CHORUS, a neutrino scale
The CHORUS experiment measures the lightest known unbound elementary particles: neutrinos. Just like electrons and their heavier counterparts, muons and tau particles, neutrinos belong to the leptons. Electrons are - since they are charged- sensitive to both the weak nuclear- and the electromagnetic force. Neutrinos on the other hand are not charged, and only experience the weak nuclear force. Because of this, they hardly interact with matter and cruise unhindered and in large amounts (more than 300 per cubic centimetre) through the universe.
There are three types of neutrinos: the electron-, the muon- and the tau neutrino. Neutrinos originate in nuclear reactions and in the core of stars such as our sun. Per second 65 billion solar neutrinos fly through every square centimetre of the earth; almost all pass unhindered through the globe. Since they hardly leave any tracks, it is not simple to do measurements on neutrinos. To detect them we need huge and heavy detectors.
In the CHORUS experiment we try to find out if neutrinos -in contradiction to what was assumed for a long time- have mass. The heavy CHORUS detector functions in this experiment as the most sensitive 'neutrino scale' on earth. The 'weighing' is done indirectly by studying high energy muon neutrinos produced in the SPS accelerator at CERN.
The neutrinos cover 800 metres before reaching the CHORUS detector (figure left). Here they first pass through a so called veto counter, which only reacts to charged particles and does not signal a passing neutrino. When this counter reacts, we ignore the interactions in the detector. Next the neutrinos reach a large amount (800 kg) of photo sensitive emulsion. Sporadically, a neutrino interacts with an atomic nucleus in this emulsion and produces a muon. Therefore a muon trail in the detector points at a neutrino interaction. We can detect muons in optic fibres which are fitted between emulsion sections. We select all potentially interesting events with scintillators which only react when charged particles such as muons pass. The energy of the charged particles is deducted from the degree in which they curve away in a magnetic field. Next, most charged particles are absorbed in the lead calorimeter (110 tons). Behind the calorimeter is the muon spectrometer which is made of 230 tons of magnetised iron and a number of wire chambers. Only muons are capable of passing through all this lead and iron and leave trails in the wire chambers. These trails are followed from the muon spectrometer to the emulsion by computers. A computer directed microscope searches for the rest of the trail in the emulsion.
In CHORUS we 'weigh' neutrinos by looking for neutrino oscillations: spontaneous transition from one type of neutrino to another. These oscillations only occur when neutrinos have a (small) mass and can be recognised by a bend in the photographic emulsion trail (figure 2). As a result of an oscillation, a muon neutrino could change into a tau neutrino during its flight to the detector. In a potential interaction with an atomic nucleus, a tau particle could originate instead of a muon. However, a tau particle decays after only one tenth to several millimetres into -amongst others- a high energy muon which causes a bend in the trail. To perceive this bend, the trail needs to be measured and reconstructed with extreme precision.
The occurrence of neutrino oscillations would yield a solution for a number of inexplicable observations. For instance, the number of electron neutrinos which reach us from the sun is smaller than expected. Also, the measured ratio of neutrino types which originate from cosmic radiation in the atmosphere is different from the theoretical predictions. Both results can be explained with neutrino oscillations. A small neutrino mass could also give an explanation for the missing dark matter in the universe.
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