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

counts/days*4 hitsDay (Red) and Night (Blue) spectrum21.81.61.41.210.80.60.40.2200 300 400 500 600nhitsFigure 4.8: Day and night spectra for the energy region outside the 210 Po peak. Nosignificant difference between the spectra is observed. Figure from [73].4.4.2 8 B NeutrinosSpectroscopic measurements of solar 8 B neutrinos have been done previously byKamiokande [11] and Super-Kamiokande [31] using a water Čerenkov detector andSNO [74] using deuterium as a target, all with an energy threshold of ∼ 5MeV orhigher. Recently, the SNO collaboration published an analysis with a threshold of3.5 MeV [29].Borexino is the first experiment succeeding in spectroscopic measurements ofthe 8 B neutrino flux using liquid scintillator. All major backgrounds above the2.614 MeV gamma from the decay of 208 Tl could be reduced sufficiently allowingto set the energy threshold for the 8 B neutrinos as low as 3MeV [75]. Taking intoaccount the 7 Be neutrino measurements described in the previous section, Borexinois the first experiment where neutrinos from both pp-chain reactions have beeninvestigated simultaneously. Very interestingly, the energy region of those neutrinoscovers the transition region from vacuum to matter dominated oscillations inthe LMA solution of the MSW effect described in Section 2.2.2, thus providing anexcellent method to experimentally challenge this prediction.Two different analyses were done for the 8 B neutrinos, one with a threshold of3MeV and another one with a higher threshold at 5 MeV to be able to compare theresults with earlier measurements from Super-Kamiokande and SNO. Recent resultsare based on 488 days of data taking, with a total exposure time after cuts of 345.3days in a fiducial target volume of 100 tons.8 B neutrinos were selected from data through the application of a set of cutsdescribed in [75]. An interaction rate of 0.217 ± 0.038 stat ± 0.008 syst counts per dayand 100 tons was found for an energy threshold of 3 MeV. Raising this threshold to+0.0085MeV, 0.134 ±0.022 stat counts per day and 100 tons remain. Fig. 4.9 shows−0.007systthe 8 B neutrino energy spectrum for the selected data. It is consistent with thehigh metallicity SSM BPS09(GS98) model presented in Section 2.3 in combination64