IceCube
IceCube Neutrino Observatory

PDD - Lower Energy Phenomena and Exotica

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5 Expected IceCube Performance

5.7 Lower Energy Phenomena and Exotica

5.7.1 Muon Neutrinos from WIMP annihilation

If Weakly Interacting Massive Particles (WIMPs) make up the dark matter of the universe, they would also populate the galactic halo of our own Galaxy. They would get captured by the Earth or the Sun where they would annihilate pairwise, producing high-energy muon neutrinos that can be searched for by neutrino telescopes. A favorite WIMP candidate is the lightest neutralino which arises in the Minimal Supersymmetric Model (MSSM).

The typical energy of the neutrino-induced muons would be of the order of ≤ 25% of the neutralino mass. Since the muon threshold for IceCube is expected to be fairly high (∼ 100 GeV), we expect IceCube to be sensitive mainly to neutralinos heavier than about 400 GeV.

In [121] the performance of IceCube was studied for two configurations, a 9 string × 9 string pattern with 125 m string spacing and a detector consisting of eight sub-modules of 10 strings each, with string spacing of 70 – 90 m within the sub-detectors (see fig. 47).

The effective area for WIMP detection depends on the WIMP mass as well as on the typical decay channel ("soft" channels from WIMP decays into many secondaries, "hard" channels from decays into a few secondaries only). Figure 48 shows effective volumes and effective areas for the rectangular pattern ("standard" geometry, left) and the modular pattern (right), separately for neutrinos from the Sun and the center of the Earth, and for soft and hard spectra, respectively.

Figure 47: Configurations studied in [121]

The standard geometry gives about 0.7 square kilometer area for both solar and terrestrial WIMPs, provided the decay channels are preferentially hard. For soft channels, the area drops down to less than 200 000 m2 for WIMP masses of 200 GeV. Compared to the standard geometry, the modular geometry gives somewhat worse results for hard decay channels and slightly better results for soft channels.

Figure 48: Effective area after a 10-degree cut on the angle between the reconstructed muon and the angle of the source, for different annihilation places, annihilation channels and WIMP masses. Results for the standard geometry to the left and for the modular geometry to the right.

Figure 49 shows the predicted muon rates from WIMPs annihilating in the Earth and the Sun as a function of the WIMP mass. Shown are results for the standard configuration with simple quality cuts after reconstruction. Each symbol in the plot corresponds to one particular combination of MSSM parameters (with the WIMP mass being one of them). Lines indicate the limits which could be achieved with IceCube after 5 years observation. Different symbols mark MSSM versions which are currently ruled out by direct detection experiments, which could be seen by direct detection experiments if the sensitivity is increased by a factor of ten, and which could not be seen by present direct detection experiments even after tenfold increase in sensitivity, respectively. All values are normalized to 10 GeV threshold for reasons of comparison6. It has been shown in [121] that IceCube could play a complementary role to future direct detection experiments (like CRESST or GENIUS) for annihilation in the Earth, and even has a slight advantage over direct detection experiments for certain low-mass WIMP models and annihilation in the Sun.

5.7.2 Neutrino oscillations

With a 10 Megaton detector of 20 GeV threshold, IceCube may also play a role in confirming the compelling indications that atmospheric neutrinos oscillate. Studies of systematics and backgrounds have revealed that significant progress requires much smaller spacing of OMs along a string than presently planned (namely 4-6 meters). For reduction of the threshold toward the horizon also a considerably smaller string spacing would be necessary. Such specialized effort is only warranted if ongoing experiments fail to conclusively prove the oscillation hypothesis. In that case one would consider to create a densely equipped region for detection of low-energy contained events as one part of IceCube. This region would be nested in the full array which acts as veto against through-going tracks. A possible solution would be to fill AMANDA-II with additional strings with small OM spacing. Note that interpretation of the feeble angular dependent oscillation effects at high neutrino energies require an extremely good understanding of the detector systematics.

In recent papers [122], the possibility is discussed to direct a neutrino beam to IceCube - either a WBB beam or a beam from a neutrino factory. The short duty time of an accelerator would allow to relax the cuts for background suppression since during the short spill times the detector is nearly free of background. The aim would be to measure mixing angle θ13 and the sign of Δm231, even though it is not possible to discriminate between charges. However, it has has to be investigated how well discrimination between muon tracks and ccascades at low energies would work.

5.7.3 MeV Neutrinos from Supernovae

Although the MeV-level energies of supernova neutrinos are far below the AMANDA/IceCube trigger threshold, a supernova could be detected by observing higher counting rates of individual PMTs over a time window of 5-10 s. The enhancement in rate of one PMT will be buried in dark noise signals of that PMT. However, summing the signals from all PMTs over 10 s, significant excesses can be observed. With background rates more than 10 times lower than ocean experiments, AMANDA and IceCube have the potential to see a supernova and to generate an alarm signal.

IceCube, with low-potassium glass spheres (40K decays in the glass sphere are the main source of external noise), might barely reach the the Large Magellanic Cloud.

AMANDA is officially, and will actively be in the near future, a member of the Supernova Early Warning System (SNEWS) [154]. The role of AMANDA will be to yield one of several coincident alarm signals from different detectors like Super-K, LVD and SNO (Macro being shut down in the mean time). On top of that, IceCube might participate in the goal to estimate the supernova direction by triangulation [153].

Figure 49: The predicted muon rates from WIMPs annihilating in the Earth (left) and the Sun (right) as a function of the WIMP mass. Each symbol in the plot corresponds to one particular combination of MSSM parameters. Lines indicate the limits which could be achieved with IceCube after 5 years observation. Different symbols mark MSSM versions which are currently ruled out by direct detection experiments (dots), which can be seen by direct detection experiments if the sensitivity is increased by a factor of ten (+), and which could not be seen by present direct detection experiments even after tenfold increase in sensitivity (×), respectively.

Figure 50 shows the increase in counting rate as seen in AMANDA-II and IceCube. Given a known template for time evolution of the pulse, the resulting accuracy in timing could be 14 ms for AMANDA-II and as good as 1-3 ms for IceCube. The resulting angular resolution depends on the orientation of the triangulation grid with respect to the supernova. For the three detectors Super-K, SNO and IceCube it would between typical values of 5 to 20 degrees. Figure 51 shows the X-2-contours of reconstructed supernova direction for a randomly choosen supernova event.

With dedicated detectors like Super-K, one can determine the neutrino direction from electron direction measurement in ν e– interaction with an accuracy of about 5°.

Figure 50: Simulated count rate data for a supernova at the center of the Galaxy, for AMANDA-II (left) and IceCube (right). The lines correspond to the simulation input.

5.7.4 Relativistic magnetic monopoles

Several theories predict the existence of magnetic monopoles with a magnetic charge which obeys the Dirac quantization rule g = n · e/(2α) with n = 1, 2, 3, ... and α = 1/137. A magnetic monopole with unit magnetic Dirac charge g = 137/2 · e and a velocity β close to 1 would emit Cerenkov radiation along its path, exceeding that of a bare relativistic muon by a factor of 8300 for β = 1. The value 8300 is obtained from (137/2)2 multiplied with nr2, with nr = 1.33 being the refractive index of water. This is a rather unique signature. Figure 52 shows the light emission from a monopole as a function of β. Note that due to the production of δ electrons, the monopole produces light even below its own Cerenkov threshold.

Figure 51: X2-contours of reconstructed supernova direction for a randomly choosen supernova event. It was assumed, that timing data from a detector at the South Pole are combined with data from SuperKamiokande and SNO. In the left figure, a time resolution of 14 ms (AMANDA-II) was assumed, in the right figure, a time resolution of 3 ms (IceCube). The nominal supernova direction was cos θ = 0.1, Φ = 2.98. Note the twofold ambiguity which even for IceCube necessarily remains if only three sites contribute.
Figure 52: Cerenkov light emission from magnetic monopoles in water (in photons per cm) as a function of velocity. The curve labeled "ά" represents the light produced by delta electrons which are generated by the monopole below its Cerenkov threshold.

Any early Universe phase transition occuring after inflation has the potential to populate the Universe with a flux of magnetic monopoles. Observations of galactic magnetic fields, as well as observations matched with models for extragalactical field lead to the conlcusion that monopoles of masses below 1015 GeV can be accelerated in these fields to relativistic velocities [147]. Figure 53 summarizes the limits obtained until now. Note that these limits are below the so-called Parker bound (1015 cm-2 s-1 sr-1). This bound is is derived from the very existence of galactic magnetic fields which would be destroyed by a too high flux of magnetic monopoles.

A cube kilometer detector could improve the sensitivity of this search by about two orders of magnitude compared to the present AMANDA limit. As mentioned above, the search could be extended to down to velocities β ∼ 0.5 by detecting the δ electrons generated along the monopole path.

Figure 53: Limits on the flux of relativistic monopoles achievable with IceCube compared to existing limits.

5.7.5 Slowly moving, bright particles

The passage of a particle with a velocity significantly below c yields a very distinct time pattern.

Candidates for particles with a velocity β < 0.1 c and with strong light emission are GUT magnetic monopoles or nuclearities [120, 148]. GUT monopoles may induce protons decays. Therefore one would expect Cerenkov light signals generated by the nucleon decay products along the path of the monopoles. For certain regions of the parameter space in monopole velocity and catalysis cross section, these signals would cause sequential hits in individual PMTs within time windows of 10-5 - 10-3 s.

Nuclearities (strange quark matter) should be aggregates of u, d and s quarks in equal proportions, and of electrons, to ensure their electrical neutrality. They should be stable for all baryon numbers in the range between ordinary nuclei and neutron stars (A ∼ 1057). They could have been produced in the primordial Universe or in violent astrophysical processes. See for recent reviews [148, 149].

The most relevant upper limits on the flux of these particles come from track etch experiments or liquid scintillator experiments (both e.g. represented by MACRO). The Baikal experiment has searched for enhanced counting rates over time intervals of 500 μs and deduced upper limits on the flux of GUT monopoles and strange quark matter [120]. The search technique relies on detectors with a low counting rate. In the Baikal case a rate as low as 100 Hz is achieved by local coincidences. With a counting rate per OM of 300 Hz (compared to typically 1 kHz for AMANDA), also IceCube may look for corresponding phenomena.