Physics Archives – Page 138 of 144

Radioactive beams drive physics forward

ion1_12-99 — Physics at CERN’s ISOLDE ion source covers an impressive range of subjects, of which this article gives only a glimpse.

CERN’s isotope factory, ISOLDE, has a history stretching back over 30 years. ISOLDE’s unique ability to produce some 600 isotopes of 70 elements has ensured it a place at the forefront of low-energy radioactive-beam research and permits a programme that ranges from basic nuclear structure and weak interaction studies to applied fields like solid-state physics and the life sciences. ISOLDE owes its longevity to the ISOLDE team’s ability to develop new and more intense isotope beams and to the resourcefulness of the scientists, who design ever-more ingenious experiments for the facility.

The heart of ISOLDE is a target station where a 1 or 1.4 GeV proton beam from CERN’s booster accelerator strikes a target to produce a range of isotopes. Those of interest are then extracted, ionized and separated before being delivered to experiments. ISOLDE has always been able to produce extremely pure radioactive beams of a variety of species by combining a range of target materials with efficient and selective ion sources.

The Laser Ion Source

An important recent development, however, has allowed ISOLDE to improve efficiency and beam purity further. The Laser Ion Source (LIS) works by shining a combination of three laser beams into the cloud of nuclei released from the ISOLDE target. The combination acts like a key in a lock and selectively ionizes the isotopes of interest. This gives an unprecedented combination of both ionization efficiency and selectivity and has already allowed important measurements to be made on key processes that are believed to play a role in supernova nucleosynthesis.

ion2_12-99 — The lithium-11 nucleus consists of a core lithium-9 nucleus orbited by two neutrons. If any one of the three bodies is removed the remaining two would be unbound, rather like heraldic Borromean rings.

More recently the LIS has been employed to produce isotopes of beryllium, which opens up a new range of possible experiments. Beryllium-7, for example, holds a key to the solar neutrino problem: the apparent deficit of neutrinos coming from the Sun that has been puzzling researchers for decades. One of the important reactions that gives rise to solar neutrinos is the capture of protons by beryllium-7. A precise knowledge of that isotope’s structure is important in understanding the expected flux of beryllium-7 related neutrinos from the Sun. An ISOLDE experiment has recently measured the quadrupole moment of the isotope. Furthermore, an experiment at Israel’s Weizmann Institute measures beryllium-7 proton capture using a beryllium-7 target made at ISOLDE.

Other, more esoteric, experiments use beryllium-11 and beryllium-14 to investigate a topic that has attracted much interest in recent years. In the 1980s, experiments on lithium-11 suggested that the nucleus might have a “halo” of two very loosely bound neutrons that surround a core lithium-9 nucleus. The proposed halo structure of lithium-11 has since been well established, both experimentally and theoretically, with much of the experimental work coming from ISOLDE.

Many similar neutron-rich nuclei have since been found and among them were beryllium-11 and beryllium-14. Beryllium-11, which has a single halo neutron, is the most simple halo nucleus. Its halo structure has recently been confirmed by the COLLAPS collaboration at ISOLDE, which has found that the magnetic dipole moment of beryllium-11 is consistent with a halo structure.

Berylium-14 is an altogether more complex object. Like lithium-11 it is a loosely bound three-body structure with two halo neutrons. Such nuclei have been dubbed Borromean because of a characteristic that they share with the heraldic Borromean rings symbol, which also features in mathematical knot theory. Just as a lithium-11 nucleus is a loosely bound state of three bodies, a Borromean ring system is an object composed of three intertwined rings. Whenever one ring is removed the remaining two are no longer bound to each other. The same is true of Borromean nuclei. These are three-body bound states in which any two-body subsystem would not be bound. The di-neutron is not bound, nor is lithium-9 plus a single neutron.

Producing beryllium-14 is particularly difficult because it has a half-life of only 4.3 ms, making it the shortest-lived isotope studied to date at ISOLDE. Few beryllium-14 nuclei survive long enough to escape the target, but a production rate of eight atoms per second has been achieved.

In lithium-11 a number of exotic decay modes involving the emission of tritons, deuterons, or up to three neutrons have been identified. These are associated with the halo structure, and the experimental goal of ISOLDE is to look for similar decay modes in beryllium-14. An early result shows that the probability of one-neutron emission is close to 100%. Further experiments are planned to search for other exotic decay modes.

Solid-state physics and life sciences

Radioactive nuclei are also used as powerful probes in solid-state physics (CERN Courier October 1998) and life sciences. One range of experiments studies the effect of hydrogen in semiconductors such as indium phosphide (InP), gallium arsenide (GaAs) and indium arsenide (InAs). Hydrogen can enter a semiconductor at many stages during production, causing passivation of intentionally introduced dopants by the formation of hydrogendopant complexes.

ion3_12-99 — Two of the many beamlines in ISOLDE’s packed experimental hall.

This is investigated at ISOLDE by implanting radioactive silver-117, which decays into cadmium-117. Cadmiumhydrogen pairs then form and, when the cadmium itself decays, the hydrogen is free to diffuse within the semiconductor. Information about the diffusion of hydrogen in the semiconductor can be gleaned by studying the gamma-ray distribution from the cadmium decay.

Another emerging area in which ISOLDE scientists are active is high-TC superconductors. Among the many that have been found, certain mercury-based materials achieve the highest critical temperature, of 135 K, at ambient pressure. These materials have a simple tetragonal lattice structure in which oxygen regulates the superconducting charge carriers and increases or decreases TC. ISOLDE researchers are able to extract information about the oxygen’s behaviour and concentration at the mercury lattice planes by measuring the electric field gradient that is caused by the charge distribution around implanted radioactive mercury probe nuclei.

In the life sciences, ISOLDE’s radionuclides are very suitable for use in biomedical research. One of the most interesting applications is cancer research. Certain modern techniques of systemic cancer therapy make use of “intelligent” molecules, which seek out specific binding sites in cancer cells with high selectivity. Such molecules can be used as vehicles to carry radioactive isotopes, produced at facilities such as ISOLDE, into the cancer tissue where they kill cancer cells. ISOLDE research into such novel forms of cancer therapy is guided by the Division of Nuclear Medicine at Geneva’s University Hospital.

Mass measurements

The mass of a nucleus is a fundamental quantity with importance in fields as diverse as nuclear physics and cosmology. This is particularly true of short-lived radioactive isotopes, for which ISOLDE has a long-standing tradition in high-precision mass measurements. The ISOLTRAP experiment, which is a combination of a recently added radiofrequency quadrupole cooler and two consecutive Penning traps, has mapped the nuclear mass surface over a sizable part of the nuclear chart with a resolution of 10^-7.

ISOLTRAP will continue to be one of the major ISOLDE experiments for many years. However, it is limited to nuclei with half-lives of around 1 s and longer. This limitation is overcome in a complementary mass-measuring experiment called MISTRAL (CERN Courier September 1997), which has measured the masses of 15 radioactive isotopes, 3 of which have half-lives of less than 50 ms, with a mass resolution better than 10^-5. These results are now providing valuable input to nuclear structure calculations.

Search for physics beyond the Standard Model

Radioactive nuclei are not only highly interesting systems in their own right but can also yield information regarding fundamental interactions, which is complementary to the results from high-energy physics. Nuclear beta decay, for example, is mediated by the exchange of W bosons having only certain (vector and axial-vector) couplings according to the Standard Model. However, theories beyond the Standard Model predict other weak couplings in nuclear beta decay. These could manifest themselves in electronneutrino correlations in super-allowed beta decays. The detection of the neutrino in the decay is practically impossible, so this correlation must be deduced from the velocity of the recoiling daughter nucleus.

ion4_12-99 — The electric field gradient (EFG1), which interacts with mercury nuclei in certain high-T_C superconductors, is revealed by a set of three frequencies that characterize the mercury lattice site. When doping oxygen merges into the mercury planes, this gradient changes in a characteristic way that is revealed by a different set of frequencies (EFG2), thus providing information about the superconducting mechanism.

This is an extremely delicate task because the velocities involved are very small and the movement of the daughter nucleus is affected by the surrounding material. In an experiment at ISOLDE, this last problem was partly circumvented by studying the decay of argon-32, which decays by beta-delayed proton emission. The energy peaks of the protons are then Doppler-broadened by the movement of the emitting daughter nucleus. Subsequently this broadening can be traced back to the electronneutrino correlation. The results were consistent with the Standard Model and have led to improved constraints on the scalar weak interaction.

Another quest for new physics is the search for right-handed currents in nuclear beta decay. This can be studied via the polarization of positrons that are emitted from a polarized radioactive source. Such an experiment has recently started taking data at ISOLDE.

REX-ISOLDE

In the past, ISOLDE has concentrated its efforts on the study of radioactive nuclei at very low energies less than 60 keV. The new REXISOLDE experiment is set to change this dramatically. A novel scheme for beam cooling, bunching, charge state breeding and subsequent acceleration will launch almost any isotope that is currently produced at ISOLDE into the 0.82.2 MeV per nucleon energy range. This opens up a completely new field of experiments with radioactive ion beams.

To convert a singly charged beam from the ISOLDE separator into a multiple-charged pulsed beam, which is suitable for acceleration, without losing too large a fraction of the painfully created radioactive nuclei, requires advanced techniques. Initially the ISOLDE beam is trapped, cooled and bunched in a linear Penning trap. The bunched beam is then transported to an electron beam ion source, where it is confined while being bombarded with a strong electron current in order to reach higher charge states. The ions are then re-accelerated and separated according to charge state before reaching a Linac. The overall efficiency is estimated to be 10%.

The REXISOLDE experiment aims to find out whether or not the magic numbers 20 and 28 are conserved when going to very neutron-rich nuclei. Other experiments have hinted at a weakening of the nuclear shell structure in the region of magnesium-32 (the most commonly occurring isotope is magnesium-24) and below calcium-48 (calcium-40 is the most commonly occurring isotope) with deduced sizable deformations.

The main detector system to be used for these experiments is the state-of-the-art gamma-detector array, MINIBALL, which is being constructed by a large European collaboration. Once REXISOLDE is ready, however, a large number of other experiments are also expected to make use of this powerful new instrument. Two more experiments are already approved. Both are concerned with investigating the unbound subsystems of halo nuclei like lithium-10. In the near future, other experiments that will take place concern the structure of unstable, medium-heavy (mass numbers between 50 and 100) nuclei with approximately the same number of neutrons and protons, proton radioactivity and nuclear astrophysics. The installation of the REX post-accelerator is well under way and the first post-accelerated radioactive beams are expected during 2000.

Outlook

With a well established physics programme, ISOLDE would be able to continue operation for many years to come with the current and foreseeable techniques, in particular with the REXISOLDE, including a possible energy upgrade to above 5 MeV per nucleon. However, the path towards new physics often relies on the availability of new or more intense radioactive beams, so the proton beam intensity delivered by the booster will become a limiting factor.

A number of new or upgraded facilities are currently being planned, built or commissioned worldwide. These will complement ISOLDE but they will still be first-generation facilities. There is considerable interest around the world in building a second-generation radioactive ion beam facility. In Europe this is spearheaded by the Nuclear Physics European Collaboration Committee (CERN Courier May p23) and in the US by the Department of Energy. Both have independently recommended the construction of a second-generation facility based on the ISOLDE principle, where radioactive beam intensities around 100 times higher than at the current facilities can be produced.

ISOLDE is well placed for further advances before such a facility comes along. ISOLDE currently uses about half of the number of protons delivered by the booster, but its target could easily withstand beam intensities more than an order of magnitude higher. ISOLDE physicists are currently studying the possibility of using a 2 GeV high-current proton Linac, which might form part of CERN’s LHC injector complex, to upgrade the facility further in years to come. A small part of the then available protons would be used by the driver beam, which would propel the ISOLDE to new scientific heights.

Handling high-energy spin

spin1_12-99 — Siberian snake – the logo of the workshop on the handling of polarized protons.

More than 10 years after the European Muon Collaboration at CERN discovered that the proton’s quarks account for only a fraction of the particle’s spin, physicists have been trying to find the “missing” component. This means exploring spin structure and content under higher-energy conditions.

The production of high-energy polarized proton beams and their subsequent long-term storage is one of the greatest challenges facing accelerator physicists. This was high on the agenda at a workshop on Polarized Protons at High Energies – Accelerator Challenges and Physics Opportunities, which was held at the DESY Laboratory in Hamburg earlier this year. Over 100 experts on the machine aspects of polarized proton beam acceleration and storage, spin physics and polarimetry were brought together.

Emphasis was placed on the polarized proton option for Brookhaven’s RHIC collider, which is expected to be commissioned next year, and the possible future storage of polarized protons in DESY’s HERA collider, in addition to the electrons and positrons that are already polarized.

The case for a fully polarized proton–electron HERA collider has been explored in two previous DESY workshops. Measurements at HERA include the structure functions g1 (the sum of the polarized quark and antiquark distributions) at low x (the quark momentum fraction) and g5 (the difference between the polarized quark and antiquark distributions), and polarized photoproduction measurements. The ability of HERA and RHIC to measure – for example, via jet or prompt photon production – the polarized gluon distribution, which is a likely source of spin, again underlined a measurement that is pivotal in understanding the spin structure of nucleons.

Previous studies have been updated, by using detailed detector simulation, for example, and several new channels have been proposed. New ideas were also presented for the HERA-N option in which the polarized proton beam would collide with a polarized, fixed target. The complementarity of the possible future physics programme at HERA and the programme at RHIC was emphasized.

Resonant depolarization during acceleration to high energy can be strongly suppressed using “Siberian snake” spin rotators. However, the snake schemes must be chosen carefully. High-intensity polarized sources, as well as high-quality polarization measurements, are needed at each step in the acceleration cycle.

Subjects for discussion included the necessary modifications to the HERA acceleration chain; the results of long-term spin-orbit tracking simulations for HERA and RHIC; progress on high-performance polarized sources; ideas for realistic snake layouts, which include plans for RHIC; experience at low-energy rings; various aspects of the theory of spin-orbit motion; and the description of helical magnetic fields. Significant advances were reported on all fronts. However, it remains clear that, to reach very high energy in HERA, a cooled beam would be desirable and that new means are needed to overcome the effects of closed-orbit distortion.

E Gabathuler, an opening plenary speaker, recalled the contribution of high-energy polarized scattering to our understanding of the structure of matter, and the surprises that emerged along the way. R Jaffe, A Deshpande and W Nowak reviewed the main aspects of the physics programme for polarized RHIC and HERA colliders. A Krisch recalled the efforts to achieve polarized proton beams at Brookhaven, and the commissioning of the Siberian snake at the IUCF ring in Indiana.

Siberian snakes for the polarized RHIC option are being constructed and will be commissioned in the near future for polarized proton collisions at an collision energy of 200 GeV. The experience that the Brookhaven Lab has in the acceleration and storage of polarized protons will be of vital use in the polarized HERA project.

The current position of the the RHIC spin option was detailed by T Roser. G Hoffstätter presented the status of the ongoing machine physics studies at DESY for HERA. The challenges in measuring the polarization of the high-energy proton beam were reviewed by G Bunce and K Kurita. Polarized ion sources, and the current spectacular developments in this field, were covered by L Anderson.

The future expectations of HERA, which has been running with polarized electron and positron beams for the HERMES experiment, were addressed by E Gianfelice-Wendt. Finally, the results, which will become available in the near future, from DESY (HERMES), CERN (COMPASS) and Jefferson (CEBAF) were reviewed by E Kinney.

A plenary session on polarimetry merited three summary talks. These were on machine aspects (A Chao), spin physics (T Gehrmann) and the general outlook (V Hughes).

A similar meeting, which will probably be held in the US, is planned for 2001, when the exciting results of RHIC will be available.

Silver celebration for Swiss pions

swiss1_12-99 — Swiss protons – 1974 proton beam spot at the thin target station. The spot was approximately 1 cm in diameter and the beam current was 30 nA.

In 1974 the first protons were accelerated in the ring accelerator and the first pions were produced at, what was then known as the Swiss Institute for Nuclear Research. Some 25 years later, Switzerland’s National User Laboratory for Nuclear and Particle Physics has grown into the Paul Scherrer Institute a multipurpose facility.

In the beginning

The story of the first Swiss pions started in earnest on 17 January 1974. This was a few days after protons had been successfully extracted from the 72 MeV Philips Injector Cyclotron and accelerated via a few turns to 92 MeV in the in-house-designed 590 MeV Isochronous Ring Cyclotron.

Despite vacuum problems, which meant running on only three of the four accelerating cavities, on 18 January the beam dynamics group was able to accelerate protons from 100 to 540 MeV in just 30 min. Thus it created a new world record for the rate of gain of energy achieved by an isochronous cyclotron.

Acceleration to the extraction energy of 590 MeV took a bit more effort. However, shortly before the filament of the ion source burned out, 4 nA of protons were extracted and hit an external copper beam dump. This was the first time that any accelerator had reached truly relativistic energies with a 100% macroscopic duty cycle.

The then director, Jean-Pierre Blaser, and the late Hans Willax, the designer of the ring cyclotron concept, were seen with “silent glows” on their faces. Many years of considerable effort had finally been rewarded with success.

The “big run”, which was dedicated to the production of the first pions on the external target station, started in the early afternoon of 23 February 1974. Again, difficulties were encountered, which necessitated running on only three acceleration cavities in the main ring. However, the machine crew took up the challenge, and between 0.1 and 0.2 µA of protons soon reached the extraction radius.

The highlight came just after midnight on 24 February, when the first extracted protons were seen as a bright spot on a glass scintillation screen at the thin target station. A few minutes later the eagerly waiting physicists had their moment of truth: the first pions were detected at the end of the ¼M3 channel (which was tuned for positive particles of 300 ± 3 MeV/c).

The characteristic signature of protons and pions was seen on the oscilloscope and was confirmed via pulseheight spectra from a scintillatorCherenkov counter telescope. The first results yielded some 3000 positive pions per second for a proton current of 30 nA. It was a moment for celebration.

Today, 25 years later, the main Ring Accelerator is routinely running at more than 1.5 mA (15 times as great as the design current) with an increase to 2 mA planned. With the present 1 MW proton beam power, PSI has the most intense pion and muon beams in the world. This has yielded numerous first-class results over the years. These include:

the most precise values of the masses of the muon and the charged and neutral pion, in addition to the best “laboratory limit” on the mass of the muon neutrino;
in the rare and forbidden decay sector, which continues to be a specialist area of research at the facility, the most precise values have been established for the branching ratios of the pion into an electron and a neutrino; and into a muon a neutrino and a photon; and a muon into three electrons and two neutrinos;
the first observation of a pion decaying into three electrons and a neutrino, and hence the determination of the vector and the two axial-vector form factors of the pion;
in the search for lepton-flavoured violating processes, the most sensitive limits have been achieved for a muon decaying into three electrons (less than 10^-12), for muon to electron conversion in nuclei (less than 6.1 x 10^-13) and for muoniumantimuonium conversion (less than 8.2 x 10^-11);
other precision measurements, one of which is the measurement of the longitudinal polarization of positrons in muon decay, which together with the results from inverse muon decay, demonstrated that the VA structure of the weak interaction follows on as a natural consequence; also, the helicity of the muon neutrino was determined for the first time;
in the hadronic sector, the most accurate measurements of the delta (1232)** and delta (1232)⁰ masses and widths and the determination of the magnetic moment of the delta (1232)** have been made. Measurements in pionic hydrogen have yielded the strong interaction shift and width to unprecedented precision, from which the pionnucleon scattering lengths at threshold could be determined.

Special symposium

The anniversary was marked by a special symposium as part of the laboratory’s traditional half-yearly Accelerator Users Meeting.

The opening address was given by deputy director Ralph Eichler, who set the scene and outlined the present range of experiments at the Ring Accelerator, with the continuing trend towards larger and longer types of precision measurements.

swiss2_12-99 — Accelerator control room elation in the early hours of the morning on 24 February 1974.

Former director Blaser recounted the “Origins and Early Days of the Ring Cyclotron” with personal recollections and anecdotes. Then it was the turn of Claude Joseph (Lausanne), a long-standing experimenter at the facility, and Florian Scheck (Mainz), former head of the SIN Theory Group, to summarize physics milestones in the strong interaction and electroweak sectors respectively. This wealth of results is underlined by the number of corresponding entries in the Particle Data Booklet.

Finally, Jerzy Sromicki (ETHZ) gave a lively overview of the present tests of fundamental symmetries and future particle physics experiments that use neutrons from PSI’s spallation neutron source, SINQ, fed by the Ring Accelerator.

As one 25 year period closes, a new one will begin when the Swiss Light Source synchrotron radiation facility is commissioned.

Experiments rechart the Big Bang

cernnews7_11-99 — These diagrams compare the total production rate of various subnuclear particles (hadrons from pions to hyperons) as seen by the NA49 experiment at CERN (which is studying central high-energy lead ion collisions), with a statistical Hagedorn-type model (developed by F Becattini). The model shows that the transition temperature, T₀, above which quarks no longer stick together to form hadrons, is about 180 MeV.

In the beginning there were quarks and gluons the “quarkgluon plasma”. Density and temperature decreased as the universe expanded and cooled. The quarks and gluons began to freeze, forming subnuclear particles (protons and neutrons). These in turn stuck together to form nuclei. So goes the dogma.

To check whether or not this is true, experiments at CERN hurl beams of high-energy nuclei at targets to create hot, dense fireballs of protons and neutrons. These may be hot and dense enough for the protons and neutrons to melt and their component quarks and gluons to be released from their nuclear habitat.

Experiments using beams of lead ions at CERN’s SPS synchrotron have recently polished their results. Included is the NA50 study, which is looking at the production of J/psi particles. These particles are composed of a heavy (charmed) quark and an antiquark, which are bound together. They are more difficult to form when quarks and antiquarks are less likely to stick together. A clear signal of J/psi suppression was, therefore, greeted as the first resynthesis of the quarkgluon plasma since the Big Bang. However, some physicists are pointing out that this is not the whole story.

In any plasma (a gas of charged particles) the charge carried by any one particle is screened by those surrounding it. This is called Debye shielding. Quarks and gluons interact via the tripartite colour charge rather than the familiar dual (positive/negative) electrical charge.

It is this screening mechanism that prevents quarks from sticking together above a certain critical temperature, T₀, and beyond a certain energy density, E₀. Subnuclear particles melt under these conditions.

However, the J/psi, as the lightest particle containing charmed quarks, is very small and tightly bound. Therefore it melts at a slightly higher temperature, about 1.3T₀. This apparently small temperature shift is amplified for the energy density, which behaves as the fourth power of temperature. Thus J/psis melt at energies of around 3E₀, which is much later than most subnuclear particles.

Furthermore, J/psis are special because their constituent charm quarkantiquark pair will never recombine once the plasma fireball expands and cools. They lose contact with each other and adhere instead to other, lighter quarks to form D mesons. Light quarks in the plasma will recombine into protons, kaons, pions, etc.

NA50 sees the onset of J/psi disappearance in the most violent “head-on” collisions of lead ions (and only lead ions) at maximum SPS energy (158 GeV per nucleon). The energy density, therefore, must have reached about 3E₀ in these collisions. Other experiments using lead ion beams, such as NA49, estimate from calorimetric measurements that the energy density achieved in these collisions is about 3 GeV per cubic fermi (1 fermi = 10^-13 cm, which is the “diameter” of a proton). Tentatively combining these observations leads to the conclusion that the critical density, E₀,is about 1 GeV per cubic fermi, which is in agreement with quarkgluon field theory calculations.

This can now be tested by the experiments using lead beams (NA44, NA49, WA89 and WA97). Reinhard Stock of Frankfurt points out that hadron production in these reactions can be reconciled with quarks and gluons combining at T₀, which is near 180 MeV, and E₀, which is near 1 GeV per cubic fermi.

Previous theoretical studies of the transition from a quarkgluon fireball to a subnuclear fireball by John Ellis of CERN and Klaus Kinder-Geiger (who died in last year’s Swissair plane crash) had set the stage. Their model set out to explain how different subnuclear particles containing quarks and gluons emerged from electronpositron annhilations.

They showed that the transition probability depends on statistical mechanics, so that the relative production levels of different subnuclear particles infers the temperature prevailing at their birth. This happens as the initial high-energy density quarkgluon fireball expands and cools towards the critical temperature at which the particles crystallize.

Thus, experiments with high-energy nuclear beams are recharting the Big Bang.

Latest news from the early universe

This month the first elements of the Very Small Array will be installed on Mount Teide in Tenerife. This is one of a number of new projects studying the cosmic microwave background.

Observations of the cosmic microwave background (CMB) are the closest that astronomers can get to the beginning of the universe. It dates from 300 000 years after the Big Bang, when radiation decoupled from matter. Fluctuations in the CMB are evidence for the first clumping of matter particles the seeds of the galaxies we see today.

The 14 antennae of the Very Small Array (VSA) will map small areas of the sky from 26 to 36 GHz with a sensitivity of 510 µK. The VSA will be capable of the two-dimensional mapping of real features and it is expected to be up and running by next summer.

By then, results from the Boomerang balloon experiment will be out. This experiment uses the polar wind to stay aloft and enables the balloon to circle the South Pole for more than 10 days. Thus it avoids the fate of other balloon experiments, which only have a short observation time. The VSA, with its greater resolution, will be able to follow up in more detail any areas of interest identified by the balloon. Balloon observations have different systematic errors than ground-based telescopes, so results are complementary.

Future projects include the US Cosmic Background Imager, to be installed in the Atacama desert, Chile, and the Degree Angular Scale Interferometer at the South Pole.

It is an interesting time for CMB observations. Following the great leap forward made by the COBE satellite in 1992, which measured the background fluctuations for the first time, years of data analysis and new ground-based experiments are providing fuller and more detailed results. A recent analysis of CMB data has even cast doubt on inflation the most stalwart theory of the early evolution of the universe. However, other investigations suggest that the discrepancy may be due to instrumental error. The VSA and Boomerang experiments will be in a position to find out.

The study of foreground microwave radiation has also progressed. New sources of microwave emission have been discovered, such as spinning dust grains in our galaxy. When this is better understood, it will make for more accurate CMB results. NASA’s Microwave Anisotropy Probe, which is scheduled for launch late next year, will perform the next all-sky survey as a follow up to COBE. At the other end of the spectrum, NASA’s X-ray satellite, Chandra, may provide crucial data with its observations of galaxy clusters, the largest scale clumping seen in the universe today. Cosmologists are hoping for some real advances. At worst, they will have to wait for the launch of ESA’s Planck satellite some time after 2007 .

The VSA is a collaboration between Cambridge University, Jodrell Bank and the Canary Islands Institute for Astrophysics. The Boomerang partners are the US and Italy, with contributions from the UK and CERN.

LEP pursues Higgs boson and greater W precision

electron1_11-99 — Ever increasing the integrated annual luminosity (a measure of the collision rate) delivered by CERN’s LEP electronpositron collider.

For the past two years, CERN’s LEP electronpositron collider has been performing better than ever. Each year sees new increases in collision energy and, just as important, new records in luminosity (a measure of the collision rate). Last year, LEP delivered about 200 inverse picobarns of luminosity to each of the four experiments and, as a result, almost quadrupled the collected data on W bosons (produced as oppositely charged pairs).

This year looks even more promising, with LEP reaching, and even slightly surpassing, the “magic” 200 GeV collision energy (100 GeV per beam). These new data have allowed the four LEP collaborations ALEPH, DELPHI, L3 and OPAL to extend and improve their studies at high energy. Of the many results presented recently at the EPS conference in Tampere, Finland, and the leptonphoton symposium at Stanford, the search for the Higgs boson and the measurement of the W mass were the highlights.

One of the first tasks in any new energy domain is to search for new particles. However, LEP is now exploring territory that could reveal the symmetry-breaking mechanism, at the heart of the Standard Model, that endows particles with mass (the “Higgs mechanism”). Although the Standard Model predicts that the Higgs particle should exist, unfortunately it says very little about its mass.

The Higgs should make its presence felt through delicate radiative corrections to the standard Z and W exchange mechanisms. These corrections have been studied in great detail by the LEP experiments, as well as by the SLD experiment at SLAC, Stanford, and the D0 and CDF experiments at Fermilab’s Tevatron protonantiproton collider. These studies indicate that the Higgs boson is relatively light. A fit to the LEP and SLC results on Z exchange, as well as the determinations of the W boson and top quark masses from LEP and the Tevatron, predict that the Higgs boson should be lighter than about 220 GeV, with a best fit at around 90 GeV. This is below the point where LEP is currently operating. A low-mass Higgs would be exciting. However, these suggestions are only indirect evidence.

The LEP experiments have been diligently searching for direct evidence of the Higgs since the beginning of LEP data taking. So far nothing has turned up. With no sign of a signal yet, the experiments express the non-observation as an upper limit on the Higgs production rate. At a given collision energy this depends on the Higgs mass and the upper limit on production rate can be converted into a lower limit on the Higgs mass.

Recently, following the example of the LEP Electroweak Working Group’s efforts to combine electroweak measurements, the LEP experiments have been combining their search results. The hope is that by combining the four experiments into a “meta-experiment”, with four times the luminosity, a signal might be detected that would be too small for an individual experiment to discover. Failing that, at least any mass limits could be increased.

Based on last year’s data, at a collision energy of 189 GeV, the combined lower limit on the Higgs mass from the four LEP experiments was 95.2 GeV. With the increased energy this year, the individual limits have also been increased to close to 100 GeV. It is too early to report on the combined limit. With the current limits so tantalizingly close to the value expected from the indirect evidence, the physicists at LEP are eagerly awaiting more data at even higher energies. The Higgs boson may be just round the corner.

W for weight

Compared with the lack of hard predictions on the Higgs boson, the Standard Model has somewhat more to say about the mass of the W boson. Via many of the same measurements that are used to constrain the Higgs boson indirectly, the experimenters (with a great deal of help from the theorists) have coaxed the Standard Model into predicting the mass of the W boson with an error of only 26 MeV (in 80 GeV). Now the challenge is to measure the W boson mass directly to the same accuracy, which will be a stringent test of the Standard Model.

electron2_11-99 — Tantalizingly close results of the Standard Model fit to the Higgs boson mass (parabolic curves) compared with the excluded region from the direct searches (yellow area).

This has been a major objective at LEP since 1996, when the energy was first raised above the threshold for producing pairs of W bosons. Since then the LEP experiments have been busy studying W bosons. Last year saw a fourfold increase in the LEP datasets, allowing the experiments to make dramatic improvements on the determination of the particle’s mass.

By now, each experiment has collected more than 4000 W pairs. With this amount of data the LEP experiments have been able to overtake their colleagues at protonantiproton colliders for the best direct W mass determination. The uncertainty on the LEP result is now 56 MeV, versus 62 MeV for the hadron collider result. Both measurements are in agreement with the indirect determination.

electron3_11-99 — The direct measurement of the W mass from LEP and protonantiproton colliders compared with the indirect determination.

This year’s data will almost double the available statistics, and there are still more data to come next year. The LEP experimenters are hopeful that the final direct LEP W mass value will have an error of around 30 MeV, which is close to the error obtained indirectly.

The very first events were recorded by the LEP experiments on 13 August 1989. At that time the LEP energy was 91 GeV, which is right on the Z peak. Now, more than 10 years later and more than 100 GeV higher in energy, both the LEP machine and the LEP experiments are still going strong and the excitement is still mounting.

CERN gears up for deceleration

antimatter1_11-99 — Getting ready for low energy CERN’s Antiproton Decelerator team in its second home, the machine’s control room.

CERN is best known for pushing the high-energy frontier of physics, but, with its new Antiproton Decelerator, the low-energy frontier is about to resume its place at the heart of the laboratory’s experimental programme.

The Antiproton Decelerator (AD) is scheduled to switch on for physics this month, and an important milestone was reached this summer when, for the first time, the AD team decelerated a beam of protons to the AD’s target momentum of just 100 MeV/c.

It might seem strange that milestones for the AD are measured in terms of achievements with protons, but, as Flemming Pedersen of the AD team explained, “We already know how to make antiprotons. The real challenge is low fields.”

With an AD beam momentum of just 100 MeV/c, the magnetic bending fields, which hold the beam in orbit, are so low that even the magnetic field of the Earth must be taken into account. Protons are used instead of antiprotons in the setting-up phase because higher intensities can be achieved, which make for easier diagnostics.

Decelerating the beam

Beam particles enter the AD with a momentum of 3.5 GeV/c, which is about 35 times as high as when they leave it. The beam is rapidly decelerated to 2000 MeV/c before undergoing stochastic cooling. Reducing the energy further is a delicate task, owing to the origin of the AD’s components. The AD is not a purpose-built machine it has been assembled using components from the Antiproton Collector, the job of which was to collect antiprotons at 3.5 GeV for CERN’s historic protonantiproton collider project of the 1980s.

Bending magnets that are designed for constant 3.5 GeV operation are not ideal for the AD, where the field is constantly cycled. In particular,eddy currents, provoked by changing the magnetic field in the AD, can become large, and these can disturb the beam. To avoid this problem the momentum is reduced from 550 MeV very slowly.

When the beam reaches 300 MeV/c there is another pause. This time the technique of electron cooling, better adapted to very low-energy beams, is used. Like the magnets,the electron cooler is recycled. It came from CERN’s previous low-energy antiproton facility, LEAR, which was the world’s second application of the technique pioneered by Gersh Budker at Novosibirsk in the late 1960s.

For the final approach to 100 MeV/c, the beam is slowed down again. Here the Earth’s magnetic field has to be considered, along with remanent fields induced in the AD’s metallic components.

The main challenge in reaching 100 MeV/c was to produce very stable power supplies that would control eddy currents in the magnets. Soon after the 100 MeV/c challenge had been met, decelerating to 100 MeV/c had become routine and the AD was shut down to allow physicists to install the three experiments that will start taking data with the new machine.

When work resumed in September, the first task was to consolidate what had already been achieved with protons and to add a further electron-cooling stage at 100 MeV/c before the beams are extracted for delivery to experiments. Next on the agenda was reversing the polarity of the bending magnets to handle antiprotons. Some further setting up is expected because, as Pedersen pointed out, “We can’t reverse the polarity of the Earth’s magnetic field!”

Zen and the art of low-energy antiproton experiments

antimatter2_11-99 — The logo of the ASACUSA experiment collaboration at CERN’s Antiproton Decelerator on a Japanese 80 ¥ postage stamp, which depicts the famous Asakusa temple in Tokyo.

As CERN’s Antiproton Decelerator comes into operation a new era begins for high-precision studies of antiprotonic atoms and antihydrogen, as well as for the antiproton itself and the way in which it interacts with ordinary atoms.

The imminent arrival of these high-quality antiproton beams of extremely low energy (5 MeV) was heralded at the International Workshop on Atomic Collisions and Spectroscopy with Slow Antiprotons, held in Tsurumi on 19-21 July.

The workshop included a Zen Buddhism course (Zazenkai). Perhaps for the first time in a physics workshop, jet-lagged attendees were woken up at 3.30 a.m. for meditation practice in the temple.

Progress on the AD programme

In the more conventional sessions, 55 participants from 25 institutions in Europe, Japan, Russia and the US discussed theoretical, technical and experimental progress on the Antiproton Decelerator (AD) experimental programme. At the moment it follows two tracks. One is the ASACUSA collaboration’s programme of antiprotonatom collisions and laser/microwave spectroscopy of antiprotonic helium (in which one of the two normal orbital electrons is replaced by an antiproton). The other track is the synthesis and spectroscopic study of antihydrogen atoms by the ATHENA and ATRAP collaborations.

antimatter3_11-99 — Physics zen the Sojiji temple.

One motivating force behind ASACUSA is that, in much the same way as the hydrogen atom spectrum revealed the properties of the proton and electron over the course of the 20th century, high-precision laser and microwave measurements of atomic transitions of the antiproton in antiprotonic helium can reveal, with commensurate precision, the properties of the antiproton itself. Such measurements constitute valuable tests of validity of underlying physics symmetries. Antiprotonic helium was chosen instead of the simpler two-body protonium (antiprotonic hydrogen a proton and an antiproton in orbit round each other) atom because it is stable against annihilation for some microseconds after formation, while protonium, under normal conditions, is not.

ASACUSA experiments that are already approved include a microwave triple resonance experiment on hyperfine splitting caused by the interaction of the electron spin and antiproton orbital moments, and a search for laser-induced transitions between, so-far unobserved, atomic levels.

In addition, a new high-resolution laser system is expected to reduce the measurement precision of all transition frequencies to less than one part per million the level at which quantum electrodynamic effects appear. This has already been achieved in experiments that were studying another transition at CERN’s LEAR low-energy antiproton ring, which closed in 1996.

The status of ASACUSA on these fronts was reported by K Komaki and M Hori (Tokyo). The interpretation of spectral features of antiprotonic helium in terms of the properties of the atomic constituents requires energy-level calculations with precision similar to that of the experimental values, and this was discussed by Y Kino (Sendai), D Bakalov (Sofia), V I Korobov (Dubna) and G Korenman (Moscow).

Another goal of ASACUSA is to extend to lower energies the Aarhus and Tokyo LEAR experiments on the atomic interactions of antiprotons. This has sparked considerable theoretical interest. Contributions came from H Knudsen (Aarhus), P Krstic (Oak Ridge) and A Igarashi (Miyazaki) on ionization and energy loss for antiprotons interacting with matter. Such experiments require 10100 keV rather than 5 MeV antiprotons. They will be produced by inserting a decelerating radiofrequency quadrupole (RFQ) in the AD beam.

This is under construction in CERN’s Proton Synchrotron division and will soon be tested in Aarhus. Antiprotons from the RFQ may be used directly or, for certain ASACUSA experiments, collected and cooled in a multiring harmonic trap (T Itchioka, Tokyo), where they will be reaccelerated to electron-volt or kilo-electron-volt energies.

Traps for charged (as well as neutral) particles feature prominently in the plans of the ATHENA and ATRAP collaborations. Here the main aim is to use the laser probes to compare identical atomic transition frequencies in hydrogen and antihydrogen.

Such experiments have an important advantage over those using antiprotonic helium. They are direct comparisons, in which symmetry-conjugate systems are compared without any need for theoretical input. On the other hand, the technical problems associated with producing antihydrogen are much more complex than is the case for antiprotonic helium, which is created in abundance whenever antiprotons are slowed to electron-volt energies in helium gas. In particular, the antihydrogen must be synthesized at micro-electron-volt rather than electron-volt energies.

For this, AD antiprotons and positrons from radioactive sources must first be collected as plasmas in suitable containers and cooled to liquid helium temperatures. M Holzscheiter (Los Alamos) reported on the progress of the ATHENA experiment, which aims to synthesize antihydrogen atoms at sub-Kelvin temperatures. K Fine (CERN) and H Totsuji (Okayama) discussed the behaviour of these plasmas in Penning traps (with hyperboloidal electrodes) and in Penning-Malmberg traps (cylindrical ones), both of which are adequate as plasma bottles for these processes.

Future proposals

As befits the promise offered by the birth of a new machine, many ideas that go beyond the current AD programme were presented in Tsurumi. They include the possibility that atomic protonium, so far ignored because of its short lifetime, may live long enough under near-vacuum conditions to be the subject of ASACUSA-type experiments (R S Hayano, Tokyo). Another possibility is the existence of metastable antiprotonic lithium (K Ohtsuki, Chofu-shi) and of antiprotons in solution in liquid helium (T Azuma, Tsukuba). H SchmidtBöcking (Frankfurt) presented a proposal for a table-sized antiproton storage ring.

The Tsurumi workshop was organized by Yasunori Yamazaki of the Tokyo University Komaba campus and RIKEN, and it was sponsored by the Antimatter Science Project of the University of Tokyo, the Danish Natural Science Research Council’s Centre for CERN-related Atomic and Nuclear Physics and the Japanese RIKEN (Rikagaku Kenkyuujo) Institute.

In his concluding remarks, Mitio Inokuti (Argonne) commented on the confidence and excitement with which this worldwide physics community awaits AD beams. Tantalizing hints of this physics were revealed during the era of LEAR, of which the AD is now a worthy successor.

The Sun: a brilliant past and an even brighter future

solar1_11-99 — Fig. 1: A Hubble Space Telescope image of a star-forming region.

Stars are formed from huge masses of gas and dust. From fluctuations, a centre forms and then, under gravity, more and more matter gradually accumulates until a core is formed, which attracts more gas and dust. This core a protostar will get hotter. Under the effect of gravity, the protostar will contract and, if the core is big enough, nuclear reactions (fusion) will cause it to ignite. Thus a star is born.

Multiple star systems

This is how I used to imagine that stars were formed. When I looked up at the sky, I saw our Sun as a solitary star and the sky full of single stars. The idea of binary star systems with two stars rotating round one another seemed quite unnecessary. It therefore came as a shock when I learned that only 15% of stars are single, while 65% are in binaries and the remaining 20% are in clusters of three or more. This seemed contrary to the concept of a big cloud of gas and dust condensing.

For me, the explanation came in an early photograph from the Hubble Space Telescope (figure 1). It showed a star-forming region of Freudian complexity that explains the birth of multiple star systems. We are fortunate that our star is a solitary one.

When the Sun was born, some 4600 million years ago, its gas was composed of 72% hydrogen, 24% helium and the remainder was a variety of elements, which had been formed almost entirely in supernova explosions and then blown out into space. The mass of the Sun is 2 x 10²⁷ tons, or 2000 million million million million tons.

When a star is formed, its contraction makes it spin faster (like an ice skater pulling in his or her arms). Such a T Tauri star is distinctively bright for its mass. However, as it spins, it throws off mass, losing some 2000 million million million tons every year. It soon slows down, and today our Sun is losing some 20 million million tons per year (or 700 000 tons per second) as solar wind.

Solar flares

This stellar erosion can be seen during a total solar eclipse. The radius of the Sun is about 700 000 km, so the easily visible part of the solar wind, seen during an eclipse, extends out from the surface by more than a million kilometres. Occasionally a large flare will occur. An example of this is shown in figure 2.

solar2_11-99 — Fig. 2: The hours following a solar flare reveal a series of loops above the surface of the Sun best seen when viewed in the light emitted by hydrogen in the red region of the solar spectrum (H-alpha emission). These loops formed after an active region flared on 26 June 1992. This image was obtained by the Swedish Solar Telescope on La Palma in the Canary Islands.

The fate of a protostar core depends on its mass. If the protostar weighs less than 0.08 solar masses, the temperature of the core is not high enough to initiate nuclear fusion reactions and the protostar remains as an inert Brown Dwarf.

If the protostar weighs more than 8 solar masses, something new happens: the density of the core becomes so great that the inner 100 km or so of the star collapses within a few milliseconds, and the resulting ionized plasma is compressed into neutrons. Such a neutron star has an enormous density about 100 million tons/cm³.

The sudden collapse of the core generates a shock wave that compresses the neutron star even further. This rebounds, and the outward-moving shock wave then hits the remainder of the star, which, meanwhile, has hardly moved. The blast blows the shell apart and ignites it to produce the visible supernova.

If the star so formed is still massive about 20-30 solar masses then the neutron star can collapse further to give a black hole.

solar3_11-99 — Fig. 3: C Cavadore from ESO, and L Bernasconi and B Gaillard from l'Observatoire de la Cote d'Azur, took this photograph of the eclipse (11 August 1999) at Vouzier, which is between Reims and Nancy (France, Champagne-Ardennes).

The basic reaction that initiates star formation is the fusion of two hydrogen nuclei (protons) to form deuterium, followed by the deuterium rapidly combining with another hydrogen nucleus to give helium-3. This happens very quickly the deuterium only exists for a second before fusing with a proton. One could say that the deuterium acts as a catalyst, which converts three nuclei of hydrogen into helium-3. The helium-3 is then converted into a stable helium-4 nucleus via several routes.

Helium-4 is very stable and is generally the end of the chain. To go further, an unusual fusion reaction needs to take place between three helium-4 nuclei to give carbon-12. This can further react to give nitrogen and oxygen. This is called the CNO cycle. The next element, fluorine, can then be formed in the NOF cycle.

solar4_11-99 — Fig. 4: The total solar eclipse of 26 February 1998. This picture comprises three radial gradient images taken by Jonathan Kern from the Willams College site (Aruba), and later processed and composited by Wendy Carlos. (Copyright 1998 Wendy Carlos and Jonathan Kern.)

The Sun, which is essentially a plasma with no rigid structure, is fuelled mainly by hydrogen in its core, which is at a temperature of 15 million degrees centigrade. Its density decreases rapidly with distance from the core, where it is about 150 g/cm3. At the surface, the density is only a millionth of a gram per cubic centimetre and the temperature is about 5500 °C.

Bright future

As the Sun continues to burn, the temperature will rise and the core will contract, which raises the temperature even higher and makes the envelope expand. Eventually the Sun will become a Red Giant, at which point most of the hydrogen in the core is burned.

Then the core will contract further and become steadily hotter and hotter until it reaches a hundred million degrees centigrade. At this point the helium-4 will ignite and carbon-12 will be formed in a thermonuclear explosion. For the following 100 million years, the star will continue to be very bright.

The carbon can provide extra nuclei of helium-4 to generate oxygen-16 and neon-20, etc (the CNO and NOF cycles will not occur because the necessary hydrogen has already been consumed). Next, the carbon and oxygen will burn, but the thermonuclear furnace will eventually cut out and the core will become inert.

However, outside the core, the remaining hydrogen and helium gas will continue to burn for some time. This will drive off much of the gas and eventually the star will become a White Dwarf with a remnant mass of about half that of the Sun. This hot star will be visible for billions of years, but will eventually cool and become an invisible Black Dwarf.

When our Sun becomes a Red Giant, its expansion will first swallow Mercury (58 million kilometres away), absorb Venus (108 million kilometres away), engulf the Earth (150 million kilometres away) and finally approach Mars (228 million kilometres away).

Fortunately for us this fate is about 5000 million years away. The Sun is a vigorous adult star that still has a brilliant future.

Heavy physics implications

heavy1_11-99 — Construction of the BaBar detector at the PEP-II B factory, SLAC, Stanford.

Heavy particles are hard to come by. As a result, precision measurements in this sector are widely admired and highly prized. This physics was a main focus at the International LeptonPhoton Symposium in Stanford. Existing experiments are homing in on the production of particles containing heavy quarks, and a series of major new experiments is being developed.

Spearheading this new generation of experiments are the B-factories at SLAC, Stanford and the Japanese KEK Laboratory. The electronpositron annihilations in these colliders PEP-II and KEKB respectively are a rich source of – particles that contain the fifth (“beauty”, “bottom” or simply “b”) quark. The decays of these B particles should reveal new insights into CP violation the subtle symmetry breaking assumed to be responsible for a matterantimatter symmetric Big Bang, which eventually produces a visible universe composed entirely of matter.

The other B physics players in the race include, notably, Cornell’s CESR electronpositron collider, which is equipped with the CLEO detector. CESR and CLEO have been working in tandem for some 20 years and have made a series of landmark contributions to B physics. Experiments at CERN’s LEP electronpositron collider have also made many valuable contributions to this work.

In the wings of the B stage is the HERA-B experiment at DESY, which uses the proton ring of the HERA collider. Fermilab’s Tevatron remains a source of a copious amount of heavy particles. Significant B physics potential is provided by the CDF and D0 detectors at Fermilab’s Tevatron protonantiproton collider, which is now fed by the new Main Injector. Detector upgrades and collision rate improvements will ensure that the Tevatron remains a focus of B physics.

For the future, the LHCb experiment at CERN’s LHC collider and the BTeV project at Fermilab are getting their acts together.

After a “Brief report from the B-factories”, an introduction to the leptonphoton symposium, attention was focused on heavy-quark physics. Ronald Poling of Minnesota showed how the experiments at CERN’s LEP electronpositron collider have charted semileptonic decays. The B physics scenarios, which have been explored at LEP at high energy and by CLEO at CESR via upsilon decays, are becoming increasingly reconciled with the LEP scenario, which is closer to theoretical predictions.

In measuring the parameters that describe the various interquark decays, CLEO has made charmless B decay (b quarks that decay directly into light quarks) its special hunting ground. In the more usual b decays (into charmed quarks) measurements from different experiments are converging.

While describing heavy-quark lifetimes and mixing, Guy Blaylock of Massachusetts highlighted valiant efforts to measure and understand why different charged states of heavy quarks have different lifetimes. In the present parametrization of quark decays, the mixing of B quarks is expected to be large, while that of D (charm) quarks is expected to be small. The former, says Blaylock, will lead to better measurements of the existing scheme, while the latter will provide a window for new physics effects.

And so to CP violation the subtle violation of a symmetry that, ideally, should reflect a particle into a mirror image of its antiparticle and vice versa. Two major experiments have recently announced new measurements of “direct” CP violation (CERN Courier September) brought about by quark transitions. Edward Blucher of Chicago spoke for the KTeV study at Fermilab and Giles Barr of CERN for the NA48 experiment. CERN experiments have had quantitative evidence of this effect since 1993, while a contemporary Fermilab experiment had published a result compatible with zero direct CP violation. After many years of anguished doubt, direct CP violation now looks here to stay. However, the years of dilemma underline the difficulty in making these measurements. With the objective of measuring direct CP violation to within 5%, both experiments have a lot more data to analyse. The CERN study continues.

Sergo Bertolucci described how the KLOE detector, at the new DAFNE phi factory at the Italian Frascati Laboratory, will explore additional aspects of neutral kaon physics via phi decays and could round off the kaon picture. Hopefully, B physics will soon open a new, and wider, window on CP violation, which has so far been confined to the strange world of the neutral kaon.

Complementary to the heavy quark is the heavy tau lepton, which is the only weakly interacting particle heavy enough to decay into strongly interacting particles. Tau specialist Antonio Pich of Valencia surveyed the tau scene at Stanford. Tau physics at electronpositron colliders, including spin effects, provides a valuable laboratory in which to explore the physics of weak interactions and the behaviour of heavy quarks.

“B decays, the unitarity triangle and the universe” was the challenging title assigned to Adam Falk of Johns Hopkins for his review of heavy-quark physics. The interrelation of the various possible quark decays has a self-consistent parametrization (the CabibboKobayashiMaskawa matrix), which gives some degree of predictive power but cannot be derived from first principles. What makes it work?

The imaginative title was a reference to the current dogma that CP violation, a mechanism that is much studied and well documented but still not entirely understood, is ultimately responsible for the disappearance of the antimatter half of the Big Bang.

The new B sector will subject this physics to much wider scrutiny and could reveal as yet unseen effects. Falk asked if the necessary formalism was ready to enable all of these processes to be analysed consistently.

At the turn of the millennium, heavy-quark physics is also poised to enter a new era.

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