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Accelerating magnet technology

A Nb3Sn cable — **Wiring the future** A Nb₃Sn cable, showing the single strands and the glass-fibre insulation, partially unwrapped. Credit: CERN

The steady increase in the energy of colliders during the past 40 years was possible thanks to progress in superconducting materials and accelerator magnets. The highest particle energies have been reached by proton–proton colliders, where beams of high-rigidity travelling on a piecewise circular trajectory require magnetic fields largely in excess of those that can be produced using resistive electromagnets. Starting from the Tevatron in 1983, through HERA in 1991 (see Constructing HERA: rising to the challenge), RHIC in 2000 and finally the LHC in 2008 (see LHC insertions: the key to CERN’s new accelerator and Superconductivity and the LHC: the early days), all large-scale hadron colliders were built using superconducting magnets.

Large superconducting magnets for detectors are just as important to large high-energy physics experiments as beamline magnets are to particle accelerators. In fact, detector magnets are where superconductivity took its stronghold, right from the infancy of the technology in the 1960s, with major installations such as the large bubble-chamber solenoid at Argonne National Laboratory, followed by the giant BEBC solenoid at CERN, which held the record for the highest stored energy for many years. A long line of superconducting magnets has provided the field to the detectors of all large-scale high-energy physics colliders (see ALEPH coil hits the road and CMS: a super solenoid is ready for business), with the last and largest realisation being the LHC experiments, CMS and ATLAS.

All past accelerator and detector magnets have one thing in common: they were built using composite Nb-Ti/Cu wires and cables. Nb-Ti is a ductile alloy with a critical field of 14.5 T and critical temperature of 9.2 K, made from almost equal parts of the two constituents and discovered to be superconducting in 1962. Its performance, quality and cost have been optimised over more than half a century of research, development and large-scale industrial production. Indeed, it is unlikely that the performance of the LHC dipole magnets, operated so far at 7.7 T and expected to reach nominal conditions at 8.33 T, can be surpassed using the same superconducting material, or any foreseeable improvement of this alloy.

The HL-LHC springboard

And yet, approved projects and studies for future circular machines are all calling for the development of superconducting magnets that produce fields beyond those produced for the LHC. These include the High-Luminosity LHC (HL-LHC), which is currently taking place, and the Future Circular Collider design study (FCC), both at CERN, together with studies and programmes outside Europe, such as the Super proton–proton Collider in China (SppC) or the past studies of a Very Large Hadron Collider at Fermilab and the US–DOE Muon Accelerator Program. This requires that we turn to other superconducting materials and novel magnet technology.

**Luca Bottura** is head of CERN’s magnets, superconductors and cryostats group. Before joining CERN in 1995, he was part of the conceptual and engineering design team for ITER. He has worked on the magnets for the LHC and now undertakes R&D for high-field future accelerator magnets. Credit: M Brice/CERN-PHOTO-201707-173-4

To reach its main objective, to increase the levelled LHC luminosity at ATLAS and CMS by a factor of five and the integrated one by a factor of 10, HL-LHC requires very large-aperture quadrupoles, with field levels at the coil in the range of 12 T in the interaction regions. These quadrupoles, currently being produced, are the main fruit of the 10-year US-DOE LHC Accelerator Research Program (US–LARP) – a joint venture between CERN, Brookhaven National Laboratory, Fermilab and Lawrence Berkeley National Laboratory. In addition, the increased beam intensity calls for collimators to be inserted in locations within the LHC “dispersion suppressor”, the portion of the accelerator where the regular magnet lattice is modified to ensure that off-momentum particles are centered in the interaction points. To gain the required space, standard arc dipoles will be substituted by dipoles of shorter length and higher field, approximately 11 T. As described earlier, such fields require the use of new materials. For HL-LHC, the material of choice is the inter-metallic compound of niobium and tin Nb₃Sn, which was discovered in 1954. Nb₃Sn has a critical field of 30 T and a critical temperature of 18 K, outperforming Nb-Ti by a factor two. Though discovered before Nb-Ti, and exhibiting better performance, Nb₃Sn has not been used for accelerator magnets so far because in its final form it is brittle and cannot withstand large stress and strain without special precautions.

In fact, Nb₃Sn was one of the candidate materials considered for the LHC in the late 1980s and mid 1990s. Already at that time it was demonstrated that accelerator magnets could be built with Nb₃Sn, but it was also clear that the technology was complex, with a number of critical steps, and not ripe for large-scale production. A good 20 years of progress in basic material performance, cable development, magnet engineering and industrial process control was necessary to reach the present state, during which time the success of the production of Nb₃Sn for ITER (see ITER’s massive magnets enter production) has given confidence in the credibility of this material for large-scale applications. As a result, magnet experts are now convinced that Nb₃Sn technology is sufficiently mature to satisfy the challenging field levels required by HL-LHC.

The present manufacturing recipe for Nb₃Sn accelerator magnets consists of winding the magnet coil with glass-fibre insulated cables made of multi-filamentary wires that contain Nb and Sn precursors in a Cu matrix. In this form the cables can be handled and plastically deformed without breakage. The coils then undergo heat treatment, typically at a temperature of around 600 to 700 °C, during which the precursor elements react chemically and form the desired Nb₃Sn superconducting phase. At this stage, the reacted coil is extremely fragile and needs to be protected from any mechanical action. This is done by injecting a polymer, which fills the interstitial spaces among cables, and is subsequently cured to become a matrix of hardened plastic providing cohesion and support to the cables.

The above process, though conceptually simple, has a number of technical difficulties that call for top-of-the-line engineering and production control. To give some examples, the electrical insulation consisting of a few tenths of mm of glass-fibre needs to be able to withstand the high-temperature heat-treatment step, but also retain dielectric and mechanical properties at liquid helium temperatures 1000 degrees lower. The superconducting wire also changes its dimensions by a few percent, which is orders of magnitude larger than the dimensional accuracy requested for field quality and therefore must be predicted and accommodated for by appropriate magnet and tooling design. The finished coil, even if it is made solid by the polymer cast, still remains stress and strain sensitive. The level of stress that can be tolerated without breakage can be up to 150 MPa, to be compared to the electromagnetic stress of optimised magnets operating at 12 T that can reach levels in the range of 100 MPa. This does not leave much headroom for engineering margins and manufacturing tolerances. Finally, protecting high-field magnets from quench, with their large stored energy, requires that the protection system has a very fast reaction – three times faster than at the LHC – and excellent noise rejection to avoid false trips related to flux jumps in the large Nb₃Sn filaments.

The CERN magnet group, in collaboration with the US-DOE laboratories participating in the LHC Accelerator Upgrade Project, is in the process of addressing these and other challenges, finding solutions suitable for a magnet production on the scale required for HL-LHC. A total of six 11 T dipoles (each about 6 m long) and 20 inner triplet quadrupoles (up to 7.5 m long) are in production. And yet, it is clear that we are not ready to extrapolate such production on a much larger scale, i.e. to the thousands of magnets required for a future hadron collider such as FCC-hh. This is exactly why HL-LHC is so critical to the development of high-field magnets for future accelerators: not only will it be the first demonstration of Nb₃Sn magnets in operation, steering and colliding beams, but by building it on a scale that can be managed at the laboratory level we have a unique opportunity to identify all the areas of necessary development, and the open technology issues, to allow the next jump. Beyond its prime physics objective, HL-LHC is the springboard into the future of high-field accelerator magnets.

The climb to higher peak fields

For future circular colliders, the target dipole field has been set at 16 T for FCC-hh, allowing proton-proton collisions at an energy of 100 TeV, while the SppC aims at a 12 T dipole field as a first step, to be followed by a 20 T dipole. Are these field levels realistic? And based on which technology?

The large-bore Nb3Sn dipole FRESCA2 at CERN — **FRESCA2** The large-bore Nb₃Sn dipole FRESCA2 at CERN, built in collaboration with CEA-Saclay, reached a record dipole field of 14.6 T at 1.9 K in 2018. FRESCA2 is planned to be used as the background field magnet for a new, high-field test station at CERN. Credit: CERN

Looking at the dipole fields produced by Nb₃Sn development magnets during the past 40 years (figure 1), fields up to 16 T have been achieved in R&D demonstrators, suggesting that the FCC target can be reached. In 2018 “FRESCA2” – a large-aperture dipole developed over the past decade through a collaboration between CERN and CEA-Saclay in the framework of the European Union project EuCARD – attained a record field of 14.6 T at 1.9 K (13.9 T at 4.5 K). Another very relevant recent result is the successful test at Fermilab of the high-field dipole MDPCT1, which reached a field of 14.1 T at 4.5 K earlier this year.

A field of 16 T seems to be the upper limit that can be reached with Nb₃Sn. Indeed, though the conductor performance can still be improved, as demonstrated by recent results obtained at NHMFL, OSU and FNAL within the scope of the US-DOE Magnet Development Program, this is the point at which the material itself will run out of steam: as for any other superconductor, the critical current density drops as the field is increased, requiring an increasing amount of material to carry a given current. This effect becomes dramatic approaching a significant fraction of the critical field. Akin to Nb-Ti in the range of 8 T, a further field increase with Nb₃Sn beyond 16 T would require an exceedingly large coil and an impractical amount of conductor. Reaching the ultimate performance of Nb₃Sn, which will be situated between the present 12 T and the expected maximum of 16 T, still requires much work. The technology issues identified by the ongoing work on the HL-LHC magnets are exacerbated by the increase in field, electro-magnetic force and stored energy. Innovative industrial solutions will be needed, and the conductor itself brought to a level of maturity comparable to Nb-Ti in terms of performance, quality and cost. This work is the core of the ongoing FCC magnet development programme that CERN is pursuing in collaboration with laboratories, universities and industries worldwide.

As the limit of Nb₃Sn comes into view, we see history repeating itself: the only way to push beyond it to higher fields will be to resort to new materials. Since Nb₃Sn is technically the low-temperature superconductor (LTS) with the highest performance, this will require a transition to high-temperature superconductors (HTS).

Brave new world of HTS

High-temperature superconductors, discovered in 1986, are of great relevance in the quest for high fields. When operated at low temperature (the same liquid-helium range as LTS), they have exceedingly large critical fields in the range of 100 T and above. And yet, only recently the material and magnet engineering has reached the point where HTS materials can generate magnetic fields in excess of LTS ones. The first user applications coming to fruition are ultra-high-field NMR magnets, as recently delivered by Bruker Biospin, and the intense magnetic fields required by material science, for example the 32 T all-superconducting user facility built by the US National High Magnetic Field Laboratory.

Diagram of record fields attained with Nb3Sn dipole magnets — **Fig. 1.** Record fields attained with Nb₃Sn dipole magnets of various configurations and dimensions, and either at liquid (4.2 K, red) or superfluid (1.9 K, blue) helium temperature. Solid symbols are short (< 1 m) demonstrator “racetracks” with no bore, while open symbols are short models (> 1 m) and long magnets with bore. For comparison, superconducting colliders past and present are shown as triangles. Credit: CERN

As for their application in accelerator magnets, the potential of HTS to make a quantum leap is enormous. But it is also clear that the tough challenges that needed to be solved for Nb₃Sn will escalate to a formidable level in HTS accelerator magnets. The magnetic force scales with the square of the field produced by the magnet, and for HTS the problem will no longer be whether the material can carry the super-currents, but rather how to manage stresses approaching structural material limits. Stored energy has the same square dependence on the field, and quench detection and protection in large HTS magnets are still a spectacular challenge. In fact, HTS magnet engineering will probably differ so much from the LTS paradigm that it is fair to say that we do not yet know whether we have identified all the issues that need to be solved. HTS is the most exciting class of material to work with; the new world for brave explorers. But it is still too early to count on practical applications, not least because the production cost for this rather complex class of ceramic materials is about two orders of magnitude higher than that of good old Nb-Ti.

It is quite logical to expect the near future to be based mainly on Nb₃Sn. With the first demonstration to come imminently, in the LHC, we need to consolidate the technology and bring it to the maturity necessary on a large-scale production. This may likely take place in steps – exploring 12 T territory first, while seeking the solutions to the challenges of ultimate Nb₃Sn performance towards 16 T – and could take as long as a decade.

Meanwhile, nurtured by novel ideas and innovative solutions, HTS could grow from the present state of a material of great potential to its first applications. The grand challenges posed by HTS will likely require a revolution rather than an evolution of magnet technology, and significant technology advancement leading to large-scale application in accelerators can only be imagined on the 25-year horizon.

Road to the future

There are two important messages to retain from this rather simplified perspective on high-field magnets for accelerators. Firstly, given the long lead times of this technology, and even in times of uncertainty, it is important to maintain a healthy and ambitious programme so that the next step in technology is at hand when critical decisions on the accelerators of the future are due. The second message is that with such long development cycles and very specific technology, it is not realistic to rely on the private sector to advance and sustain the specific demands of HEP. In fact, the business model of high-energy physics is very peculiar, involving long investment times followed by short production bursts, and not sustainable by present industry standards. So, without taking the place of industry, it is crucial to secure critical know-how and infrastructure within the field to meet development needs and ensure the long-term future of our accelerators, present and to come.

The galaxy that feeds three times per day

All galaxies are thought to contain a super-massive black hole (SMBH) at their centres, one of which was famously pictured for the first time by the Event Horizon Telescope only a few months ago (CERN Courier May/June 2019 p10). Both the size and activity of such SMBHs differ significantly from galaxy to galaxy: some galaxies contain an almost dormant black hole at their centre, while in others the SMBH is accumulating surrounding matter at a vast rate resulting in bright emission with energies ranging from the radio to the X-ray regime.

While solar-mass black holes can show dramatic variations in their emission on the time scale of days or even hours, such time scales increase with size, meaning that for an SMBH one would not expect much change during years or even centuries. However, observations during the past decade have revealed sudden increases. In 2010 the X-ray emission from a galaxy called GSN 069, which has a relatively small SMBH (400,000 solar masses), became 240 times brighter compared to observations in 1989 – turning it into an active galaxy. In such objects the matter falling into the central SMBH releases radiation when it approaches the event horizon (the boundary beyond which nothing can escape the black hole’s gravitational field).

The brightness of the emission produced as the SMBH feeds on the surrounding disk of matter typically varies randomly on short time scales, a result of a change in accretion rate and turbulence in the disk. But subsequent observations with the European Space Agency’s X-ray satellite XMM-Newton in 2018 revealed never-before-seen behavior. The object emitted strong bursts of X-rays lasting about one hour. Even more surprising was that the bursts appeared to occur at very consistent intervals of nine hours. Follow-up observations in 2019 with both XMM-Newton and NASA’s Chandra X-ray telescope have now confirmed this picture. While simultaneous observations at radio wavelengths showed no variability, the intensity of the bursts at X-ray wavelengths decreased. An extrapolation of this decrease indicates that, by now, the bursts should have fully disappeared, although further observations are needed to confirm this.

XMM-Newton data — The X-ray emission of the galaxy GSN 069 measured by the XMM-Newton telescope, showing a burst every nine hours. Credit: Nature.

The team behind the latest observations, published in Nature, has no clear explanation of what causes such extreme periodic behaviour from such a massive object. One possibility, claims the paper, is that it is the result of a second SMBH orbiting the main one: each time it crosses the disk of matter a burst would be expected. However, the associated variation would be expected to be more smooth than is observed. Furthermore, no such bursts were seen in the 2010 observations, making this theory implausible. Another explanation is that a semi-destroyed star is currently orbiting the SMBH, disturbing the accretion rate. The last and most probable hypothesis is that the quasi-periodic explosions are a result of complex oscillations in the disk of hot matter surrounding the SMBH induced by instabilities. The authors make it clear, however, that deeper studies are required to fully explain this new phenomenon.

Although only observed for the first time in GSN 069, it could very well be that other galaxies exhibit a similar behaviour. Other SMBHs with masses many orders of magnitude larger could exhibit the same periodic burst but on time scales of months or years, explaining why no one has ever noticed them. So while it could be that GSN 069 is simply a strange galaxy, the finding could have large implications for galaxies in general.

Further reading:
G Miniutti et al. 2019 Nature 573 381.

European strategy enters next phase

The European Strategy for Particle Physics got under way last year. Credit: CERN

Physicists in Europe have published a 250-page “briefing book” to help map out the next major paths in fundamental exploration. Compiled by an expert physics-preparatory group set up by the CERN Council, the document is the result of an intense effort to capture the status and prospects for experiment, theory, accelerators, computing and other vital machinery of high-energy physics.

Last year, the European Strategy Group (ESG) — which includes scientific delegates from CERN’s member and associate-member states, directors and representatives of major European laboratories and organisations and invitees from outside Europe — was tasked with formulating the next update of the European strategy for particle physics. Following a call for input in September 2018, which attracted 160 submissions, an open symposium was held in Granada, Spain, on 13-16 May at which more than 600 delegates discussed the potential merits and challenges of the proposed research programmes. The ESG briefing book distills input from the working groups and the Granada symposium to provide an objective scientific summary.

“This document is the result of months of work by hundreds of people, and every effort has been made to objectively analyse the submitted inputs,” says ESG chair Halina Abramowicz of Tel Aviv University. “It does not take a position on the strategy process itself, or on individual projects, but rather is intended to represent the forward thinking of the community and be the main input to the drafting session in Germany in January.”

Collider considerations
An important element of the European strategy update is to consider which major collider should follow the LHC. The Granada symposium revealed there is clear support for an electron–positron collider to study the Higgs boson in greater detail, but four possible options at different stages of maturity exist: an International Linear Collider (ILC) in Japan, a Compact Linear Collider (CLIC) or Future Circular Collider (FCC-ee) at CERN, and a Circular Electron Positron Collider (CEPC) in China. The briefing book states that, in a global context, CLIC and FCC-ee are competing with the ILC and with CEPC. As Higgs factories, however, the report finds all four to have similar reach, albeit with different time schedules and with differing potentials for the study of physics topics at other energies.

Also considered in depth are design studies in Europe for colliders that push the energy frontier, including a 3 TeV CLIC and a 100 TeV circular hadron collider (FCC-hh). The briefing book details the estimated timescales to develop some of these technologies, observing that the development of 16 T dipole magnets for FCC-hh will take a comparable time (about 20 years) to that projected for novel acceleration technologies such as plasma-wakefield techniques to reach conceptual designs.

“The Granada symposium and the briefing book mention the urgent need for intensifying accelerator R&D, including that for muon colliders,” says Lenny Rivkin of Paul Scherrer Institut, who was co-convener of the chapter on accelerator science and technology. “Another important aspect of the strategy update is to recognize the potential impact of the development of accelerator and associated technology on the progress in other branches of science, such as astroparticle physics, cosmology and nuclear physics.”

The bulk of the briefing book details the current physics landscape and prospects for progress, with chapters devoted to electroweak physics, strong interactions, flavour physics, neutrinos, cosmic messengers, physics beyond the Standard Model, and dark-sector exploration. A preceding chapter about theory emphasises the importance of keeping theoretical research in fundamental physics “free and diverse” and “not only limited to the goals of ongoing experimental projects”. It points to historical success stories such as Peter Higgs’ celebrated 1964 paper, which had the purely theoretical aim to show that Gilbert’s theorem is invalid for gauge theories at a time when applications to electroweak interactions were well beyond the horizon.

“While an amazing amount of progress has been made in the past seven years since the Higgs boson discovery, our knowledge of the couplings of the Higgs-boson to the W and Z and to third-generation charged fermions is quite imprecise, and the couplings of the Higgs boson to the other charged fermions and to itself are unmeasured,” says Beate Heinemann of DESY, who co-convened the report’s electroweak chapter. “The imperative to study this unique particle further derives from its special properties and the special role it might play in resolving some of the current puzzles of the universe, for example dark matter, the matter-antimatter asymmetry or the hierarchy problem.”

Readers are reminded that the discovery of neutrino oscillations constitutes a “laboratory” proof of physics beyond the Standard Model. The briefing book also notes the significant role played by Europe, via CERN, in neutrino-experiment R&D since the last strategy update concluded in 2013. Flavour physics too should remain at the forefront of the European strategy, it argues, noting that the search for flavour and CP violation in the quark and lepton sectors at different energy frontiers “has a great potential to lead to new physics at moderate cost”. An independent determination of the proton structure is needed if present and future hadron colliders are to be turned into precision machines, reports the chapter on strong interactions, and a diverse global programme based on fixed-target experiments as well as dedicated electron-proton colliders is in place.

Europe also has the opportunity to play a leading role in the searches for dark matter “by fully exploiting the opportunities offered by the CERN facilities, such as the SPS, the potential Beam Dump Facility, and the LHC itself, and by supporting the programme of searches for axions to be hosted at other European institutions”. The briefing book notes the strong complementarity between accelerator and astrophysical searches for dark matter, and the demand for deeper technology sharing between particle and astroparticle physics.

Scientific diversity
The diversity of the experimental physics programme is a strong feature of the strategy update. The briefing book lists outstanding puzzles that did not change in the post-Run 2 LHC era – such as the origin of electroweak symmetry breaking, the nature of the Higgs boson, the pattern of quark and lepton masses and the neutrino’s nature – that can also be investigated by smaller scale experiments at lower energies, as explored by CERN’s dedicated Physics Beyond Colliders initiative.

Finally, in addressing the vital roles of detector & accelerator development, computing and instrumentation, the report acknowledges both the growing importance of energy efficiency and the risks posed by “the limited amount of success in attracting, developing and retaining instrumentation and computing experts”, urging that such activities be recognized correctly as fundamental research activities. The strong support in computing and infrastructure is also key to the success of the high-luminosity LHC which, the report states, will see “a very dynamic programme occupying a large fraction of the community” during the next two decades – including a determination of the couplings between the Higgs boson and Standard Model particles “at the percent level”.

Following a drafting session to take place in Bad Honnef, Germany, on 20-24 January, the ESG is due to submit its recommendations for the approval of the CERN Council in May 2020 in Budapest, Hungary.

“Now comes the most challenging part of the strategy update process: how to turn the exciting and well-motivated scientific proposals of the community into a viable and coherent strategy which will ensure progress and a bright future for particle physics in Europe,” says Abramowicz. “Its importance cannot be overestimated, coming at a time when the field faces several crossroads and decisions about how best to maintain progress in fundamental exploration, potentially for generations to come.”

Hadron therapy to get heavier in Southeast Europe

Montenegro prime minister Duško Marković marks the start of the SEEIIST design phase on 18 September. Credit: gov.me/B Ćupić

A state-of-the-art facility for hadron therapy in Southeast Europe has moved from its conceptual to design phase, following financial support from the European Commission. At a kick-off meeting held on Wednesday 18 September in Budva, Montenegro, more than 120 people met to discuss the future South East European International Institute for Sustainable Technologies (SEEIIST) – a facility for tumour therapy and biomedical research that follows the founding principles of CERN.

“This is a region that has no dilemma regarding its European affiliation, and which, I believe, will be part of a joint European competition for technological progress. Therefore, the International Institute for Sustainable Technologies is an urgent need of our region,” said Montenegro prime minister Duško Marković during the opening address. “I am confident that the political support for this project is obvious and indisputable. The memorandum of understanding was signed by six prime ministers in July this year in Poznan. I believe that other countries in the region will formally join the initiative.”

The idea for SEEIIST germinated three years ago at a meeting of trustees of the World Academy of Art and Science in Dubrovnik, Croatia. It is the brainchild of former CERN Director-General Herwig Schopper, and has benefitted from a political push from Montenegro minister of science Sanja Damjanović, who is also a physicist who works at CERN and GSI-FAIR in Darmstadt, Germany. SEEIIST aims to create a platform for internationally competitive research in the spirit of the CERN model “science for peace”, stimulating the education of young scientists, building scientific capacity and fostering greater cooperation and mobility in the region.

SEEIIST event — Montenegro science minister Sanja Damjanović (left) and prime minister Duško Marković (right) with SEEIIST founder Herwig Schopper at the Budva event. Credit: gov.me/B Ćupić

In January 2018, at a forum at the International Centre for Theoretical Physics in Italy held under the auspices of UNESCO, the International Atomic Energy Agency and the European Physical Society, two possibilities for a large international institute were presented: a synchrotron X-ray facility and a hadron-therapy centre. Soon afterwards, the 10 participating parties of SEEIIST’s newly formed intergovernmental steering committee chose the latter.

Europe has played a major role in the development of hadron therapy, with numerous centres currently offering proton therapy and four facilities offering proton and more advanced carbon-ion treatment. But currently no such facility exists in Southeast Europe despite a growing number of tumours being diagnosed there. SEEIIST will follow the idea of the “PIMMS” accelerator design started at CERN two decades ago, profiting from the experience at the dual proton–ion centres CNAO in Italy and MedAustron in Austria, and also centres at GSI and in Heidelberg. It will be a unique facility that splits its beam time 50:50 between treating patients and performing research with a wide range of different ions for radiobiology, imaging and treatment planning. The latter will include studies into the feasibility of heavier ions such as oxygen, making SEEIIST distinct in this rapidly growing field.

The next steps are to prepare a definite technical design for the facility, to propose a structure and business plan and to define the conditions for the site selection. To carry out these tasks, several working groups are being established in close collaboration with CERN and GSI-FAIR. “This great event was a culmination of the continuous efforts invested since 2017 into the project,” says Damjanović. “If all goes well, construction is expected to start in 2023, with first patient treatment in 2028.”

KATRIN sets first limit on neutrino mass

Based on just four weeks of running, researchers at the Karlsruhe Tritium Neutrino (KATRIN) experiment in Germany have set a new model-independent bound on the mass of the neutrino. At a colloquium today, the collaboration reported an upper limit of 1.1 eV at 90% confidence, almost halving the previous bound.

Neutrinos are among the least well understood particles in the Standard Model. Their three known mass eigenstates do not match up with the better-known flavour eigenstates, but mix according to the PMNS matrix, resulting in the flavour transmutations seen by neutrino-oscillation experiments. Despite their success in constraining neutrino mixing, such experiments are sensitive only to squared mass differences between the eigenstates, and not to the neutrino masses themselves.

Physicists have pursued direct mass measurements since Reines and Cowan observed electron antineutrinos in inverse beta decays in 1956. The direct mass measurement method hinges on precisely measuring the energy spectrum of beta-decay electrons, and is considered model independent as the extracted neutrino mass depends only on the kinematics of the decay. KATRIN is now the most precise experiment of this kind. It builds on the invention of gaseous molecular tritium sources and spectrometers based on the principle of magnetic adiabatic collimation with electrostatic filtering. The combination of these methods culminated in the previous best limits of 2.3 eV at 95% confidence in 2005, and 2.05 eV at 95% confidence in 2011, by physicists working in Mainz, Germany and Troitsk, Russia, respectively. The KATRIN analysis improves on these experimental results, with systematic uncertainties reduced by a factor of six and statistical uncertainties reduced by a factor of two.

These are exciting times for the collaboration
Guido Drexlin

“These are exciting times for the collaboration,” said KATRIN co-spokesperson Guido Drexlin. “The first KATRIN result is based on a measurement campaign of only four weeks at reduced source activity, equivalent to five days at nominal activity.” To reach its final sensitivity, KATRIN will collect data for 1000 days, and systematic errors will be reduced. “This will allow us to probe neutrino masses down to 0.2 eV,” continued Drexlin, “as well as many other interesting searches for beyond-the-Standard-Model physics, such as for admixtures of sterile neutrinos from the eV up to the keV scale.”

Conceived almost two decades ago, KATRIN operates using a high-resolution, large-acceptance and low-background measurement of the decay spectrum of tritium ³H → ³He e^– ν̄_e. Electrons are transported to the spectrometer via a beamline that was completed in autumn 2016, allowing experimenters to search for distortions in the tail of the electron energy distribution that depend on the absolute mass of the neutrino. KATRIN collaborators are now looking forward to a two-month measurement campaign, which will start in a few days. It will feature a signal-to-background ratio that is expected to be about one order of magnitude better than the initial measurements, due to an increase in source activity, and a decrease in background due to hardware upgrades. The goal is to achieve an activity of 10¹¹ beta-decay electrons per second, while reducing the current background level by about a factor of two.

Direct measurements are not the only handle on neutrino masses available to physicists, though they are certainly the most model independent. Experiments searching for neutrinoless double beta-decay offer a complementary limit, but must assume that the neutrino is a Majorana fermion.

The tightest limit on neutrino masses comes from cosmology. Comparing data from the Planck satellite with simulations of the development of structure in the early universe yields an upper limit on the sum of all three neutrino masses of 0.17 eV at 95% confidence.

The Planck limit is fairly robust, and one would have to go to great lengths to avoid it
Joachim Kopp

“The Planck limit is fairly robust, and one would have to go to great lengths to avoid it – but it’s not impossible to do so,” says CERN theorist Joachim Kopp. For example, it would be invalidated by a scenario where as-yet-undiscovered right-handed neutrinos couple to a new scalar field with a vacuum expectation value that evolves over cosmological timescales. “Planck data tell us what neutrinos were like in the early universe,” says Kopp. “The value of KATRIN lies in testing neutrinos now.”

Black-hole snap scoops 2020 Breakthrough Prize in Fundamental Physics

The first direct image of a black hole, obtained by the Event Horizon Telescope (EHT) collaboration earlier this year, has been recognized by the 2020 Breakthrough Prize in Fundamental Physics. The $3 million prize will be shared equally between 347 researchers who were co-authors of the six papers published by the EHT collaboration on 10 April.

The EHT is a network of eight radio dishes in Antarctica, Chile, Mexico, Hawaii, Arizona and Spain that creates an Earth-sized interferometer. Its ultra-high angular resolution images of radio emission from a supermassive black hole at the heart of galaxy M87* opened a new window on black holes and other phenomena. Recently, a team at Brookhaven National Laboratory used the EHT image to disfavour “fuzzy” models of ultra-light boson dark matter.

Also announced were six New Horizons Prizes worth $100,000 each, which recognize early-career achievements in physics and mathematics. In physics, Jo Dunkley (Princeton); Samaya Nissanke (University of Amsterdam) and Kendrick Smith (Perimeter Institute) were awarded for the development of novel techniques to extract fundamental physics from astronomical data. Simon Caron-Huot (McGill University) and Pedro Vieira (Perimeter Institute) were recognized for their “profound contributions to the understanding of quantum field theory”.

The Breakthrough Prize was founded in 2012 by former physicist and entrepreneur Yuri Milner, with sponsors including Google’s Sergey Brin and Facebook’s Mark Zuckerberg. In August, a Special Breakthrough Prize in Fundamental physics was awarded to Sergio Ferrara, Daniel Freedman and Peter van Nieuwenhuizen for the discovery of supergravity.

All prize recipients, along winners in mathematics and biology, will receive their awards at a ceremony in California on 3 November.

Black-hole image constrains ultra-light dark matter

EHT black hole — Supermassive black hole M87*. Credit: EHT Collaboration

Hooman Davoudiasl and Peter Denton of Brookhaven National Laboratory have used the recent Event Horizon Telescope image of supermassive black hole M87* to disfavour “fuzzy” models of ultra-light boson dark matter with masses of the order of a few 10^-21 eV (Phys. Rev. Lett. 123 021102). The inferred mass, spin and age of the black hole are incompatible with the existence of such fuzzy dark matter given the principle of superradiance, whereby quantum fluctuations deplete the angular momentum of a rotating black hole by populating a cloud of bosons around it. The effect depends only on the bosons’ mass, and does not presuppose any non-gravitational interactions. Future measurements of M87* and other spinning supermassive black holes have the potential to exclude the entire parameter space for fuzzy dark matter.

An intriguing alternative to cold dark matter, fuzzy dark matter could address the “core-cusp problem”, wherein observations of an approximately constant dark matter density in the inner parts of galaxies conflict with the steep power-law-like behaviour of cosmological simulations. The particles’ long de Broglie wavelengths, of the order of a kiloparsec, would suppress structure at this scale.

A new centre for astroparticle theory

Gian Giudice, Teresa Montaruli, Eckhard Elsen and Job de Kleuver — **Merging minds** Gian Giudice (head of CERN-TH), Teresa Montaruli (APPEC chair), Eckhard Elsen (CERN director for research and computing) and Job de Kleuver (APPEC secretary-general) with the new agreement. Credit: CERN-PHOTO-201907-190-1

On 10 July, CERN and the Astroparticle Physics European Consortium (APPEC) founded a new research centre for astroparticle physics theory called EuCAPT. Led by an international steering committee comprising 12 theorists from institutes in France, Portugal, Spain, Sweden, Germany, the Netherlands, Italy, Switzerland and the UK, and from CERN, EuCAPT aims to coordinate and promote theoretical physics in the fields of astroparticle physics and cosmology in Europe.

Astroparticle physics is undergoing a phase of profound transformation, explains inaugural EuCAPT director Gianfranco Bertone, who is spokesperson of the Centre for Gravitation and Astroparticle Physics at the University of Amsterdam. “We have recently obtained extraordinary results such as the discovery of high-energy cosmic neutrinos with IceCube, the direct detection of gravitational waves with LIGO and Virgo, and we have witnessed the birth of multi-messenger astrophysics. Yet we have formidable challenges ahead of us: understanding the nature of dark matter and dark energy, elucidating the origin of cosmic rays, understanding the matter-antimatter asymmetry problem, and so on. These are highly interdisciplinary problems that have ramifications in cosmology, particle, and astroparticle physics, and that are best addressed by a strong and diverse community of scientists.”

The construction of experimental astroparticle facilities is coordinated by APPEC, but until now there was no Europe-wide coordination of theoretical activities, says Bertone. “We want to be open and inclusive, and we hope that all interested scientists will feel welcome to join this new initiative.” On a practical level, EuCAPT aims to coordinate scientific and training activities, help researchers attract adequate resources for their projects, and promote a stimulating and open environment in which young scientists can thrive. CERN will act as the central hub of the consortium for the first five years.

It is not a coincidence that CERN has been chosen as the central hub of EuCAPT, says Gian Giudice, head of CERN’s theory department. “The research that we are doing at CERN-TH is an exploration of the possible links between physics at the smallest and largest scales. Creating a collaborative network among European research centres in astroparticle physics and cosmology will boost activities in these fields and foster dialogue with particle physics,” he says. “Dark matter, dark energy, inflation and the origin of large-scale structures are big questions regarding the universe. But there are good hints that suggest that their explanation has to be looked for in the domain of particle physics.”

CERN and ESA join forces in harsh environments

The effects of radiation on electronics for the JUICE mission — **Europa** A recent project at CERN evaluated the effects of radiation on electronics for the Jupiter Icy Moons Explorer (JUICE) mission. Credit: Galileo Project/JPL/NASA

Strengthening connections between particle physics and related disciplines, CERN signed a collaboration agreement with the European Space Agency (ESA) on 11 July to address the challenges of operating equipment in harsh radiation environments. Such environments are found in both particle-physics facilities and outer space, and the agreement identifies several high-priority projects, including: high-energy electron tests; high-penetration heavy-ion tests; assessment of commercial components and modules; radiation-hard and radiation-tolerant components and modules; radiation detectors, monitors and dosimeters; and simulation tools for radiation effects. Important preliminary results have already been achieved in some areas, including high-energy electron tests of electronics for the Jupiter Icy Moons Explorer (JUICE) mission performed at CERN’s CLEAR/VESPER facility.

CMS revisits rare and beautiful decays

Two muons emerge from a Bs → μμ decay candidate — **Beauty and strangeness** Two muons (red) emerge from a B_s → μμ decay candidate in the CMS Run 2 data. The inset shows the displacement of the decay vertex from the collision point (yellow). Credit: CERN

The B_s meson is a bound state of a strange quark and a beauty antiquark – as such it possesses both beauty and strangeness. For many years the search for its extremely rare decay to a μ⁺μ^– pair was a holy grail of particle physics, because of its sensitivity to theories that extend the Standard Model (SM). The SM predicts the decay rate for B_s → μ⁺μ^– to be only about 3.6 parts per billion (ppb). Its lighter cousin, the B⁰, which is made from a down quark and a beauty antiquark, has an even lower predicted branching fraction for decays to a μ⁺μ^– pair of 0.1 ppb. If beyond-the-SM particles exist, however, the predictions could be modified by their presence, giving the decays sensitivity to new physics that rivals and might even exceed that of direct searches.

It took more than a quarter of a century of extensive effort to establish B_s → μ⁺μ^–, and the first observation was presented in 2013, in a joint publication by the CMS and LHCb collaborations based on LHC Run 1 data. The same paper reported evidence for B⁰ → μ⁺μ^– with a significance of three standard deviations, however, this signal has not subsequently been confirmed by CMS, LHCb or ATLAS analyses. A new CMS Run 2 analysis now looks set to bolster interest in these intriguing decays.

Diagram of probability contours — **Fig. 1.** Probability contours representing the simultaneous measurement of the relative probabilities for B_s → μμ and B⁰ → μμ decays. The contours correspond to one to five standard deviations, and the red point indicates the SM predictions. Credit: CERN

The CMS collaboration has updated its 2013 analysis with higher centre-of-mass-energy Run 2 data from 2016, permitting an observation of B_s → μ⁺μ^– with a significance of 5.6 standard deviations (figure 1). The results are consistent with the latest results from ATLAS and LHCb, and while no significant deviation from the SM is observed by any of the experiments, all three decay rates are found to lie slightly below the SM prediction. The slight deficit is not significant, but the trend is intriguing because it could be related to so-called flavour anomalies recently observed by the LHCb experiment in other rare decays of B mesons (CERN Courier May/June p9). This makes the new CMS measurement even more exciting. The new analysis showed no sign of B⁰ → μ⁺μ^–, and a stringent 95% confidence limit of less than 0.36 ppb was set on its rate.

CMS also managed to measure the effective lifetime of the B_s meson using the several dozen B_s → μ⁺μ^– decay events that were observed. The interest in measuring this lifetime is that, just as for the branching fraction, new physics might alter its value from the SM expectation. This measurement yielded a lifetime of about 1.7 ps, consistent with the SM. The measured CMS value is also consistent with the only other such lifetime measurement, performed by LHCb.

With three times more Run 2 data yet to be analysed by CMS, the next update – based on the full Run 1 and Run 2 datasets – may shed more light on this fascinating corner of physics, and move us closer to the ultimate goal, which is the observation of the B⁰ → μ⁺μ^– decays.

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