1. Tau Neutrinos in IceCube
Advantages of tau neutrinos
Tau neutrino signatures in IceCube
Or: Double Bangs Are Just the Tip of the Iceberg
Results from initial “toy” Monte Carlo studies
1 PeV nttX, tmnn
D. Cowen/Penn State

2. Advantages of Tau Neutrinos
At high energies (E > ~1 TeV), nt are a virtually background-free source of cosmological neutrinos
Sources of nt which will give negligibly small fluxes:
atmospheric nt from atmospheric ne and/or nm`oscillations
oscillations small at these energies
“prompt” atmospheric nt from charm decay
Only faraway accelerators that produce neutrinos as ne:nm:nt::1:2:0 can, through neutrino oscillations, produce appreciable numbers of tau neutrinos at IceCube
flux ratio at earth is ~1:1:1
Tau flavor is a very clean tag for cosmological neutrino origin
D. Cowen/Penn State

3. More Advantages of Tau Neutrinos
Energy resolution
can be comparable to that of ne
Pointing resolution
can be comparable to that of nm
Acceptance
varies from ~2p to ~4p, depending on tau decay channel
tau neutrino regeneration in the earth allows UHE nt to penetrate and emerge at ~ eV
leads to 4p acceptance at E(nt) < ~ eV
Rich set of signatures allows for
better background rejection
self-consistency checks
e.g., measurements of the same neutrino flux with different systematics
D. Cowen/Penn State

4. Quick Overview of IceCube
Over 70 strings, L~1km, total V~1km3
60 Digital Optical Modules (DOMs) per string
Deployed at depths of m at South Pole
Completion slated for 2011
Currently have 9 strings deployed
partially surrounding AMANDA; eventually will completely surround
in principle already sensitive to some nt channels
[see talk by K. Hanson for more details about IceCube detector]
D. Cowen/Penn State

5. Capabilities of IceCube DOMs
Each DOM, a standalone computer, has
built-in set of digitizers (very important for detection of tau neutrinos)
fast ones: 3 different gain levels, ~3ns sampling period, ~400ns depth
(128 samples)
slow one: 25ns sampling period,
6.4ms depth (256 samples)
built-in, remotely
programmable, calibration
light source (can be used to
simulate tau neutrinos)
few nanosecond time resolution
distinguish light pulses from
individual nt–induced cascades
D. Cowen/Penn State

6. Tau Neutrino Signatures in IceCube: Overview
Cartoon
Description
Lollipop
Tau created outside (un- detected), decayscascade
Inverted Lollipop
Tau created insidecascade, decays outside (undetected)
Sugardaddy
(see talk by T. DeYoung)
Tau created outside (un- detected), decaysmuon, see D in light level along track
Double Bang
Tau created and decays inside, cascades well-separated
Double Pulse
Double bang, w/cascades un-resolvable, but nearby DOM(s) see double pulsed waveform
Low Et m Lollipop
Inverted lollipop but low-E tau decays quickly to m; Study ratio Esh/Etr nt t
Decreasing IceCube Acceptance Energy  nt t nt t m nt t t nt
DOM
Waveform t m nm nt
D. Cowen/Penn State

7. Minimal; maybe downgoing m?
Lollipop nt t
Energy range
(tau decay length)
E > ~5 PeV
Lt > 200 m
Acceptance
~2p
Energy resolution
Better than nm
Pointing resolution
Slightly worse than nm
Background
Minimal; maybe downgoing m?
Tau branching ratio
82%
D. Cowen/Penn State

8. Inverted Lollipop Energy range (tau decay length) E > ~5 PeVnt t
Energy range
(tau decay length)
E > ~5 PeV
Lt > 200 m
Acceptance
~2p
Energy resolution
Better than nm
Pointing resolution
Slightly worse than nm
Background
Downgoing m and nm CC
Tau branching ratio
100%
D. Cowen/Penn State

9. Minimal, maybe downgoing m?
Sugardaddy nt t m
See talk by T. DeYoung
Energy range
(tau decay length)
~5 PeV < E < ~EeV
Lt > 200 m
Acceptance
~2p
Energy resolution
Similar to nm
Pointing resolution
Background
Minimal, maybe downgoing m?
Tau branching ratio
18%
D. Cowen/Penn State

10. Minimal; maybe downgoing m?
Double Bang nt t
Energy range
(tau decay length)
~2 PeV < E < ~20PeV
100 m < Lt < 1km
Acceptance
~2p
Energy resolution
Similar to ne
Pointing resolution
Similar to nm
Background
Minimal; maybe downgoing m?
Tau branching ratio
82%
D. Cowen/Penn State

11. Minimal; Coincident downgoing m?
Double Pulse t nt
DOM
Waveform
Energy range
(tau decay length)
~100 TeV < E < ~5 PeV
~5m < Lt < ~100 m
Acceptance
~4p
Energy resolution
Similar to ne
Pointing resolution
Background
Minimal; Coincident downgoing m?
Tau branching ratio
82%
D. Cowen/Penn State

12. Low Et m Lollipop Energy range (tau decay length) E < ~1 PeVnm nt
Energy range
(tau decay length)
E < ~1 PeV
Lt < 50 m
Acceptance
~4p
Energy resolution
Better than nm
Pointing resolution
Similar to nm
Background
Downgoing m and nm CC
Tau branching ratio
18%
D. Cowen/Penn State

13. Tau Channels in IceCube
D. Cowen/Penn State

14. “Toy” MC Studies of Tau Neutrinos in IceCube
Many of the channels mentioned here are under active investigation
Using very simple MC at present
no actual tau decay—we fake it for now
no full detector simulation—but geometry and timing resolution are reasonably accurate
Initial goal is to do feasibility studies
if a signal is not detectable under these idealized circumstances, it will not be detectable under more realistic circumstances
D. Cowen/Penn State

15. Double Pulse Channel
Look at tagging efficiency using a toy simulation, full km3:
place first cascade randomly in box ±200m from detector center with E = 0.25 E(nt)
Tau travels in same direction as initial nt and then decays following the expected lifetime
Tau decays to an electron with E = 0.42 E(nt)
Look at variety of energies and zenith angles
Calculate time separation Dt detected at one (or more) DOMs purely geometrically (i.e. no scattering);
For this study, we require large enough Dt to consider a two-pulse waveform to be detectable and
we crudely simulate scattering by varying a cut on the shower-to-DOM distance t nt
DOM
Waveform
D. Cowen/Penn State

16. Double Pulse Channel Cuts (>=1 or >=2 DOMs):
Pat Toale, Penn State
(Efficiency is basically flat as a function of zenith angle to tau track)
Cuts (>=1 or >=2 DOMs):
cut1: r<70m && 30<Dt<300ns (~ignores scattering, optimistic Dt)
cut2: r<70m && 60<Dt<300ns (~ignores scattering, conservative Dt)
cut3: r<35m && 30<Dt<300ns (~no scattering, optimistic Dt)
cut4: r<35m && 60<Dt<300ns (~no scattering, conservative Dt)
D. Cowen/Penn State

17. Double Pulse Channel
Here is what a fully simulated waveform looks like for a 75 TeV tau (~300 TeV nt)
designing a robust algorithm for identifying the two separate pulses is underway (and should not be terribly hard for cases like this)
  cascade 1
  cascade 2
 sum
 MC truth
Light from two cascades from 75 TeV tau in a single DOM (5mV=1p.e.)
D. Cowen/Penn State

18. Lollipop Channels
The lollipop channels consist of a cascade and a track in the same event
For an initial feasibility study, we simulate a cascade followed by a muon, using the average Ec and Em energies expected for a tmnn decay
Investigate whether or not we can reconstruct such a “hybrid” event
reconstruct cascade and muon as distinct entities
Use full detector simulation nt t nt t
50 TeV nt
D. Cowen/Penn State

19. Lollipop Channels In the topology under study
the early high- multiplicity- photon hits will come mainly from the cascade
the later low-multiplicity hits will come mainly from the muon
This is borne out by the MC: nt t
multiplicity (p.e.)
time (ns)
D. Cowen/Penn State

20. Lollipop Channels Initial findings are that
the muon reconstructs well even if the fitter is given all hit DOMs (including those from the cascade)
here, “tagged” = space angle is within ~6o of true direction
the cascade reconstructs better if it is only given the earlier hits
here, “tagged” = vertex position within ~50 m of true vertex
D. Cowen/Penn State

21. Lollipop Channels Estimate of tagging efficiency vs. E
Seon-Hee Seo, Penn State
D. Cowen/Penn State

22. Sugardaddy Channel
This channel relies on seeing an increase in track brightness produced by tmnn
probably background-free signal
tracks from background processes should only decrease in brightness along their lengths
expect brightness increase of 3x to 7x
see Ty DeYoung’s talk for details
Toy simulation uses single muon track that is overlaid with 2 or 6 additional collinear muon tracks about halfway along its length
D. Cowen/Penn State

23. distance along track (m)
Sugardaddy Channel
“decay”
at -100m
Toy simulation of 10 PeV tau lepton
use 1 PeV muon
overlay with additional 1PeV m tracks to mimic decay tmnn
Look at number of hit DOMs as a function of length along the track(s)
number of DOMs hit 7x 4x
Dawn Williams, Penn State
no “decay”
distance along track (m)
D. Cowen/Penn State

24. Conclusions
Many different tau decay channels are accessible to large-scale UHE neutrino detectors (not just IceCube)
tau neutrinos can be relatively background-free as a signal for cosmological neutrino detection
tagging efficiencies are reasonably high
different tau neutrino channels can be compared to one another as a valuable systematic check
Initial studies are encouraging
more detailed Monte Carlo studies are underway
Ultimately, expect to have sensitivity to tau neutrinos at energies 1-2 orders of magnitude below and many orders of magnitude above the better-known double bang channel
D. Cowen/Penn State