Thirty years of LEP’s Z0 line shape
Thirty years ago this week, the four experiments at CERN’s LEP collider published the first of their famous results: the Z0 line shape, which told us that there are three, and only three, families of fundamental particles in nature. For most of us, it’s hard to imagine a time before the Standard Model – not surprisingly, since the theory that underpins particle physics is already into its 50s.
Yet for those of us old enough to remember, the 1960s was a time of painstaking theoretical work, culminating in the emergence of the Standard Model in the 70s, and paving the way for the ambitious project to build a 27-kilometre-circumference electron–positron collider at CERN. The Large Electron Positron collider (LEP) duly switched on in 1989, and it was not long before it established itself as the machine that would put the theory developed through the 60s and 70s to the test, making the 1990s the decade that put the Standard Model on unassailably solid experimental foundations.
Despite its great size and complexity, the start-up of LEP went with metronomic precision. First beams circulated on 14 July, the first collisions were recorded on 13 August and, on 13 October, 30 years ago this week, the first of LEP’s big results was published: unassumingly known as the Z0 line shape, it changed our view of the universe.
When LEP switched on, the number of families of particles was unknown. Theory told us that there had to be at least three, but set no upper bound. We already knew about three families, and only two members of those families remained to be discovered: the top quark and the tau neutrino, which made their first appearances at Fermilab in 1995 and 2000 respectively. But would there be more? The LEP experiments had the means to find out by measuring the production and decay of Z particles, the neutral carriers of the weak interaction.
Z particles decay into pairs of quarks or leptons, all of which can be measured by the detectors, with the exception of the very light neutrinos, which escape unseen. Since each family of particles counts a neutrino as a member, the prediction for what LEP would see was different for two, three, four or more families. The Z0 line shape is the peak in the distribution of particles produced in collisions as the collision energy scans through the Z production energy. Rather than being a sharp peak, it is a distribution around the mass of the Z0 particle, influenced by the experimental resolution and, more importantly, the lifetime of the Z0: the more ways the Z0 can decay, the shorter its lifetime and the larger the width of the peak. This is a direct manifestation of the famous Heisenberg uncertainty principle.
By October 1989, it was clear that the LEP data corresponded with the prediction for three families of particles. To be precise, the LEP experiments gave the number of light neutrinos to be 2.9840, plus or minus 0.0082. Such precision for the number three may seem pedantic, but it still has important consequences today. At the time LEP began, transitions between quarks, known as mixing, were well established, but neutrino mixing was not. Now we have a much better understanding of neutrino mixing, but the LEP measurement remains a very important constraint on the precise form that mixing takes.
As with many discoveries, a question answered is a new question opened. We now know that there are three families, the minimum required by theory and no more, but we do not yet know why. This was an important milestone in physics, and part of a long tradition in neutrino research, which really took off after this measurement. Today, 30 years on, with the LHC in the former LEP tunnel, and as we prepare for a new and exciting era in neutrino physics with our partners in the US and Japan, it’s timely to reflect on the significance of those early days at LEP.