The world’s largest data sample of J/psi particles produced directly from electron-positron annihilation has been accumulated with the BESII spectrometer at the BEPC collider in Beijing. The discovery of this particle in 1974 heralded a revolution in particle physics. In the remarkable progress made since then, the J/psi, composed of a charmed quark bound to a charmed antiquark, continues to provide a useful benchmark.
The BES collaboration successfully completed a scan of R (the ratio of hadron to muon pair production) over 85 scan points in the important collision energy region of 2-5 GeV in 1999. Precision R values in this energy region are crucial for the accurate determination of the quark-gluon coupling constant alpha at the Z mass and the interpretation of the muon (g-2) measurement at Brookhaven, which are essential for precision tests of the Standard Model and for narrowing the mass window for Higgs particle searches.
After finishing the R scan, the collaboration turned its attention to other charmonium (charmed quark-antiquark bound states) physics with the goal of accumulating 5 x 107 J/psi events, about six times higher than the world’s existing largest J/psi sample.
The detector was turned on in mid-November 1999. Data-taking started in December 1999 and ended in mid May with about 2.2 x 107 J/psi events accumulated as planned.
In addition, special data runs were taken at 3.0 GeV and at the J/psi resonance peak for the study of quantum electrodynamics background and the trigger efficiency. Data runs were also taken at the peak of the psi(2S) resonance to help in the determination of the total number of J/psi events.
The J/psi run shows that with BESII and the upgraded BEPC, both the hadronic rate and the integrated luminosity accumulated per day have been increased by a factor of two to three compared with BESI. Also, the upgraded barrel time-of-flight system with a time resolution of about 180 ps significantly improves particle identification.
All the accumulated data has been reconstructed. Preliminary physics analysis shows that the data quality is excellent and that the detection efficiency is higher than the J/psi data collected with the BESI detector.
BES continued accumulating J/psi events in the autumn and hopes to reach the total of about 5 x107 events before next summer.
With this world’s largest J/psi event sample, the BES collaboration can systematically study light hadron spectroscopy; excited baryonic states such as N*, L* and S*; search for glueballs, chiral partners and exotic states; and probe lepton flavour violation and CP violation using J/psi decays. The collaboration is very excited about the physics that can be done with this unique huge sample.
At the end of August nearly 100 physicists met in Ambleside in England’s beautiful Lake District for the Photon 2000 conference organized by Lancaster University. Held roughly every two years, this conference concentrates on theoretical and experimental advances in the understanding of the high-energy behaviour of the photon, particularly the way it interacts with quarks – the production of matter from light.
In the first talk, Maria Krawzyck pointed out that this year is the 100th anniversary of Planck’s quantization of electromagnetic phenomena, which led to the concept of the photon.
The large volume of data coming from CERN’s LEP electron-positron collider in the last few years provides a unique tool to study the collisions of high-energy photons, and all four LEP experiments presented exciting new results in this field.
These results are complemented by the study of photons in their collisions with protons in data coming from the two experiments at the HERA electron-proton collider at DESY, and the very high volumes of data from the CLEO detector at Cornell’s CESR electron-positron ring which provide precise measurements at lower energies.
At the end of the conference delegates looked forward to their “dream machine”, a dedicated high-energy photon collider, which is one option available for the new generation of linear electron-positron colliders now being planned.
In July 1994 Swedish musical personality Martin Engstrom launched the Verbier Festival and Academy in Valais, Switzerland, which has gone on to become a regular feature of the late-July arts calendar. This festival has attracted prominent figures from the musical and theatrical world, such as Zubin Mehta, Isaac Stern and Isabelle Huppert. It is now a valuable step on the ladder for aspiring young artists.
CERN physicist André Martin and his wife Schu knew Aspen, in the Rocky Mountains, where there is a very successful annual summer symbiosis of music, mountains and physics, with the famous Music Festival on one hand and the Aspen Center for Physics on the other. It was tempting to propose scientific activities in conjunction with the Verbier Music Festival.
In the summer of 2000 this was realized for the first time through a conference entitled CAPP (Cosmology and Astroparticle Physics) 2000, organized by Ruth Durrer, Juan Garcia-Bellido, André Martin and Misha Shaposhnikov. About 100 participants came from as far afield as Australia and Korea, to Verbier’s “Centre Culturel du Hameau”.
Prestigious lecturers also came from all over the world. The programme covered both theoretical and experimental physics. One focus was the extremely accurate measurements of the structure of cosmic microwave background radiation by the balloon experiments Boomerang and Maxima at the South and North poles, respectively. This, combined with new measurements of the Hubble galactic recession parameter, leads to a picture of the universe which is asymptotically flat (W = 1), with an accelerating expansion, a non-vanishing cosmological constant and an age of between 14 and 18 billion years, fitting most inflationary models.
W = 1 is the result of W = 0.3 for matter and 0.7 for the vacuum. The former retains a need for invisible “dark matter”, also needed to explain the observed rotation of galaxies. Although definite cases of gravitational lensing have been seen (see Not enough stellar mass objects to fill the galactic halo?), their interpretation does not seem to fit with the Massive Astronomical Compact Halo Object (MACHO) picture.
On the other hand the Weakly Interacting Massive Particle (WIMP) interpretation of dark matter is still possible, which would also be an indication in favour of supersymmetry.
Among the projects for the future, more refined detectors of the cosmic microwave background such as the Planck mission and the Virgo project for detecting gravitational waves were described. Tremendous progress has been made in recent years thanks to the new instruments, and this looks set to continue. In particular the continued detailed analysis of fluctuations in the cosmic microwave background radiation will lead to a further confirmation of the inflationary models.
Returning to the music festival, a public lecture “L’Univers, passé, présent et futur” given in “Café Schubert” (where musicians attending the festival are habitually interviewed), was well received by an audience including Swiss Federal Councillor Pascal Couchepin.
For all its spectacular experimental successes, the Standard Model (SM) fails to give us solutions to such basic problems as why there are three copies (generations) of quarks and leptons, why there are three different gauge forces (the strong, weak and electromagnetic, with differing strengths), and how gravity should be included in a consistent quantum theory along with the gauge forces.
Supersymmetry (SUSY) is the leading contender for physics beyond the SM. Although SUSY has been around for some time and has so far had no direct experimental support, indirect experimental hints and progress in understanding the theoretical possibilities allowed for in a SUSY world have led to a new feeling of excitement. With these new ideas on the market, the Supersymmetry 2000 (SUSY2K) conference, held recently at CERN, attracted a large crowd and showed how the new SUSY ideas can help.
SUSY makes precise predictions for the quantum numbers and selection rules for many new particles. What is much more difficult is predicting the masses of these additional supersymmetric particles. The reason for this is that SUSY must be a so-called “broken” or hidden symmetry, and the mechanism of communication of SUSY breaking to the SM and its superpartners is inevitably indirect, not well constrained, and is poorly understood.
As a comparison, the unification of weak and electromagnetic gauge forces in the electroweak sector is also “broken” or hidden – with the Higgs mechanism leading to very different masses for the electromagnetic photon and the W and Z carriers of the weak force.
For SUSY, such a direct coupling to the sector that breaks SUSY (analogous to the direct coupling of the electroweak force to the Higgs) is not possible, because such a coupling leads to sum rules for the masses of the unobserved superpartners (see box) that are definitively excluded. Thus an indirect communication of SUSY breaking must be employed.
Many attractive new communication mechanisms for SUSY breaking were reviewed at the SUSY2K conference. In “archetypal” SUSY breaking, gravity takes on the role of communicating between the SUSY breaking sector and the conventional world, and, until recently, this gravity-mediated SUSY breaking was considered as the most plausible possibility.
However, during the last few years many innovative new mechanisms have been proposed – “gauge mediation” (with heavy messenger particles communicating the breaking), “anomaly mediation” (via symmetries that are broken at the quantum but not at the classical level), and “gaugino mediation” (when the SUSY partners of the SM gauge bosons take on the mediating role).
These different mechanisms have characteristic mass spectra and experimental signatures. Supersymmetry might not manifest itself as neutrino-like invisible events detectable only through “missing” energy, but in several other ways, for example in events producing additional photons or stable charged particles, or models with supersymmetric particles that are nearly degenerate in mass. Experiments at LEP and elsewhere have been looking for these various possibilities, but without any luck so far (see “Particles and sparticles” below).
Particles and sparticles
Standard Model (SM) particles come off the shelf in two kinds – fermions (matter particles) such as quarks, electrons, muons, etc.) and bosons (force carriers) such as photons, gluons, Ws and Zs. A feature of SUSY is that every matter particle (quark, electron…) has a boson counterpart (squark, selectron…) and every force carrier (photon, gluon) has a fermion counterpart (photino, gluino, chargino, neutralino…).
This doubling of the spectrum is due to the fact that SUSY is a quantum-mechanical enhancement of the properties and symmetries of the space-time of our everyday experience – such as translations, rotations and Lorentz boosts.
SUSY introduces a new form of dimension – one that is only defined quantum mechanically, and does not possess the classical properties we associate with a new dimension, such as continuous “extent”.
The doubling of the particle population can fix several of the problems afflicting today’s SM, for instance why the different forces – gravity, electromagnetism, weak and strong – appear to operate at such vastly different and apparently arbitrary scales (the “hierarchy problem”). The extra particles provided by SUSY are also natural candidates for exotica such as the missing “dark matter” of the universe.
One of the theoretical motivations for these new models is the “flavour problem”, namely that of understanding the relations between the different generations of particles. Experiments observe many approximate flavour symmetries in today’s non-SUSY SM; however, these symmetries are usually violated in typical gravity-mediated SUSY breaking schemes.
Another motivation for some of the new communication ideas (anomaly and gaugino mediation) has been provided by new ideas for physics beyond the SM, such as extra dimensions beyond those accessible to us and multidimensional “branes” (see Superstrings, black holes and gauge theories).
Many new ideas have also been stimulated by the exact non-perturbative results that have allowed theorists to construct explicit models of SUSY breaking, and motivated attempts to merge SUSY breaking with the visible particles. One approach involves composite (sub-quark) models, where some of the SM states are composites of a strongly interacting sector.
Extra dimensions – are we the scum of the universe?
A natural focus of the workshop was extradimensional models, in which the world we experience is complemented by extra (but to us invisible) spatial dimensions. These models have the common feature that our SM world is realized as localized degrees of freedom living on a generalized 3-spatial-dimensional membrane (“3 brane”) embedded in a universe possessing a larger number of dimensions.
In this approach, it is possible that the fundamental scale of gravity might be the TeV scale, rather than the embarrassingly distant Planck scale (1019 GeV), potentially eliminating the hierarchy problem (see “Particles and sparticles”).
This requires a fundamental rethinking of cosmology and the high-energy behaviour of SM physics. Many questions are being reformulated in terms of the geometry of the extra dimensions – their sizes and shapes, and the fields localized on them. In the same way that general relativity introduced geometry as the natural explanation of gravity, so concepts of geometry and locality replace the ideas of symmetry usually used in field theory.
Superstring theory naturally incorporates such branes and gives, at least in toy models, explicit realizations of the brane-world idea. One major question is the radiative stability of such models – that their predictions are compatible with accompanying virtual quantum effects.
Without SUSY, the apparently haphazard hierarchy of the different forces of nature, with each force having very different associated mass scales, is not stable (or rather requires fine tuning). SUSY can take care of this problem, and new light may be cast by brane physics.
At the moment there are two main approaches to the construction of extra-dimensional models. Originally, it was thought that the geometry of the extra coordinates should be distinct from our space – the universe at large could be viewed as the product of two spaces. In this case, a solution to the hierarchy problem requires large extra dimensions and quantum gravity physics at the TeV scale.
In a more recent approach, highly-curved geometries have been proposed, which tightly constrain the brane in which we live. In this very different geometry, gravity is concentrated away from our world, explaining its observed weakness for us. Both schemes have very specific signatures for experiments at high-energy colliders.
Seeing SUSY
All current major high-energy collider experiments are desperately seeking SUSY and/or extra dimensions. One of the crucial searches is for a Higgs boson: SUSY suggests that one might well be visible at CERN’s LEP electron positron collider.
Future collider experiments are also gearing up to look for new particles. The Fermilab Tevatron will resume the sparticle and Higgs searches after LEP is retired, and has quite good prospects. In the longer run, the LHC is expected to produce Higgs bosons and any supersymmetric particles. It will also be able to probe for extra dimensions at shorter scales than any previous experiments. There is optimism that the next generation of collider experiments will break out of the SM straitjacket.
The issue of the cosmological constant – the energy density of free space – has been the most striking problem in quantum field theory for many years. Experimentally, it has long been known that it is very close to zero. According to the latest observations a (very) small non-zero value is now preferred, and this is further supported by cosmic microwave background observations by the BOOMERANG and MAXIMA collaborations.
However, the result of theoretical calculations in quantum field theory is naturally a number at least 60 orders of magnitude bigger. SUSY has long held out the promise of a resolution to this dilemma, but so far has not been able to claim a solution. However, many new ideas of how to approach this problem are also suggested by brane theories and were discussed at SUSY2K.
Dark matter
If SUSY is correct then it would have played an important role in the Big Bang. For example, SUSY might have played a role in the generation of the observed matter in the universe. However, one of the most important issues is that of possible SUSY remnants of the Big Bang, which could play the role of the invisible “dark matter” known to pervade our universe. One of the most attractive features of SUSY is that it provides quite naturally a candidate, the “neutralino”. Experimental searches for such particle dark matter are just beginning to reach the range suggested by theory. However, SUSY must also contend with the strong upper limits on various unwanted supersymmetric particles such as gravitinos.
SUSY2K showed that supersymmetry is assured of an exciting future.
The Sun is a typical main sequence star that generates its energy via the fusion of hydrogen into helium in two chains of nuclear reactions: the so-called pp chain and the CNO chain. If the nucleon number, electric charge, lepton flavour and energy are conserved and the Sun is in a steady state, then the total solar neutrino flux is fixed, to a good approximation, by the solar luminosity (approximately 65 billion neutrinos/cm2/s at Earth), independent of the specific nuclear reactions that power the Sun and produce neutrinos by beta decay or the electron capture of reaction products.
The neutrinos from the dominant pp chain are produced by the beta decay of proton pairs (pp), boron-8 and lithium-4, and by electron capture by pp pairs and beryllium-7. Their spectra can be measured directly in the laboratory or calculated from the standard theory of electroweak interactions.
To a very good approximation, they are independent of the conditions in the Sun. Only their relative contributions depend on the detailed chemical composition, temperature and density distributions in the Sun. Solar neutrino experiments can therefore test both the standard theory of stellar evolution and neutrino properties over a long distance, much larger than the diameter of Earth.
By the turn of the last century, solar neutrinos had been detected by radiochemical methods in three underground solar neutrino experiments in the US (Homestake) and Europe (SAGE and GALLEX) and in real time by the water Cherenkov techniques in two experiments in Japan (Kamiokande and Superkamiokande). These studies have confirmed that the Sun is powered by the fusion of hydrogen into helium – a milestone achievement in physics.
However, the combined results also suggested that the solar neutrino fluxes differ significantly from that expected from the standard solar models. This discrepancy has become known as the solar neutrino problem (SNP).
Neutrino oscillations
Many scientists have argued that this discrepancy is due to neutrino properties beyond the minimal standard electroweak model. In 1968, Gribov and Pontecorvo suggested that “oscillations” of electron neutrinos to other neutrino flavours may reduce the solar electron neutrino flux at the Earth. Later, Mikheyev and Smirnov elaborated on work by Wolfenstein on the propagation of neutrinos in matter and found that matter amplification of these oscillations in the Sun can provide an elegant solution (the so-called MSW solution) to the SNP.
The widespread belief in this solution of the SNP was strengthened by the accumulating data from the deep underground experiments on the atmospheric neutrino anomaly (fewer muons than expected) and, most recently, also from the first terrestrial long distance neutrino experiment, K2K, which were reported by Kenzo Nakamura from the KEK laboratory at Neutrino 2000 – the 19th international conference on neutrino physics and astrophysics which was held this summer in Sudbury, Canada.
Both the atmospheric neutrino anomaly and the K2K results can be explained by the hypothesis of nearly maximal strength oscillations of muon neutrinos to tau neutrinos if their squared masses differ by some 3 x 10-3 eV2. However, conclusive solar neutrino evidence for electron neutrino properties beyond the standard electroweak model can be provided only by detecting at least one of the following signals:
* neutrinos other than electron-type visible by neutral current interactions;
* spectral distortion of the fundamental beta-decay spectra;
* a neutrino flux different from that expected from the solar luminosity;
* modulations of the solar neutrino flux, such as a day-night or summer-winter difference, other than that expected from the seasonal variation in the Earth’s distance from the Sun.
Looking for smoking guns
It was hoped that these “smoking gun signals” would be found before Neutrino 2000 with the two currently operating solar neutrino telescopes: the 50 kt Superkamiokande underground light-water Cherenkov detector that has been collecting data on the boron-8 solar neutrino flux, its spectrum, and seasonal and day-night possible variations, with a lower energy threshold and lower background; and the 1 kt Sudbury Neutrino Observatory (SNO) heavy-water detector in a 2 km deep Canadian mine that started taking data half a year ago and is expected to detect the conversion of solar electron neutrinos to mu or tau neutrinos through their dissociation of the deuterium into a proton and a neutron in the heavy water.
However, no such signals have been detected. At Neutrino 2000, Yoichiro Suzuki from the Kamioka Observatory presented data from 1117 days running of Superkamiokande which show no day-night effect, no spectral distortion of the boron-8 solar neutrino spectrum, and the expected variation due to the annual variation in the distance between the Earth and the Sun.
In fact, the use of a new and more precise laboratory measurement of the neutrino spectrum from boron-8 beta decay and a new estimate of the cross-section for proton capture on helium-3 yield an excellent agreement between the expected and observed Superkamiokande solar neutrino spectra as seen in figure 1.
When combined with the other solar neutrino experiments, the Superkamiokande data rule out, with 95% confidence, the small mixing angle MSW solution and a “sterile” neutrino oscillation solution to the SNP. It leaves only a small region in the mass-mixing exclusion plot with a large mixing angle as a possible simple oscillation solution to the SNP as can be seen from figure 2. Fortunately, this solution will be tested in the near future in a terrestrial experiment – KamLAND, a long base-line neutrino oscillations experiment in Kamioka using nuclear reactor neutrinos, and with new solar neutrino experiments such as BOREXINO.
What if?
But what if the large mixing angle oscillation solution to the SNP will also be ruled out by KamLAND and BOREXINO, and SNO will not detect conversion of solar electron neutrinos into mu or tau neutrinos? At Neutrino 2000, E Belloti, spokesman of the Gallium Neutrino Observatory (GNO), and V Gavrin, spokesman of SAGE, reported updated results for the solar neutrino capture rate in gallium. Their measured rates, some 78 ± 7 standard solar neutrino units (SNU), are above the minimal signal expected from the observed solar luminosity, if solar neutrinos do not oscillate.
These results seem to leave only a little room for solar neutrinos from electron capture by beryllium-7 in the Sun. This is also suggested by the results from the pioneering chlorine experiment of Ray Davis at Homestake, the counting rate of which, 2.56 ± 0.23 SNU, is consistent with the solar neutrino flux (2.37 x 1010 cm2/s) measured by Superkamiokande, but leaves very little room for beryllium-7 neutrinos.
However, this conclusion heavily relies on the accuracy of the theoretically-deduced cross-sections for neutrino capture in gallium and chlorine. If the results of GALLEX and SAGE are calibrated with their chromium source experiments, they leave more space for beryllium-7 solar neutrinos, perhaps sufficient to accommodate a solar electron-capture rate in beryllium-7 consistent with the solar proton-capture rate in beryllium-7 that produces the observed boron-8 solar neutrino flux in Superkamiokande.
A direct calibration experiment for the chlorine detector was described by Ken Lande at Neutrino 2000. Improved calibration experiments are also under consideration by the GNO and SAGE collaborations. Altogether, it will still require long, challenging and innovative experiments to give a complete spectroscopy of the elusive solar neutrinos and pin down the origin of the SNP.
It is difficult to explain what the biennial neutrino conferences mean for the neutrino physics community. Every two years, hundreds of scientists from all over the world meet to update and compare results, conclusions, opinions, claims and contentions.
Neutrino physics is different from any other field due to the variety of experiments, techniques, measurements, approaches, theories and prejudices associated with it.
Neutrino physics today involves many different fields – the detection of remnant neutrinos from the Big Bang; galactic neutrinos at extreme energies; neutrinos from supernovae, the Sun, the Earth’s atmosphere and emitted from the Earth; artificial neutrinos produced from accelerators and nuclear reactors; and laboratory neutrinos from tabletop sources.
These experiments, spanning the 0.1 eV-1 PeV (1000 TeV or 1015 eV) energy region and providing information on the same particle, give a good idea of the spirit of the conference – a scientific bazaar where a cosmologist’s opinion is confronted with that of a solid-state physicist, and a 50 000 t Cherenkov detector’s results have to be understood in the light of the claims from a 10 kg calorimeter. Neutrino 2000, the 19th iteration of the biennial neutrino forum, took place in Sudbury, Ontario.
According to the results presented, the only possible conclusion is that neutrinos have mass. The solar neutrino deficit, the atmospheric neutrino oscillation pattern and the Los Alamos Neutrino Detector (LSND) claim in the region of cosmological interest have been confirmed and reinforced.
However, the interpretation of the results is ambiguous and the emergence of a unique picture strongly depends on experimental confirmation and improvements. A wide ongoing programme aims to improve our current experimental knowledge – new solar neutrino experiments such as SNO and Borexino, long-baseline programmes at Fermilab at CERN addressing the atmospheric neutrino signal, and MiniBoone on the verification of the LSND claim.
A worldwide programme for the study of a neutrino factory is also challenging the unprecedented possibility of precision measurements on neutrinos and is advancing rapidly to overcome the difficulties of exploitation.
In addition to the beautiful Canadian environment and the conference itself, interest was focused on the Sudbury Neutrino Observatory (SNO), a 1 kt heavy-water detector that has been in operation since September 1999. SNO has the unique ability to be able to say if the solar neutrino problem – a neutrino deficit with respect to the theoretical predictions and now observed by a variety of experiments – is indeed due to neutrino oscillations. (Classically, the different neutrino types – electron, muon and tau – lead separate lives. However, with a mass these species can “oscillate” or transform into each other.)
To achieve this goal at SNO meant solving formidable technological problems – the deepest observatory in the world, shielded by 2000 m of rock in an active nickel mine, with a cosmic ray reaching the detector once every 20 min. The environmental conditions are particularly critical, and the entire laboratory has been constructed by transporting single components down a vertical elevator and a 1.3 km horizontal tunnel. The SNO is a “ship in a bottle”.
SNO has an unprecedented purity for a large-scale detector – the purification system extracts daily seven atoms of radon for every ton of heavy water. To achieve this, strict precautions are taken. The laboratory is completely shielded from the still-active mine environment (blasting occurs in neighbouring tunnels), and different levels of cleanliness and dust purity allow the inner core to be a class 10 000 clean room (fewer than 10 000 dust particles per cubic foot).
Since everything reaching the detector goes through the mine, the major source of radon contamination comes from the transfer of material. For example, even Neutrino 2000 visitors in special clothing meant a significant effort to restore normal purity conditions.
SNO uses heavy water – a very precious liquid. However, Canada is the major world producer. Heavy water (produced for nuclear power plants) is usually extracted from freshwater lakes.
The SNO experiment has demonstrated that the design goals have been achieved, and solar neutrino interactions above 2 MeV are indeed measured – less than one per hour – with the expected backgrounds. However, no quantitative statement on the solar neutrino problem was made by SNO at the conference, and scientists are waiting for additional data and improvements that will make SNO the only experiment capable of also detecting the interactions from muon or tau neutrinos coming from the Sun. Since only electron neutrinos are produced in solar nuclear reactions, simultaneous detection of an electron neutrino deficit and a muon or tau neutrino excess would be the final proof that electron neutrinos oscillate.
The conference, as usual, provided new and exciting results. For the first time in such a meeting, long-baseline data were presented. In the K2K project, artificial neutrinos from the Japanese KEK laboratory are detected 250 km away in the Superkamiokande 50 000 t underground detector.
The observed rate is compared with predictions extrapolated from the interaction rate registered by detectors near the source to cross-check, in an independent way, the result claimed by Superkamiokande based on “natural” neutrinos produced by cosmic-ray interactions in the atmosphere.
As with the solar neutrino effect, a neutrino deficit would imply neutrino disappearance, and thus an indication of neutrino oscillation. Still, the meagre data so far (17 events observed while 29 were expected; September p8) did not allow K2K to claim a deficit that would confirm the atmospheric neutrino oscillation signal, but data will naturally increase in the coming years.
Superkamiokande is also able to detect solar neutrinos, and on this subject data were meaningful, excluding possible hypotheses on the solar neutrino problem. In particular, they were able to collect 15 000 solar neutrino interactions with an improved signal-to-noise ratio and lower energy threshold (5 MeV).
These data could disfavour the simple hypothesis of neutrino oscillation in vacuum (space), and instead point more directly to additional oscillations as the neutrinos traverse the Sun, significantly refining the known oscillation parameters.
Limits on neutrino mass?
The neutrino oscillation phenomenon cannot occur if neutrinos are massless. On the contrary, if neutrinos are massive, it is possible that they oscillate. The oscillation is such that not all neutrino oscillation experiments could actually observe it: this depends on a few parameters, such as the neutrino flavour (electron, muon or tau) they can detect, the neutrino flavour emitted from the source, the distance from the neutrino source and the energy of the neutrinos.
Many experiments are therefore currently looking for neutrino masses but are not observing a positive signal, without being in contradiction with the fact that neutrinos have mass and can be seen to oscillate under other conditions. Among these are the CHORUS and NOMAD experiments at CERN, which are currently exploring the oscillation parameters with a sensitivity higher than any other experiment (their new results are about 1000 times as sensitive as Superkamiokande).
All of these experiments are nevertheless contributing to our understanding of neutrino properties. CHOOZ and Palo Verde, for example, have measured the neutrino emission from nuclear power plants but did not observe any oscillation phenomenon. However, their results are crucial, showing that the atmospheric neutrino oscillation claimed by Superkamiokande does not take place between electron neutrinos and muon neutrinos, but between muon neutrinos and something else.
Another group of experiments is pursuing a different strategy – to detect the neutrino masses directly, that is without assuming the oscillation hypothesis but directly “weighing” their mass, as has been done for all other known particles.
Unfortunately, neutrino masses are extremely small, and the current experimental sensitivity of the Mainz and Troitsk experiments allows us to say that the electron neutrino mass is less than a few electronvolts. This is again compatible with all neutrino masses currently claimed (even with the massless neutrino hypothesis) but does not rely at all on the oscillation hypothesis.
Neutrino astrophysics and cosmology
No less important were the neutrino-related conclusions from astrophysics and cosmology. For the first time, supernova modelling was able to describe the stellar explosion mechanism, which is crucial to understand the time distribution of the emitted neutrinos.
At the same time, cosmological measurements are constraining more and more the way neutrinos can be distributed in the universe, pointing to a significant dark matter contribution by neutrinos of a mass of about 1 eV.
Neutrino scientists are eagerly awaiting the next nearby supernova explosion. The 1987 event, were it to happen today, would yield far more data on neutrino properties due to detector improvements. The supernova rate in our galaxy is about three per century, so one of the next neutrino meetings will be more interesting than ever.
The year 2000 sees the debut of a new kind of precision physics. It formally began with the opening plenary talks at the International Conference on High Energy Physics, the biennial particle physics jamboree, held this year in Osaka.
On 31 July the initial plenary speakers at Osaka – David Hitlin of Caltech and Hiroaki Aihara of Tokyo – presented the first physics results from the BaBar and BELLE detectors respectively. A small but paradoxically important effect called CP violation has now been seen and measured outside its traditional hunting ground, which had been limited to studies of neutral kaons.
BaBar and BELLE operate at new electron_positron colliders – BaBar at the PEP-II machine at SLAC, Stanford, and BELLE at KEKB at the Japanese KEK laboratory. These colliders started operating for physics in 1999 with the aim of achieving high luminosities (collision rates) to mass-produce B-mesons – particles containing the fifth “beauty” quark, hence their name “B factories”. The commissioning of both machines has been impressive, and they are routinely delivering luminosities in excess of 1033/cm2/s – figures previously unheard of.
Physicists think that the delicate charge-parity (CP) violation effect, or matter-antimatter asymmetry, played a major role in shaping the particle scenario that emerged from the Big Bang. The initial explosion that created the universe presumably produced equal amounts of matter and antimatter. The convention is that CP violation then moulded the universe so that it eventually emerged with antimatter apparently erased from the map.
For CP symmetry the physics of left-handed particles is the same as that of right-handed antiparticles. In 1964, physicists discovered that, in the decays of one of the neutral kaons (the long-lived variety), CP symmetry is respected only 99.997% of the time. This tiny violation is enough to define which is positive and which is negative electric charge – the positive-negative assignment is not just convention.
Subsequent precision experiments have probed CP violation in depth, revealing even smaller effects that operate at the quark level. At a few parts per million, these effects are difficult to see and even harder to measure, and it has taken some 20 years for experiments to approach a consensus.
Quarks transform into each other under the action of the weak force, and the various possible quark transformations have been studied extensively and documented in the Cabibbo-Kobayashi-Maskawa (CKM) matrix of quark coupling strengths. According to these numbers, CP violation in the decays of B mesons should be easier to see than in the neutral kaon case.
The results
At Osaka it was announced that PEP-II has delivered an impressive 16 inverse femtobarns of accumulated luminosity since last May, of which the BaBar team has 14 on tape. The collider is running well and is already exceeding the design goal of 135 inverse picobarns per day. For the measurements presented at Osaka, BaBar uses only this year’s data, corresponding to about 10 inverse femtobarns.
The CP violation measurement uses B decays into J/psi and a short-lived kaon, with several different kaon decay modes, and into psi prime and a short-lived kaon. A total of 120 reconstructed events are used to determine the sin(2b) CP violating parameter. The Babar result is 0.12 ± 0.37 ± 0.09. For a check, CP asymmetries of channels that should not have any CP violation, for instance J/psi and a positive kaon, are consistent with zero.
The PEP-II plan is to extend this year’s run until October, collecting 25 inverse femtobarns (BaBar aimed for 30 inverse femtobarns, so they are right there from the start). BELLE at KEKB uses the full data sample since the start-up, corresponding to 6.8 inverse femtobarns. The luminosity reached so far is 2.04 × 1033. The decay channels used for the determination of the sin(2b) CP-violating parameter include, in addition to those used at PEP-II, B decays into chi-c1 and decays giving long-lived kaons. (BaBar has about 89 of these but doesn’t include them yet.) A total of 98 events enter the final BELLE CP fit. For J/psi Ks events only, the BELLE result is sin(2b) = 0.49 + 0.53 – 0.57. Combining this with a result from other events yields sin(2ß) = 0.45 + 0.43 – 0.44 + 0.07 – 0.09. Statistically, this is within one standard deviation of previous determinations/expectations. BELLE checked that decay channels not expected to show CP asymmetry give a null result. Plans are for BELLE to resume running with higher currents in October until summer 2001.
The upshot
The first results from BaBar and BELLE show that CP violation happens in B decays. Forget briefly about comparing the results with predictions. The fact that the effect can be measured for B-particles so soon after the debut of two innovative machines and two challenging new experiments is a major achievement and bodes well for the continued vigour of this new branch of particle physics. It look a long time before all of the CP violation parameters in neutral kaon decay were known with confidence. The B sector will probably consolidate much faster.
Fermilab’s Tevatron proton-antiproton collider is also a prolific source of B particles, and the CDF experiment there made an earlier brave attempt to measure CP violation in B decays. This environment is cluttered and the signal difficult to measure, but an effect was almost certainly there. With the advent of the PEP-II and KEKB B-factories, the study of matter-antimatter asymmetry enters a new era. For the longer-term future, other experiments are setting their sights on B physics – HERA-B at DESY, Hamburg, BTeV at Fermilab and LHCb at CERN’s LHC collider.
A neutrino experiment at Fermilab has seen the first direct evidence for the tau neutrino, the most elusive of the 12 particles that make up the Standard Model picture of the fundamental structure of matter.
Using the intense neutrino beam from Fermilab’s Tevatron, the DONUT (Direct Observation of the Nu Tau) experiment has seen four examples of neutrinos producing slightly kinked tracks, the tell-tale sign that an unstable tau particle has been produced.
According to the Standard Model, all of the matter we know in nature can be explained in terms of six quarks – the ultimate constituents of nuclear matter – and six other particles (leptons). The quarks are arranged in three pairs – up and down, heavy strange and charm, and still heavier beauty and top. The six leptons are also arranged in three pairs – three electron-like particles; the electron, the muon and the tau; and three ghostly neutrinos – each associated with one of the electron-like particles.
The Standard Model
Quarks and leptons can thus be arranged in three “families” of four: the first contains the up and down quarks, the electron and the electron neutrino; the second contains the strange and charm quarks, the muon and the muon neutrino; and the third contains the beauty and top quarks and the tau and tau neutrino.
It has been known for a long time that the Standard Model contains these 12 particles, but initially not all of them had been seen. In 1995, experiments at Fermilab’s Tevatron collider saw evidence for particles containing the long-awaited sixth “top” quark. Now, with the evidence for the tau neutrino, all of the direct evidence for the 12 particles is finally in place.
For the DONUT experiment, Fermilab’s 800 GeV proton beam (effectively the highest energy in the world) is slammed into a huge target or “beam dump”, which produces a dense fog of highly unstable secondary particles. One of these is a D meson, containing both strange and charmed quarks (the Ds particle), which can decay to produce tau neutrinos. (Conventionally, neutrinos are produced by the decay of secondary pions and kaons. However, with a beam dump, many of these are otherwise
absorbed by the surrounding material before they have a chance to decay and produce neutrinos. The fraction of the neutrino content produced via other decays is therefore increased.)
After the beam dump, an obstacle course of magnets sweeps away charged particles, while thick shielding absorbs many of the rest. However, the ethereal neutrinos continue almost unaffected.
Downstream of the magnets and shielding is the DONUT detector, a sandwich of iron plates and photographic emulsion. In this target, one in a trillion tau neutrinos hits an iron plate, releasing an unstable tau lepton.
The tau leptons (which like the electron carry electric charge) leave a sub-millimetre track in the emulsion before decaying. The DONUT experiment set out to look for these tiny track stubs. Of the 100 or so tau neutrino collisions, just four track stubs have been unearthed so far. Isolating these signals from the mass of accumulated data is a triumph of painstaking analysis. Emulsion technology developed at Nagoya plays a major role in this work, and the Nagoya team handles DONUT’s crucial emulsion analysis.
When CERN’s LEP electron-positron collider came into operation in 1989, one of its first results was to show that particle decays allow for three, and only three, kinds of neutrino. The first of these had been seen by Clyde Cowan and Fred Reines in a reactor experiment in 1955, and for this the latter received the Nobel Prize for Physics in 1995 (Cowan died in 1974). In the 1950s, seeing the neutrino (in this case the electron-type particle) was considered a major accomplishment.
Soon the decay patterns of the muon suggested that the neutrino had to come in two different kinds, one preferring to associate with electrons, the other with muons. In 1962 an experimental team led by Leon Lederman, Mel Schwartz and Jack Steinberger
at Brookhaven revealed muon tracks emerging from neutrino interactions. For this discovery the trio received the 1988 Nobel Prize.
In 1975 Martin Perl at the SPEAR electron-positron collider at SLAC, Stanford, discovered the third lepton, the tau. Before this discovery only two families of fundamental particles had been known. Perl’s breakthrough suggested that there are three. For the tau discovery he was awarded the 1995 Nobel Prize, sharing it with neutrino pioneer Reines.
For the tau to fit into the picture it also had to be accompanied by its own neutrino. Physicists learned to live with the elusiveness of this particle, and could infer its existence directly. For example, in 1987 the UA1 experiment at CERN’s proton-antiproton collider studied decays of the W particle, the electrically charged carrier of weak interactions, which was discovered at CERN four years previously. Setting to one side the W decays producing electrons and muons, they found 29 decays that could be designated as candidate decays producing a tau (and a tau neutrino). Although the neutrino could not be seen, energy-momentum accounting revealed “missing energy, showing that an invisible particle – the tau neutrino – had escaped in the W decays”.
Tau physics, with the tau neutrino playing an essential but invisible role, went on to become a precision science in the hands of experiments at electron-positron colliders – LEP at CERN and CESR at Cornell.
The recent Chorus neutrino experiment at CERN also used Nagoya emulsion technology. This study (and the companion Nomad experiment) explicitly set out to look for the transformation of muon neutrinos into tau neutrinos (neutrino oscillations). These experiments used a conventional neutrino target rather than a beam dump. At the lower proton energies available at CERN, few Ds particles containing heavy quarks are produced directly. The experiments did not see any tau neutrinos, either through oscillations or via direct production.
DONUT is a collaboration between the US, Greece, Japan and Korea.
Announced at the recent Neutrino 2000 meeting in Sudbury, Canada, were the first results from the K2K long-distance neutrino beam experiment in Japan. For the first time, synthetic neutrinos made in a physics laboratory are seen to disappear.
In the K2K study, neutrinos (of the muon-like variety) generated at the Japanese KEK laboratory are directed towards the Superkamiokande underground detector 250 km away. In the detector, 22.5 kT of water are monitored by sensitive photomultipliers to pick up the tiny flashes of light produced by particle interactions.
The experiment, which began running last year, was able to announce at Sudbury that 17 neutrino counts had been picked up. The pulses of parent protons at the source accelerator can be used to clock the arrival of the neutrinos, so the results are essentially free of spurious background.
About 29 neutrino counts were expected, assuming the neutrinos despatched from the KEK laboratory arrived unscathed at Superkamiokande. Such a deficiency, if it continues to be seen, implies that something happens to the particles along their 250 km flight path.
This is not a surprise. In 1998, initial results from Superkamiokande on muon signals generated by neutrinos produced via cosmic-ray collisions in the atmosphere showed that the signal from muon-like neutrinos arriving from the atmosphere directly above the detector was very different from the signal arriving from below.
This is not a result of absorption in the Earth – 99.9999…% of neutrinos pass through the Earth as though it were not there. The effect was interpreted as neutrino metamorphosis – “oscillations” – as the particles passed through the planet.
Neutrinos come in three varieties – electron, muon and tau – according to the particles that they are associated with. For a long time, physicists thought that these neutrino allegiances were immutable – a neutrino produced in an electron environment could remain an electron-like neutrino for ever.
However, this is only so if the neutrinos have no mass and travel at the speed of light. If the particles do have a tiny mass, they can, in principle, switch their electron/muon/tau allegiance en route. The 1998 atmospheric neutrino effects seen in Superkamiokande provided the first firm evidence for such neutrino oscillations. These Superkamiokande data have now been consolidated, while other evidence has also appeared.
With neutrino oscillations, neutrinos of a certain type are more likely to change into neutrinos of another type than to interact with matter. If the viewing detector is sensitive only to neutrinos of a certain type, then some neutrinos disappear from view.
Such neutrino disappearance also correlates with the long-observed dearth of neutrino signals from the Sun, where measurements using a variety of detectors (including Superkamiokande) give only a fraction of the number of expected electron-type solar neutrinos. The Superkamiokande detector is sensitive to electron-like and muon-like neutrinos. However, if the parent KEK muon-like neutrinos change into tau-like neutrinos, Superkamiokande would not see them. This is the interpretation of the initial deficiency logged by the experiment in Japan. However, these are only the first results to appear from the K2K experiment, and it usually takes a long time to assemble reliable neutrino data.
The transmutation of muon-like into tau-like neutrinos is good news for long-distance neutrino experiments now under construction. The MINOS experiment in the US will send neutrinos 730 km from Fermilab to a detector in the Soudan mine, Minnesota, while neutrinos from CERN will be despatched towards new detectors in the Italian Gran Sasso underground laboratory, also 730 km distant. The detectors in these projects hope to pick up signs of tau-like neutrinos not present when the beams left the parent laboratories.
Because of the scant affinity of neutrinos for matter, intercepting them in a detector is always a challenge. The DONUT experiment at Fermilab recently presented possible evidence for the production of tau-like neutrinos in particle interactions.
The K2K experiment is a collaboration involving Japan, Korea and the US.
Now coming into action for physics is CERN’s new Antiproton Decelerator (AD), opening another chapter of CERN’s tradition of physics with antiprotons. With the AD, the focus switches from exploiting beams of antiprotons to capturing the precious nuclear antiparticles.
When CERN’s low-energy antiproton ring (LEAR) was closed in 1996 after more than 10 years of operation, it had supplied 1.3 x1014 antiprotons – enough to supply about 10 000 particles to everyone on the planet, but representing a theoretical accumulation of only 0.2 ng of antimatter.
Although LEAR slowed down the particle beams supplied by CERN’s antiproton factory from 26 GeV by a factor of about 10 (itself no mean feat), its antiprotons were nevertheless still moving extremely fast. A particle with 100 MeV momentum corresponds to a temperature of billions of degrees.
Of all LEAR’s antiprotons, just a few were privileged to be selected and eventually cooled down to temperatures approaching absolute zero. The techniques learned in this work opened up substantial economies for antiparticles – probably one of the rarest, and therefore most expensive, commodities in the world.
Cooling antiprotons is a tricky business. They quickly annihilate with ordinary matter such as liquid helium, the conventional ultra-refrigeration medium. Instead, antiprotons have to be supercooled by a gas of electrons (negatively charged antiprotons can peacefully coexist with electrons).
In this way the TRAP Bonn-Harvard-Seoul collaboration was able to stack several thousand ultracold antiprotons at a time. Antiprotons cooled to such a low energy by the electrons were locked in a shallow trap using electric and magnetic fields to contain the valuable antiparticles. Meanwhile a large electromagnetic well was opened alongside to receive a fresh batch of antiprotons, which were then similarly cooled. The energies of the individual antiparticles were then just one ten-millionth of what they were in LEAR.
Interesting antiproton physics thus became feasible using a less ambitious antiproton source. This is the motivation behind CERN’s new AD, which supplies antiprotons to several experiments – ATRAP (son of TRAP at LEAR), ATHENA and ASACUSA.
One ultimate physics objective at LEAR was to isolate a lone antiproton and study it carefully. Gradually reducing the electromagnetic “depth” of its snare, the TRAP team spilled out excess antiparticles until just a single antiproton survived.
Like any other captive electrically charged particle, an antiproton orbits in a magnetic field – the principle of the cyclotron. Comparing the frequencies of this rotation for an antiproton and a proton gives a direct comparison of the masses of the particle and its antiparticle.
The TRAP team at LEAR was able to ascertain that the proton and antiproton masses are equal with increasing precision, eventually to just one part in 10 billion. Making a measurement to such astonishing accuracy is equivalent to fixing the position of an obect on the surface of the Earth to within a few millimetres.
This is by far (a factor of a million) the most incisive comparison yet of proton and antiproton properties. According to the fundamental theorems of physics, a particle and an antiparticle should be exactly equal and opposite so that their scalar quantities, like mass, are the same, but quantum numbers, like electron charge, should have opposite signs.
The major objective of ATRAP and ATHENA at the new AD is to synthesize and study antihydrogen – the simplest electrically neutral atoms of antimatter, each made up of a positron orbiting a lone antiproton.
Antihydrogen was first produced by experiment PS210 at LEAR in 1995. Synthesizing atomic antimatter was a major achievement, but no measurements were made – the antihydrogen was too hot and dissociated quickly into its component positrons and antiprotons.
Using electromagnetic traps, ATRAP and ATHENA aim to collect supercold antihydrogen that can be stored for further study. Comparing the properties of this antihydrogen with hydrogen under the same conditions will provide a much more stringent test of whether matter and antimatter behave in exactly the same way.
ASACUSA uses antiprotons for collision and annihilation studies, particularly to form exotic atoms, in which the negatively charged antiproton is captured in a target atom, replacing the electron of everyday atoms.
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