2019 to 2022. While the bulk of HL-LHC upgrades for ATLAS and CMS will take place during LS3, beginning in 2026, the ALICE and LHCb detectors underwent significant transformations during LS2. In the final weeks leading to the LHC restart, the LHCb collaboration completed the last element of its Upgrade 1 – the upstream tracker.
This advanced silicon-strip detector, located at the entrance of the LHCb bending magnet, allows fast determination of track momenta. This speeds up the LHCb trigger by a factor of three, which is vital to operate the newly installed 40 MHz fully software-based trigger. The new tracker will also improve the reconstruction efficiency of long-lived particles that decay after the vertex locator (VELO), and will provide better coverage overall, especially in the very forward regions. It is composed of 968 silicon-hybrid modules arranged in four vertical planes to handle the varying occupancy over the detector acceptance. A dedicated front-end ASIC, the “SALT chip”, provides pulse shaping with fast baseline restoration and digitisation, while nearby detector electronics implement the transformation to optical signals that are transmitted to the remote data-acquisition system in LHCb’s new data centre. Institutes from the US, Italy, Switzerland, Poland and China were involved in designing, building and testing the upstream tracker. Assembly began in 2021 and intensive work took place underground throughout the recent YETS, so the device installation was successfully completed by cavern closure on 27 March.
Under pressure
However, earlier in the year, there was an incident that affected another LHCb subdetector, the VELO. This occurred on 10 January, when there was a loss of control of the LHC primary vacuum system at the interface with the VELO. At the time, the primary and secondary vacuum volumes were filled with neon as the installation of the upstream tracker was taking place. A failure in one of the relays in the overpressure safety system not only prevented the safety system from triggering at the appropriate time, but also led to an issue with the power supply that supports some of the machine instrumentation, causing the pressure balancing system to mistakenly pump on the primary volume. The subsequent pressure build-up went beyond specification limits and led to a plastic deformation of the mechanical interface – an ultrathin aluminium shield called the “RF box” – between the LHC and detector volumes. The RF box is mechanically linked to the VELO and a change in its shape affects the degree to which the VELO can be moved and centred around the colliding beams.
To minimise any risk of impact on the other LHC experiments, the LHCb collaboration will wait until this year’s YETS to replace the RF box. In the meantime, the collaboration has been developing ways to mitigate the impact on data-taking, explains LHCb spokesperson Chris Parkes of the University of Manchester: “Initially we were very concerned that the VELO could have been damaged, but fortunately this is not the case. After much careful recovery work, we will be able to operate the system in 2023, and after the RF box is replaced, we will be back to full performance.”
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Jonathan Edelen, president, earned a PhD in accelerator physics from Colorado State University, after which he was selected for the prestigious Bardeen Fellowship at Fermilab. While at Fermilab he worked on RF systems and thermionic cathode sources at the Advanced Photon Source. Currently, Jon is focused on building advanced control algorithms for particle accelerators including solutions involving machine learning.
