First Year Performance Paper - Section 4.2
4.2 Candidate Neutrino Search with a Single IceCube string
About 99 % of the muon data collected with the IceCube string in 2005 was reconstructed and searched for upward-going muon candidates possibly induced by neutrinos.
A set of parameter cuts was applied to remove more uncertain, harder to reconstruct tracks, e.g., those that passed far from the string. These were satisfied by 70% of reconstructed tracks, corresponding to about 164 days of run time.
Additional cuts were developed to remove most of the remaining misreconstructed tracks by rejecting events for which the most likely upgoing hypothesis was comparable to the most likely downgoing hypothesis.
Two upgoing tracks were found with high hit multiplicity, one with 35 and the other with 50 out of 60 DOMs hit. These two events are reconstructed as muons traveling up with nadir angles of 1.16 ± 0.06° and 0.87 ± 0.05° degrees from the vertical, respectively. (The stated errors are statistical only; systematic errors may be larger.) Residual arrival times, i.e., time delays from reconstructed unscattered photon expectation, are plotted in Fig. 19 for one of these events. Almost every hit DOM received a direct (unscattered) hit, and many also recorded late (scattered) photons, more so in the dustier ice, and less in the clearer ice. Observation of two nearly vertical upward muons with String-21 in six months is consistent with expectations based on preliminary simulations .