Investigative science must progress or die. For Experimental Particle Physics this is especially true. Governments will tolerate continuing to pay our bills only if they see continued vigour and advance. Decisions to build new accelerators must be driven by theoretical insights that point the way forward. The need for high energies to study neutrinos impelled the construction of the SPS. LEP was impelled by precisely testable predictions of electroweak theory, HERA by questions on the structure of matter and its interactions, and the Large Hadron Collider by theoretical insight (the Higgs mechanism) on the origin of mass and hints of higher symmetries.
The financial climate has become less congenial. In the 1970's the CERN SPS was built on a dramatically increased annual budget. In the 1980's LEP was built within a constant annual budget. In 1994 CERN Council insisted that the Large Hadron Collider be built within a shrinking budget, with special contributions from the two countries in which the LHC will sit (across the Franco-Swiss border) and with substantial contributions to help and accelerate the construction of the collider from non-member states who wish to share in its exploitation.
One must therefore be ruthless in moving on when a current venture has fulfilled its potential. LEP ran from 1989-1995 as a Z0 factory and made measurements of unparalleled impact over a range of topics. The precision that has been achieved is legendary and has transformed the style of investigation. The beauty of the measurements that established the sensitivity of the measurement of the Z0 mass to the tidal distortion of the earth's crust by the moon has subsequently revealed effects due to rainfall, to the height of the water in Lake Geneva, and to the daily passage of trains between Paris and Geneva! The precision of the detectors allowed the identification of processes such as that shown in figure 2, but did not provide sufficient statistical power or sufficient energy to pursue the now-visible goals in fermion and boson physics.
To pursue these goals further two different machines are needed. LEP is being upgraded in stages from 1995 to 1999 to study the W±-boson and to search for new phenomena that are hinted at in the Higgs and supersymmetry sectors. The LEP2 machine (as it is called) is a bargain. The LEP accelerator and all its infrastructure is reused with the addition of superlative new superconducting accelerating cavities. (The quality of these can be expressed numerically. An organ playing concert-pitch A and built to the same standard would reverberate for a year after the note was sounded - an acoustic that puts any cathedral in the shade.) The nineteen member states that together pay for CERN can take pride in the centre of excellence they have created that can make these.
The detectors already built for Z0 studies needed only modest upgrading to handle the higher energies and rates. But the upgrading is also of the highest quality, and its realisation is an example of the way international collaboration works. Consider the ALEPH vertex detector upgrade (an example with which I am familiar.) Compared to the vertex detector used for Z0 work, this was devised to extend the acceptance so as to make most efficient use of the accelerator, to reduce the material intercepting outgoing particles whilst providing additional measurements, and to withstand higher radiation doses. The upgraded system has Italian detector elements, French mechanics and British electronics. To minimise material whilst maximising thermal conduction to carry away heat beryllia substrates and diamond-loaded glue were used.
But the trail of quark-physics found by LEP whilst studying the Z0 cannot be pursued to the needed statistical precision at LEP. This needs a new accelerator optimised for high intensity rather than high energy. The energy (10 GeV mass rather than 90 GeV as at LEP) is optimised to produce B-mesons in a particular controlled way. The topic addressed is vital for our understanding of the Universe. It offers a clue as to how a Big Bang dominated by radiation (`let there be light') could have evolved into one of matter such as we live in now. And it demands a purpose-built accelerator.
A number of European groups are moving to new facilities, most to California to the Babar experiment being constructed at Stanford. Competition with another facility under construction in Japan will guarantee the vitality and quality in the work. Cooperation across the Atlantic is well-developed amongst scientists, though not, it appears, so easily addressed by certain governments whose modes of authorisation and funding are rather different. Cooperation between Europe and Japan has been highly successful (for example at PETRA and at LEP), but the scale has so far been more limited. It is good to see the level of cooperation now begun between Japan and CERN as an augur for the future.