The biannual International Conference on High Energy Physics was this year located in Osaka, third biggest city in Japan with about 2.5 million inhabitants. It is a centre of business and industry and does not have the old flavour of nearby Kyoto or Nara. The site of the Conference was in the International House, a modern and well equipped conference centre.
This year some extra attention was given the three new colliders, the b-factories in SLAC and KEK, with its experiments BaBar and Belle, and the Relativistic Heavy Ion Collider in Brookhaven were Au-Au collisions are studied in four different experiments. The main purpose of BaBar and Belle is to determine if there is CP violation in the decays of the neutral B-mesons (the favoured channel is the decay into J/Psi and Koshort ). The two B-factories have started up very nicely and give almost design luminosity after less than a year of running. One of the main goals are to look for CP violation in the system of B-meson. The first results show no indication of CP violation which is a surprise. If the preliminary result presists there is need for new interpretations of this phenomena. But it takes several years of data taking before a significant answer to the CP violation question can be given. The RICH experiments only had a short period with beams so far so no results were available yet on the quark matter formation except for the multiplicity (which was on the low side of predictions). Previous CERN experiments have given several indications on this quark-gluon plasma formation from Pb-Pb collisions, e.g. in the suppression of charmed hadrons and a not yet understood enhancement of strange hadron production.
Many new results from the experiments at CERN-LEP and FNAL-Tevatron were presented, while DESY-HERA this time was less well represented. The Standard Model (SM) stands up to all the refined analysis from the LEP experiments. The latest errors on the masses of Z and W are 2.1 MeV and 62 MeV; the W mass will still be improved somewhat. The missing link of the SM is the Higgs particle where fits to indirect measurements point strongly at the existence of such a particle, at a preferred mass which is almost excluded by the lower limit of the direct search, where the LEP limit is now 113 GeV. LEP is supposed to end by October 1 this year if no Higgs discovery is made before that in the narrow range 113-115 GeV (a search for higher masses will have to await the Large Hadron Collider experiments at CERN starting in 2006).
The running QED coupling constant is measured at LEP with rather big errors – a significant improvement to the overall fit from low energy up to LEP energies can now be made since BESII at Beijing presented precise results on the ratio R in the energy range 2-5 GeV, with a precision of 7% compared to 15% previously. The improved value of alpha(QED) also has implications for the prediction of the mass of the Higgs to a somewhat higher value, more in line with the non-observation so far. The 95% upper bound of the Higgs mass, quoted to be 170 GeV from the LEP results, is pushed up to 210 GeV using these new data.
The limitation of the SM at very high energy requires Beyond the SM theories. The most popular one, minimal Super symmetry, was proposed about twenty years ago. The SUSY model assumes a universal scale at very high energy, about 1016 GeV, where all the running coupling constants of the electromagnetic, weak and strong interactions assume the same values. The model predicts a lightest stable particle (LSP), but no super symmetrical particles have been seen yet within the range available to LEP; the lower limit is 38 GeV at the moment. Vincent Hedberg from Lund gave the best LEP limits for the neutralino alternative for the LSP. The model has a more complicated Higgs sector with several observable Higgs particles. So far LEP has seen no super symmetric Higgs which is supposed to be even lighter than the SM Higgs. During the last two years a competing theory of extra dimensions has become popular where there is no very high energy scale but rather a few TeV energy scale at which all the coupling constants extrapolate, not linearly but exponentially. Lawrence Hall, LBL, gave a very good talk on this and claimed this to be the most important theoretical development since the SUSY days.
Much attention was given to the one of the favourite topics during the last ten years, i.e. neutrinos. Super Kamiokande, which is a giant underground water Cerenkov detector, has improved its solar and atmospheric neutrino detection signals. In particular the ratio of electron to muon neutrinos from cosmic ray interaction in the upper atmosphere has given clear evidence of finite masses for the neutrino mass. Particularly impressive are the zenith angle variation of the muon neutrino flux which gives many standard deviations for the disappearance of upward going muon neutrinos, (Nup-Ndown)/(Nup+Ndown) = -0.296+-0.032+-0.01. The Super Kamiokande findings are supported by recent results reported at the conference from the Soudan2 and MACRO underground experiments. Another strong indication of the finite neutrino mass are the solar neutrino data, but we still miss the information on the energy of the missing solar neutrinos (from the SNO and Borexino experiments). The SM can very well accommodate neutrino masses although they were assumed to be zero up till now. The three family structure of the leptons can explain a muon to tau neutrino oscillation seen in the atmospheric data while the solar data would involve electron neutrino oscillation to muon (or tau) neutrino. However the Los Alamos neutrino disappearance experiment does not fit this picture but those data need confirmation.
Particle physics and cosmology go hand in hand. The understanding of the cosmic structure formation is improving with new input from Boomerang and MAXIMA. The Universe is very close to flat and the cosmological parameters are better determined. The best value of the Hubble constant is now 62-83 and the cosmological constant is definitely not zero or one, the value given is 0.25-0.48. For a complete understanding one needs input from particle physics on vacuum energy, cold dark matter and scalar fields.
For the future one is looking forward to the upgrade of the Tevatron at FNAL and to the LHC at CERN. However this does not exclude active studies of other accelerator schemes, in particular the Linear electron colliders. The first step in this direction will be the TESLA proposal by DESY which is ready next year. It aims at an energy of 500 GeV which would be suitable for Higgs physics. The advice given at the conference was to build only one such machine, as quickly as possible, but not to make it extendable in energy which would increase the cost. The final speaker, Prof. Hirotaka Sugawara of KEK, warned that constructive international collaboration was required to avoid the fate of the physicstaurus (SSC) and being completely overrun by the biologistals.
Lund, August 14, 2000
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