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A 30-year adventure with heavy ions

Collision in ALICE — A collision between two lead ions at a nucleon–nucleon centre- of-mass energy of 5.02 TeV recorded by the LHC’s ALICE experiment.
Image credit: F Ronchetti/CERN.

Heavy-ion and proton–proton collisions at ultrarelativistic energies provide a unique system with which to investigate the dynamics of matter in the early universe. By generating an incredibly hot and dense “fireball” of fundamental particles, such collisions allow us to recreate the extreme conditions of the universe during its first tens of microseconds of existence.

Given that the universe did not become transparent until roughly 370,000 years after the Big Bang, this epoch in our history lies completely out of reach to observational astronomy. According to the Standard Model of particle physics, the emergence of elementary particles and forces took place via a succession of symmetry-breaking mechanisms at different energy scales as the universe expanded and cooled. In the early universe, matter was made of freely roaming quarks – which formed the quark–gluon plasma (QGP) – in addition to leptons and gauge bosons. The QGP cooled down until hadrons including baryons such as neutrons and protons were formed. Photons continued interacting with charged particles until most of the matter became bound in neutral atoms, after which they were set free to form today’s cosmic microwave background.

During the past 30 years, a succession of collider experiments and impressive theoretical achievements have driven immense progress in the field of high-energy heavy-ion physics. Not only do these results shed new light on the dynamics of matter in the early universe, they probe fundamental predictions about the strong nuclear force governed by quantum chromodynamics (QCD).

Surprises galore

We have come a long way from the early belief in the 1970s that this early phase in the universe, recreated by colliding heavy ions at continuously increasing energies, comprised a gas of quarks and gluons. This is what was expected following asymptotic freedom, a feature of QCD that explains how the interaction between two quarks becomes asymptotically weaker as the distance between them decreases. But it took three major colliders on both sides of the Atlantic to find out what was really going on during these extreme initial moments.

Thirty years in detection: the collision of a 200 GeV/nucleon sulphur ion on a gold target from the CERN SPS seen by the NA35 streamer chamber in 1986.
Image credit: CERN EX-38-7-91.

The first big result came from CERN in 2000, when it was announced that heavy-ion collisions generated by the Super Proton Synchrotron (SPS) had created a new state of matter. CERN’s then Director-General, Luciano Maiani, worded the discovery as follows: From the combined data presented by the seven CERN experiments dedicated to the heavy-ion programme has emerged the clear picture that a new state of colour-deconfined matter has been created in the early stage of the collision that develops into a collective expansion of the fireball in the later stages.

This finding confirmed a fundamental prediction of QCD: above a critical temperature, quarks are no longer confined in hadrons. The CERN announcement was, however, only the beginning of our exploration into strongly interacting matter. The same year, the baton was passed to the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory in the US. Just five years after the CERN announcement, the remarkable data collected at RHIC demonstrated that a change of paradigm for strongly interacting matter was needed. The QGP that had been created in RHIC’s STAR and PHENIX experiments did not have the properties of a perfect gas. Rather, it showed all the properties of a perfect liquid: a strongly interacting fluid with minimal mean free path.

With RHIC continuing to produce data, in 2010 CERN rejoined the heavy-ion programme with the newly operational Large Hadron Collider (LHC) and the dedicated heavy-ion experiment ALICE, ATLAS, CMS and, more recently, LHCb. This machine marked a factor 25 jump in collision energy compared with RHIC, and its experiments confirmed with unprecedented precision the STAR and PHENIX findings. The LHC also offered new opportunities to explore deconfined matter in great detail, with the goal of understanding how the dynamics of matter emerge from the fundamental properties of the strong interaction and from the quark infrastructure of particles. More recently, and surprisingly, LHC data are pointing to unexpected similarities between observables measured in heavy-ion collisions and those measured in proton–lead or in high-multiplicity proton–proton collisions, perhaps hinting at yet another change of paradigm.

During hadronisation of the QGP, final states range from non-strange and strange mesons and baryons to light nuclei. The data were recorded by ALICE in central lead–lead collisions at an energy of 2.76 TeV and fitted with a grand canonical thermal model with a temperature of 156 MeV.
Image credit: arXiv:1611.01347.

The past 30 years have been an arduous path where every step both reveals more knowledge to us while simultaneously generating new riddles. To mark the important achievements so far and to discuss the long and thrilling future of heavy-ion physics, more than 400 physicists met at CERN on 9 November last year to review what can be considered as one of the most vigorous fields at the forefront of the high-energy physics programme.

A fitting celebration

Although accelerators had been working with electrons and protons for many decades, it was in 1974 when the Bevalac at Lawrence Berkeley Laboratory accelerated the first ions to relativistic energies (approximately 2 GeV per nucleon) and led to further programmes at BNL and CERN. The Bevalac beams were not energetic enough to create the necessary energy densities for the QGP to form, and it required the ingenuity of accelerator physicists and the remarkable development of electron cyclotron resonance (ECR) sources during the 1980s to take the decisive step toward “ultrarelativistic” energies.

The idea to launch an experimental heavy-ion programme at CERN came shortly after the SPS had enabled the discovery of the W and Z bosons in 1983. As the then CERN Director-General Herwig Schopper recalled at the November workshop, the 1980s were not the best time to initiate new projects. The CERN budget was severely cut and the laboratory was very much focused on the construction of the Large Electron–Positron Collider (LEP). Despite this, Schopper bravely decided to give heavy-ion physics a chance. He was motivated by arguments put forth by Reinhardt Stock, Hans Specht, Rudolf Bock, William Willis and several other leading physicists, but the main arguments that convinced him came from Tsung Dao Lee during a VIP lunch at CERN. Schopper recalled: “I knew [Lee] from the parity-violation experiment. He had no direct personal interest and his physics motivation sounded convincing. The main argument he put forward was to find the theoretically predicted quark–gluon plasma, which played an important role in the development of the universe.”

When, in October 1986, oxygen-16 ions were successfully accelerated by the SPS and fired into a fixed target of gold, the heavy-ion programme began with a disparate ensemble of detectors recuperated from earlier high-energy experiments. Six different experiments were hatched, each with a different profile adapted to hunt the variety of observables predicted to accompany the QGP phase transition: WA80 “Plastic Ball”; the NA34/2 HELIOS; the NA35 streamer chamber; NA36; WA85/94; and the NA38 muon-pair spectrometer.

Measurements of the nuclear modification factors as a function of transverse momentum in central heavy-ion collisions at four different centre-of-mass energies, for neutral pions (SPS, RHIC), charged hadrons (h^±) (SPS, RHIC), and charged particles (LHC). Values of R_AA < 1 indicate that high-energy partons lose energy while traversing the QGP and thus carry information about its colour-charge density.
Image credit: arXiv:1611.01664.

In 1987, together with the increase of energy and the acceleration of sulphur beams, second-generation experiments containing innovative detector technologies were launched. Among these were: NA49 with an ambitious time projection chamber; CERES and its double ring imaging Cherenkov detectors; NA57 and its silicon tracking; and NA44, which contained a focusing spectrometer that made use of cesium-iodide photocathodes for the first time. The number of aficionados of this new and intriguing field of investigation grew rapidly from a few hundred initial physicists to the several thousand from all over the world who work on today’s LHC, SPS and RHIC heavy-ion facilities.

State of the art

Today, heavy-ion science is a thriving field of research, and it is notable that it is the common denominator in the physics programmes of all four major LHC experiments. On one hand, we have entered a phase of precision measurements of the QGP properties, while on the other hand the surprising similarities between proton–proton and proton–nucleon collisions observed at the LHC lead us to question if the same dynamics are at work in light and heavy systems. As demonstrated at the Quark Matter 2017 conference (see “Highlights from Quark Matter 2017” in Faces and Places), many new results are generating discussion. On the experimental side, these are based on a wealth of high-quality data collected both at LHC and RHIC for a variety of collision systems and energies, coupled with inventive analysis tools. On the theory side, particular progress has been made in relativistic hydrodynamics calculations. Among many new and creative theoretical concepts is the non-perturbative formulation of string theory, which along with the “AdS/CFT” correspondence provides tools to perform calculations for the QGP in the strongly coupled regime.

Macroscopic properties of the QGP such as its density and viscosity can now be determined with increasing precision by studying how the QGP, modelled by hydrodynamics, transports a perturbation. Measurements include the value of high-order flow coefficients and nonlinear mode mixing, while the value of η/S (shear viscosity over entropy density) and its temperature dependence have been pinned down within a factor two or less to 1/4π – which is the conjectured minimal value for a perfect quantum fluid.

On 9 November 2016, more than 400 physicists came to CERN to celebrate three decades of heavy-ion physics.

The microscopic structure of the QGP remains to be established, with the help of hard probes to provide the required resolving power. Here, jet quenching has already become a mundane phenomenon with which to study the content and dynamics of the QGP. In turn, the same studies also hint at the ability of the QGP to resolve the partonic shower. Quarkonia states, another hard probe, have also revealed rich dynamics. Their collision energy and transverse-momentum-dependent production can be understood in terms of two competing mechanisms: suppression due to resonance melting by colour screening and regeneration due to coalescence of free heavy-flavour quarks, both providing evidence for deconfinement. In addition, a flow signal has been measured for open and hidden charmed mesons, raising the question of whether charm quarks participate in the collective dynamics of the medium.

In general, the composition of the final hadronic state of the collision is quite well explained, assuming hadrons are formed in a thermalised state with a temperature that closely matches the temperature predicted for the QGP phase transition to the hadronic phase. Surprisingly, fragile objects such as light nuclei appear to be produced and to survive at temperatures several times larger than their binding energy. The possibility that nuclei were formed at the phase transition to hadrons, and not later via coalescence, would be an interesting complement to baryogenesis.

Strong future

As far as the next decades are concerned, in view of the achievements realised in the past years and the remaining open questions, it is a safe bet that heavy-ion physics will continue to be a vigorous field of research on both sides of the Atlantic. What are the relevant degrees of freedom of the QGP: perturbative partons, pseudo-particles, collective excitation of colour fields? Which dynamics drive the collision towards the formation of the QGP on timescales of a trillionth of a trillionth of a second, and in systems as small as a proton–proton collision? At which energy and which size does collectivity and statistical behaviour step in, and is chiral symmetry restored in the QGP?

These are some of the unanswered questions in the heavy-ion field. Existing and planned facilities that offer varying collision systems combined with ever more sophisticated detectors and strong collaborations between the theory and experiment communities are key to answering them. Based on what we are seeing currently, heavy-ion veteran Reinhard Stock commented that we could be about to enter a new paradigm with impact across high-energy physics. That would perhaps reveal QCD to be some sort of low-energy limit to a more fundamental theory.

Accelerating gender equality

We all need to join forces to assure CERN’s ongoing commitment to diversity.

When I started working at CERN in 1976, women were a relatively rare sight. The few women who did work here generally held administrative roles, many having started with the incongruous job title of “scanning girls”, regardless of the age at which they had been recruited. Back then it was quite normal to walk into a workshop and find pictures of naked females on the walls, and everyday sexism was common. I recall once being told that women couldn’t possibly do night shifts in the control room. The reason, a male colleague explained, was to avoid mysterious calls in the middle of the night: “What if there was a problem and she has to call a physicist? What would his wife think?”

Such attitudes were not just true of CERN, of course, and things have changed significantly since then. Even as recently as 1995, less than three per cent of CERN research and applied physicists were female, whereas today that number is around 18 per cent. Similar increases have been seen across engineering and technician roles, and CERN now has its first female Director-General.

It was in 1996 that CERN launched its equal opportunities (EO) programme. I was appointed as the first EO officer, and the following year an EO advisory panel was created. Many a meeting was taken up by educating male colleagues about the lasting effects of sexist behaviour through the personal experiences of their female counterparts. The EO programme adopted a four-pronged strategy focusing on recruitment, career development, work environment and harassment. On recruitment we took a firm stand against quotas, recommending instead thorough monitoring that would ensure reasonable proportions of qualified women were shortlisted for interview.

Equitable recruitment practices that we take for granted today were then the subject of much debate. The multicultural nature of CERN brought added complexity, as people’s notions of acceptable behaviour varied greatly. We were often accused of exaggerating the need for gender-neutral language or reproached for no longer having a sense of humour. Although some women colleagues found themselves in the uncomfortable situation of wishing to support EO initiatives while not wishing to risk the perception of tokenism or positive discrimination, many became vital allies in moving the EO agenda forward. Whether it was a question of work–life balance or simply accepting women in all job categories, a great deal of effort was invested to overcome resistance born of years of habit. It has only been over time that the proportion of female scientists at CERN has risen to match the numbers in society, as reflected by our world-wide user community.

CERN’s EO programme itself has also evolved into today’s diversity programme, which was launched in 2010 together with a newly created ombudsperson function and a formal harassment investigation panel. The CERN code of conduct was also produced at this time. The growing numbers of female colleagues in all fields at CERN is living proof that we have come a long way in the last two decades. But gender equality means more than just gender parity. While continuing our efforts to encourage female students to pursue science and to employ our colleagues through equitable recruitment practices, we should ask if we are doing everything possible to promote a mindset that enables all our colleagues to contribute as equals.

The last six years have seen approximately equal numbers of male and female visitors to the ombud office. However, when mapped against the corresponding staff-member populations, there are proportionally three-to-four times more women than men consulting the ombudsperson. A similar pattern is seen in other international organisations where women are a minority, and is mirrored by the proportionally higher number of females who participate in CERN’s “diversity in action” workshops. Although the issues raised by women are essentially the same as those faced by their male colleagues, a closer examination reveals examples of stereotyping and unconscious bias that suggests ours is not yet a completely level playing field.

Not only is it difficult for the majority to recognise the insidious barriers of organisational culture faced by minority groups, it is sometimes equally difficult for those within the minority to bring these aspects to light. If we are to ensure that our work environment is equally supportive to all, the experience of women needs to be shared with a wider audience including their male colleagues. We all need to join forces to assure CERN’s ongoing commitment to diversity.

Exotic hadrons bend the rules

The LHCb detector at CERN is one of several experiments where unusual hadrons have appeared in the past few years.
Image credit: C Marcelloni/CERN.

Fifty years have passed since Dick Dalitz presented his explicit constituent-quark model at the 1966 International Conference on High Energy Physics in Berkeley, US. Murray Gell Mann and George Zweig independently introduced the quark concept in 1964, and the idea had also been anticipated by André Petermann in a little-known paper received by Nuclear Physics in 1963. But it was Dalitz who developed the model and considered excitations of quarks by analogy with the behaviour of nucleons in atomic nuclei. His primary focus was on the spectroscopy of baryons, which were interpreted as bound states of three quarks. Dalitz realised that the restrictions enforced by the Pauli exclusion principle led to a distinct pattern of supermultiplets. Today, this simple model remains in excellent agreement with experiments, in particular for mesons that comprise a quark–antiquark pair.

Despite its success in matching empirical data, the theoretical underpinning of this non-relativistic model for light hadrons has always been unclear. One of the remarkable features of hadron spectroscopy is that, half a century after the invention of the constituent-quark model, the particle data tables are filled with states that fit with a non-relativistic spectrum almost to the exclusion of anything else. Quarks are but a few MeV in mass, and are therefore surely relativistic when confined within the 1 fm radius of a proton, yet the constituent-quark model treats them as if relativity plays no role.

In the case of mesons, which fit the quark model arguably even better than baryons, this incongruity is especially significant. When Dalitz spoke in 1966, it made sense to emphasise baryons because they outnumbered the known mesons at that time. Following the discovery of charm and heavy flavours in the late 1970s, however, the spectroscopy of mesons flourished and the correlations among a meson’s spin (J), parity (P) and charge conjugation (C) were also found to be in accord with those of a non-relativistic system.

Following Dalitz’s description of the baryon spectrum, Greenberg, Nambu, Lipkin and others noted that the model’s ad-hoc correlation of baryon spins with the constraints of the Pauli principle required some novel degree of freedom, which we call “colour”. The advent of quantum chromodynamics (QCD) in the 1970s provided the rationale for this concept, explaining the existence of quark–antiquark or three-quark combinations in terms of colour-singlet clusters. But QCD did not explain the non-relativistic pattern of states. Feynman, who in his final years devoted his attention to this issue, asserted: “The [non-relativistic] quark model is correct as it explains so much data. It is for theorists to explain why.” Today, physicists still await this explanation. Yet the empirical guide of the quark model is so well established that hadrons outside of this straitjacket are deemed “exotic”.

Fig. 1. Some of the ways in which QCD allows quarks to arrange into hadrons: (a) a red and blue quark triplet produces a magenta (anti-green) antitriplet and sextet; (b) three anticoloured diquark antitriplets; (c) exotic, colour-singlet states formed from quarks, antiquarks, diquarks and diantiquarks; and (d) other possible multiquark/gluon systems.
Image credit: *Front. Phys.* 10 101401.

Although the restriction to colour singlets within QCD explains the existence of qq and qqq hadrons, it raised the question of why the spectroscopy of QCD is so meagre. Colour singlets also allow combinations of pairs of quarks and antiquarks (“tetraquark” mesons), four quarks and an antiquark (“pentaquark” baryons), in addition to states comprised solely of gluons (“glueballs”). Furthermore, combinations called “hybrids” in which the gluonic fields entrapping the quark and antiquark are themselves excited are also theoretically possible within QCD (figure 1). Glueballs, tetraquarks and hybrid mesons, predicted in the late 1970s, can form correlations among a meson’s J, P and C quantum numbers that are forbidden by the non-relativistic model. Indeed, it is the lack of any empirical evidence for such exotic states in the meson spectrum that helped to establish the constituent-quark model in the first place. It is therefore ironic that searches for such states at modern experiments are now being used to establish the dynamic role of gluonic excitations in hadron spectroscopy.

Although QCD is well tested to high precision in the perturbative regime, where it is now an essential tool in the planning and interpretation of experiments, its implications for the strong-interaction limit are far less understood. Forty years after its discovery, and notwithstanding the advent of lattice QCD, hadron physics is still led by empirical data, from which clues to novel properties in the strong interactions may emerge. The search for exotic hadrons is an essential part of this strategy, and in recent years several new hadrons have been discovered that do not fit well within the traditional quark model.

Strange sightings

With hindsight, one of the first clues to the existence of quarks came in the 1950s from measurements of cosmic-ray interactions in the atmosphere, which revealed hadrons with unusual production and decay properties. These “strange” hadrons, we now know, contain one or more strange quarks or strange antiquarks, yet history has left us with a perverse convention whereby strange quarks are deemed to carry negative strangeness, and strange antiquarks are positive. Thus mesons can have one unit of strangeness, in either positive or negative amounts, while baryons can have strangeness –1, –2 or –3 (antibaryons, in turn, can have positive strangeness).

A baryon with positive strangeness (or an antibaryon with negative strangeness) is therefore classed as exotic. The minimal configuration for such a baryon would involve four quarks together with the strange antiquark, giving a total of five and the technically incorrect name of “pentaquark”. A claim to have found such a state – the θ(1540) – made headlines nearly two decades ago but is now widely disregarded. The scepticism was not that a pentaquark exists, since QCD can accommodate such a state, but that it appeared to be anomalously stable. More recently, the LHCb experiment at CERN’s Large Hadron Collider (LHC) reported decays of the Λ_bpentaquark-like baryon that revealed similar structures with a mass of around 4.4 GeV (CERN Courier September 2015 p5). These have normal strong-interaction lifetimes and have been interpreted as clusters of three quarks plus a charm–anticharm pair. Whether these are genuinely compact pentaquarks, or instead bound states of a charmed baryon and a meson or some other dynamic artefact, they do appear to qualify as “exotic” in that they do not fit easily into a traditional three-constituent picture.

Fig. 2. At 4.6 GeV, e⁺e^– annihilation produces DD₁ at threshold. The (cu̅)(c̅u) content rearranges into (cc̅)(uu̅) → J/ψππ.

There have also been interesting meson sightings at lepton colliders in recent decades. Electron–positron annihilation above energies of 4 GeV in numerous experiments reveals a series of peaks in the total cross-section that are consistent with radial excitations of the fundamental cc J/ψ meson: the ψ(2S), ψ(4040), ψ(4160) and ψ(4415), which are non-exotic and fit within the non-relativistic spectrum. Evidence for exotic mesons has come from data on specific final states, notably those containing a J/ψ with one or more pions, which have revealed several novel states. Historically, the first clue for an exotic charmonium meson of this type above a mass of 4 GeV came around a decade ago from the BaBar experiment at SLAC in the US. Analysing the process e⁺e^– → J/ψππ, researchers there found a clear resonant-like structure dubbed Y(4260), which has no place in the qq spectrum because its mass lies between the ψ(4160) and ψ(4415) cc states. More remarkably, this state decays into charmonium and pions with a standard strong-interaction width of the order of 100 MeV rather than 100 keV, which is more typical for such a channel.

The clue to the nature of this meson appears to be that the mass of the Y meson (4260 MeV) is near the threshold for the production of DD₁ – the combination of pseudoscalar (D) and axial (D₁) charmed mesons (figure 2). This is the first channel in e⁺e^– annihilation where charmed meson pairs can be produced with no orbital angular momentum (i.e. via S-wave processes). Thus at threshold there is no angular-momentum barrier against a DD₁ pair being created effectively at rest, and rearranging their constituents into the form of J/ψ and light flavours (the latter then seeding pions). Thus the structure could simply be a threshold effect rather than a true resonance, or an exotic “molecule” made of D and D₁ charmed mesons.

The decay of the Y(4260) into J/ψππ reveals a manifestly exotic structure. The J/ψπ^± channel is electrically charged with a pronounced peak called Z(3900), as reported by both the BESIII experiment in China and Belle in Japan in 2013. Another sharp peak observed by BESIII – the Z(4020) – appears in the flavour-exotic channel containing a pion and a charmonium meson. Since it can carry electric charge, this state must contain ud (or du) in addition to its cc content, and therefore cannot be explained as a bound state of a single quark and antiquark. In principle, these states should be accessible in decays of B mesons, but there is no sign of them so far.

Nonetheless, B decays are a source of further exotic structures. For example, the invariant-mass spectrum of B → K π^±ψ(2S) contains a structure called the Z(4430) observed by Belle and LHCb in the ψ(2S)π invariant-mass spectrum, which contains both hidden charm and isospin and hence must contain (at least) two quarks and two antiquarks. These features first need to be established as genuine and not artefacts associated with some specific production process. Their appearance and decay in other channels would help in this regard, while the observation of analogous signals for other combinations of flavour may also signpost the underlying dynamics. If real, these states are the product of charmonium cc– and light-quark basis states (a summary of charmonium candidates can be seen in figure 3).

Proceed with caution

It is clear that peaks are being found that cannot be interpreted as qqq or qq clusters. But one should not leap to the conclusion that we have discovered some fundamentally novel state built from, say, diquarks and antidiquarks or, for baryons, a pentaquark. A qq qq “tetraquark”, for example, looks less exotic when trivially rewritten as qqqq, which is suggestive of two bound conventional mesons. Indeed, these could be the two mesons in the invariant mass of which the peak was seen. Unless the peak is seen in different channels, and ideally in different production mechanisms, one should be cautious.

Fig. 3. The spectrum of charmonium and charmonium-like candidates.
Image credit: *Front. Phys.* **10</b 101401.**

For example, when three or more hadrons are produced in a single decay it is common to discover peaks in invariant-mass spectra just above the two-body thresholds. These are not resonances, although papers on the arXiv preprint server are full of models built on the assumption that they are. Instead, the peaks likely arise due to competition between two effects. First, phase space opens up for the production of the two-body channel, but as the invariant mass increases, the chance of this exclusive two-body mode dies off because the probability for the wavefunctions of the two hadrons to overlap decreases. Any peak seen within a few hundred MeV of such a threshold is most likely to be the accidental result of this phenomenon. Such “cusps” have been proposed as explanations of several recent exotic candidates, such as the Z(3900) and Z(10610) spotted at BESIII and Belle, among others. Whether the tetraquark candidates X(4274), X(4500) and X(4700) recently observed at LHCb, in addition to the X(4140) found by the CDF experiment at Fermilab in 2009, herald the birth of a new QCD spectroscopy or are examples of more mundane dynamics such as cusps, is also the subject of considerable debate. In short, if a peak occurs above a two-body threshold in a single channel: beware.

Enter the deuson

More interesting for exotic-hadron studies are peaks that lie just below threshold. Such states are well known in the baryon sector, the deuteron being a good example. The nuclear force driven by pion exchange that binds neutrons and protons inside the atomic nucleus should also occur between pairs of mesons, at least for those that are stable on the timescale of the strong interaction. Thus on purely phenomenological and conservative grounds, we should anticipate meson molecules (or, by analogy with the deuteron, “deusons”), which would take us beyond the simple quark-model spectroscopy. The Y(4260) could be an example of such a state, since both DD₁ and D*D₀ S-wave thresholds lie in this region and pion exchange may play a role in linking the two channels (figure 4). If these states are indeed deusons then there should also be partners with isospin. Establishing whether these structures are singletons or have siblings is therefore another important step in identifying their dynamical origins.

The first sign of deusons may be expected in the axial-vector channel formed from a pseudoscalar and vector charmed (or bottom) meson. This is because pion exchange can occur between a pair of vector mesons or as an exchange force between a pseudoscalar-vector combination, but not within a state of two pseudoscalars as this would violate parity conservation. The enigmatic state X(3872), which was first observed in B decays by Belle in 2003 and occurs at the D₀ D*⁰ + cc threshold, has long been a prime candidate for a deuson. If so, there should be analogous states in the BB* as well as charm-bottom flavour mixtures and perhaps siblings with two units of charm or bottom. Whether these states have charged partners is one of many model-dependent details. That some of these states should occur seems unavoidable, however, and if doubly charmed states exist they should be produced at the LHC.

Fig. 4. “Deusons” arise from pion exchange. Pion exchange is the source of an attractive force between D and D̅* or DD̅ and D* charmed mesons. This also occurs between D and D* analogous to the case of the deuteron.

Whereas for baryons the attractive forces arise in the exchange or “t channel”, for pairs of mesons there can also be contributions due to qq annihilation in the direct s-channel. In QCD this can also mask the search for glueballs: for example, the scalar glueball of lattice QCD predicted at a mass of around 1.5 GeV mixes with the nonet of scalar qq states in this very region. The pattern of these scalars empirically is consistent with such dynamics.

Scalar mesons are interesting not least because the theoretical interest in multiquark or molecular states originated in such particles 40 years ago, after Robert Jaffe noticed that the chromo-magnetic QCD forces are powerfully attractive in the nonet of light-flavoured scalar mesons. Intriguingly, this idea has remained consistent with the observed nonet of scalars below 1 GeV ever since. The main question that remains unresolved is to what extent these states are dominantly formed from coloured diquarks and their antidiquarks, or are better described as molecular states formed from colour-singlet π and K mesons.

LHCb in particular has shown that it is possible to identify light scalars among the decay debris of heavy-flavoured mesons, offering a new opportunity to investigate their nature and dynamics. Indeed, the kinematic reach of the LHC potentially enables a multitude of information to be obtained about heavy-flavoured mesons in both conventional and exotic combinations. We might therefore hope that information about exotic mesons will be extended into different flavour sectors to help identify the source of the binding.

Remarkably robust

In general, the simple qq picture of mesons appears to remain remarkably robust so long as there are no nearby prominent channels for pair production of hadrons in the S-wave channel. “Exotic” mesons and baryons seem to correlate with some S-wave channel sharing quantum numbers with a nominal qq state and causing the appearance of a state near the corresponding S-wave threshold. In some of these cases, but not all, the familiar forces of conventional nuclear physics play a role, and the multi-particle events at the LHC have the kinematic reach to include all combinations of non-strange, strange, charm and bottom mesons. How many of these can in practice be identified is the challenge, but identifying the dynamics of states “beyond qq” may depend on it.

In conclusion, these exotic states need to be studied in different production mechanisms and in a variety of decay channels. A genuine resonant state should appear in different modes, whereas a structure that appears in a single production mechanism and a unique decay channel is suggestive of some dynamical feature that is not truly resonant. While interesting in its own right, such a state is not “exotic” in the sense of hadron spectroscopy.

As for truly exotic states, there are different levels of exoticity. For flavoured hadrons: the least exotic are meson analogues of nuclei – “deusons” driven by pion exchange between pairs of mesons. Next are “hybrids”: states anticipated in QCD where the gluonic degrees of freedom are excited in the presence of quarks and/or antiquarks. Finally, the most exotic of all would be colour-singlet combinations of compact diquarks, which are allowed in principle by QCD and would lead to a rich spectroscopy. At present their status is like the search for extraterrestrial life: while one feels that in the richness of nature such entities must exist, they seem reluctant to reveal themselves.

Position-Sensitive Gaseous Photomultipliers: Research and Applications

By Tom Francke and Vladimir Peskov
IGI Global

Gaseous photomultipliers are gas-filled devices capable of detecting single photons (in the visible and UV spectrum) with a high position resolution. They are used in various research settings, in particular high-energy physics, and are among several types of contemporary single-photon detectors. This book provides a detailed comparison between photosensitive detectors based on different technologies, highlighting their advantages and disadvantages of them for diverse applications.

After describing the main principles underlying the conversion of photons to photoelectrons and the electron avalanche multiplication effect, the characteristics (and requirements) of position-sensitive gaseous photomultipliers are discussed. A long section of the book is then dedicated to describing and analysing the development of these detectors, which evolved from photomultipliers filled with photosensitive vapours to devices using liquid and then solid photocathodes. UV-sensitive photodetectors based on caesium iodide and caesium telluride, which are mainly used as Cherenkov-ring imaging detectors and are currently employed in the ALICE and COMPASS experiments at CERN, are presented in a dedicated chapter. The latest generation of gaseous photomultipliers, sensitive up to the visible region, are also discussed, as are alternative position-sensitive detectors.

The authors then focus on the Cherenkov light effect, its discovery and the way it has been used to identify particles. The introduction of ring imaging Cherenkov (RICH) detectors was a breakthrough and led to the application of these devices in various experiments, including the Cosmic AntiParticle Ring Imaging Cherenkov Experiment (CAPRICE) and the former CERN experiment Charge Parity violation at Low Energy Antiproton Ring (CP LEAR).

The latest generation of RICH detectors and applications of gaseous photomultipliers beyond RICH detectors are also discussed, completing the overview of the subject.

17 Big Bets for a Better World

By S Tackmann, K Kampmann and H Skovby (eds)
Forlaget Historika/Gad Publishers

This book, which includes a contribution by CERN Director-General Fabiola Gianotti, presents 17 radical and game-changing ideas to help reach the 2030 Global Goals for Sustainable Development identified by the United Nations General Assembly.

Renowned and influential leaders propose innovative solutions for 17 “big bets” that the human race must face in the coming years. These experts in the environment, finance, food security, education and other relevant disciplines share their vision of the future and suggest new paths towards sustainability.

In the book, Gianotti replies to this call and shares her ideas about the importance of basic science and research in science, technology, engineering and maths (STEM) to underpin innovation, sustainable development and the improvement of global living conditions. After giving examples of breakthrough innovations in technology and medicine that came about from the pursuit of knowledge for its own sake, Gianotti contends that we need science and scientifically aware citizens to be able to tackle pressing issues, including drastic reduction of poverty and hunger, and the provision of clean and affordable energy. Finally, she proposes a plan to secure STEM education and funding for basic scientific research.

Published as part of the broader Big Bet Initiative to engage stakeholders around new and innovative ideas for global development, this book provides fresh points of view and credible solutions. It would appeal to readers who are interested in innovation and sustainability, as well as in the role of science in such a framework.

Probability for Physicists

By Simon Širca
Springer

Also available at the CERN bookshop

This book aims to deliver a concise, practical and intuitive introduction to probability and statistics for undergraduate and graduate students of physics and other natural sciences. The author attempts to provide a textbook in which mathematical complexity is reduced to a minimum, yet without sacrificing precision and clarity. To increase the appeal of the book for students, classic dice-throwing and coin-tossing examples are replaced or accompanied by real physics problems, all of which come with full solutions.

In the first part (chapters 1–6), the basics of probability and distributions are discussed. A second block of chapters is dedicated to statistics, specifically the determination of distribution parameters based on samples. More advanced topics follow, including Markov processes, the Monte Carlo method, stochastic population modelling, entropy and information.

The author also chooses to cover some subjects that, according to him, are disappearing from modern statistics courses. These include extreme-value distributions, the maximum-likelihood method and linear regressions using singular-value decomposition. A set of appendices concludes the volume.

Introduction to Quantum Physics and Information Processing

By Radhika Vathsan
CRC Press

An introduction to the novel and developing field of quantum information, this book aims to provide undergraduate and beginning graduate students with all of the basic concepts needed to understand more advanced books and current research publications in the field. No background in quantum physics is required because its essential principles are provided in the first part of the book.

After an introduction to the methods and notation of quantum mechanics, the authors explain a typical two-state system and how it is used to describe quantum information. The broader theoretical framework is also set out, starting with the rules of quantum mechanics and the language of algebra.

The book proceeds by showing how quantum properties are exploited to develop algorithms that prove more efficient in solving specific problems than their classical counterparts. Quantum computation, information content in qubits, cryptographic applications of quantum-information processing and quantum-error correction are some of the key topics covered in this book.

In addition to the many examples developed in the text, exercises are provided at the end of each chapter. References to more advanced material are also included.

Modern Atomic Physics

By Vasant Natarajan
CRC Press

This book collates information from various literature to provide students with a unified guide to contemporary developments in atomic physics. In just 400 pages it largely succeeds in achieving this aim.

The author is a professor of physics at the Indian Institute of Science in Bangalore. His research focuses on laser cooling and trapping of atoms, quantum optics, optical tweezers, quantum computation in ion traps, and tests of time-reversal symmetry using laser-cooled atoms. He received a PhD from the Massachusetts Institute of Technology under the supervision of David Pritchard, a leader in modern atomic physics and a mentor of two researchers – Eric Cornell and Wolfgang Ketterle – who went on to become Nobel laureates.

The book addresses the basis of atomic physics and state-of-the-art topics. It explains material clearly, although the arrangement of information is quite different to classical atomic-physics textbooks. This is clearly motivated by the importance of certain topics in modern quantum-optics theory and experiments. The physics content is often accompanied by the history behind concepts and by explanations of why things are named the way they are. Historical notes and personal anecdotes give the book a very appealing flair.

Chapter one covers different measurement systems and their merits, followed by universal units and fundamental constants, with a detailed explanation of which constants are truly fundamental. The next chapter is devoted to preliminary materials, starting with the harmonic oscillator and moving to concepts – namely coherent and squeezed states – that are important in quantum optics but not explicitly covered in some other books in the field. The chapter ends with a section on radiation, even including a description of the Casimir effect.

Chapter three is called Atoms. Alongside classical content such as energy levels of one-electron atoms, interactions with magnetic and electric fields, and atoms in oscillating fields, this chapter explains dressed atoms and also, unfortunately only briefly, includes a description of the permanent atomic electric dipole moment (EDM).

The following chapter is devoted to nuclear effects, the isotope shift and hyperfine structure. At this point it would have been nice to see some mention of the flourishing field of laser spectroscopy of radioactive nuclei, which exploits the two above effects to investigate the ground-state properties of nuclei far from the valley of stability.

Chapter five is about resonance, which is often scattered around in other books about atomic physics. Here, interestingly, nuclear magnetic resonance (NMR) plays a central role, and the chapter connects this topic very naturally to atomic physics. The chapter closes with a description of the density matrix formalism. After this comes a chapter devoted to interactions, including the electric dipole approximation, selection rules, transition rates and spontaneous emission. The last section is concerned with differences in saturation intensities by broadband and monochromatic radiation.

Multiphoton interactions are the topic of chapter seven, which is clearly motivated by their importance in modern quantum-optics laboratories. Two-photon absorption and de-excitation, Raman processes and the dressed atom description are all explained. Another crucial concept in modern quantum optics is coherence. Thus it is included as a full chapter, which includes coherence in a single atom and in ensembles of atoms, as well as coherent control in multilevel atoms. Spin echo appears as well, showing again how close the topics presented in the book are to NMR.

Chapter nine is devoted to lineshapes, which is clearly a subject relevant for modern atomic spectroscopists. Spectroscopy is the next chapter, which starts with alkali atoms – used extensively in laser cooling and Bose–Einstein condensates. The rest of the material is aimed at experimentalists. Uniquely for such a book, it includes a description of the key experimental tools, followed by Doppler-free techniques and nonlinear magneto-optic rotation.

The last chapter covers cooling and trapping, with so many relevant concepts already presented in the preceding chapters. The content includes different cooling approaches, principles of atom and ion traps, the cryptic and equally common Zeeman slower, and even more intriguing optical tweezers.

Each chapter ends with a problems section, in which the problems are often relevant to a real quantum-optics lab, for example concerning quantum defects, RF-induced magnetic transitions, Raman scattering cross-sections, quantum beats or the Voigt line profile. The problems are worked out in detail, allowing readers to follow how to arrive at the solution.

The appendices cover the standards and the frequency comb, which is one of the ingenious devices to come from the laboratory of Nobel laureate Theodor Hänsch and which can be now found in an ever-growing number of laser-spectroscopy and quantum-optics labs. Two other appendices are very different: they have a philosophical flair and deal with the nature of the photon and with Einstein as nature’s detective.

The presented theoretical basis leads to state-of-the art experiments, especially related to ion and atom cooling and to Bose–Einstein condensates. The selection of topics is thus clearly tailored for experimentalists working in a quantum optics lab. One small criticism is that it would be good to read more about the EDM experiments and laser spectroscopy of radioactive ions, which are currently two very active fields. Readers interested in different classic subjects, like atomic collisions, should turn to other books such as Bransden and Joachain’s Physics of Atoms and Molecules.

The level of the book makes it suitable for undergraduate level, but also for new graduate students. It can also serve as a quick reference for researchers, especially concerning the topics of general interest: metrology, what is a photon or how a frequency comb works, and how to achieve a Bose–Einstein condensate. Overall, the book is a very good guide to the topics relevant in modern atomic physics and its style makes it quite unique and personal.

BASE boosts precision of antiproton magnetic moment

BASE spokesperson Stefan Ulmer standing next to the experimentʼs trapping apparatus at CERNʼs Antiproton Decelerator facility.
Image credit: M Brice/CERN.

The Baryon Antibaryon Symmetry Experiment (BASE) collaboration at CERN has made the most precise direct measurement of the magnetic moment of the antiproton, allowing a fundamental comparison between matter and antimatter.

The BASE measurement shows that the magnetic g-factors (which relate the magnetic moment of a particle to the nuclear magneton) of the proton and antiproton are identical within the experimental uncertainty of 0.8 parts per million: 2.7928465(23) for the antiproton, compared to 2.792847350(9) for the proton. The result improves the precision of the previous best measurement by the ATRAP collaboration in 2013, also at CERN, by a factor of six.

Comparisons of the magnetic moments of the proton and antiproton at this level of precision provide a powerful test of CPT invariance. Were even slight differences to be found, it would point to physics beyond the Standard Model. It could imply, for example, the existence of a new vector boson that couples only to antimatter, which could have a direct effect on the lifetime of baryons. Such effects more generally could also shed light on the mystery of the missing antimatter observed on cosmological scales.

Six measurements of the antiproton g-factor were made (average shown in red, with 95% confidence levels in green), to be compared with the accepted value for the proton (blue).

BASE uses antiprotons from CERN’s Antiproton Decelerator (AD), which serves several other experiments making rapid progress in precision antimatter measurements (CERN Courier December 2016 p16). By trapping the particles in electromagnetic containers called Penning traps and cooling them to temperatures below 1 K, the BASE team can measure the cyclotron and Larmor frequencies of single trapped antiprotons. By measuring the ratio of these two frequencies the magnetic moment of the antiproton is obtained in units of the nuclear magneton.

Similar techniques have been successfully applied in the past to electrons and positrons. However, antiprotons present a much bigger challenge because their magnetic moments are considerably weaker, requiring BASE to design Penning traps with about 2000 times higher sensitivity with respect to magnetic moments. BASE now plans to measure the antiproton magnetic moment using a new double-Penning trap technique, which should enable a precision at the level of a few parts per billion in the future.

SESAME sees first beam

SESAME was established under the auspices of UNESCO before becoming a fully independent intergovernmental organisation in its own right in 2004.
Image credit: N Caraban/CERN.

Late in the evening of 12 January, a beam of electrons circulated for the first time in the SESAME light source in Jordan. Following the first single turn, the next steps will be to achieve multi-turns, store and then accelerate a beam. This is an important milestone towards producing intense beams of synchrotron light at the pioneering facility, which is the first light-source laboratory in the Middle East.

SESAME, which stands for Synchrotron-light for Experimental Science and Applications in the Middle East, will eventually operate several beamlines at different wavelengths for wide-ranging studies of the properties of matter. Experiments there will enable SESAME users to undertake research in fields ranging from medicine and biology, through materials science, physics and chemistry to healthcare, the environment, agriculture and archaeology.

CERN has a long-standing involvement with SESAME, notably through the European Commission-funded CESSAMag project, coordinated by CERN. This project provided the magnet system for SESAME’s 42 m-diameter main ring and brought CERN’s expertise in accelerator technology to the facility in addition to training, knowledge and technology transfer.

The January milestone follows a series of key events, beginning with the establishment of a Middle East Scientific Collaboration group in the mid-1990s. This was followed by the donation of the BESSY1 accelerator by the BESSY laboratory in Berlin. Refurbished and upgraded components of BESSY1 now serve as the injector for the completely new SESAME main ring, which is a competitive third-generation light source built by SESAME with support from the SESAME members, as well as the European Commission and CERN through CESSAMag, and Italy.

There is still a lot of work to be done before experiments can get underway. Beams have to be accelerated to SESAME’s operating energy of 2.5 GeV. Then the synchrotron light emitted as the beams circulate has to be channelled along SESAME’s two initial beamlines and optimised for the experiments that will take place there. This process is likely to take around six months, leading to first experiments in the summer of 2017.

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