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

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

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

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

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

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

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

Inverted Lollipop Energy range (tau decay length) E > ~5 PeV 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 Downgoing m and nm CC Tau branching ratio 100% D. Cowen/Penn State

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

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

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

Low Et m Lollipop Energy range (tau decay length) E < ~1 PeV nm 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

Tau Channels in IceCube D. Cowen/Penn State

“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

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

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

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

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

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

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

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

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

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

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