Neutrinos, on your marks...!

As the Bulletin was about to be printed, the CNGS team was ready to produce its first neutrinos. The gradual commissioning of the installation should result in the production of a nominal beam during the month of August.

The aim of the CNGS (CERN Neutrinos to Gran Sasso) Project is to penetrate some of the mysteries surrounding neutrinos. Neutrinos, which are very light, neutrally charged subatomic particles, interact very little with matter. The fact that they are extremely hard to intercept goes some way to explaining the mystery that surrounds them (see box). We know that there are three types - or flavours - of neutrino: the electron neutrino, the muon neutrino and the tau neutrino. But, among various mysteries to be unravelled, physicists want to find out why the flux of neutrinos from the sun is much smaller than theory predicts. The puzzle could be explained by the transformation of neutrinos from one flavour into another, a process which has been observed in recent experiments. This phenomenon, known as oscillation, is directly linked to another fundamental question, that of the neutrino mass. Oscillation has shown that neutrinos have a mass, but it has yet to be determined.

The CNGS project is expected to provide direct evidence of the neutrino oscillation thought to occur over long distances. At the end of their journey through the earth's crust, the neutrinos created at CERN will pass through the OPERA detector. The experiment should identify particles transformed from muon neutrinos into tau neutrinos during the journey, thus providing evidence of oscillation.

A complicated installation linked to CERN's accelerators has been set up 80 metres underground to produce a high intensity neutrino beam with a nominal flux of 10¹⁷ (100 million billion!) neutrinos a day. A proton beam accelerated to 400 GeV by the SPS is ejected into an 800-metre-long transfer tunnel pointing towards Gran Sasso, which is equipped with around a hundred magnets. The protons then strike a 2-metre-long target consisting of a series of small graphite rods. The interactions inside the target produce pions and kaons.

The pions and kaons then pass through a system comprising two magnetic horns, which act as lenses that guide and focus them into a parallel beam pointing towards Gran Sasso. The pions and kaons decay as they travel along the tunnel in a 1000-metre decay tube under vacuum and, in the majority of cases, produce pairs of muons and muon neutrinos. At the tunnel exit, a beam dump consisting of a 3-metre-thick graphite wall and a 15-metre steel wall absorbs all the unwanted particles, i.e. protons that have survived the target, or pions or kaons that have failed to decay. All that remains is a beam of muons, which is quickly absorbed by the rock, and muon neutrinos, which continue on their way to Gran Sasso. Behind the beam dump, two muon detectors measure the flux of muons that are produced. As the muons and muon neutrinos are produced in pairs, it is possible to work out the characteristics of the muon neutrino beam by measuring the trajectory of the muons. This information is important for checking the quality of the beam sent from CERN to the Gran Sasso Laboratory.

The CNGS team has been gradually commissioning the whole installation since the beginning of July, starting with the testing of some 200 items of equipment without the beam. Since 10 July, protons from the SPS have been progressively sent through all the components of the installation. 'We are using a beam that is 100 times less intense than the nominal beam for this first commissioning phase', explains Konrad Elsener, CNGS Project Leader. All the systems will undergo a detailed analysis at the end of July before we resume the commissioning in August with a nominal intensity beam.

All this represents the crowning touch to years of work by the CNGS team, from the initial design in the 1990s to the first steps in civil engineering in October 2000 and the subsequent construction and installation of components, in which some 150 members of the CERN personnel plus all the CERN departments and members of other laboratories have been involved.

Did you know?

Neutrinos are among the most pervasive particles in space but they are also among the most evasive. Four hundred thousand billion neutrinos from the sun pass through us every second but only one or two of them are diffused by our bodies throughout our entire lives! Of the 10¹⁷ (100 million billion) muon neutrinos sent on their way from CERN every day, 10¹¹ (100 billion) should pass through the OPERA detector at Gran Sasso. Of these 100 billion, only 25 muon neutrinos should be intercepted and detected each day when the experiment is complete. Tau neutrinos produced by oscillation will be even more of a rare phenomenon as only fifteen or so of them are expected to be detected over five years.