LEP II physics

Abstract

The LEP II machine at CERN provides ee collisions above the WW production threshold and the four LEP experiments have collected data at collision energies growing from 161 GeV to 183 GeV over the last two years. Examples are presented of data analysis leading to new measurements of gauge boson masses and couplings and increased lower mass limits for new particles. 1 THE LEP II PROJECT The LEP tunnel was made 27 km long, the largest dimension allowed by the geology of Pay de Gex and the economy of the 1980’es. Even with this formidable circumference, it would still have been impossible to carry out the LEP II project, doubling the LEP I beam energy of 45.6 GeV, without using superconducting (Sc) accelerating cavities in the ring. The energy lost to synchrotron radiation grows with the fourth power of the beam energy. With conventional technology one would thus need many more warm Cu cavities than there is room for. Moreover, the power dissipated in the cavity walls would have been at least 50 MW [1]. Instead, new Sc cavities were installed where the walls have a thin layer of Nb sputtered into the Cu and a cryostat maintaining the boiling temperature of Helium. Hereby the performance is improved dramatically: the accelerating gradient is about 6 MV/m (instead of 1.4 MV/m) and the efficiency for converting RF power into beam energy is 75% (instead of 15% at LEP I) [1]. The limitations to further energy increases comes from space constraints, the high cost of the intricate production of Sc cavities and from limited cooling power. By exploiting the extra cooling power being installed for the LHC magnets, it may be possible to run with a maximum beam energy of 100 GeV in 1999 and 2000, if the economy allows it. The four LEP experiments: ALEPH [2], DELPHI [3], L3 [4] and OPAL [5] are well equipped for LEP II physics with four hermetic detectors that are well understood from the LEP I experience. The detectors have different strong points, but also many similarities. In particular, all experiments have a silicon tracking system close to the beam, which is crucial for detecting the short lifetime of b hadrons. The ee annihilation cross-section at LEP II energies is more than two orders of magnitude lower than at the Z peak, and the cross-section for the four-fermion final states of particular interest is yet another order of magnitude lower. For reasons of statistics alone, it is therefore important to have four experiments measuring the same quantities. In this lecture some examples of the measurements are discussed, mainly at the collision energies 161 and 172 GeV, where each experiment collected 10 pb 1 at each energy in 1996. The examples cover measurements of gauge boson masses and couplings and searches for Higgs bosons and the supersymmetric partners of Higgs and gauge bosons. 2 TWO-FERMION PROCESSES 2.1 Lineshape For each class of standard model processes, the experiments at LEP II have to their disposal several Monte Carlo codes (see Ref. [6], Vol.II) simulating the final state. These codes are interfaced to a full apparatus simulation and used to correct the measured data and to compare the results with the standard model expectations. Figure 1 shows the p s 0 spectrum in LEP II hadronic ee annihilation events 191 10 20 30 40 50 60 70 80 60 80 100 120 140 160 ECMeff [GeV] N um be r of E ve nt s