capabilities of a hybrid optical-radio-acoustic neutrino detector at the south pole

Capabilities of a Hybrid Optical-Radio-Acoustic

Neutrino Detector at the South Pole

Justin VandenbrouckeSebastian BöserRolf Nahnhauer

Dave BessonBuford Price

ARENA Workshop, DESY-Zeuthen, May 19, 2005

Vandenbroucke et al ARENA Workshop May 20, 2005

The goal

• ~EeV neutrinos, particularly GZK neutrinos, could be a valuable source for astro- and particle physics

• IceCube or Auger could detect ~1 GZK neutrino per year, but• 10-100 GZK events (eg 10 yrs @ 10/yr) would give a quantitative

measurement including energy, angular, and temporal distributions allowing tests of cosmic ray production models and new physics [cross section measurements! See A. Connolly’s talk]

• Other projects (e.g. ANITA, SalSA, …) are actively seeking this goal. Should IceCube also seek it?

• If acoustic ice properties are measured to be as good as predicted [S. Boeser’s talk], proceed from a South Pole Acoustic Test Setup to a hybrid detector (IceCube + Acoustic + Radio EeV Neutrino Array)


Why a hybrid extension to IceCube (in addition to ANITA, SalSA et al)?

• Like Auger and detectors at accelerators, use >1 technique monitoring the same interaction region

• Difficult to reach 10 GZK events/yr with optical alone• No (?) scattering for radio and acoustic• At ~EeV, radio and acoustic methods could outdo optical• Detecting events in coincidence between 2-3 methods more

convincing than detections with one method alone• Coincident events allow calibration/cross-check of the radio and

acoustic methods with the optical method• Hybrid reconstruction gives superior energy and direction

resolution than with one method, or allows reconstruction of coincident events that cannot be reconstructed with one method alone

• Extended IceCube could be a sensitive neutrino telescope at all cosmic energies?

• [Halzen & Hooper “IceCube Plus” JCAP 01 (2004) 002]


EeV fluxes

• Z-burst and topological defect models predict large EeV fluxes but are observationally disfavored

• The GZK flux is a fairly conservative EeV source• Optimize the hybrid detector for a high rate of events from

the Engel, Seckel, Stanev (ESS) GZK flux model, but• Do not only seek GZK events. Measure whatever is there at

~EeV and design to detect events over a wide energy range• Then the IceCube Observatory measures the neutrino

spectrum over ~10 orders of magnitude!


The ESS GZK flux model

zmax = 8, n = 3

Unclear which to use (unclear effect on star formation rate) For now use the lower rate

Log(Ethr/eV) ~Veff for 1 evt/yr

16 7

17 8

18 14

19 50


Simulation of hybrid-detector GZK event rate (first pass: keep it simple)

• Assume exactly the 2 downgoing neutrinos make it to the detector, independent of energy, within our 1016 - 1020 eV range

• For radio and acoustic: assume the LPM effect completely washes out signal from EM component of e CC events, so

• For all flavors and both CC and NC we detect only the hadronic shower, with

• Esh = 0.2E for all events, independent of energy

• Generate incident directions uniformly in downward

2, and vertices uniformly in a fiducial cylinder• At each of a set of discrete energies, expose each of the 3

detector components to the same set of Monte Carlo events


An example hybrid array

Optical: 80 IceCube + 13 IceCube-Plus holes at a 1 km radius

Radio/Acoustic: 91 holes, 1 km spacing; ~5 radio + ~200 acoustic receivers per hole


Optical simulation

• Check Halzen & Hooper’s rate estimate with standard simulation tools; run a common event set through optical, radio, and acoustic simulations

• For now, only simulate the muon channel (showers in progress)

• Use standard AMANDA simulation tools: muon propagation, ice properties, detector response

• Define a coincidence to be hits at 2 out of 5 neighboring modules on one string within 1000 ns

• Require 10 coincidences in the entire array within 2.5 s• For optical-only events, require > 182 channels hit (a muon

energy cut proxy) to reject atmospheric background• Do not apply Nch requirement when seeking coincidence

with radio or acoustic


Radio simulationUsing RICE MC - see D. Besson’s talk

• Dipole antennas in pairs to resolve up-down ambiguity• 30% bandwidth, center frequency = 300 MHz in air• Effective height = length/• Radio absorption model: based on measurements by

Besson, Barwick, & Gorham (accepted by J. Glac.)• Trigger: require 3 pairs in coincidence• Use full radio MC


Interlude

Reminder: Signal ~10x higher than water [P.B. Price]Noise >10x lower? [limited by sensor self-noise, not ambient?]

Notes on acoustic neutrino detection in ice


Firn (uncompactified snow) in top 200 m: Vsound increasing with density refraction. Rcurvature ~200 m!

Sound velocity profile in South Pole ice

measured in firn (J. Weihaupt)

predicted in bulk (using IceCube-measured temperature profile and A. Gow temperature

coefficient) - measure with SPATS?

Sound channel

ridge


10 m depth:Only downward ~40° penetrate

1 m depth:Only downward ~10° penetrate

Acoustic ray traces

source in firn

source in bulk


Strong refraction in firnAcoustic: upward Radio: downward [D. Besson]

Signals always bend toward minimum propagation speed, but:Sound abhors vacuum [c =0]

Radio adores vacuum [c = 3e8 m/s]


Predicted depth (temperature)-dependent acoustic absorption at ~10 kHz

In simulation, integrate over

absorption from source to receiver

See P.B. Price’s talk: absorption frequency-independent but

temperature (depth)-dependent


Acoustic detection contours in ice

Contours for Pthr = 9 mPa:

raw discriminator, no filter


Acoustic event rate depends on threshold (noise level) and hole spacing

RMS Noise, (mPa)

Hole spacing, km (91 string hexagonal array)

0.25 0.5 1 2

15 1.7 2.6 4.5 4.0

6 3.6 5.5 9.6 9.1

3 5.6 8.6 15 15

Trigger: ≥ 3 strings hit

ESS GZK events per year:

Need low-noise sensors (DESY) and low-noise ice (South Pole?)

Frequency filtering may lower effective noise level

For hybrid MC, set threshold at 9 mPa = a few sigma


Acoustic neutrino direction and vertex reconstruction

- With 3 strings hit, it’s easy:

- Fit a plane to hit receivers.

- Upward normal points to neutrino source.

- Within that plane, only 2D vertex reconstruction is necessary, done by intersecting 2 hyperbola determined by 3 arrival times.


Acoustic angular resolutionResolution due to pancake thickness: expose array (0.5 km hole spacing) to isotropic 1019 eV flux, determine hit receiver, fit plane to hit receivers,

compare plane normal with true MC neutrino direction:

Result (not including noise hits):


Hybrid reconstruction• Typical UHE vertices are outside the optical detector - optical

might measure muon energy at detector but needs muon energy at vertex and doesn’t know the vertex

• Get the vertex from radio/acoustic shower detection. Combining them gives good energy and pointing resolution

• Very little radio or acoustic scattering - hits are always prompt and timing information straightforward

• So hybrid sets of 4 receivers hit (e.g. 3+1, 2+2, 2+1+1) may be sufficient for vertex reconstruction using time differences of arrival

• Different radiation patterns between the methods leads to non-degenerate hit geometry for good reconstruction

• Not a problem that timing resolutions are different:


Can we combine acoustic and radio timestamps on equal footing?

Problem: Acoustic timing resolution (pulse width) ~10 us.Radio ~ few nsCan we combine them for reconstruction?

Yes! [R. Porrata]: convert times to distances using respective signal speeds.

Then they have the same resolution and the analytical TDOA matrix equations (with SVD) can be used.

Verification with simulated hybrid event set in progress…


Optical, radio, acoustic independent effective volumes

Preliminary!


Coincident effective volumes

RA, AO, ORA curves in preparation

Preliminary!


Event ratesLog(E/eV) ESS Events per year with E > E

Optical (muons only) Radio Acoustic R-O hybrid

16.5 0.6 8.1 7.6 0.4

17.5 0.4 8.0 7.6 0.4

18.5 0.1 4.7 7.6 0.2

19.5 0.0 0.7 1.3 0

cf. Halzen & Hooper IceCube-Plus muon rate: 1.2

These results depend on a wide parameter space:

- Acoustic ice properties and noise level

- Optimizing the array (eg hierarchical spacing such as adding R/A receivers to the optical holes) could increase rates

- Adding the optical shower channel will increase rates.

First results are encouraging


O(91) radio/acoustic strings for a fraction of the IceCube cost?

• Holes: ~3 times smaller in diameter and ~1.5 km deep• Don LeBar (Ice Coring and Drilling Services) drilling estimate: $33k per

km hole length after $400k drill upgrade (cf. SalSA ~$600k/hole)• Sensors: simpler than PMT’s• Cables and DAQ: Only ~5 radio channels per string (optical fiber).

~200 acoustic modules per string, but:• Cable channel reduction: Send acoustic signals to local in-ice DAQ

module (eg 16 sensor modules per DAQ module) which builds triggers and sends to surface

• Acoustic bandwidth and timing requirements are easy (csound ~10-5 clight!)

• Acoustic data bandwidth per string = 0.1-1 Gbit, could fit on a single ethernet cable per string

capabilities of a hybrid optical-radio-acoustic neutrino detector at the south pole

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