Tau Neutrinos in IceCube

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 ~1014-15 eV • leads to 4p acceptance at E(nt) < ~1014-15 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 1450-2450m 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 nt nt t t nt t nt t nt t m DOM Waveform nt t m nm nt Decreasing IceCube Acceptance Energy  D. Cowen/Penn State

7. Lollipop nt t D. Cowen/Penn State

8. Inverted Lollipop nt t D. Cowen/Penn State

9. Sugardaddy nt t m See talk by T. DeYoung D. Cowen/Penn State

10. Double Bang nt t D. Cowen/Penn State

11. Double Pulse nt t DOM Waveform D. Cowen/Penn State

12. Low Etm Lollipop t m nm nt 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 nt t DOM Waveform • 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 D. Cowen/Penn State

16. Double Pulse Channel • 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) Pat Toale, Penn State • (Efficiency is basically flat as a function of zenith angle to tau track) 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 nt nt t t 50 TeV nt • 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 D. Cowen/Penn State

19. Lollipop Channels nt t • 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: 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. 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) 7x number of DOMs hit 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