While it is now generally accepted that dark matter makes up the majority of the mass in the universe, little is known about what it is. A favoured hypothesis among particle physicists has long been that dark matter is made of new elementary particles. However, experiments searching for such particles face a serious challenge: neither the particles’ mass nor the strength of their interaction with normal matter is known. So the experiments must cast an ever-widening net in search of these elusive particles.
At the end of February, the Cryogenic Dark Matter Search collaboration announced new results, obtained with the SuperCDMS detector. They expanded their search down to a previously untested dark-matter particle-mass range of 4–6 GeV/c2 and a dark-matter nucleon cross-section range of 1 × 10–40–1 × 10–41 cm2. Their exclusion results contradict recent hints of dark-matter detection by another experiment, CoGeNT, which uses particle detectors made of germanium – the same material used by SuperCDMS.
For their new results, CDMS employed a redesigned cryogenic detector known as iZIP that has ionization and phonon sensors interleaved on both sides of the germanium crystals. This substantially improves rejection of surface events from residual radioactivity, which have limited dark-matter sensitivity in previous searches. The collaboration operated these detectors 0.7 km underground in the Soudan mine in northern Minnesota, to shield them from cosmic-ray backgrounds.
There have been several recent hints for low-mass dark-matter particle detection, from previous data using silicon instead of germanium detectors in CDMS, and from three other experiments—DAMA, CoGeNT and CRESST—all finding their data compatible with the existence of dark-matter particles between 5 and 20 GeV/c2. But such light dark-matter particles are hard to pin down. The lower the mass of the dark-matter particles, the less energy they leave in detectors, and the more likely it is that background noise will drown out any signals.
The new CDMS iZIP detectors, with their improved background rejection, are continuing this search at Soudan, and hopefully soon in the lower background environment at SNOLAB. Confirmation of a signal of the direct detection of dark matter, and understanding of the interaction of dark matter with normal matter, is likely to require spotting these particles with different target nuclei in at least two different experiments.
The Standard Model predicts that the photons emitted in b → sγ transitions, which can only occur through loop-level processes, are predominantly left-handed. This means that the asymmetry between the amplitudes with right- and left-handed photons – photon polarization – is close to its minimum value of –1. This quantity has never been observed in a direct measurement and remains largely unexplored. As a consequence, there still exist several extensions of the Standard Model that predict a photon polarization significantly closer to zero but have not been ruled out by other measurements of b → sγ transitions.
The LHCb collaboration has exploited B+ →K+ π–π+γ decays, which are governed by the b → sγ transition, to probe the photon polarization. The “up–down” asymmetry between the number of photons detected above and below the plane defined by the momenta of the kaon and the two pions in their centre-of-mass frame is proportional to the photon polarization. So, a measurement of a nonzero asymmetry implies observation of photon polarization. The investigation is conceptually similar to the experiment that discovered parity violation in 1957 by measuring a nonzero up–down asymmetry for the electrons emitted in the weak decay of 60Co nuclei with respect to their spin direction.
Using the full data sample collected with the LHCb detector in 2011 and 2012, the collaboration has reconstructed almost 14,000 B+ →K+ π–π+γ events. Their angular distribution has been studied in four regions of the K+ π–π+ system’s mass, where different kaon resonances and their interferences can result in different sensitivities to the photon polarization. From determination of the up–down asymmetry, Aud, in each of these mass regions, LHCb finds a combined significance with respect to the null hypothesis of 5.2σ, and therefore observes photon polarization for the first time in such decays (LHCb collaboration 2014). This important result opens the door to the future determination of the value of the polarization of the photon, which will provide a strong new test of the validity of the Standard Model.
The fusion of two weak bosons is an important process that can be used to probe the electroweak sector of the Standard Model. Measurements of Higgs production via weak-boson fusion are crucial for precise extraction of the Higgs-boson couplings and have the potential to help pin down the charge conjugation and parity of the Higgs boson. A similar process, weak-boson scattering, is sensitive to alternative electroweak symmetry-breaking models and to anomalous weak-boson gauge couplings. These processes are extremely rare and the experimental observation of the production of heavy bosons via weak-boson fusion has become possible only recently with the large centre-of-mass energy and luminosity provided by the LHC. Extracting the signals from the huge backgrounds in the high pile-up conditions at the LHC is a major challenge.
The production of a Z boson via weak-boson fusion (figure 1) is an excellent benchmark for these rare processes. Weak-boson fusion has the characteristic signature of two low-angle jets, one on each side of the detector. These “tagging” jets typically have transverse momentum of the order of the W mass, because they arise from quarks in each proton recoiling against the two W bosons that fuse to produce the Z boson. Another interesting feature is the lack of colour flow between the tagging jets, which means there is little hadronic activity in that region. These features have been exploited by the ATLAS collaboration to extract the purely electroweak contribution to Z-plus-two-jet production, which includes the weak-boson fusion process.
The analysis was carried out using proton–proton collisions at a centre-of-mass energy of 8 TeV recorded by the ATLAS detector in 2012. Events containing a Z boson candidate in association with two high-transverse-momentum jets were selected in the e+e– and μ+μ– decay channels. The electroweak component was extracted by a fit to the dijet invariant mass spectrum in an electroweak-enhanced region that was defined, in part, by a veto on additional jet activity in the interval between the tagging jets. The background model was constrained using data in a signal-suppressed control region that was defined by reversing the jet-veto requirement. This data-driven constraint reduced the experimental and theoretical modelling uncertainties on the background model, allowing the electroweak signal to be extracted with a significance above the 5σ level. Figure 2 clearly demonstrates that the background-only model is inconsistent with the data in the electroweak-enhanced region. The cross-section measured for electroweak Z-plus-two-jet production, σ = 54.7±4.6 (stat.) +9.8–10.4 (syst.) ±1.5 (lumi.)fb, is in good agreement with the Standard Model prediction of 46.1±1.2 fb.
As the LHC experiments improve the precision of their measurements of Standard Model processes, the extent of possibilities for new physics open to exploration is becoming ever more apparent. Even within a constrained framework for new physics, such as the phenomenological minimal supersymmetric standard model (pMSSM), there is an impressive variety of final-state topologies and unique phenomena. For instance, in regions of the pMSSM where the chargino–neutralino mass difference is small, the chargino can become metastable and exhibit macroscopic lifetimes, potentially travelling anywhere between a few centimetres and many kilometres before it decays. An experiment like CMS can identify these heavy stable charged particles (HSCPs) through specialized techniques, such as patterns of anomalously high ionization in the inner tracker, as well as out-of-time signals in the muon detectors.
The CMS collaboration recently released a reinterpretation of a previously published search for HSCPs that used these techniques to constrain several broad classes of new physics models (CMS 2013a). There are two purposes for this reinterpretation. The first is to provide a simplified description of the acceptance and efficiency of the analysis as a function of a few key variables. This simplified “map” allows theorists and others interested to determine an approximate sensitivity of the CMS experiment to any model that produces HSCPs. This is an essential tool for the broader scientific community, because HSCPs are predicted in a large variety of models and it is important to understand if the gaps in their coverage are still present.
The second purpose is to provide a concrete example of a reinterpretation in terms of the pMSSM. In this analysis, CMS chose a limited subspace of the full pMSSM, requiring, among other things, that sparticle masses extend only up to about 3 TeV. The figure shows the number of points in this restricted pMSSM subspace that are excluded as a function of the average decay length, cτ, of the chargino. The red points are excluded by the HSCP interpretation described here (CMS 2013b). The blue points are excluded by another CMS search dedicated to “prompt” chargino production (CMS 2012a). The bottom panel shows the fraction of parameter points excluded by each of these two searches. Only a few parameter points, with chargino cτ >1 km, are still not excluded. This is because the theoretical cross-section for these parameter points is small – around 0.1 fb.
This analysis demonstrates the power of the CMS search for HSCPs to cover a broad range of models of new physics. By mapping the sensitivity of the analysis as a function of the HSCP kinematics and the detector geometry, it also makes the results from the search accessible for studies by the broader scientific community.
Although this analysis searches for metastable particles, another open possibility is the production of new, exotic particles that traverse a short distance – around 1 mm to 100 cm – before decaying to visible particles within the detector. CMS has also released results from two searches for such particles. One search looks for decays of these long-lived particles into two jets, and another into two oppositely charged leptons (CMS 2012b and 2012c). The results from these searches exclude production cross-sections for such particles as low as about 0.5 fb, depending on the lifetime and kinematics of the decay.
Knowledge of the electron mass has been improved by a factor of 13, thanks to a clever extension of previous Penning-trap experiments. A team from the Max-Planck-Institut für Kernphysik in Heidelberg, GSI and the ExtreMe Matter Institute in Darmstadt, and the Johannes Gutenberg-Universität in Mainz, used a Penning trap to measure the magnetic moment of an electron bound to a carbon nucleus in the hydrogen-like ion 12C5+. The cyclotron frequency of the combined system allowed precise determination of the magnetic field at the position of the electron, while the precession frequency allowed for measurement of the mass of the electron.
The result, in atomic-mass units, is 0.000548579909067(14)(9)(2) where the last error is theoretical. This new value for the electron’s mass value will allow comparison of the magnetic moment of the electron to theory – which is good to about 0.08 parts in 1012 – to better than one part in 1012.
Studies of the centrality-dependence of jet production in proton–lead collisions in ATLAS at the LHC are yielding surprising results.
In both proton–ion and ion–ion collisions, many interesting phenomena are influenced by the initial geometry of the collision system. In proton–nucleus collisions, protons that strike the centre of the nucleus (central collisions) see a thicker nuclear target than those that strike the edge (peripheral collisions) and are more likely to undergo a hard scattering. Traditionally, the geometry of the collisions or “centrality” is characterized by a measure of the event activity. For this measurement in ATLAS, the centrality is defined by the total transverse energy in the forward calorimeter in the direction of the lead beam. A model that describes the expected geometric enhancement of hard-scattering rates is used to relate this experimental measure to a factor TA (Miller et al. 2007).
The quantity RCP is defined as the ratio of the per-event jet yields in different centrality bins divided by the ratio of corresponding TA factors, which account for the expected geometric enhancement. The left panels of the figure show RCP values as a function of the jet transverse-momentum, pT, for different ranges in jet rapidity, y*, in the centre-of-mass frame. Negative y* indicates the proton-going direction. The normalized jet ratio, RCP, is suppressed at high pT and large negative y* compared with the expectation from known nuclear effects, which would correspond to a value near unity in RCP. The suppression of jets is strongest in the most central collisions (0–10%) at the highest pT values. Additional studies, independent of the centrality definition, indicate that final-state energy-loss like that observed in ion–ion collisions (jet quenching) is unlikely to be the main source of the effect.
These results suggest either a correlation between hard-jet production and soft-particle production that breaks the traditional geometric paradigm, or that in proton–lead collisions the energy in the forward calorimeter is not a good measure of the centrality of the collision in events with jets.
The right panel of the figure shows the jet RCP as a function of pTcosh(y*), which corresponds closely to the jet’s energy. When recast in terms of this quantity, the RCP values from different y* intervals all fall along the same line. Whether this is coincidence or related to the underlying dynamics is not yet known.
These observations are among the most striking preliminary ATLAS results from the 2013 proton–lead run of the LHC. Further measurements are needed to uncover the mechanism underlying the observed soft–hard correlations.
The top quark remains, nearly 20 years after its discovery by the experiments at Fermilab’s Tevatron, the heaviest particle known. Its production and decays continue to be the subjects of extensive studies, both at the Tevatron and at the LHC. While top quarks are predominantly produced in pairs through the strong interaction, the production of single top quarks is also possible by virtue of the electroweak interaction, albeit with a much smaller production cross-section. This latter production mechanism provides a unique window into the dynamics of the top quark.
Three different production channels for single top quarks can be distinguished: the t-channel, the s-channel and the W-associated channel (figure 1). After many years of intensive searches, the Tevatron experiment collaborations first reported the observation of singly produced top quarks in 2009. These initial searches were optimized for the t- and s-channel production modes combined. Later, the t-channel mode was individually established by experiments at the Tevatron and the LHC, while evidence for s-channel production was reported only recently by the D0 collaboration at the Tevatron, in July 2013.
Meanwhile, the third production mechanism, W-associated production, remained out of the Tevatron’s reach because of its small cross-section and the lack of a sufficiently distinctive signature. At the LHC, however, the production rate for top quarks is much higher. This allowed ATLAS and CMS to report evidence of W-associated production already with the data collected at a centre-of-mass energy of 7 TeV in 2011, although these measurements did not reach the 5σ gold standard for the solid observation of a new signal. Now, thanks to the data collected in 2012 with the LHC operating at a centre-of-mass energy of 8 TeV, the CMS collaboration has been able to complete the experimental picture of the family of single top-quark production mechanisms.
The main background to W-associated production comes from pair-produced top quarks, where the decay of one of the top quarks appears W-like, because the b quark into which it decays fails the b-tagging requirements. No single defining feature separates the two processes sharply, so several kinematic properties have been combined into a single multivariate discriminant by a boosted decision tree – a machine-learning technique that is particularly suited to separating tiny signals from overwhelming backgrounds. Figure 2 shows the discriminant from the boosted decision tree.
The amount of signal in the selected sample is inferred by a fit to the discriminant distribution. To constrain tightly the amount of top-quark pair background with the data, two complementary samples with one additional jet, separated into cases with one or both jets b-tagged, are also included in the fit. The relative population of the samples with one or two b-tagged jets – both of which are almost pure in top-quark pair events – proves to be a powerful handle to reduce the uncertainty on the b-tagging efficiency, and therefore improve the precision of the analysis.
The excess of events with respect to the background-only hypothesis is quantified at a significance of 6.1σ (with 5.4±1.4σ expected from simulation), and the cross-section is found to be 23.4±5.4 pb, in agreement with the Standard Model prediction. Two cross-check analyses, less sensitive but relying on fewer modelling assumptions, confirm the result, therefore further supporting the first-time observation of this new single-top production mode.
This result opens the door to future searches for anomalous interactions of the top quark with the W boson, in a production mode that brings complementary information to studies that have so far been performed only with selections optimized for the more abundant top-pair and single top-quark t-channel events.
The LHCb collaboration has released updated measurements of central exclusive production of the J/ψ and ψ(2S) mesons (LHCb collaboration 2014).
Central exclusive production is a class of reactions in which one or two particles are produced from a beam collision, but the colliding hadrons emerge intact. At the LHC this leads to an unusual and distinctive topology of low-multiplicity events contained in a small rapidity interval with large rapidity gaps on either side. J/ψ and ψ(2S) mesons are produced when a photon emitted from one proton interacts with a pomeron (a colourless combination of gluons) from the other. Measurements of the process can be used to test QCD predictions – to improve our understanding of the distribution of gluons inside the proton – and are also sensitive to saturation effects.
LHCb’s ability to trigger on low-momentum particles and the low number of proton–proton interactions per beam crossing provide an ideal environment to study these processes with particularly low multiplicity. Using data collected in 2011, around 56,000 central exclusive J/ψ and 1500 ψ(2S) mesons have been identified by reconstructing their decays to pairs of muons. While non-resonant backgrounds are very small, the challenge in the analysis is to estimate the larger background that arises when J/ψ and ψ(2S) mesons are produced and one or both of the colliding protons dissociate. As LHCb is instrumented in the forward region mainly, this effect often cannot be detected directly. Instead the collaboration has developed methods to estimate the background rate from the portion of events that are detected.
The measured cross-sections are compared to theoretical predictions, as well as to photoproduction measurements from the HERA electron–proton collider and from fixed-target experiments. Although these environments are quite different from collisions at the LHC, the underlying process is the same. In the former a photon is emitted from an incoming electron beam, while the latter use photon beams directly.
The figure shows a model-dependent comparison of the LHCb results with those from the other types of experiment. It plots the photoproduction cross-section as a function of the photon–proton centre-of-mass energy (W). There is a two-fold ambiguity in converting LHCb’s proton–proton differential cross-section to a photoproduction cross-section, corresponding to the photon being either an emitter or a target. This is resolved using recent results from the H1 experiment at HERA (H1 collaboration 2013). The data in the figure show broad consistency over two orders of magnitude, but are in marginal agreement with a single power-law distribution expected from leading-order QCD. Better agreement is provided either at next-to-leading order QCD (Jones et al. 2013) or by including saturation effects (Gay Ducati et al. 2013).
More than 180 physicists from around the world gathered at the Paul Scherrer Institut (PSI) last year for the 3rd workshop on the “Physics of fundamental Symmetries and Interactions” at low energies and the precision frontier – PSI2013. Broadly speaking, the focus was on high-precision experiments, with results complementary to those at the LHC, often covering a parameter space in physics beyond the Standard Model that is inaccessible to direct searches at the LHC or even at future colliders.
PSI’s particle-physics laboratory fosters cutting-edge research using the unmatched high power of its 590 MeV, 2.2 mA proton cyclotron to produce the brightest low-momentum beams of muons and pions and, since 2011, ultracold neutrons. This environment set the scene for lively discussions on the latest results and the future direction of worldwide low-energy precision experiments. Among the many workshop contributions, there were several major topical areas of interest.
Fundamental physics probed with antiprotons and antihydrogen featured prominently, with recent results from experiments at CERN’s Antiproton Decelerator. The now regular production of antihydrogen has moved these experiments closer to final physics measurements. Among the main goals are sensitive tests of CPT symmetry and measurements in antihydrogen spectroscopy, such as determination of the ground-state hyperfine splitting, together with tests of antihydrogen free fall. A recent result is the Penning-trap measurement by the ATRAP collaboration of the antiproton’s magnetic moment to 5 ppm precision. A further highlight, involving Penning traps but with ordinary matter, is determination of the electron’s mass with unprecedented precision by the MPI-Heidelberg group, achieving an order-of-magnitude improvement.
Many presentations covered experiments using cold (CN) or ultracold (UCN) neutrons. A full session was devoted to the neutron lifetime and worldwide progress on improving its precision, to resolve the significant outstanding discrepancy between results from neutron-storage experiments and those using beams. For the latter, a new result from the National Institute of Standards and Technology in the US was presented, consolidating the existing discrepancy.
Neutron-decay parameters and spin correlations of the decay particles are sensitive to physics beyond the Standard Model. Competing CN and UCN experiments using improved experimental techniques such as precision neutron polarimetry at the 100 ppm level were presented, with future plans for UCNs at Los Alamos National Laboratory (LANL) and the Proton Electron Radiation Channel project at the FRM II neutron source at the Technische Universität München. Other parity-violation experiments were also discussed, with a new result for neutron capture on hydrogen by the NPDG experiment at the Spallation Neutron Source (SNS) at Oak Ridge, trapped radium ions at KVI Groningen, and neutron spin rotation in helium.
UCN production with new-generation sources – either in existence or under construction – was extensively covered, including the use of superfluid helium (at Institut Laue–Langevin (ILL) and TRIUMF) and solid deuterium (Mainz, LANL and PSI) as superthermal converters. UCN densities are steadily increasing, despite experimental and technical difficulties that have slowed down the expected progress. The main thrust for these high-intensity UCN sources comes from the search for a permanent electric dipole moment (EDM) of the neutron. Because it is the focus of an experiment at PSI, there was intensive discussion on this topic at the workshop. Several talks elaborated on efforts to search for the neutron EDM by international collaborations at various institutions. These are mainly based on UCN-storage measurements that employ either Ramsey’s Oscillatory Field method (at ILL, SNS, PSI, the Petersburg Nuclear Physics Institute, TRIUMF, Osaka University and FRM II) or crystal diffraction (at ILL).
Complementary atomic (Fr, Ra, Xe) and molecular (YbF, ThO) EDM searches have even higher experimental sensitivities, but sometimes suffer from being more difficult to interpret in terms of the fundamental EDMs. Diamagnetic atoms are usually interpreted in terms of searches for nuclear EDMs, whereas measurements in polar molecules and paramagnetic atoms give limits on the electron EDM. However, the workshop was a little too early to see the result of the new ThO experiment ACME, by a Harvard/Yale University group, which appeared shortly afterwards. Proposed storage-ring-based EDM measurements with protons and deuterons are also being pursued actively.
Common to all of the EDM searches are the many challenging experimental difficulties, especially in terms of magnetic shielding and the control and measurement of the magnetic field. Presentations from the theoretical side underlined that EDM studies in different systems are complementary and necessary in helping to identify the underlying models of CP or T violation. Also in this context, recent results on CP violation were presented from the NA62 experiment at CERN, on the kaon system, and from LHCb at the LHC.
UCNs also allow study of the quantization of gravitational bound states of the neutron, which are sensitive to non-Newtonian gravity and hypothetical extra forces, mediated by, for example, axions, axion-like particles, or chameleons. Such forces can also be probed in clock-comparison experiments, as explained at the workshop for the 3He/129Xe case. These are sensitive to possible Lorentz violations, which can be accommodated in the framework of the so-called Standard Model Extension (SME). In the SME, Lorentz violation stems from an underlying background field in the universe, resulting, for example, in day/night or annual variations of fundamental parameters. Recently calculated effects in neutron decay, as well as in muonium and positronium spectroscopy, were also discussed, with experimental efforts.
Charged-lepton flavour violation was another key topic where increasing worldwide efforts are under way. Lepton flavour violation involving muons is predicted by various models that go beyond the Standard Model, at levels that might be within reach of the next generation of experiments. Nevertheless, major progress is needed, both in experimental techniques and in increased muon-beam intensities, and is being pursued actively.
The international PSI-based MEG collaboration presented its new limit of 5 × 10–13 on the μ → eγ branching ratio. The project to search for the decay μ → 3e at a sensitivity level of 10–16 was presented by the Mu3e collaboration. Impressive efforts towards the construction of the Muon Campus at Fermilab were also shown, with the goal of a new, more precise (g–2)muon measurement to help solve or confirm the present discrepancy with the Standard Model calculation. There are also plans to search for μ → e conversion within Project-X, at a sensitivity of 10–17 and beyond. Similar efforts in Japanese projects that are ongoing at Osaka University and the Japan Proton Accelerator Research Complex (J-PARC) were also detailed. These involve huge efforts in the muon sector towards, for example, μ → e conversion and muon g–2 experiments. The progress shown at J-PARC following repairs of the extensive earthquake damage was impressive.
There was great encouragement on the part of all participants to meet again at PSI for PSI2016
The new result on the pseudoscalar coupling between the muon and the proton from the MuCAP experiment at PSI was presented and discussed, finally solving a long-standing puzzle and providing the first precise value of this Standard Model parameter. Interpretations within recent calculations based on effective field theory were presented, together with relevant ongoing precision measurements in the deuterium system.
In the light of the current construction of the free-electron laser – SwissFEL – at PSI, the possible use of such high photon-intensities or electron beams for particle-physics experiments attracted much interest, for example in using high-intensity lasers for “light shining through the wall experiments”, which search for weakly interacting sub-electronvolt particles. The final session of the workshop – held with the detector workshop of the Swiss Institute of Particle Physics, CHIPP – provided an overview of state-of-the-art detector technology, which is under development to cope with future high-intensity experiments.
Aside from fundamental science, the Hochrhein Bigband jazz concert delighted participants, as did the workshop dinner featuring a “fundamental classic” of Swiss cusine – raclette. A workshop summary of the year 2034 provided an amusing outlook from a theoretician’s point of view of what might be important in particle physics 20 years from now. In the meantime, there was great encouragement on the part of all participants to meet again at PSI for PSI2016.
Aristotle said that ‘‘An iron ball of one hundred pounds, falling from a height of one hundred cubits [about 5.2 m], reaches the ground before a one-pound ball has fallen a single cubit.” Galileo Galilei replied, “I say that they arrive at the same time.” The universality of free fall illustrated by the latter’s legendary experiment at the tower of Pisa was formulated by Isaac Newton in his Principia and became, with Albert Einstein, the weak equivalence principle (WEP): the motion of any object under the influence of gravity does not depend on its mass or composition. This principle is the cornerstone of general relativity.
The WEP has been verified to incredible precision by dropping experiments and Eötvös-type torsion balances, the latter reaching an amazing accuracy of one part in 1013. The acceleration of the Earth and the Moon towards the Sun has also been determined to the same accuracy by measuring the transit time of laser pulses between the planet and the reflectors left on the Moon by the Apollo and Soviet space missions. But does the WEP also hold for antimatter for which no direct measurement has been performed, in particular for antimatter particles such as positrons or antiprotons? Or does antimatter even fall up?
The purpose of the 2nd International Workshop on Antimatter and Gravity, which took place on 13–15 November, was to review the experimental and theoretical aspects of antimatter interaction with gravity. The meeting was hosted by the Albert Einstein Center for Fundamental Physics of the University of Bern, following the success of the first workshop held in 2011 at the Institut Henri Poincaré in Paris. The highlights are summarized here.
Free-fall experiments with charged particles are notoriously difficult because they must be carefully shielded from electromagnetic fields
Free-fall experiments with charged particles are notoriously difficult because they must be carefully shielded from electromagnetic fields. For example, the sagging of the gas of free electrons in metallic shielding induces an electric field that can counterbalance the effect of gravity. Indeed, measurements based on dropping electrons led to a value of the acceleration of gravity, g, consistent with zero (instead of g = 9.8 m/s2). A free-fall experiment with positrons has not yet been performed, owing to the lack of suitable sources of slow positrons. In the 1980s, a team proposed a free-fall measurement of g with antiprotons at CERN’s Low Energy Antiproton Ring (LEAR), but it could not be performed before the closure of LEAR in 1996.
Using neutral antimatter such as antihydrogen can alleviate the disturbance from electromagnetic fields. The ALPHA collaboration at CERN’s Antiproton Decelerator (AD) has set the first free-fall limit on g with a few hundred antihydrogen atoms held for more than 400 ms in an octupolar magnetic field. The results exclude a ratio of antimatter to matter acceleration larger than 110 (normal gravity) and smaller than −65 (antigravity). Plans to measure this ratio at the level of 1% by using a vertical trap are under way.
Positronium matters
The AEgIS collaboration at the AD uses positronium produced by bombarding a nanoporous material with a positron pulse derived from a radioactive sodium source. Positronium (Ps) is then brought to highly excited states with lasers and mixed with captured antiprotons to produce antihydrogen (H) through the reaction Ps + p → e– + H. The highly excited antihydrogen atoms possess large electric dipole moments and can be accelerated with inhomogeneous electric fields to form an antihydrogen beam. The sagging of the beam over a distance of typically 1 m is measured with a two-grating deflectometer by observing the intensity pattern with high-resolution (around 1 μm) nuclear emulsions. AEgIS is currently setting up, with antiprotons (around 105) and positrons (3 × 107) successfully stacked. A first measurement of g is planned in 2015 and the initial goal is to reach 1% uncertainty.
As a neutral system, positronium is also suitable for gravity measurements, but free-fall experiments are not easy because positronium lives for 140 ns only. Such studies require sufficiently cold positronium in long-lived, highly excited states and the appropriate atom optics. Preparations for a free-fall experiment at University College London are under way.
At ETH Zurich, a team is measuring the 1s → 2s atomic transition in positronium with a precision better than one part per billion (1 ppb) by using a high-intensity positron beam that traverses a solid neon moderator and impinges on a porous silica target. The positronium ejected from the target is laser-excited to the 2s state and the γ-decay rate is measured by scintillating crystals, as a function of laser frequency. The 1s → 2s frequency can be calculated from hydrogen data. For hydrogen, the frequency is redshifted in the gravitational potential of the Sun, but the shift cannot be observed because the clocks used to measure the frequency are equally redshifted. However, for positronium (equal amounts of matter and antimatter) and assuming antigravity, measurements should yield a higher frequency than is calculated from hydrogen. At the level of 0.1 ppb, such studies could even test the hypothesis of antigravity as the Earth revolves around the Sun.
A similar experiment with muonium – an electron orbiting a positive muon – is planned at PSI in Switzerland. Ultra-slow muon beams with sub-millimetre sizes and sub-electronvolt energy for re-acceleration could also be used in a free-fall experiment employing gratings (a Mach–Zehnder interferometer).
Free-fall experiments
At CERN, the AD delivers bunches of 5.3 MeV antiprotons (3 × 107) every 100 s. However, storing antiprotons requires lower energies, which are reachable by inserting thin foils, albeit at the expense of substantial losses and degradation in beam size. Prospects for improved experiments are now bright with ELENA, a 30 m circumference electron-cooled ring that decelerates the AD beam further to 100 keV (figure 1). ELENA will be installed in 2015 and will be available for physics in summer 2017.
The first free-fall experiment to profit from this new facility will be GBAR. Antihydrogen atoms will be obtained by the interaction of antiprotons from ELENA with a positronium cloud. The positrons will be produced by a 4.3 MeV electron linac. In contrast to AEgIS, the antihydrogen atom will capture a further positron to become a positively charged ion, which can be transferred to an electromagnetic trap, cooled to 10 mK with cold beryllium ions and then transported to a launching trap where the additional positron will be photodetached. The mean velocity of the antihydrogen atoms will be around 1 m/s and the fall distance will be about 30 cm. GBAR will be commissioned in 2017 with the initial goal of reaching 1% accuracy on g.
The sensitivity of GBAR, limited by the velocity distribution of the antihydrogen atoms, could be improved substantially by using quantum reflection, a fascinating effect that was discussed at the workshop. Antihydrogen atoms dropped towards a surface experience a repulsive force, which leads to gravitational quantum states. A similar phenomenon was observed with cold neutrons at the Institut Laue–Langevin (ILL) in Grenoble. Now, the ILL team proposes to bounce the atoms in GBAR between two layers – a smooth lower surface to reflect slow enough antihydrogen atoms and a rough upper surface to annihilate the fast ones. Transition frequencies between the gravitational levels – which depend on g – could also be measured by recording the annihilation rate on the bottom surface. Provided that the lifetime of these antihydrogen levels is long enough, orders of magnitude improvements could be obtained on the determination of g.
Atom interferometers might be able to measure g to within 10–6. In a Ramsey–Bordé interferometer, the falling atom interacts with pulses from two counter-propagating vertical laser beams. Having absorbed a photon from the first beam, the atom is stimulated to emit another photon with the frequency of the second beam, thereby modifying its momentum. The signal from the annihilating antihydrogen atom, for example at the top of the interferometer, interferes with the one from another atom that has equal momentum but was not subject to the laser kick. The interference pattern will depend on the value of g.
At FLAIR the antiproton flux will be an order of magnitude higher than at ELENA
In the more distant future, the Facility for Low-energy Antiproton and Ion Research (FLAIR) will become operational at GSI. As an extension to the high-energy antiproton facility, FLAIR will consist of a low-energy storage ring decelerating antiprotons from 30 MeV to 300 keV, followed by an electrostatic ring capable of reducing the energy even further, down to 20 keV. At FLAIR the antiproton flux will be an order of magnitude higher than at ELENA, and slow extracted antiproton beams will be available for experiments in nuclear and particle physics.
The question of how large an effect these free-fall experiments could measure cannot be answered without theoretical assumptions, such as exact symmetry between matter and antimatter (the CPT theorem). However, string theory can break CPT. The standard model extension proposed by the Indiana/Carleton group involves Lorentz and CPT violation. Also, atoms and nuclei contain virtual antiparticles in amounts that depend on the atomic number. The calculable quantum corrections agree with measurements, arguing against antigravity. However, there is a huge discrepancy in the value of the cosmological constant estimated from vacuum particle–antiparticle pair fluctuations, which might question our understanding of the interaction between gravity and virtual particles. As pointed out at the workshop, if all of the theoretical assumptions are valid, then antimatter experiments should not expect to see discrepancies in g at a level larger than 10–7. Ultimately, the issue must be settled by experiments.
To compare with matter, a presentation was given on the 10–9 precision achievable on g at the Swiss Federal Institute of Metrology (METAS) using a free-fall interferometer. Together with improved measurements of Planck’s constant with a watt balance, this might lead to a re-definition of the kilogram based on natural units.
The workshop also included a session on antimatter in the universe. Is there any antimatter and could it repel matter (the Dirac–Milne universe) and provide the accelerating expansion? Can the excess of positrons observed above 10 GeV by balloon experiments, the PAMELA satellite experiment and, more recently, the Fermi Gamma-ray Space Telescope and the Alpha Magnetic Spectrometer (AMS-02), be explained by antimatter annihilation?
In his summary talk, Mike Charlton of Swansea University concluded that “the challenge of measuring gravity on antihydrogen remains formidable”, but that “in the past decade the prospects have advanced from the totally visionary to the merely very difficult”.
The workshop, with 28 plenary talks, was attended by 70 participants. A visit to the house where Einstein spent the years 1903–1905 and dinner at Altes Tramdepot were part of the social programme.
To provide the best experiences, we use technologies like cookies to store and/or access device information. Consenting to these technologies will allow us to process data such as browsing behavior or unique IDs on this site. Not consenting or withdrawing consent, may adversely affect certain features and functions.
Functional
Always active
The technical storage or access is strictly necessary for the legitimate purpose of enabling the use of a specific service explicitly requested by the subscriber or user, or for the sole purpose of carrying out the transmission of a communication over an electronic communications network.
Preferences
The technical storage or access is necessary for the legitimate purpose of storing preferences that are not requested by the subscriber or user.
Statistics
The technical storage or access that is used exclusively for statistical purposes.The technical storage or access that is used exclusively for anonymous statistical purposes. Without a subpoena, voluntary compliance on the part of your Internet Service Provider, or additional records from a third party, information stored or retrieved for this purpose alone cannot usually be used to identify you.
Marketing
The technical storage or access is required to create user profiles to send advertising, or to track the user on a website or across several websites for similar marketing purposes.
