Interferometry in Dense Media
with ARA
Carl Pfendner
Ohio State University
4/22/13 - 4/24/13
Interferometry Workshop
1
Outline
• Background
• Interferometry
• Preliminary analysis and results
4/22/13 - 4/24/13
Interferometry Workshop
2
Outline
• Background
• Interferometry
• Preliminary analysis and results
4/22/13 - 4/24/13
Interferometry Workshop
3
Cosmic Messengers
• Cosmic rays measured
over ~13 orders of
magnitude
• Gamma rays: Egret,
Fermi
• Only extraterrestrial
neutrinos:
plot by A. Connolly
adapted from Swordy CR plot
4/22/13 - 4/24/13
– Sun
– Supernova 1987a
Interferometry Workshop
4
GZK Process
• Greisen-Zatsepin-Kuzmin (GZK): Cosmic rays
> 1019.5 eV slowed by cosmic microwave
background (CMB) photons within ~50 Mpc:
ν’s from GZK
process first
pointed out by
Berezinsky and
Zatsepin (1969)
Neutrinos can also be produced in the sources
themselves through photohadronic interactions
4/22/13 - 4/24/13
Interferometry Workshop
5
UHE Neutrinos
•
•
-
• Cosmic ray data points to
cutoff at GZK threshold
• If this is indeed due to
GZK process, UHE
neutrinos have to be
there!
UHE neutrinos unique messengers to cosmos:
Travel cosmological distances unattenuated
No magnetic deflections - point back
Particle physics probes exceeding LHC energies:
1018 eV neutrino interaction → 45 TeV!
4/22/13 - 4/24/13
Interferometry Workshop
6
Need for large volume detectors
• Consider a few models for
neutrinos from GZK
• Consider Antarctic ice
• Fold in Earth shadowing,
νN cross sections σ(E)
• Less than:
1/km3/year/energy decade
A. Connolly
• Discovery experiments must
exceed Veff ~ O(1) km3
• UHE Observatory would ~1 interaction/second occurring
need ~O(100) km3
somewhere in Antarctic ice
# / km3 / year for a detector in Antarctic ice
4/22/13 - 4/24/13
Interferometry Workshop
7
Detection Techniques
• <1018 eV: visible dominates
current constraints
• >1018 eV: radio dominates
– Radio thresholds dropping with
experiments coming online
Cascades in
atmosphere
4/22/13 - 4/24/13
Interferometry Workshop
8
Radio Cerenkov Technique
(Radio Technique in Dense Media)
• Coherent Cerenkov
Idea by Gurgen
signal from net
Askaryan (1962)
“current,” instead of
from individual tracks
• A ~20% charge asymmetry develops (mainly
Compton scattering)
• Excess moving with v > c/n in matter
–→ Cherenkov Radiation dP ∝ ν dν
This effect has been
• If λ >> RMoliere → Coherent Emission
confirmed
P ~ N2 ~ E2
experimentally
PRL 86, 2802 (2002)
– λ > RMoliere → Radio/Microwave Emission PRD
72, 023002 (2005)
Macroscopic size: RMoliere ≈ 10 cm,
L ~ meters
4/22/13 - 4/24/13
Interferometry Workshop
PRD 74, 043002 (2006)
PRL 99, 171101 (2007)
9
Askaryan Radio Array (ARA)
• Array of antennas designed to detect UHE neutrinos using
radio Cherenkov technique (Askaryan effect) at South Pole
• Deployed a shallow testbed and 3 deep stations
– 16 borehole antennas / station at 200MHz to 800MHz
• 8 vertically polarized (Vpol), 8 horizontally polarized (Hpol)
– ARA2, ARA3 drilled to design depth of 200 m
• Proposed full 37-station array covering ~200 km2 area of ice
4/22/13 - 4/24/13
Interferometry Workshop
10
ARAARA
Testbed
Testbed Station
Station
• First prototype station
deployed Jan 2011
• Total 16 antennas, 8
borehole antennas at 150
MHz to 1 GHz
• Maximum depth of
antennas ~ 30 m
• 3 sets of calibration pulsers
– Each set has a Vpol and an
Hpol pulser
Calibration pulser event waveform
from 8 deep antennas in testbed
• First ARA neutrino search
being carried out with
testbed station data
4/22/13
- 4/24/13
Sunday, April 14, 13
4
Interferometry Workshop
11
Outline
• Background
• Interferometry
• Preliminary analysis and results
4/22/13 - 4/24/13
Interferometry Workshop
12
Importance of Event Position
• Want to trace events
back to a point in the
sky
– Source? Diffuse?
• Pointing direction of
incoming neutrino
needs
– Reconstruction
direction
– Polarization
• Rejection of known
sources and clusters of
events
4/22/13 - 4/24/13
Interferometry Workshop
13
Interferometry
• Antenna just receives the
radio signal
– cannot locate on its own the
direction of the signal
– Use correlation of signal
between pairs of antennas
• Signal travels different
lengths to reach different
antennas
– Different times of arrival
– Difference in timing of the
signals -> approximate location
– Akin to astrometry
4/22/13 - 4/24/13
Interferometry Workshop
d1
+∞
(f
d
g)(t) 2=
f ∗ (τ )g(t + τ )dτ
−∞
∆t =
(d2 − d1 )n
c
14
Interferometric Technique
•
•
•
•
•
•
•
Impulsive waveform – ~1-10 ns time scale
Correlation factor - Convolution of the two waveforms including a timing offset
Only Vpol-to-Vpol comparison and Hpol-to-Hpol comparison
Calculate timing delays for all angles of approach
Sample correlation plot at these delays
Many positions will produce the same timing delays for a pair of antennas
Solution: Use more antennas - Add up all the correlation values from all the
pairs of antennas
4/22/13 - 4/24/13
Interferometry Workshop
15
Noiseless simulated event
4/22/13 - 4/24/13
Generated maximum at θ
= -45°, φ = -60°
Interferometry Workshop
16
Dense Media
• Most interferometry performed in medium
where index of refraction of source and
detector are both ~1
– Air, vacuum
• With ARA, source and detector are located in
regions with different indices of refraction
– Calibration pulser is located close enough and at
the same depth so that these effects are minimal
– Further away, effects become more prominent
4/22/13 - 4/24/13
Interferometry Workshop
17
• Top layer of ice is densely
packed snow AKA firn
– Quickly changing n
down to 100-150 m
• Not directly measured at
all points
– Modeled with different
functions
• Need to get
measurements of this
profile
– Direct - drill to different
depths and get
measurements
– Indirect - Place pulsers
at depths at a distance
to get timing response
in detector - planned
4/22/13 - 4/24/13
Index of Refraction, n
Changing Index of Refraction
1.7
1.6
1.5
1.4
500
1000
1500
2000
Depth in Ice (m)
Models based on data collected by RICE
- different fits to this data
Interferometry Workshop
18
Ray Tracing
• Changing n -> Snell’s
law
– Rays bend
• Plane wave
approximation no
longer valid in all cases
• Areas are excluded
from ray tracing
– Increased effective
volume at greater
depth
• Ray tracing algorithm
developed by Chris
Weaver
4/22/13 - 4/24/13
Interferometry Workshop
19
Concerns for Reconstruction
• Anything that affects timing delays will
affect the correlation map
• The index of refraction of the ice
– The values themselves
– How they change in the ice
• Produces curvature in the path
• Cable delays - measure them
• Geometric assumptions - plane-wave vs
spherical vs other (ray tracing)
• Also noise over the signal can severely
wash out the correlation
4/22/13 - 4/24/13
Interferometry Workshop
20
Simulated event
Observed maximum at θ
= -42°, φ = -58°
4/22/13 - 4/24/13
Generated maximum at θ
= -45°, φ = -60°
Interferometry Workshop
21
Reconstructions Using Different Ice Models
• Center of radius shifted to
surface so that calpulser (θ, ϕ)
shifted
• Expected at:
Changing n
– (θ = -28.6°, ϕ = -93.6°)
• Changing n -> mostly
reconstructed at
– (θ = -26°, ϕ = -90°)
• Constant n ->
Constant n
– (θ = 2°, ϕ = -96°)
• Focuses better and onto
correct point
• But not perfect
4/22/13 - 4/24/13
Interferometry Workshop
22
Reconstruction of Calpulser Event
• Hpol only, 30m radius
• Maximum is at (θ = 3°, ϕ = -90°)
• Expected at (θ = 2.81°, ϕ = -93.6°)
7/2/2016
ARA Phone Call
Using ray tracing Get expected Δt from points at a set
radius from the center and plot
correlation values sampled as before
23
Zoomed Peak
Zoomed to
1/10 scale
• Poor reconstruction resolution – different fringes do not align “properly”
• Sources of systematic error:
– Cable delays, position information, index of refraction model
7/2/2016
ARA Phone Call
24
Additional Technique
• Developed by D. Besson at U.
Kansas
• Pseudo Χ2 – fit –
– difference between
expected and observed time
• uses point with maximum
correlation factor
– No ray tracing
– Assume either spherical or
plane-wave
• Goal: combine ray tracing
and fit algorithm
– Will ray tracing slow this
down too much to be
useful?
• Also fits radius rather than
trying to use a fixed value
– Currently has large error
4/22/13 - 4/24/13
Simple 2-D simulation of reconstruction fit
Empty circles = antennas, black circle = pulser location
Interferometry Workshop
25
Calibration Pulser: Reconstruction
Voltage (mV)
• Local Cal pulsers (~30 m away)
• Repetition rate 1 Hz
• For trigger monitoring, timing,
amplitude reference
• Amplitude gradient across array
• Uncertainties in delays:
Error on mean: 4.5 ps →
Array positions to 1mm
Standard dev. : 136 ps
→Origin of RF to 2.7 cm
-
4/22/13 - 4/24/13
Number of Events
Time (ns)
θ (reconstr.) - θ(pulser),
degrees
Jan 29th Event Reconstruction
very small fraction misreconstruct
in elevation
-1° < Φ <
1°
Zoom
Φ (reconstr.) - Φ (pulser),
degrees
Interferometry Workshop
26
Interferometry Plans
• Obtain pulser information at far distances and
differing depths - help characterize ice properties
• Improve ray tracing reconstruction method
– Combine with fitting method
– Use coherently summed waveform?
– Use Hilbert transform to get timing information?
– Use Gaussian envelope to account for smaller errors?
• Important to reconstruct position for directional
reconstruction and further information about
event
4/22/13 - 4/24/13
Interferometry Workshop
27
Outline
• Background
• Interferometry
• Preliminary analysis and results
– Eugene Hong recently presented these at the APS
Meeting in Denver
4/22/13 - 4/24/13
Interferometry Workshop
28
Strategy
• Searching for neutrino events with data from testbed February to June 2012
• Using blinding technique: 10% not blinded data to
characterize backgrounds
• Cut-based analysis guided by simulation and 10% not
blinded data set
• Will result in either neutrino events or neutrino limit
• Will analyze data from other deployed stations
building on experience from testbed analysis
Alternative analysis from Univ. of Kansas uses crosscorrelation technique to search for unique impulsive
events
4/22/13 - 4/24/13
Interferometry Workshop
29
ARA simulation - AraSim
• Simulates
full trigger
• Simulates
full
triggerchain
and signal
and signal
for
chainevents
for neutrino
neutrino
events
detected
bydetected
ARA by
ARA stations
stations
• Writes simulated
• Writes
simulated
events in data
events
in data format
format
4/22/13 - 4/24/13
Sunday, April 14, 13
Calibration pulser event waveform
Voltage (mV)
Voltage (mV)
• Official
Monte
• Official
Monte
Carlo
Carlo simulation
simulation for
for assessing
assessing
sensitivity
sensitivity
and for
and for
general
general
use use
AraSim
Testbed
Time (ns)
Time (ns)
VRMS distribution
Thermal
noise
calibration
in AraSim
6
Interferometry Workshop
Environmental
background
Testbed
AraSim
0
40
80
120 Voltage (mV)
30
AraSim Description
• Simulated full chain includes
– Parameterized Askaryan signal
by J. Alvarez-Muniz et al*
– Ray tracing in Antarctic ice
– Antenna properties
– Electronics chain (amplifier,
trigger, waveform readout)
* J. Alvarez-Muniz, R.A. Vazquez and E. Zas,
astro-ph/0003315
4/22/13 - 4/24/13
Interferometry Workshop
31
Backgrounds
• Thermal Noise
– Dominates triggered events
– Randomly triggered, no coincidence between channels
• Continuous Wave (CW)
– Anthropogenic noise, less than 3% livetime
– Dominated by one frequency
– Weather balloon (400 MHz, twice a day during Antarctic
summer)
– Airplane-ground communication (130 MHz, occasionally
during Antarctic summer)
• Anthropogenic impulsive background
– Signals come form same location repeatedly
4/22/13 - 4/24/13
Interferometry Workshop
32
Rejecting Thermal Noise Background
• Require two or more channels show impulsive signal
• Will optimize parameters for neutrino sensitivity with
AraSim and not blinded data subset
4/22/13 - 4/24/13
Interferometry Workshop
33
Reconstruction Direction Cuts
• Reject known
background
locations from
reconstruction
plot
• Current cut
locations :
Cal1
rejection
area
cal pulser 1 event 30m Hpol reconstruction
3000m Vpol reconstruction
– cal pulser
– south pole station
– unidentified
clustering
locations (1)
South Pole
Station area
Unidentified
clustering
location
15
4/22/13 - 4/24/13
Sunday, April 14, 13
Interferometry Workshop
34
Rejecting CW Background
Fraction of Events that Pass
1
0.8
0.6
0.4
Thermal
Noise
Events
Calpulser
Events
400 MHz
Balloon
Events
Cut Threshold (dB)
Baseline Average
for Example Run
3.5 dB above Baseline
400 MHz Balloon Event
200 400 600 800 1000
Frequency (MHz)
0 1 2 3 4 5 6
0
34
32
30
28
26
24
22
20
0
18
0.2
Voltage, dB (Arb. scale)
• Design cut based on ANITA experience
• Make average spectrum for each run (1 run = 18000 evts ~ 30 minutes)
• Reject events whose Fourier transformed voltage waveform exceeds 3.5 dB
above baseline anywhere in frequency space
• Will optimize the cut using simulation and not blinded data subset
4/22/13 - 4/24/13
Interferometry Workshop
35
First pass of neutrino search with Testbed 10% data set
Cuts are not yet optimized
• No events pass at first analysis with 10% not blinded
testbed data set.
4/22/13 - 4/24/13
Interferometry Workshop
36
Analysis Summary
• We are performing a neutrino search in the
ARA Testbed data set
• Analysis cut parameters will be optimized for
neutrino efficiency with calibrated AraSim
– We are working on calibrating AraSim to the
Testbed’s properties (trigger threshold, antenna
model)
• We will complete the neutrino search with the
full unblinded Testbed data set this year
4/22/13 - 4/24/13
Interferometry Workshop
37
Questions?
4/22/13 - 4/24/13
Interferometry Workshop
38
Deep Pulser: Radio Loss
• Observed signals 3.2 km distance
at 40° from vertical
• Largest horiz. component yet for an
in-ice radio loss measurement
• Received, transmitted powers Pr,
Pt, effective areas Ar, At, distance R,
frequency ν
• Antenna responses, insertion
losses, beam patterns, first order
estimate of partial saturation,
average two analyses
• Deconvolve depth dependence
based on ice temp profile
0.2 km < depth <1.5 km:
300 MHz
Reduced
density at the
surface
Errors dominated by received
amplitude and pulse shape
Radio Interference
• Yearly average livetime
loss ~3% at most
30
Trigger Rate (Hz)
– Meteorological balloon
launches @ 400 MHz (2x
daily until early March
launch, then once daily)
– Incoming, departing
aircraft @ 129.3 MHz
Trigger Rate (Hz)
• Higher levels of sporadic
interference during
summer, as expected
• Daily ~1/2 hour periods
where interference from
a source is strong
enough to interfere
operations
20
10
0
Decline of ambient trigger rate due to
temperature dependent thresholds
Hz
• Thermal noise
Outside temp
Testbed temp
Rate
Day of Year
fluctuations trigger the •
system during quiet
periods at 0.5-1 Hz
•
• No correlation with
wind speed
Average Noise Power (dBm/Hz)
Max wind speed
°C
°C m/s
Thermal Noise Rates
Borehole Antenna
130-850 MHz passband
notch filter
Surface Antenna
300 MHz lowpass
notch filter
Frequency (MHz)
Average power ~ -173.5 dBm (room
temp: -174 dBm)
Measured antenna temperatures
consistent with ambient noise temps
within 10% errors