On 8 January the Republic of Lithuania formally became an associate Member State of CERN, following the completion of its internal approval procedures (CERN Courier July/August 2017 p7). Lithuania’s relationship with CERN dates back to an International Cooperation Agreement signed in 2004, with Lithuanian researchers contributing to the CMS experiment since 2007. Its new status strengthens the long-term partnership between CERN and the Lithuanian scientific community, makes Lithuanian scientists eligible for staff appointments, and entitles Lithuanian industry to bid for CERN contracts.
Since 4 December, around 500 technicians and engineers have been working flat-out to maintain and upgrade the Large Hadron Collider (LHC) and other parts of the CERN accelerator complex. The current year-end technical stop will last until 9 March, and preparations for the machine and its infrastructure for the High Luminosity LHC (HL-LHC) have been a focus of activities.
Collimators are key to operating the HL-LHC, which will have roughly twice the stored energy (700 MJ) as the present machine. These devices control losses from the circulating proton beams so that they can be constrained to a small section of the machine’s circumference. Continuing work undertaken during last year’s extended year-end stop (CERN Courier March 2017 p9), two new collimators are being installed at point 1 containing a wire that generates an electromagnetic field to compensate for long-range beam–beam effects.
Higher performing injectors that can produce more intense particle beams are another demand of the HL-LHC, and this aspect is being managed by the LHC Injector Upgrade (LIU) project (CERN Courier October 2017 p32). An upgraded kicker magnet, one of eight fast-pulsed magnets that inject particle beams coming from the Super Proton Synchrotron (SPS) into the LHC, will be installed at point 8. A special coating applied to the inner wall of the ceramic pipe of the magnet is one of several techniques developed to reduce the heating of components in the harsher HL-LHC environment.
While work steps up on the LHC, which has been temporarily emptied of its 120 tonnes of helium coolant, brand-new accelerator technology that will help the HL-LHC achieve its unprecedented luminosities is being prepared for tests in the SPS. Two prototype radiofrequency crab cavities – designed to tilt particle bunches before they collide to maximise the overlapping of the beams and increase the probability of collisions – have been installed for testing during 2018. In around five years from now, during Long Shutdown 3 (LS3), the full system will be installed in the LHC.
Further down the accelerator chain, a major de-cabling campaign is taking place in the Proton Synchrotron (PS) to create space for the deployment of the LIU project during Long Shutdown 2 (LS2) beginning next year. The transfer line linking the PS to the SPS is also having all of its 43 quadrupole magnets replaced, among numerous other works. The whole CERN injector chain is undergoing an annual check-up, in particular concerning the cooling, ventilation, cryogenics and electrical supply systems. Other important activities are taking place to consolidate the infrastructure, such as the installation of a new lift at LHC point 8, and to update the beam control systems.
During the 2018 LHC performance workshop, held in Chamonix from 29 January to 2 February, the performance of the LHC during 2017 was reviewed and operational scenarios for 2018 were discussed. A particular focus of the workshop was on the status of the LIU and HL-LHC projects, which will be rolled-out in LS2 and LS3, respectively. There was lively discussion about the organisation and planning of activities for LS2, and the final session of the workshop covered the full energy exploitation of the LHC. Until LS2 the machine will run at a centre-of-mass energy of 13 TeV, but prospects for running at 14 TeV after LS2 and eventuallly even 15 TeV were also discussed.
The LHCb collaboration has shed light on a long-standing anomaly in the very rare hyperon decay Σ+→ pµ+µ– first observed in 2005 by Fermilab’s HyperCP experiment. The HyperCP team found that the branching fraction for this process is consistent with Standard Model (SM) predictions, but that the three signal events observed exhibited an interesting feature: all muon pairs had invariant masses very close to each other, instead of following a scattered distribution.
This suggested the existence of a new light particle, X0, with a mass of about 214 MeV/c2, which would be produced in the Σ+ decay along with the proton and would decay subsequently to two muons. Although this particle has been long sought in various other decays and at several experiments, no experiment other than HyperCP has so far been able to perform searches using the same Σ+ decay mode.
The large rate of hyperon production in proton–proton collisions at the LHC has recently allowed the LHCb collaboration to search for the Σ+→ pµ+µ– decay. Given the modest transverse momentum of the final-state particles, the probability that such a decay is able to pass the LHCb trigger requirements is very small. Consequently, events where the trigger is activated by particles produced in the collisions other than those in the decay under study are also employed.
This search was performed using the full Run 1 dataset, corresponding to an integrated luminosity of 3 fb–1 and about 1014 Σ+ hyperons. An excess of about 13 signal events is found with respect to the background-only expectation, with a significance of four standard deviations. The dimuon invariant- mass distribution of these events was examined and found to be consistent with the SM expectation, with no evidence of a cluster around 214 eV/c2. The signal yield was converted to a branching fraction of (2.1+1.6–1.2) × 10–8 using the known Σ+→ pπ0 decay as a normalisation channel, in excellent agreement with the SM prediction. When restricting the sample explicitly to the case of a decay with the putative X0 particle as an intermediate state, no excess was found. This sets an upper limit on the branching fraction at 9.5 × 10–9 at 90% CL, to be compared with the HyperCP result (3.1+2.4–1.9± 1.5) × 10–8.
This result, together with the recent search for the rare decay KS→ μ+μ– shows the potential of LHCb in performing challenging measurements with strange hadrons. As with a number of results in other areas reported recently, LHCb is demonstrating its power not only as a b-physics experiment but as a general-purpose one in the forward region. With current data, and in particular with the upgraded detector thanks to the software trigger from Run 3 onwards, LHCb will be the dominant experiment for the study of both hyperons and KS mesons, exploiting their rare decays to provide a new perspective in the quest for physics beyond the SM.
A new national facility at La Silla Observatory in Chile, operated by the European Southern Observatory (ESO), made its first observations at the beginning of the year. ExTrA (Exoplanets in Transits and their Atmospheres) will search for Earth-sized planets orbiting nearby red dwarf stars, its three 0.6 m-diameter near-infrared telescopes (pictured) increasing the sensitivity compared to previous searches. ExTrA is a French project also funded by the European Research Council and the telescopes will be operated remotely from Grenoble.
Measuring the production of the top quark with vector bosons can provide fresh insight into the Standard Model (SM), in particular by testing the top quark and heavy vector boson vertices, which may be modified by extensions to the SM. In two new results, ATLAS presents strong evidence for the production of a single top quark in association with a Z boson (tZ) and has for the first time extracted differential cross-sections for the production of a top quark in association with a W boson (tW). While tW production was already measured during LHC Run 1, the next in line, the tZ process, is much harder to observe because its production rate is about one hundredth lower.
For both the tZ and tW processes, separating them from background events is critical. ATLAS searched for events containing leptons (electrons or muons), jets and transverse momentum imbalance. All the information from the measured particles is condensed into one multivariate discriminator (MVA) trained to separate the signal from the background.
The new ATLAS results use data collected in 2015 and 2016, corresponding to an integrated luminosity of 36.1 fb–1. For the tZ analysis, 25 signal events are found after selection, together with 120 background events. Applying the MVA allows the signal and background to be better separated (see figure, left), leading to a signal significance of 4.2 standard deviations. This constitutes strong evidence that the associated production of a single top quark and a Z boson has been seen, and the observed production rate agrees with that predicted by the SM.
The extraction of differential cross-sections for tW is particularly challenging, as top quarks almost always decay into a b quark and a W boson, leaving two W bosons in the final state. The dominant background from the production of a top quark with a top antiquark has an 11 times larger inclusive production rate. Applying the MVA it is possible to select events with a signal to background ratio of about 1:2, which allows the signal cross-section to be extracted as a function of kinematic observables. Differential cross-sections have been measured as a function of several variables and measured and compared to predictions implemented in different Monte Carlo programmes (see figure). The uncertainty on the measurements is at the 20–50% level, dominated by statistical effects. While the analysis was not able to exclude particular models, the data tend to have more events with high-momentum particles than predicted.
With the additional data to be collected over the next years, ATLAS will study both tW and tZ production in more detail, and improve its searches for the even rarer and more elusive production of a (single) top quark in association with a Higgs boson.
The quest to search for new physics inspires searches in CMS for very rare processes, which, if discovered, could open the door to a new understanding of particle physics.
One such process is the production and decay of heavy sterile Majorana neutrinos, a type of heavy neutral lepton (HNL) introduced to describe the very small neutrino masses via the so-called seesaw mechanism. Two further fundamental puzzles of particle physics can be solved by adding three HNLs to the Standard Model (SM) particle spectrum: the lightest (with a mass of a few keV) can serve as a dark-matter candidate; the two heavier ones (heavier than about a GeV) could, when mass-degenerate, be responsible for a sizable amount of CP violation and thus help explain the cosmological matter–antimatter asymmetry.
Through their mixing with the SM neutrinos (see figure, left), the heavier HNLs could be produced at the LHC in leptonic W-boson decays. Subsequently, the HNL can decay to another W boson and a lepton, leading to a signal containing three isolated leptons. Depending on how weakly the new particles couple to the SM neutrinos, characterised by the parameters |VeN|2, |VμN|2 and |VτN|2, they can either decay shortly after production, or after flying some distance in the detector.
A new search performed with data collected in 2016 by CMS focuses on prompt trilepton (electrons or muons) signatures of HNL production. It explores a mass range from 1 GeV to 1.2 TeV, more than doubling the scope of LHC results so far. It also probes a mass regime that was unexplored since the days of the Large Electron-Positron collider (LEP), indicating that eventually the LHC will supersede these results with more data.
The trilepton final state does not lead to a sharp peak in an invariant mass spectrum, and therefore the search has to employ various kinematic properties of the events to be able to detect a possible presence of HNLs. To be sensitive to very low HNL masses, the search uses soft muons (with pT > 5 GeV) and electrons (pT > 10 GeV). While no signs of HNL have been found so far (see figure, right), the constraints on |VμN|2 (|VeN|2 is similar) in the high-mass region are the strongest to date. In the low mass region, the analysis has comparable sensitivity to previous searches.
Using dedicated analysis techniques, it is foreseen to extend this search to explore the parameter space where HNLs have longer lifetimes and so travel large distances in the detector before they decay. Together with more data this will enable CMS to significantly improve the sensitivity at low masses and eventually probe unexplored territory in this important region of HNL parameter space.
In two publications submitted to the Journal of High Energy Physics and Physics Letters B in December, the ALICE collaboration reports new production cross-section measurements of the charmed baryons Λ+c and Ξ0c in proton–proton collisions at an energy of 7 TeV and in proton–lead collisions at a collision energy of 5.02 TeV per nucleon–nucleon pair. The Λ+c were reconstructed in the hadronic decay modes Λ+c→ pK− π+ and Λ+c→ p K0S, and in the semileptonic channel Λ+c→ e+ νe Λ (and charge conjugates). For the Ξ0c analysis, the semi-leptonic channel Ξ0c→ e+ νeΞ– was used.
The comparison of charm baryon and meson cross-sections provides information on c-quark hadronisation. Surprisingly, the measured values of the Λ+c/D0 baryon-to-meson ratio were significantly larger than those previously measured in other experiments in collisions involving electron beams at different centre-of-mass energies, rapidity and pT intervals.
The results (see figure) are compared with the expectations obtained from perturbative QCD calculations and Monte Carlo event generators. None of the models reproduce the data, indicating that the fragmentation of charm quarks is not well understood. A similar pattern is seen when comparing the Ξ0c/D0 baryon-to-meson ratio with predicted values (see figure, right), where the latter have a sizable uncertainty due to the unknown branching ratio of the decay.
These two results suggest that charmed baryon formation might not be universal, and that the baryon/meson ratio depends on the collision system. Hints of non-universality of the fragmentation functions are also seen when comparing beauty-baryon production measurements at the Tevatron and LHC with those at LEP. The ratios measured in pPb collisions are similar to the result in pp collisions.
The statistical precision of the Λ+c and Ξ0c measurements is expected to be improved with data collected during the LHC Run 2, and with data from Run 3 and Run 4 following a major upgrade of the ALICE apparatus. This set of measurements also provides a reference for future investigation of Λ+c and Ξ0c production in lead–lead collisions, where the formation and kinematic properties of charm baryons are expected to be affected by the presence of the quark–gluon plasma.
Many questions remain about what happened in the first billion years of the universe. At around 100million years old, the universe was a dark place consisting of mostly neutral hydrogen without many objects emitting detectable radiation. This situation changed as stars and galaxies formed, leading to a phase transition known as reionisation where the neutral hydrogen was ionised. Exactly when reionisation started and how long it took is still not fully clear, but a recent discovery of the oldest massive black hole ever found can help answer this important question.
Up to about 300,000years after the Big Bang, the universe was hot and dense, and electrons and protons were fully separated. As the universe started to expand, it cooled down and underwent a first phase transition where electrons and protons formed neutral gases such as hydrogen. The following period is known as the cosmic dark ages. During this period, protons and electrons were mostly combined into neutral hydrogen, but the universe had to cool much further before matter could condense to the level where light-producing objects such as stars could form. These new objects started to emit both the radiation we can now detect to study the early universe and also the radiation responsible for the last phase transition – the reionisation of the universe. Some of the brightest and therefore easiest-to-detect objects are quasars: massive black holes surrounded by discs of hot accreting matter that emit radiation over a wide but distinctive spectrum.
Using data from a range of large-area surveys by different telescopes, a group led by Eduardo Bañados from the Carnegie Institution for Science has discovered a distant quasar called J1342+0928, with the black hole at its centre found to be eight million solar masses. After the radiation was emitted by J1342+0928, it travelled through the expanding universe, increasing its wavelength or “red shifting” in proportion to its travel time. Using known spectral features of quasars, the redshift (and therefore the moment at which the radiation was emitted) can be calculated.
The spectrum of J1342+0928, shown in the figure, demonstrates that the universe was only 690million years old – just 5% of its current age – at the time we see J1342+0928. The spectrum also shows a second interesting feature: the absorption of a part of the spectrum by neutral hydrogen, which implies that at the time we are observing the black hole, the universe was not fully ionised yet. By modelling the emission and absorption, Bañados and co-workers found that the spectrum from J1342+0928 is compatible with emission in a universe where half the hydrogen was ionised, putting the time of emission right in the middle of the epoch of reionisation.
The next mystery is to explain how a black hole weighing eightmillion solar masses could form so early in the universe. Black holes grow as they accrete mass surrounding them, but the accreting mass radiates and this radiation pushes other accreting mass away from the black hole. As a result, there is a theoretical limit on the amount of matter a black hole can accrete. Forming a black hole the size of J1342+0928 with such accretion limits would require black holes in the very early universe with sizes that challenge current theoretical models. One possible explanation, however, is that this particular black hole is a peculiar case and was formed by a merger of several smaller black holes.
Thanks to continuous data taking from a range of existing telescopes and upcoming new instrumentation, we can expect more objects like J1342+0928 or even older to be discovered, offering a probe of the universe at even earlier stages. The discovery of further objects would allow a more exact date for the period of reionisation, which can be compared with indirect measurements coming from the cosmic microwave background. At the same time, more measurements will show if black holes of this size in the early universe are just an anomaly or if there are more. In either case, such observations would provide important input for research on early black hole formation.
When deciding on the shape of a particle accelerator, physicists face a simple choice: a ring of some sort, or a straight line? This is about more than aesthetics, of course. It depends on which application the accelerator is to be used for: high-energy physics, advanced light sources, medical or numerous others.
Linear accelerators (linacs) can have denser bunches than their circular counterparts, and are widely used for research. However, for both high-energy physics collider experiments and light sources, linacs can be exceedingly power-hungry because the beam is essentially discarded after each use. This forces linacs to operate at an extremely low current compared to ring accelerators, which in turn limits the data rate (or luminosity) delivered to an experiment. On the other hand, in a collider ring there is a limit to the focusing of the bunches at an interaction point as each bunch has to survive the potentially disruptive collision process on each of millions of turns. Bunches from a linac have to collide only once and can therefore be focused to aggressively collide at a higher luminosity.
Linacs could outperform circular machines for light-source and collider applications, but only if they can be operated with higher currents by not discarding the energy of the spent beam. Energy-recovery linacs (ERLs) fill this need for a new accelerator type with both linac-quality bunches and the large currents more typical of circular accelerators. By recovering the energy of the spent beam through deceleration in superconducting radio-frequency (SRF) cavities, ERLs can recycle that energy to accelerate new bunches, combining the dense beam of a linear accelerator with the high current of a storage ring to achieve significant RF power savings.
A new facility called CBETA (Cornell-Brookhaven ERL Test Accelerator) that combines some of the best traits of linear and circular accelerators has recently entered construction at Cornell University in the US. Set to become the world’s first multi-turn SRF ERL, with a footprint of about 25 × 15 m, CBETA is designed to accelerate an electron beam to an energy of 150 MeV. As an additional innovation, this four-turn ERL relies on only one return loop for its four beam energies, using a single so-called fixed-field alternating-gradient return loop that can accommodate a large range of different electron energies. To further save energy, this single return loop is constructed from permanent Halbach magnets (an arrangement of permanent magnets that augments the magnetic field on the beam side while cancelling the field on the outside).
Initially, CBETA is being built to test the SRF ERL and the single-return-loop concept of permanent magnets for a proposed future electron-ion collider (EIC). Thereafter, CBETA will provide beam for applications such as Compton-backscattered hard X-rays and dark-photon searches. This future ERL technology could be an immensely important tool for researchers who rely on the luminosity of colliders as well as for those that use synchrotron radiation at light sources. ERLs are envisioned for nuclear and elementary particle-physics colliders, as in the proposed eRHIC and LHeC projects, but are also proposed for basic-research coherent X-ray sources, medical applications and industry, for example in lithography sources for the production of yet-smaller computer chips.
The first multi-turn SRF ERL
The theoretical concept of ERLs was introduced long before a functional device could be realised. With the introduction of the CBETA accelerator, scientists are following up on a concept first introduced by physicist Maury Tigner at Cornell in 1965. Similarly, non-scaling fixed-field alternating-gradient optics for beams of largely varying energies were introduced decades ago and will be implemented in an operational accelerator for only the second time with CBETA, after a proof-of-principle test at the EMMA facility at Daresbury Laboratory in the UK, which was commissioned in 2010.
The key behind the CBETA design is to recirculate the beam four times through the SRF cavities, allowing electrons to be accelerated to four very different energies. The beam with the highest energy (150 MeV) will be used for experiments, before being decelerated in the same cavities four times. During deceleration, energy is taken out of the electron beam and is transferred to electromagnetic fields in the cavities, where the recovered energy is then used to accelerate new particles. Reusing the same cavities multiple times significantly reduces the construction and operational costs, and also the overall size of the accelerator.
The energy-saving potential of the CBETA technology cannot be understated, and is a large consideration for the project’s funding agency the New York State Energy Research and Development Authority. By incrementally increasing the energy of the beam through multiple passes in the accelerator section, CBETA can achieve a high-energy beam without a high initial energy at injection – characteristics more commonly found in storage rings. CBETA’s use of permanent magnets provides further energy savings. The precise energy savings from CBETA are difficult to estimate at this stage, but the machine is expected to require about a factor of 20 less RF power than a traditional linac. This saving factor would be even larger for future ERLs with higher beam energy.
SRF linacs have been operated in ERL mode before, for example at Jefferson Lab’s infrared free-electron laser, where a single-pass energy recovery has reclaimed nearly all of the electron’s energy. CBETA will be the first SRF ERL with more than one turn and is unique in its use of a single return loop for all beams. Simultaneously transporting beam at four very different energies (from 42 to 150 MeV) requires a different bending field strength for each energy. While traditional beamlines are simply unable to keep beams with very different energies on the same “track”, the CBETA design relies on fixed-field alternating-gradient optics. To save energy, permanent Halbach magnets containing all four beam energies in a single 70 mm-wide beam pipe were designed and prototyped at Brookhaven National Laboratory (BNL). The special optics for a large energy range had already been proposed in the 1960s, but a modern rediscovery began in 1999 at the POP accelerator at KEK in Japan. This concept has various applications, including medicine, nuclear energy, and in nuclear and particle physics, culminating so far with the construction of CBETA. Important aspects of these optics will be investigated at CBETA, including the following: time-of-flight control, maintenance of performance in the presence of errors, adiabatic transition between curved and straight regions, the creation of insertions that maintain the large energy acceptance, the operation and control of multiple beams in one beam pipe, and harmonic correction of the fields in the permanent magnets.
Harmonic field correction is achieved by an elegant invention first used in CBETA: in order to overcome the magnetisation errors present in the NdFeB blocks and to produce magnets with 10–3 field accuracy, 32 to 64 iron wires of various lengths are inserted around the magnet bore, with lengths chosen to minimise the lowest 18 multipole harmonics.
A multi-turn test ERL was proposed by Cornell researchers following studies that started in 2005. Cornell was the natural site, given that many of the components needed for such an accelerator had been prototyped by the group there. A collaboration with BNL was formed in the summer of 2014; the test ERL was called CBETA and construction started in November 2016.
CBETA has some quite elaborate accelerator elements. The most complex components already existed before the CBETA collaboration, constructed by Cornell’s ERL group at Wilson Lab: the DC electron source, the SRF injector cryomodule, the main ERL cryomodule, the high-power beam stop, and a diagnostic section to map out six-dimensional phase-space densities. They were designed, constructed and commissioned over a 10-year period and hold several world records in the accelerator community. These components have produced the world’s largest electron current from a photo-emitting source, the largest continuous current in an SRF linac and the largest normalised brightness of an electron bunch.
Setting records
Meanwhile, the DC photoemission electron gun has set a world record for the average current from a photoinjector, demonstrating operation at 350 kV with a continuous current of 75 mA with 1.3 GHz pulse structure. It operates with a KCsSb cathode, which has a typical quantum efficiency of 8% at a wavelength of 527 m and requires a large ceramic insulator and a separate high voltage, high current, power supply to be able to support the high voltage and current. The present version of the Cornell gun has a segmented insulator design with metal guard rings to protect the ceramic insulator from punch-through by field emission, which was the primary limiting factor in previous designs. This gun has been processed up to 425 kV under vacuum, typically operating at 400 kV.
The SRF injector linac, or injector cryomodule (ICM), set new records in current and normalised brightness. It operates with a bunch train containing a series of five two-cell 1.3 GHz SRF cavities, each with twin 50 kW input couplers that receive microwaves from high-power klystrons, and the input power couplers are adjustable to allow impedance matching for a variety of different beam currents. The ICM is capable of a total energy gain of around 15 MeV, although CBETA injects beam at a more modest energy of 6 MeV. The high-current CW main linac cryomodule, meanwhile, has a maximum energy gain of 70 MeV and a beam current of up to 40 mA, and for CBETA will accelerate the beam by 36 MeV on each of the four beam passes.
Several other essential components that have also been commissioned include a high-power beam stop and diagnostics tools for high-current and high-brightness beams, such as a beamline for measuring 6D phase-space densities, a fast wire scanner for beam profiles and beam-loss diagnostics. All these components are now being incorporated in CBETA. While the National Science Foundation provided the bulk funding for the development of all these components, the LCLS-II project contributed funding to investigate the utility of Cornell’s ERL technology, and the company ASML contributed funds to test the use of ERL components for an industrial EUV light source.
Complementary development work has been ongoing at BNL, and last summer the BNL team successfully tested a fixed-field alternating-gradient beam transport line at the Accelerator Test Facility. It uses lightweight, 3D-printed frames to hold blocks of permanent magnets and uses the above-mentioned innovative method for fine-tuning the magnetic field to steer multiple beams at different energies through a single beam pipe. With this design, physicists can accelerate particles through multiple stages to higher and higher energies within a single ring of magnets, instead of requiring more than one ring to achieve these energies. The beams reached a top momentum that was more than 3.8 times that of the lowest transferred momentum, which is to be compared to the previous result in EMMA, where the highest momentum was less than twice that of the lowest one. The properties of the permanent Halbach magnets match or even surpass those of electromagnets, which require much more precise engineering and machining to create each individual piece of metal. The success of this proof-of-principle experiment reinforces the CBETA design choices.
The initial mission for CBETA is to prototype components for BNL’s proposed version of an EIC called eRHIC, which would be built using the existing Relativistic Heavy Ion Collider infrastructure at BNL. JLAB also has a design for an EIC, which requires an ERL for its electron cooler and therefore also benefits from research at CBETA. Currently, the National Academy of Sciences is studying the scientific potential of an EIC. More than 25 scientists, engineers and technicians are collaborating on CBETA and they are currently running preliminary beam tests, with the expectation of completing CBETA installation by the summer of 2019. Then we will test and complete CBETA commissioning by the spring of 2020, and begin to explore the scientific applications of this new acceleration and energy-saving technique.
The enigma of why the universe contains more matter than antimatter has been with us for more than half a century. While charge–parity (CP) violation can, in principle, account for the existence of such an imbalance, the observed matter excess is about nine orders of magnitude larger than what is expected from known CP-violating sources within the Standard Model (SM). This striking discrepancy inspires searches for additional mechanisms for the universe’s baryon asymmetry, among which are experiments that test fundamental charge–parity–time (CPT) invariance by comparing matter and antimatter with great precision. Any measured difference between the two would constitute a dramatic sign of new physics. Moreover, experiments with antimatter systems provide unique tests of hypothetical processes beyond the SM that cannot be uncovered with ordinary matter systems.
The Baryon Antibaryon Symmetry Experiment (BASE) at CERN, in addition to several other collaborations at the Antiproton Decelerator (AD), probes the universe through exclusive antimatter “microscopes” with ever higher resolution. In 2017, following many years of effort at CERN and the University of Mainz in Germany, the BASE team measured the magnetic moment of the antiproton with a precision 350 times better than by any other experiment before, reaching a relative precision of 1.5 parts per billion (figure 1). The result followed the development of a multi-Penning-trap system and a novel two-particle measurement method and, for a short period, represented the first time that antimatter had been measured more precisely than matter.
Non-destructive physics
The BASE result relies on a quantum measurement scheme to observe spin transitions of a single antiproton in a non-destructive manner. In experimental physics, non-destructive observations of quantum effects are usually accompanied by a tremendous increase in measurement precision. For example, the non-destructive observation of electronic transitions in atoms or ions led to the development of optical frequency standards that achieve fractional precisions on the 10–18 level. Another example, allowing one of the most precise tests of CPT invariance to date, is the comparison of the electron and positron g-factors. Based on quantum non-demolition detection of the spin state, such studies during the 1980s reached a fractional accuracy on the parts-per-trillion level.
The latest BASE measurement follows the same scheme but targets the magnetic moment of protons and antiprotons instead of electrons and positrons. This opens tests of CPT in a totally different particle system, which could behave entirely differently. In practice, however, the transfer of quantum measurement methods from the electron/positron to the proton/antiproton system constitutes a considerable challenge owing to the smaller magnetic moments and higher masses involved.
The idea is to store single particles in ultra-stable, high-precision Penning traps, where they oscillate at characteristic frequencies. By measuring those frequencies, we can access the cyclotron frequency, νc, which defines the particle’s revolutions per second in the trap’s magnetic field. Together with a measurement of the spin precession frequency νL, the g-factor can be extracted from the relation:
To determine νc we use a technique called image-current detection. The oscillation of the antiproton in the trap induces tiny image currents in the trap electrodes, which are picked up by highly sensitive superconducting tuned circuits.
The measurement of νL, on the other hand, relies on single-particle spin-transition spectroscopy – comparable to performing NMR with a single antiproton. The idea is to switch the spin of the individual antiproton from one state to the other and then detect the flip. To this end a smart trick is used: the continuous Stern–Gerlach effect, which imprints the collapsed spin state of the single antiproton on its axial oscillation frequency (a parameter that can be measured non-destructively). We use a special Penning trap configuration in which an inhomogeneous magnetic bottle is superimposed on the homogeneous magnetic field of the ideal Penning trap (figure 2, top). The inhomogeneousfield adds a spin-dependent quadratic magnetic potential to the axial electrostatic trapping potential and, consequently, the continuously measured axial oscillation frequency of the trapped antiproton becomes a function of the spin eigenstate.
In practice, to detect spin quantum-transitions we first measure the axial frequency, then inject a magnetic radio-frequency to drive spin transitions, and finally measure the axial frequency again. The observation of an axial frequency jump corresponds to the clear signature that a spin-transition was driven, and by repeating such measurements many times and for different drive frequencies, we obtain the spin-flip probability as a function of the drive frequency. The corresponding resonance curve gives νL (figure 2, bottom).
Doubling up
This challenge has become the passion of the members of the BASE collaboration for the past decade. A trap was developed at Mainz with a superimposed magnetic inhomogeneity of 300,000 T/m2, which corresponds to a magnetic field change of about 1 T over a distance of about 1.5 mm! In this extreme magnetic environment, a proton/antiproton spin transition induces an axial frequency shift of only 170 mHz when driven at a frequency of around 650 kHz.
Using this unique device, in 2011 we reported the first observation of spin flips with a single trapped proton. This was followed by the unambiguous quantum-non-demolition detection of proton spin-transitions, which was later also demonstrated with antiprotons (figure 3). The high-fidelity detection of the spin state, however, requires the particle to be cooled to temperatures of the order of 100 mK. This was achieved by sub-thermal cooling of the particle’s cyclotron mode by means of cryogenic resistors, but is an inconceivably time-consuming procedure.
The high-fidelity resolution of single-spin quantum transitions is the key to measuring the antiproton magnetic moment at the parts-per-billion level. The elegant double-trap technique that makes this possible was invented at Mainz and applied with great success in tests of bound-state quantum electrodynamics, in collaboration with GSI Darmstadt and the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, both institutes also being part of the BASE collaboration. This double Penning-trap technology separates the sensitive frequency measurements of νL and νc, and the spin analysis measurements into two traps: a homogeneous “precision trap” (PT) and the spin state “analysis trap” (AT) with the superimposed strong magnetic bottle. The magnetic field in the PT is about 100,000 times more homogeneous than that of the AT and allows sampling of the spin-flip resonance at much higher resolution, compared to measurements solely carried out in the inhomogeneous AT.
The single-particle “double-trap method”, however, comes with the drawback that each frequency measurement in the PT heats the particle’s radial mode to about room-temperature and requires repeated particle preparation to sub-thermal radial energy, a condition that is ultimately required for the high-fidelity detection of spin transitions. Each of these sub-thermal-energy preparation cycles takes several hours, while a well resolved g-factor resonance contains at least 400 individual data points. We applied this method at BASE to measure the proton magnetic moment with parts-per-billion precision in a measurement campaign that took, including systematic studies and maintenance of the instrument, about half a year.
To reduce the total measurement time, we invented the novel two-particle method in which the precision frequency measurements and the high-fidelity spin-state analysis are carried out using two particles: a hot “cyclotron particle” and a cold “Larmor particle”, in addition to adding a third trap called the “park trap” (figure 4). We first identify the spin state of the cold antiproton in the AT. Then we measure the cyclotron frequency with the hot particle in the PT, move this particle to the park trap and transport the cold antiproton to the PT, where spin-flip drives are irradiated. Afterwards, the cold particle is shuttled back to the AT and the hot particle to the PT. There, the cyclotron frequency is measured again, and in a last step the spin state of the cold particle in the AT is identified. By repeating this scheme many times and for different drive frequencies, the spin-flip probability as a function of the spin-flip drive frequency, normalized to the measured cyclotron frequency, is obtained – a g-factor resonance – with all the required frequency information sampled in the homogeneous PT. This novel two-particle scheme drastically reduces the measurement time, since it avoids the time-consuming preparation of sub-thermal radial energy-states.
Successfully implementing this new method, we were able to sample about 1000 data points over a period of just two months. From this campaign we extracted the antiproton magnetic moment as µp̅ = –2.792 847 344 1 (42) μN, the value having a fractional precision of 1.5 parts per billion and thereby improving the previous best value by BASE by a factor of 350. The result is consistent with our most precise measurement of the proton magnetic moment, μp = 2.792 847 350 (9) µN, and thus supports CPT invariance.
Trappings of success
Underpinning this rapid achievement of the initially defined major experimental goal of the BASE collaboration was another BASE invention called the reservoir trap (RT) method. This RT, being one of four traps in the BASE trap-stack, is loaded with a shot of antiprotons and provides single particles to the precision measurement traps on request. The method allows BASE to operate antiproton experiments even during the winter shut-down of CERN’s accelerators and practically doubles the available experiment time. Indeed, we have demonstrated antiproton trapping and experiment optimisation for a period of more than 400 days and operated the entire 2016 run with antiprotons captured in 2015. This long storage time also allows us to set limits on directly measured antiproton lifetime.
Together with the proton-to-antiproton charge-to-mass ratio comparison with a fractional precision of 69 parts in a trillion CERN Courier September 2015 p7), which was carried out during the 2014 antiproton run, BASE has set tighter constraints on all the fundamental antiproton parameters that are directly accessible by this type of experiment. So far, all the BASE results are consistent with CPT invariance.
The latest triple-trap measurement of the antiproton magnetic moment sets new constraints on CPT violating coefficients in the Standard Model extension (SME) – an effective theory that allows the sensitivities of different experiments at different locations to be compared with respect to CPT violation. The recent BASE magnetic-moment measurement addresses a total of six combinations of SME coefficients and improves the limits on all of them by more than two orders of magnitude. Finding a non-zero coefficient would, for example, indicate the discovery of a new type of exchange boson that couples exclusively to antimatter and immediately raise the question of its role in the universal baryon asymmetry.
Although up to now all results are CPT-consistent, this not-yet-understood asymmetry is one of the motivations to further improve the experimental resolution of the AD experiments. The recent successes reported by the ALPHA collaboration herald the first ultra-high-precision measurements on the optical spectrum of antihydrogen. Improved methods in measurements on antiprotonic helium by the ASACUSA collaboration will lead to even higher resolution results in comparisons of the antiproton-to-electron mass ratio, while the ATRAP collaboration continues to contribute independent measurements of antiprotons and antihydrogen.
Gravitational sensitivity
A new branch of experiments at CERN’s AD, AEgIS, GBAR and ALPHA-g, will soon investigate the gravitational acceleration of antimatter in Earth’s gravitational field – which has never been directly observed before. Indirect measurements were carried out with antiprotons by the TRAP collaboration at the AD’s predecessor, LEAR, and by BASE, which set constrains on antigravity effects.
The AD community aims to verify the laws of physics with antimatter in various ways, thereby testing fundamental CPT invariance. The experiments are striving to access yet unmeasured quantities, or to improve their sensitivities to new physics. In this respect, the BASE–Mainz experiment succeeded recently in measuring the proton magnetic moment at an 11-fold improved precision, reaching a fractional uncertainty of 0.3 parts per billion. By applying these even further advanced methods to the antiproton, BASE will improve the sensitivity of the CPT invariance test by at least another factor of five.
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