Poster excerpt 7: A Km3Net Prototype
(The following text is from R.Bormouth).
The first string prototype of the KM3NeT experiment equipped with only 3 detection units already allows for a muon reconstruction. As well as establishing the benefits of the multi-PMT layout for background suppression and in-situ calibration.
The KM3NeT experiment is a next generation neutrino telescope in the Mediterranean sea. The KM3NeT Phase 2.0 is planned with two building blocks: ORCA for low energy and ARCA for high energy neutrino detections. Both blocks will be build using 115 string-type detection units holding 18 optical modules. Each optical module (DOM) holds 31 photo multiplier tubes (PMT), evenly distributed in zenith and azimuth in the DOM.
A first string prototype (PPM-DU) with three instrumented DOMs as sketched in the illustration was deployed on the coast of Sicily in May 2014. The three DOMs of the prototype are equipped with PMTs from different producers. The top DOM holds Hamamatsu PMTs of the type R12199-2 while the other two hold ETEL PMTs of the type D783KFL.
Since deployment 680h of data have been recorded. Although the goal of KM3NeT will be the detection of neutrinos, due to the low statistics the PPM-DU focuses on the detection of the far more abundant muons produced in interaction in the earth atmosphere. In order to be able to track these muons, a nano-second precision time calibration of DOMs and PMTs is required.
The DOM time calibration is performed by using LEDs installed on the top of each DOM illuminating the DOM above it. By operating the LEDs in special data runs the relative signal travel time differences between the 3 DOMs can be measured.
The relative PMT time calibration is performed on each DOM by using the light originating from Potassium decays in the sea salt. The correlated signals from these decays on different PMTs allow for a global fit from which the relative time offsets and efficiencies can be determined.
In order to detect atmospheric muon signals we have to be able to distinguish them from the backgrounds. The two main backgrounds are Potassium decays and bioluminescence. Both can be distinguished from atmospheric muons by selecting time coincident light hits on a DOM. By selecting events with at least two hits within 25ns on each DOM a background free atmospheric muon sample is obtained.
A simple algorithm for muon tracking allows for the reconstruction of angular information. Only the information of every first hit on a DOM is taken into account for this fit. Since we are fitting 4 parameters this causes a under-determined system and therefore leads to degeneracies. By selecting a phase space with low degeneracies a good angular reconstruction is possible for 60% of the atmospheric muons. Illustration 2 shows the distribution of the cosine of the zenith for an atmospheric muon simulation and the taken data where excellent agreement can be seen.
The first string prototype delivered in depth results on the performances of a KM3NeT detection unit. The obtain results are encouraging and the deployment of the first full string in 2015 is highly anticipated.