When you eliminate the illogical, what ever remains, however improbable, must be the truth (or ask folk around you for ideas).
The mass of the Z boson is taken as an input parameter in studies of electroweak particle physics. Small changes in this particle's estimated mass have large effects on the standard physics model of how our universe works.
In the 1990s, the CERN (Centre for European Research Nuclear) Lab's LEP circular particle collider was being used to generate lots of Z bosons, in order to improve the accuracy of the Z's mass. The estimated error on the value of the Z's mass is directly related to the precision with which the beam energy in the LEP was measured, so for the experimental physicists involved, accurate measurement of the LEP particle collider beam energy was very important and exciting work.
There are lots of external factors that can effect electron/positron beam energy drift. This includes any change in the 26.67km circumference of the underground circular ring - leading to an offset in position of the revolving beam. It is the strength of the applied magnetic bending field that keeps the beam within the confines of the ring's vacuum chamber.
Previous measurements had taken into account the phases of the moon and even the weight of neighboring Lake Leman on the LEP ring. Tidal effects due to the combined gravitational attraction of the Sun and the Moon cause deformations of the local Earth radius, therefore having an influence on the accelerator circumference.
In the local Geneva area the vertical component of the 'tidal' motion could distort the circumference of LEP by about ±0.5 mm. LEP was also subject to much slower long-term changes in circumference, over 6 months the ring experienced changes in circumference of up to 2 mm, some of which were clearly correlated to rainfall and to fluctuations in the height of the underground water table and lake.
Corrections for all these slow-moving physical changes where added using the LEP control and measurement systems, but there were still short-term protuberances of beam energy measured over a period of about fifteen hours, the results showing calm and noisy periods, appearing roughly on a daily basis but with no repeatable pattern.
The fact that the calm period fell into the time between midnight and early morning did however make a correlation with human activities very likely. Magnet power supplies and field strengths were monitored, instruments re-calibrated and a list of possible external effects compiled. Many items of on-site electrical equipment were subject to trial for effects on the beam energy, but engineers could not find any correlation with the noisy periods.
Meanwhile, a bright wag decided that it would be fun to punt the problem to all CERN employees for solutions, so an announcement was made in the in-house newsletter for people to contribute ideas about the source of the disturbance. The game was afoot!
A CERN employee, who was also a user of the Swiss railway system, had a brainwave and ran off to grab his copy of the local train timetable.
Following further investigation and measurements, the noise was proven to be caused by so-called 'vagabond currents' originating from neighboring electric railways. These zombie currents flowing through the beam pipe were perturbing the magnetization of the iron by producing both short-term fluctuations on the scale of a few seconds, as well as long-term (lasting hours) field increases.
The currents were arising from the trains passing along the Geneva - Bellegarde track. A fraction of the direct current powering the trains leaks to earth when returning to the power generator station, some of this leakage current passed through the earth and the LEP beam pipe - buried in a tunnel approx 150m underground from the surface. The phantom current was just doing what the laws of physics tell it to do: traveling through the path of least resistance, where the nice LEP metalwork sometimes performed better than the local ground connectivity. The quiet period was a consequence of there being no trains running in the area at that time.
The final measurements of the Z mass made using LEP included offsets to compensate for the external effects, including corrections to incorporate the timetable of the local railways (the Swiss trains always run on time!).
LEP was closed in 2000 to make way for the LHC, a much higher energy machine which is now being commissioned with the aim of making new physics discoveries. I expect that those zombie vagabond currents are still roaming through the machine.
By the way, Z does not stand for Zombie.
About Author Grover Mercer: "My background is 30 years of electronics and system engineering, including the manufacturing industry (wound component factory), physics research lab work and defence engineering in naval and land combat systems. Through industry trend I have been sucked into software development, but I am at my happiest building little boxes crammed with electronic hardware that makes lights flash and things move."