Research in particles is carried out with broad international participation. Experimental facilities are located at large centers. The membership of C11 is distributed geographically so as to represent broadly the regions involved in the field.
The composition of C11, approved at the 1999 General assembly is as follows: China 1; Japan 1; Russia and Eastern Europe 2; Western Europe 4; USA 2; rest of the world (‘fourth region’) 2. In addition there is a Vice Chair from industry. The Western European members are rotated as follows: three of the four are rotated from the large CERN member states – France, Germany and the UK, and the fourth is chosen from the other countries.
There is overlap between C11 and some other commissions, in particular Cosmic Ray Physics (C4), Nuclear Physics (C12) and Astrophysics (C19). C11 has an Associate Member from each of these commissions. A subcommittee, Particle and Nuclear Astrophysics and Gravitational Interactions Committee (PANAGIC), covers the emerging fields of non-accelerator or particle astrophysics and provides an important link between these commissions.
The general aims of the Commission are:
C11 has an annual meeting which is always held at the large IUPAP sponsored conferences, ICHEP in even years and LP in odd years. The rest of business is conducted electronically. The four conferences in 1999, 2000, 2001 and 2002 were well attended, had good and balanced international representation in both speakers and participants. These meetings provide the intellectual focus for particle physics and are generally regarded as the premier meetings in the field
IUPAP SPONSORED CONFERENCES 1999-2002
The commission normally only supports one conference per year. The following were held since the last IUPAP General Assembly
1999 International Symposium on Lepton-Photon Physics Stanford, California, USA 9 – 14 August 1999
2000 International Conference on High Energy Physics Osaka, Japan 27 July – 2 August 2000
2001 International Symposium on Lepton-Photon Physics Rome, Italy 23 -27 July 2001
2002 International Conference on High Energy Physics Amsterdam, Netherlands 24 – 31 July 2002
In addition the following conference was approved by C11, but no IUPAP resources were requested:-
2001 International Conference on High Energy Accelerators Tsukuba, Japan 26-30 March 2001.
Conferences sponsored by C11 have met the IUPAP regulations concerning international participation, free circulation of scientists, visas etc.
The Commission has sponsored the International Committee for Future Accelerators (ICFA) since 1976. The membership of this is drawn from all regions having or using accelerators. The guidelines for this Committee approved by the C11 Commission (1985) are as follows:
ICFA has 16 members, selected primarily from the regions most deeply involved in high energy physics. The chairman of C11 serves as an ex-officio member of ICFA.
ICFA meets twice per year, once during the main particle physics conference, and once at an enlarged meeting which includes directors of all high energy accelerator laboratories. Once every three years there is an ‘ICFA Seminar’ lasting three or four days, attended by about 150 people, which can take a broad look at the field and its future. There are also several active subpanels. ICFA has an impressive record of leadership in the development of future accelerators. From time to time ICFA issues statements. In August 1999 it strongly supported the concept of a large electron-positron linear collider to be built by international cooperation.
NEW DEVELOPMENTS IN THE FIELD
The building blocks of nature.
The realization that the great diversity of the world stems from a handful of elementary particles acting under the influence of a few fundamental forces is one of the triumphs of twentieth century physics.
According to our present understanding there are two classes of elementary building blocks, called quarks and leptons. Quarks feel the strong interaction, leptons do not. In our normal surroundings where energies per particle are low, we have only two of each. The up and down quarks have electric charges of +2/3 and -1/3. The electron has charge -1 and the electron-type neutrino has zero charge. At higher energies, this simple pattern of two leptons and two quarks with the above charges is repeated, but only twice, leading to three generation of quarks and leptons. Also every quark and lepton has an antiparticle, so we are left with six each of quarks, antiquarks, leptons and antileptons. Quarks and leptons are spin 1/2 fermions. Quarks have an additional property called colour which exists in three varieties.
The forces through which the particles interact are transmitted by the exchange of another type of object. The force-carriers are bosons. The carrier of electromagnetism is the spin-1 photon. In electroweak theory this is joined by three massive bosons, the W+, W- and Zo which mediate the weak force. The Electroweak theory has been tested to high precision, particularly at the CERN LEP machine. This machine finished its programme at the end of 2000 and has been dismantled to allow the construction of the Large hadron Collider (LHC) in the existing tunnel. The strong force is mediated by gluons. The theory of strong interactions, known as quantum chromodynamics (QCD), is well developed and consistent with experiments, although the tests are less precise.
THE STANDARD MODEL AND ITS SHORTCOMINGS
Electroweak theory and QCD have been incorporated into what is known as the standard model of particle physics. Although this model works very well it suffers from a number of defects. There are rather a lot of arbitrary numbers which are not intrinsic to the theory but have to be obtained from experiment. The theory predicts nonsensical results at energies slightly higher than now available – equivalent to processes having a probability greater than unity! In addition, the theory requires that the W and Z particles, like the photon, should be massless. A mechanism which gives mass to the particles by allowing them to interact with a field was first suggested by Peter Higgs. This would have a carrier object – the Higgs boson. Just before the closure of LEP a handful of collisions gave results which were ‘candidates’ for Higgs bosons, but more detailed analyses have made this interpretation less likely. There is a possibility that the Higgs will be found at the Fermilab Tevatron which has had a significant upgrade. If not then the LHC, which is now expected to start in 2007, will either produce Higgs, or some new physics to overcome the above anomaly.
Symmetries play a significant role in particle physics. Up till 50 years ago it was thought that the symmetries: parity P, charge conjugation C, and time reversal T, were each conserved in all interactions. However in the 1950s it was shown that P and C were violated maximally in the weak interaction, and in the 1960s the combined operation CP was found to be violated, but only by a small amount, and only in the decays of neutral kaons. The origin of CP violation is not properly understood, but it is one of the ingredients required in the very early universe to produce the present tremendous preponderance of matter over antimatter.
One of the outstanding discoveries during the past three years has been the observation of CP violation in another system, that of the neutral B mesons. This has been seen at the ‘B factories’ at KEK in Japan, and at SLAC in the USA. These machines are asymmetric electron-positron colliders, tuned to produce Bo pairs copiously and give them a relativistic boost so that their short decay paths can be detected. Both machines started up rapidly, have exceeded their design luminosity, and are giving interesting results on various B decays. These experiments have allowed further tests of the Cabibbo-Kobayashi-Maskawa (CKM) theory which relates the interactions between the quark generations.
This has been one of the most exciting areas of research in the past three years. The most important results have come from the SuperKamiokande project in Japan and the Sudbury Neutrino Observatory (SNO) in Canada, both involving international collaborations, but several other experiments have contributed. There is now compelling evidence that neutrinos from the three generations undergo quantum mechanical mixing. From this it follows that neutrinos are not massless, and the mass eigenstates are mixtures of the weak interaction states. The solar neutrino problem, first detected 30 years ago, has now been pinned down. The total number of solar neutrinos detected is as expected from the best model of fusion reaction in the solar core, but only about one third of the neutrinos survive as the electron-type produced by solar fusion. Neutrino mixing has also been observed with atmospheric neutrinos, which are the decay products of cosmic ray interactions. Downward going neutrinos, which have traveled only a few kilometers have been contrasted with upward neutrinos which have traversed large portions of the Earth before interacting in the underground detector. The experiments give preferred values for the square of the mass difference between neutrino types. Additional neutrino experiments are just starting or being prepared, using a variety of techniques. These include using large volumes of seawater or Antarctic ice as detectors for atmospheric or astrophysical neutrinos, and long baseline neutrino beams from accelerator and reactor sources to detectors at various distances.
As mentioned, the LHC will start in 2007. Although a CERN facility, it will in many ways be a world machine, as there is significant input both to the machine and to experiments from non-member states. There is now general agreement that the next machine should be a linear electron-positron collider with ~ 0.5 TeV in the center of mass. This should be built as a true world machine, perhaps using the global accelerator network concept where different regions or laboratories take responsibility for the construction, operation and maintenance of the various components. There is also R & D on other possible future machines: neutrino factories using decay neutrinos from muon storage rings, muon colliders and very large hadron colliders, but plans for these are less well developed.
The compatibility of the standard model with all experiments is both a triumph and a frustration. Since it has to break down at energies which will be achieved in the next decade, there was hope that some discrepancies will have been found which would indicate directions for future theories. Theoretical work continues with various unifying schemes. Grand unified theories unite the strong with the electroweak interactions. Supersymmetry unites the building blocks, the quarks and the leptons, with the force carriers. This requires new partner particles for all these objects, none of which have so far been discovered. Superstring theories, and their recent extension, M-theories, which require supersymmetry, are exciting and fashionable. They treat particles as excitations of tiny strings. This avoids objectionable infinities which arise when particles are treated as point objects. Superstring theories do however require more than the usual three space and one time dimension. The unobserved dimensions are assumed to be compactified – curled up so that they are too small to be observable. Superstring theories have the potential to provide a quantum theory of gravity and to unite it with the other forces, and there is much activity in this field. Some recent work has suggested that gravity which at the particle level appears vastly weaker than the other forces, might actually be comparable in strength, but leaks out into some higher dimensions. Consequently the inverse square law may not be correct at very short distances. Ingenious experiments to test this at submillimetre distances are in progress.