Tau Neutrinos in IceCube

D. Cowen/Penn State 1Tau Neutrinos in IceCube

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 X,


Advantages of Tau Neutrinos• At high energies (E > ~1 TeV), are a

virtually background-free source of cosmological neutrinos• Sources of which will give negligibly small

fluxes:• atmospheric from atmospheric e and/or

oscillations• oscillations small at these energies

• “prompt” atmospheric from charm decay

• Only faraway accelerators that produce neutrinos as e::::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


More Advantages of Tau Neutrinos• Energy resolution

• can be comparable to that of e

• Pointing resolution• can be comparable to that of

• Acceptance• varies from ~2 to ~4depending on tau

decay channel• tau neutrino regeneration in the earth allows UHE

to penetrate and emerge at ~1014-15 eV• leads to 4acceptance at E() < ~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


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 channels

• [see talk by K. Hanson for more details about IceCube detector]


Mu-metalgrid

Penetrator HV Divider

LEDFlasherBoard

PMT

DelayBoard

DOMMainboard

RTVgel

Glass Pressure Housing

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, 6.4s 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 –induced cascades


Tau Neutrino Signatures in IceCube: Overview

Signature Cartoon Description

Lollipop Tau created outside (undetected), decayscascade

Inverted Lollipop

Tau created insidecascade, decays outside (undetected)

Sugardaddy(see talk by T.

DeYoung)

Tau created outside (undetected), decaysmuon, see in light level along track

Double BangTau created and decays inside, cascades well-separated

Double Pulse

Double bang, w/cascades un-resolvable, but nearby DOM(s) see double pulsed waveform

Low E Lollipop

Inverted lollipop but low-E tau decays quickly to ; Study ratio Esh/Etr

DOM Waveform

Decre

asin

g Ice

Cu

be A

ccep

tan

ce E

nerg

y


Lollipop

Energy range(tau decay length)

E > ~5 PeVL > 200 m

Acceptance ~2

Energy resolution Better than

Pointing resolution Slightly worse than

Background Minimal; maybe downgoing ?

Tau branching ratio 82%


Inverted Lollipop


E > ~5 PeVL > 200 m

Acceptance ~2Energy resolution Better than

Pointing resolution Slightly worse than

Background Downgoing and CC



Sugardaddy


~5 PeV < E < ~EeVL > 200 m

Acceptance ~2

Energy resolution Similar to

Pointing resolution Similar to

Background Minimal, maybe downgoing ?


See talk by T. DeYoung


Double Bang


~2 PeV < E < ~20PeV

100 m < L1km

Acceptance ~2

Energy resolution Similar to e


Background Minimal; maybe downgoing ?



Double Pulse


~100 TeV < E < ~5 PeV

~5m < L < ~100 m

Acceptance ~4

Energy resolution Similar to e

Pointing resolution Similar to e

Background Minimal; Coincident downgoing ?


DOM Waveform


Low E Lollipop


E < ~1 PeVL < 50 m

Acceptance ~4

Energy resolution Better than


Background Downgoing and CC



Tau Channels in IceCube


“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


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()• Tau travels in same direction as initial and

then decays following the expected lifetime• Tau decays to an electron with E = 0.42 E()• Look at variety of energies and zenith angles• Calculate time separation t detected at one

(or more) DOMs purely geometrically (i.e. no scattering);

• For this study, we require large enough t to consider a two-pulse waveform to be detectable and

• we crudely simulate scattering by varying a cut on the shower-to-DOM distance

DOM Waveform


Double Pulse Channel

• Cuts (>=1 or >=2 DOMs):• cut1: r<70m && 30<t<300ns (~ignores scattering, optimistic t)• cut2: r<70m && 60<t<300ns (~ignores scattering, conservative t)• cut3: r<35m && 30<t<300ns (~no scattering, optimistic t)• cut4: r<35m && 60<t<300ns (~no scattering, conservative t)

Pat

Toale

, Penn

Sta

te

• (Efficiency is basically flat as a function of zenith angle to tau track)


Double Pulse Channel• Here is what a

fully simulated waveform looks like for a 75 TeV tau (~300 TeV )• 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.)


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 E energies expected for a decay• Investigate whether or not

we can reconstruct such a “hybrid” event

• reconstruct cascade and muon as distinct entities

• Use full detector simulation 50 TeV


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:

time (ns)

mu

ltip

licit

y

(p.e

.)


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


Lollipop Channels

• Estimate of tagging efficiency vs. E

Seon-H

ee S

eo,

Penn

Sta

te


Sugardaddy Channel

• This channel relies on seeing an increase in track brightness produced by • 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


Sugardaddy Channel• Toy

simulation of 10 PeV tau lepton• use 1 PeV

muon• overlay with

additional 1PeV tracks to mimic decay

• Look at number of hit DOMs as a function of length along the track(s)

“decay”at -100m

no “decay”

4x

7x

distance along track (m)

num

ber

of

DO

Ms

hit

Daw

n W

illia

ms,

Penn

Sta

te


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

Tau Neutrinos in IceCube

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