Following severe damage caused by flooding on 9 November, the INFN-CNAF Tier-1 data centre of the Worldwide LHC Computing Grid (WLCG) in Bologna, Italy, has been fully repaired and is back in business crunching LHC data. The incident was caused by the burst of a large water pipe at high pressure in a nearby street, which rapidly flooded the area where the data centre is located. Although the centre was designed to be waterproof against natural events, the volume of water was overwhelming: some 500 m3 of water and mud entered the various rooms, seriously damaging electronic appliances, computing servers, network and storage equipment. A room hosting four 1.4 MW electrical-power panels was filled first, leaving the centre without electricity.
The Bologna centre, which is one of 14 Tier-1 WLCG centres located around the world, hosts a good fraction of LHC data and associated computing resources. It is equipped with around 20,000 CPU cores, 25 PB of disk storage, and a tape library presently filled with about 50 PB of data. Offline computing activities for the LHC experiments were immediately affected. About 10% of the servers, disks, tape cartridges and computing nodes were reached by floodwater, and the mechanics of the tape library were also affected.
Despite the scale of the damage, INFN-CNAF personnel were not discouraged, quickly defining a roadmap to recovery and then attacking one by one all the affected subsystems. First, the rooms at the centre had to be dried and then meticulously cleaned to remove residual mud. Then, within a few weeks, new electrical panels were installed to allow subsystems to be turned back on.
Although all LHC disk-storage systems were reached by the water, the INFN-CNAF personnel were able to recover the data in their entirety, without losing a single bit. This was thanks in part to the available level of redundancy of the disk arrays and to their vertical layout. Wet tape cartridges hosting critical LHC data had to be sent to a specialised laboratory for data recovery.
A dedicated computing farm was set up very quickly at the nearby Cineca computing centre and connected to INFN-CNAF via a high-speed 400 Gbps link to enable the centre to reach the required LHC capacity for 2018. During March, three months since the incident, all LHC experiments were progressively put back online. Following the successful recovery, INFN is planning to move the centre to a new site in the coming years.
The NA62 collaboration at CERN has found a candidate event for the ultra-rare decay K+→ π+ ν ν, demonstrating the experiment’s potential to test heavily-suppressed corners of the Standard Model (SM).
The SM prediction for the K+→ π+ ν ν branching fraction is 0.84 ± 0.03 × 10–10. The very small value arises from the underlying coupling between s and d quarks, which only occurs in loops and is suppressed by the couplings of the quark-mixing CKM matrix. The SM prediction for this process is very clean, so finding even a small deviation would be a strong indicator of new physics.
NA62 was approved a decade ago and builds on a long tradition of kaon experiments at CERN (CERN Courier June 2016 p24). The experiment acts as a kaon factory, producing kaon-rich beams by firing high-energy protons from the Super Proton Synchrotron into a beryllium target and then using advanced Cherenkov and straw trackers to identify and measure the particles (see figure). Following pilot and commissioning runs in 2014 and 2015, the full NA62 detector was installed in 2016 enabling the first analysis of the K+→ π+ ν ν channel.
Finding one candidate event from a sample of around 1.2 × 1011 events allowed the NA62 team to put an upper limit on the branching fraction of 14 × 10–10 at a confidence level of 95%. The result, first presented at Moriond in March, is thus compatible with the SM prediction, although the statistical errors are too large to probe beyond-SM physics.
Several candidate K+→ π+ ν ν events have been previously reported by the E949 and E787 experiments at Brookhaven National Laboratory in the US, inferring a branching fraction of 1.73 ± 1.1 × 10–10 – again consistent, within large errors, with the SM prediction. Whereas the Brookhaven experiments observed kaon decays at rest in a target, however, NA62 observes them in-flight as they travel through a large vacuum tank and therefore creates a cleaner environment with less background events.
The NA62 collaboration expects to identify more events in the ongoing analysis of a 20-fold-larger dataset recorded in 2017. In mid-April the experiment began its 2018 operations with the aim of running for a record number of 218 days. If the SM prediction is correct, the experiment is expected to see about 20 events with the data collected before the end of this year.
“The K+→ π+ ν ν decay is special because, within the SM, it allows one to extract the CKM element |Vtd| with a small theoretical uncertainty,” explains NA62 spokesperson Augusto Ceccucci. “Developing the necessary experimental sensitivity to be able to observe this decay in-flight has involved a long R&D programme over a period of five years, and this effort is now starting to pay off.”
The ALPHA collaboration at CERN’s Antiproton Decelerator (AD) has reported the most precise direct measurement of antimatter ever made. The team has determined the spectral structure of the antihydrogen 1S–2S transition with a precision of 2 × 10–12, heralding a new era of high-precision tests between matter and antimatter and marking a milestone in the AD’s scientific programme (CERN Courier March 2018 p30).
Measurements of the hydrogen atom’s spectral structure agree with theoretical predictions at the level of a few parts in 1015. Researchers have long sought to match this stunning level of precision for antihydrogen, offering unprecedented tests of CPT invariance and searches for physics beyond the Standard Model. Until recently, the difficulty in producing and trapping sufficient numbers of delicate antihydrogen atoms, and acquiring the necessary optical laser technology to interrogate their spectral characteristics, has kept serious antihydrogen spectroscopy out of reach. Following a major programme by the low-energy-antimatter community at CERN during the past two decades and more, these obstacles have now been overcome.
“This is real laser spectroscopy with antimatter, and the matter community will take notice,” says ALPHA spokesperson Jeffrey Hangst. “We are realising the whole promise of CERN’s AD facility; it’s a paradigm change.”
ALPHA confines antihydrogen atoms in a magnetic trap and then measures their response to a laser with a frequency corresponding to a specific spectral transition. In late 2016, the collaboration used this approach to measure the frequency of the 1S–2S transition (between the lowest-energy state and the first excited state) of antihydrogen with a precision of 2 × 10–10, finding good agreement with the equivalent transition in hydrogen (CERN Courier January/February 2017 p8).
The latest result from ALPHA takes antihydrogen spectroscopy to the next level, using not just one but several detuned laser frequencies with slightly lower and higher frequencies than the 1S–2S transition frequency in hydrogen. This allowed the team to measure the spectral shape, or spread in colours, of the 1S–2S antihydrogen transition and get a more precise measurement of its frequency (see figure). The shape of the spectral line agrees very well with that expected for hydrogen, while the 1S–2S resonance frequency agrees at the level of 5 kHz out of 2.5 × 1015 Hz. This is consistent with CPT invariance at a relative precision of 2 × 10−12 and corresponds to an absolute energy sensitivity of 2 × 10−20 GeV.
Although the precision still falls short of that for ordinary hydrogen, the rapid progress made by ALPHA suggests hydrogen-like precision in antihydrogen is now within reach. The collaboration has also used its unique setup at the AD to tackle the hyperfine and other key transitions in the antihydrogen spectrum, with further seminal results expected this year. “When you look at the lineshape, you feel you have to pinch yourself – we are doing real spectroscopy with antimatter!” says Hangst.
Direct searches for particles beyond the Standard Model (SM) have so far come up empty handed, but perhaps physicists can get luckier with indirect searches. Quantum mechanics allows neutral flavoured mesons to transform (or oscillate) into their anti-meson counterparts and back via weak interactions. Novel particles may contribute to the amplitude that governs such oscillations, thus altering their rate or introducing charge-parity (CP) violating rate differences between mesons and anti-mesons. Depending on the flavour structure of what lies beyond the SM, precision studies of such effects can probe energies up to 105 TeV – far beyond the reach of direct searches at the maximum energy currently achievable at colliders.
Oscillations, first posited in 1954 by Gell-Mann and Pais, have been measured precisely for kaons and beauty mesons. But there is room for improvement for D mesons, which contain a charm quark. Neither a nonzero value for the mass difference between mass eigenstates of neutral D mesons, nor a departure from CP symmetry, have yet been established. Charm oscillations are especially attractive because the D-meson flavour is carried by an up-type (i.e. with an electric charge of +2/3) quark. Charm-meson oscillations therefore probe phenomena complementary to those probed by strange- and beauty-meson oscillations.
LHCb recently determined charm- oscillation parameters using 5 fb–1 of proton–proton collision data collected at the LHC in 2011–2016. About 5–10% of LHC collisions produce charm mesons; approximately 10,000per second are reconstructable. Oscillations are studied by comparing production and decay flavour (i.e. whether a charm or an anti-charm is present) as a function of decay time. The charge of the pion from the strong-interaction decay D*+→ D0π+ determines the flavour at production. The decay flavour is inferred by restricting to K±π∓ final states because charm (anti-charm) neutral mesons predominantly decay into so-called right-sign K–π+ (K+π–) pairs. Hence, a decay-time modulation of the wrong-sign yields of D0→ K–π+ and D0→ K+π– decays indicates oscillations. In addition, differing modulations between charm or anti-charm mesons indicate CP violation. Backgrounds and instrumental effects that induce a decay-time dependence in the wrong-sign yield, or a difference between charm and anti-charm rates, may introduce harmful biases.
LHCb used track-quality, particle identification, and D0 and D*+ invariant masses to isolate a prominent signal of 0.7 million wrong-sign decays overlapping a smooth background. Decays of mesons produced as charm or anti-charm were analysed independently. The wrong-sign yield as a function of decay time was fitted to determine the oscillation parameters. Statistical uncertainties dominate the precision. Systematic effects include biases from signal candidates originated from beauty hadrons, residual peaking backgrounds, and instrumental asymmetries associated with differing K+π– and K–π+ reconstruction efficiencies. With about 10–4–10–5 absolute (10% fractional) precision, the results are twice as precise as the previous best results (also by LHCb) and show no evidence of CP violation in charm oscillations.
Decays of the Higgs boson to vector bosons (WW, ZZ, γγ) provide precise measurements of the boson’s coupling strength to other Standard Model (SM) particles. In new analyses, ATLAS has measured these decays for different production modes using the full 2015 and 2016 LHC datasets recorded at a centre-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 36.1 fb–1.
With a predicted branching fraction of 21%, the Higgs-boson decay to two W bosons (H → WW) is the second most common decay mode after its decay to two b quarks. The new analysis follows a similar strategy to the earlier ones carried out using the LHC datasets recorded at 7 and 8 TeV. It focuses on the gluon-gluon fusion (ggF) and vector-boson fusion (VBF) production modes, with the subsequent decay to an electron, a muon and two neutrinos (H → WW → eνμν). The main backgrounds come from SM production of W and top-quark pairs; other backgrounds involve Z →ττ with leptonic τ decays and single-W production with misidentified leptons from associated jets.
Events are classified according to the number of jets they contain: events with zero or one jet are used to probe ggF production, while events with two or more jets are used to target VBF production. Due to the spin-zero nature of the Higgs boson, the electron and muon are preferentially emitted in the same direction. The ggF analysis exploits this and other kinematic information via a sequence of selection requirements, while the VBF analysis combines lepton and jet variables in a boosted decision tree to separate the Higgs-boson signal from background processes.
The transverse mass of the selected events from the zero and one-jet signal regions is shown in the left figure, with red denoting the expectation from the Higgs boson and other colours representing background processes. These events are combined with those from the two-jet signal region to derive cross sections times branching fractions for ggF and VBF production of 12.3 +2.3–2.1 pb and 0.50+0.30–0.29 pb, respectively, to be compared to the SM predictions of 10.4 ± 0.6 pb and 0.81 ± 0.02 pb.
ATLAS also performed a combination of inclusive and differential cross-section measurements using Higgs-boson decays to two photons and two Z bosons, where each Z decays to a pair of oppositely charged electrons or muons. The combination of the two channels allows the study of Higgs-boson production rates versus event properties with unprecedented precision. For example, the measurement of the Higgs-boson rapidity distribution can provide information about the underlying parton density functions. The transverse momentum distribution (figure) is sensitive to the coupling between the Higgs boson and light quarks at low transverse momentum, and to possible couplings to non-SM particles at high values. The measured cross sections are found to be consistent with SM predictions.
In a new publication submitted to the Journal of High Energy Physics, the ALICE collaboration has reported transverse momentum (pT) spectra of charged hadrons in proton–proton (pp), proton–lead (pPb) and lead–lead (PbPb) collisions at an energy of 5.02 TeV per nucleon pair. The results shed further light on the dense quark-gluon plasma (QGP) thought to have existed shortly after the Big Bang.
At high transverse momentum, hadrons originate from the fragmentation of partons produced in hard-scattering processes. These processes are well understood in pp collisions and can be modelled using perturbative quantum chromodynamics.
In PbPb collisions, the spectra are modified by the energy loss that the partons suffer when propagating in the QGP. Proton–lead collisions serve as a baseline for initial-state effects such as the modification of the gluon density of the nucleons of colliding lead nuclei.
To characterise the change of spectra in nuclear collisions with respect to the expectation from pp collisions, the nuclear modification factors RPbPb (RpPb) are calculated by dividing the pT spectra from PbPb (pPb) collisions by the spectra measured in pp collisions, scaled by the number of binary nucleon–nucleon collisions in the PbPb (pPb) collisions (see figure).
The nuclear modification factor in proton–lead collisions is consistent with unity at high transverse momentum. This shows that initial-state effects from the parton density in the lead nucleus are small and that the strong suppression observed in PbPb collisions is caused by final-state parton-energy loss in the QGP. The new results with higher statistics have much improved systematic uncertainties compared to the earlier publications based on Run 1 data. This is possible because of the improvements in the particle reconstruction and its description in Monte Carlo simulations, as well as data-driven corrections based on identified particles.
The suppression in PbPb collisions at 5.02 TeV is found to be similar to that at the collision energy of 2.76 TeV despite the harder spectrum at the higher energy, which indicates a stronger parton-energy loss and a larger energy density of the medium at the higher energy.
Theoretical models are able to describe the main features of the ALICE data; the improved precision of the measurements will allow researchers to constrain theoretical uncertainties further and to determine transport coefficients in the QGP. The upcoming PbPb run scheduled for November this year and the large pp reference sample collected at the end of 2017 will improve the statistical precision substantially and further extend the covered range of the transverse momentum.
The amazing performance of the LHC provides CMS with a large sample of Z bosons. With such high statistics, the CMS collaboration can now probe rare decay channels that were not accessible to experiments at the former Large Electron Positron (LEP) collider. One of these channels, first theoretically studied in the early 1990s, is the decay of the Z boson to a J/ψ meson and two additional leptons. Theoretical calculations of this process, illustrated in the top figure, predict a branching fraction of 6.7–7.7 × 10–7.
The new analysis was performed using proton–proton collision data collected during 2016, corresponding to an integrated luminosity of 35.9 fb–1. To separate signal and background events, a 2D unbinned maximum likelihood fit was used which exploits as discriminating variables the invariant masses of the reconstructed J/ψ and Z states. Due to the limited separation sensitivity of the prompt J/ψ decays from ψ(2S) → J/ψ X decays, the sum of the two modes is indicated with ψ. The decay modes Z →ψμ+μ– and Z →ψ e+e– were searched for, resulting in a yield of 13 and 11 reconstructed candidates in the two channels, respectively. The significance of the Z →ψℓ+ℓ– observation (where ℓ = μ, e) is greater than five standard deviations.
Using the Z →μ+μ–μ+μ– decay mode as a reference sample and after removing the (ψ2S) → J/ψ X contribution, the branching fraction ratio B(Z → J/ψℓ+ℓ–)/B(Z →μ+μ–μ+μ–) in the fiducial phase space of the CMS detector is measured to be 0.70 ± 0.18 (stat) ± 0.05 (syst), assuming null J/ψ polarisation.
Extrapolating from the fiducial volume to the full space and assuming that the extrapolation uncertainties of the two channels cancel in the ratio, a qualitative estimate of B(Z → J/ψℓ+ℓ–) can be extracted. The measured value of approximately 8 × 10–7 is consistent with the prediction of the Standard Model.
This is the first observation of this decay mode, and is the rarest Z-decay channel observed to date. With this analysis, CMS has started a new era of rare Z decay measurements. Looking forward, the full Run 2 data can lead to a more precise measurement of this decay’s branching fraction. This is particularly important since this process is a background to the even rarer process whereby a Higgs boson decays into a J/ψ and lepton pair, and rare decays are a rich target in which to detect new physics.
In the 1920s, Edwin Hubble discovered that the universe is expanding by showing that more distant galaxies recede faster from Earth than nearby ones. Hubble’s measurements of the expansion rate, now called the Hubble constant, had relatively large errors, but astronomers have since found ways of measuring it with increasing precision. One way is direct and entails measuring the distance to far-away galaxies, whereas another is indirect and involves using cosmic microwave background (CMB) data. However, over the last decade a mismatch between the values derived from the two methods has become apparent. Adam Riess from the Space Telescope Science Institute in Baltimore, US, and colleagues have now made a more precise direct measurement that reinforces the mismatch and could signal new physics.
Riess and co-workers’ new value relies on improved measurements of the distances to distant galaxies, and builds on previous work by the team. The measurements are based on more precise measurements of type Ia supernovae within the galaxies. Such supernovae have a known luminosity profile, so their distances from Earth can be determined from how bright they are observed to be. But their luminosity needs to be calibrated – a process that requires an exact measurement of their distance, which is typically rather large.
To calibrate their luminosity, Riess and his team used Cepheid stars, which are closer to Earth than type Ia supernovae. Cepheids have an oscillating apparent brightness, the period of which is directly related to their luminosity, and so their apparent brightness can also be used to measure their distance. Riess and colleagues measured the distance to Cepheids in the Milky Way using parallax measurements from the Hubble Space Telescope, which determine the apparent shift of the stars against the background sky as the Earth moves to the other side of the Sun. The researchers measured this minute shift for several Cepheids, giving a direct measurement of their distance. The team then used this measurement to estimate the distance to distant galaxies containing such stars, which in turn can be used to calibrate the luminosity of supernovae in those galaxies. Finally, they used this calibration to determine the distance to even more distant galaxies with supernovae. Using such a “distance ladder”, the team obtained a value for the Hubble constant of73.5 ± 1.7 km s–1 Mpc–1. This value is more precise than the 73.2 ± 1.8 km s–1 Mpc–1 value obtained by the team in 2016, and it is 3.7 sigma away from the 66.9 ± 0.6 km s–1 Mpc–1 value derived from CMB observations made by the Planck satellite.
Future data could also potentially help to identify the source of the discrepancy
Reiss and colleagues’ results therefore reinforce the discrepancy between the results obtained through the two methods. Although each method is complex and may thus be subject to error, the discrepancy is now at a level that a coincidence seems unlikely. It is difficult to imagine that systematic errors in the distance-ladder method are the root cause of the tension, says the team. Figuring out the nature of the discrepancy is pivotal because the Hubble constant is used to calculate several cosmological quantities, such as the age of the universe. If the discrepancy is not due to errors, explaining it will require new physics beyond the current standard model of cosmology. But future data could also potentially help to identify the source of the discrepancy. Upcoming Cepheid data from ESA’s Gaia satellite could reduce the uncertainty in the distance-ladder value, and new measurements of the expansion rate using a third method based on observations of gravitational waves could throw new light on the problem.
Two key parameters underpin the physics reach of a particle collider: its collision energy and its luminosity, which is the number of potential collisions per unit area per unit time at the interaction point of the colliding beams. Accelerator physicists have devised numerous ways to boost the luminosity and thus scientific reach of colliders, via innovative magnet and radio-frequency (RF) technologies. One such RF innovation is the crab cavity, which is a key feature of the high-luminosity upgrade to the Large Hadron Collider (HL-LHC) now in its construction phase at CERN.
The roots of crab cavities can be traced to 1988, when Bob Palmer at Brookhaven National Laboratory proposed the crab crossingscheme for an electron-positron linear collider. The first implementation in an accelerator was much later, in 2006, at the KEKB electron–positron collider in Japan. One crab cavity per ring operated for approximately four years and, in conjunction with other accelerator elements, helped the collider reach record luminosities. Crab-crossing was considered as one of several potential LHC upgrade paths as early as 2002 and, in 2006, the first proposal for the HL-LHC crab cavities was made. Soon afterwards, crab cavities – along with high-field niobium–tin magnets – were adopted as one of the key technologies allowing the HL-LHC to multiply the integrated luminosity of the present LHC by a factor of 10.
An extensive R&D programme followed, and the first tests of the HL-LHC crab cavities took place in spring last year (CERN Courier May 2017 p7). Beginning in early 2018, two cavities were installed in CERN’s Super Proton Synchrotron (SPS) to study how they behave with real beam (image left). This will be the first time that a crab cavity has ever been used for manipulating protons, paving the way not just for the HL-LHC but a variety of other accelerator applications.
Crab-wise crossings
The advanced niobium-tin “inner-triplet quadrupole” magnets for the HL-LHC (CERN Courier March 2017 p23 and September 2017 p17) will be placed on either side of the ATLAS and CMS experiments to squeeze the incoming proton beams into smaller transverse beam sizes at the collision point than those presently obtained at the LHC. To avoid unwanted parasitic collisions in the single beam pipe either side of the interaction points, the HL-LHC will operate with a crossing angle, which has the negative effect of reducing the luminosity with respect to that obtained with head-on collisions. Smaller beam sizes at the interaction point of the HL-LHC imply larger beams in the inner triplet magnets and, consequently, a larger crossing angle is necessary to ensure sufficient separation of the beams. At the nominal HL-LHC, the large crossing angle coupled with the small bunch dimensions would result in a luminosity reduction of around 66% if not corrected.
To recover some of this potential loss of luminosity while maintaining the necessary beam separation, an elegant crab-crossing scheme has been proposed. Independent superconducting crab cavities for each beam are positioned around 160 m upstream and downstream of a given collision point. The crab cavities provide a time-dependent transverse kick to the protons in the head and tail of a bunch. As a bunch moves towards the interaction point, the kick serves to rotate the bunch in the crossing plane so that it effectively collides head-on with its incoming counterpart (figure 1). The downstream crab cavity then reverses the kick to confine the rotation to the interaction region and leave the particle orbit in the rest of the machine untouched. A total of 16 cavities – eight (two per beam per side) near ATLAS and eight near CMS – will be required for the project.
The tight spatial constraints of the HL-LHC upgrade meant a long journey of technological challenges. The large crossing angle (implying a large transverse kick) between the HL-LHC beams required a radically new RF concept for particle deflection with a novel shape and significantly smaller cavities than those used in other accelerators. In less than two years, more than 10 concurrent designs from RF experts across three continents were being discussed as potential options. By 2013, three designs stemming from a worldwide collaboration between CERN, the US and the UK were considered to be most adapted to the HL-LHC: double quarter wave (DQW), RF dipole (RFD) and four rod (4R). The results of RF tests of these designs were highly promising and, in 2014, an international panel recommended that efforts be focused on the first two (figure 2) with the aim of making a full validation with real proton beams. Both designs will be used, one around ATLAS and the other around CMS.
The cavities are made from sheets of high-purity niobium, a type II superconductor commonly used for very high-field superconductors. A sheet thickness of 4 mm is necessary to cope with the strict mechanical constraints; advanced shaping, machining and ultra-precise electron-beam welding are required to produce the complex shapes with mechanical tolerances well below 1 mm. Once “dressed”, each cavity is equipped with: a helium tank, an internal magnetic shield, a precision frequency tuning system, a fundamental RF power coupler, a field probe and two or three higher-order mode couplers (figure 3).
The helium tank serves as an enclosing body to cool the cavity surface with saturated superfluid helium at 2 K and its geometry was chosen to limit the maximum stress on the cavity. Owing to the unconventional geometries of the crab cavities, titanium was selected because it has nearly the same thermal contraction as niobium. An active frequency tuning system structurally integrated with the tank with a resolution of a few nanometres allows the cavity frequency to be precisely synchronised to the beam. Two identical cavities are inserted into a special cryomodule that serves as a high-performance thermos flask under high vacuum, thus reducing the heat load and stray magnetic fields from the outside environment and keeping the cavities stable at 2 K. This is made feasible with different layers of materials all properly thermalised in a staged fashion and under high vacuum conditions to minimise the total footprint. The most external layer of the cryomodule is the vacuum vessel. A complex cryogenic circuit with staged temperatures also allows the passage of the cold helium (2 K and 50 K) to cool the cavity ancillaries such as higher-order mode couplers and RF lines. The cryomodule also serves as the support structure and keeps the two cavities precisely aligned.
Successful operation of the crab cavities depends on their correct position and orientation, and the HL-LHC places tight constraints on the cavity alignment; in particular, the transverse displacement of one cavity with respect to another should not exceed 500 μm. To determine the cavity alignment relative to targets positioned outside the cryostat, a position monitoring system five to ten times more precise is needed. Frequency scanning interferometry was chosen for this task, whereby a laser beam is sent inside the cryomodule and reflected off several reflectors placed on the cavity interfaces to track their movements. The optical path of the measurement beam is then compared with a beam from a reference interferometer, offering absolute interferometric distance measurements with sub-micrometre precision.
Early last year, two superconducting prototype DQW-type crab cavities manufactured at CERN underwent RF tests in a superfluid helium bath at a temperature of 2 K. These first cavity tests demonstrated a maximum transverse-kick voltage exceeding 5 MV, surpassing the nominal operational voltage of 3.4 MV. The corresponding electric and magnetic fields on the cavity surfaces were 57 MV/m and 104 mT, respectively (for comparison, the KEKB cavities reached a maximum of 2.5 MV kick voltage in similar tests). Prototypes of the DQW and the RFD cavities were built in the US and reached even higher fields than the CERN prototypes, demonstrating the robustness of the RF design.
By the end of 2017, the two crab cavities were assembled at CERN into a special cryomodule to allow operation in an accelerator environment. Its design was a joint effort between CERN and the UK. The module was successfully RF-tested at 2 K in December at CERN’s SM18 facility (figure 4, lower right), validating the mechanical, cryogenic and RF functioning prior to its installation in the SPS for beam tests. The complete installation of the cryomodule and support infrastructure, requiring a new high-power RF and cryogenic system, had to be finished in a period of eight weeks during the year-end technical stop of the SPS. The rush was dictated by the operation schedule of the CERN accelerator complex: after 2018, all the accelerators of the injector complex will be stopped for two years, to undergo a major upgrade in preparation for the HL-LHC.
Ready for beam
Proton beam tests of the compact crab cavities in the SPS are considered a prerequisite before installation into the LHC itself. The aim is to demonstrate the operational performance of the cavityand transparency throughout the energy cycle and to study long-term effects on proton beams and failure modes.
A 15 m long section of the SPS ring was identified as suitable for installation of the in-beam crab-cavity test stand. The beam line was equipped with two articulated, Y-shaped vacuum chambers to provide a bypass to the circulating beam, with highly flexible bellows allowing for a lateral displacement of approximately 51 cm (figure 5, middle). Via this articulated continuous connection of the vacuum beam pipe, the crab-cavity test module can remain parked out of the beamline during regular operation of the SPS and be transferred back into the beamline during periods dedicated to crab-cavity testing. This is essential because the entrance diameter of the crab cavities is smaller than what is needed for beam extraction to the LHC, plus there is an inherent operational risk associated with having a prototype element inserted in the beamline of the main injector to the LHC. The motorised transfer table, produced by Added Value Solutions (Spain), also supports the Y-chambers, the two RF circulators and passive loads, and a cryogenic valve box – corresponding to a total load of about 15 tonnes, which is moved in and out of the beam with a positioning precision of a few micrometres.
Due to limited space and accessibility in the underground areas of the SPS, the RF amplifiers required to power the cavities were placed in a surface building and RF power routed via coaxial lines to the cryomodule. The cold box for the cryogenic refrigeration system, by contrast, was installed underground because transporting liquid helium along a vertical pipeline increases losses by evaporation. Helium is liquefied in the cold-box and routed to the cryomodule via a 110 m long cryogenic distribution line. A comprehensive beam-test programme is now under preparation. In parallel to the installation of the prototype cryomodule in the SPS, an effort has started to fabricate at CERN two RFD prototype cavities. They will then be assembled at Daresbury (UK) into a cryomodule that will be tested in the SPS after the second LHC long shutdown. An industrial contract for the production of DQW cavities for the full crab-cavity system for the HL-LHC was finalised in December 2017 with Research Instruments in Germany, and the industrial production of the RFD cavities, which is now under the responsibility of a collaboration with the US, is also ramping up.
Superconducting crab cavities and RF deflectors have a wide range of applications other than high-energy physics. The significant contribution of the HL-LHC developments to ultra-compact and very high-field cavities are already influencing new proposals for luminosity improvements similar to the HL-LHC for electron–hadron colliders, bunch compression in light sources to produce sub-picosecond photon pulses, and ultrafast particle separators in proton linacs as a means to separate bunches of secondary particles for different experiments.
It is now also evident that crab cavities are useful beyond the HL-LHC for even higher energy colliders. In the proton-proton version of the Future Circular Collider (FCC) study, the bunches would be squeezed at the collision point by a factor of 2–4 compared with that of the HL-LHC. Without crab cavities to compensate, only 20% of the available peak luminosity would be exploited by the machine. It is clear that this advanced RF technology, taken further than before by its adaption to the HL-LHC, has an extremely bright future for high-energy physics and beyond.
Superconductors are poor thermal conductors. Whenever a superconducting state is established in a given material, a large fraction of the conduction electrons are frozen in Cooper pairs and become unavailable for heat transport. This can have serious practical implications: at low temperatures, even a small amount of localised heating can drive the material into a normal conducting state, triggering an avalanche process (or quench) that destroys superconductivity in the whole device. It is therefore good practice in applied superconductivity to stabilise superconductors with a high-thermal-conductivity metal. The superconducting niobium–titanium filaments in strands used for accelerator magnets such as those in the LHC, for example, are usually embedded in a copper matrix to spread out any small fluctuation in temperature.
In the world of superconducting radio-frequency (SRF) cavities, which are used to accelerate charged particles in accelerators around the globe, this technique is the exception rather than the rule. Today, mainstream SRF technology makes use of bulk niobium sheets to build the entire resonator structure, circumventing the problem of poor thermal conductivity by using material of very high purity. In the past 40 years, great leaps forward have brought bulk-niobium cavity performances close to what is considered the intrinsic limit of the material, with accelerating fields of the order 50 MV/m in elliptical structures.
However, things were not always this way. In the 1960s, lead (which is a type-I superconductor) was electroplated on copper RF resonators used for beam acceleration at several high-energy physics facilities around the world. As the RF currents only penetrate a few tens of nanometres in the cavity wall, a few-micron-thin superconducting layer on a high-thermal-conductivity copper substrate provided an elegant solution to the problem of thermal stabilisation, in perfect analogy to what happens in superconducting strands for magnets.
Coated RF cavities (figure 1) offered another advantage: they allowed the function of producing high electric fields to be easily decoupled from that of giving enough mechanical stability, which is required to control the field amplitude and phase sufficiently for beam acceleration. The main drawback of lead-plated cavities was their relatively low accelerating field, limited by the critical magnetic field of lead. A natural step forwards was to use niobium, whose critical field is about 2.5 times higher. Unfortunately, the synthesis of good-quality niobium films on copper was much more difficult, and in the 1970s the research quickly turned to using bulk niobium as a cavity material.
Niobium-coated cavities
In 1984, Cristoforo Benvenuti, Nadia Circelli and Max Hauer from CERN (figure 2) published a seminal paper on niobium films for superconducting accelerating cavities. These films were deposited on copper cavity substrates by sputtering, which was reported to give encouraging results on real cavities. It was the start of a successful development, which in only a few years led to the greatest achievement of niobium–copper technology: the SRF system of the upgraded Large Electron Positron collider (LEP), which operated at CERN in the mid- to late-1990s. This consisted of 288 four-cell elliptical cavities working at a frequency of 352 MHz, the vast majority of which were produced with niobium–copper technology in three European industries. During the early times of LEP, niobium–copper cavities could outperform their bulk niobium counterparts at LEP’s nominal fields. Besides being cheaper and free from quenches, they also revealed unexpected insensitivity to trapped magnetic flux. This is a peculiar problem of superconducting cavities that can spoil their performance unless great care is used to shield the cavity from any magnetic fields when it undergoes the superconducting transition. The need for magnetic shielding added to the cost of the bulk niobium systems, whereas LEP’s niobium–copper cavities could operate without shielding.
In the meantime, as coated-cavity technology took off at CERN, bulk niobium technology was progressing fast: all over the world in national laboratories and in industry, the SRF community removed all of the obstacles one after the other in the quest for higher accelerating fields and lower power dissipation (expressed by the unloaded quality factor, Q). Nowadays, state-of-the-art bulk niobium cavities are cutting-edge technology objects made from high residual-resistivity-ratio (RRR) niobium sheets that are shaped and electron-beam welded with the utmost precision. They must be assembled according to high cleanliness standards borrowed from the semiconductor industry to prevent electron-field emission. They need to be carefully shielded from the Earth’s and other parasitic magnetic fields, operated in superfluid helium, and employ complex feedback systems and high installed RF power to combat the effects of microphonics (vibrations that detune the cavities). The result is outstanding in terms of accelerating field and Q, now approaching the theoretical limits. Bulk niobium cavities can be produced in industry and are operating reliably in accelerators at several facilities worldwide, such as the European X-ray free electron laser (XFEL) in Hamburg (CERN Courier July/August 2017 p25).
In contrast, niobium–copper cavities suffered from a problem that had been present since the start: at high accelerating fields, the cavity Q value decreased faster than it did in the case of bulk niobium. This phenomenon is still not fully explained today. During and after the LEP era, a great deal of research was carried out at CERN on 1.5 GHz niobium–copper cavities to tackle the issue. Despite significant progress in understanding and remarkable cavity results, the gap in performance compared with bulk niobium was not bridged. This prevented niobium-coated copper cavities from being considered for the high-energy linear colliders under study at the time – namely TESLA, the technology of which has since morphed into that underpinning the European XFEL and the International Linear Collider proposal (CERN Courier September 2017 p27).
At medium accelerating fields, like those required for circular hadron machines and many other applications, the drop in Q was not a showstopper. For the LHC, the niobium–copper technology was applied to build the 16 single-cell 400.8 MHz elliptical cavities required in the machine’s accelerating sections. The LHC cavities, like their LEP ancestors, also work at a temperature of 4.5 K – the cryogenics for which is cheaper and more robust than that used for bulk niobium devices.
In the 1990s, niobium–copper cavities of another shape, adapted for heavy-ion acceleration, were employed for the ALPI linac in INFN Legnaro, Italy. Here, Vincenzo Palmieri and collaborators developed niobium-sputtered quarter-wave resonators (QWRs) on copper substrates. A total of 58 niobium–copper cavities are operational today in ALPI, replacing old lead-plated cavities and considerably extending the energy reach of the machine.
Revival at ISOLDE
During the construction of the LHC in the early 2000s, the SRF activities and infrastructures at CERN were down-sized as resources were focused on the production of the LHC’s superconducting magnets. However, the need for a new SRF system came back in a CERN project in 2009, when a proposal was approved for a high-energy upgrade of the ISOLDE facility using a superconducting linac booster for the radioactive ion beams. For this application, niobium–copper technology was considered particularly well suited because the absence of beam loading allows the stiff, niobium-coated copper cavities to be operated at very narrow RF bandwidths, leading to significant savings in installed RF power. To support the high-energy HIE-ISOLDE upgrade, CERN invested in rebuilding its SRF infrastructure and expertise.
Today, thanks to the collective effort of several CERN teams, the high-beta section of the HIE-ISOLDE linac is complete (figure 3). The work for HIE ISOLDE also offered an opportunity to advance understanding of the limitations to niobium–copper cavity performance. One particular issue was the frequent appearance of defects on the copper substrate, especially close to the electron-beam weld. To overcome this problem, towards the end of the production, a new design for the RF cavity was proposed, which made possible machining the whole resonator out of a copper billet and thus avoided any weld (figure 1). The results of the change were very encouraging.
The first two cavities, manufactured with this technique in industry and coated at CERN, were tested at the end of 2017. Their RF performance scored top of a series of 20 units. Even more strikingly, when cooled down close to superfluid helium temperatures with active shielding of the ambient field, a cavity reached unprecedented peak fields for niobium–copper technology (figure 4). Incredibly, the RF performance of this cavity is comparable to bulk niobium cavities with the same shape, at least as far as the Q-slope is concerned. This result is now the basis of exploring new possible applications, notably for the acceleration of higher-beta beams like those required in spallation sources for accelerator-driven systems.
At about the same time, another excellent result was achieved in the context of the LHC spare-cavities programme. Newly coated cavities are giving results that lie on an upward learning curve, and have already surpassed the LHC specifications. Achieving such good RF performances is only possible if high quality standards are maintained along the whole production chain, from manufacturing of the copper substrate, to chemical polishing, ultra-pure water rinsing, cleanroom assembly and coating, to the final RF test at cryogenic temperatures. This requires a close collaboration of various teams of specialists, and CERN is an ideal place for that.
These two recent achievements are proof that the potential of niobium–copper technology is not exhausted, and that these cavities could be as high performing as their bulk niobium counterparts. Indeed, this technology is already being considered within the Future Circular Collider study, led by CERN to explore the feasibility of a 100 km-circumference machine. Clearly, the long march of niobium on copper is far from over.
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