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

(typically below 50 cm), the location of the cosmogenic induced carbon nuclei canbe found in the intersection of a sphere around the location of the neutron captureand a cylinder around the muon track. Any signal occurring within this regionduring a few half-lives of the 11 C can now be vetoed thus reducing the cosmogenicbackground. However, it is desirable to keep the excluded region of fiducial volumesmall, in order to not reduce the overall statistics too much. Therefore, a goodmuon tracking is needed. Fig. 4.4 shows the schematic principle of the three foldcoincidence. For a detailed description see [72].4.3.4 Muon Identification and TrackingMuons crossing Borexino are identified using several techniques. Light from the outerdetector Čerenkov veto is a strong indicator for muons. Also the inner detector canbe used by performing a pulse shape and timing analysis of the PMT signals: muonpulses feature longer rise and decay times than neutrino events of comparable energy.Thus, the outer and inner detector provide two independent and yet redundant tagsfor cosmic muons.For the reconstruction of muon tracks, two different algorithms are used. One isbased on the data from the outer detector and the other uses the information fromthe inner detector. A combination of the OD and ID fits is then further used for athird, global tracking. A brief overview of the tracking algorithms is presented here.The characteristic light patterns created by muons crossing Borexino are usedto reconstruct the muon tracks. For the OD tracking, the disk-like shapes from theČerenkov cone of a crossing muon are identified by the PMTs close to the muon’sentry point to the detector and after a time delay also close to the exit point.PMTs close to the entry point show significantly more photons registered than thosefurther away. Also, the Čerenkov light reaches these PMTs earlier. Depending onthe inclination of the track, the time profile of the hits corresponds to a circle or anellipse around the entry point. Once the two clusters of PMTs indicating a crossingmuon have been identified, the entry and exit points are determined by the chargebarycenters of these clusters. A detailed description of the tracking algorithm ispresented in [71].The inner detector tracking relies mainly on the distinct patterns of the photonarrival times at the PMTs. Due to the high light output from crossing muons, thecharge distribution of the PMTs cannot be used for the track reconstruction. Amuon crossing the inner detector produces both scintillation and Čerenkov light.The Čerenkov light is emitted as the typical forward bound Čerenkov cone whereasthe scintillation light evolves in all directions. First, the entry point is identified bythe backward running scintillation light. For a first approximation, the time patternof the hits registered within the first 5 ns of the event is analyzed. The exit pointis then identified by determining the symmetry plane dividing the photon arrivalpattern of the PMTs in half. This plane has to go through both the entry andexit point of the tracks, as well as through the center of the SSS, of which two arealready known. Having found this plane already allows a rough determination ofthe azimuth angle of the muon track. To get an even better result, twelve planesperpendicular to the symmetry plane in rising distance from the entry point areinvestigated. The arrival times at the PMTs in each plane show a minimum aroundthe track’s azimuth angle ϕ when plotted against the azimuth location of the PMT.60