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CERN’s prowess in nothingness

A miniature robot for in situ surface treatments of LHC beam screens, pictured inside the 74 mm-aperture beam screen of a superconducting magnet. The whole robot moves axially along the beam screen via inchworm steps while its “head” turns to direct a laser in the radial direction. Image credit: CERN.

From freeze-dried foods to flat-panel displays and space simulation, vacuum technology is essential in many fields of research and industry. Globally, vacuum technologies represent a multi-billion-dollar, and growing, market. However, it is only when vacuum is applied to particle accelerators for high-energy physics that the technology displays its full complexity and multidisciplinary nature – which bears little resemblance to the common perception of vacuum as being just about pumps and valves.

Particle beams require extremely low pressure in the pipes in which they travel to ensure that their lifetime is not limited by interactions with residual gas molecules and to minimise backgrounds in the physics detectors. The peculiarity of particle accelerators is that the particle beam itself is the cause of the main source of gas: ions, protons and electrons interact with the wall of the vacuum vessels and extract gas molecules, either due to direct beam losses or mediated by photons (synchrotron radiation) and electrons (for example by “multipacting”).

Nowadays, vacuum technology for particle accelerators is focused on this key challenge: understand, simulate, control and mitigate the direct and indirect effects of particle beams on material surfaces. It is thanks to major advances made at CERN and elsewhere in this area that machines such as the LHC are able to achieve the high beam stability that they do.

Since it is in the few-nanometre-thick top slice of materials that vacuum technology concentrates most effort, CERN has merged in the same group: surface-physics specialists, thin-film coating experts and galvanic-treatment professionals, together with teams of designers and colleagues dedicated to the operation of large vacuum equipment. Bringing this expertise together “under one roof” makes CERN one of the world’s leading R&D centres for extreme vacuum technology, contributing to major existing and future accelerator projects at CERN and beyond.

Intersecting history

Vacuum technology for particle accelerators has been pioneered by CERN since its early days, with the Intersecting Storage Rings (ISR) bringing the most important breakthroughs. At the turn of the 1960s and 1970s, this technological marvel – the world’s first hadron collider – required proton beams of unprecedented intensity (of the order of 10 A) and extremely low vacuum pressures in the interaction areas (below 10–11 mbar). The former challenge stimulated studies about ion instabilities and led to innovative surface treatments – for instance glow-discharge cleaning – to mitigate the effects. The low-vacuum requirement, on the other hand, drove the development of materials and their treatments – both chemical and thermal – in addition to novel high-performance cryogenic pumps and vacuum gauges that are still in use today. The technological successes of the ISR also allowed a direct measurement in the laboratory of the lowest ever achieved pressure at room temperature, 2 × 10–14 mbar, a record that still stands today.

The Large Electron Positron collider (LEP) inspired the next chapter in CERN’s vacuum story. Even though LEP’s residual gas density and current intensities were less demanding than those of the ISR, the exceptional length and the intense synchrotron-light power distributed along its 27 km ring triggered the need for unconventional solutions at reasonable cost. Responding to this challenge, the LEP vacuum team developed extruded aluminium vacuum chambers and introduced, for the first time, linear pumping by non-evaporable getter (NEG) strips.

Fig. 1. The copper substrate of the HIE-ISOLDE superconducting radio-frequency cavities. The surface treatment of copper is one of the most important competences needed to achieve the required performance. Image credit: CERN

In parallel, LEP project leader Emilio Picasso launched another fruitful development that led to the production of the first superconducting radio-frequency (RF) cavities based on niobium thin-film coating on copper substrates. The ability to attain very low vacuum gained with the ISR, the acquired knowledge in film deposition, and the impressive results obtained in surface treatments of copper were the ingredients for success. The present accelerating RF cavities of the LHC and HIE-ISOLDE (figure 1) are essentially based on the expertise assimilated for LEP (CERN Courier May 2018 p26).

The coexistence in the same team of both NEG and thin-film expertise was the seed for another breakthrough in vacuum technology: NEG thin-film coatings, driven by the LHC project requirements and the vision of LHC project leader Lyn Evans. The NEG material, a micron-thick coating made of a mixture of titanium, zirconium and vanadium, is deposited onto the inner wall of vacuum chambers and, after activation by heating in the accelerator, provides pumping for most of the gas species present in accelerators. The Low Energy Ion Ring (LEIR) was the first CERN accelerator to implement extensive NEG coating in around 2006. For the LHC, one of the technology’s key benefits is its low secondary-electron emission, which suppresses the growth of electron clouds in the room-temperature part of the machine (figure 2).

Fig. 2. The central beam pipe of CMS is entirely coated with NEG materials on the inside to ensure the lowest possible beam-gas background to the detector and suppress electron-cloud development. Image credit: CERN

Studying clouds

Electron clouds had to be studied in depth for the LHC. CERN’s vacuum experts provided direct measurements of the effect in the Super Proton Synchrotron (SPS) with LHC beams, contributing to a deeper understanding of electron emission from technical surfaces over a large range of temperatures. New concepts for vacuum systems at cryogenic temperatures were invented, in particular the beam screen. Conceived at BINP (Russia) and further developed at CERN, this key technology is essential in keeping the gas density stable and to reduce the heat load to the 1.9 K cold-mass temperature of the magnets. This non-exhaustive series of advancements is another example of how CERN’s vacuum success is driven by the often daunting requirements of new projects to pursue fundamental research.

Preparing for the HL-LHC

As the LHC restarts this year for the final stage of Run 2 at a collision energy of 13 TeV, preparations for the high-luminosity LHC (HL-LHC) upgrade are getting under way. The more intense beams of HL-LHC will amplify the effect of electron clouds on both the beam stability and the thermal load to the cryogenic systems. While NEG coatings are very effective in eradicating electron multipacting, their application is limited for room-temperature beam pipes that needed to be heated (“bakeable” in vacuum jargon) to around 200 °C to activate them. Therefore, an alternative strategy has to be found for the parts of the accelerators that cannot be heated, for example those in the superconducting magnets of the LHC and the vacuum chambers in the SPS.

Thin-film coatings made from carbon offer a solution. The idea originated at CERN in 2006 following the observation that beam-scrubbed surfaces – those that have been cleared of trapped gas molecules which increase electron-cloud effects – are enriched in graphite-like carbon. During the past 10 years, this material has been the subject of intense study at CERN. Carbon’s characteristics at cryogenic temperatures are extremely interesting in terms of gas adsorption and electron emission, and the material has already been deposited on tens of SPS vacuum chambers within the LHC Injectors Upgrade project (CERN Courier October 2017 p32). By far, the HL-LHC project presents the most challenging activity in the coming years, namely the coating of the beam screens inserted in the triplet magnets to be situated on both sides of the four LHC experiments to squeeze the protons into tighter bunches. A dedicated sputtering source has been developed that allows alternate deposition of titanium, to improve adherence, and carbon. At the end of the process, the latter layer will be just 50 nm thick.

Another idea to fight electron clouds for the HL-LHC, originally proposed by researchers at the STFC Accelerator Science and Technology Centre (ASTeC) and the University of Dundee in the UK, involves laser-treating surfaces to make them more rough: secondary electrons are intercepted by the surrounding surfaces and cannot be accelerated by the beam. In collaboration with UK researchers and GE Inspection Robotics, CERN’s vacuum team has recently developed a miniature robot that can direct the laser onto the LHC beam screen (“Miniature robot” image). The possibility of in situ surface treatments by lasers opens new perspectives for vacuum technology in the next decades, including studies for future circular colliders.

An additional drawback of the HL-LHC’s intense beams is the higher rate of induced radioactivity in certain locations: the extremities of the detectors, owing to the higher flux of interaction debris, and the collimation areas due to the increased proton losses. To minimise the integrated radioactive dose received by personnel during interventions, it is necessary to properly design all components and define a layout that facilitates and accelerates all manual operations. Since a large fraction of the intervention time is taken up by connecting pieces of equipment, remote assembling and disassembling of flanges is a key area for potential improvements.

One interesting idea that is being developed by CERN’s vacuum team, in collaboration with the University of Calabria (Italy), concerns shape-memory alloys. Given appropriate thermomechanical pre-treatment, a ring of such materials delivers radial forces that tighten the connection between two metallic pipes: heating provokes the clamping, while cooling generates the unclamping. Both actions can be easily implemented remotely, reducing human intervention significantly. Although the invention was motivated by the HL-LHC, it has other applications that are not yet fully exploited, such as flanges for radioactive-beam accelerators and, more generally, the coupling of pipes made of different materials.

Synchrotron applications

Technology advancement sometimes verges off from its initial goals, and this phenomenon is clearly illustrated by one of our most recent innovations. In the main linac of the Compact Linear Collider (CLIC), which envisages a high-energy linear electron-positron collider, the quadrupole magnets need a beam pipe with a very small diameter (about 8 mm) and pressures in the ultra-high vacuum range. The vacuum requirement can be obtained by NEG-coating the vacuum vessel, but the coating process in such a high aspect-ratio geometry is not easy due to the very small space available for the material source and the plasma needed for its sputtering.

Fig. 3. Prototype of a NEG-coated vacuum chamber produced by copper electroforming and coated using a sacrificial mandrel. The technique is applicable to a wide range of diameters and lengths. Image credit: M Brice/CERN

This troublesome issue has been solved by a complete change of the production process: the NEG material is no longer directly coated on the wall of the tiny pipe, but instead is coated on the external wall of a sacrificial mandrel made of high-purity aluminium (figure 3). On the top of the coated mandrel, the beam pipe is made by copper electroforming, a well-known electrolytic technique, and on the last production step the mandrel is dissolved chemically by a caustic soda solution. This production process has no limitations in the diameter of the coated beam pipe, and even non-cylindrical geometries can be conceived. The flanges can be assembled during electroforming so that welding or brazing is no longer necessary.

It turns out that the CLIC requirement is common with that of next-generation synchrotron-light sources. For these accelerators, future constraints for vacuum technology are quite clear: very compact magnets with magnetic poles as close as possible to the beam – to reduce costs and improve beam performance – call for very-small-diameter vacuum pipes (less than 5 mm in diameter and more than 2 m long). CERN has already produced prototypes that should fit with these requirements. Indeed, the collaboration between the CERN vacuum group and vacuum experts of light sources has a long history. It started with the need for photon beams for the study of vacuum chambers for LEP and beam screens for the LHC, and continued with NEG coating as an efficient choice for reducing residual gas density – a typical example is MAX IV, for which CERN was closely involved (CERN Courier September 2017 p38). The new way to produce small-diameter beam pipes represents another step in this fruitful collaboration.

Further technology transfer has come from the sophisticated simulations necessary for the HL-LHC and the Future Circular Collider study. A typical example is the integration of electromagnetic and thermomechanical phenomena during a magnet quench to assess the integrity of the vacuum vessel. Another example is the simulation of gas-density and photon-impingement profiles by Monte Carlo methods. These simulation codes have found a large variety of applications well beyond the accelerator field, from the coating of electronic devices to space simulation. For the latter, codes have been used to model the random motion and migration of any chemical species present on the surfaces of satellites at the time of their launch, which is a critical step for future missions to Mars looking for traces of organic compounds.

Of course, the main objective of the CERN vacuum group is the operation of CERN’s accelerators, in particular those in the LHC chain. Here, the relationship with industry is key because the vacuum industry across CERN’s Member and Associate Member states provides us with state-of-art components, valves, pumps, gauges and control equipment that have contributed to the high reliability of our vacuum systems. On the other hand, the LHC gives high visibility to industrial products that, in turn, can be beneficial for the image of our industrial partners. Collaborating with industry is a win–win situation.

Fig. 4. NEG thin-film coating of the ELENA vacuum chambers at CERN’s Antiproton Decelerator. The blue light is the effect of the ionisation of the coating process gas. The target material is three intertwisted elemental wires of titanium, zirconium and vanadium. Image credit: CERN

The variety of projects and activities performed at CERN provide us with a continuous stimulation to improve and extend our competences in vacuum technology. The fervour of new collider concepts and experimental approaches in the physics community drives us towards innovation. Other typical examples are antimatter physics, which requires very low gas density (figure 4), and radioactive-beam physics that imposes severe controls on contamination and gas exhausting. New challenges are already visible at the horizon, for example physics with gas targets, higher-energy beams in the LHC, and coating beam pipes with high-temperature superconductors to reduce beam impedance.

An orthogonal driver of innovation is reducing the costs and operational downtime of CERN’s accelerators. In the long term, our dream is to avoid bakeout of vacuum systems so that very low pressure can be attained without the heavy operation of heating the vacuum vessels in situ, principally to remove water vapour. Such advances are possible only if the puzzling interaction between water molecules and technical materials is understood, where again only a very thin layer on top of material surfaces makes the difference. Achieving ultra-high vacuum in a matter of a few hours at a reduced cost would also have an impact well beyond the high-energy physics community. This and other challenges at CERN will guarantee that we continue to push the limits of vacuum technology well into the 21st century.

Colliding visions drive success

A simulation of the 100 km-circumference tunnel that could host different particle colliders explored by the FCC study. Image credit: FCC study/CERN

With each new high-energy accelerator, a question arises: is this the largest facility that we can conceive of being built? Although accelerator experts have never lost their astounding capacity for innovation when it comes to building the next collider, it is the necessary political will required to fund multi-billion-dollar science projects that remains the big unknown. Yet, giant facilities have succeeded in the past.

CERN’s Large Hadron Collider (LHC) was Europe’s answer to a long-standing transatlantic competition for the high-energy frontier. The US Superconducting Super Collider (SSC) had been designed to operate at 40 TeV – an enormous step – while the LHC was proposed at lower energy to fit into the existing LEP tunnel, compensating with higher luminosity. The cancellation of the SSC in 1993 ensured that the LHC would take up the high-energy mantle. Meanwhile, the 2 TeV Tevatron at Fermilab continued operations, giving the LHC real competition in the search for the Higgs boson.

Overcoming many challenges to the LHC’s construction, CERN wrestled back the energy frontier with magnificent success, crowned in 2012 by the Higgs-boson discovery. The machine and its approved high-luminosity upgrade will maintain Europe’s leadership into the 2030s. But what then?

The proposed Future Circular Collider (FCC) aims to keep CERN at the energy frontier via a 100 km-circumference ring capable of housing a 100 TeV proton collider (see CERN thinks bigger). It may well proceed via an intermediate 90–365 GeV electron–positron collider (FCC-ee), bringing incredible precision to measurements of the Higgs boson and backing up the discovery at the LHC in much the same way that LEP did after the discoveries of the Z and W bosons at the SppS.

Geoffrey Taylor is chair of the International Committee for Future Accelerators and director of the ARC Centre of Excellence for Particle Physics at the Terascale (CoEPP) at the University of Melbourne.

Whilst CERN physicists and partners wish for continued leadership from their stable base, global competition is tilting towards Asia, where two major proposals are progressing towards approval: the 250 GeV International Linear Collider (ILC) in Japan, and the 250 GeV Circular Electron-Positron Collider (CEPC) in China (see China’s bid for a circular electron–positron collider). The ILC requires major international participation, whilst CEPC (which, like FCC, could proceed to a high-energy proton collider) will be largely nationally resourced.

In principle, FCC-ee and the CEPC are direct competitors. For that matter the ILC is too, and CERN is also developing the Compact Linear Collider (CLIC) with a much higher energy reach. All would produce a very large sample of Higgs bosons in a clean environment. Uniquely, linear machines can in principle be upgraded by extending their length or increasing the gradient of their accelerating cavities. Circular machines, with the radius fixed at construction, require stronger magnets and increased power to push up their energy.

The existence of the Chinese and CERN bids is reminiscent of the competitive LHC–SSC era. Again, while the physics potential of each machine is similar, their political, economic and social environments are quite different. This time it is the new economic power of China, with a government focussed on international leadership in a range of endeavours, that is impacting future planning in the field.

The ILC and CEPC have such different development pathways – and with China increasing the size of the international high-energy physics pie, not just re-slicing it – that both could be important. For a 100 TeV proton collider, perhaps the massive development and production of the necessary superconducting magnets can be limited to one facility, freeing up international resources to explore more compact and efficient acceleration techniques for the future. CERN clearly has the experience and leadership in high-energy proton colliders.

The lesson from the LHC–SSC story is the need for persistence, international collaboration and endorsement, stability, innovation and a long-term vision – characteristics that underpin CERN’s successes. In Asia too, where long-term vision is the cultural norm, effective decadal planning is expected and recognised as critical amongst high-energy physics leaders. Surely international resources can be optimised to ensure our field remains active and relevant in the decades to come.

The Standard Model in a Nutshell

By Dave Goldberg
Princeton University Press

The Standard Model in a Nutshell
The Standard Model in a Nutshell

This book is an excellent source for those interested in learning the basic features of the Standard Model (SM) of particle physics – also known as the Glashow–Weinberg–Salam (GSW) model – without many technical details. It is a remarkably accessible book that can be used for self learning by advanced undergraduates and beginning graduate students. All the basic building blocks are provided in a self-contained manner, so that the reader can acquire a good knowledge of quantum mechanics and electromagnetism before reaching the boundaries of the SM, which is the theory that best describes our knowledge of the fundamental interactions.

The topics that the book deals with include special relativity, basic quantum field theory and the action principle, continuous symmetries and Noether’s theorem, as well as basic group theory – in particular, the groups needed in the SM: U(1), SU(2) and SU(3). It also covers the relativistic treatment of fermions through the Dirac equation, the quantisation of the electromagnetic field and a first look at the theory of gauge transformations in a familiar context. This is followed by a reasonable account of quantum electrodynamics (QED), the most accurate theory tested so far. The quantisation rules are reviewed with clarity and a number of useful and classic computations are presented to familiarise the reader with the technical details associated with the computation of decay rates, scattering amplitudes, phase-space volumes and propagators. The book also provides an elementary description of how to construct and compute Feynman rules and diagrams, which are later applied to electron–electron scattering and electron–positron annihilation, and how the latter relates to Compton or electron–photon scattering. This lays the basic computational tools to be used later in the sections about electroweak and strong interactions.

At this point, before starting a description of the SM per se, the author briefly describes the historical Fermi model and then presents the main actors. The reader is introduced to the lepton doublet (including the electron, the muon, the tau and their neutrinos), the weak charged and neutral currents, and the vector bosons that carry the weak force (the Ws and the Z). This is followed by an analysis of electroweak unification and the introduction of the weak angle, indicating how the electromagnetic interaction sits inside the weak isospin and hypercharge. Then, the author deals with the quark doublets and the symmetry breaking pattern, using the Brout–Englert–Higgs mechanism, which gives mass to the vector bosons and permits the accommodation of masses for the quarks and leptons. We also learn about the Cabibbo–Kobayashi–Maskawa mixing matrix, neutrino oscillations, charge and parity (CP) violation, the solar neutrino problem, and so on. To conclude, the author presents the SU(3) gauge theory of the strong interactions and provides a description of some theories that go beyond the SM, as well as a short list of important open problems. All this is covered in just over 250 pages: a remarkable achievement. In addition, the book includes many interesting and useful computations.

This work is a very welcome addition to the modern literature in particle physics and I certainly recommend it, in particular for self study. I hope, though, that in the second edition the correct Weinberg is portrayed on p184… an extremely hilarious blunder.

A Student’s Guide to Dimensional Analysis

By Don S Lemons
Cambridge University Press

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Dimensional analysis is a mathematical technique that allows one to deduce the relationship between different physical quantities from the dimensions of the variables involved in the system under study. It provides a method to simplify – when possible – the resolution of complex physical problems.

This short book provides an introduction to dimensional analysis, covering its history, methods and formalisation, and shows its application to a number of physics and engineering problems. As the author explains, the foundation principle of dimensional analysis is essentially a more precise version of the well known rule against “adding apples and oranges”; nevertheless, the successful application of this technique requires physical intuition and some experience. Most of the time it does not lead to the solution of the problem, but it can provide important hints about the direction to take, constraints on the relationship between physical variables and constants, or a confirmation of the correctness of calculations.

After a chapter covering the basics of the method and some historical notions about it, the book offers application examples of dimensional analysis in several areas: mechanics, hydrodynamics, thermal physics, electrodynamics and quantum physics. Through the solution of these real problems, the author shows the possibilities and limitations of this technique. In the final chapter, dimensional analysis is used to take a few steps in the direction of uncovering the dimensional structure of the universe.

Aimed primarily at physics and engineering students in their first university courses, it can also be useful to experienced students and professionals. Being concise and providing problems with solutions at the end of each chapter, the book is ideal for self study.

A Primer on String Theory

By Volker Schomerus
Cambridge University Press

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This textbook aims to provide a concise introduction to string theory for undergraduate and graduate students.

String theory was first proposed in the 1960s and has become one of the main candidates for a possible quantum theory of gravity. While going through alternate phases of highs and lows, it has influenced numerous areas of physics and mathematics, and many theoretical developments have sprung from it.

It was the intention of the author to include in the book just the fundamental concepts and tools of string theory, rather than to be exhaustive. As Schomerus states, there are already various textbooks available that cover this field in detail, from its roots to its most modern developments, but these might be dispersive and overwhelming for students approaching the topic for the first time.

The volume is composed of a brief historical introduction and two parts, each including various chapters. The first part is dedicated to the dynamics of strings moving in a flat Minkowski space. While these string theories do not describe nature, their study is helpful to understand many basic concepts and constructions, and to explore the relation between string theory and field theory on a two-dimensional “world”.

The second part deals with string theories for four-dimensional physics, which can be relevant to the description of our universe. In particular, the motion of superstrings on backgrounds in which some of the dimensions are curled up is studied (this phenomenon is called compactification). This part, in turn, includes three sections devoted to as many subtopics.

First, the author discusses conformal field theory, also dealing with the SU(2) Wess–Zumino–Novikov–Witten model. Then, he passes on to treat Calabi–Yau spaces and the associated string compactification. Finally, he focuses on string dualities, giving special emphasis to the AdS/CFT correspondence and its application to gauge theory.

Technology Meets Research: 60 Years of CERN Technology, Selected Highlights

By Christian Fabjan, Thomas Taylor, Daniel Treille and Horst Wenninger (eds.)
World Scientific

Technology Meets Research: 60 Years of CERN Technology, Selected Highlights
Technology Meets Research

This book, the 27th volume in the “Advanced Series on Directions in High Energy Physics”, presents a robust and accessible summary of 60 years of technological development at CERN. Over this period, the foundations of today’s understanding of matter, its fundamental constituents and the forces that govern its behaviour were laid and, piece by piece, the Standard Model of particle physics was established. All this was possible thanks to spectacular advances in the field of particle accelerators and detectors, which are the focus of this volume. Each of the 12 chapters is built using contributions from the physicists and engineers who played key roles in this great scientific endeavour.

After a brief historical introduction, the story starts with the Synchrocyclotron (SC), CERN’s first accelerator, which allowed – among other things – innovative experiments on pion decay and a measurement of the anomalous magnetic dipole moment of the muon. While the SC was a development of techniques employed elsewhere, the Proton Synchroton (PS), the second accelerator constructed at CERN and now the cornerstone of the laboratory’s accelerator complex, was built using the new and “disruptive” strong-focusing technique. Fast extraction from the PS combined with the van der Meer focussing horn were key to the success of a number of experiments with bubble chambers and, in particular, to the discovery of the weak neutral current using the large heavy-liquid bubble chamber Gargamelle.

The book goes on to present the technological developments that led to the discovery of the Higgs boson by the ATLAS and CMS collaborations at the LHC, and the study of heavy-quark physics as a means to understand the dynamics of flavour and the search for phenomena not described by the SM. The taut framework that the SM provides is evident in the concise reviews of the experimental programme of LEP: the exquisitely precise measurements of the properties of the W and Z bosons, as well as of the quarks and the leptons – made by the ALEPH, DELPHI, OPAL and L3 experiments – were used to demonstrate the internal consistency of the SM and to correctly predict the mass of the Higgs boson. An intriguing insight into the breadth of expertise required to deliver this programme is given by the discussion of the construction of the LEP/LHC tunnel, where the alignment requirements were such that the geodesy needed to account for local variations in the gravitational potential and measurements were verified by observations of the stars.

The rich scientific programme of the LHC and of LEP before it have their roots in the systematic development of the accelerator and detector techniques. The accelerator complex at CERN has grown out of the SC.

The book concisely presents the painstaking work required to deliver the PS, the Intersecting Storage Rings (ISR) and the Super Proton Synchrotron (SPS). Experimentation at these facilities established the quark-parton model and quantum chromodynamics (QCD), demonstrated the existence of charged and neutral weak currents, and pointed out weaknesses in our understanding of the structure of the nucleon and the nucleus. The building of the SPS was expedited by the decision to use single-function magnets that enabled a staged approach to its construction. The description of the technological innovations that were required to realise the SPS includes the need for a distributed, user-friendly control-and-monitoring system. A novel solution was adopted that exploited an early implementation of a local-area network and for which a new, interpretative programming language was developed.

The book also describes the introduction of the new isotope separation online technique, which allows highly unstable nuclei to be studied, and its evolution into research on nuclear matter in extreme conditions at ISOLDE and its upgrades. The study of heavy-ion collisions in fixed target experiments at the SPS collider and now in the ALICE experiment at the LHC, has its roots in the early nuclear-physics programme as well. The SC, and later the PS, were ideal tools to create the intense low-energy beams used to test fundamental symmetries, to search for rare decays of hadrons and leptons, and to measure the parameters of the SM.

Reading this chronicle of CERN’s outstanding record, I was struck by its extraordinary pedigree of innovation in accelerator and detector technology. Among the many examples of groundbreaking innovation discussed in the book is the construction of the ISR which, by colliding beams head on, opened the path to today’s energy

frontier. The ISR programme created the conditions for pioneering developments such as the multi-wire proportional chamber, and the transition radiation detector as well as large-acceptance magnetic spectrometers for colliding-beam experiments. Many of the technologies that underpin the success of the proton–antiproton (Spp S) collider, LEP and the LHC, were innovations pioneered at the ISR. For example, the discovery of the W and Z bosons at the Spp S relied on the demonstration of stochastic cooling and antiproton accumulation. The development of these techniques allowed CERN to establish its antiproton programme, which encompassed the search for new phenomena at the energy frontier, as well as the study of discrete symmetries using neutral kaons at CPLEAR and the detailed study of the properties of antimatter.

The volume includes contributions on the development of the computing, data-handling and networking systems necessary to maximise the scientific output of the accelerator and detector facilities. From the digitisation and handling of bubble- and spark-chamber images in the SC era, to the distributed processing possible on the worldwide LHC computing grid, the CERN community has always developed imaginative solutions to its data-processing needs.

The book concludes with thoughtful chapters that describe the impact on society of the technological innovations driven by the CERN programme, the science and art of managing large, technologically challenging and internationally collaborative projects, and a discussion of the R&D programme required to secure the next 60 years of discovery.

The contributions from leading scientists of the day collected in this relatively slim book document CERN’s 60-year voyage of innovation and discovery, the repercussions of which vindicate the vision of those who drove the foundation of the laboratory – European in constitution, but global in impact. The spirit of inclusive collaboration, which was a key element of the original vision for the laboratory, together with the aim of technical innovation and scientific excellence, are reflected in each of the articles in this unique volume.

Industry rises to FCC conductor challenge

Superconductivity underpins large particle accelerators such as the LHC. It is also a key enabling technology for a future circular proton–proton collider reaching energies of 100 TeV, as is currently being explored by the Future Circular Collider (FCC) study. To address the considerable challenges of this project, a conductor development workshop was held at CERN on 5 and 6 March to create momentum for the FCC study and bring together industrial and academic partners.

The first Future Circular Collider conductor-development workshop at CERN in March. Image credit: A Koufidou

The alloy niobium titanium is the most successful practical superconductor to date, and has been used in all superconducting particle accelerators and detectors. But the higher magnetic fields required for the high-luminosity LHC (11 T) and FCC (16 T) call for new materials. A potential superconducting technology suitable for accelerator magnets beyond fields of 10 T is the compound niobium tin (Nb3Sn), which is the workhorse of the 16 T magnet-development programme at CERN.

The FCC conductor programme aims to develop Nb3Sn multi-filamentary wires with a critical current-density performance of at least 1500 A/mm2 at 16 T and at a temperature of 4.2 K. This is 30 to 50% higher than the conductor for the HL-LHC, and a significant R&D effort – including fundamental research on superconductors – is needed to meet the magnet requirements of future higher-energy accelerators. The FCC magnets will also require thousands of tonnes of superconductor, calling for a wire design suitable for industrial-scale production at a considerably lower cost than current high-field conductors.

CERN is engaged in collaborative conductor development activities with a number of industrial and academic partners to achieve these challenging targets, and the initial phase of the programme will last for four years. Representatives from five research institutes and seven companies from the US, Japan, Korea, Russia, China and Europe attended the March meeting to discuss progress and opportunities. Firms already producing Nb3Sn superconducting wire for the FCC programme are Kiswire Advanced Technology (KAT); the TVEL Fuel Company working with the Bochvar Institute (JSC VNIINM); and, from Japan, Furukawa Electric and Japan Superconductor Technology (JASTEC), both coordinated by the KEK laboratory. Columbus Superconductor SpA is participating in the programme for other superconducting materials, while two additional companies – Luvata and Western Superconducting Technologies (WST) – expressed their interest in the CERN conductor programme and attended the workshop.

The early involvement of industry is crucial and the event provided an environment in which industrial partners were free to discuss their proposed technical solutions openly. In the past, most companies produced a bronze-route Nb3Sn superconductor, which has no potential to reach the target for FCC. Thanks to their commitment to the programme, and with CERN’s support, companies are now investing in a transition to internal tin processes. Innovative approaches for characterising superconducting wires are also coming out of academia. Developments include the correlation of microstructures, compositional variations and superconducting properties at TU Wien, research into promising internal-oxidation routes at the University of Geneva, phase transformation studies at TU Bergakademie Freiberg and research of novel superconductors for high-fields at SPIN in Genova.

The FCC initiative is of key importance for future high-energy accelerators. Participants agreed that this could result in a new class of high-performance Nb3Sn material suitable not only for accelerator magnets, but also for other large-scale applications such as high-field NMR and laboratory solenoids.

• Panos Charitos, CERN.

Standard Model gets annual check up at Moriond

The 2018 Moriond sessions took place in La Thuile, Italy, from 10 to 24 March. The annual conference is an opportunity to review the progress taking place over the breadth of particle physics, from B physics to gravitational waves and from advances in electroweak precision tests to exploratory searches for dark matter. The quest for new particles covers an impressive 40 orders of magnitude, from the 10–22 eV region explored via neutron-spin precession to the 13 TeV energy of the LHC and the highest-energy phenomena in cosmic rays.

Photo of a talk at the 2018 Moriond conference
Participants during a talk at the 2018 Moriond conference in Italy. Image credit: V Varanda

Anomalies in the decays of beauty quarks found by the LHCb and B-factory experiments continue to entice theorists to look for explanations for these possible hints of lepton non-universalities, and experimental updates are eagerly awaited (CERN Courier April 2018 p23). Progress continues in the field of CP violation in B and D mesons, while quantitative tests of the CKM matrix are being helped by precise calculations in lattice QCD. Progress on leptonic and semi-leptonic D-meson decays was reported from BES-III, while Belle showed hints of the decay B+ → μ+ ν and evidence of isospin violation. In the classic field of rare kaon decays, the CERN SPS experiment NA62 showed its first results, presenting one candidate event for the elusive decay K+ → π+ ν ν– obtained using a novel in-flight technique.

Fundamental parameters of the Standard Model (SM), such as the masses of the top quark and W boson, are being measured with increasing precision. The SM is in very good shape, apart from the long-standing exception of forward–backward asymmetries. These asymmetries are also being studied at the LHC, and precise results continue to be produced at the Tevatron.

Results on top-quark production and properties are constantly being improved, while hadron spectroscopy is as lively as ever, both in the light meson sector (BESIII) and in heavy quarks (BaBar, Belle and LHCb). Data from HERA are still providing new inputs into structure functions, with c and b quarks now being included. Heavy-ion collisions at LHC and RHIC continue to explore the behaviour of the hot, dense quark–gluon plasma, while proton–ion collisions at fixed-target experiments (LHCb) provide useful inputs to constrain Monte Carlo event generators.

The news on the Brout–Englert–Higgs mechanism is good, with progress on many fronts. The amount of new results presented by ATLAS and CMS, including evidence of ttH production, and global combinations of production and decay channels shows that the precision on the couplings between the Higgs and other particles is improving fast. The study of rare decays of the Higgs boson is advancing rapidly, with the H → μ+μ–decay within reach.

The search for heavy resonances is continuing at full speed, with CMS presenting one Z´ analysis employing the full, available LHC data set (77.3 fb–1), including 2017 data. Is supersymmetry hiding somewhere? Several analyses at ATLAS and CMS are now being recast to include more elusive signatures with various amounts of R-parity violation and degenerate spectra, and there is an emerging interest in performing searches beyond narrow-width approximations.

The search for dark matter is on, with WIMP direct searches maturing rapidly (XENON1T) and including novel experiments like DARKSIDE which, with just 20 l of very pure liquid argon, presented a new best limit at low masses. This field shows that, with ingenuity, there is still room to have an impact. Bounds on extremely light axion-like particles were presented by ADMX for QCD axions, and for neutron electric dipole moments. The interplay between these dedicated experiments and the search for directly produced dark matter at the LHC are highly complementary.

The field of neutrinos continues to offer steady progress with new and old puzzles being addressed. The latest results from T2K disfavour CP-conservation at the level of two sigma, while NOvA disfavours the inverted hierarchy at a similar level. A revival of decay-at-rest techniques and the measurement of coherent elastic neutrino–nucleus scattering by COHERENT (CERN Courier October 2017 p8) were noticeable. The search for heavy neutral leptons is taking place at both fixed-target and collider experiments, while reactor experiments (like DAYA BAY and STEREO) are meant to clarify the reactor antineutrino anomaly. The puzzle of sterile neutrinos is not yet completely clarified after 20 years. Deep-sea (ANTARES) and South Pole (IceCube) experiments are now mature, with ANTARES showing, among other things, searches for point-like sources. IceCube presented a brand new analysis looking for tau-neutrino appearance that is competitive with existing results. Neutrinoless double-beta decay experiments are now biting into the sensitivity of the inverted mass hierarchy (CUORE and EXO-200), with promising developments in the pipeline (CUPID).

Completing the programme of the electroweak session was a glimpse into the physics of cosmic rays and gravitation. The sensitivity of AUGER is now such that mapping the origin of the cosmic rays on the sky becomes feasible. With the observation of a binary neutron-star collapse by LIGO and VIRGO, 2017 saw the birth of multi-messenger astronomy.

On the theory side, one continues to learn from the abundance of experimental results, and there is still so much to be learned by the study of the Higgs and further high-energy exploration. SM computations are breaking records in terms of the numbers of loops and legs involved. Electroweak and flavour physics can indicate the way to new physics scales and extend the motivation to search for dark matter at very low energies. The case to study neutrinos remains as compelling as ever, with many outstanding questions still waiting for answers.

• Augusto Ceccucci, CERN.

Beams back in LHC for final phase of Run 2

The LHC tunnel photographed in February. Image credit: M Brice, J Ordan/CERN

Shortly after midday on 30 March, protons circulated in the Large Hadron Collider (LHC) for the first time in 2018. Following its annual winter shutdown for maintenance and upgrades, the machine now enters its seventh year of data taking and its fourth year operating at a centre-of-mass energy of 13 TeV.

The LHC restart, which involves numerous other links in the CERN accelerator chain, went smoothly. At the beginning of March, the first protons were injected into Linac2, and then into the Proton Synchrotron (PS) Booster. On 8 March the PS received beams, followed by the Super Proton Synchrotron (SPS) one week later. In parallel, the teams had been checking all the LHC hardware and safety installations. No fewer than 1560 electrical circuits had to be powered and about 10,000 tests performed before the LHC was deemed ready to accept protons.

The first beams circulating contain just one bunch, each of which contains 20 times fewer protons than in normal operation; the energy of the beam is also limited to the SPS injection energy of 450 GeV. Further adjustments and tests were undertaken in early April to allow the energy and density of the bunches to be increased.

Bunching up

As the Courier went to press, a few bunches had been injected and accelerated at full energy for optics and collimator commissioning. The first stable beams with only a few bunches are scheduled for 23 April, but could take place earlier thanks to the good progress made so far. This will be followed by a period of gradual intensity ramp-up, during which the number of bunches will be increased stepwise. Between each step, a formal check and validation will take place. The target is to fill each ring with 2556 bunches, and the experiments will be able to undertake serious data collection as soon as the number rises above 1200 bunches – which is expected in early May.

Since early December 2017, when the CERN accelerator complex entered its end of year technical stop, numerous important activities were completed on the LHC and other accelerators. Alongside standard maintenance, the LHC injectors underwent significant preparatory work for the LHC Injector Upgrade project (LIU) foreseen for 2019 and 2020 (CERN Courier October 2017 p32). In the LHC, an important activity was the partial warm-up of sector 1-2 to solve the so-called 16L2 issue, wherein frozen air from an accidental ingress caused beam instabilities and losses during last year’s run: a total of 7 l of frozen air was removed from each beam vacuum chamber during the warm up.

The objective for the 2018 run is to accumulate more data than was collected last year, targeting an integrated luminosity of 60 fb–1 (as opposed to the 50 fb–1 recorded in 2017). While the intensity of collisions is being ramped up in the LHC, data taking is already under way at various fixed-target experiments at CERN that are served by beams from the PS Booster, PS and SPS. The first beams for physics at the n_TOF experiment and the PS East Area started on 30 March. The nuclear-physics programme at ISOLDE restarted on 9 April, followed closely by that of the SPS North Area and, later, the Antiproton Decelerator.

2018 is an important year for the main LHC experiments (ALICE, ATLAS, CMS and LHCb) because it marks the last year of Run 2. In December, the accelerator complex will be shut down for a period of two years to allow significant upgrade work for the High-Luminosity LHC, with the deployment of the LIU project and the start of civil-engineering work. Operations of the HL-LHC will begin in earnest in the mid-2020s, promising an integrated luminosity of 3000 fb–1 by circa 2035.

DESY sets out vision for the future

The DESY 2030 strategy is detailed in a 74-page brochure.

On 20 March, the DESY laboratory in Germany presented its strategy for the coming decade, outlining the areas of science and innovation it intends to focus on. DESY is a member of the Helmholtz Association, a union of 18 scientific-technical and medical-biological research centres in Germany with a workforce of 39,000 and annual budget of €4.5 billion. The laboratory’s plans for the 2020s include building the world’s most powerful X-ray microscope (PETRA IV), expanding the European X-ray free-electron laser (XFEL), and constructing a new centre for data and computing science.

Founded in 1959, DESY became a leading high-energy-physics laboratory and today remains among the world’s top accelerator centres. Since the closure of the HERA collider in 2007, the lab’s main accelerators have been used to generate synchrotron radiation for research into the structure of matter, while DESY’s particle-physics division carries out experiments at other labs such as those at CERN’s Large Hadron Collider.

Together with other facilities on the Hamburg campus, DESY aims to strengthen its role as a leading international centre for research into the structure, dynamics and function of matter using X rays. PETRA IV is a major upgrade to the existing light source at DESY that will allow users to study materials and other samples in 100 times more detail than currently achievable, approaching the limit of what is physically possible with X rays. A technical design report will be submitted in 2021 and first experiments could be carried out in 2026.

Together with the international partners and operating company of the European XFEL, DESY is planning to comprehensively expand this advanced X-ray facility (which starts at the DESY campus and extends 3.4 km northwest). This includes developing the technology to increase the number of X-ray pulses from 27,000 to one million per second (CERN Courier July/August 2017 p18).

As Germany’s most important centre for particle physics, DESY will continue to be a key partner in international projects and to set up an attractive research and development programme. DESY’s Zeuthen site, located near Berlin, is being expanded to become an international centre for astroparticle physics, focusing on gamma-ray and neutrino astronomy as well as on theoretical astroparticle physics. A key contribution to this effort is a new science data-management centre for the planned Cherenkov Telescope Array (CTA), the next-generation gamma-ray observatory. DESY is also responsible for building CTA’s medium-sized telescopes and, as Europe’s biggest partner in the neutrino telescope IceCube located in the Antarctic, is playing an important role in upgrades to the facility.

The centre for data and computing science will be established at the Hamburg campus to meet the increasing demands of data-intensive research. It will start working as a virtual centre this year and there are plans to accommodate up to six scientific groups by 2025. The centre is being planned together with universities to integrate computer science and applied mathematics.

Finally, the DESY 2030 report lists plans to substantially increase technology transfer to allow further start-ups in the Hamburg and Brandenburg regions. DESY will also continue to develop and test new concepts for building compact accelerators in the future, and is developing a new generation of high-resolution detector systems.

“We are developing the campus in Hamburg together with partners at all levels to become an international port for science. This could involve investments worth billions over the next 15 years, to set up new research centres and facilities,” said Helmut Dosch, chairman of DESY’s board of directors, at the launch event. “The Zeuthen site, which we are expanding to become an international centre for astroparticle physics, is undergoing a similarly spectacular development.”

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