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The electron neutrinos produced by the neutrino source ISIS ineract with the carbon nuclei of the scintillator in the KARMEN detector by interchange of a charged W-boson (thats why it is called charged current). Thereby the carbon nucleus is transformed to a nitrogen nucleus emitting a positron. The positron can be detected in the KARMEN detector as prompt event. The produced nitrogen nucleus with mass number 12 (the same as carbon) is radioactive and decays with a decay time of 15.9 ms back to carbon. The nitrogen nucleus does not move within this short time, so that the electron emitted during the radioactive decay of the nitrogen, can be detected practically at the same positon in the detector as the positron. Thus the neutrino reaction is identified by the detection of a pair of events, strongly correlated in time and space.
During the measuring time from July 1990 until December 1995, the ISIS target was bombarded with protons equivalent to a total charge of 9122 Coulomb. During this bombardment so many neutrinos were produced, that 60,000 Billion neutrinos crossed every square centimetre of the detector cross section. Despite of this, the probability that an interaction of the neutrinos with matter occurs, is so low, that during these five years only about 500 events with the given signature could be clearly identified. The measurement is practically free of background (signal to background ratio is 35:1), i.e. the number of events in the data, which are not neutrino induced, is very low. The following picture shows the time and energy distributions of the prompt and sequential events. The numbers of measured events is plotted in red, a Monte Carlo simulation, representing the expectation for a very long measuring time, is shown in blue. The time distribution of the prompt events (relative to proton beam on ISIS target) shows clearly the decrease as it is expected from the 2.2 microsecond decay time of the muon in the ISIS target, producing the electron neutrinos. The difference time between prompt and sequential event reveals the decay time of the radioactive nitrogen.
The background is (for a neutrino experiment) extremely low, so that one can perform a precision test of the standard model of the weak interaction. If one reconstructs the energy distribution of the electron neutrinos from the energy distribution of the detected electrons, one can compare this distribution with the theoretical expectation. Deviations from the standard model would show up as an increase of the event rate at the high energy tail of the neutrino spectrum.
If one does not look for the detection of a seqential event, one can analyse several other neutrino reactions beside the charged current reaction introduced above. Here the detection signature is given just by the detection of a single, isolated prompt event, which is not followed by a sequential event. From now on such a signature is called a single prong reaction. The picture below shows an analysis of the energy spectrum of the events measured in the years 1990-95 with KARMEN. The complex spectrum consists of several different neutrino reactions wich will be explained below in more detail.
By the analysis of the single prong energy spectrum of the roughly 2000 neutrino events detected in the years 1990-95 a large number of topics in neutrino-nucleus physics could be addressed. The results can be found in our Publications, especially in a recent overview article .
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