On 2 May, the Thomas Jefferson National Accelerator Facility in Virginia, US, celebrated the completion of the 12 GeV CEBAF upgrade project. The $338 million upgrade has tripled CEBAF’s original operating energy and will allow, among other studies, more in-depth investigations of nuclear confinement.
CEBAF (Continuous Electron Beam Accelerator Facility) provides high-quality beams of polarised electrons that allow physicists to extract information on the quark and gluon structure of nucleons.
The CEBAF accelerator started up in 1994 and originally delivered 4 GeV beams, which were later pushed to 6 GeV thanks to efficiencies in the machine’s design and extensive experience gained during operation. Previously, CEBAF operated as a pair of superconducting radio-frequency linear accelerators in a “racetrack” configuration, capable of delivering 6 GeV electron beams simultaneously to three experimental halls. In 2008 work began on a major upgrade project to double the maximum energy and add new
experimental setups.
The 12 GeV CEBAF upgrade project required 10 high-performance, superconducting radio-frequency cryomodules, doubling the capacity of the existing cryogenics plant, and the addition of eight superconducting magnets and other system upgrades. The upgrade also includes the construction of a new experimental area (“hall D”) for dedicated research on exotic mesons produced by energetic photons incident on a target. CEBAF’s newly energetic and precise beams will enable the first 3D views of the structure of protons and neutrons, the study of the origin of confinement in QCD, and the investigation of physics beyond the Standard Model by testing the theory’s completeness at low energies.
A new facility called the Higgs Centre for Innovation opened at the Royal Observatory in Edinburgh on 25 May as part of the UK government’s efforts to boost productivity and innovation. The centre, named after Peter Higgs of the University of Edinburgh, who shared the 2013 Nobel Prize in physics for his theoretical work on the Higgs mechanism, will offer start-up companies direct access to academics and industry experts. Space-related technology and big-data analytics are the intended focus, and up to 12 companies will be based there at any one time. According to a press release from the UK Science and Technology Facilities Council (STFC), the facility incorporates laboratories and working spaces for researchers, and includes a business incubation centre based on the successful European Space Agency model already in operation in the UK.
“Professor Higgs’ theoretical work could only be proven by collaboration in different scientific fields, using technology built through joint international ventures,” said principal and vice-chancellor of the University of Edinburgh Peter Mathieson. “This reflects the aims and values of the Higgs Centre for Innovation, which bring scientists, engineers and students together under one roof to work together for the purpose of bettering our understanding of space-related science and driving technological advancement forward.”
The Higgs Centre for Innovation was funded through a £10.7 million investment from the UK government via STFC, which is also investing £2 million over the next five years to operate the centre.
On 28 May, the world’s largest and most sensitive detector for direct searches of dark matter in the form of weakly interacting massive particles (WIMPs) released its latest results. XENON1T, a 3D-imaging liquid-xenon time projection chamber located at Gran Sasso National Laboratory in Italy, reported its first results last year (CERN Courier July/August 2017 p10). Now, the 165-strong international collaboration has presented the results from an unprecedentedly large exposure of approximately one tonne × year.
The results are based on 1300 kg out of the total 2000 kg active xenon target and 279 days of data-taking, improving the sensitivity by almost four orders of magnitude compared to XENON10 (the first detector of the XENON dark-matter project, which has been hosted at Gran Sasso since 2005). The data are consistent with background expectations, and place the most stringent limit yet on spin-independent interactions of WIMPs with ordinary matter for a WIMP mass higher than 6 GeV/c².
XENON1T spokesperson Elena Aprile of Columbia University in the US describes the result as a milestone in dark-matter exploration. “Showing the result after a one tonne × year exposure was important in a field that moves fast,” she explains. “It is also clear from the new result that we will win faster with a yet-larger mass and lower radon background, which is why we are now pushing the XENONnT upgrade.”
CERN’s radioactive ion-beam facility ISOLDE has stamped a new coin in its impressive collection. Long considered the domain of high-energy, in-flight rare-isotope facilities, chromium has now been produced at ISOLDE in prodigious quantities, thanks to a new resonant ionisation laser-ion source (RILIS) scheme. Together with the latest calculations based on chiral effective field theory, the result provides important guidance for improving theoretical approaches that bridge the gap between nuclear matter and the low-energy extension of quantum chromodynamics (QCD).
Certain configurations of protons and neutrons are more bound than others, revealing so-called magic numbers. Chromium has 24 protons, situating it squarely between magic calcium (with 20 protons) and nickel (with 28). Of particular interest to nuclear physics are isotopes with a large excess of neutrons.
The RILIS is a chemically selective ion source which relies on resonant excitation of atomic transitions using a tunable laser. In the new ISOLDE experiment, Maxime Mougeot of CSNSM/Université Paris-Saclay and collaborators used RILIS to venture 10 neutrons further on the nuclear chart to 63Cr. With a total of 39 neutrons, 63Cr lies exactly between the magic neutron numbers 28 and 50 and has a half-life of just 130 ms.
The masses of the newly forged chromium isotopes, as measured by ISOLDE’s precision Penning-trap mass spectrometer ISOLTRAP, offer insights into its shape and structure. Magic-number nuclides have filled orbitals that favour spherical shapes, but not so the chromium nuclides weighed by ISOLTRAP, which are deformed.Whereas in some areas of the nuclear chart deformation sets in very suddenly with the addition of a further neutron, the remarkably smooth neutron binding energies of chromium show that deformation sets in very gradually – contrary to previous conclusions.
The ISOLDE measurements were compared with different theoretical results, including a very first attempt by a new ab-initio approach called valence-space in-medium similarity renormalization group (VS-IMSRG). While several ab-initio approaches exist, until now they have been restricted to the near-spherical cases that have very few valence protons and neutrons. The latest VS-IMSRG results are the first for such open-shell nuclides.
“It turns out that the ab-initio VS-IMSRG, an interaction derived from chiral effective field theory which reduces QCD to its relevant degrees of freedom at the nuclear scale, failed to predict these results,” explains Mougeot. “So the recent chromium measurements are constructive and important for advancing this promising technique, which bridges the gap between first-principle calculations and the structure of nuclei at the extremes of the nuclear landscape.”
Bottomonium mesons, composed of beauty quark–antiquark pairs bound to each other through the strong force, play a special role in our understanding of hadron formation because the large quark mass allows important simplifications in the relevant theoretical calculations.
The spectroscopy of the bottomonium family has now been significantly upgraded, thanks to the first observation of the individual χb1(3P) and χb2(3P) states by the CMS collaboration. Identified via the decay χb(3P) → Υ(3S) γ, and adding for the first time all the LHC data collected at an energy of 13 TeV (corresponding to a staggering 80 fb–1 of integrated luminosity), CMS detected 16.5 million Υ mesons in the dimuon decay channel. The corresponding invariant mass distribution shows well-resolved Υ(1S), Υ(2S) and Υ(3S) resonances (figure 1, inset), which constitute the starting point for the reconstruction of the p-wave bottomonia through the radiative decay χb(mP) → Υ(nS) γ.
The main challenge in this study is the low energy of the photons. The CMS analysis uses photons that convert into e+e– pairs and are reconstructed in the silicon tracker with very high precision, leading to clear χb(mP) peaks in the resulting Υ(nS) γ invariant mass distributions. The resolution of the χb mass measurement scales with the photon energy, or the difference between the masses of the P- and S-wave mesons. The Υ(3S) γ invariant mass is measured with a remarkable resolution, enabling the first observation of a double-peak structure in the χb(3P) resonance, which corresponds to the states of total angular momentum J = 1 and J = 2 (figure 2).
The existence of two peaks is established with a significance exceeding nine standard deviations and the two masses are measured to be 10,513.42 ± 0.41 (stat) ± 0.18 (syst) MeV and 10,524.02 ± 0.57 (stat) ± 0.18 (syst) MeV. The measured mass splitting, 10.60 ± 0.64 (stat) ± 0.17 (syst) MeV, can be used to improve the theoretical calculations, which currently predict values between 8 and 18 MeV depending on the potentials describing the quark–antiquark non-perturbative interaction. The only exception predicts a value of –2 MeV, the negative sign meaning that the χb2(3P) has a mass smaller than the χb1(3P).
The new measurement is a step forward in completing the spin-dependent bottomonium spectroscopy diagram, and should significantly contribute to an improved understanding of the non-perturbative QCD processes that lead to the binding of quarks and gluons into hadrons.
Last year, the LHCb collaboration announced the first observation of the Ξcc++ baryon, a doubly charmed particle (CERN Courier July/August 2017 p8). It was identified via the decay Ξcc++→ Λc+ K–π+π+, with the Λc+ baryon subsequently decaying to pK–π+. Since then, LHCb has carried out a campaign of further studies to pinpoint the properties of this special particle, namely looking for additional Ξcc++ decays and, more importantly, measuring its lifetime.
LHCb has now reported a first measurement of the Ξcc++ lifetime, exploiting the same decay mode and using a data sample and an event selection similar to those used in the first observation. The experimental technique used is to measure the decay-time distribution relative to that of another decay with a similar topology, Λb0→ Λc+π–π+π–. As the lifetime of the Λb0 is already known with high precision from previous measurements, once the ratio of efficiencies for reconstructing the Ξcc++ and Λb0 decays is determined, it is possible to derive the lifetime of the Ξcc++ baryon from its decay-time distribution (see figure).
The lifetime value that is obtained is 256+24–22 (stat) ± 14 (syst) fs. Relatively large lifetimes like this are a distinctive feature of weak interactions. In addition, LHCb has also observed a new Ξcc++ decay: Ξcc++→ Ξc+π+, with a statistical significance of about six standard deviations, thus confirming the first observation of Ξcc++ in an independent analysis. The baryon’s mass is measured to be 3620.6 ± 1.5 (stat) ± 0.5 (syst) MeV/c2, which is consistent with the previous result.
Turning to a separate analysis, a puzzling result has emerged at LHCb while measuring the lifetime of another charmed baryon: the Ωc0. The sample of LHCb data used for this measurement comprises about 1000 Ωb−→ Ωc0μ−νμX signal decays, where the Ωc0 baryon is detected via the decay Ωc0→ pK−K−π+ and X represents possible additional undetected particles in the decay. The Ωc0 lifetime is determined from the observed decay-time distribution to be 268 ± 24 (stat) ± 10 (syst) fs (see figure).
Quite surprisingly, this value is nearly four times larger than, and inconsistent with, the current world average of 69 ± 12 fs. This average is based on three experimental results from fixed-target experiments, namely E687, FOCUS and WA89, where each experiment observed only a few dozen events with relatively large background. The new measurement from LHCb redefines the lifetime hierarchy of charmed baryons, placing the Ωc0 baryon as having the second largest lifetime after the Ξc+ baryon, i.e. τ(Ξc+) > τ(Ωc0) > τ(Λc+) > τ(Ξc0). This result may lead to reconsideration of the relative importance of the roles of spectator quarks and of non-perturbative effects in the decay dynamics of hadrons containing heavy quarks.
According to the Standard Model (SM), fermions acquire their mass through coupling to the Higgs field. New results released by the ATLAS collaboration firmly establish and measure these so-called Yukawa couplings to third-generation fermions. The Higgs-boson coupling to top quarks has been observed in associated production with a top quark pair (ttH production), and the Higgs-boson coupling to tau leptons has been observed in Higgs-boson decays to two tau leptons (H → ττ). Data from LHC proton–proton collisions at a centre-of-mass energy of 13 TeV recorded during 2015, 2016 and 2017 were analysed for these results.
The measurement of H → ττ, which is based on 2015 and 2016 data, was challenging because the tau lepton is short- lived and can only be observed through its decay products, of which at least one is always an invisible neutrino. The unknown momentum taken away by the neutrino makes the tau reconstruction incomplete and thus susceptible to backgrounds. Events with tau leptons are difficult to select online when the visible tau decay products are hadrons. Moreover, the Z boson, which also decays to a tau-lepton pair and is relatively close in mass to the Higgs boson but much more abundant, represents a large source of background. Good reconstruction of the di-tau invariant mass is therefore essential, using information from all detector systems to account for the missing energy.
The measured H → ττ signal has an observed (expected) statistical significance of 6.4 (5.4) standard deviations when combined with previous measurements using 7 and 8 TeV data. In 13 TeV data, the total cross-section times branching fraction was measured to be 3.71 ± 0.59 (stat) +0.87–0.74 (syst) pb. In addition, separate measurements of the gluon fusion and weak-boson-fusion Higgs-boson production cross sections were performed (figure, top). SM predictions agree with these measurements.
The production of ttH was measured from a combination of channels involving Higgs-boson decays to a pair of W or Z bosons (WW* or ZZ*), tau leptons, b-quarks or photons. The analyses exploiting the H → γγ and H → ZZ* → 4l decays used the full 80 fb–1 proton–proton dataset collected by ATLAS between 2015 and 2017, and deployed improved reconstruction algorithms and new analysis procedures based on machine learning. The H → γγ analysis alone observed a ttH signal with a significance of 4.1 standard deviations for 3.7 expected in the SM. The H → 4l analysis expected less than one event from ttH production in the 80 fb–1 dataset and observed no event.
These results, combined with those from the other ttH channels based on 2015 and 2016 data, led to an observed (expected) significance of 5.8 (4.9) standard deviations for ttH production at 13 TeV, with a ratio of measured to predicted cross section of 1.32 . Further combination with the results from Run 1 based on data taken at 7 and 8 TeV centre-of-mass energies yielded an observed significance of 6.3 standard deviations for 5.1 expected. The measured total cross-section for ttH production at 13 TeV is 670 ± 90 (stat) ± 110 (syst) fb, in agreement with the SM prediction of 507 fb. The corresponding result at 8 TeV is 220 ± 100 (stat) ± 70 (syst) fb (figure, bottom).
With further data being collected at the LHC, more precise measurements of cross-sections and differential distributions will allow the study of the structure of Yukawa couplings in great detail and thus provide more stringent tests of the SM and increased sensitivity to physics beyond it.
One of the key goals in exploring the properties of QCD matter is to determine the minimum value of the shear viscosity to entropy density ratio (η/s) for an ideal fluid. In heavy-ion collisions at the LHC, a quark-gluon plasma (QGP) is created, which is a state of hot and dense matter where quarks and gluons become deconfined. The plasma is formed at early times in the collisions and subsequently cools down to a temperature where the quarks and gluons cluster together into hadrons. The value of η/s is of particular interest, as weak coupling QCD and anti-de-Sitter/conformal field theory (AdS/CFT) theories predict different values. AdS/CFT is a technique from string theory that can be used to understand a strongly coupled system. The value of η/s implied by AdS/CFT is approximately 0.05–0.08, with calculations based on perturbative QCD techniques giving larger values.
The ALICE collaboration has recently released results of anisotropic-flow measurements from xenon–xenon (Xe–Xe) collisions at a per-nucleon centre of mass energy of 5.44 TeV, which offer additional constraints for the viscosity of the QGP. The anisotropic flow observed in a heavy-ion collision results from the spatial anisotropy of the initial collision zone, which is converted to momentum anisotropies via pressure gradients during the system̓s evolution. The magnitudes of momentum anisotropies are quantified by the harmonic coefficients νn of a Fourier expansion of the azimuthal distribution of particles; ν2 is generated by initial states with an elliptic shape, ν3 a triangular shape, and so on. The magnitude of νn depends not only on η/s, but also depends on the magnitude of the azimuthal asymmetries in the initial density distribution in the collisions. Comparing the new results from Xe–Xe collisions to those from lead–lead (Pb–Pb) collisions is expected to provide stronger constraints in the initial matter distribution, which will, in turn, provide a more precise determination of η/s.
The figure shows measurements of νn vs centrality for both Xe–Xe and Pb–Pb collisions. Centrality is a measure of the degree of overlap in heavy-ion collisions, where 0% corresponds to collisions that are head-on, and for 100% the heavy-ions do not overlap enough to interact. For mid-central collisions (20–70%), the second harmonic coefficients of the initial matter distributions are predicted to be very similar for Xe–Xe and Pb–Pb from various initial-state models. At the same centrality, however, the Xe–Xe system size is smaller than Pb–Pb and the impact of a finite η/s suppresses ν2 by 1/R, where R corresponds to the transverse size of the system. Therefore, ratios of Xe–Xe/Pb–Pb ν2 coefficients in the mid-centrality range could be directly sensitive to η/s, with larger values of η/s leading to a greater suppression of this ratio. When comparing our data to two different hydrodynamic models, which use parameters of η/s close to the values from AdS/CFT calculations, we find a good agreement with the data.
This shows that η/s is small, which implies a short mean-free path for the quarks and gluons in the QGP, or strong interactions. In central collisions, the ν2 in Xe–Xe collisions is larger than in Pb–Pb collisions. This is due to the 129Xe nucleus not being exactly spherical and to larger fluctuations of the initial density distributions for the smaller Xe nucleus. The latter also gives rise to larger values of ν3 in the centrality range of 0–50%.
For decades, theoretical models of galaxy evolution have predicted that the supermassive black hole lying at the heart of the Milky Way is surrounded by thousands of smaller black holes left behind by dying stars. Testing such theories is important to understand our own galaxy and, more generally, to understand how galaxies evolve and how black holes are produced. Now, observations by NASA’s Chandra X-ray Observatory have revealed a dozen stellar-mass black holes at the centre of the galaxy, providing the first observational evidence for such a black-hole cluster.
Black holes emit virtually no radiation, so it’s not possible to detect them when they are isolated and located at large distances from Earth. But many black holes have close stellar companions from which they accrete matter and, as this matter is sucked into the black hole, it heats up and emits X-rays that can be detected on Earth. If only a few of the thousands of the stellar-mass black holes that are predicted to exist in the galactic centre had a companion star, at least this binary fraction of the total black-hole population would be detectable by X-ray telescopes.
Using Chandra data, a group led by Chuck Hailey of Columbia University in New York searched for such black-hole binary systems in a region extending several light years from the galactic centre. This type of search is confounded by two aspects: the high density of other X-ray-emitting objects in the same region, such as binary systems containing neutron stars or white dwarfs instead of black holes; and the relatively low intensity of the X-ray binary sources in the region. But in their study, Hailey and colleagues were able to distinguish between the different types of weak X-ray binary system in the region by studying their spectra.
The researchers examined the Chandra spectra of 415 weak X-ray point sources, containing as few as 100 counts, and looked for the expected spectral features of black-hole binaries. They found 12 sources that have the expected spectral characteristics of black-hole binaries, all within a radius of three light years from the supermassive black hole (see figure). Other X-ray sources whose spectra match well with those of white-dwarf binary systems were found to be distributed at larger distances from the galactic centre.
The researchers went on to estimate the total number of black-hole binary systems in the observed region, assuming that the 12 sources are the brightest in their family and using the known fluxes of brighter and well-studied black-hole binary systems. This resulted in about 300–1000 binary black holes, which is a lower limit on the total number because it only includes those with companion stars. According to theoretical follow-up work by Aleksey Generozov of Columbia and colleagues, the total number of black holes should be between 10,000 and 40,000.
The results, published in Nature, agree with the theoretical predictions and therefore confirm the existing models of galaxy evolution. What’s more, the findings allow astronomers to predict the number of black-hole mergers – and thus the number of gravitational waves – from this region.
Fundamental insights into the constituents of matter have been gained by observing what happens when beams of high-energy particles collide. Electron–positron, proton–proton, proton–antiproton and electron–proton colliders have all contributed to the development of today’s understanding, embodied in the Standard Model of particle physics (SM). The Large Hadron Collider (LHC) brings 6.5 TeV proton beams into collision, allowing the Higgs boson and other SM particles to be studied and searches for new physics to be carried out. To reach physics beyond the LHC will require hadronic colliders at higher energies and/or lepton colliders that can deliver substantially increased precision.
A variety of options are being explored to achieve these goals. For example, the Future Circular Collider study at CERN is investigating a 100 km-circumference proton–proton collider with beam energies of around 50 TeV the tunnel for which could also host an electron–positron collider (CERN Courier June 2018 p15). Electron–positron annihilation has the advantage that all of the beam energy is available in the collision, rather than being shared between the constituent quarks and gluons as it is in hadronic collisions. But to reach very high energies requires either a state-of-the-art linear accelerator, such as the proposed Compact Linear Collider or the International Linear Collider, or a circular accelerator with an extremely large bending radius.
Muons to the fore
A colliding-beam facility based on muons has a number of advantages. First, since the muon is a lepton, all of the beam energy is available in the collision. Second, since the muon is roughly 200 times heavier than the electron and thus emits around 109 times less synchrotron radiation than an electron beam of the same energy, it is possible to produce multi-TeV collisions in an LHC-sized circular collider. The large muon mass also enhances the direct “s-channel” Higgs-production rate by a factor of around 40,000 compared to that in electron–positron colliders, making it possible to scan the centre-of-mass energy to measure the Higgs-boson line shape directly and to search for closely spaced states.
Stored muon beams could also serve the long-term needs of neutrino physicists (see box 1). In a neutrino factory, beams of electron and muon neutrinos are produced from the decay of muons circulating in a storage ring. It is straightforward to tune the neutrino-beam energy because the neutrinos carry away a substantial fraction of the muon’s energy. This, combined with the excellent knowledge of the beam composition and energy spectrum resulting from the very well-known characteristics of muon decays, makes the neutrino factory the ideal place to make precision measurements of neutrino properties and to look for oscillation phenomena that are outside the standard, three-neutrino-mixing paradigm.
Given the many benefits of a muon collider or neutrino factory, it is reasonable to ask why one has yet to be built. The answer is that muons are unstable, decaying with a mean lifetime at rest of 2.2 microseconds. This presents two main challenges: first, a high-intensity primary beam must be used to create the muons that will form the beam; and, second, once captured, the muon beam must be accelerated rapidly to high energy so that the effective lifetime of the muon can be extended by the relativistic effect of time dilation.
One way to produce beams for a muon collider or neutrino factory is to harness the muons produced from the decay of pions when a high-power (few-MW), multi-GeV proton beam strikes a target such as carbon or mercury. For this approach, new proton accelerators with the required performance are being developed at CERN, Fermilab, J-PARC and at the European Spallation Source. The principle of the mercury target was proved by the MERIT experiment that operated on the Proton Synchrotron at CERN. However, at the point of production, the tertiary muon beam emerging from such schemes occupies a large volume in phase space. To maximise the muon yield, the beam has to be “cooled” – i.e. its phase-space volume reduced – in a short period of time before it is accelerated.
The proposed solution is called ionisation cooling, which involves passing the beam through a material in which it loses energy via ionisation and then re-accelerating it in the longitudinal direction to replace the lost energy. Proving the principle of this technique is the goal of the Muon Ionization Cooling Experiment (MICE) collaboration, which, following a long period of development, has now reported its first observation of ionisation cooling.
An alternative path to a muon collider called the Low Emittance Muon Accelerator (LEMMA), recently proposed by accelerator physicists at INFN in Italy and the ESRF in France, provides a naturally cooled muon beam with a long lifetime in the laboratory by capturing muon–antimuon pairs created in electron–positron annihilation (see box 2).
Cool beginnings
The benefits of a collider based on stored muon beams were first recognised by Budker and Tikhonin at the end of the 1960s. In 1974, when CERN’s Super Proton Synchrotron (SPS) was being brought into operation, Koshkarev and Globenko showed how muons confined within a racetrack-shaped storage ring could be used to provide intense neutrino beams. The following year, the SPS proton beam was identified as a potential muon source and the basic parameters of the muon beam, storage ring and neutrino beam were defined. It was quickly recognised that the performance of this facility—the first neutrino factory to be proposed – could be enhanced if the muon beam was cooled. In 1978, Budker and Skrinsky identified ionisation cooling as a technique that could produce sufficient cooling in a timeframe short compared to the muon lifetime and, the following year, Neuffer proposed a muon collider that exploited ionisation cooling to increase the luminosity.
The study of intense, low-emittance muon beams as the basis of a muon collider and/or neutrino factory was re-initiated in the 1990s, first in the US and then in Europe and Japan. Initial studies of muon production and capture, phase-space manipulation, cooling and acceleration were carried out and neutrino- and energy-frontier physics opportunities evaluated. The reduction of the tertiary muon-beam phase space was recognised as a key technological challenge and at the 2001 NuFact workshop the international MICE collaboration was created, comprising 136 physicists and engineers from 40 institutes in Asia, Europe and the US.
The MICE cooling cell, in common with the cooling channels studied since the seminal work of the 1990s, is designed to operate at a beam momentum of around 200 MeV/c. This choice is a compromise between the size of the ionisation-cooling effect and its dependence on the muon energy, the loss rate of muon-beam intensity through decay, and the ease of acceleration following the cooling channel. The ideal absorber has, at the same time, a large ionisation energy loss per unit length (to maximise ionisation cooling) and a large radiation length (to minimise heating through multiple Coulomb scattering). Liquid hydrogen meets these requirements and is an excellent absorber material; a close runner-up, with the practical advantage of being solid, is lithium hydride. MICE was designed to study the properties of both. The critical challenges faced by the collaboration therefore included: the integration of high-field superconducting magnets operating in a magnetically coupled lattice; high-gradient accelerating cavities capable of operation in a strong magnetic field; and the safe implementation of liquid-hydrogen absorber modules – all solved through more than a decade of R&D (see box 3).
In 2003 the MICE collaboration submitted a proposal to mount the experiment (figure 1) on a new beamline at the ISIS proton and muon source at the Science and Technology Facilities Council’s (STFC) Rutherford Appleton Laboratory in the UK. Construction began in 2005 and first beam was delivered on 29 March 2008. The detailed design of the spectrometer solenoids was also carried out at this time and the procurement process was started. During the period from 2008 to 2012, the collaboration carried out detailed studies of the properties of the beam delivered to the experiment and, in parallel, designed and fabricated the focus-coil magnets and a first coupling coil.
Delays were incurred in addressing issues that arose in the manufacture of the spectrometer solenoids. This, combined with the challenges of integrating the four-cavity linac module with the coupling coil, led, in November 2014, to a reconfiguration of the MICE cooling cell. The simplified experiment required two, single-cavity modules and beam transport was provided by the focus-coil modules. An intense period of construction followed, culminating with the installation of the spectrometer solenoids and the focus-coil module in the summer of 2015. Magnet commissioning progressed well until, a couple of months later, a coil in the downstream solenoid failed during a training quench. The modular design of the apparatus meant the collaboration was able to devise new settings rapidly, but it proved not to be possible to restore the downstream spectrometer magnet to full functionality. This, combined with the additional delays incurred in the recovery of the magnet, eventually led to the cancellation of the installation of the RF cavities in favour of the extended operation of a configuration of the experiment without the cavities (see schematic in box 3).
It is interesting to reflect, as was done in a recent lessons-learnt exercise convened by the STFC, whether a robust evaluation of alternative options for the cooling-demonstration lattice at the outset of MICE might have identified the simplified lattice as a “less-risky” option and allowed some of the delays in implementing the experiment to be avoided.
The bulk of the data-taking for MICE was carried out between November 2015 and December 2017, using lithium-hydride and liquid-hydrogen absorbers. The campaign was successful: more than 5 × 108 triggers were collected over a range of initial beam momentum and emittance for a variety of configurations of the magnetic channel for each absorber material. The key parameter to measure when demonstrating ionisation cooling is the “amplitude” of each muon – the distance from the beam centre in transverse phase space, reconstructed from its position and momentum. The muon’s amplitude is measured before it enters the absorber and again as it leaves, and the distributions of amplitudes are then examined for evidence of cooling: a net migration of muons from high to low amplitudes. As can be seen (figure 2), the particle density in the core of the MICE beam is increased as a result of the beam’s passage through the absorber, leading to a lower transverse emittance and thereby providing a higher neutrino flux or a larger luminosity.
The MICE observation of the ionisation-cooling of muon beams is an important breakthrough, achieved through the creativity and tenacity of the collaboration and the continuous support of the funding agencies and host laboratory. The results match expectations, and the next step would be to design an experiment to demonstrate cooling in all six phase-space dimensions.
Completing the MICE programme
Having completed its experimental programme, MICE will now focus on the detailed analysis of the factors that determine ionisation-cooling performance over a range of momentum, initial emittance and lattice configurations for both liquid-hydrogen and lithium-hydride absorbers. MICE was operated such that data were recorded one particle at a time. This single-particle technique will allow the collaboration to study the impact of transverse-emittance growth in rapidly varying magnetic fields and to devise mechanisms to mitigate such effects. Furthermore, MICE has taken data to explore a scheme in which a wedge-shaped absorber is used to decrease the beam’s longitudinal emittance while allowing a controlled growth in its transverse emittance. This is required for a proton-based muon collider to reach the highest luminosities.
With the MICE observation of ionisation cooling, the last of the proof-of-principle demonstrations of the novel technologies that underpin a proton-based neutrino factory or muon collider has now been delivered. The drive to produce lepton–antilepton collisions at centre-of-mass energies in the multi-TeV range can now include consideration of the muon collider, for which two routes are offered: one, for which the R&D is well advanced, that exploits muons produced using a high-power proton beam and which requires ionisation cooling; and one that exploits positron annihilation with electrons at rest to create a high-energy cold muon source. The high muon flux that can be achieved using the proton-based technique has the potential to serve a neutrino-physics programme of unprecedented sensitivity, and the MICE collaboration’s timely results will inform the coming update of the European Strategy for Particle Physics.
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