tau neutrinos in icecube
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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,
D. Cowen/Penn State 2Tau Neutrinos in IceCube
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
D. Cowen/Penn State 3Tau Neutrinos in IceCube
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
D. Cowen/Penn State 4Tau Neutrinos in IceCube
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]
D. Cowen/Penn State 5Tau Neutrinos in IceCube
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
D. Cowen/Penn State 6Tau Neutrinos in IceCube
Tau Neutrino Signatures in IceCube: Overview
Signature Cartoon Description
Lollipop Tau created outside (un- detected), decayscascade
Inverted Lollipop
Tau created insidecascade, decays outside (undetected)
Sugardaddy(see talk by T.
DeYoung)
Tau created outside (un- detected), 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
D. Cowen/Penn State 7Tau Neutrinos in IceCube
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%
D. Cowen/Penn State 8Tau Neutrinos in IceCube
Inverted Lollipop
Energy range(tau decay length)
E > ~5 PeVL > 200 m
Acceptance ~2Energy resolution Better than
Pointing resolution Slightly worse than
Background Downgoing and CC
Tau branching ratio 100%
D. Cowen/Penn State 9Tau Neutrinos in IceCube
Sugardaddy
Energy range(tau decay length)
~5 PeV < E < ~EeVL > 200 m
Acceptance ~2
Energy resolution Similar to
Pointing resolution Similar to
Background Minimal, maybe downgoing ?
Tau branching ratio 18%
See talk by T. DeYoung
D. Cowen/Penn State 10Tau Neutrinos in IceCube
Double Bang
Energy range(tau decay length)
~2 PeV < E < ~20PeV
100 m < L1km
Acceptance ~2
Energy resolution Similar to e
Pointing resolution Similar to
Background Minimal; maybe downgoing ?
Tau branching ratio 82%
D. Cowen/Penn State 11Tau Neutrinos in IceCube
Double Pulse
Energy range(tau decay length)
~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 ?
Tau branching ratio 82%
DOM Waveform
D. Cowen/Penn State 12Tau Neutrinos in IceCube
Low E Lollipop
Energy range(tau decay length)
E < ~1 PeVL < 50 m
Acceptance ~4
Energy resolution Better than
Pointing resolution Similar to
Background Downgoing and CC
Tau branching ratio 18%
D. Cowen/Penn State 13Tau Neutrinos in IceCube
Tau Channels in IceCube
D. Cowen/Penn State 14Tau Neutrinos 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
D. Cowen/Penn State 15Tau Neutrinos in IceCube
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
D. Cowen/Penn State 16Tau Neutrinos in IceCube
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)
D. Cowen/Penn State 17Tau Neutrinos in IceCube
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.)
D. Cowen/Penn State 18Tau Neutrinos in IceCube
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
D. Cowen/Penn State 19Tau Neutrinos in IceCube
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
.)
D. Cowen/Penn State 20Tau Neutrinos in IceCube
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 21Tau Neutrinos in IceCube
Lollipop Channels
• Estimate of tagging efficiency vs. E
Seon-H
ee S
eo,
Penn
Sta
te
D. Cowen/Penn State 22Tau Neutrinos in IceCube
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
D. Cowen/Penn State 23Tau Neutrinos in IceCube
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
D. Cowen/Penn State 24Tau Neutrinos in IceCube
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
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