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The Higgs adventure: five years in

*In Search of the Higgs Boson*, a series of works produced by artist Xavier Cortada and physicist Pete Markowitz.
Image credit: X Cortada.

Where were you on 4 July 2012, the day the Higgs boson discovery was announced? Many people will be able to answer without referring to their diary. Perhaps you were among the few who had managed to secure a seat in CERN’s main auditorium, or who joined colleagues in universities and laboratories around the world to watch the webcast. For me, the memory is indelible: 3.00 a.m. in Watertown, Massachusetts, huddled over my laptop at the kitchen table. It was well worth the tired eyes to witness remotely an event that will happen once in a lifetime.

“I think we have it, no?” was the question posed in the CERN auditorium on 4 July 2012 by Rolf Heuer, CERN’s Director-General at the time. The answer was as obvious as the emotion on faces in the crowd. The then ATLAS and CMS spokespersons, Fabiola Gianotti and Joe Incandela, had just presented the latest Higgs search results based on roughly two years of LHC operations at energies of 7 and 8 TeV. Given the hints for the Higgs presented a few months earlier in December 2011, the frenzy of rumours on blogs and intense media interest during the preceding weeks, and a title for the CERN seminar that left little to the imagination, the outcome was anticipated. This did not temper excitement.

Since then, we have learnt much about the properties of this new scalar particle, yet we are still at the beginning of our understanding. It is the final and most interesting particle of the Standard Model of particle physics (SM), and its connections to many of the deepest current mysteries in physics mean the Higgs will remain a focus of activities for experimentalists and theorists for the foreseeable future.

Speculative theories

The Higgs story began in the 1960s with speculative ideas. Theoretical physicists understood how the symmetries of materials can spontaneously break down, such as the spontaneous alignment of atoms when a magnet is cooled from high temperatures, but it was not yet understood how this might happen for the symmetries present in the fundamental laws of physics. Then, in three separate publications by Brout and Englert, by Higgs, and by Guralnik, Hagen and Kibble in 1964, the broad particle-physics structures for spontaneous symmetry breaking were fleshed out. In this and subsequent work it became clear that a scalar field was a cornerstone of the general symmetry-breaking mechanism. This field may be excited and oscillate, much like the ripples that appear on a disturbed pond, and the excitation of the Higgs field is known as the Higgs boson.

Possible Higgs boson mass and the relevant method for discovery, as considered in the landmark 1975 paper by Ellis, Gaillard and Nanopoulos.
Image credit: *Nucl. Phys. B* **106** 292.

As the detailed theoretical structure of symmetry breaking in nature was later developed, in particular by Weinberg, Glashow, Salam, ’t Hooft and Veltman, the precise role of the Higgs in the SM evolved to its modern form. In addition to explaining what we see in modern particle detectors, the Higgs plays a leading role in the evolution of the universe. In the hot early epoch an infinitesimally small fraction of a second after the Big Bang, the Higgs field spontaneously “slipped” from having zero average value everywhere in space to having an average value equivalent to about 246 GeV. When this happened, any field that was previously kept massless by the SU(2) × U(1) gauge symmetries of the SM instantly became massive.

Before delving further into the vital role of the Higgs, it is worth revisiting a couple of common misconceptions. One is that the Higgs boson gives mass to all particles. Although all of the known massive fundamental particles obtain their mass by interacting with the pervasive Higgs field, there are non-elementary particles, such as the proton, whose mass is dominated by the binding energy of the strong force that holds its constituent gluons and quarks together. So very little of the mass we see in nature comes directly from the Higgs field. Another misconception is that the Higgs boson gives mass to everything it interacts with. On the contrary, the Higgs has very important interactions with two massless fundamental fields: the photon and the gluon. The Higgs is not charged under the forces associated with the photon and the gluon (quantum electrodynamics and quantum chromodynamics), and therefore cannot give them mass, but it can still interact with them. Indeed, somewhat ironically, it was precisely its interactions with massless gluons and photons that revealed the existence of the Higgs boson in the summer of 2012.

Fig. 2. Possibilities for the Higgs boson discovery at the LHC before 2012. (Left) Different Higgs cross-sections as a function of mass, with lower lines showing scenarios where a Higgs boson is produced in association with another particle and the top line showing the single-Higgs production cross-section, which is dominated by gluon fusion. (Right) The relative rates for the Higgs boson to decay into different particles for different Higgs boson mass values. Lighter Higgs bosons can observably decay to a variety of final states.

The one remaining unmeasured free parameter of the SM at that time, which governs which production and decay modes the particle can have, was the Higgs boson mass. In the early days it was not at all clear what the mass of the Higgs boson would be, since in the SM this is an input parameter of the theory. Indeed, in 1975, in the seminal paper about its experimental phenomenology by Ellis, Gaillard and Nanopoulos, it is notable that the allowed Higgs mass range at that time spanned four orders of magnitude, from 18 MeV to over 100 GeV, with experimental prospects in the latter energy range opaque at best (figure 1).

How the Higgs was found

By 4 July 2012 the picture was radically different. The Higgs no-show at previous colliders, including LEP at CERN and the Tevatron at Fermilab, had cornered its mass to be greater than 114 GeV and not to lie between 147–180 GeV, while theoretical limits on the allowed properties of W- and Z-boson scattering required it to be below around 800 GeV. If nature used the SM version of the Higgs mechanism, there was nowhere left to hide once CERN’s LHC switched on. In the end, the Higgs weighed in at the relatively light mass of 125 GeV. How the different Higgs cross-sections, which are related to the production rate for various processes, depend on the mass are shown in figure 2, left.

Producing the Higgs would alone not be sufficient for discovery. It would also have to be observed, which depends on the different fractional ways in which the Higgs boson will decay (figure 2, right). If heavy, one would have to search for decays to the weak gauge bosons, W and Z; if lighter, a cocktail of decays would light up detectors. Going further, if thousands of Higgs bosons could be produced, then decays to pairs of photons may show up. Thus, by the time of the LHC operation, the basic theoretical recipe was relatively simple: pick a Higgs mass, calculate the SM predictions and search.

On the other hand, the experimental recipe was far from simple. The LHC, a particle accelerator capable of colliding protons at energies far beyond anything previously achieved, was a necessity. But energy alone was not enough, as sufficient numbers of Higgs bosons also had to be produced. Although occurring at a low rate, Higgs decays into pairs of massless photons would prove to be experimentally clean and furnish the best opportunity for discovery. Once detection efficiencies, backgrounds, and requirements of statistical significance are folded into the mix, on the order of 100,000 Higgs bosons would be required for discovery. This is no short order, yet that is what the accelerator teams delivered to the detectors.

Fig. 3. The discovery of the Higgs boson at ATLAS and CMS, as reported in two papers (arXiv:1207.7214 and arXiv:1207.7235) published after the 4 July announcement. Black lines show the local “p-value”, which is the probability that the observation is a statistical fluctuation and not the Higgs boson. This p-value is less than one part in a million, similar to the probability of flipping a coin 21 times and it coming up heads on every occasion, and the significance is peaked at the same mass for both experiments.

With the accelerator running, it remained to observe the thing. This would push ingenuity to its limits. Physicists on the ATLAS and CMS detectors would need to work night and day to filter through the particle detritus from innumerable proton–proton collisions to select data sets of interest. The search set tremendous challenges for the energy-resolution and particle-identification capabilities of the detectors, not to mention dealing with enormous volumes of data. In the end, the result of this labour reduced to a couple of plots (figure 3). The discovery was clear for each collaboration: a significance pushing the 5σ “discovery” threshold. In further irony for the mass-giving Higgs, the discovery was driven primarily by the rare but powerful diphoton decays, followed closely by Higgs decays to Z bosons. Global media erupted in a science-fuelled frenzy. It turns out that everyone gets excited when a fundamental building block of nature is discovered.

The hard work begins

The joy in the experimental and theoretical communities in the summer of 2012 was palpable. If we were to liken early studies of the electroweak forces to listening to a crackling radio, LEP had given us black and white TV and the LHC was about to show us the world in full cinematic colour. Particle physicists now had the work they had waited a lifetime to do. Is it the SM Higgs boson, or something else, something exotic? All we knew at the time was that there was a new boson, with mass of roughly 125 GeV, that decayed to photons and Z bosons.

Despite the huge success of the SM, there was every reason to hope that the new boson would not be of the common variety. The Higgs brings us face-to-face with questions that the SM cannot answer, such as what constitutes dark matter (observed to make up roughly 80% of all the matter in the universe). Unlike the other SM particles, it is uncharged and without spin, and can therefore interact easily with any other neutral scalar particles. This makes it a formidable tool in the hunt for dark matter – a possibility we often call the “Higgs portal”. The ATLAS and CMS collaborations have been busy exploring the Higgs portal and we now know that the Higgs decay rate into invisible new dark particles must be less than 34% of its total rate into known particles. This is an incredible thing to know for a particle that is itself so elusive, and a significant early step for dark-sector physics.

Another deep puzzle, even more esoteric than dark matter and which has driven the theoretical community to distraction for decades, is called the hierarchy problem. We know that at higher energies (smaller sizes) there must be more structure to the laws of nature: the scale of quantum gravity, the Planck scale, is one example, but there are hints of others. For any other SM particle, this new physics at high energies has no dramatic effect, since fundamental particles with nonzero spin possess special protective symmetries that shield them from large quantum corrections. But the Higgs possesses no such symmetry, and is thus a sensitive creature: quantum-mechanical effects will give large corrections to its mass, pulling it all the way up to the masses of the new particles it is interacting with. That has clearly not happened, given the mass we measure in experiments, so what is going on?

Fig. 4. (Left) The mass the Higgs boson bestows on other particles depends on its interaction strength with that particle. Knowing the mass of a particle we may therefore predict (dashed blue line) how strongly it interacts with the Higgs boson. LHC measurements are shown in black, corroborating the predictions of the SM over many orders of magnitude in interaction strength. (Right) The strength of the Higgs coupling to fermions versus vector bosons, as measured by ATLAS and CMS based on Run 1 data for individual decay channels (colours) and their global combination (grey).

Thus the discovery of the Higgs brings the hierarchy problem to the fore. If the Higgs is composite, being made up of other particles, in a similar fashion to the ubiquitous QCD pion, then the problem simply goes away because there is no fundamental scalar in the first place. Another popular theory, supersymmetry, postulates new space–time symmetries, which protect the Higgs boson from these quantum corrections and could modify its properties. Measurements of the Higgs interactions thus indirectly probe this deepest of questions in modern particle physics. For example, we now know the interaction between the Higgs boson and the Z boson to an accuracy at the level of 10%, a significant constraint on these theories.

It is also crucial that we understand the way the Higgs interacts with fermions. Anyone who has ever looked up the masses of the quarks and leptons will see that they follow cryptic hierarchical patterns, while families of fermions can also mix into one another through the emission of a W boson in peculiar patterns that we do not yet understand. By playing a star role in generating particle masses, and as a supporting actor by also generating the mixings, the Higgs could shed light on these mysteries.

At the time of the Higgs discovery in 2012, the only interactions we were certain of concerned bosons: photons, W and Z bosons, and, to a certain degree, gluons. There was emerging evidence for interactions with top quarks, but it was circumstantial, coming from the role of the top quark in the quantum-mechanical process that generates Higgs interactions with gluons and photons. After a four-year wait, in 2016 ATLAS and CMS combined forces to reach the first 5σ direct discovery of Higgs interactions with a fermion: the τ lepton, to be precise. This was a significant milestone, not least because it also happened to give the first direct evidence of Higgs interactions with leptons.

CChig6_06_17 — Fig. 5. The Higgs field sits at the bottom of its so-called “Mexican-hat” scalar potential. The second derivative of this potential is its mass, the third is its self-interaction, and so on to the fourth derivative, which allows four Higgs bosons to interact. Thus by measuring these interactions we would directly probe the shape of the scalar potential, and hence the dynamical mechanism by which the Higgs field spontaneously broke electroweak symmetry in the early universe.

The scope of the Higgs programme has also broadened since the early days of the discovery. This applies not only to the precision with which certain couplings are measured, but also to the energy at which they are measured. For example, when the Higgs boson is produced via the fusion of two gluons at the LHC, additional gluons or quarks may be emitted at high energies. By observing such “associated production” we may gain information about the magnitude of a Higgs interaction and about its detailed structure. Hence, if new particles that influence Higgs boson interactions exist at high energies, probing Higgs couplings at high energies may reveal their existence. The price to be paid for associated production is that the probability, and hence the rate, is low (figure 2). As an ever increasing number of Higgs production events have been recorded at the LHC in the past five years, this has allowed physicists to begin mapping the nature of the Higgs boson’s interactions.

What’s next?

We have much to anticipate. Although the Higgs is too light to be able to decay into pairs of top quarks, experimentalists will study its interactions with the top quark by observing Higgs produced in association with pairs of top quarks. Another anticipated discovery, which is difficult to pick out above other background processes, is the decay of the Higgs to bottom quarks. Amazingly, despite the incredibly rare signal rate, the upgraded High-Luminosity LHC will be able to discover Higgs decays to muons. This would be the first observation of Higgs interactions with the second generation of fermions, pointing a floodlight towards the flavour puzzle. These measurements will bring the overall picture of how the Higgs generates particle masses into closer focus. Even now, after only five years, the picture is becoming clear: Higgs physics is becoming a precision science at the LHC (figure 4).

There is more to Higgs physics than a shopping list of couplings, however. By the end of the LHC’s operation in the mid-2030s, more than one hundred million Higgs bosons will have been produced. That will allow us to search for extremely rare and exotic Higgs production and decay modes, perhaps revealing a first crack in the SM. On the opposing flank, by observing the standard production processes in extreme kinematic corners, such as Higgs production at very high momentum, we will be able to measure its interactions over a range of energies. In both cases the challenge will not only be experimental, as the SM predictions must also keep pace with the accuracy of the measurements – a fact which is already driving revolutions in our theoretical understanding.

Setting our sights on the distant future of Higgs physics, it would be remiss to overlook the “white whale” of Higgs physics: the Higgs self-interaction. In yet another unique twist, the Higgs is the only particle in the SM that can scatter off itself (figure 5). In contrast, gluons only interact with other non-identical gluons. If we could access the Higgs self-interactions, by determining how a Higgs boson scatters on itself in measurements of Higgs boson pair-production processes, we would be measuring the shape of the Higgs scalar potential. This is tremendously important because, in theory, it determines the fate of the entire universe: if the scalar potential “turns back over” again at high field values, it would imply that we live in a metastable state. There is mounting evidence, in the form of the measured SM parameters such as the mass of the top quark, that this may be the case. Unfortunately, with the LHC we will not be able to measure this interaction well enough to definitively determine the shape of the Higgs scalar potential, and so we must ultimately look to future colliders to answer this question, among others.

The Higgs is the keystone of the SM and therefore everything we learn about this new particle is central to the deepest laws of nature. When huddled over my laptop at 3.00 a.m. on 4 July 2012, I was 27 years old and in the first year of my first postdoctoral position. To me, and presumably the rest of my generation, it felt like a new scientific continent had been discovered, one that would take a lifetime to explore. On that day we finally knew it existed. Today, after five years of feverish exploration, we have in our hands a sketch of the coastline. We have much to learn before the mountains and valleys of the enigmatic Higgs boson are revealed.

FAIR forges its future

Eighty foundation piles are currently being driven into the ground to create the retaining walls of the GSI facilities.
Image credit: G Otto/GSI.

Supernova explosions, neutron-star mergers and rare radioactive ions might not seem to have much connection to terrestrial matters. Yet, while the lightest elements were synthesised immediately after the Big Bang, and elements up to iron were created in stellar cores, all of the heavy elements beyond gold and platinum were produced via complex production paths during extreme astrophysical events. Experiments with intense heavy-ion beams produced at the international Facility for Antiproton and Ion Research (FAIR), which is under construction at Darmstadt in Germany, promise new and detailed insights into the nuclear reactions and rare radioactive ion species that underpin the synthesis of heavy elements in the universe.

FAIR is a multipurpose accelerator facility that will provide beams, from protons up to uranium ions, with a wide range of intensities and energies, in addition to secondary beams of antiprotons and rare isotopes. Complementary to CERN’s Large Hadron Collider or Super Proton Synchrotron, FAIR is pushing the intensity rather than the energy frontier for hadron beams. It will enable scientists to produce and study reactions involving rare exotic hadronic states or rare, very short-lived radioactive nuclei. It will enable the investigation of processes under the extreme temperatures and pressures that prevail in large planets, stars and stellar explosions. FAIR will also allow physicists to produce and study dense hadronic matter and its transition to quark matter, and permit tests of quantum electrodynamics in the regime of very strong electromagnetic fields, to name but a few goals.

The existing GSI accelerators UNILAC, SIS18 and ESR (blue) and the FAIR facilities (red). FAIR comprises the SIS100 synchrotron; the antiproton separator and the super fragment separator (Super-FRS); the collector ring (CR); high-energy storage ring (HESR); and experimental stations for APPA, CBM, NUSTAR and PANDA. The proton linac and the CRYRING also belong to the FAIR instrumentation portfolio.
Image credit: GSI/FAIR.

Overall, FAIR’s scientific programme comprises hadron physics, nuclear structure and astrophysics, atomic physics, plasma physics, materials research, and radiation biophysics and its applications in cancer therapy and space research. Its science is divided between four main pillars (see panel “FAIR’s four scientific pillars” below), including experiments similar in design to those in high-energy physics. After a lengthy and complex phase of development, a groundbreaking ceremony held on 4 July 2017 marked the start of construction of the FAIR facility.

Project evolution

FAIR was developed by the international science community and the GSI laboratory (the Helmholtz Centre for Heavy Ion Research) around the turn of the millennium. GSI, founded in 1969, has a long tradition in nuclear and atomic physics and, more generally, heavy-ion research, and was therefore a natural site on which to develop the next generation of accelerators and experiments for these fields. The initial start date of FAIR was 7 October 2010, when nine partner countries (Finland, France, Germany, India, Poland, Romania, Russia, Slovenia and Sweden) signed an intergovernmental agreement for its construction and operation. The UK joined FAIR as an associate member in 2013.

During late 2014, the then FAIR management reported difficulties surrounding new construction requirements. Although not unusual for a complex, one-of-a-kind facility such as FAIR, this caused major modifications of the civil-construction design and resulted in a delay and cost increase of the overall project. In September 2015 the FAIR Council, representing the nine shareholders, unanimously agreed to adapt the FAIR construction budget and timeline according to the necessary design modifications.

Following this key decision, FAIR was completely reorganised and consolidated: the FAIR and GSI GmbH companies aligned their managerial and administrative structures and processes, and a joint management team was installed in a stepwise process, with former spokesperson for the ALICE experiment at CERN, Paolo Giubellino, appointed as scientific managing director and spokesperson for FAIR-GSI in January 2017. Thanks to these and other changes, civil-construction work for the tunnel that will house FAIR’s main accelerator began on schedule this summer, with the goal to finish all FAIR buildings by the end of 2022. In parallel, procurement of the FAIR accelerator systems and construction of the FAIR detector instrumentation is progressing well. Following the installation and commissioning of the accelerators and experiments starting in 2020/2021, the FAIR science programme is expected to start operation in 2025.

A journey through FAIR

The FAIR accelerator complex is optimised to deliver intense and energetic beams of particles to different production targets. The resulting beams will then be steered to various fixed-target experiments or injected into storage-cooler rings for novel in-ring experiments with beams of secondary antiprotons or radioactive ions at the highest beam qualities. The central machines of FAIR are: the fast-ramping SIS100 synchrotron, which provides intense primary beams; the large-aperture Super Fragment Separator (Super-FRS), which filters out the exotic ion beams; and the cooler storage rings CR and HESR (see image). The SIS100 is the heart of FAIR. With a circumference of 1.1 km and a maximum magnetic bending power of 100 Tm, the machine will accelerate ion beams with maximum intensities ranging from 4 × 10¹³ protons at 29 GeV to 5 × 10¹¹ uranium (28+) ions at 2.7 GeV/u. The existing GSI accelerators UNILAC and SIS18 will serve as injectors and pre-accelerators for SIS100, while a new proton linac will be installed for high-intensity injection into the SIS18/SIS100 synchrotron chain.

A rendering of the completed GSI-FAIR facility.
Image credit: GSI/FAIR.

To maximise the luminosity of the SIS100, fast-ramped superconducting superferric magnets with a maximum field of 1.9 T and ramp rates up to 4 T per second have been developed to enable cycle times of the same order as the cooling rates in the storage rings (see image). Together with the upgraded SIS18 pre-accelerator, the SIS100 will provide uranium ion beams 10 times more intense than previously available beams at GSI. The cold machine design has a further advantage: the SIS100 beam pipe enables heavy residual gas components to be pumped, potentially stabilising the dynamic pressure. Due to the tight beam-loss budget, the iron yoke of the superconducting magnets must be built with the highest precision and reproducibility. Production has already started for the SIS100 dipole magnets, and the first beams from SIS100 are foreseen for 2025. Three test facilities at GSI, JINR/Dubna and CERN have been established to assess the different types of superconducting magnets.

Two production targets for rare isotope and antiproton beams will be served by the SIS100. A primary ion beam can either be slowly extracted to the Super-FRS over a period of many seconds to produce radioactive secondary beams for fixed-target experiments, or it can be extracted quickly in the form of a single, compressed, short bunch to produce a secondary beam of antiprotons or exotic ions. The in-flight-generated rare isotopes, produced via projectile fragmentation of all primary beams up to uranium-238 or alternatively via fission of uranium-238 beams, are efficiently separated in the large aperture of the Super-FRS. Due to the large acceptance of this machine, the gain in primary-beam intensities for uranium ions in the SIS100 translates into a factor of more than 1000 for secondary-beam intensities of rare, radioactive isotopes.

After production and separation, the hot secondary ion beams drive three experimental scenarios: they can be stopped to allow studies of their ground-state properties; used in in-flight and secondary reactions to produce even more exotic species; or stored and pre-cooled in the collector ring (CR). The fast stochastic cooling process in the CR relies on a fast de-bunching of the injected short bunch. Pre-cooled secondaries will then be transferred from the CR to the high-energy storage ring (HESR), where they can be accumulated and accelerated up to an energy of 15 GeV for antiprotons and about 5–6 GeV/u for very heavy ions. The HESR can also store and cool stable high-charge-state heavy-ion beams, directly injected from the SIS100 via the CR, for precision studies in atomic, nuclear and fundamental physics, such as tests of quantum electrodynamics (QED) in strong fields or tests of special relativity.

FAIR science ahead

About 3000 scientists including more than 500 PhD students from around the world will carry out experiments at FAIR to understand the fundamental structure of matter, explore its exotic forms, and to understand how the universe evolved from its primordial state. FAIR’s science programme is structured into four pillars and organised in four large collaborations with several hundred members each: APPA, serving communities in atomic, plasma physics and applications; CBM, the Compressed Baryonic Matter experiment; NUSTAR, the NUclear STructure, Astrophysics and Reactions programme; and PANDA (antiProton ANihilation in DArmstadt), which aims to study hadrons using antiproton beams. APPA and NUSTAR consist of several sub-collaborations, while CBM and PANDA are rather monolithic experiments involving large detectors (see panel on previous page).

The newly developed fast-cycling superconductiong dipole magnet for FAIR’s SIS100 synchrotron.
Image credit: Babcock Noell GmbH.
Image credit: Babcock Noell GmbH.

Well before the start of the SIS100 operation in 2025, an upgrade of the GSI accelerators due for completion this year will allow extensive testing of FAIR components. This upgrade will also allow researchers to trial novel FAIR instrumentation for an attractive intermediate research programme, named FAIR phase 0. For instance, the NUSTAR “R3B” spectrometer, the CRYRING and the HITRAP facility will be available and will enable, in combination with the intensity-upgraded SIS18 synchrotron and GSI’s fragment separator, novel experiments in nuclear structure and reactions in so far unexplored areas of the nuclear chart.

The CRYRING and the HITRAP facility will enable physicists to further increase the precision of both atomic-physics measurements of QED effects in highly charged heavy ions and of measurements of fundamental constants. Moreover, the hadronic-matter experimental programme of HADES (High Acceptance Di-Electron Spectrometer) will benefit from the higher intensities from the SIS18. HADES is a versatile detector for the study of dielectron (e⁺e⁻) and hadron production in heavy-ion collisions, as well as in proton- and pion-induced reactions in the energy range of 1–4 GeV. These are just a few examples from the intermediate research programme, which will start in 2018 and offer about three months of beam time per year, thereby bridging the gap until the commissioning of the SIS100.

The FAIR phase 0 programme intends to maintain and further establish the FAIR-GSI community by offering attractive science before the full complex is up and running. It will also educate and train the next generation of scientists and engineers for FAIR and, last but not least, maintain and extend the technical skills required to operate such a large accelerator complex. While FAIR phase 0 is an important and necessary step offering new and excellent research opportunities for users, full exploitation of the unique science potential opened up by FAIR has to await the start of SIS100 operation in 2025.

Depending on how rich the scientific harvest from FAIR will be and in which specific directions it will be most prominent, one can conceive of several upgrade options. One is a further increase of intensities by up to two orders of magnitude for nuclear structure, reactions and astrophysics, which will also benefit dense-plasma research. Another option is a further increase of beam energy by a factor of 3–6 for hadron- and quark-matter research. Other upgrade possibilities include strengthening the antiproton research programme, via cooled low-energy antiproton beams, for the study of fundamental interactions and symmetries. FAIR is expected to be the flagship facility for hadron, nuclear and atomic physics – as well as related science fields exploiting intense beams of antiprotons and heavy ions – until around 2040.

FAIR's four scientific pillars

Atomic and Plasma Physics, and Applied sciences (APPA)
With about 700 participants, APPA is an umbrella for several sub-collaborations working across atomic physics, plasma physics and applied sciences, with specific programmes in biophysics, medical physics and materials science. Several experimental stations, in addition to the CRYRING and HESR storage rings and the trapping facility HITRAP, will allow the APPA community to tackle a variety of challenges. In atomic physics, for example, high-precision tests of bound-state QED in the non-perturbative regime become possible. A precise determination of fundamental constants such as the fine-structure constant is also a target, which involves very precise measurements of the bound-state g-factors in medium to high-Z hydrogen-like ions confined in a trap. Plasma physicists will be able to create and probe dense plasmas to test models of planetary and stellar structure. By means of FAIR beams, the high-energy component of galactic cosmic radiation can also be simulated to assess the risk of space missions for astronauts and electronic equipment by dedicated irradiation experiments. Finally, the material science and geoscience communities will be able to test how materials respond to the simultaneous application of irradiation and pressure, which is of interest for the synthesis of new materials from highly non-equilibrium conditions and for understanding processes in the Earth’s mantle.

The Compressed Baryonic Matter experiment (CBM)
The CBM experiment, which has more than 500 participants and is organised similarly to the LHC experiments at CERN, will use high-energy nucleus–nucleus collisions to investigate highly compressed nuclear matter. The fixed-target experiment is 10 m long and comprises a large-aperture superconducting dipole magnet and seven subsequent detector systems providing tracking and particle identification. CBM collisions will recreate the matter densities found in supernova explosions, the cores of neutron stars and neutron-star mergers. In contrast to the very high temperatures and low net-baryon densities reached at the Relativistic Heavy Ion Collider in Brookhaven and the LHC at CERN (conditions that are similar to the conditions that prevailed microseconds after the Big Bang), the energies of the FAIR beams are perfectly suited to study the QCD phase diagram of strongly interacting matter at large net baryon densities and low temperatures. Here, it is expected that the QCD phase diagram exhibits a rich structure such as a critical point, a first-order phase transition between hadronic and partonic matter, or new phases such as quarkyonic matter. Discovering these landmarks would be a breakthrough in our understanding of the strong interaction. The CBM experiment is designed to run at interaction rates of up to 10 MHz, which is 3–4 orders of magnitude higher than the rates reached in other high-energy heavy-ion experiments. It has very fast and radiation-hard detectors, a novel data read-out and analysis concept, and a high-performance computing cluster for online event reconstruction and selection.

The PANDA experiment
The antiProton ANihilation in DArmstadt (PANDA) collaboration is a co-operation of more than 400 scientists from 19 countries, similar to but smaller than the LHC experiments at CERN. Its goal is to understand hadrons using the power of an antiproton beam on fixed hydrogen or other nuclear targets. Antiproton–proton annihilations have enormous advantages compared to proton–proton collisions, such as small momentum-transfer at maximum released energy with well-defined initial states and high-precision mass scanning. The vast difference in mass between the proton and its individual quark constituents is a result of the binding among quarks in the confinement regime, and exotic hadrons such as tetra- and pentaquarks, hybrids and glueballs will reveal uncharted properties of this binding. PANDA will use proton form-factor measurements, deep virtual Compton scattering and quark dynamics, as well as the behaviour of hadrons inside nuclear media, as highly complementary tools with which to understand the very nature of hadrons. Strange quarks in hyperons, for instance, can be used as tags to trace quark dynamics with very high cross-sections and spin degrees of freedom. The PANDA experiment features a modern multipurpose detector with excellent tracking, calorimetry and particle-identification capabilities. Together with the high-quality antiproton beam at FAIR’s high-energy storage ring (HESR), an unprecedented annihilation rate and sophisticated event filtering, it will be ideally suited to address important questions in all aspects of this field.

NUclear STructure, Astrophysics and Reactions (NUSTAR)
The NUSTAR collaboration at FAIR has more than 800 participants from 180 institutes located in 38 countries. Similar to APPA, NUSTAR does not represent a single monolithic experiment but is structured in several sub-collaborations across different experimental set-ups tailored to various aspects of secondary radioactive ions, such as mass and lifetime measurements. A major goal of NUSTAR is to improve our knowledge of the synthesis and abundance of chemical elements, for which the collaboration will explore the structure and reaction properties of very rare radioactive ions produced for the first time by FAIR. Although much has been learnt about the behaviour of stable and unstable nuclei in past decades, we are still far from understanding how the very heavy elements are formed through reactions involving rare nuclei at the limit of stability. FAIR will allow scientists to artificially produce the nuclei that occur as radioactive intermediate products in the formation of stable isotopes, measuring directly in the laboratory the different processes involved. FAIR offers unique tools for such studies. The Super-FRS will make very efficient use of the highly intense beams at high energies to separate beams of the heaviest and most neutron-rich nuclei, while FAIR’s complex network of storage rings will allow mass and lifetime measurements. This will place NUSTAR at the forefront of this branch of science. Many of NUSTAR’s experimental set-ups are already complete, and the collaboration plans to transfer them into the new buildings starting from 2023.

Data privacy concerns us all

National institutions, companies, laboratories and universities in Europe will soon be subject to the General Data Protection Regulation.
Image credit: D Foster.

It is perhaps no coincidence that many dystopian visions of the future in popular fiction, such as Nineteen Eighty-Four, Brave New World and Fahrenheit 451, have breach of data privacy at the core of their plots. With an ever growing level of interaction between humans and a global infrastructure tied together by the internet, there is always the fear that others know more about you than you would like. How can we save ourselves from such a bleak future?

The answer has been to create, over the past 20 years, a number of strict legal obligations and rights when dealing with the personal data of individuals. You will notice that you are increasingly asked for consent for use of your personal data on websites and to allow software to store cookies on your computer. Such legislation is sometimes criticised for generating bureaucracy that gets in the way of “real work”. But for those who work in the data-privacy arena, it is clear that we need to adapt quickly to a rapidly evolving digital environment. What you do, where you go and how long you spend there are valuable assets in the information world.

In 2012 the European Union (EU) proposed new data-protection reforms to strengthen the fundamental rights of citizens. Three years later, EU institutions reached agreement on the rules, and in May 2016 a new regulation was issued called the General Data Protection Regulation (GDPR), which enters into force in all European Economic Area (EEA) countries from 25 May 2018.

You probably haven’t heard much about the GDPR until now, yet it is almost certain to impact the way our field deals with personal data. The central idea is that your personal data is truly yours: it cannot be taken or processed without safeguards to its privacy, and any data collection or processing must have an appropriate legal basis. The new laws offer a very broad interpretation of what “personal data” and “processing” mean, and offer a number of legal bases that must be considered. Personal data is anything that could be used to identify you, including obvious things like name and address but also more subtle information like GPS location or IP address. Processing is equally loosely defined, from storing data in a database to viewing data on a screen and even copying a file.

Although in practice there are many details to be determined, the intention of the regulators is evident: to stop the use of people’s personal data except for well-defined purposes that must be clear when the data are collected to be fair to the individual. Crucially, the new regulations aim to be technology agnostic and therefore apply equally to online databases as well as a filing cabinet full of paper.

All EEA institutions, companies, labs and universities will be subject to the GDPR. Although CERN, as an international organisation, is not directly subject to EU regulations, in light of the coming changes it is reviewing its internal legislation to offer equivalent levels of personal-data protection. Consequently, in January this year CERN established the Office of Data Privacy Protection to assist services that process personal data and to help anyone who is concerned about how their personal data is being handled by the Organization.

Given the broad scope of personal data and data processing, it can be complicated and somewhat burdensome to comply with these new practices. For instance, it will require us to review how passport information should be sent, how records such as medical information and personal attributes are secured, as well as how photos and CCTV are used. At the same time, we need to recognise that protecting privacy is important and that adopting a “nothing to hide, nothing to fear” approach does not protect us from future unknown uses of our personal data.

So, if in any doubt, simply adopt the golden rule of personal data: if you don’t really need it, don’t collect and store it; if you do, delete it as soon as possible.

A First Course in Mathematical Physics

By Colm T Whelan
Wiley-VCH

The aim of this book is to provide undergraduate students taking classes in the physical sciences with the fundamental-mathematics tools they need to proceed with their studies.

In the first part the author introduces core mathematics, starting from basic concepts such as functions of one variable and complex numbers, and moving to more advanced topics including vector spaces, fields and operators, and functions of a complex variable.

The second part shows some of the copious applications of these mathematics tools to physics. When introducing complex physics laws and theories, including Maxwell’s equations, special relativity and quantum theory, the author tries to present the material in an easily intelligible way. The author also emphasises the direct connection between the conceptual basis of these physics topics and the mathematical tools provided in the first part of the text.

Two appendices of formulas conclude the book. A large number of problems are included but the solutions are only made available on a password-protected website for lecturers.

Thorium Energy for the World

By J-P Revol, M Bourquin, Y Kadi, E Lillestol, J-C de Mestral and K Samec (eds)
Springer

This book contains the proceedings of the Thorium Energy Conference (ThEC13), held in October 2013 at CERN, which brought together some of the world’s leading experts on thorium technologies. According to them, nuclear energy based on a thorium fuel cycle is safer and cleaner than the one generated from uranium. In addition, long-lived waste from existing power plants could be retrieved and integrated into the thorium fuel cycle to be transformed into a stable material while generating electricity.

The technology required to implement this type of fuel cycle is already being developed, nevertheless much effort and time is still needed.

The ThEC13 conference saw the participation of high-level speakers from 30 countries, such as the Nobel prize laureates Carlo Rubbia and Jack Steinberger, the then CERN Director-General Rolf Heuer, and Hans Blix, former director-general of the International Atomic Energy Agency (IAEA), to name a few.

Collecting the contributions of the speakers, this book offers a detailed technical review of thorium-energy technologies from basic R&D to industrial developments, and is thus a tool for informed debates on the future of energy production and, in particular, on the advantages and disadvantages of different nuclear technologies.

Bose–Einstein Condensation and Superfluidity

By L Pitaevskii and S Stringari
Oxford University Press

CCboo2_05_17 — Bose–Einstein Condensation and Superfluidity

This book deals with the fascinating topics of Bose–Einstein condensation (BEC) and superfluidity. The main emphasis is on providing the formalism to describe these phases of matter as observed in the laboratory. This is far from the idealised studies that originally predicted BEC and are essential to interpret the experimental observations.

BEC was predicted in 1925 by Einstein, based on the ideas of Satyendra Nath Bose. It corresponds to a new phase of matter where bosons accumulate at the lowest energy level and develop coherent quantum properties at a macroscopic scale. These properties may correspond to phenomena that seem impossible from an everyday perspective. In particular, BEC lies behind the theory of superfluids, which are fluids that flow without dissipating energy and rotate without generating vorticity – if we except quantised vortices, which are a sort of topological defect.

Experimentally, the first BEC from dilute gases was observed in the laboratory in 1995, recognised by the 2001 Nobel Prize in Physics. Since then, there has been an explosion of interest and new results in the field. It is thus timely that two of its leading experts have updated and extended their volume on BEC to summarise the theoretical aspects of this phase of matter. The authors also describe in detail how superfluid phenomena can occur for Fermi gases in the presence of interactions.

The book is relatively heavy in formalism, which is justified by the wide range of phenomena covered in a relatively concise volume. It starts with some basics about correlation functions, condensation and statistical mechanics. Next, it delves into the simplest systems for which BEC can occur: weakly coupled dilute gases of bosonic particles. The authors describe different approaches to the BEC phase, including the works of Landau and Bogoliubov. They also introduce the Gross–Pitaevskii equation and show its importance in the description of superfluids. Superfluidity is explained in great detail, in particular the occurrence of quantised vortices.

The second part describes how to adapt the theoretical formalism introduced in the first part to realistic traps where BEC is observed. This is very important to connect theoretical descriptions to laboratory research, for instance to predict in which experimental configurations a BEC will appear and how to characterise it.

Part three deals with BEC in fermionic systems, which is possible if the fermions interact and pair-up into bosonic structures. These fermionic phases exhibit superfluid properties and have been created in the laboratory, and the authors consider fermionic condensates in realistic traps. The final part is devoted to new phenomena appearing in mixed bosonic–fermionic systems.

The book is a good resource for the theoretical description of BEC beyond the idealised configurations that are described in many texts. The concise style and large amount of notation requires constant effort from the reader, but seems inevitable to explain many of the surprising phenomena appearing in BECs. The book, perhaps combined with others, will provide the reader with a clear overview of the topic and latest theoretical developments in the field. The text is enhanced by the many figures and plots presenting experimental data.

Making Sense of Quantum Mechanics

By Jean Bricmont
Springer

CCboo1_05_17 — Making Sense of Quantum Mechanics

In this book, Jean Bricmont aims to challenge Richard Feynman’s famous statement that “nobody understands quantum mechanics” and discusses some of the issues that have surrounded this field of theoretical physics since its inception.

Bricmont starts by strongly criticising the “establishment” view of quantum mechanics (QM), known as the Copenhagen interpretation, which attributes a key role to the observer in a quantum measurement. The quantum-mechanical wavefunction, indeed, predicts the possible outcomes of a quantum measurement, but not which one of these actually occurs. The author opposes the idea that a conscious human mind is an essential part of the process of determining what outcome is obtained. This interpretation was proposed by some of the early thinkers on the subject, although I believe Bricmont is wrong to associate it with Niels Bohr, who relates the measurement with irreversible changes in the measuring apparatus, rather than in the mind of the human observer.

The second chapter deals with the nature of the quantum state, illustrated with discussions of the Stern–Gerlach experiment to measure spin and the Mach–Zender interferometer to emphasise the importance of interference. During the last 20 years or so, much work has been done on “decoherence”. This has shown that the interaction of the quantum system with its environment, which may include the measuring apparatus, prevents any detectable interference between the states associated with different possible measurement outcomes. Bricmont correctly emphasises that this still does not result in a particular outcome being realised.

The author’s central argument is presented in chapter five, where he discusses the de Broglie–Bohm hidden-variable theory. At its simplest, it proposes that there are two components to the quantum-mechanical state: the wavefunction and an actual point particle that always has a definite position, although this is hidden from observation until its position is measured. This model claims to resolve many of the conceptual problems thrown up by orthodox QM: in particular, the outcome of a measurement is determined by the position of the particle being measured, while the other possibilities implied by the wavefunction can be ignored because they are associated with “empty waves”. Bricmont shows how all the results of standard QM – particularly the statistical probabilities of different measurement outcomes – are faithfully reproduced by the de Broglie–Bohm theory.

This is probably the clearest account of this theorem that I have come across. So why is the de Broglie–Bohm theory not generally accepted as the correct way to understand quantum physics? One reason follows from the work of John Bell, who showed that no hidden-variable theory can reproduce the quantum predictions (now thoroughly verified by experiment) for systems consisting of two or more particles in an entangled state unless the theory includes non-locality – i.e. a faster-than-light communication between the component particles and/or their associated wavefunctions. As this is clearly inconsistent with special relativity, many thinkers (including Bell himself) have looked elsewhere for a realistic interpretation of quantum phenomena. Not so Jean Bricmont: along with other contemporary supporters of the de Broglie–Bohm theory, he embraces non-locality and looks to use the idea to enhance our understanding of the reality he believes underlies quantum physics. In fact he devotes a whole chapter to this topic and claims that non-locality is an essential feature of quantum physics and not just of models based on hidden variables.

Other problems with the de Broglie–Bohm theory are discussed and resolved – to the author’s satisfaction at least. These include how the de Broglie–Bohm model can be consistent with the Heisenberg uncertainty principle when it appears to assume that the particle always has a definite position and momentum; he points out that the statistical results of a large number of measurements always agree with conventional predictions, and these include the uncertainty principle.

Alternative ways to interpret QM are presented, but the author does not find in them the same advantages as in the de Broglie–Bohm theory. In particular, he discusses the many-worlds interpretation, which assumes that the only reality is the wavefunction and that, rather than collapsing at a measurement, this produces branches that correspond to all measurement outcomes. One of the consequences of decoherence is that there can be no interaction between the possible measurements, and this means that no branch can be aware of any other. It follows that, even if a human observer is involved, each branch can contain a copy of him or her who is unaware of the others’ presence. From this point of view, all the possible measurement outcomes co-exist – hence the term “many worlds”. Apart from its ontological extravagance, the main difficulty with many-worlds theory is that it is very hard to see how the separate outcomes can have different probabilities when they all occur simultaneously. Many-worlds supporters have proposed solutions to this problem, which do not satisfy Bricmont (and, indeed, myself), who emphasises that this is not a problem for the de Broglie–Bohm theory.

A chapter is also dedicated to a brief discussion of philosophy, concentrating on the concept of realism and how it contrasts with idealism. Unsurprisingly, it concludes that realists want a theory describing what happens at the micro scale that accounts for predictions made at the macro scale – and that de Broglie–Bohm provide just such a theory.

The book concludes with an interesting account of the history of QM, including the famous Bohr–Einstein debate, the struggle of de Broglie and Bohm for recognition, and the influence of the politics of the time.

This is a clearly written and interesting book. It has been very well researched, containing more than 500 references, and I would thoroughly recommend it to anyone who has an undergraduate knowledge of physics and mathematics and an interest in foundational questions. Whether it actually lives up to its title is for each reader to judge.

LHC back with a splash

“Splash” events, whereby protons that have scattered off a collimator in the LHC beam pipe spray debris into the detectors, were recorded by ATLAS (left) and CMS (middle) at the end of April. (Right) A single low-intensity proton bunch strikes a screen in the LHC twice, signifying that it has made one full turn of the collider.

On 29 April, just after 8.00 p.m., the Large Hadron Collider (LHC) began circulating beams of protons for the first time this year. Extensive technical and maintenance work was undertaken since its end-of-year shutdown in early December, yet the restart of the 27 km-circumference superconducting collider has proceeded smoothly.

Magnet powering tests, which ensured the machine can be operated at an energy of 6.5 TeV per beam, were completed during the last week of April. This was followed by the machine-checkout phase, during which all equipment is placed in its operational state and the four LHC experiment caverns are patrolled and closed.

In the meantime, the crew of the Super Proton Synchrotron (SPS), which feeds protons to the LHC, worked hard to extract the single-bunch beam so that the LHC could be commissioned with beam. By the end of the afternoon on Friday 28 April, protons had been sent successfully down both transfer lines and were knocking at the LHC’s door. The following day, at 6.00 p.m., beam 1 (clockwise direction) was injected and threaded through the LHC’s eight sectors one at a time, circulating the entire machine after a period of 45 minutes. Beam 2 (anticlockwise direction) then went through the same process, and at 8.12 p.m. both beams were circulating. On Sunday 30 April, the single-bunch, low-intensity beams were successfully ramped to an energy of 6.5 TeV.

The next task, which was well under way as the Courier went to press, was to continue with detailed setting up of the machine while stepping up to higher bunch intensities and then multiple bunches. Each step in the intensity ramp up that follows has to be validated by circulating the beams from three fills for up to 20 hours, and the team is aiming for a configuration of 2550 bunches per beam, with each bunch containing of the order 1.2 × 10¹¹ protons. Once stable beams have been declared, expected in the second half of May, the beams will be brought into collision and the second chapter of LHC Run 2 will be under way.

Workshop puts advanced accelerators on track

CERN’s AWAKE experiment. A possible application of this technology would be to use a proton beam from the LHC to accelerate electrons in a few-hundred-metre-long plasma for electron–proton collisions.
Image credit: M Brice/CERN.

Researchers working on advanced and novel accelerator technologies met at CERN on 25–28 April to draw up an international roadmap for future high-energy particle accelerators. Organised by the International Committee for Future Accelerators (ICFA) to trigger a community effort, the Advanced and Novel Accelerator for High Energy Physics Roadmap Workshop saw 80 experts from 11 countries discuss the steps needed to include these new technologies in strategies for future machines in Asia, Europe and the US. It follows recent discussions of national roadmaps in the US and elsewhere, and was timed so that advanced accelerator development can be taken into account in the coming update of the European Strategy for Particle Physics in 2020.

Given the scale of the cost of traditional accelerator technologies, which require large circular or long linear accelerators to reach the highest energies, the past two decades have seen significant progress to find alternative approaches. These include dielectrics and plasmas driven by laser pulses or particle beams, which are able to accelerate particles 1000 times more than the radio-frequency structures used in todayʼs accelerators. Major laboratories including CERN, SLAC, Argonne, DESY and INFN-Frascati are working on various techniques. CERN has recently started the AWAKE experiment, demonstrating that high-energy protons from the SPS can drive large accelerating fields in a plasma.

The next step is to apply these methods to high-energy physics. For example, the acceleration schemes must be tuned to determine their real potential for producing high-energy and high-quality particle bunches; the former has been demonstrated, but the latter remains a challenge. This requires experimental facilities that can only be hosted by international laboratories and a strong, united and co-ordinated community that merges the advanced and traditional accelerator communities.

The April event has now set this process in motion, focusing on the technical milestones that are needed to progress towards an intermediate-size particle accelerator and on the strategies needed to bring communities together. A new working group dedicated to the development of a roadmap will be included in the European Advanced Accelerators Concept Workshop in September 2017 in Elba, Italy.

“This is the first time that the advanced accelerator field is co-ordinated at the international level, and will pull the community together towards the first great challenge ahead, i.e. the achievement of reliable and high-quality particle bunches,” says workshop chair Brigitte Cros. “Further workshops will be organised to strengthen and sustain this co-ordination.”

CAST experiment constrains solar axions

The latest constraints on the two-photon coupling, g_a_γ, of axions and other similar particles as a function of mass, showing the new CAST limits.

In a paper published in Nature Physics, the CERN Axion Solar Telescope (CAST) has reported important new exclusion limits on coupling of axions to photons. Axions are hypothetical particles that interact very weakly with ordinary matter and therefore are candidates to explain dark matter. They were postulated decades ago to solve the “strong CP” problem in the Standard Model (SM), which concerns an unexpected time-reversal symmetry of the nuclear forces. Axion-like particles, unrelated to the strong-CP problem but still viable dark-matter candidates, are also predicted by several theories of physics beyond the SM, notably string theory.

A variety of Earth- and space-based observatories are searching possible locations where axions could be produced, ranging from the inner Earth to the galactic centre and right back to the Big Bang. CAST looks for solar axions using a “helioscope” constructed from a test magnet originally built for the Large Hadron Collider. The 10 m-long superconducting magnet acts like a viewing tube and is pointed directly at the Sun: solar axions entering the tube would be converted by its strong magnetic field into X-ray photons, which would be detected at either end of the magnet. Starting in 2003, the CAST helioscope, mounted on a movable platform and aligned with the Sun with a precision of about 1/100th of a degree, has tracked the movement of the Sun for an hour and a half at dawn and an hour and a half at dusk, over several months each year.

In the latest work, based on data recorded between 2012 and 2015, CAST finds no evidence for solar axions. This has allowed the collaboration to set the best limits to date on the strength of the coupling between axions and photons for all possible axion masses to which CAST is sensitive. The limits concern a part of the axion parameter space that is still favoured by current theoretical predictions and is very difficult to explore experimentally, allowing CAST to encroach on more restrictive constraints set by astrophysical observations. “Even though we have not been able to observe the ubiquitous axion yet, CAST has surpassed even the sensitivity originally expected, thanks to CERN’s support and unrelenting work by CASTers,” says CAST spokesperson Konstantin Zioutas. “CAST’s results are still a point of reference in our field.”

The experience gained by CAST over the past 15 years will help physicists to define the detection technologies suitable for a proposed, much larger, next-generation axion helioscope called IAXO. Since 2015, CAST has also broadened its research at the low-energy frontier to include searches for dark-matter axions and candidates for dark energy, such as solar chameleons.

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