Setup of a Drift Tube Muon Tracker and Calibration of Muon ...

Chapter 5CMT for BorexinoAs described in Chapter 4.3, uncorrelated cosmogenic induced background dominatesthe energy range between 1 and 2 MeV in Borexino. This is exactly the energyrange of CNO and pep neutrinos. In order to detect these neutrinos it is thus necessaryto understand well the effects of muons crossing the detector and to find aneffective method to suppress the background.Identifying cosmogenic background events is not trivial since the actual backgroundsignal is not correlated to the crossing muon. Thus a simple muon veto isinsufficient. Instead, a three-fold coincidence of a crossing muon, a neutron captureand the delayed radioactive decay is used to reduce the cosmogenic background.The produced cosmogenic radionuclide is expected to be within the intersection ofa cylinder around the initiating muon track and a sphere around the captured neutron.The affected volume can then be taken from the fiducial volume for a timeperiod of a few half-lives [72]. However, with more than 4000 muons crossing thedetector each day resulting in approximately 25 11 C decays, this could quickly leadto a large part of the fiducial volume blinded. Therefore the coordinates of the muontrack need to be known with a high precision to apply this cut. In Borexino, tracksare reconstructed from the time and pulse shape information from the PMTs whena muon crosses the detector (cf. Section 4.3.4). This method allows to reconstructtracks with a resolution of approximately 50 cm.An exact determination of the resolution of the Borexino muon tracking is hencea key prerequisite for keeping the vetoed volume small. The angular resolution hasbeen estimated before by a comparison of the angular muon distribution to the datacollected at LNGS by the MACRO detector [70]. However, for an exact determinationof the resolution on an event by event basis, the true coordinates of the passingmuon have to be known. The CMT presented in Chapter 3 is capable of reconstructionmuon tracks with a precision close to their true coordinates. Operating theCMT together next to the Borexino experiment allows to collect a sample of cosmicmuons that pass both detectors. This sample can thus be used as a reference fordetermining the resolution of the Borexino tracking.This chapter describes the setup of the CMT at the LNGS laboratory. It is nowused to sample those cosmic muons crossing both the CMT and Borexino. Thus,some muon tracks will be known with high precision and can then be compared tothe Borexino reconstruction allowing to test the quality and the efficiency of theBorexino track reconstruction. Possible systematic errors can be identified and thus69