Scientific essays well suited to the interested layperson are notoriously difficult to write. It is then not surprising that various popular books, articles and internet sites recycle similar analogies – or even entire discussions – to explain scientific concepts with the same standardised, though very polished, language. CERN theorist Alvaro De Rújula recently challenged this unfortunate and relatively recent trend by proposing a truly original and unconventional essay for agile minds. There are no doubts that this book will be appreciated not only by the public but also by undergraduate students, teachers and active scientists.
Enjoy our Universe consists of 37 short chapters accounting for the serendipitous evolution of basic science in the last 150 years, roughly starting with the Faraday–Maxwell unification and concluding with the discovery of the Higgs boson and of gravitational waves. While going through the “fun” of our universe, the author describes the conceptual and empirical triumphs of classical and quantum field theories without indulging in excessive historic or technical details. Those who had the chance to attend lectures or talks given by De Rujula will recognise the “parentheses” (i.e. swift digressions) that he literally opens and closes in his presentations with gigantic brackets on the slides. A rather original glossary is included at the end of the text for the benefit of general readers.
This book is also a collection of opinions, reminiscences and healthy provocations of an active scientist whose contributions undeniably shaped the current paradigm of fundamental interactions. This is a bonus for practitioners of the field (and for curious colleagues), who will often find the essence of long-standing diatribes hidden in a collection of apparently innocent jokes or in the caption of a figure. As the author tries to argue in his introduction, science should always be discussed with that joyful and playful attitude we normally use when talking about sport and other interesting matters not immediately linked to the urgencies of daily life.
One of the most interesting subliminal suggestions of this book is that physics is not a closed logical system. Basic science in general (and physics in particular) can only prosper if the confusion of ideas is tolerated and encouraged, at least within certain reasonable limits.
The text is illustrated with drawings by the author himself and this aspect, among others, brings to mind an imaginative popular essay by George Gamow (Gravity 1962), where the author drew his own illustrations (unfortunately not in colour) with a talent comparable to De Rújula’s. The inspiration in this book is also a reminder of the autobiographical essay of Victor Weisskopf written almost 30 years ago, entitled The Joy of Insight, which echoes the enjoyment of the universe and suggests that the true motivation for basic science is the fun of curiosity: all the rest is irrelevant. So, please, enjoy our universe since you have no other choice!
Enrico Fermi can be considered as one of the greatest physicists of all time due to his genius creativity in both theoretical and experimental physics. This book describes his prodigious story, as a man and a scientist.
Born in Rome in 1901, Fermi spent the first part of his life in Italy, where he made his brilliant debut in theoretical physics in 1926 by applying statistical mechanics to atomic physics in a quantum framework, thus sealing the birth of what is now known as Fermi–Dirac statistics. In 1933 he postulated the original theory of weak interactions to explain the mysterious results on nuclear ß decays. Having soon become a theoretical “superstar”, he then switched to experimental nuclear physics, leading a celebrated team of young physicists at the University of Rome, known as the “boys”. Among them were Edoardo Amaldi, Ettore Majorana, Bruno Pontecorvo, Franco Rasetti and Emilio Segrè. They nicknamed him “the Pope” since he knew and understood everything and was considered to be simply infallible. His discoveries on neutron-induced radioactivity and on the neutron slowing-down effect earned him the Nobel Prize in Physics in 1938.
Those were, however, difficult years for Fermi because of Italy’s inconsistent research strategy and harsh political situation of fascism and antisemitism. Fermi left with his family to go to the US in December 1938, using the Nobel ceremony as a chance to travel abroad. Initially at Columbia University, Fermi then moved to the “Met Lab” of the University of Chicago, which was the seed of the Manhattan Project. There, he created the first self-sustained nuclear reactor in December 1942. The breakthrough ushered in the nuclear age, leaving a lasting impact on physics, engineering, medicine and energy – not to mention the development of nuclear weapons. In 1944 Fermi moved to the Manhattan Project’s secret laboratory in Los Alamos. Within this project, he collaborated with some of the world’s top scientists, including Hans Bethe, Niels Bohr, Richard Feynman, John von Neumann, Isidor Rabi, Leo Szilard and Edward Teller. These were terrible times of war.
When the Second World War concluded, Fermi resumed his research activities with energy and enthusiasm. On the experimental front he focused on nuclear physics, particle accelerators and technology, and early computers. On the theoretical front he concentrated on the origin of extreme high-energy cosmic rays. He also campaigned on the peaceful use of nuclear physics. As in Rome, in Chicago he was also the master of a wonderful school of pupils, among whom were several Nobel laureates. Fermi sadly died prematurely in 1954.
This book is about the epic life of Fermi, mostly known to the general public for the first ever nuclear reactor and the Manhattan Project, but to scientists for his theoretical and experimental discoveries – all diverse and crucial in modern physics – which always resulted in major advances. He remains less known as a personality or a public figure, and his scientific legacy is somehow underestimated. The merit of this book is therefore to bring Fermi’s genius within everyone’s reach.
Many renowned texts have been dedicated to Fermi until now, offering various perspectives on his life and his work. First of all on the personal life of Fermi, there is Atoms in the Family (1954) by his widow, Laura. Exhaustive information about Fermi’s outstanding works in physics can be found in the volume Enrico Fermi, Physicist (1970) by his friend and colleague Emilio Segrè, Nobel laureate and Gino Segrè’s uncle, and in Enrico Fermi: Collected Papers, two volumes published in the 1960s by the University of Chicago. Also worth mentioning are: Fermi Remembered (2004), edited by Nobel laureate James W Cronin; Enrico Fermi: His Work and Legacy (2001, then 2004), edited by C Bernardini and L Bonolis, and The Lost Notebook of Enrico Fermi by F Guerra and N Robotti (2015, then 2017), both published by the Italian Physical Society–Springer. Finally, published almost at the same time as Segrè and Hoerlin’s book, is another biography of Fermi: The Last Man Who Knew Everything by D N Schwartz, the son of Nobel laureate Melvin Schwartz. In their “four-handed” book, Segrè and Hoerlin have highlighted with expertise the scientific biography of Fermi and his extraordinary achievements, and described with emotion the human, social and political aspects of his life.
Readers familiar with Fermi’s story will enjoy this book, which is as scientifically sound as a textbook but at the same time bears the gripping character of a novel.
The High-Luminosity LHC (HL-LHC) has reached its halfway point. The upgrade project was launched eight years ago and is scheduled to start up in 2026, following major interventions to the CERN accelerator complex. From 15 to 18 October, representatives of the institutes contributing to the HL-HLC gathered at CERN for the 8th annual meeting of the HL-LHC to assess progress as the project moves from prototyping to the series production phase for much of the equipment.
The HL-LHC annual meeting is a chance to conduct a global review of the project. The civil-engineering work has progressed since it began in the spring: excavations have reached a depth of 30 m at Point 1 (ATLAS) and 25 m at Point 5 (CMS). The two 80 m shafts should be fully excavated by the beginning of 2019. As for the accelerator, one of the key tasks is the production of around 100 magnets of 11 different types. Some of these, notably the main superconducting quadrupole magnets that will replace the LHC’s triplets and focus the beams very strongly before they collide, are made from the conductor niobium tin, which is particularly difficult to work with. The short prototype phase is already nearing completion for the quadrupole magnets: the long (7.15 m) quadrupoles are being produced at CERN, while shorter (4.2 m) quadrupoles are being developed in the framework of the US LHC-AUP (LHC Accelerator Upgrade Project) collaboration. Several short prototypes have already reached the required intensities on both sides of the Atlantic.
New dipole magnets at the interaction points, which divert the beams before and after the collision point, are being developed in Japan and Italy. One short model has been successfully tested at the KEK laboratory in Japan and a second is in the process of being tested. INFN in Italy is also assembling a short model. Finally, progress is being made on the development of the corrector magnets at CERN and in Spain (CIEMAT), Italy (INFN) and China (IHEP), with several prototypes already tested. In 2022, a test line will be installed at CERN’s SM18 hall to test the first magnet chains.
One of the major successes of 2018 is the installation in the Super Proton Synchrotron (SPS) of a test bench with an autonomous cryogenic unit. The test bench houses two DQW (double-quarter wave) crab cavities, one of two designs under study (CERN Courier May 2018 p18). The two cavities rotated the proton bunches as soon as the tests began in May, marking a world first. The construction of the DQW cavities will continue while the second architecture, the radiofrequency dipole, is being developed in the US.
Many other developments were presented during the symposium: new collimators have been tested in the LHC; a beam absorber for the injection points from the SPS was tested over the summer and will be installed during the LHC’s second long shutdown; a demonstrator for a magnesium-diboride superconducting link is currently being validated; and studies have been undertaken to test and adjust the remote alignment of all the equipment in the interaction regions.
Over the four days that the meeting took place, some 180 presentations covered a wide range of technologies developed for the HL-LHC and beyond.
The ambitious upgrade programme for the Large Hadron Collider (LHC) will result in significant information and communications technology (ICT) challenges over the next decade and beyond. It is therefore vital that members of the HEP research community keep looking for innovative computing technologies so as to continue to maximise the discovery potential of the world-leading research infrastructures at their disposal (CERN Courier November 2018 p5).
On 5–6 November, CERN hosted a first-of-its kind workshop on quantum computing in high-energy physics (HEP). The event was organised by CERN openlab, a public–private partnership between CERN and leading ICT companies established to accelerate the development of computing technologies needed by the LHC research community.
More than 400 people followed the workshop, which provided an overview of the current state of quantum-computing technologies. The event also served as a forum to discuss which activities within the HEP community may be amenable to the application of quantum-computing technologies.
“In CERN openlab, we’re always looking with keen interest at new computing architectures and trying to understand their potential for disrupting and improving the way we do things,” says Alberto Di Meglio, head of CERN openlab. “We want to understand which computing workflows from HEP could potentially most benefit from nascent quantum-computing technologies; this workshop was the start of the discussion.”
Significant developments are being made in the field of quantum computing, even if today’s quantum-computing hardware has not yet reached the level at which it could be put into production. Nevertheless, quantum-computing technologies are among those that hold future promise of substantially speeding up tasks that are computationally expensive.
“Quantum computing is no panacea, and will certainly not solve all the future computing needs of the HEP community,” says Eckhard Elsen, CERN’s director for research and computing. “Nevertheless, quantum computers are starting to be available; a breakthrough in the number of qubits could emerge at any time. Fundamentally rethinking our algorithms may appear as an interesting intellectual challenge today, yet may turn out as a major benefit in addressing computing challenges in the future.”
The workshop featured representatives of the LHC experiments, who spoke about how computing challenges are likely to evolve as we approach the era of the High-Luminosity LHC. There was also discussion of work already undertaken to assess the feasibility of applying today’s quantum-computing technologies to problems in HEP. Jean-Roch Vlimant provided an overview of their recent work at the California Institute of Technology, with collaborators from the University of Southern California, to solve an optimisation problem related to the search for Higgs bosons. Using an approach known as quantum annealing for machine learning, the team demonstrated some advantage over traditional machine-learning methods for small training datasets. Given the relative simplicity of the algorithm and its robustness to error, they report, this technique may find application in other areas of experimental particle physics, such as real-time decision making in event-selection problems and classification in neutrino physics.
Several large-scale research initiatives related to quantum-computing technologies were presented at the event, including the European Union’s €1 billion Quantum Technologies Flagship project, which involves universities and commercial partners across Europe. Presentations were also given of ambitious programmes in the US, such as the Northeast Quantum Systems Center at Brookhaven National Laboratory and the Quantum Science Program at Fermilab, which includes research areas in superconducting quantum systems, quantum algorithms for HEP, and computational problems and theory.
Perhaps most importantly, the workshop brought members of the HEP community together with leading companies working on quantum-computing technologies. Intel, IBM, Strangeworks, D-Wave, Microsoft, Rigetti and Google all presented their latest work in this area at the event. Of these companies, Intel and IBM are already working closely with CERN through CERN openlab. Plus, Google also announced at the event that they have signed an agreement to join CERN openlab.
“Now is the right time for the HEP community to get involved and engage with different quantum-computing initiatives already underway, fostering common activities and knowledge sharing,” says Federico Carminati, CERN openlab CIO and chair of the event. “With its well-established links across many of the world’s leading ICT companies, CERN openlab is ideally positioned to help drive this activity forward. We believe this first event was a great success and look forward to organising future activities in this exciting area.”
Recordings of the talks given at the workshop are available via the CERN openlab website at: openlab.cern.
CERN hosted a workshop on high-energy theory and gender on 26–28 September. It was the first activity of the “Gen-HET” working group, whose goals are to improve the presence and visibility of women in the field of high-energy theory and increase awareness of gender issues.
Most of the talks in the workshop were on physics. Invited talks spanned the whole of high-energy theory, providing an opportunity for participants to learn about new results in neighbouring research areas at this interesting time for the field. Topics ranged from the anti-de-Sitter/conformal field theory (AdS/CFT) correspondence and inflationary cosmology to heavy-ion, neutrino and beyond-Standard Model physics.
Agnese Bissi (Uppsala University, Sweden) began the physics programme by reviewing the now-two-decades-old AdS/CFT correspondence, and discussing the use of conformal bootstrap methods in holography. Korinna Zapp (LIP, Lisbon, Portugal and CERN) then put three recent discoveries in heavy-ion physics into perspective: the hydrodynamic behaviour of soft particles; jet quenching; and surprising similarities between soft particle production in high-multiplicity proton–proton and heavy-ion collisions.
JiJi Fan (Brown University, USA) delved into the myriad world of beyond-the Standard Model phenomenology, discussing the possibility that the Higgs is “meso-tuned” but that there are no other light scalars. Elvira Gamiz (University of Granada, Spain) reviewed key features of lattice simulations for flavour physics and mentioned significant tensions with some experimental results that are as high as 3σ in certain B-decay channels. The theory colloquium, by Ana Achucarro (University of Leiden, Holland, and UPV-EHU Bilbao, Spain), was devoted to the topic of inflation, which still presents a major challenge to theorists.
The importance of parton distribution functions in an era of high-precision physics was the focus of a talk by Maria Ubiali (University of Cambridge, UK), who explained the state-of-the-art methods used. Reviewing key topics in cosmology and particle physics, Laura Covi (Georg-August-University Göttingen, Germany) then described how models with heavy R-parity violating supersymmetry lead to scenarios for baryogenesis and gravitino dark matter.
In neutrino physics, Silvia Pascoli (Durham University, UK) gave an authoritative overview of the experimental and theoretical status, while Tracy Slatyer (MIT, USA) did the same for dark matter, emphasising the necessity of search strategies that test many possible dark-matter models.
Closing the event, Alejandra Castro (University of Amsterdam, the Netherlands) talked about black-hole entropy and its fascinating connections with holography and number theory. The final physics talk, by Eleni Vyronidou (CERN), covered Standard Model effective field theory (SMEFT), which provides a pathway to new physics above the direct energy-reach of colliders.
The rest of the workshop centred on talks and discussion sessions about gender issues. The full spectrum of issues was addressed, a few examples of which are given here.
Julie Moote from University College London, UK, delivered a talk on behalf of the Aspires project in the UK, which is exploring how social identities and inequalities affect students continuing in science, while Marieke van den Brink from Radboud University Nijmegen, the Netherlands, described systematic biases that were uncovered by her group’s studies of around 1000 professorial appointments in the Netherlands. Meytal Eran-Jona from the Weizmann Institute of Science, Israel, reviewed studies about unconscious bias and its implications for women in academia, and described avenues to promote gender equality in the field.
The last day of the meeting focused on actions that physicists can take to improve diversity in their own departments. For example, Jess Wade from Imperial College London, UK, discussed UK initiatives such as the Institute of Physics Juno and Athena SWAN awards, and Yossi Nir from the Weizmann Institute gave an inspiring account of his work on increasing female participation in physics in Israel. One presentation drawing on bibliometric data in high-energy theory attracted much attention beyond the workshop, as has been widely reported elsewhere.
This first workshop on high-energy theory and gender combined great physics, mentoring and networking. The additional focus on gender gave participants the opportunity to learn about the sociological causes of gender imbalance and how universities and research institutes are addressing them.
We are very grateful to many colleagues for their support in putting together this meeting, which received help from the CERN diversity office and financial support from the CERN theory department, the Mainz “cluster of excellence” PRISMA, Italy’s National Institute for Nuclear Physics (INFN), the University of Milano-Bicocca, the ERC and the COST network.
Similar activities are planned in the future, including discussions on other scientific communities and minority groups.
Karlheinz Meier, a visionary experimental particle physicist and co-founder of the Human Brain Project, unexpectedly passed away on 24 October, much too early, at the age of 63.
Karlheinz’s career began at the University of Hamburg in Germany, where he studied physics. He completed his PhD there in 1984, with Gus Weber and Wulfrin Bartel as his supervisors, working for the JADE experiment at the PETRA electron–positron collider at DESY. During the following six years, he worked at CERN for the UA2 project, for two years as a CERN fellow and then as staff scientist. Returning to DESY in 1990, he joined the H1 collaboration. In 1992 he accepted a full professorship at Heidelberg University, where in 1994 he founded the Heidelberg ASIC Laboratory for Microelectronics and later, in 1999, the Kirchhoff Institute for Physics; during this period he also joined the ATLAS collaboration at CERN’s Large Hadron Collider (LHC). He was vice-rector at Heidelberg University from 2001 to 2004, chair of the European Committee for Future Accelerators (ECFA) from 2007 to 2009, and a member of the governing board of the German Physical Society (DPG) from 2009 to 2013. Within the Human Brain Project – a major 10 year-long effort harnessing cutting-edge research infrastructure for the benefit of neuroscience, computing and brain-related medicine – he initiated the European Institute for Neuromorphic Computing (EINC) at Heidelberg. Sadly, the completion of the facility cannot be witnessed by him anymore.
Karlheinz was an extremely enthusiastic, visionary and energetic scientist. He made fundamental contributions to the instrumentation and data analysis of large particle-physics experiments, especially concerning calorimeter systems. Early on, during his PhD, he developed advanced algorithms for identifying photons with the lead-glass calorimeter of JADE, an essential ingredient for his analysis of the inclusive production of photons, pions and η-mesons in multi-hadronic final states, but also for many studies of hadronisation and jet production, which JADE became famous for. Later, at CERN’s UA2 experiment, he participated in the first analyses of the newly discovered W and Z bosons.
Back at DESY, he was one of the advocates and initiators of the H1 scintillating fibre “spaghetti” calorimeter, which was decisive for precise measurements of the proton structure. In addition, his research group built another specialised backward calorimeter for H1, the VLQ; by analysing the VLQ data, he was able to refute the then theoretical predictions on special multi-gluon states, such as the odderon (CERN Courier April 2018 p9). Karlheinz recognised early on the need for developing highly integrated electronic circuits for experimental physics, and his group – together with colleagues from the Heidelberg ASIC Laboratory – developed the pre-processor system of the ATLAS level-1 calorimeter trigger, which played a pivotal role in the discovery of the Higgs boson.
Since 2001, Karlheinz became increasingly interested in fundamental questions related to the physics of complex systems and information processing, with a focus on the development of neuromorphic hardware for decoding the functioning of the brain. In contrast to normal, programme-oriented Turing machines, neuromorphic systems are extremely energy efficient, error tolerant and self-adaptive – just like the human brain. His research results received special international recognition through the Human Brain Project, which he initiated together with Henry Markram and Richard Frackowiak, and which was selected by the European Union in 2012 as one of two so-called Flagship Projects of European research funding.
Karlheinz was also exceptional in supervising and motivating young researchers. He was a highly gifted teacher, whose lectures and seminars were loved by his students. Through his renowned “Team-Anderthalb” 90-second movies on a wide variety of basic physics topics, he became known to the wider public; they are available, like many other of his lectures and talks, on YouTube.
Curiosity for the fundamental questions of physics and technological innovation were the two driving forces that accompanied Karlheinz throughout his research life. He not only contributed significantly to the expansion of our knowledge about nature, but also gave new impetus to technological development, especially in the field of microelectronics and computing. His commitment to both research and teaching was outstanding and special. His passion, humanity, humour, overall guidance and inspiration will be sorely missed and not forgotten.
“I think the Courier is excellent; it’s sort of ‘frozen in time’, but in a rather appropriate and appealing way.” Of all the lively comments received from the 1400 or so readers who took part in our recent survey (see below), this one sums things up for the CERN Courier. “Excellent” might be a stretch for some, but, coming up for its 60th anniversary, this well-regarded periodical is certainly unique. It has been alongside high-energy physics as the field has grown up, from the rise of the Standard Model to the strengthening links with cosmology and astrophysics, the increasing scale and complexity of accelerators, detectors and computing, the move to international collaborations involving thousands of people, and other seismic shifts.
In terms of presentation, though, the Courier is indeed ripe for change. The website preview-courier.web.cern.ch was created in 1998 when the magazine’s production and commercial dimensions were outsourced to IOP Publishing in the UK. Updated only 10 times per year with the publication of each print issue, the website has had a couple of makeovers (one in 2007 and one earlier this year) but its functionality remains essentially unchanged for 20 years.
A semi-static, print-led website is no longer best suited to today’s publishing scene. The sheer flexibility of online publishing allows more efficient ways to communicate different stories to new audiences. Our survey concurs: a majority of readers (63%) indicated that they were willing to receive fewer print copies per year if preview-courier.web.cern.ch was updated more regularly – a view held most strongly among younger responders. It is this change to its online presence that drives the new publishing model of CERN Courier from 2019, with a new, dynamic website planned to launch in the spring.
At the same time, there is high value attached to a well-produced print magazine that worldwide readers can receive free of charge. And, as the results of our survey show, a large section of the community reads the Courier only when they pick up a copy in their labs or universities to browse over lunch or while travelling. That’s why the print magazine is staying, though at a reduced frequency of six rather than 10 issues per year. To reflect this change, the magazine will have a new look from next year. Among many improvements, we have adopted a more readable font, a clearer layout and other modern design features. There are new and revised sections covering careers, opinion and reviews, while the feature articles – the most popular according to our survey – will remain as the backbone of the issue.
It is sometimes said that the Courier can be a bit too formal, a little dry. Yet our survey did not reveal a huge demand to lighten things up – so don’t expect to see Sudoku puzzles or photos of your favourite pet any time soon. That said, the Courier is a magazine, not an academic journal; in chronicling progress in global high-energy physics it strives to be as enjoyable as it is authoritative.
Another occasional criticism is that the Courier is a mere mouthpiece for CERN. If it is, then it is also – and unashamedly – a mouthpiece for other labs and for the field as a whole. Within just a few issues of its publication, the Courier outgrew its original editorial remit and expanded to cover activities at related laboratories worldwide (with the editorially distinct CERN Bulletin serving the internal CERN community). The new-look Courier will also retain an important sentence on its masthead demarcating the views stated in the magazine from those of CERN management.
A network of around 30 laboratory correspondents helps to keep the magazine updated with news from their facilities on an informal basis. But the more members of the global high-energy physics community who interact, the better the Courier can serve them. Whether it’s a new result, experiment, machine or theorem, an event, appointment or prize, an opinion, review or brazen self-promotion, get in touch at cern.courier@cern.ch.
Reader survey: the results are in
To shape the Courier’s new life in print and online, a survey was launched this summer in conjunction with IOP Publishing to find out what readers think of the magazine and website, and what changes could be made. The online survey asked 21 questions and responders were routed to different sections of the survey depending on the answers they provided. Following promotion on preview-courier.web.cern.ch, CERN’s website, CERN Bulletin, social media channels and e-mails to CERN users, there were a total of 1417 responses.
Responders were split roughly 3:1 male to female, with a fairly even age distribution. Geographically, they were based predominantly in France, the US, Italy, Switzerland, Germany and the UK. Some 43% of the respondents work at a university, followed by a national or international research institute (34%), with the rest working in teaching (5%) and various industries. While three-quarters of the respondents named experimental particle physics as their main domain of work, many have other professional interests ranging from astronomy to marketing.
Responders were evenly split between those that read the printed magazine and those that don’t. Readers tend to read the magazine on a regular basis and, overall, have been reading for a significant period of time. A majority (54.1%) do not read the magazine via a direct subscription, and the data suggest that one copy of the Courier is typically read by more than one person.
In terms of improving the CERN Courier website, there was demand for a mobile-optimised platform and for video content, though a number of respondents were unaware that the website even existed. Importantly for the future of CERN Courier, a majority of readers (63%) indicated that they were willing to receive fewer print copies per year if preview-courier.web.cern.ch was updated more regularly; this trend was sharpest in the under-30 age group.
When it comes to the technical level of the articles, which is a topic of much consideration at the Courier, the responses indicate that the level is pitched just right (though, clearly, a number of readers will find some topics tougher than others given the range of subfields within high-energy physics). Readers also felt that their fields were well represented, and agreed that more articles about careers and people would be of interest.
Many written comments were provided, a few of which are listed here: “More investigative articles please”; “I would like that it has a little glossary”; “A column about people themselves, not only the physics they do”; “More debate on topics on which there is discussion in the field”; “Please do NOT modify CERN Courier into a ‘posher’ version”; “Leave out group photos of people at big meetings”; and “Make a CERN Courier kids edition”. The overwhelming majority of comments were positive, and the few that weren’t stood out: “The whole magazine reads like propaganda for CERN and for the Standard Model”; “The Courier style is intentionally humourless, frigid, stale and boring. Accordingly, almost everybody agrees that the obituaries are by far its best part”; and, curiously, “The actual format is so boring that I stop to read it!”
It only remains to thank participants of the survey and to congratulate the winners of our random prize draw (V Boudry, J Baeza, S Clawson, V Lardans and M Calvetti), who each receive a branded CERN hoodie.
Measurements of b-hadron decays with neutrinos in the final state are one of the best ways to understand how quarks decay, and in particular how they couple to leptons. With recent results from LHCb, BaBar and Belle raising questions about whether the Standard Model (with its assumption of lepton-flavour universality) is able to explain these couplings fully, further experimental results are needed.
At first glance, measuring fully leptonic decays such as B+ → τ+ντ and B+ → μ+νμ seems a step too far, since there is only one charged particle as a signature and no reconstructed B-decay vertex.
However, studying these decays is notoriously tricky at a hadron collider, where the busy collision environment makes it challenging to control the background. Despite this, the LHCb collaboration has made unexpected progress in this area over the last few years, with a comparison of decays with taus and muons, and measurements the CKM element ratio |Vub/Vcb| that originally seemed impossible.
At first glance, measuring fully leptonic decays such as B+ → τ+ντ and B+ → μ+νμ seems a step too far, since there is only one charged particle as a signature and no reconstructed B-decay vertex. The key to accessing these processes is to allow additional particles to be radiated, while preserving the underlying decay amplitude. The decay B+ → μ+μ−μ+νμ is a good example of this, where a hard photon is radiated and converts immediately into two additional muons. Such a signature is significantly more appealing experimentally: there is a vertex to reconstruct and the background is low, as there are not many B decays that produce three muons.
B decays with a well-defined vertex and only one missing neutrino are becoming LHCb’s “bread and butter” thanks to the so-called corrected mass technique. The idea behind the corrected mass is that if you are only missing one neutrino, then adding the momentum perpendicular to the B flight direction is enough to recover the B mass. This technique is only possible thanks to the precise vertex resolution provided by the LHCb’s innermost detector, the VELO. Using this technique, LHCb expects to have a very good sensitivity for this decay, at a branching fraction level of 2.8 × 10−8 (equivalent to around one in 40 million B+ decays) with the 2011–2016 data sample.
The LHCb collaboration searched for this decay using 5 fb–1 of data (see figure). The main backgrounds come from reconstructed muons that originate from different decays (“combinatorial”) or from hadrons misidentified as muons (“misidentified”). No evidence for the signal is seen and an upper limit on the branching fraction of 1.6 × 10−8 is set at a confidence level of 95%.
The figure also shows a projected signal expected from a recent Standard Model prediction, which is based on the vector meson dominance model. This prediction includes two contributions to the decay: one in which two muons originate from a photon, and another in which they originate from the annihilation of a hadron (such as ρ0→ μ+μ− or ω → μ+μ−). As can be seen, the data disfavour this prediction, which motivates further theoretical work to understand the discrepancy. The good sensitivity for this decay is encouraging, and raises interesting prospects for observing the signal with future datasets collected at the upgraded LHCb detector.
Deep in a mine in Greater Sudbury, Ontario, Canada, you will find the deepest flush toilets in the world. Four of them, actually, ensuring the comfort of the staff and users of SNOLAB, an underground clean lab with very low levels of background radiation that specialises in neutrino and dark-matter physics.
Toilets might not be the first thing that comes to mind when discussing a particle-physics laboratory, but they are one of numerous logistical considerations when hosting 60 people per day at a depth of 2 km for 10 hours at a time. SNOLAB is the world’s deepest cleanroom facility, a class-2000 cleanroom (see panel below) the size of a shopping mall situated in the operational Vale Creighton nickel mine. It is an expansion of the facility that hosted the Sudbury Neutrino Observatory (SNO), a large, heavy-water detector designed to detect neutrinos from the Sun. In 2001, SNO contributed to the discovery of neutrino oscillations, leading to the joint award of the 2015 Nobel Prize in Physics to SNO spokesperson Arthur B McDonald and Super-Kamiokande spokesperson Takaaki Kajita.
Initially, there were no plans to maintain the infrastructure beyond the timeline of SNO, which was just one experiment and not a designated research facility. However, following the success of the SNO experiment, there was increased interest in low-background detectors for neutrino and dark-matter studies.
Building on SNO’s success
The SNO collaboration was first formed in 1984, with the goal of solving the solar neutrino problem. This problem surfaced during the 1960s, when the Homestake experiment in the Homestake Mine at Lead, South Dakota, began looking for neutrinos created in the early stages of solar fusion. This experiment and its successors, using different target materials and technologies, consistently observed only 30–50% of the neutrinos predicted by the standard solar model. A seemingly small nuisance posed a large problem, which required a large-scale solution.
SNO used a 12 m-diameter spherical vessel containing 1000 tonnes of heavy water to count solar neutrino interactions. Canada had vast reserves of heavy water for use in its nuclear reactors, making it an ideal location for such a detector. The experiment also required an extreme level of cleanliness, so that the signals physicists were searching for would not be confused with background events coming from dust, for instance. The SNO collaboration also had to develop new techniques to measure the inherent radioactivity of their detector materials and the heavy water itself.
Using heavy water gave SNO the ability to observe three different neutrino reactions: one reaction could only happen with electron neutrinos; one was sensitive to all neutrino flavours (electron, muon and tau); and the third provided the directionality pointing back to the Sun. These three complementary interactions let the team test the hypothesis that solar neutrinos were changing flavour as they travelled to Earth. In contrast to previous experiments, this approach allowed SNO to make a measurement of the parameters describing neutrino oscillations that didn’t depend on solar models. SNO’s data confirmed what previous experiments had seen and also verified the predictions of theories, implying that neutrinos do indeed oscillate during their Sun–Earth journey. The experiment ran for seven years and produced 178 papers accumulating more than 275 authors.
In 2002, the Canadian community secured funding to create an extended underground laboratory with SNO as the starting point. Construction of SNOLAB’s underground facility was completed in 2009 and two years later the last experimental hall entered “cleanroom” operation. Some 30 letters of interest were received from different collaborations proposing potential experiments, helping to define the requirements of the new lab.
SNOLAB’s construction was made possible by capital funds totalling CAD$73 million, with more than half coming from the Canada Foundation for Innovation through the International Joint Venture programme. Instead of a single giant cavern, local company Redpath Mining excavated several small and two large halls to hold experiments. The smaller halls helped the engineers manage the enormous stress placed on the rock in larger underground cavities. Bolts 10 m long stabilise the rock in the ceilings of the remaining large caverns, and throughout the lab the rock is covered with a 10 cm-thick layer of spray-on concrete for further stability, with an additional hand-troweled layer to help keep the walls dust-free. This latter task was carried out by Béton Projeté MAH, the same company that finished the bobsled track in the 2010 Vancouver Winter Olympics.
In addition to the experimental halls, SNOLAB is equipped with a chemistry laboratory, a machine shop, storage areas, and a lunchroom. Since the SNO experiment was still running when new tunnels and caverns were excavated, the connection between the new space and the original clean lab area was completed late in the project. The dark-matter experiments DEAP-1 and PICASSO were also already running in the SNO areas before construction of SNOLAB was completed.
Dark matter, neutrinos, and more
Today, SNOLAB employs a staff of over 100 people, working on engineering design, construction, installation, technical support and operations. In addition to providing expert and local support to the experiments, SNOLAB research scientists undertake research in their own right as members of the collaborations.
With so much additional space, SNOLAB’s physics programme has expanded greatly during the past seven years. SNO has evolved into SNO+, in which a liquid scintillator replaces the heavy water to increase the detector’s sensitivity. The scintillator will be doped with tellurium, making SNO+ sensitive to the hypothetical process of neutrinoless double-beta decay. Two of tellurium’s natural isotopes (128Te and 130Te) are known to undergo conventional double-beta decay, making them good candidates to search for the long-sought neutrinoless version. Detecting this decay would violate lepton-number conservation, proving that the neutrino is its own antiparticle (a Majorana particle). SNO+ is one of several experiments currently hunting this process down.
Another active SNOLAB experiment is the Helium and Lead Observatory (HALO), which uses 76 tons of lead blocks instrumented with 128 helium-3 neutron detectors to capture the intense neutrino flux generated when the core of a star collapses at the early stages of a supernova. Together with similar detectors around the world, HALO is part of a supernova early-warning system, which allows astronomers to orient their instruments to observe the phenomenon before it is visible in the sky.
With no fewer than six active projects, dark-matter searches comprise a large fraction of SNOLAB’s physics programme. Many different technologies are employed to search for the dark-matter candidate of choice: the weakly interacting massive particle (WIMP). The PICASSO and COUPP collaborations were both using bubble chambers to search for WIMPS, and merged into the very successful PICO project. Through successive improvements, PICO has endeavoured to enhance the sensitivity to WIMP spin-dependent interactions by an order of magnitude every couple of years. Its sensitivity is best for WIMP masses around 20 GeV/c2. Currently the PICO collaboration is developing a much larger version with up to 500 litres of active-mass material.
DEAP-3600, successor to DEAP-1, is one of the biggest dark-matter detectors ever built, and it has been taking data for almost two years now. It seeks to detect spin-independent interactions between WIMPs and 3300 kg of liquid argon contained in a 1.7 m-diameter acrylic vessel. The best sensitivity will be achieved for a WIMP mass of 100 GeV/c2. Using a different technology, the DAMIC (Dark Matter In CCDs) experiment employs CCD sensors, which have low intrinsic noise levels, and is sensitive to WIMP masses as low as 1 GeV/c2.
Although the science at SNOLAB primarily focuses on neutrinos and dark matter, the low-background underground environment is also useful for biology experiments. REPAIR explores how low radiation levels affect cell development and repair from DNA damage. One hypothesis is that removing background radiation may be detrimental to living systems. REPAIR can help determine whether this hypothesis is correct and characterise any negative impacts. Another experiment, FLAME, studies the effect of prolonged time spent underground on living organisms using fruit flies as a model. The findings from this research could be used by mining companies to support
a healthier workforce.
Future research
There are many exciting new experiments under construction at SNOLAB, including several dark-matter experiments. While the PICO experiment is increasing its detector mass, other experiments are using several different technologies to cover a wide range of possible WIMP masses. The SuperCDMS experiment and CUTE test facility use solid-state silicon and germanium detectors kept at temperatures near absolute zero to search for dark matter, while the NEWS-G experiment will use gasses such as hydrogen, helium and neon in a 1.4 m-diameter copper sphere.
SNOLAB still has space available for additional experiments requiring a deep underground cleanroom environment. The Cryopit, the largest remaining cavern, will be used for a next-generation double-beta-decay experiment. Additional spaces outside the large experimental halls can host several small-scale experiments. While the results of today’s experiments will influence future detectors and detector technologies, the astroparticle physics community will continue to demand clean underground facilities to host the world’s most sensitive detectors. From an underground cavern carved out to host a novel neutrino detector to the deepest cleanroom facility in the world, SNOLAB will continue to seek out and host world-class physics experiments to unravel some of the universe’s deepest mysteries.
Two new experiments at CERN, ALPHA-g and GBAR, have begun campaigns to check whether antimatter falls under gravity at the same rate as matter.
The gravitational behaviour of antimatter has never been directly probed, though indirect measurements have set limits on the deviation from standard gravity at the level of 10–6 (CERN Courier January/February 2017 p39). Detecting even a slight difference between the behaviour of antimatter and matter with respect to gravity would mean that Einstein’s equivalence principle is not perfect and could have major implications for a quantum theory of gravity.
ALPHA-g, a close model of the ALPHA experiment, combines antiprotons from CERN’s Antiproton Decelerator (AD) with positrons from a sodium-22 source and traps the resulting antihydrogen atoms in a vertical magnetic trap about 2 m tall. To measure their free-fall, the field is switched off so that the atoms fall under gravity and the position where the antiatoms annihilate with normal matter allows the rate to be determined precisely.
GBAR adopts a similar approach but takes antiprotons from the new and lower-energy ELENA ring attached to the AD (CERN Courier December 2016 p16) and combines them with positrons from a small linear accelerator to make antihydrogen ions. Once a laser has stripped all but one positron, the neutral antiatoms will be released from the trap and allowed to fall from a height of 20 cm.
ALPHA-g began taking beam on 30 October, while ELENA has been delivering beam to GBAR since the summer, allowing the collaboration to perfect the beam-delivery system. Both experiments are being commissioned before CERN’s accelerators are shut down on 10 December for a two-year period. The ALPHA-g team hopes to be able to gather enough data during this short period to make a first measurement of antihydrogen in free fall, while the brand new GBAR experiment aims to make a first measurement when antiprotons are back in the machine in 2021. A third experiment at the AD hall, AEgIS, which has been in operation for several years, is also measuring the effect of gravity on antihydrogen using yet another approach, based on a beam of antihydrogen atoms. AEgIS is also hoping to produce its first antihydrogen atoms this year.
So far, most efforts at the AD have focused on looking for charge–parity–time violation by studying the spectroscopy of antihydrogen and comparing it with that of hydrogen (CERN Courier March 2018 p30). This latest round of experiments opens a new avenue in antimatter exploration.
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