A team in Germany has made the most precise measurement to date of the mass of a single proton, achieving a precision of 32 parts-per-trillion (ppt). The result not only improves on the precision of the accepted CODATA value by a factor of three but also disagrees with its central value at a level of 3.3 standard deviations, potentially shedding light on other mysteries surrounding the proton.
The proton mass is a fundamental parameter in atomic and particle physics, influencing atomic spectra and allowing tests of ultra-precise QED calculations. In particular, a detailed comparison between the masses of the proton and the antiproton offers a stringent test of the fundamental CPT invariance of the Standard Model.
The team at the Max Planck Institute for Nuclear Physics (MPIK) in Heidelberg and collaborators from RIKEN in Japan used a bespoke electromagnetic Penning trap cooled to 4 K to store individual protons and highly charged carbon ions. By measuring the characteristic cyclotron frequencies of the trapped particles using ultra-sensitive image-current detectors, the mass of the proton in natural units follows directly.
For the new measurement, the team stored one proton and one highly charged carbon ion in separate compartments of the apparatus and then transported them alternately into the central measurement compartment. Purpose-built electronics allowed the proton to be interrogated under identical conditions as the carbon ion, despite its 12-fold lower mass and six-fold smaller charge, and the ratio of the two measured values results directly in the proton mass in atomic units: 1.007276466583±15 (stat)±29 (syst).
The sensitive single-particle detectors were partly developed by the RIKEN group, drawing on experience gained with similar traps for antimatter research at CERN’s Antiproton Decelerator (AD) – specifically the BASE experiment. “The group around Sven Sturm and Klaus Blaum from MPIK Heidelberg, which did the measurement, has great expertise with carbon, whereas the BASE group contributed proton expertise based on 12 years dealing with protons and antiprotons,” explains RIKEN group leader and BASE spokesperson Stefan Ulmer. “We shared knowledge such as know-how on ultra-sensitive proton detectors and the ‘fast-shuttling’ method developed by BASE to perform the proton–antiproton charge-to-mass ratio measurement.”
Interestingly, the new value of the proton mass is significantly smaller than the accepted one and could therefore be linked to well-known discrepancies in the mass of the heaviest hydrogen isotope, tritium. “Our result contributes to solving this puzzle, since it corrects the proton’s mass in the proper direction,” says Blaum. The result also improves the proton–electron mass ratio by a factor two, achieving a relative precision of 43 ppt, where the uncertainty arises nearly equally from the proton and the electron mass.
Although carefully conducted cross-check measurements confirmed a series of previously published values of the proton mass and showed that no unexpected systematic effects were imposed by the new method, such a striking departure from the accepted value will likely challenge other teams to revisit the proton mass. The discrepancy has already inspired the MPIK-RIKEN team to further improve the precision of its measurement, for instance by storing a third ion in the trap and measuring it simultaneously to eliminate uncertainties originating from magnetic-field fluctuations, which are the main source of the systematic error using the new technique.
“It is also planned to tune the magnetic field to even higher homogeneity, which will reduce additional sources of systematic error,” explains BASE member Andreas Mooser. “The methods that will be pioneered in the next step of this experiment will have immediate positive feedback to future BASE measurements, for example to improve the precision in the antiproton-to-proton charge-to-mass ratio.”
The KEDR collaboration has used the VEPP-4M electron–positron collider at the Budker Institute in Russia to make the most precise measurement of the quantity “R” in the low-energy range. R is defined as the ratio of the radiatively corrected total hadronic cross-section in electron–positron annihilation to the Born cross-section of muon pair production. The dependence of R on the centre-of-mass energy is critical for determining the running strong coupling constant and heavy-quark masses, the anomalous magnetic moment of the muon and the value of the electromagnetic fine structure constant at the Z peak. A substantial contribution to the uncertainties on these quantities comes from the energy region below charm threshold, where KEDR measurements were made.
The KEDR team performed a precise measurement of R at 20 points: in the energy ranges 1.84–3.05 and 3.12–3.72 GeV the weighted averages of R are 2.225±0.051 and 2.189±0.047, respectively, in good agreement with perturbative QCD. At present, it is the most accurate measurement of R in this energy range, to which more than 10 experiments have contributed. It involved a challenging analysis in which the hadronisation of light quarks at low energies was modelled by tuning distributions of parameters essential for the event selection in the various generator codes.
The collaboration now plans to measure R in the range 5–7 GeV, where the last similar experiment was carried out more than a quarter of a century ago.
On 14 July, the Square Kilometre Array (SKA) organisation signed an agreement with CERN to formalize their collaboration in the area of extreme-scale computing. The agreement will address the challenges of “exascale” computing and data storage, with the SKA and the Large Hadron Collider (LHC) to generate an overwhelming volume of data in the coming years.
When completed, SKA will be the world’s largest radio telescope with a total collecting area of more than 1 km2 using thousands of high-frequency dishes and many more low- and mid-frequency aperture array telescopes distributed across Africa, Australia and the UK. Phase 1 of the project, representing approximately 10% of the final array, will generate around 300 PB of data every year – 50% more than has been collected by the LHC experiments in the last seven years. As is the case at CERN, SKA data will be analysed by scientific collaborations distributed across the planet. The acquisition, storage, management, distribution and analysis of such volumes of scientific data is a major technological challenge.
“Both CERN and SKA are and will be pushing the limits of what is possible technologically, and by working together and with industry, we are ensuring that we are ready to make the most of this upcoming data and computing surge,”says SKA director-general Philip Diamond.
CERN and SKA have agreed to hold regular meetings to discuss the strategic direction of their collaborations, and develop demonstrator projects or prototypes to investigate concepts for managing and analysing exascale data sets in a globally distributed environment. “The LHC computing demands are tackled by the Worldwide LHC computing grid, which employs more than half a million computing cores around the globe interconnected by a powerful network,” says CERN’s director of research and computing Eckhard Elsen. “As our demands increase with the planned intensity upgrade of the LHC, we want to expand this concept by using common ideas and infrastructure into a scientific cloud. SKA will be an ideal partner in this endeavour.”
On 19 June, the Nuclear Physics European Collaboration Committee (NuPECC) released its long-range plan for nuclear research in Europe, following 20 months of work involving extensive discussions with the scientific community. The previous long-range plan was issued in 2010.
Today, nuclear physics is a broad field covering nuclear matter in all its forms and exploring their possible applications. It encompasses the origin and evolution of the universe, such as the quark deconfinement at the Big Bang, the physics of neutron stars and nucleosynthesis, and addresses open questions in nuclear structure, among other topics.
A number of research programmes are at the interface between nuclear and particle physics, where CERN plays an important role. This can be seen clearly in the six chapters of the NuPECC report devoted to: hadron physics; properties of strongly interacting matter; nuclear structure and dynamics; nuclear astrophysics; symmetries and fundamental interactions; and applications. For symmetries and fundamental interactions, particular emphasis is given to experiments (such as those with antihydrogen) where nuclei are sensitive to physics beyond the Standard Model. Concerning broader societal benefits, CERN’s MEDICIS facility, based on expertise from ISOLDE, is of particular interest (CERN Courier October 2016 p28).
The Facility for Antiproton and Ion Research (FAIR), a major investment that just entered construction in Germany (CERN Courier July/August 2017 p41), also has high prominence. Three other prominent recommendations have particular relevance to CERN: support for world-leading isotope-separation facilities (ISOLDE at CERN together with SPIRAL2 in France and SPES in Italy); support for existing and emerging facilities (including the new ELENA synchrotron at CERN’s Antiproton Decelerator); and support for the LHC’s heavy-ion programme, in particular ALICE. Emerging facilities – the extreme-light source ELI-NP in Bucharest, and NICA and a superheavy element factory in Dubna – are also highlighted.
Research in nuclear physics involves several facilities of different sizes that produce complementary scientific results, and in Europe they are well co-ordinated. International collaborations beyond Europe, mainly in the US (JLAB in particular) and in Asia (Japan in particular), add much value to this field.
NuPECC’s latest report is expected to help co-ordinate and guide this rich field of physics for the next 6–7 years. Its recommendations were extensively discussed and can be read in full at nupecc.org/pub/lrp2017.pdf.
Five years ago, the ATLAS and CMS collaborations at the LHC announced the discovery of a new particle with properties consistent with those of a Standard Model Higgs boson. Since then, based on proton–proton collision data collected at energies of 7 and 8 TeV during LHC Run 1 and at 13 TeV during Run 2, many measurements have confirmed this hypothesis. Several decay modes of the Higgs boson have been observed, but the dominant decay into pairs of b quarks, which is expected to contribute at a level of 58%, had up to now escaped detection – largely due to the difficulty in observing this decay mode at a hadron collider.
On 6 July, at the European Physical Society conference in Venice, the ATLAS collaboration announced that they had found evidence for H → bb, representing an immense analysis achievement. By far the largest source of Higgs bosons is their production via gluon fusion, gg → H → bb, but this is overwhelmed by the huge background of bb events, which are produced at a rate 10 million times higher. The associated production of a Higgs with a W or Z vector boson (jointly denoted V) offers the most sensitive alternative, despite having a production rate roughly 20 times lower than H→ bb, because the vector bosons are detected via their decay to leptons and therefore allow efficient triggering and background rejection. Nevertheless, the signal remains orders of magnitude smaller than the backgrounds, which arise from the associated production of vector bosons with jets and from top-quark production.
To find evidence for the H → bb decay in the VH production channel, it is necessary to use detailed information on the properties of the decay products. The jets arising from b quarks contain b hadrons, whose long lifetime can be used in sophisticated b-tagging algorithms to discriminate them from jets originating from the fragmentation of gluons or other quark species. These algorithms have benefitted significantly from the new innermost pixel layer installed in ATLAS before Run 2. The kinematic properties of the decay products can also be used to enhance the signal-over-background ratio. The property with the most discriminatory power is the invariant mass of the two-b-jet system, which for the signal accumulates at the mass of the Higgs boson (see figure). To increase the sensitivity of the analysis, this mass is used together with several other kinematic variables as input to a multivariate analysis.
Based on data collected during the first two years of LHC Run 2 in 2015 and 2016, evidence for the H → bb decay is obtained at the level of 3.5σ, slightly increased to 3.6σ after combination with the Run 1 results (compared to an expected significance of 4σ). The measured signal yield is in agreement with the Standard Model expectation, within an uncertainty of 30%. The associated VZ production, with Z → bb, allows for a powerful cross-check of the analysis, as the final states are very similar except for the location of the two-b-jet mass peak (see figure); VZ production is observed with a significance of 5.8σ in the Run 2 data, in agreement with the Standard Model prediction.
This analysis opens a way to study about 90% of the Higgs boson decays expected in the Standard Model, which is a sharp increase from the approximately 30% observed previously. With much more data expected by the end of Run 2 in 2018, a definitive 5σ observation of the H → bb decay may be in sight, with the increased precision providing new opportunities to challenge the Standard Model.
The quest to find dark matter (DM) has inspired new searches at CMS, specifically looking for interactions between DM and quarks mediated by particles of previously unexplored mass and width. If the DM mediator is a leptophobic vector resonance coupling only to quarks with a universal coupling g′q, for instance, its decay also produces a dijet resonance (see bottom figure, left) and the value of g′q determines the width of the mediator.
CMS has traditionally searched for peaks from narrow resonances on the steeply falling dijet invariant mass spectrum predicted by QCD. This search has been updated with the full 2016 data set and limits set on a DM mediator, constraining g′q for resonances with a mass between 0.6 and 3.7 TeV and width less than 10% of the resonance mass. Two additional dijet searches have now been released: a boosted-dijet search sensitive to lower mediator masses, and an angular-distribution search sensitive to larger couplings and widths.
The first search gets round the limitations of the narrow-resonance search, which only applies above a minimum mass that satisfies the dijet trigger requirements, by requiring resonance production in association with a jet (bottom figure, middle). In such events the resonance is highly boosted and by analysing the jet substructure the QCD background can be highly suppressed, making the search sensitive in a lower mass range. The mass spectrum of the single jet was used to search for resonances over a mass range of 50–300 GeV, and the corresponding constraints on g′q and the mediator width from boosted dijets explore the lowest mediator masses.
For large couplings and widths, the sensitivity of searches for dijet resonance peaks is strongly reduced. However, a search for a very wide resonance can be performed by studying dijet angular distributions such as the scattering angle between the incoming and outgoing partons. These distributions differ significantly, depending on whether a new particle is produced in the s-channel or from the QCD dijet background, which is dominated by t-channel production (bottom figure, right). Being sensitive to both large-width resonances and non-resonant signatures, this search also sets lower limits on the scale of contact interactions that may arise from quark compositeness in the range 6–22 TeV, as well as signatures of large extra dimensions and quantum black holes. The same search, when interpreted in the context of a vector mediator coupling to DM, excludes values of g′q greater than 0.6, corresponding to widths higher than 20% of the resonance mass, and extending to mediator masses as high as 5 TeV.
Using these three complementary techniques, CMS has now explored a large range in mass, coupling and width, extending the scope of searches for DM mediators. The expected volume of data from the LHC in upcoming years will allow CMS to extend this reach even further, with the study of three-jet topologies allowing the uncovered mass range of 300–600 GeV to be explored.
In 2016 the LHC collided protons and lead nuclei for the first time at a centre-of-mass energy of 8.16 TeV per nucleon–nucleon pair. In lead–lead collisions, the formation of the quark–gluon plasma (QGP), a deconfined system where quarks and gluons can move freely, is a subject of intense studies at the LHC. By contrast, proton–lead collisions represent the best available environment to quantify nuclear effects that are not related to the QGP.
Our knowledge of the partonic content of nuclei suffers from large uncertainties, particularly at low momentum where large modifications of the partonic flux with respect to the free nucleon are expected. The particular design of the LHCb experiment, with its fully instrumented forward acceptance, offers a unique opportunity to access production processes in which one parton carries a momentum fraction of the incoming nucleon inside the lead nucleus of approximately 10–5–10–4 (covering the proton fragmentation region) and 10–3–10–1 for the lead fragmentation region.
The LHCb collaboration recently submitted the first paper at the LHC based on results obtained with the 2016 proton–lead data sample. This measurement of J/ψ production profits from an integrated luminosity about 20 times larger than the proton–lead sample collected by LHCb during the 2013 run. The nuclear modification factor RpPb as a function of transverse momentum is shown in the figure: J/ψ mesons produced in the interaction point (prompt) are found to be suppressed by about a factor two at low transverse momentum, while RpPb approaches unity at higher transverse momenta. Those arising from the decays of long-lived beauty hadrons (non-prompt) follow a similar pattern. This is the most precise measurement to date of inclusive beauty production in nuclear collisions.
The results can be compared with perturbative QCD calculations based on collinear nuclear parton distribution functions (nPDFs) or with calculations within the colour-glass condensate (CGC) framework, which takes into account gluon saturation. The large uncertainties on the nPDFs compared to the data show the importance of new experimental data to better constrain them, while the CGC-based calculation reproduces the observed dependence accurately.
The large 2016 data set will allow for a precise study of heavy-flavour production with different hadron species, and also of cleaner electromagnetic/electroweak probes. These measurements will test which frameworks adequately describe the modification of the partonic flux in nuclear collisions. Additionally, other mechanisms such as partonic energy loss due to gluon radiation, which is very relevant for nuclear modifications.
Quarkonium states, such as the J/ψ meson, are prominent probes of the quark–gluon plasma (QGP) formed in high-energy nucleus–nucleus (AA) collisions. That bulk J/ψ production is suppressed in AA collisions with respect to proton–proton collisions had been reported by ALICE five years ago. However, measurements of J/ψ production in proton–lead collisions, where the formation of the QGP is not expected, are essential to quantify effects that are present in AA collisions but not associated with the QGP. In a recent study, ALICE has shown that the production of J/ψ mesons in proton–lead collisions is strongly correlated with the total number of produced particles in the event (event multiplicity), and that this correlation varies as a function of rapidity.
In ALICE, the J/ψ measurements are performed at forward (proton direction), mid- and at backward-rapidity (lead direction). An increase of the J/ψ yield relative to the event-averaged value with the relative charged-particle multiplicity is observed for all rapidity domains, with a similar slope at low multiplicities (see figure). At multiplicities a factor two above the event average, the trend at forward rapidity is very different from those at mid- and backward-rapidity. In the forward rapidity window, a saturation of the relative yield sets in at high multiplicities, which is interesting because the forward region with low parton fractional momentum is in the domain of gluon shadowing/saturation.
Models incorporating nuclear parton distribution functions with significant shadowing have previously been shown to describe J/ψ measurements performed in event classes selected according to the centrality of the collision. The present measurement, exploring significantly more “violent” events (below 1% of the total hadronic interaction cross-section), suggests that effective gluon depletion in the colliding lead nucleus is larger in high-multiplicity events. However, there are additional concepts to describe this regime of QCD, and it remains to be seen whether such models can also describe the saturation of the yields at forward rapidities.
The reason why some stars are born in pairs while others are born singly has long puzzled astronomers. But a new study suggests that no special conditions are required: all stars start their lives as part of a binary pair. The result has implications not only in the field of star evolution but also for studies of binary neutron-star and binary black-hole formation. It also suggests that our own Sun was born together with a companion that has since disappeared.
Stars are born in dense molecular clouds measuring light-years across, within which denser regions can collapse under their own gravity to form high-density cores opaque to optical radiation, which appear as dark patches. When the densities reach the level where hydrogen fusion begins, the cores can form stars. Although young stars already emit radiation before the onset of the hydrogen-burning phase, it is absorbed in the dense clouds that surround them, making star-forming regions difficult to study. Yet, since clouds that absorb optical and infrared radiation re-emit it at much longer wavelengths, it is possible to probe them using radio telescopes.
Sarah Sadavoy of the Max Planck Institute for Astronomy in Heidelberg and Steven Stahler of the University of California at Berkeley used data from the Very Large Array (VLA) radio telescopes in New Mexico, together with micrometre-wavelength data from the James Clerk Maxwell Telescope (JCMT) in Hawaii, to study the dense gas clumps and the young stars forming in them in the Perseus cluster – a star-forming region about 600 light-years away. Data from the JCMT show the location of dense cores in the gas, while the VLA provides the location of the young stars within them.
Studying the multiplicity as well as the location of the young stars inside the dense regions, the researchers found a total of 19 binary systems, 45 single-star systems and five systems with a higher multiplicity. Focusing on the binary pairs, they observed that the youngest binaries typically have a large separation of 500 astronomical units (500 times the Sun–Earth distance). Furthermore, the young stars were aligned along the long axis of the elongated cloud. Older binary systems, with an age between 500,000 and one million years, were found typically to be closer together and separated around a random axis.
Subsequent to cataloguing all the young stars, the team compared the observed star multiplicity and the features seen in the binary pairs to simulations of stars being formed either as single or binary systems. The only way the model could reproduce the data was if its starting conditions contained no single stars but only stars that started out as part of wide binaries, implying that all stars are formed as part of a binary system. After formation, the stars either move closer to one another into a close binary system or move away from each other. The latter option is likely to be what happened in the case of the Sun, its companion having drifted away long ago.
If indeed all stars are formed in pairs, it would have big implications for models of stellar birth rates in molecular clouds as well as for the formation of binary systems of compact objects. The studied nearby Perseus cluster could, however, just be a special case, and further studies of other star-forming regions are therefore required to know if the same conditions exist elsewhere in the universe.
Particle physicists try to understand the environment that existed fractions of a second after the Big Bang by studying the behaviour of particles at high energies. Early studies relied on cosmic rays emanating from extraterrestrial sources, but the invention of the circular accelerator by Ernest Lawrence in 1931 revolutionised the field. Further advances in accelerator technology gave physicists more control over their experiments, in particular thanks to the invention of the synchrotron and the development of storage rings. By capturing particles via a ring of magnets and accelerating them with radio-frequency cavities, these facilities finally reached energies of a few hundred GeV. But storage rings are limited by the maximum magnetic field achievable with resistive magnets, which is around 2 T. To go further into the heart of matter, particle physicists required higher energies and a new technology to get them there.
The maximum field of an electromagnet is roughly determined by the amount of current in a conductor multiplied by the number of turns the conductor makes around its support structure. Over the years, the growing scale of accelerators and the large number of magnets needed to reach the highest energies demanded compact and affordable magnets. Conventional electromagnets, which are usually based on a copper conductor, are limited by two main factors: the amount of power required to operate them due to resistive losses and the size of the conductor. Typical conventional-magnet windings therefore tended to use conductors with a cross-sectional area of the order of a few square centimetres, which is not optimal for generating high magnetic fields.
Superconductivity, which allows certain materials at low temperatures to carry very high currents without any resistive loss, was just the transformational technology needed. It powered the Tevatron collider at Fermilab in the US to produce the top quark, and CERN’s Large Hadron Collider (LHC) to unearth the Higgs boson. Advanced superconducting magnets are already being developed for future collider projects that will take physicists into a new phase of subatomic exploration beyond the LHC (figure 1).
Maintaining the state
Discovered in 1911, superconductivity didn’t immediately lead to broad applications, particularly not high-field accelerator magnets. As far as accelerators were concerned, the possibility of using superconducting magnets to produce higher fields started to take root in the mid-1960s. The big challenge was to maintain the superconducting state in a bulk object in which tremendous forces are at work: the slightest microscopic movement of the conductor would cause it to transition to the normal state (a “quench”) and result in burn-up, unless the fault was detected quickly and the current turned off.
Early superconductors were mostly formed into high-aspect-ratio tapes measuring a few tenths of a millimetre thick and around 10 mm wide. These are not particularly useful for making magnets because precise geometry and current distribution are necessary to achieve a good field quality. Intense studies led to the development of multi-filamentary niobium-zirconium (NbZr), niobium-titanium (Nb-Ti) and niobium-tin (Nb3Sn) wires, propelling interest in superconducting technology. In 1961, Kunzler and colleagues at Bell Labs produced a 7 T field in a solenoid, a relatively simple coil geometry compared with the dipoles or quadrupoles needed for accelerators. This swiftly led to higher-field solenoids, and a number of efforts to utilise the benefits of superconductivity for magnets began. But it was only in the early 1970s that the first prototypes of superconducting dipoles and quadrupoles demonstrated the potential of superconducting magnet technology for accelerators.
A turning point came during a six-week-long study group at Brookhaven National Laboratory (BNL) in the US in the summer of 1968, during which 200 physicists and engineers from around the world discussed the application of superconductivity to accelerators (figure 2). Considerable focus was directed towards the possibility of using superconducting beam-handling magnets (such as dipoles and quadrupoles for transporting beams from accelerators to experimental areas) for the new 200–400 GeV accelerator being constructed at Fermilab. By that time, several high-field superconducting alloys and compounds had been produced.
Hitting the mainstream
It could be argued that the unofficial kick-off for superconducting magnets in accelerators was a panel discussion at the 1971 Particle Accelerator Conference held in Chicago, although there was a clear geographical divide on key issues. The European contingent was reluctant to delve into higher-risk technology when it was clear that conventional technology could meet their needs, while the Americans argued for the substantial cost savings promised by superconducting machines: they claimed that a 100 GeV superconducting synchrotron could be built in five or six years, while the Europeans estimated a more conservative seven to 10 years.
In the US, work on furthering the development of superconducting magnets for accelerators was concentrated in a few main laboratories: Fermilab, the Lawrence Radiation Laboratory, Brookhaven National Laboratory (BNL) and Argonne National Laboratory. In Europe, a consortium of three laboratories – CEA Saclay in France, Rutherford Appleton Laboratory in the UK and the Nuclear Research Center at Karlsruhe – was formed to enable future conversion of the recently approved 300 GeV accelerator, to become CERN’s Super Proton Synchrotron (SPS), to higher energies using superconducting magnets. Of particular historical note, a short paper written at this time referred to a “compacted fully transposed cable” produced at the Rutherford Lab, and the “Rutherford cable” has since become the standard conductor configuration for all accelerator magnets (figure 3).
Rapid progress followed, reaching a tipping point in the 1970s with the launch of several accelerator projects based on superconducting magnets and a rapidly growing R&D community worldwide. These included: the Fermilab Energy Doubler; Interaction Region (IR) quadrupoles (used to bring particles into collision for the experiments) for the Intersecting Storage Rings at CERN; and IR quadrupoles for TRISTAN at KEK in Japan and UNK in the former USSR. The UNK magnets were ambitious for their time, with a desired operating field of 5 T, but the project was cancelled in the years following the breakup of the USSR.
Although superconducting magnet technology was one of the initial options for the SPS, it was rapidly discarded in favour of resistive magnets. This was not the case at Fermilab, which at that time was pursuing a project to upgrade its Main Ring beyond 500 GeV. The project was initially presented as an Energy Doubler, but rapidly became known by the very modern name of Energy Saver, and is now known as the Tevatron collider for protons and antiprotons, which shut down in 2011. The Tevatron arc magnets were the result of years of intense and extremely effective R&D, and it was their success that triggered the application of superconductivity for accelerators.
As superconducting technology matured during the 1980s, its applications expanded. The electron–proton collider HERA was getting under way at DESY in Germany, while ISABELLE was reborn as the Relativistic Heavy Ion Collider (RHIC) at BNL. Thanks to intensive development by high-energy physics, Nb-Ti was readily available from industry. This allowed the construction of magnets with fields in the 5 T range, while multi-filamentary conductors made from niobium-titanium-tantalum (Nb-Ti-Ta) and Nb3Sn were being pursued for fields up to 10 T. The first papers on the proposed Superconducting Super Collider (SSC) in the US were published in the mid-1980s, with R&D for the SSC ramping up substantially by the start of the 1990s. Then, in 1991, the first papers on R&D for the LHC were presented. The LHC’s 8 T Nb-Ti dipole magnets operate close to the practical limit of the conductor, and the collider now represents the largest and most sophisticated use of superconducting magnets in an accelerator.
The niobium-tin challenge
With the success of the LHC, the international high-energy physics community has again turned its attention to further exploration of the energy frontier. CERN has launched a Future Circular Collider (FCC) study that envisages a 100 TeV proton–proton collider as the next step for particle physics, which would require a 100 km-circumference ring of superconducting magnets with operating fields of 16 T. This will be an unprecedented challenge for the magnet community, but one that they are eager to take on. Other future machines are based on linear accelerators that do not require magnets to keep the beams on track, but demand advanced superconducting radio-frequency structures to accelerate them over short distances.
Thanks to superconducting accelerator magnets wound with strands and cables made of Cu/Nb-Ti composites, the energy reach of particle colliders has steadily increased. After nearly half a century of dominance by Nb-Ti, however, other superconducting materials are finally making their way into accelerator magnets. Quadrupoles and dipoles using Nb3Sn will be installed as part of the high-luminosity upgrade for the LHC (the HL-LHC) in the next few years, for example, and the high-temperature superconductor Bi2Sr2CaCu2O8 (BSCCO), iron-based superconductors and rare-earth bismuth copper oxide (REBCO) have recently been added to the list of candidate materials. Proposals for new large circular colliders has boosted interest in high-field dipole magnets but, despite the tantalising potential for achieving dipole fields more than twice that of Nb-Ti, there are many problems that still need to be overcome.
Although Nb3Sn was one of the early candidates for high-field magnets, and has much better performance at high fields than Nb-Ti, its processing requirements, mechanical properties and costs present difficulties when building practical magnets. Nb3Sn comes as a round wire from industry vendors, which is excellent for making multi-wire cables but requires the reaction of a copper, niobium and tin composite at 650 °C to develop the superconducting Nb3Sn cable. Unfortunately, Nb3Sn is a brittle ceramic, unlike Nb-Ti, which requires only modest heat treatment and drawing steps and is mechanically very strong. Years of effort worldwide have overcome these limitations and fields in the range of 16 T have recently been achieved – first in 2004 by a US R&D programme and more recently at CERN – and this is close to the practical limit for this conductor. In addition to the near-term use in the HL-LHC, and despite currently costing 10 times more than Nb-Ti, it is the material of choice for a future high-energy hadron collider, and is also being used in enormous quantities for the toroidal-field magnets and central solenoid of the ITER fusion experiment (see “ITER’s massive magnets enter production”).
High-temperature superconductors represent a further leap in magnet performance, but they also raise major difficulties and could cost an additional factor of 10 more than Nb3Sn. For fields above 16 T there are currently only two choices for accelerator magnets: BSCCO and REBCO. Although these materials become superconductors at a higher temperature than niobium-based materials, their maximum current density is achieved at low temperatures (in the vicinity of 4.2 K). BSCCO has the advantage of being obtainable in round wire, which is perfect for making high-current cables but requires a fairly precise heat treatment at close to 900 °C in oxygen at high pressures. This is not a simple engineering task, especially when dealing with large coils. Much progress has been made recently, however, and there is a vibrant programme in industry and academia to tackle these challenges. REBCO has excellent high-field performance, high current density and requires no heat treatment, but it only comes in tape form, presenting difficulties in winding the required coil shapes and producing acceptable field quality. Nevertheless, the performance of this high-temperature superconductor is too tantalising to abandon it, and many people are working on it. Even after half a century, progress in the development of high-field accelerator magnet R&D continues, and indeed is critical for future discoveries in particle physics.
CERN breaks records with high-field magnets for High-Luminosity LHC
To keep the protons on a circular track at the record-breaking luminosities planned for the LHC upgrade (the HL-LHC) and achieve higher collision energies in future circular colliders, particle physicists need to design and demonstrate the most powerful accelerator magnets ever. The development of the niobium-titatnium LHC magnets, currently the highest-field dipole magnets used in a particle accelerator, followed a long road that offered valuable lessons. The HL-LHC is about to change this landscape by relying on niobium tin (Nb3Sn) to build new high-field magnets for the interaction regions of the ATLAS and CMS experiments. New quadrupoles (called MQFX) and two-in-one dipoles with fields of 11 T will replace the LHC’s existing 8 T dipoles in these regions. The main challenge that has prevented the use of Nb3Sn in accelerator magnets is its brittleness, which can cause permanent degradation under very low intrinsic strain. The tremendous progress of this technology in the past decade led to the successful tests of a full-length 4.5 m-long coil that reached a record nominal field value of 13.4 T at BNL. Meanwhile at CERN, the winding of 7.15 m-long coils has begun.Several challenges are still to be faced, however, and the next few years will be decisive for declaring production readiness of the MQFX and 11 T magnets. R&D is also ongoing for the development of a Nb3Sn wire with an improved performance that would allow fields beyond 11 T. It is foreseen that a 14–15 T magnet with real physical aperture will be tested in the US, and this could drive technology for a 16 T magnet for a future circular collider. Based on current experience from the LHC and HL-LHC, we know that the performance requirements for Nb3Sn for a future circular collider require a large industrial effort to make very large-scale production viable.
• Panagiotis Charitos, CERN.
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Preferences
The technical storage or access is necessary for the legitimate purpose of storing preferences that are not requested by the subscriber or user.
Statistics
The technical storage or access that is used exclusively for statistical purposes.The technical storage or access that is used exclusively for anonymous statistical purposes. Without a subpoena, voluntary compliance on the part of your Internet Service Provider, or additional records from a third party, information stored or retrieved for this purpose alone cannot usually be used to identify you.
Marketing
The technical storage or access is required to create user profiles to send advertising, or to track the user on a website or across several websites for similar marketing purposes.
