WANF

From SydneyHEPWiki

The West Area Neutrino Facility at CERN

WANF: The West Area Neutrino Facility (WANF) at CERN was the facility that produced the neutrino beam that was used for the two neutrino oscillation experiments, NOMAD and CHORUS. This facility was significantly revamped around 1993 for the neutrino oscillation experiments which ran from 1994 to 1998. A schematic of a neutrino beam line is shown below.

Neutrinos were produced at CERN by colliding a beam of high energy protons (450 GeV) accelerated at the Super Proton Synchroton (SPS) onto a beryllium target consisting of 11 rods of beryllium, each of them 10 cm long. The high energy interactions produce secondary particles, predominantly pions and kaons, that were allowed to decay in a long (290 m) decay tunnel under vacuum. The secondary pions and kaons were focused by two magnetic elements called the horn and the reflector that selected predominantly positive particles (that decay to neutrinos rather than antineutrinos). Since other charged particles, like muons, are also produced from the decays, these were absorbed in about 370 metres of iron and earth shielding. In addition, a toroidal magnet was also introduced inside the shielding to deflect the muons away from the neutrino beam. At a distance of 835 metres from the original target was the position of the NOMAD detector that received only neutrinos and a small number of the most energetic muons that were able to cross the shield.

There were two proton extractions, each separated by 2 seconds and lasting about 6 milliseconds, that bombarded the beryllium target during every 14.4 second SPS cycle. Each of the extractions typically dumped about (1 - 1.5)X10E13 protons on the target, so the neutrino flux obtained was the largest to that time.

Energy Spectra: Extensive simulations of the beam line were performed to estimate the number (flux) of neutrinos that arrived and the shape of their energy spectrum. Results of these simulations are shown in the image below. It can be seen that the majority were muon neutrinos, but there remained backgrounds of approximately 7% of muon antineutrinos, around 1% of electrons neutrino and 0.3% of electron antineutrinos. There was a 20% uncertainty in these flux estimates but results from the SPY experiment were be able to significantly improve these estimates. It is also worth noting that the estimated number of tau neutrinos in the beam was negligible for the expected sensitivity of NOMAD. The intrinsic tau neutrino component of the beam was due to production and decay of strange-charm mesons (Ds) and wass estimated to be present in the beam at a level of 5X10E-6.