Design studies for the Double Chooz trigger

The main characteristic of the neutrino mixing effect is assumed to be the coupling between the flavor and the mass eigenstates. Three mixing angles (θ12, θ23, θ13) are describing the magnitude of this effect. Still unknown, θ13 is considered very small, based on the measurement done by the CHOOZ experiment. A leading experiment will be Double Chooz, placed in the Ardennes region, on the same site as used by CHOOZ. The Double Chooz goal is the exploration of ∝80% from the currently allowed θ13 region, by searching the disappearance of reactor antineutrinos. Double Chooz will use two similar detectors, located at different distances from the reactor cores: a near one at ∝150 m where no oscillations are expected and a far one at 1.05 km distance, close to the first minimum of the survival probability function. The measurement foresees a precise comparison of neutrino rates and spectra between both detectors. The detection mechanism is based on the inverse β-decay. The Double Chooz detectors have been designed to minimize the rate of random background. In a simplified view, two optically separated regions are considered. The target, filled with Gd-doped liquid scintillator, is the main antineutrino interaction volume. Surrounding the target, the inner veto region aims to tag the cosmogenic muon background which hits the detector. Both regions are viewed by photomultipliers. The Double Chooz trigger system has to be highly efficient for antineutrino events as well as for several types of background. The trigger analyzes discriminated signals from the central region and the inner veto photomultipliers. The trigger logic is fully programmable and can combine the input signals. The trigger conditions are based on the total energy released in event and on the PMT groups multiplicity. For redundancy, two independent trigger boards will be used for the central region, each of them receiving signals from half of the photomultipliers. A third trigger board will handle the inner veto signals and the additional trigger inputs. The work presented in this thesis establishes the trigger algorithm as result of the trigger efficiency optimization. The efficiency parameters are obtained from fits of Monte Carlo simulation data. Various possible influences are considered, the resulted algorithm being able to sustain the trigger goals for all kinds of events. Also presented is a method for measuring the trigger efficiency based on the redundancy of the two target trigger boards. Cosmogenic muons are the dominant source of the Double Chooz triggered events. For the near detector, the foreseen muon rate is ∝250 Hz. The DAQ system is unable to sustain the full read-out of the detector at such high frequency. As consequence, the triggered events are treated differently, regarding their importance for future analysis. For physics events, the full available information is saved, the offline data for background muons will contain only summary information. The trigger algorithm is able to identify ''special'' muons classes, for which the full detector read-out is performed. The muon recognition is based on the energy depositions from all detector regions and on the ''topological'' information provided by groups of inner veto photomultipliers. (orig.)