IceCube
IceCube Neutrino Observatory

PDD - Calibration of High-Level Detector Response Variables

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5 Expected IceCube Performance

5.9 Calibration of High-Level Detector Response Variables

Detector geometry, angular response, vertex resolution and energy response can be calibrated using in situ light sources, IceTop-tagged or untagged downgoing cosmic-ray muons, and atmospheric neutrinos. Examples of in situ light sources include blue and UV LEDs, lasers buried in the ice or placed on the surface and connected via optical fiber to diffusing elements placed in the ice, buried DC light sources, etc. Lower-level parameters like cable lengths will be calibrated online by the DAQ system and can be verified by adapting some of the techniques described below.

5.9.1 Geometry Calibration

IceCube simulations indicate that downgoing muons can be used to determine the relative positions of the OMs to an accuracy of less than 1 m, which is sufficient for accurate reconstruction of neutrino-induced muons and cascades. The algorithm entails using fitted downgoing cosmic-ray muon tracks and calculating the probability for a photon from the track to arrive at each of a number of imaginary spatial grid points surrounding the putative OM position. Averaging over many tracks, each set of grid points around each OM describes a paraboloid whose maximum can be used to estimate the true OM position. Using these new position estimates for each OM we can then repeat the entire procedure until the iterative procedure converges.

Figure 54: Calculated correct position of a single OM. The OM was intentionally displaced from its true position by 1.5 m The procedure demonstrates that convergence occurs in three iterations.
Figure 55: Left plots: Intentional displacements of OMs (straight lines), all applied at the same time, and their calibrated positions (dots connected by lines). Right plots: Calibrated OM positions relative to original correct values. The left plots demonstrate that the calibration procedure can cope with string displacements which are larger than what the IceCube drill is designed to deliver. The right plots show that the resolution of the calibration is roughly 30 cm. The offset in ΔZ is believed to be an artifact of inaccuracies in the Monte Carlo parameterization of the photon arrival times.

Tests using simulated data have shown that the procedure converges quickly enough that it can be used to determine geometry positions of OMs almost immediately after deployment. A plot showing the convergence of the procedure for a single OM, whose position was intentionally displaced from its correct position by 1.5 m, is shown in fig. 54. Figure 55 shows how well the procedure calibrates the correct OM positions after intentional three-dimensional displacements of a string's position. This procedure will be a useful high-level test of the success of the deployment at the full-string level, and will permit the running of subsequent calibrations which require detector geometry as input. More computationally intensive refinements of the technique can be used to fine-tune the positions for use by the online filters and downstream data analyses. (Bright in situ light sources may also be useful for geometry calibrations, and indeed this is how the AMANDA geometry has been calibrated. However, IceCube's larger interstring spacing may limit the effectiveness of this techinque.)

5.9.2 Calibration of Angular Response

Events simultaneously triggering IceTop and IceCube constitute a data sample which can be used to calibrate IceCube's angular response. A similar technique has been used successfully by AMANDA with SPASE-2 triggers, where only a limited solid angle was available for study. With substantially larger solid angle coverage, IceTop should enable IceCube to calibrate its angular response more completely and accurately.

Over a smaller solid angle, IceTop can provide a tagged single muon beam for calibration of IceCube response to single muons and their direction reconstruction. This will require a denser subarray component, either in the center or perhaps also at one or more other locations on the surface, that triggers at a much lower threshold for showers with energies of roughly 10 TeV that are most likely to have only zero or one high energy muon capable of reaching IceCube.

5.9.3 Calibration of Vertex Resolution

The vertex resolution for cascades can be studied with pulsed in situ light sources located at known positions in the ice. Varying the intensity of these sources provides a way to study the vertex resolution as a function of energy. In AMANDA, sources have been determined to produce light outputs equivalent to a cascade event with several hundred TeV of energy.

5.9.4 Energy Calibration

The IceCube energy response can be calibrated using a number of independent techniques. Atmospheric neutrinos provide a test-beam with a known energy spectrum against which Ice-Cube can be calibrated. Downgoing muons may also be of use here, although often such events are composed of mulitple muons. (This may be addressed by IceTop, as discussed below.) Bremsstrahlung events from downgoing muons provide an energy calibration for cascades at the lower end of the energy scale, up to several hundred GeV. In situ light sources, preferably with precisely controllable output intensities, offer a way to calibrate IceCube energy response to cascades at higher energies, up to roughly 500 TeV.