4. The IceCube Neutrino Telescope 33
4.4. Background Events
The direction of the track can be reconstructed very well due to the long lever arm in the detector. Interactions can happen outside of the detector so that the muon enters the instrumented volume of IceCube from outside. It is also possible that the muon leaves the instrumented volume and continues its path. Thus the total interaction is not fully contained in the detector, makes the energy estimation challenging.
Charged current muon neutrino interactions are the event signature that is utilized in this thesis to perform a point source analysis. The following discussion is restricted only to this type of events since they are the one of interest for this analysis.
Figure 4.9.: Event view of a simulated track event. The spheres mark hit DOMs; the size represents the accumulated amount of light and the color the time of the first detection of light going from red to blue.
4.4. Background Events
The signatures of astrophysical muon neutrino µν interactions are buried in several types of background processes. A key challenge in neutrino astronomy is to extract the astrophysical neutrino interactions from the background. Since the development of this event selection is a common problem in point source analysis, there is a dedicated
event selection developed inside the IceCube collaboration for point source analysis to select atmospheric muon neutrinos and suppress any background events [20]. The development of this sample was not part of this work and thus is only briefly discussed in the following sections.
The atmosphere is constantly bombarded by cosmic ray particles. The interaction of a highly energetic cosmic ray particle with a nucleus of the atmosphere produces particle showers of mainly unstable particles. Most particles decay in the upper at-mosphere, leaving only a small fraction reaches the surface. The reaction is similar to hadronuclear interactions of cosmic rays with the difference that the atmosphere is typically much thicker than typical astrophysical cosmic ray sources, see section 2.3. The largest population of particles which reach the Earth surface (and eventually can also travel several kilometers below the surface and reach IceCube) are neutrinos and muons. Thus the main background events in IceCube are atmospheric muon and neutrino-induced events. Both are discussed in the following. Figure 4.10 shows a sketch of an atmospheric particle shower.
4.4.1. Atmospheric Muons
Muons are unstable particles with a lifetime of about 2.2µs [9]. Mouns only reach earth and the IceCube detector if they do not decay before. If they are highly en-ergetic, they benefit from relativistic time dilatation ∆t0 =γ∆t withγ =1/√
1−v2/c2
and survive the journey from their production side at the height of about 20 to 30 kilometers to the surface and even further. Thus, the atmospheric muons reaching IceCube are typically high energetic. If such an atmospheric muons reach the detec-tor, it mimics the signature of a muon produced in a muon neutrino interaction. The two are not distinguishable since they are both just muons.
To suppress the atmospheric muon background, there are several strategies in place depending on the direction of the muons. The most energetic muons can reach the surface and also travel few kilometers below the surface, but not hundreds of kilome-ters. This fact is used in IceCube as a directional cut: Muons that are reconstructed to originate from below the detector must have traveled through the entire earth to reach the detector from this direction. The probability is neglectable for muons to do this. Neutrinos, on the other hand, are capable of traveling through the Earth without interaction and then undergo a CC interaction close to the IceCube detector.
4.4. Background Events
Figure 4.10.: Atmospheric particle shower with the electromagnetic, hadronic and mesonic components. Figure is taken from http://www.antarcticglaciers.org/glacial-geology/
dating-glacial-sediments-2/cosmic-rays/.
In this interaction, a muon is produced which then enters the detector from below (or is produced inside of the detector).
The result is a strong background suppression as a function of material that has to be passed on the way to the detector and thus as a function of zenith angle θ of the event. For muons originating below the horizon, the chance of atmospheric origin is negligible. The detector is split into an up-going region (direction coming from below of the detector) and down-going region (events originating above the horizon). The up-going region is essentially free of atmospheric muons, whereas in the down-going region they dominate the sample.
The up-going region (up-going is defined in this analysis everything up above the
horizon (δ > 0◦) and down-going accordingly) is dominated by misreconstructed down-going events. To reduce this background, strict requirements on the quality of event reconstruction is required. These cuts remove badly reconstructed and thus miss reconstructed downgoing events from the sample. This is done utilizing a set of straight cuts and a boosted decision trees (BDT), see [20]. The remaining irreducible background consists of atmospheric neutrinos which are discussed in the next section.
For the down-going region, there are many high energetic and well reconstructed at-mospheric muons. Quality cuts on the reconstruction quality and energy are applied, again utilizing a BDT in the final step. Still, cosmic ray interactions in the atmosphere can produce bundles of multiple muons which are moving in parallel due to the large relativistic boosting if the initial cosmic ray particle was highly energetic. These muon bundles can mimic the signature of single, highly energetic neutrino-induced muon.
It has been shown that muon bundles lose energy more constantly than single, highly energetic muons. They undergo so-called stochastic losses when losing a large amount of energy in a very short distance [50]. The difference is thus given by the smoothness of light yield along the track. This light yield smoothness is used as an additional parameter in the BDT.
The final event rate as function of zenith angle θis shown in figure 4.11. One can see that the down-going region is dominated by atmospheric muons and the contribution of atmospheric neutrinos can be neglected. In the up-going region, the opposite is the case: Atmospheric muons are negligible, and most of the background comes from atmospheric neutrinos. Also, the expected rate of astrophysical neutrinos is larger in the up-going region.
Since the total event rate is essentially constant over all declination bands, the signal to background ratio in the up-going region is much better than in the down-going region. This will also be discussed later regarding point source sensitivity.
4.4.2. Atmospheric Neutrinos
Similar to the production of production of atmospheric muons, neutrinos are also produced in cosmic ray showers in the upper atmosphere. The process is similar to hadronuclear interaction expected at astrophysical neutrino sources (see section 2.3).
The atmospheric neutrinos are, except for the highest energies, not absorbed by the earth and should thus be isotropic in the detector. There is no way to suppress the
4.4. Background Events
Figure 4.11.: Zenith (cos(θ)) or declination (−sin(δ)) distribution of the through-going track sample after event selection (2012 to 2015 data). Values of
−1 correspond to vertically up-going events. Shown is the experimental data (black), compared to the atmosphericνµ+ ¯νµ expectation of con-ventional atmospheric (solid gray) and astrophysical neutrinos (dashed gray), and atmospheric muons (dotted gray) from Monte Carlo simula-tion [50].
atmospheric neutrino background.
Conventional Atmospheric Neutrinos In section 2.3, it was claimed that the neu-trinos follow the primary cosmic ray spectrum. The difference between astrophysical neutrino source environments and the atmosphere is the thickness, as the atmosphere is several orders of magnitude thicker. Thus charged pionsπ±and muonsµ±undergo energy losses according to the Bethe-Bloch formula [51] before they decay. In the high-energy regime above 104GeV, pair production, and bremsstrahlung dominate the energy losses. The energy loss can be approximated by
dE
dx ≈A·E+B (4.3)
where E is the energy andA(E) and B(E) are parameterizations of the energy loss.
The energy loss increases linearly with energy (the energy dependence of A and B
can be neglected to first order). The initial cosmic ray spectrum with E−2.7 thus transforms into an E−3.7 energy spectrum of atmospheric neutrinos. This spectrum is much softer than the expected astrophysical neutrino spectrum (E−2 toE−2.5).
Most of atmospheric neutrinos are produced with the decay of the relatively long living π± (2.2·10−8s) and µ± (2.2·10−6s). They are called the conventional atmospheric neutrinos and make up the dominant part of the atmospheric neutrino background.
Prompt Atmospheric Neutrinos Atmospheric neutrinos can also be produced by the decay of heavier mesons, especially charmed mesons. Charmed mesons have a typical lifetime of 10−13s. In contrast to the production of conventional atmospheric neutrinos, charmed mesons decay before they can lose significant fractions of their energy. Therefore, the neutrino spectrum from the decay of charmed mesons follows the initial cosmic ray spectrum with E−2.7. Since the meson decay happens quasi-instantaneously, the neutrinos are called prompt. The expected prompt atmospheric neutrino flux is about two orders of magnitude lower than the conventional one at energies of 104GeV (see [52, discussion and especially figure 3]).
Prediction of prompt atmospheric neutrino flux is strongly model dependent. The flux can be confused with a diffuse astrophysical neutrino flux since both would have a similar energy spectrum and isotropic characteristics.