LUNAR TIDAL EFFECTS 

are the main cause of error in determining the mass of the Z boson.
Scientists at the CERN laboratory in Geneva, Switzerland, with help from 
workers at SLAC and the University of Lausanne, have found that the 
Moon's gravitational pull warps the LEP (Large Electron Positron) collider 
by as much as a millimeter (out of a total circumference of 27 km).
This blurs the Z mass estimates by about 10 MeV. Hereafter, calibrations of
the beam energy will take into account the phase of the Moon. 
                                         (CERN press release, 23 Nov. 1992.) 


To improve the precision measurements of the Z boson mass and its width,
the 1995 LEP run was devoted to a three point energy scan.  After the
"discoveries" of the effects of the tidal motion from the moon and the
height of the water level of Lake Geneva on the beam energy, the LEP Energy
Working Group found in 1995 that the LEP beam energy is also sensitive to high
speed trains passing nearby, when some return current finds its way into the 
LEP ring. Various new features in the Energy Calibration analysis in '95 will
be mentioned.


Amongst the most important results from LEP this year is the new combined 
measurement of the mass of the Z, using the data from all four LEP 
experiments. The results of the individual experiments together with the 
combined result are shown in Table 1. On the combined result, the second error
quoted (± 6 MeV) comes from uncertainty in the energy of LEP. An interesting 
side-note, illustrative of the many minute effects which experiments have to 
take into account, is the determination of the energy of the beams by the
machine physicists in CERN. The best measurement of the LEP energy relies on 
the fact that after the beam has been stored for some time it becomes 
transversely polarised. The energy can then be determined using a technique 
based on resonant depolarisation, when a suitable frequency horizontal
magnetic field is applied. The accuracy of this energy determination is such 
that the variations due to tidal forces from the Moon and the Sun are readily
observable (see Fig. 3). This variation corresponds to changes in the 
circumference of the machine by as little as 1mm in 27km. In fact, more 
sophisticated tidal models which include the effects of Jupiter and the other
planets are now needed.