Beam Dump: Where Nu Taus are Born
Before physicists can detect neutrinos, they first have to create them. In order to do so, DONUT physicists use an 800 GeV beam of protons from the TeVatron, which they collide with a block of tungsten. The beam used consists of 8 x 1012 protons per 20 second spill (amount of time beam is on) per minute for a total of 3.6 x 10 17 protons on target (pot). Due to its density and accessibility, tungsten is an excellent proton target, allowing for a high probability for proton interaction. In the dump the proton collides with a tungsten nucleus, producing a Ds, along with other charm particles such as D+. The primary source of tau neutrinos is the decay of Ds into a tau and tau neutrino; the rest, approximately 14% of the total, comes from secondary interactions of Ds in the dump, D+ decays and B meson decays. In addition to producing taus and tau neutrinos, the Ds also decays into other lepton pairs and other various particles. The neutrinos (both tau and others) produced from charm meson decays form a prompt neutrino beam consisting of tau neutrinos and an approximately equal number of muon and electron neutrinos. Additionally, the muon neutrinos produced in pion and kaon decays comprise a non-prompt beam that contributes about 23% to the total number of observed reactions. The mean energy of the not interacted neutrinos is 56 GeV for the taus and 54 GeV for the muon and electron neutrinos. Physicists calculate that there are 2.1 x 10-5 tau neutrinos per pot per m2 and 2.5 x 10-4 muon and electron neutrinos per pot per m2. There are several uncertainties in the calculations of the expected number of neutrinos. For example, there is a +/- 20% uncertainty in the Ds production and +/- 15% uncertainty due to the Ds , the Ds branching ratio constant (the branching ratio, BR, is the probability the particle will decay to a specific particle). DONUT is interested in only the prompt beam of neutrinos because the ratio between the tau, electron and muon neutrinos are well known.
Last updated: 6/29/01 comments