Praxis Archives – Page 47 of 181

Leading physicists back future circular collider

An artist’s impression of a particle collision taking place at the Future Circular Collider. Credit: CERN.

The next major European project after the LHC should be a 100-km circumference circular collider, argue more than 50 senior particle physicists in a preprint posted on arXiv at the end of last year. The authors — who include two previous CERN Council presidents, two former CERN Directors-General, leading members of the LHC experiments and high-energy theorists – say that the sequential electron-positron and hadron-hadron programme of the CERN-led Future Circular Collider (FCC) offers the most promising way to explore in full detail the Higgs sector and extend substantially the reach for new physics, and is the best option to maintain Europe’s place at the high-energy frontier during the coming decades.

“The combination of FCC-ee and FCC-hh will provide a forefront scientific programme for CERN for many decades, just as the combination of LEP and LHC has done,” says coauthor and former CERN Council president Michel Spiro. “We consider FCC to be a visionary programme for the future of CERN.”

“We consider FCC to be a visionary programme for the future of CERN”

The 15-page long preprint comes as the update of the European strategy for particle physics enters its final stages, and notes that several important new facts have emerged during the past year: the FCC conceptual design reports were published in January; in March, Japan postponed the decision about an International Linear Collider to an indefinite date; in May, Europe discussed its particle physics strategy at an open symposium in Granada, where several high-energy options were presented; and in September, the European Strategy Group (ESG) published a Physics Briefing Book and prepared a supporting note which included five possible scenarios for major new accelerator facilities and raised a number of important issues. In Europe the options for a post-LHC collider are the FCC and Compact Linear Collider (CLIC) projects, both proposed to be located at CERN. “The supporting note had the cardinal virtue of posing directly the central question: linear or circular?” write the authors. “We summarise our view on the key issues, which contain the answer to this question.”

The estimated physics reach of both machines is explored in detail. In terms of an initial-stage 380 GeV CLIC or 365 GeV FCC-ee, the report finds that both machines cover in comparable ways the number-one priority of the particle physics community: exploring more fully the Higgs sector, and covering top-quark physics. FCC compares favourably with CLIC on the expected accuracy of the Higgs couplings, it claims, and its much higher luminosity means it can operate as a “tera-Z” and WW facility, providing a new generation of precision electroweak measurements. FCC-ee combines several new accelerator technologies, but “will be built using the vast experience accumulated with previous circular electron-positron colliders,” notes the report. CLIC would require “a vertical beam size in the collision region at the nanometre level”, and the authors raise concerns that CLIC would be restricted to electron–positron collisions with only a single interaction point and one experimental facility.

The differences between the physics reach of a linear and circular machine become sharper for “stage 2”: a 1– 3 TeV CLIC or a 100 TeV FCC-hh. Here, the authors conclude that CLIC will have very interesting capabilities for physics exploration, such as double-Higgs production, assuming that the design performance is achieved, whereas a 100 TeV FCC-hh opens a new energy regime, provided the 16 T magnet technology can be mastered technically and cost-effectively. FCC would have the last word on Standard Model measurements, and “an unrivalled discovery potential, with an increased reach for direct discovery at the highest masses”.

Whichever project is chosen, the necessary time and resources will require a new style for CERN

Both CLIC and FCC require a new scale of investment, the report notes, and success in this formidable task may be achieved “only if the particle physics community at large shows overwhelming support for the recommended programme”. The authors note that the integrated FCC-ee and FCC-hh programme is estimated to be a factor 1.5 more expensive than a 3 TeV CLIC, but will provide a greater range of research opportunities for a larger physics community over a longer time span. The costs are also to be seen in the perspective of the long timeframes of these programmes, each of which will extend over several decades, as well as the expected physics advances.

Whichever project is chosen, conclude the authors, the necessary time and resources will require a new style for CERN, for the particle-physics community – including innovative ways of guiding the careers of young researchers – and for the interaction between politics and society. “The FCC programme that we support will keep particle physics at the high-energy frontier vibrant, but it will require a deep and lasting commitment by society to fundamental research, which the high-energy community must strive to merit and justify,” says Spiro.

The next step in the European strategy update is the ESG drafting session to take place in Bad Honnef, Germany, from 20-24 January. Recommendations to CERN will be formally presented at an event in Budapest in May.

Particle physics inspires all

Collage of images from CERN Open Days 2019 — Director-General Fabiola Gianotti with visitors at LHC Point 4 (left). Sparking imagination in the power-converters lab (middle). All smiles at LHC Point 6 (right). Credits: CERN-PHOTO-201909-247-9; CERN-PHOTO-201909-255-14; CERN-PHOTO-201909-243-9

“It’s a huge place full of ideas to try in case something revolutionary turns up.”

“There is high-tech science going on. You’re trying to make applications to real life, such as non-destructive testing.”

“We’re not here for the science, we’re here for the machines!”

“I thought it was all about programming, but you actually build things.”

“CERN is here to test out how particles behave and exploring the limit of the universe, like matter (which we know) and antimatter (which we don’t).”

“If you understand materials and energy at the basic scale, you have a better chance of creating new energy sources and materials for the future.”

“I read an article recently that said this was all a waste of taxpayers’ money, but now I am less sure because I have seen today that there are a lot of applications.”

Gaspar Barreira 1940–2019

Experimental particle physicist Gaspar Barreira, co-founder of the Portuguese Laboratory for Instrumentation and Experimental Particle Physics (LIP), passed away on 1 June. He was the Portuguese delegate to the CERN Council and to the SESAME Council, and was a strong proponent of international cooperation.

Gaspar’s life proceeded in cycles, each lived intensely with great energy and focus. He had a vision to foster progress, to change the world here and now. Each time, despite arriving as an outsider, he was able to make great impact thanks to his intelligence and capability to transmit enthusiasm. He always chose grand objectives: let’s build something that doesn’t exist at all in the country; let’s do something that was never done before. He was not afraid of dreaming, nor of obstacles.

Born in Braga, in the north of Portugal, Gaspar arrived in Lisbon at the age of 18 to study physics and mathematics. He fought against the dictatorship of Salazar, which gagged Portugal for more than 40 years until the Carnation Revolution of 25 April 1974, and was imprisoned more than once. In the early 1970s he taught himself electronics, and soon found himself at the Nuclear Physics Centre in Lisbon, saving the day for many colleagues with his ability to fix the scarce equipment or assemble non-existing parts. He also entered into pioneering collaborations with archaeologists to date ancient artefacts – a path that in 1980 led him to the International Centre for Theoretical Physics in Trieste, Italy, where he soon become director of the microprocessors laboratory.

In 1985 Gaspar retuned to Portugal to get involved in the county’s accession to CERN, founding LIP with José Mariano Gago and Armando Policarpo, and building LIP’s instrumentation division. NA38 at the SPS was the first experiment in which LIP participated as an institution. He greatly contributed to establish LIP as a reference laboratory in particle and astroparticle physics, instrumentation, technology and computing.

Gaspar was a strong believer in CERN and international cooperation. He had a fundamental role in bringing Portugal into the DELPHI experiment at LEP, and was a strong supporter of the LHC from the early days. He was a strong advocate of distributed computing, and did not spare efforts to have Portugal and LIP in the main projects in this area, at CERN and at a European level. Gaspar was responsible for the creation of the Portuguese Tier-2 in the CERN Worldwide LHC Computing Grid, and was active in several related initiatives.

From the turn of the century, Gaspar was fully involved in science policy. He was the Portuguese representative to a variety of international organisations and boards, and coordinated the Portuguese participation in the Alpha Magnetic Spectrometer for its shuttle flight in 1998. Gaspar was always particularly concerned with knowledge-transfer to society. He co-coordinated the training programmes for young Portuguese engineers at CERN, ESA and ESO, and the creation of the Portuguese language teachers programme.

Before and after the revolution of 1974, Gaspar worked towards the construction of a world where knowledge, freedom and rationality were decisive. We have lost a great friend of CERN, LIP and physics, an excellent scientist and a truly unique personality. Though departed, Gaspar leaves us an immense legacy of vision, endurance and resilience. His last big project, the installation in Portugal of a treatment and research centre for cancer therapy with protons, is not yet accomplished. For this we will strive.

Giovanni Muratori 1924–2019

Giovanni Muratori received a double degree in naval and mechanical engineering at the University of Genoa in 1949, after which he worked at ENI-AGIP on the construction of instruments for oil exploration. He started at CERN in August 1959 in the PS division, where he worked on the heavy-liquid bubble chamber designed to study neutrino physics. Giovanni oversaw the design of the cameras – not an easy task in view of the strong magnetic field that precluded the use of electric motors – and, after some initial setbacks, the chamber was ready for data-taking in early 1961. Finding the event rate to be insufficient, a crash programme was set in motion to improve the beam (using van der Meer’s magnetic horn) and to increase the total mass of detectors (by adding spark chambers downstream). Giovanni embarked on the design of the mechanics and optics for these spark chambers, which were operational in 1963.

At the end of 1961 he was transferred to the nuclear physics division and in April 1966 was appointed leader of the technical assistance group, which was involved in the design and construction of optical and mechanical equipment. The group developed and constructed a wide variety of detectors and associated equipment, including the R-108 experiment at the ISR where the group built a set of novel cylindrical drift chambers allowing track positions along the wire to be measured using the difference of arrival times of the signal at the ends of each wire. For NA31 the group built drift chambers installed in a helium-filled tank as well as a lightweight Kevlar window separating the helium from a vacuum tank.

Early-on, the group designed and constructed an automatic machine for winding large wire spark chambers and soon became specialised in the construction of arrays for the new multiwire proportional chambers. Led by Giovanni, the group developed equipment and facilities for Cherenkov detectors, including a dry lab for handling lithium foil and methods of producing precision glass spherical mirrors coated with highly reflecting aluminium coatings. Mirrors made using these techniques were later used in the RICH detector at LEP’s DELPHI experiment.

Towards the end of his CERN career he worked on the initial designs of the TPC detector for another LEP detector, ALEPH. He also started a collaboration with a group searching for the existence of a “fifth force” and designed and built a rotor that generated a dynamic gravitational field at around 450 Hz, which was used in the first absolute calibration of the gravitational wave detector EXPLORER at CERN.

Giovanni remained at CERN for several years after his retirement in 1986, during which time he worked on several problems including the initial design of a prototype liquid argon chamber for use in underground experiments at Gran Sasso. He was a superb engineer. His work was highly appreciated and his opinions respected. He participated actively in the design of equipment with innovative and ingenious ideas. He also loved solving machining and manufacturing problems, whether on a large or Swiss-watch scale. With his common-sense attitude and his warm and generous spirit, his advice was often sought on personal matters. Giovanni will be remembered with respect and affection by all.

European astroparticle, nuclear and particle physicists join forces

Not impossibly tribal A Penrose triangle was used to symbolise how astroparticle, particle and nuclear physics are intertwined. Credit: JENAS

The first joint meeting of the European Committee for Future Accelerators (ECFA), the Nuclear Physics European Collaboration Committee (NuPECC), and the Astroparticle Physics European Consortium (APPEC) took place from 14 – 16 October in Orsay, France. Making progress in domains such as dark matter, neutrinos and gravitational waves increasingly requires interdisciplinary approaches to scientific and technological challenges, and the new Joint ECFA-NuPECC-APPEC Seminar (JENAS) events are designed to reinforce links between astroparticle, nuclear and particle physicists.

Jointly organised by LAL-Orsay, IPN-Orsay, CSNSM-Orsay, IRFU-Saclay and LPNHE-Paris, the inaugural JENAS meeting saw 230 junior and senior members of the three communities discuss overlapping interests. Readout electronics, silicon photomultipliers, big-data computing and artificial intelligence were just a handful of the topics discussed. For example, the technological evolution of silicon photomultipliers, which are capable of measuring single-photon light signals and can operate at low voltage and in magnetic fields, will be key both for novel calorimeters and timing detectors at the high-luminosity LHC. They will also be used in the Cherenkov Telescope Array – an observatory of more than 100 telescopes which will be installed at La Palma in the northern hemisphere, and in the Atacama Desert in the southern hemisphere, becoming the world’s most powerful instrument for ground-based gamma-ray astronomy.

As chairs of the three consortia, we issued a call for novel expressions of interest

Organisational synergies related to education, outreach, open science, open software and careers are also readily identified, and a diversity charter was launched by the three consortia, whereby statistics on relevant parameters will be collected at each conference and workshop in the three subfields. This will allow the communities to verify how well we embrace diversity.

As chairs of the three consortia, we issued a call for novel expressions of interest to tackle common challenges in subjects as diverse as computing and the search for dark matter. Members of the high-energy physics and related communities can submit their ideas, in particular those concerning synergies in technology, physics, organisation and applications. APPEC, ECFA and NuPECC will discuss and propose actions in advance of the next JENAS event in 2021.

Maxwell’s enduring legacy

In 1871, James Clerk Maxwell undertook the titanic enterprise of planning a new physics laboratory for the University of Cambridge from scratch. To avoid mistakes, he visited the Clarendon Laboratory in Oxford, and the laboratory of William Thomson (Lord Kelvin) in Glasgow – then the best research institutes in the country – to learn all that he could from their experiences. Almost 150 years later, Malcolm Longair, a renowned astrophysicist and the Cavendish laboratory’s head from 1997 to 2005, has written a monumental account of the scientific achievements of those who researched, worked and taught at a laboratory which has become an indispensable part of the machinery of modern science.

The 22 chapters of the book are organised in ten parts corresponding to the inspiring figures who led the laboratory through the years, most famously among them Maxwell himself, Thomson, Rutherford, Bragg, Mott and few others. The numerous Nobel laureates who spent part of their careers at the Cavendish are also nicely characterised, among them Chadwick, Appleton, Kapitsa, Cockcroft and Walton, Blackett, Watson and Crick, Cormack, and, last but not least Didier Queloz, Nobel Laureate in 2019 and professor at the universities of Cambridge and Geneva. You may even read about friends and collaborators as the exposition includes the most recent achievements of the laboratory.

Rutherford and Thomson managed the finances of the laboratory almost from their personal cheque book

Besides the accuracy of the scientific descriptions and the sharpness of the ideas, this book inaugurates a useful compromise that might inspire future science historians. So far it was customary to write biographies (or collected works) of leading scientists and extensive histories of various laboratories: here these two complementary aspects are happily married in a way that may lead to further insights on the genesis of crucial discoveries. Longair elucidates the physics with a competent care that is often difficult to find. His exciting accounts will stimulate an avalanche of thoughts on the development of modern science. By returning to a time when Rutherford and Thomson managed the finances of the laboratory almost from their personal cheque book, this book will stimulate readers to reflect on the interplay between science, management and technology.

History is often instrumental in understanding where we come from, but it cannot reliably predict directions for the future. Nevertheless the history of the Cavendish shows that lasting progress can come from diversity of opinion, the inclusiveness of practices and mutual respect between fundamental sciences. How can we sum up the secret of the scientific successes described in this book? A tentative recipe might be unity in necessary things, freedom in doubtful ones and respect for every honest scientific endeavour.

ATLAS – A 25 Year Insider Story of the LHC Experiment

ATLAS – A 25 Year Insider Story of the LHC Experiment is a comprehensive overview of one of the most complex and successful scientific endeavours ever undertaken. 117 authors collaborated to write on diverse aspects of the ATLAS project, ranging from the early days of the proto-collaboration, to the test-beam studies to verify detector concepts, the design, building and installation of the detector systems, building the event selection and computing environment required, forming the organisation, and finally summarising the harvest of physics gathered thus far. Some of the chapters cover topics that are discussed elsewhere – the description of the detector summarises more extensive journal publications, the major physics achievements have been covered in recent review articles and the organisational structure is discussed on the web – but this volume usefully brings these various aspects together in a single place with a unified treatment.

Despite the many authors who contributed to this book, the style and level of treatment is reasonably coherent. There are many figures and pictures that augment the text. Those showing detector elements that are now buried out of sight are important complements to the text descriptions: the pictures of circuit boards are less helpful, besides demonstrating that these electronics exist. A most engaging feature is the inclusion of one-page “stories” at the ends of the chapters, each giving some insight into the ups and downs of how the enterprise works. Among these vignettes we have such stories as the ATLAS collaboration week that almost no one attended and the spirit of camaraderie among the experimenters and accelerator operators at the daily 08:30 run meetings.

One could imagine several audiences for this book, and I suspect that, apart from ATLAS collaborators themselves, each reader will find different chapters most suited to their interests. The 26-page chapter “The ATLAS Detector Today” offers a more accessible overview for students just joining the collaboration than the 300-page journal publication referenced in most ATLAS publications. Similarly, “Towards the High-Luminosity LHC” gives a helpful brief introduction to the planned upgrades. “Building up the Collaboration” will be useful to historians of science seeking to understand how scientists, institutions and funding agencies engage in a project whose time is ripe. Those interested in project management will find “Detector Construction Around the World” illuminating: this chapter shows how the design and fabrication of detector subsystems is organised with several, often geographically disparate, institutions joining together, each contributing according to its unique talents. “From the LoI to the Detector Construction” and “Installation of the Detectors and Technical Coordination” will appeal to engineers and technical managers. The chapters “Towards the ATLAS Letter of Intent” and “From Test Beams to First Physics” catalogue the steps that were necessary to realise the collaboration and experiment, but whose details are primarily interesting to those who lived through those epochs. Finally, “Highlights of Physics Results (2010 – 2018)” could have offered an exciting story for non-scientists, and indeed the thrill of the chase for the Higgs boson comes through vividly, but with unexplained mentions of leptons, loops and quantum corrections, the treatment is at a more technical level than would be ideal for such readers, and the plots plucked from publications are not best suited to convey what was learned to non-physicists.

What makes a collaboration like this tick?

Given the words in the foreword that the book is “intended to provide an insider story covering all aspects of this global science project,” I looked forward to the final chapter, “ATLAS Collaboration: Life and its Place in Society”, to get a sense of the human dimensions of the collaboration. While some of that discussion is quite interesting – the collaboration’s demographics and the various outreach activities undertaken to engage the public – there is a missing element that I would have appreciated: what makes a collaboration like this tick? How did the large personalities involved manage to come to common decisions and compromises on the detector designs? How do physicists from nations and cultures that are at odds with each other on the world stage manage to work together constructively? How does one account for the distinct personalities that each large scientific collaboration acquires? Why does every eligible author sign all ATLAS papers, rather than just those who did the reported analysis? How does the politics for choosing the collaboration management work? Were there design choices that came to be regretted in the light of subsequent experience? In addition to the numerous successes, were there failures? Although I recognise that discussing these more intimate details runs counter to the spirit of such large collaborations, in which one seeks to damp out as much internal conflict as possible, examining some of them would have made for a more compelling book for the non-specialist.

The authors should be commended for writing a book unlike any other I know of. It brings together a factual account of all aspects of ATLAS’s first 25 years. Perhaps as time passes and the participants mellow, the companion story of the how, in addition to the what and where, will also be written.

Particle physicists challenge EC rebranding

An open letter addressed to the presidents of the European Parliament and the European Commission (EC) demanding better recognition for education and research has closed, having attracted around 13,600 signatories during the past two months.

Published on 17 September by a group of eight prominent particle physicists in Europe – Siegfried Bethke (MPI for Physics), Nora Brambilla (TU-München), Aldo Deandrea (U-Lyon 1), Carlo Guaraldo (INFN Frascati), Luciano Maiani (U-Roma La Sapienza), Antonio Pich (U-València), Alexander Rothkopf (U-Stavanger) and Johanna Stachel (U-Heidelberg) – the letter followed the announcement of a new EC organisational structure on 10 September in which former EC directorates for education, culture, sports and youth, as well as that for research, science and innovation, have been subsumed under a single commissioner with the titular brief “innovation and youth”.

“Words are important,” says Maiani, who was CERN Director-General from 1999–2003. “Omitting ‘research’ from the logo of the EC is reason for concern. The response we received, including from prestigious personalities, reassured us that this concern is widely shared.”

With signatories including hundreds of university and laboratory leaders, 19 Nobel laureates and many institutions including the European, French and German physical societies, the letter demands that the EC revises the title of the brief to “Education, Research, Innovation and Youth”. It states: “We, as members of the scientific community of Europe, wish to address this situation early on and emphasise both to the general public, as well as to relevant politicians on the national and European Union level, that without dedication to education and research there will neither exist a sound basis for innovation in Europe, nor can we fulfill the promise of a high standard of living for the citizens of Europe in a fierce global competition.”

Of course we are disappointed that the voices of more than 13,600 scientists went unheard
Johanna Stachel, University of Heidelberg

The letter closed on 13 November after the EC issued a press release stating that it will rename three commissioner portfolios, but that the title of commissioner designate for innovation and youth, Mariya Gabriel, is not among those three being changed. “Naturally we are disappointed, and even frightened as the decisions about non-renaming prove that the omission of research and education in the title signals how low these fields may be valued by the new commission,” says Bethke.

“We will keep pushing,” adds Stachel. “But of course we are disappointed that the voices of more than 13,600 scientists went unheard, despite many prominent voices and also significant press coverage.”

On 18 November, Rothkopf responded to European parliament president David Sassoli on behalf of the initial signatories with a letter, stating: “It is with great disappointment that we recognize that the voice of science has not reached the ears of the Commission. The intention of the Commission is to stimulate innovation. But we reiterate with force that without research and education there is no future to innovation… We are counting on you, Mr. President, to represent the voice of all European citizens who have signed up as supporter of the open letter and in the interest of European research.”

Update 28th November: Speaking at a plenary session of the European Parliament in Strasbourg on 27 November, European Commission (EC) president-elect Ursula von der Leyen announced that the brief of commissioner Mariya Gabriel would be been renamed “Innovation, research, culture, education and youth”. The addition of “education” and “research” to the initial title of the brief announced on 10 September was met with applause in the chamber.

The Weil conjectures

“I am less interested in mathematics than in mathematicians,” wrote Simone Weil to her brother André, a world-class mathematician who was imprisoned in Rouen at the time. The same might be said about US novelist and onetime mathematics student Karen Olsson. Despite the title, her new book, The Weil Conjectures, stars the extraordinary siblings at the expense of André’s mathematical creation.

First conceived by André in prison, and finally proven three decades later by Pierre Deligne in 1974, the Weil conjectures are foundational pillars of algebraic geometry. Linking the continuous and the discrete, and the realms of topology and number theory, they are pertinent to efforts to unite gravity with the quantum theories of the other forces. Frustratingly, though, mathematical hobbyists hoping for insights into the conjectures will be disappointed by this book, which instead zeroes in on the people in orbit around the maths.

Olsson is particularly fascinated by Simone Weil. An iconoclastic public intellectual in France, and possessed by an intensely authentic humanity that the author presents as quite alien to André, Simone was nevertheless envious of her brother’s mathematical insight, writing that she “preferred to die than to live without that truth”. Olsson is clearly empathetic, and so, one would suspect, will be most readers in a profession where intellect is all. Whether one is a grad student or a foremost expert in the field, there is always someone smarter, whose insights seem inaccessible.

Physicists may also detect echoes of the current existential crisis in theoretical physics (see Redeeming the role of mathematics) in Simone’s thinking. While she feels that “unless one has exercised one’s mind at the gymnastics of mathematics, one is incapable of precise thought, which amounts to saying that one is good for nothing,” she criticises “the absolute dominion that is exercised over science by the most abstract forms of mathematics.”

Peppered with anecdotes about other mathematicians – Girolamo Cardano is described as a “total dick” – and more a succession of scenes than a biography, the book is as much about Olsson herself as the Weils. The prose zig-zags between vignettes from the author’s own life and the Weils without warning, leaving the reader to search for connections. Facts are unsourced, and readers are left to guess what is historical and what is the author’s impressionistic character portrait. Charming and quirky, the text transforms dusty perceptions of the meetings of the secret Bourbaki society of French mathematicians into scenes of lakeside debauchery and translucent camisoles that are almost reminiscent of Pride and Prejudice. Olsson even takes us into Simone’s dreams, with the conjectures only cropping up at the end of the book. If you limit your reading to the maths and the Weils, the resulting slim volume is a page turner.

Ernst-Wilhelm Otten 1934–2019

Ernst Otten initiated the KATRIN neutrino experiment, among many other achievements. Credit: University of Mainz

Ernst-Wilhelm Otten received his doctorate in 1962 at the University of Heidelberg under the supervision of atomic and nuclear physicist Hans Kopfermann. From 1972 until his retirement in 2002, he headed the department of experimental atomic and nuclear physics (EXAKT) at the University of Mainz. Ernst spent numerous research stays abroad, including at CERN and at the Ecole Normale Supérieure in Paris. After his retirement Ernst continued his research activities, especially for the KATRIN neutrino experiment. The hallmark of his work was the extraordinary breadth across almost all disciplines of physics, which earned him a large number of distinctions and prizes.

In Heidelberg, Ernst developed the method of optical pumping for polarising the nuclear spins of radioactive isotopes to determine their nuclear moments. He also recognised, from its start-up in the late 1960s, the opportunities offered by the on-line isotope separator ISOLDE at CERN. He became a pioneer of optical spectroscopy with accelerators. The discovery of unexpected nuclear-shape coexistence and nuclear-size changes in neutron-deficient mercury isotopes is one of the most outstanding results obtained at ISOLDE, as early as 1972. In Mainz, his group developed the high-resolution method of collinear laser spectroscopy – now a workhorse at ISOLDE for the determination of nuclear ground-state properties of short-lived nuclei – and with his collaborators initiated laser-based trace analysis for the detection of radionuclides in the environment.

The electron accelerators at Mainz enabled spectacular experiments: the test of parity non-conservation by neutral currents in polarised–electron nucleon scattering, and the determination of the neutron electric form factor using polarised ³He targets at high density. With hyperpolarised ³He gas, Ernst performed lung diagnostics by magnetic resonance imaging in collaboration with the German Cancer Research Centre in Heidelberg and the department of radiology at the University of Mainz.

In the 1980s, when a group reported a 30 eV mass of the antineutrino, Ernst developed a novel high-resolution beta spectrometer at Mainz to determine the neutrino mass very precisely from tritium decay. Together with his team he succeeded in setting an upper limit of 2 eV. After the discovery of neutrino oscillations in 1998, proving the existence of finite neutrino masses, Ernst initiated the KATRIN experiment at the Karlsruhe Institute of Technology to measure the neutrino mass. The construction of this technically extremely difficult spectrometer started in 2001, and Ernst was very actively involved until his death on 8 July. As such, he was able to witness the first successful result: the setting of a new upper limit on the neutrino mass of 1 eV (see KATRIN sets first limit on neutrino mass).

Ernst leaves deep traces in science and in the physics community. We will remember him as a great scientist, teacher, mentor and friend.

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