First Year Performance Paper - Section 5.3
5.3 Single station coincidences with String-21
Air-showers containing one or more muons with energy sufficient to reach the deep detector can be detected in coincidence by IceTop and String-21. Because of the steep cosmic-ray spectrum, most of these coincident events will produce signals at only one IceTop station. In a small fraction three or more IceTop stations will be hit, satisfying the IceTop trigger described above. Events in which both tanks at one station are hit with no hits in any other stations are highly correlated with single muons at string depth and therefore of special interest for calibration and background tagging. The rate of singlestation coincidences with String-21 triggers is 0.05 Hz. By comparison with a preliminary simulation, this rate is consistent with expectation if String-21 sees vertical muons out to 30 m with its present 8-fold coincidence requirement; this 30 m radius is consistent with the measured In-Ice muon rate. As the array grows, the rate of such tagged single muons will increase quadratically, allowing a large sample of tagged background events to be accumulated. It will be possible to tag about 5% of the In-Ice muons, including some accidental coincidences within a 2 μsec trigger window.
Because of the large spacing between the DOMs in IceTop and those in String-21, the coincident events allow us to verify the absolute timing of IceCube on the kilometer scale, using a selection of tagged, nearly vertical muons. Fig. 25b is a distribution of times for events in which at least one of 3 specific DOMs on String-21 (1, 25 and 55) is hit in coincidence with a specific high gain DOM in a surface tank. (DOM 1 is at the top of String-21 and DOM 60 at the bottom.) Times on String-21 are measured relative to the time of the first signal in IceTop (which may occasionally be earlier than the time of the specific IceTop DOM plotted). By definition, there are at most 4 entries for each event. The systematic decrease in population of the deeper DOMs on String-21 is an expected consequence of muons ranging out between the top and bottom of the string.