Physics on the large scale is based on Einstein’s theory of general relativity, which interprets gravity as the curvature of space–time. Despite its tremendous success as an isolated theory of gravity, general relativity has proved problematic in its integration with physics as a whole, and in particular with the physics of the very small, which is governed by quantum mechanics. There can be no unification of physics that does not include both general relativity and quantum mechanics. Superstring theory and its recent extension to the more general theory of branes is a popular candidate for a unified theory, but the links with experiment are very tenuous. The approach known as loop quantum gravity attempts to quantize general relativity without unification, and has so far received no obvious experimental verification. The lack of experimental guidance has made the issue extremely hard to pin down.
One hundred years ago, when Max Planck introduced the constant named after him, he also introduced the Planck scales, which combined his constant with the velocity of light and Isaac Newton’s gravitational constant to give the fundamental Planck time around 10–43 s, the Planck length around 10–35 m and the Planck mass around 10–8 kg. Experiments on quantum gravity require access to these scales, but direct access using accelerators would require machines that reach an energy of 1019 GeV, well beyond the reach of any experiments currently conceivable.
For almost a century it has been widely perceived that the lack of experimental evidence for quantum gravity presents a major barrier to a breakthrough. One possible way of investigating physics at the Planck scale, however, is to use the kind of approach developed by Albert Einstein in his study of thermal fluctuations of small particles through Brownian motion, where he showed that the visible motion provided a window onto the invisible world of molecules and atoms. The idea is to access the Planck scale by observing decoherence in matter waves caused by quantum fluctuations, as first proposed using neutrons more than 20 years ago by CERN’s John Ellis and colleagues (Ellis et al. 1984). Since then, ultra-cold atom technologies have advanced considerably, and armed with the sensitivity of modern atomic matter-wave interferometry we are now in a position to consider using “macroscopic” instruments to access the Planck scales, a possibility that William Power and Ian Percival outlined more recently (Power and Percival 2000).
Our recent work represents a new approach to gravitationally produced decoherence near the Planck scale (Wang et al. 2006). It has been made possible by the recent discovery by one of us of the conformal structure – the scaling property of geometry – of canonical gravity, one of the earliest important approaches to quantum gravity. This leads to a theoretical framework in which the conformal field interacts with gravity waves at zero-point energy using a conformally decomposed Hamiltonian formulation of general relativity (Wang 2005). Working in this framework, we have found that the effects of ground-state gravitons on the geometry of space–time can lead to observable effects by causing quantum matter waves to lose coherence.
The basic scenario is that near the Planck scale, ground-state gravitons constantly stretch and squash the geometry of space–time causing conformal fluctuations in space–time. This process is analogous to the Brownian motion of a pollen particle interacting with ambient molecules of much smaller sizes. It means that information on gravitons near the Planck scale can be extracted by observing the conformal fluctuations of space–time, which can be done by analysing their blurring effects on coherent matter waves.
The curvature of space–time produces changes in proper time, the time measured by moving clocks. For sufficiently short time intervals, near the Planck time, proper time fluctuates strongly owing to quantum fluctuations. For longer time intervals, proper time is dominated by a steady drift due to smooth space–time. Proper time is therefore made up of the quantum fluctuations plus the steady drift. The boundary separating the shorter time-scale fluctuations from the longer time-scale drifts is marked by a cut-off time, τcut-off, which defines the borderline between semi-classical and fully quantum regimes of gravity. It is given by τcut-off = λTPlanck, for quantum-gravity theories, where TPlanck is the Planck time, and λ is a theory-dependent parameter determined by the amplitude of zero-point gravitational fluctuations. A lower limit on λ is given by noting that the quantum-to-classical transition should occur at length scales λLPlanck that are greater than the Planck length LPlanck by a few orders of magnitude, so we can expect λ > 102.
A matter-wave interferometer can be used to measure quantum decoherence due to fluctuations in space–time, and hence provide experimental guidance to the value of λ. In an atom interferometer an atomic wavepacket is split into two wavepackets, which follow different paths before recombining (see “Atom interferometer”). The phase change of each wavepacket is proportional to the proper time along its path, resulting in an interference pattern when the wavepackets recombine. The detection of the decoherence due to space–time fluctuations on the Planck scale would provide experimental access to quantum-gravity effects analogous to accessing to atomic scales provided by Brownian motion.
In our analysis we found an equation that gives λ (See “equation”).
M is the mass of the quantum particle; T is the separation time before two wavepackets recombine; and Δ denotes the loss of contrast of the matter wave and is a measure of the decoherence (Wang et al. 2006). Existing matter-wave experiments set limits on the size of λ, their sensitivity depending on both Δ and M. Results using caesium atom interferometers (Chu et al. 1997) and also from a fullerene C70 molecule interferometer (Hackermueller et al. 2004) with its larger value of M, both set a lower bound for λ of the order of 104, well within the theoretical limits of λ > 102. This suggests that the sensitivities of advanced matter-wave interferometers may well be approaching the fundamental level due to quantum space–time fluctuations. Investigating Planck-scale physics using matter-wave interferometry may therefore become a reality in the near future.
Further improved measurements will confirm and refine this bound on λ, pushing it to higher values. An atom interferometer in space, such as the proposed HYPER mission, could provide such improvements. However, the lower bound of λ calculated using current experimental data is already within the expected range. This is a very good sign and strongly suggests that the measured decoherence effects are converging towards the fundamental decoherence due to quantum gravity. Therefore, a space mission flying an atom-wave interferometer with significantly improved accuracy will be able to investigate Planck-scale physics.
As well as causing quantum matter waves to lose coherence at small scales, the conformal gravitational field is responsible for cosmic acceleration linked to inflation and the problem of the cosmological constant. Our formula, which relates the measured decoherence of matter waves to space–time fluctuations, is “minimum” in the sense that ground-state matter fields have not been taken on board. Their inclusion may further increase the estimated conformal fluctuations and result in an improved “form factor” in our formula. In this sense, the implications go beyond quantum gravity to more generic physics at the Planck scale. Furthermore, it opens up new perspectives of the interplay between the conformal dynamics of space–time and vacuum energy due to gravitons, as well as elementary particles. (A well known example of vacuum energy is provided by the Casimir effect.) These may have important consequences on cosmological problems such as inflation and dark energy.
The electron’s magnetic moment has recently been measured to an accuracy of 7.6 parts in 1013 (Odom et al. 2006). As figure 1a indicates, this is a six-fold improvement on the last measurement of this moment made nearly 20 years ago (Van Dyck et al. 1987). The new measurement and the theory of quantum electrodynamics (QED) together determine the fine structure constant to 0.70 parts per billion (Gabrielse et al. 2006). This is nearly 10 times more accurate than has so far been possible with any rival method (figure 1b). Higher accuracies are expected, based upon convergence of many new techniques – the subject of a half-dozen Harvard PhD theses during the past 20 years. A one-electron quantum cyclotron, cavity-inhibited spontaneous emission, a self-excited oscillator and a cylindrical Penning trap contribute to the extremely small uncertainty. For the first time, researchers have achieved spectroscopy with the lowest cyclotron and spin levels of a single electron fully resolved via quantum non-demolition measurements, and a cavity shift of g has been directly observed.
A circular storage ring is the key to these greatly improved measurements, but the storage ring is unusual compared with those at CERN, for example. To begin with it uses only one electron, stored and reused for months at a time. The radius of the storage ring is much less than 0.1 µm, and the electron energy is so low that we use temperature units to describe it – 100 mK. Furthermore, the electron does not orbit in a familiar circular orbit even though it is in a magnetic field; instead, it makes quantum jumps between only the ground state and the first excited states of its cyclotron motion – non-orbiting stationary states. It also makes quantum jumps between spin up and spin down states. Blackbody photons stimulate transitions between the two cyclotron ground states until we cool our storage ring to 100 mK to essentially eliminate them. The spontaneous emission of synchrotron radiation is suppressed because of its low energy and by locating the electron in the centre of a microwave cavity. The damping time is typically about 10 seconds, about 1024 times slower than for a 104 GeV electron in the Large Electron–Positron collider (LEP). To confine the electron weakly we add an electrostatic quadrupole potential to the magnetic field by applying appropriate potentials to the surrounding electrodes of a Penning trap, which is also a microwave cavity (figure 2a).
The lowest cyclotron and spin energy levels for an electron in a magnetic field are shown in figure 2b. (Very small changes to these levels from the electrostatic quadrupole and special relativity are well understood and measured, though they cannot be described in this short report.) Microwave photons introduced into our trap cavity stimulate cyclotron transitions from the ground state to the first excited state. The long cyclotron lifetime allows us to turn on a detector to count the number of quantum jumps for each attempt as a function of cyclotron frequency νc (figure 3d). A similar quantum jump spectroscopy is carried out as a function of the frequency of a radiofrequency drive at a frequency νa = νs – νc, which stimulates a simultaneous spin flip and cyclotron excitation, where νs is the spin precession frequency (figure 3c). The lineshapes are understood theoretically. One-quantum cyclotron transitions (figure 3b) and spin flips (figure 3a) are detected with good signal-to-noise from the small shifts that they cause to an orthogonal, classical electron oscillation that is self-excited.
The dimensionless electron magnetic moment is the magnetic moment in units of the Bohr magneton, ehbar/2m, where the electron has charge –e and mass m. The value of g is determined by a ratio of the frequencies that we measure, g/2 = 1 + νa/νc, with the result that g/2 = 1.00115965218085(76) [0.76 ppt]. The uncertainty is nearly six times smaller than in the past, and g is shifted downwards by 1.7 standard deviations (Odom et al. 2006).
What can be learned from the more accurate electron g? The first result beyond g itself is the fine structure constant, α = e2/4πε0hbarc – the fundamental measure of the strength of the electromagnetic interaction, and also a crucial ingredient in our system of fundamental constants. A Dirac point particle has g = 2. QED predicts that vacuum fluctuations and polarization slightly increase this value. The result is an asymptotic series that relates g and α:
According to the Standard Model, hadronic and weak contributions are very small and believed to be well understood at the accuracy needed. Impressive QED calculations give exact C2, C4 and C6, a numerical value and uncertainty for C8, and a small aµτ. Using the newly measured g in equation 1 gives α–1 = 137.035999710(96) [0.70 ppb] (Gabrielse et al. 2006). The total uncertainty of 0.70 ppb is 10 times smaller than for the next most precise methods (figure 1b), which determine α from measured mass ratios, optical frequencies, together with rubidium (Rb) or caesium (Cs) recoil velocities.
The second use of the newly measured electron g is in testing QED. The most stringent test of QED – which is one of the most demanding comparisons of any calculation and experiment – continues to come from comparing measured and calculated g-values, the latter using an independently measured α as an input. The new g, compared with equation 1 with α(Cs) or α(Rb), gives a difference δg/2 < 15 × 10sup>–12 (see Gabrielse 2006 for details and a discussion.) The small uncertainties in g/2 will allow a 10 times more demanding test if ever the large uncertainties in the independent α values can be reduced. The prototype of modern physics theories is thus tested far more stringently than its inventors ever envisioned – as Freeman Dyson remarks in his letter at the beginning of the article – with better tests to come.
The third use of the measured g is in probing the internal structure of the electron – limiting the electron to constituents with a mass m* > m/√(δg/2) = 130 GeV/c2, corresponding to an electron radius R <1 × 10–18 m. If this test was limited only by our experimental uncertainty in g, then we could set a limit m* > 600 GeV. This is not as stringent as the related limit set by LEP, which probes for a contact interaction at 10.3 TeV. However, the limit is obtained quite differently, and is somewhat remarkable for an experiment carried out at 100 mK.
The fourth use of the new electron g concerns measurements of the muon g – 2 as a way to search for physics beyond the Standard Model. Even though the muon g values have nearly 1000 times larger uncertainties than the new electron g, heavy particles – possibly unknown in the Standard Model – are expected to make a contribution that is much larger for the muon. However, this contribution would still be very small compared with the calculated QED contribution, which depends on α and must be subtracted out. The electron g provides α and a confidence-building test of the QED, both needed for the large subtraction.
CERN has long embraced particle physics at whatever energy scales are most appropriate for learning about fundamental reality. It is impressive that CERN is replacing the highest energy electron–positron collider, LEP, with the world’s highest energy proton collider, the Large Hadron Collider. Also at CERN, however, the lowest energy antiproton storage rings are also operating. One antiproton cooled to 4.2 K was used to show that the magnitudes of q/m for the proton and antiproton were the same to better than nine parts in 1011 – the most stringent test of CPT invariance with a baryon system.
Now, these low-energy antiproton techniques are being used to make the coldest possible antihydrogen atoms, to be used for higher-precision tests of fundamental symmetries. It is fitting that the new measurement of the electron magnetic moment and the fine structure constant were carried out in the lab of a long-time CERN researcher, since they illustrate the power of low-energy techniques of the sort that we are applying to antihydrogen studies at CERN’s Antiproton Decelerator facility, the unique source of low-energy antiprotons.
The D0 Collaboration at Fermilab has announced the first measurement of the cross-section for WZ pair production in proton–antiproton collisions. The cross-section times branching ratio for the process is the smallest ever measured at a hadron collider. The data for this result were taken from more than 1 fb–1 of total collision data at the Tevatron, and a sample of 1.5 thousand million events.
Making this measurement requires events in which both the W and Z boson decay to leptons, but while such events provide the cleanest signature of WZ events, they constitute only 1.4% of all WZ decays. D0 found 12 events, each containing three charged leptons with high transverse momentum together with missing transverse energy (indicating an undetected neutrino), with an expected background of 3.6±0.2 events. The probability that the background accounts for these 12 events is 4.1 × 10–4, which constitutes 3.3 σ evidence for WZ pair production. With these events D0 measures the WZ production cross-section to be 4.0 +1.9–1.5 pb, which is consistent with the Standard Model prediction of 3.6±0.3 pb.
The coupling of the weak vector bosons is an important consequence of the non-Abelian nature of the Standard Model, and the rate for the associated production of W and Z bosons in proton–antiproton collisions allows this coupling to be probed. The kinematics of the Z boson decay can also be used to characterize the interaction between the W and Z and provide further constraints on the nature of the electroweak force. In addition, measuring the cross-section times branching ratio for Standard Model processes with such low rates is an important stepping stone in the search for the Higgs boson at the Tevatron.
A team at Harvard University has made a new precise measurement of the electron magnetic moment, which in turns allows the fine structure constant to be determined with an uncertainty 10 times smaller than previously attained.
Gerald Gabrielse and colleagues have measured the value of the constant g of the electron, which relates its magnetic moment to the Bohr magneton, ehbar/2m, where e is the size of the charge on the electron, and m is the electron’s mass. For a Dirac point particle of spin 1/2, g should have a value of 2, but quantum electrodynamics (QED) predicts a value slightly higher, owing to vacuum fluctuations and polarizations effects.
To measure g more precisely than before, the Harvard team has resolved the cyclotron and spin energy levels of an electron confined for several months in a cylindrical Penning trap cooled to 100 mK (Odum et al. 2006). The value they obtained is g/2 = 1.00115965218085(76); the uncertainty of 0.76 ppt is nearly six times lower than the most recent accepted value, measured nearly 20 years ago (Van Dyck et al. 1987).
Working with Cornell University and RIKEN in Japan, Gabrielse and colleagues have used this new value of g with a prediction from QED involving 891 eighth-order Feynman diagrams, to determine a new value for the fine structure constant, α. They obtain α–1 = 137.035999710(96), that is, with an uncertainty of 0.70 ppb – an uncertainty that is about 10 times smaller than for any rival method to determine a (Gabrielse et al. 2006).
On 14 February 2006 the first fully instrumented ANTARES detector line was deployed and placed at a predetermined place on the bottom of the Mediterranean Sea, about 40 km off the coast of Toulon, and at a depth of 2500 m. On 2 March, a remote-controlled submarine connected the line to the junction box, the terminal at the end of the 40 km telecommunications cable that leads to the shore station at La Seyne-sur-Mer. On the same day the line recorded the first cosmic-ray tracks. This is the first of 12 lines to be deployed over the next 18 months, after many years of tests that have investigated conditions at the detector site and parts of the detector set-up. An instrumentation line has been taking data smoothly since April 2005 (Aguilar et al. 2006).
ANTARES is one of only a few detectors employing natural seawater or ice as the detector medium to search for neutrinos of extraterrestrial origin. These neutrinos may have been produced in high-energy events in the cosmos, travelling towards us undisturbed by intervening matter or magnetic fields. If their direction can be determined then their origin in the universe can be identified. High-energy neutrinos could also be indicators for certain types of dark matter.
The neutrino is weakly interacting and this sets the scale of the detectors. Only truly gargantuan sizes allow the detection of neutrinos with sufficient sensitivity to be useful. It was an idea of Moissey Markov in 1960 that gave impetus to the possibility of neutrino astronomy. He reasoned that if one concentrated on muon-neutrinos through the detection of a muon produced in a charged-current interaction, then the large range of the muon in matter would allow for large effective volumes. The direction of the muon is closely related to the direction of the neutrino, and if the detection medium is water or transparent ice then the muon can be tracked through its emission of Cherenkov radiation.
For ANTARES, the Mediterranean Sea and the rock below the seabed provide the interaction volume, and the water provides the detection medium. Because of its large scattering length for Cherenkov light, the seawater allows excellent timing and, consequently, good directional accuracy can be obtained. The Mediterranean was chosen because it is in the Northern Hemisphere and provides complementary sky coverage, including the centre of our galaxy, to the AMANDA and IceCube detectors that are operating in the Antarctic ice.
The overall detector consists of storeys suspended at intervals of 14.5 m along a 500 m vertical cable, which is anchored to the sea floor and held vertical by a buoy at the top of the cable. The storeys begin at 100 m above the seabed and there are 25 such storeys on a line. Placing more of these cables at distances of 70 m increases the volume of the detector.
Figure 1 shows a storey in situ in the sea, with the glass pressure spheres housing the 25 cm diameter photomultiplier tubes (PMTs) that are used to detect the Cherenkov light. The PMTs point downwards at an angle of 45° with respect to the vertical. Two are clearly visible, whereas the third is hidden behind the titanium cylinder that contains the electronics for the readout and control of the storey, and an electronic compass, plus a tilt meter. A hydrophone, used for acoustic positioning, is located at the bottom of the storey.
The PMTs operate at a threshold of 0.4 photoelectrons. All the data produced by the tubes are transferred via an optical cable to the shore, where a farm of computers processes the data to extract interesting events. Because of radioactive potassium present in the seawater, each PMT has a base rate of about 60 kHz; bioluminescent life in the seawater may increase this rate. It is the task of the software running on the computer farm to recognize the presence of a muon track among the background hits. At present the software is able to perform this task up to about five times the base rate. However, conditions at the bottom of the sea can vary significantly. There was a period of relative calm just after deployment, followed by two months of high bioluminescent activity, making data-taking difficult; now the background activity has subsided and normal data-taking has resumed.
The present software selects slices in time and searches for the passage of muons through their time patterns in the PMTs along the string. The final stage of the process is a χ2 minimization fit to the height versus time pattern. The reconstructed data set is dominated by down-going muons originating from high-energy cosmic-ray showers in the atmosphere. The main signature of a neutrino-induced muon is that it originates from below, in which case the Earth acts as a very effective filter against the directly produced muons.
Figure 2 shows two examples of reconstructed tracks from data from the first ANTARES line. Figure 2a shows a vertical muon track, where the signal propagates down the line with the velocity of the muon, and figure 2b shows a slightly more inclined track identified by the change in the signal’s vertical propagation velocity, before and after the closest approach. So far several thousand tracks of down-going muons have been reconstructed and a few candidates for up-going muons have also been observed.
The ANTARES Collaboration is now in full swing analysing the data coming from this first detector line. The experience of the first line shows that we are on track towards a full neutrino telescope in the Mediterranean and we look forward to several years of data-taking.
Understanding our universe from basic physics is an ambitious goal involving many disciplines in physics. One key ingredient is nuclear astrophysics, with its focus on explaining energy production and chemical evolution in the universe – topics that are coupled through nuclear reactions that transform elements and may also release energy. The first overview of the synthesis of elements was about 50 years ago, with the work of Geoffrey and Margaret Burbidge, Willy Fowler and Fred Hoyle, and, independently, Al Cameron. Although there had been some important work earlier in the 20th century, this was the defining moment for nuclear astrophysics.
At the end of June, nearly 250 astronomers, astrophysicists, cosmologists and nuclear physicists met at CERN for the ninth Nuclei in the Cosmos meeting to summarize the status of the field. Organized by a team from the Isotope Separator On Line (ISOLDE) and Neutron Time-of-Flight (n_TOF) facilities at CERN, it was dedicated to the memory of Al Cameron, Ray Davis and John Bahcall, all of whom have recently died and had played major roles in helping to understand the production and role of nuclei in the cosmos.
Nuclei, of course, consist of smaller particles and the meeting reviewed recent developments in cosmology and their possible connection to particle physics. While at one time particle physics provided input for calculating the abundances of elements created during the early universe in Big Bang nucleosynthesis, nowadays the results from the Wilkinson Microwave Anisotropy Probe yield the baryonic density of the universe, which is used in calculating the abundances. However some problems in reconciling observations and calculations for the primordial elements remain, in particular for the two stable lithium isotopes, 6Li and 7Li.
Analysing nuclear ashes
Optical observations of stars reveal their element abundances. Old stars are metal-poor (following the convention in astronomy that all elements above helium are metals) and the heavy elements in them appear to be made exclusively by rapid neutron capture – the r-process. Only later in galactic evolution does the s-process – slow neutron capture – begin to contribute as well. The relative abundances for elements above barium fit well with r-process abundances deduced for solar-system material, but for lighter elements there are differences that could indicate the presence of a second (“weak”) r-process. The coming years should bring clarification as the amount of observational data will increase significantly owing to the large-scale surveys, the Hamburg/ESO R-process Enhanced Star survey and the Sloan Digital Sky Survey.
Another fruitful source of abundance data comes from presolar grains embedded in primitive meteorites. Here a recent breakthrough has been the extraction of isotopic ratios for many different grains. Such detailed information about isotope abundance helps in constraining the astrophysical conditions in which the grains were formed.
Refinements in knowledge of element abundances are not restricted to distant stars. An improved modelling of the solar atmosphere indicates that solar abundances of most “metals” should be decreased by more than a third. This change arises from a more careful and more dynamic treatment of the outer solar layers.
More direct evidence for nuclear processes in stars comes from the observation of the radioactive isotopes that are produced. High-resolution gamma-ray spectrometers have operated in space for some years, for example on the INTEGRAL satellite (figure 1). Several galactic radioactive decays have been observed through their gamma-emission lines, such as those of 26Al, 44Ti and 60Fe, although the decay of 22Na and positron emission both remain to be seen. Many of the radioactive isotopes found on Earth also stem from stellar events. A recent addition to the list is 60Fe, which has been identified in deep-ocean material by the highly sensitive accelerator mass spectrometer technique, indicating that a supernova exploded near the Earth about 2.4 million years ago.
Modelling the evolution of stars and their sometimes violent end requires the coordinated work of many people. It is rather like assembling a giant multi-dimensional jigsaw puzzle, but one in which the pieces have first to be found. Some researchers concentrate on finding these pieces, while others focus on how to fit them together to form a coherent picture. However, we have still not identified the important ingredients for all stellar events.
For normal, “quiet” stellar burning, the nuclear reactions take place between stable isotopes, but with extremely small cross-sections that are hard to reproduce in the laboratory. Major efforts during the past decade have improved the situation, and participants at the meeting heard of progress on one of the remaining challenges, the reaction 12C(α,γ)16O, which is a key reaction for the processes responsible for the production of many elements.
At higher temperatures reaction rates increase and reactions involving radioactive nuclei need also to be known. A major highlight at the meeting in terms of nuclear data is a recent good estimate of the reaction rate for the key radiative alpha-capture process on 15O, which has been pursued for more than 20 years. The direct measurement of radiative proton-capture on 26gAl and alpha-capture on 40Ca was also presented. Lastly, reaction rates for several neutron-induced reactions were reported; these will have important implications for understanding the synthesis of elements heavier than iron – the “neutron capture elements”.
Very high temperatures belong to the domain of cosmic explosions – novae, supernovae, X-ray bursts and gamma-ray bursts – which have a fascinating history and rightly continue to attract attention. Numerical results presented at the meeting indicate that first-generation nova explosions occur at higher temperatures than classical novae. They could therefore be more important players in the early universe, but more studies are needed to confirm this.
A highlight of the conference was the presentation of successful computer simulations of supernova explosions (figure 2). Here, the key new ingredient is to follow the two-dimensional hydrodynamic evolution to long durations after the core-collapse, when the inner part has become a highly non-spherical object with significant fluctuations. The violent conditions in a supernova are perfect for cooking elements and the meeting heard about a new mechanism, the np-process, in which the strong neutrino fluxes play a more versatile role in the nucleosynthesis than imagined earlier. An exciting report on quantitative calculations of the r-process suggested that nuclear fission, and in particular neutron-induced fission, might play a very important role for the dynamics in later stages of the r-process.
It was clear from the meeting that nuclear astrophysics is rapidly evolving. The next meeting in the series will provide another snapshot of the status of the field, when it takes place in the US in 2008, hosted by the Michigan State University/National Superconducting Cyclotron Laboratory and the Joint Institute for Nuclear Astrophysics.
Coimbra, in central Portugal, was the country’s capital from 1143 to 1255 and in historical importance ranks behind only Lisbon and Oporto. Its university was founded in 1290 and was the only one in Portugal until the beginning of the 20th century. Its ancient setting contrasted well with the central theme of TOP2006: the top quark, discovered only in 1995 in experiments at Fermilab’s Tevatron.
The workshop itself grew from the idea of developing a strong collaboration between theorists and experimentalists who are interested in studying the properties of the top quark. The first properties of this unique particle were measured during Run I of the Tevatron by the CDF and D0 experiments; with Run II more data are now becoming available. Though not yet sufficient to perform the precision tests required to challenge (once again) the Standard Model, the data acquired so far are already providing valuable information on top-quark physics. The knowledge of the physics of the top quark will then enter a totally new phase – the precision era – with the start-up of the Large Hadron Collider (LHC) at CERN, foreseen towards the end of 2007.
The top quark is the heaviest quark found (mt = 172.5±2.3 GeV/c2) and is still believed to be a fundamental particle. It completes the third-generation structure of the Standard Model, as the isospin partner of the b (bottom) quark. Why it is so heavy and why its Yukawa coupling to the Higgs field (after spontaneous symmetry breaking) is of the order of 1 is a mystery. Its solution requires an answer to the question: does the top quark play a special role in the electroweak symmetry-breaking mechanism of the Standard Model?
Although mainly produced via the strong interaction at particle colliders (double production via gluon–gluon fusion or qqbar annihilation), the top quark decays through the weak force to a b quark and a W boson with a branching ratio of almost 100%. Because of their large mass and decay rate (Γ = 1.42 GeV at next-to-leading order), top quarks, unlike any other quark, are produced and decay as free particles. With a very short lifetime (around 10–25 s), the top quark decays before hadronization can take place. For the same reason no toponium bound states with sharp binding energy are expected in the Standard Model; any evidence of a ttbar bound state would be a sign of physics beyond the model. The flavour-changing neutral-current decays of the top quark are also highly suppressed in the Standard Model, with branching ratios at the level of around 10–12 to 10–14; any evidence of decays such as t → qZ, qγ or qg would therefore constitute a sign of new physics.
Top-quark properties
The first day of the workshop was dedicated to the current theoretical and experimental status of top-quark physics, in the morning and afternoon sessions, respectively. C P Yuan of Michigan State University recalled the need for a precise measurement of the top-quark mass to constrain the Higgs mass when combined with the measurement of the W mass. Within the context of current theoretical knowledge, the day also covered the importance of the rate of single top production at colliders (not yet observed) as a probe for the element Vtb in the Cabibbo–Kobayashi–Maskawa matrix. He also stressed the fact that the different channels (s, t and Wt) that contribute to single top production are important processes for the search for physics beyond the Standard Model.
Aurelio Juste from Fermilab reviewed the current experimental status of the top quark starting from the total cross-section measurement at the Tevatron, with a relative precision of around 25% in Run I, dominated essentially by statistics. In Run II, with a luminosity of 2 fb–1, this error is expected to be reduced to about 10%. The mass is by far the most precisely measured property of the top quark, with a relative error less than 2%. The top charge, anomalous couplings and single top production were also discussed.
The second day examined the experimental methods used to select top quarks at colliders, and the leading-order and next-to-leading-order generators and theoretical methods available for understanding the data. Evelyn Thomson of the University of Pennsylvania presented the experimental methods that are used in the selection and analysis of top-quark decays at hadron colliders. In particular, she discussed the importance of the trigger, the difficult question of the background rejection and estimation (as W+jets and Z+jets), the need for a detailed calibration and determination of the jet energy scale (a major source of systematic error), and b-tagging, a key tool to reduce the background. She stressed the need to fine-tune the available Monte Carlo to reproduce data accurately. Available top-selection tools involve multivariate analysis and different statistical techniques.
Werner Bernreuther, of RWTH (Rheinisch-Westfälische Technische Hochschule) Aachen, described spin effects in hadronic top-pair production and polarized top decays, ttbar spin correlations (which are transferred to the decay products), and the possible existence of heavy ttbar resonances. As the top polarization is reliably calculable, it is well suited for experimental checks of the predictions of the Standard Model and its extensions. Bernreuther concluded that the top-quark physics is an excellent probe to test electroweak symmetry breaking and that it provides powerful observations to determine the structure of the tbW vertex. Sergey Slabospitsky of the Institute for High Energy Physics, Protvino, and Borut Kersevan of the Josef Stefan Institute presented the status of the important event generators that are being developed and used at the Tevatron and LHC to simulate top production and decays.
Top prospects
The prospects for top physics on the up-coming colliders were discussed on the third day of the workshop. In the morning, Dominique Pallin of Blaise Pascal University presented the expected performance of the LHC as a top factory. In particular, he showed the work going on for early top-quark studies, such as the measurement of the ttbar production cross-section and the top mass, as well as the determination of the W and top polarizations, in the lepton+jets channel. The top quark is a very useful calibration tool for early data (for the jet energy scale, b-tagging, trigger etc), which can also be used to check detector performance. With the increase of luminosity at the LHC many precision measurements of top-quark properties will be possible.
In the afternoon, Lynne Orr of the University of Rochester gave a talk about top physics at the LHC and a future International Linear Collider (ILC). She described the electroweak symmetry breaking mechanism and the hierarchy problem. She also discussed top-quark physics in models beyond the Standard Model, which are possible solutions to this problem: supersymmetry, little Higgs, technicolour and its descendents, and modified space–time models with extra dimensions. Finally, the sensitivity of different top-quark couplings at the LHC and ILC was reviewed. Brian Foster of Oxford University presented the status of the ILC.
Finally John Womersley, of the CCLRC, Rutherford Appleton Laboratory, presented a lively and appealing workshop summary talk. He also covered the status and the open questions in particle and astroparticle physics. All in all, the workshop was a fruitful opportunity for interesting discussions on the exciting subject of top-quark physics. The participants are looking forward to the next workshop, which will probably take place two years from now, where the latest results of the Tevatron’s Run II and the first results from the LHC in top-quark physics will be presented and discussed, and new challenges to the Standard Model will be tested.
The Major Atmospheric Gamma-ray Imaging Cherenkov (MAGIC) Telescope has discovered variable very-high-energy gamma-ray emission from a microquasar. The telescope, on the island of La Palma, observed the microquasar called LS I +61 303 between October 2005 and March 2006. The observations show a clear variation with time and suggest that gamma-ray production may be a common property of microquasars.
Microquasars are gravitationally bound binary-star systems consisting of a massive ordinary star and a compact object of a few solar masses that is either a neutron star or a black hole. The two stars orbit a common centre and when close enough the mutual tides can cause a sudden transfer of mass from the normal star onto the compact companion. Some of the gravitational energy released in this exchange gives rise to jets of particles ejected at close to the speed of light, together with spectacular emission of radiation. Microquasars appear to be scaled-down versions of quasars, but in this case the small mass of the compact object means that events occur on a much smaller timescale – days rather than years – making them interesting objects to study. They are also a possible source of high-energy cosmic rays.
MAGIC detected LS I +61 303, one of about 20 known microquasars, at a rate of one gamma ray per square metre per month (Albert 2006). The telescope registers gamma rays through the Cherenkov radiation produced by the showers of particles created by the gamma rays as they enter the atmosphere.
LS I +61 303 was observed over six orbital cycles and a clear variability was found that is consistent with the orbital changes in aspect of the compact object (see figure). There is also evidence of periodicity. This shows that the very-high-energy gamma-ray emission is directly related to the interaction between the two stars.
The Japanese-European ASACUSA team at CERN has measured the antiproton-to-electron-mass ratio to record-breaking accuracy. The answer is 1836.153674, with an error margin of 5 in the last decimal place, which is equivalent to measuring the distance between Paris and London to within 1 mm. The corresponding ratio for the proton is 1836.15367261, so the new result shows that the mass of the antiproton is the same as that of the proton to nine significant figures (Hori 2006). This precision has been achieved using the “frequency comb” technique, development of which earned John Hall and Theodor Hänsch, the Nobel prize in 2005.
In the ASACUSA experiment, samples of antiprotonic helium – an atom with an antiproton and an electron orbiting a normal helium nucleus – were produced using CERN’s Antiproton Decelerator facility, and irradiated with a tunable laser beam, the frequency of which could be measured very precisely with the Hall-Hänsch frequency-comb technique. The laser beam could be tuned to one of several characteristic frequencies of the antiprotonic atoms, each frequency corresponding to an atomic transition of the antiproton. Since these frequencies were determined by the properties of the antiproton, the ratio of the antiproton mass to the electron mass could then be calculated from the measured values.
The results can also be combined with an earlier high-precision measurement of the antiproton’s cyclotron frequency (which determines the curvature of its path in a magnetic field). This shows that there is no difference in the proton and antiproton charges either, apart from the sign. Still more precise experiments are planned with the optical comb, and may soon give an even smaller margin of error for the antiproton than the best one obtained for the proton itself (currently about five times smaller). Surprisingly, the antiproton may soon be known better than the proton.
One of the biggest mysteries of science is the nature of dark matter, which first became apparent as astronomer Fritz Zwicky’s “dunkle Materie” in 1933. The two leading particle candidates for this “missing matter” are weakly interacting massive particles (WIMPs) and axions – hypothesized uncharged particles that have a very small but unknown mass, which barely interact with other particles. To bring together the widespread axion community, the Integrated Large Infrastructure for Astroparticle Science (ILIAS), the CERN Axion Solar Telescope (CAST) collaboration and CERN have organized a series of training workshops on current axion research, including open discussions between theorists and experimentalists. The first two of these were held at CERN in November and at the University of Patras in Greece, in May. This article highlights the presentations at both meetings.
The idea of the axion has been around for some 30 years, proposed as a solution to the strong charge-parity (CP) problem in quantum chromodynamics (QCD), the theory of strong interactions. According to the basic field equations of QCD, strong interactions should violate CP symmetry, rather as weak interactions do. However, strong interactions show no sign of CP violation. In 1977, Roberto Peccei and Helen Quinn suggested that to restore CP conservation in strong interactions, a new symmetry must be present, compensating the original CP-violating term in QCD almost exactly – to at least one part in 1010. The breakdown of this gives rise to the so-called axion field proposed by Steven Weinberg and Frank Wilczek, and the associated pseudo-scalar particle – the axion. Appropriately, Peccei, from the University of California Los Angeles, gave the first lecture of the workshop series and described the theoretical raison d’être of the Peccei-Quinn symmetry.
Evidence for strong CP violation should in particular appear in an electron dipole moment (EDM) for the neutron, but this has not yet been detected. Instead, we know from a high-precision measurement using polarized ultracold neutrons at the Institut Laue Langevin (ILL) in Grenoble that the neutron EDM is at least some 10 orders of magnitude below expectation. Peter Geltenbort of ILL presented the recently announced limit of 3 × 10-26 e cm. This is part of a series of experiments started by Nobel laureates Norman Ramsey and Edward Purcell in the 1950s, which continues today with the ambitious goal of reaching 10-28 e cm by the end of the decade. Other proposed neutron EDM experiments include those at the Paul Scherrer Institut and at the Spallation Neutron Source in Oak Ridge with goals of 10-27 e cm and 10-28 e cm, respectively. A new technique with the deuteron may provide the route for the next sensitivity scale, reaching 10-29 e cm, as Yannis Semertzidis of Brookhaven explained.
CP violation seems to be necessary to explain the survival of matter at the expense of antimatter after the Big Bang. Thus the creation of relic axions shortly after the dawn of time could have been enormous, perhaps amounting to some six times more in mass than ordinary matter. In addition to the scenario of relic axions, Georg Raffelt, an axion pioneer from the Max Planck Institute, introduced the connections between astrophysics and axions, with the stars as axion sources as his central topic. The effect of such an energy-loss channel on stellar physics provides constraints on the interaction strength of axions with ordinary particles. The Sun, our best known star, should be a strong axion source in the sky, allowing a direct search for these almost-invisible particles.
This is precisely the objective of the CAST helioscope at CERN, which searches for solar axions using a recycled LHC test dipole magnet pointing at the Sun for some three hours a day. The signal of solar axions will be an excess of X-rays detected during solar tracking. While the relic axions are expected to move slowly at about 300 km/s, those escaping from the solar core must be super relativistic, despite their assumed kinetic energy of only about 4 keV. CAST is the first helioscope ever built with an imaging X-ray optical system, whose working principle was explained by Peter Friedrich from Max-Planck-Institut für extraterrestrische Physik and Regina Soufli from Lawrence Livermore National Laboratory (LLNL) in their lectures on X-ray optics. For axion detection, the X-ray optics act as a concentrator to enhance the signal-to-noise ratio by focusing the converted solar X-rays into a small spot on a CCD chip or a micromesh gaseous structure (Micromegas), as developed by Yannis Giomataris and Georges Charpak. CAST has been taking data since the end of 2002 and has already published first results.
The possible existence of axions in the universe means that they are a candidate for (very) cold dark matter, as another axion pioneer, Pierre Sikivie, from the University of Florida explained. He also described the technique that he invented in 1983 for detecting axions. The idea is that axions in the galactic halo may be resonantly converted to microwave photons in a cavity permeated by a strong magnetic field. The expected signals are extremely weak, measured in yoctowatts, or 10-24 W. The same holds also for the solar axions inside the CAST magnet, whose energies of a few kilo-electron-volts (keV) are several orders of magnitude higher. The process depends on various parameters, such as the magnetic-field vector and size, the plasma density, the (unpredictable) axion rest mass and the photon polarization – all of which provide the multiparameter space in which axion hunters search for their quarry.
Sikivie also described the search for relic axions at LLNL, the topic of the CERN seminar at the start of the first workshop, presented by Karl van Bibber from LLNL. The Axion Dark Matter eXperiment (ADMX), which uses a microwave cavity to look for axionic dark matter as proposed by Sikivie, has been taking data for a decade. It is now undergoing an upgrade to use near-quantum-limited SQUID amplifiers. In his review, van Bibber also described CARRACK, a similar experiment in Kyoto, which uses a Rydberg-atom single-quantum detector as the back-end of the experiment.
The axion, together with the Higgs boson – another so-far undetected particle required by theory – may contribute not only to dark matter but also to dark energy, as Metin Arik from Istanbul explained. This leads to the question of why the dark-energy density is so small.
Light polarization
Giovanni Cantatore presented the Polarizzazione del Vuoto con LASer (PVLAS) experiment at the INFN Legnaro National Laboratory, which has recently caused a stir in the axion community. In a recent paper in Physical Review Letters, the PVLAS collaboration reports that a magnetic field can be used to rotate the polarization of light in a vacuum. The detected rotation is extremely small, about 0.00001°. The slight twist in the polarization, the result of photons of a given polarization disappearing from the beam, could suggest the existence of a light, new neutral boson, as the signal strength observed by PVLAS is much larger than would be expected on the basis of quantum electrodynamics alone.
The particle suggested by PVLAS is not exactly the expected axion; its coupling to two photons is so strong that experiments searching for axions, such as CAST, should have seen many of them coming from astrophysical sources. It would need peculiar properties not to conflict with the current astrophysical observations, but there is no fundamental reason barring it from having such properties. Eduard Masso from the University of Barcelona reviewed the theoretical motivation for axions and the importance of an axion-like coupling to photons, and addressed the apparent conflict between the PVLAS results and CAST and the astrophysically derived bounds.
Andreas Ringwald from DESY pointed out that the possible interpretation of the PVLAS anomaly in terms of the production of an axion-like particle has triggered a revisit of astrophysical considerations. Models exist in which the production of axion-like particles in stars is suppressed compared with the production in a vacuum. In these models, the bounds derived from the age of stars or from CAST may be relaxed by some orders of magnitude. The workshop participants agreed unanimously that the PVLAS result needs direct confirmation of the particle hypothesis with laboratory-based experiments.
Semertzidis spoke about a PVLAS-type experiment that was performed at Brookhaven more than 15 years ago, with most of the PVLAS collaborators as major players. They also observed large signals, which they attributed however to the laser light motion at the magnet frequency. He went on to suggest that laser motion at the magnet rotation frequency might also produce signals at the second harmonic that would look like axion signals. The PVLAS collaboration has spent five years looking for a systematic artifact that might explain their observations, and plans to attempt to settle the question in a new photon-regeneration experiment. Here, any particles produced from photons in a first magnet, would propagate into a second magnet blocked to photons, where they would convert back into photons.
The solar-axion energy range less than 0.5-1 KeV remains a challenging new territory
Detection of such regenerated photons would provide a very robust confirmation of the particle interpretation of the PVLAS result, and similar regeneration experiments are in preparation elsewhere. Keith Baker presented the plans by the Hampton University-
Jefferson Lab collaboration to use the world’s highest-power tunable free-electron laser (FEL), in the LIght Pseudoscalar-Scalar Particle Search (LIPSS) experiment, which will run during the coming months. As Ringwald pointed out, there are a number of experiments based either on photon polarization or on photon regeneration measurements that should soon exceed the sensitivity of PVLAS. At DESY, there is a proposal to exploit the photon beam from the Free-electron LASer in Hamburg (FLASH) for the Axion Production at the FEL (APFEL) experiment, which will take advantage of unique properties of the FLASH beam. The available photon energies (around 40 eV) are just in the range where photon regeneration is most sensitive to masses in the milli-electron-volt range. In addition, the tuning possibilities of FLASH will allow a mass determination, and the pulsed nature of the photon beam allows noise reduction by timing.
Two linked experiments to search for axions proposed by a team from CERN and several other institutes are also well advanced. These were presented by Pierre Pugnat from CERN, who explained how this approach allows for simultaneous investigations of the magneto-optical properties of the quantum vacuum and of photon regeneration. The team could start next year to check the PVLAS result. The two experiments are integrated in the same LHC superconducting dipole magnet and so can provide solid results via mutual cross-checks.
Carlo Rizzo from Université Paul Sabatier/Toulouse presented a different detection concept in the Biréfringence Magnétique du Vide experiment at the Laboratoire National des Champs Magnétiques Pulsées in Toulouse. The goal is to study quantum vacuum magnetism and the experiment will be in operation this summer to test the PVLAS result.
Frank Avignone from South Carolina reviewed possibilities that go beyond the current experimental searches for axions, such as the use of coherent Bragg-Primakoff conversion in single crystals, coherence issues in vacuum and gas-filled magnetic helioscopes, and novel proposals to detect hadronic axions with suppressed electromagnetic couplings. Emmanuel Paschos of the University of Dortmund addressed possible coherence phenomena in low-energy axion scattering and its potential use for axion detection. This could be an important application of light-sensitive detectors used in underground dark-matter experiments, where they may allow the first low-energy axion searches, as reported by Klemens Rottler from the University of Tübingen and the CRESST dark-matter experiment. After all, the solar-axion energy range less than 0.5-1 keV remains a challenging new territory.
From the Sun and beyond
The signatures of axions are not confined to the solar system, and there were a number of interesting presentations on searches for axions or axion-like particles with telescopes on the ground or in orbit. A cosmologically interesting topic concerns axion-photon conversion induced by intergalactic magnetic fields, which offers an alternative explanation for the dimming of distant supernovae, without the need for cosmic acceleration. However, the same mechanism would cause excessive spectral distortion of the cosmic microwave background (CMB). Alessandro Mirizzi of Bari concludes that owing to the spectral shape of the CMB, photon-axion oscillation can play only a relatively minor role in supernova dimming. Nevertheless, a combined analysis of all the observables affected by the photon-axion oscillations would be required to give a final verdict on this model.
In related work, Damien Hutsemékers from the University of Liège has investigated the potential for photon-axion conversion within a magnetic field over cosmological distances, as it can affect the polarization of light from distant objects such as quasars. He reported on the remarkable observation, using the ESO telescopes in Chile, of alignments of quasar polarization vectors that might be due to axion-like particles along the line of sight.
Rizzo also discussed potential axion signatures in astrophysical observations, presenting an impressive movie. He reported that axion and quantum vacuum effects have been studied in the double neutron-star system J0737-3039. Astrophysical observations of such effects will be possible in 2007 with the ESA XMM/Newton and NASA GLAST telescopes in orbit.
Coming nearer to Earth, Hooman Davoudiasl from the University of Wisconsin-Madison showed that solar axion conversion to photons in the Earth’s magnetosphere can produce an X-ray flux, with average energy about 4 keV, which is measurable on the dark side of the Earth. (The low strength of the Earth’s magnetic field is compensated for by a large magnetized volume.) The signal has distinct features: a flux of X-rays coming from the dark Earth, pointing back to the core of the Sun, with a thermal distribution characteristic of the solar core, and orbital as well as annual modulations. For axion masses less than 10-4 eV, a low-Earth-orbit X-ray telescope could probe the axion-photon coupling well below the current laboratory bounds, with a few days of data-taking. Also, the question was discussed as to whether axion-photon oscillations occur inside solar magnetic fields, sufficient to give the enhanced X-ray emission from places such as in sunspots.
Another possibility is the detection of the radiative decay of massive axions, predicted in extra dimensional models, which change drastically their mass, lifetime and detection, as Emilian Dudas from Ecole Polytechnique argued. In this context, Juhani Huovelin from Helsinki Observatory presented space-borne X-ray observations of the Sun and the sky background with ESA’s SMART-1, the first European mission to the Moon, which began operation in 2004 and will continue data-taking until September 2006. The important instruments onboard for axion research are an X-ray camera from CCLRC Rutherford Appleton Laboratory in the UK, and the X-ray Solar Monitor (XSM) from the University of Helsinki. The XSM measures solar X-ray spectra with high time resolution in the 1-20 keV energy range.
Extensive data have already been accumulated, including a series of lengthy observations of the X-ray Sun during quiescence and flares, as well as various observations of the background sky. Preliminary analysis of the data indicates possible residual emission at several intervals in the 2-10 keV range after fitting known solar and sky-background emission components. A future more-refined analysis will show whether the residual emission is statistically significant, and possibly related to X-rays from the decay of gravitationally trapped massive axions. The NASA solar mission RHESSI has also entered this kind of research, with the aim of detecting the same sort of particles near the surface of the Sun, as we published with Luigi di Lella at CERN five years ago. SMART and RHESSI use the Moon and Sun respectively to block out the background sky, thereby creating a large fiducial volume to search for axion radiative decay. The 1 m3 DRIFT detector operating in the Boulby Mine in the UK provides a similar capability in an underground experiment, as Eirini Tziaferi and Neil Spooner from Sheffield explained.
The friendly atmosphere of the two workshops saw plenty of fruitful discussions in which new ideas could emerge. For example, Ringwald has recently suggested a laboratory photon-regeneration experiment with X-rays. It seems that the ESRF at Grenoble offers one of the best opportunities worldwide for such an experiment, with photon energies in the 3-70 keV range. Also, as Sikivie highlighted, there is strong scientific interest in building a next-generation microwave cavity embedded in a large-bore superconducting solenoid to detect galactic-halo axions. CERN, together with several collaborating institutes, for example, could build a microwave cavity of around 1 m3 integrated inside a 8-10 T magnetic field.
The workshop participants unanimously concluded with a call to CERN to become a focal point for axion physics. There will be more ideas and new results by the next workshop in June 2007 in Patras.
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