IceCube
IceCube Neutrino Observatory

PDD - Tau Neutrinos

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5 Expected IceCube Performance

5.5 Tau Neutrinos

In this section we expand on the discussion found in secs. 3.2.4 on ντ kinematics, detection probability and event rates in IceCube. High energy tau neutrinos can be distinguished from other signals by either their "double bang" or "lollipop" topologies. These topologies are diagrammed in fig. 31. Preliminary studies of the lollipop topology have only just begun, so we will focus on the double bang topology in what follows. Double bang events are those in which the production and decay of a τ lepton can be identified [118]. Figure 32 shows a simulated CC ντ; double bang event in IceCube.

Figure 31: The "double bang" (A) and "lollipop" (B) signatures which tau neutrino interactions can create. These signatures are also discussed in section 5.6.1.

We have calculated the probability of detecting a double bang event in a neutrino telescope of linear dimension D = 1 km such as IceCube. We have developed a very simple Monte Carlo for this purpose in which the detector is considered as one dimensional. We have taken the energy threshold for detecting showers to be Eshower ∼ 1 TeV. We have conservatively fixed to 250 m the minimum distance the τ has to travel to clearly distinguish the Cherenkov light from both showers. This number is determined by the separation between strings which is 125 m. (In principle this number could go much lower if other double bang topologies are considered.) Our Monte Carlo accounts for the fluctuations in the fraction of energy transferred to the τ in the interaction, as well as for the fluctuations in its range. The result of the Monte Carlo is shown in fig. 33 along with the probability of observing a νμ-induced muon. Also shown is the

Figure 32: A double-bang ντ event with an energy of a few PeV in IceCube. Colored circles represent hit channels, with red circles hit earliest and violet circles latest. Circle size corresponds to number of detected photons.

probability of observing a double bang event as obtained in [119] using the expression:

Pτ(E,Eτmin) = ρNA ∫1-Eτmin/E dy[D-Rτ]dσCC(E,y),
dy

where ρ is the density of the medium and NA is Avogadro's number. Eτmin is the minimum energy of the τ , it is automatically fixed imposing that Rτ ≥ 250 m. This expression uses average values for Rτ and hence at Eντ ≤ Eτmin the probability of observing a double bang vanishes since the average range of the τ is smaller than 250 m. The probability calculated with this expression has a broad maximum which is determined by the condition that Rτ has to be larger than 250 m but smaller than the size of the detector. Assuming an average value of y ∼ 0.25 at these energies and using eq. 2 this corresponds to Eντ between roughly 7 PeV and 27 PeV.

Figure 33: Probability of detecting ντ induced double bang events in IceCube obtained with Monte Carlo simulation and using the analytic expression given by of [119] (Eq.2). The probability is shown for Rτ = 100 m and 250 m. Also shown is the probability of detecting a νμ-induced muon.

Due to fluctuations in Rτ , even ντ's with energy below Eτmin are going to produce τ's that travel more than 250 m. This explains why our probability doesn't vanish at Eντ < Eτmin as happens to the one given by eq. (3). It also has a longer tail at high energies because fluctuations in the τ range and in its energy prevent a fraction of high energy ντ's from producing a τ that travels a distance larger than the dimension of the detector.

5.5.1 Tau Neutrino Event Rates

We calculate the number of ντ events in the standard way [93], convoluting the probability of observing the double bang events with the ντ + ντ flux:

Nevents=Aeff∫Φντ+ντ(Eντ)Pντ → double bang(Eντ)dEντ

where we will take Aeff = 1 km2. Pντ → double bang is the probability of observing a double bang event as shown in fig. 33.

By far the largest uncertainty in this expression is the ντ + ντ flux. Tau neutrino production is expected to be very small in most astrophysical scenarios where ν's are produced in pp or pγ collisions. Several calculations suggest a ratio [119] Fντ/νμ ∼ 10-5. Oscillation scenarios in which νμ's convert themselves into ντ's may be the most promising sources of ντ's. Making the assumption that Fντ/νμ = 0.5 we obtain the number of events in table 10 for representative νμ + νμ fluxes in the literature. In fig. 34 we also show the energy distribution of the events. They all resemble the shape of the probability function of fig. 33. For more details and references, see reference [83].

It is important to note that with neutrino oscillations a generic cosmic accelerator which produces neutrinos in the flux ratio νe : νμ : ντ :: 1 : 2 : 0 will result in neutrinos in the flux ratio of 1 : 1 : 1 at the detection point. With a 1 : 1 : 1 ratio the numbers of double bang events in table 10 would grow by a factor of two.

5.5.2 Tau Neutrino Simulations

Although tau neutrinos are clearly a promising analysis to pursue, at present no IceCube simulations of tau neutrinos have been run, due to lack of time and manpower. Many of the simulation tools used for muons and cascades are applicable, so it is anticipated that this deficit will be remedied in the near future.