Properties of Antarctic Ice
|The optical properties of in situ ice
beneath the south pole are measured by a combination of in situ
N2 lasers, DC lamps, and YAG laser pulses from the surface.
The properties vary with depth due to climatological variation
such as ice ages. We have not measured the scattering length
and absorption length over all relevant optical wavelengths, so
they are inferred by a model developed by P. Buford Price (UC-Berkeley)
and his collaborators. The two properties that most strongly
affect the reconstruction capabilities of AMANDA-II are absorption
right figure shows the absorption length as a function of depth.
The bulk of the scientifically-useful optical sensors in AMANDA
are embedded between 1500 and 1900 m beneath the surface.
strongly depends on wavelength. Notice that the absorption also
depends on depth at wavelengths where the absorption coefficient
is relatively small. For short wavelengths, the absorption coefficient
is small and dust contributes significantly, which is responsible
for the depth dependence. At 532nm, the absorption coefficient
is large, and the value is largely determined by intrinsic properties
of ice (i.e, the roll of dust is less obvious).
left figure shows the average scattering coefficent (1/scattering_length)
as a function of depth. Note that the effective scattering
length, L_eff, is (approximately) the average length to isotropize
the direction of all but 1/e of the photons. This important
parameter for diffusion calculations is related to the geometric
scattering length by L_eff=L_geo/(1-<cos(angle)>). The
solid curve shows the coefficient of the scattering length
for 400 nm light. Other colors behave differently due to the
slight dependence of the scattering length on wavelength.
depths below 1400m, dust is responsible for light scattering
in ice. The rapid rise in scattering at shallow depths (relative
to the surface) is due to onset of air bubbles trapped in
the ice. The dashed blue line shows the intrinsic scattering
from dust in the region dominated by air bubbles.
|To summarize, the maximum absorption length is slightly more
than 100m at AMANDA-II depths, but the scattering
length is only 20m for wavelengths that correspond
to the longest absorption lengths.
Neutrinos in AMANDA-II
neutrinos measured by AMANDA-B10
|The atmospheric neutrino analysis concentrates on signal purity.
The distribution show on the right is from our 2000 data sample.
It is consistent with expectations from atmospheric neutrinos
because 1) the observed events are distributed approximately
isotropically, 2) the distribution of the number of OMs participating
in the event (which is correlated with energy) is consistent
with a soft spectra, 3) the shape of the zenith angle distribution
of events is consistent with expectation (see figure for Diffuse
Flux), 4) the absolute number of events is within 30% of expectation,
consistent with systematic uncertainty of the predictions, and
5) upon visual inspection, the events topology is consistent
with upgoing neutrino events. Recently, we have developed an
accurate energy estimator, allowing us to plot the differential
flux of the sum of atmospheric muon neutrinos and anti-neutrinos.
figure below shows the differential flux of atmospheric neutrinos.
The enormous effective volume allows AMANDA-II to measure the
flux to much higher energies than any previous detector. The
solid black lines show the theoretical expectation for both
horizontal (upper) and vertical (lower) orientations.
The zenith angle distribution
is compared with MC predictions in the figure below.
Horizontal events have sin(declination)=0, and upward vertical
events have sin(declination)= +1. AMANDA-II has vastly improved
sensitivity near the horizon compared to AMANDA-B10 due to
the fact that it is much wider. Note that AMANDA-II has very
little sensitivity to neutrino oscillations due to its relatively
large energy threshold, although we have included such effects
in the predicted flux.
Source Search using AMANDA-II
The search for point sources
The point source analysis optimizes the selection
criteria on hard spectra (differential energy spectra proportionally
to E-2), although it has reasonable sensitivity to
softer spectra. The critical features of the point source analysis:
demonstrate good angular resolution and absolute pointing and
maintain good effective size for as much of the sky as possible.
For the point source analysis, the AMANDA-II detector achieves:
~25,000 m^2 if the muon energy at the
detector is greater than 10 TeV.
space angle resolution of 2 deg (median)
Neutrino flux sensitivity of ~0.22 x10^-7
achieved by the AMANDA-II point source analysis. The plot shows
90% CL upper limits (averaged over right ascension) for a source
with an assumed differential spectrum of E^-2. The neutrino
signal is then integrated for energies above 10 GeV to determine
the limit on the neutrino flux. The units for the vertical scale
are 10-7 cm-2s-1.
limits on muon fluxes from high energy neutrinos, and projected
sensitivity for AMANDA-II assuming data is analyzed from several
years of operation.
AMANDA-II sky map:
The left figure below, first
presented at scientific conferences this spring (03), below
shows the neutrino sky as seen by AMANDA-II using data from
just the first year of operation (Feb -Oct of 2000). The point
source analysis was developed by randomizing the true azimuthal
(or right ascension, RA) distribution of events to insure that
human expectation does not bias the analysis. Nearly all
events in the northern sky are compatible with atmospheric neutrinos
(plus a small admixture poorly reconstructed atmospheric muons).
While the angular distribution of this data reveals NO evidence
for extraterrestrial neutrino sources, it provides important
constraints on theoretical models.
of 1555 events are shown in the figure to the left (in
equatorial coordinates). Compared to the previous
sky map from AMANDA-B10, the coverage near the horizon
(declination = 0 degrees) is markedly improved. The angular
resolution of AMANDA-II is much improved so the angular
dimensions of a search bin are likely to be reduced to
6x6 square degrees. The downgoing atmospheric muon background
is responsible for the thick band of events below the
horizon. Less than 3% of the events above a declination
of 5 degrees are due to atmospheric muon contamination.
angular distribution of events in the preceeding skyplot
was examined for statistically significant excesses. None
were found, leading to preliminary limits shown in the
contour plot and reported in the table below.
The units of the color legend are 10-7
cm-2s-1.The horizontal units are hours of right ascension
and vertical units are degrees of declination.
above figure, we show the differential flux limit for
two different assumptions of the spectral index, alpha.
We compare this result to AMANDA-B10 and to the average
gamma ray flux from Mrk 501 as observed in 1997, a period
of high activity. Also shown is the intrinsic source flux
after correction for IR absorption (de Jager and Stecker).
Although Mrk 501 was not very active
in 2000, the figure suggests that AMANDA-II has achieved
the requisite sensitivity to observe neutrinos during
the active period if the neutrino to photon ratio is one.
|The following table provides
upper limits for a selection of candidate sources, and compares
the results to limits previously published for AMANDA-B10. The
units for columns 3 and 4 are 10-8 cm^(-2)s^(-1).
||For a more complete list of sources and flux
limits, click here.
|Gamma Ray Bursts (or GRBs) are
the most spectacular and powerful explosions in the sky. Their
duration lasts from milliseconds to hundreds of seconds, and they
tend to cluster into two distinct populations. They are
also extremely distant, with redshifts exceeding Z=1, although
most of what is known about distances is deduced from the longer
duration GRBs. We search for correlation between the GRB event
time and location provided by gamma ray satellites. Unfortunately,
one of the most powerful detectors (called BATSE on the Compton
Gamma Ray Observatory) ceased operation in May of 2000.
To increase our search exposure, we have examined BATSE bursts
that were uncovered offline by additional analysis (termed "non-triggered"
bursts) and GRBs identified by the Interplanetary Network (IPN)
Simulation of GRB explosion
which shows strong jet features (dark) and predicts strong relativistic
flows. High energy neutrino production may occur along
the barrel of the jets. |
The most critical
feature of GRB analysis is the very modest background rejection
requirements. GRB emit gamma radiation over short duration,
typically less than 100s. Satellites provide time and direction
information so the analysis needs to reject only non-neutrino
events that happen to occur at the right time and reconstruct
with the right direction. The upshot: the effective size of
AMANDA-II for GRB analysis approaches 50,000 m2 for muons created
by neutrinos. Note that the figure does not include the effects
of attenuation by the earth.
table below summarizes the number of GRB sources observed by
AMANDA (as of Aug 03), the observed number of signal events
(5th column), the expected background (4th column), and the
muon flux upper limit (90% CL).
result is clear, and widely reported, - neither AMANDA-B10 nor
AMANDA-II observes correlated emission of high energy neutrinos
from any burst sample.
the interpretation of this result in the context of models is
nontrivial. We are consulting with scientists worldwide to provide
input on how we can best report our results.
From the last row in the table above, we derive a
preliminary neutrino flux limit of E^2*(dN/dE)
< 4x10^-8 GeV/cm^2/s/sr, assuming a broken
power law spectral form. More generically, we follow the method outlined
by SuperKamiokande to derive an energy dependent Greens Function for
the fluence limit, shown next. The turn-up at high energies is due
to the attenuation of the neutrino flux by the earth.
The most sensitive mode to search for diffuse
flux of HE neutrinos uses the muon signals.
(July 2003) limits are shown in the next figure for AMANDA-B10
and Baikal. Note that the AMANDA limits are published.
Systematic studies have not yet been fully completed.
AMANDA-II and IceCube are also shown. The curve labeled
(down) utilizes the EHE analysis to search for neutrino-induced
muons in the downgoing direction (from above the local horizon
of the detector). The sensitivity of AMANDA of AMANDA-II is
sufficient to search for sources below the "evolved" Waxman-Bahcall
limit of ~5x10-8 GeV/cm2/s/sr.
The main background to the upgoing
muon flux is atmospheric neutrinos generated by cosmic ray collisions.
At high energy energies, direct production by charm decay becomes
important, although the predicted fluxes have large uncertainty.
As the neutrino energies increase beyond 10^7 GeV, most of the
signal comes from above or very near the horizon and perhaps
prompt MUONs from charm quark decay become a significant background.
The importance of the background depends on the magnitude of
the flux and detector energy and angular resolution. Fortunately,
the sensitivity of AMANDA-II is not impacted by charm contributions
for the most of the favored models of charm production in the
The figure above
and to the right are taken from our paper
published in PRL. Nch refers to the number of modules that
participate in a given event, and crudely related to total energy
of the particle.
Detection by AMANDA-II
High energy neutrinos may interact to produce a large cascade of particles.
In this case, the production of Cherenkov light remains localized
and the photons propagation radially outward (well, almost).
The effective volume of AMANDA is much smaller for cascades than muons
and the angular resolution is very poor, but there are several interesting
features of cascades that make them useful to study. First,
the energy resolution of the cascade event can be measured with much
better precision relative to the muon signature if the vertex of the
interaction is contained. Second, the backgrounds from atmospheric
electron neutrinos is much smaller than muon neutrinos at these energies
because the decay of atmospheric muons is suppressed by time dilation.
Third, cascades are produced by electron neutrinos and tau neutrinos
so the ratio of cascades to muon neutrino events provides insight
on the properties of neutrino oscillation. Fourth, even downgoing
neutrinos can be observed if the energy is larger than ~10 TeV. Thus,
the cascade technique is sensitive to neutrinos from any direction.
The search for a diffuse astronomical source was
performed using data from 1997 collected by AMANDA-B10 and 2000, which
was collected by AMANDA-II.
events as a function of reconstructed energy. Experimental data
is compared to background predictions (Background MC) and signal
expected for a diffuse source emitting electron neutrinos with
a differential energy spectrum proportional to E^(-2).
flux limits at the source on the sum of all neutrino flavors
as derived from AMANDA-B10 using muons (lower blue dotted line)
and cascades (lower red). These limits were derived assuming
neutrinos oscillate so the flux of all flavors is equal by the
time they reach the earth.
Correcting for neutrino oscillations increases
the deduced flux limits for the muon channel. This is the appropriate
value to constrain model predictions.
Like with accelerator physics,
most of the interest neutrino astrophysics is at the extreme energy
frontier, which we label "extreme high energy" or EHE. Perhaps the
most reliable flux predictions (after atmospheric neutrinos) involve
the GZK mechanism. Neutrinos are produced by the inevitable
collisions between cosmic rays and the cosmic microwave background.
Although the physics required by GZK mechanism is straightforward,
the GZK mechanism is still in doubt because one of its predictions
has not yet been confirmed. The GZK mechanism predicts are rather
strong upper limit on the energy of the cosmic ray, but this upper
limit has not yet been clearly identified.
Perhaps the most intriguing
aspect of neutrino physics at extreme energies is the potential to
study fundamental theories of particle physics. For example,
Jonathan Feng (UCI) and his colleagues predict that extremely energetic
neutrinos create micro-black holes as the collide with nuclei of atoms
in the earth, if certain models of strong gravity are correct.
at EHE energies are difficult to detect. As the neutrino energies
increase to 1 PeV (1015 eV), the earth becomes opaque except
near the horizon. We have developed a new technique to search
for "downgoing" nearly horizontal muons. They can be distinguished
from the blizzard of downgoing muons from cosmic ray collisions because
the energies are much higher than muons generated by cosmic ray collisions. Consequently,
the topologies of the events (see next figure on the left) are quite
different from the typical event. Notice the large number of OMs that
observe Cherenkov light. The effective detection area for muons at
these energies is very large, typically 0.2 km2 for AMANDA-B10!
In the next few figures, we show preliminary results from our study
of AMANDA-B10 data from 1997.
Using data from 1997, the
preliminary limit (90% CL) on the differential flux proportional to
x 10-7 GeV/cm2/s/sr.
was derived using only 3 months of data from 1997 (the remaining
data from that year was not suitable for this technique).
The horizontal extent of the black experimental limit indicates
the energy interval that contains ~90% of the detected events
if you assume a differential energy spectrum proportional to
E-2. For E-2, the energy interval
is between about 2x1015 eV and 5x1018
eV, but since few EHE models predict E-2, we studied
a much larger energy interval.
The effective detection
area of AMANDA-II is even larger than B10. In addition,
we upgraded the data acquisition system of AMANDA-II.
The new system records the complete waveform from all (usable)
OMs in the array. This should dramatically improve the
dynamic range of the photon measurement and allow far better
energy reconstruction for these high energy events. The expected
sensitivity of AMANDA-II is shown on the diffuse
The EHE analysis
relies on training a neural net (NN2) to separate background
from signal. As the above figure shows, the data agree with
background expectations (using the program CORSIKA to generate
atmospheric particles created by cosmic ray collisions. There
is no evidence for an astrophysical source of high energy neutrinos.
A class of dark matter candidates are hypothetical particles known
Weakly Interacting Massive Particles (WIMPs). One of the
mostly widely studied WIMP is provided by an extension to the standard
model of particle physics known as Supersymmetry (or SUSY to its friends).
If supersymmetry ideas are correct, then the lightest stable supersymmetric
particle could be the dark matter. AMANDA can search for WIMP dark
matter indirectly by searching for high energy neutrino emission from
the core of the earth or the sun. The idea is that dark matter
particles would occasionally inelastically collide with atoms and
lose enough energy to become gravitationally bound to the sun or earth.
Eventually, the WIMPs spiral down to the core. As the density
of dark matter WIMPs increases in the core, they begin to interact
and annihilate with each other. Their annihilation produces
high energy neutrinos (among other particles).
This graphic (from Joakim Edsjo)
shows the capture of dark matter WIMPs by the sun, but a similar
picture applies to capture of the WIMPs by the core of the earth.
The annihilations of WIMPs in the core produce muon neutrinos,
and they can be observed by AMANDA if they interact near the
Figure shows the AMANDA-B10
limit (solid black line) deduced by searching for high energy
neutrinos from the center of the earth. Other experimental
limits are shown for comparison. For this science, all experimental
techniques are limited by irreducible background from atmospheric
neutrinos. The green dots represent SUSY models that are
already excluded by direct search techniques.
We are just beginning to search for WIMPs from the
sun. It is relatively difficult for AMANDA because the sun remains
near the horizon where most of the residual background events appear
to come from. Therefore, we expect the first substantial result
from AMANDA-II due to its superior background rejection near the horizon.
The extremely low ambient photon
flux in deep ice provides the opportunity to monitor the galaxy for
supernova explosions. Supernova events are expected to generate
neutrinos at low energies (< 20 MeV), nominally too low of an energy
to trigger AMANDA electronics. However, a nearby supernova blast
would generate so many neutrinos that enough of them would interact
within 10m of each AMANDA OM and produce Cherenkov light. The
extra photons could contribute the average "noise" rate from each
OM. By summing the signals from each OM, a statistically significant
signature of a supernova can be obtained. We have installed
special electronics to read out and sum the "noise" rates from each
||AMANDA-B10 can monitor about 68% of the stars in our galaxy,
and AMANDA-II can reach 95% of the stars in our galaxy.
The AMANDA collaboration is working with SNEWS (supernova early
warning system) members to provide timing information to help
pinpoint the position of the supernova by triangulation.
By virtue of it large volume, AMANDA can search for relativistic magnetic
monopoles with unprecedented sensitivity. Our technique relies
on fact that the equivalent charge of a magnetic monopole is 68.5,
and since the amount of Cherenkov light depends on the square of the
charge, the light produced by monopoles is prodigious. By constraining
the search to monopoles that pass through the earth, we simplify the
analysis. The downside of this restriction is that the mass
of the monopole must be large to possess enough kinetic energy to
pass through the earth.
Atmospheric muons constitute the dominant type
of events that trigger the AMANDA telescope. Recently, we have used
this data to deduce the muon flux as a function of (slant) depth.
The predicted (Monte Carlo) and observed atmospheric muon
flux agree to within 30%, which is consistent with the uncertainties
in the prediction. To simplify the comparison of spectra shape,
the AMANDA-II data in the figure on the left has been normalized
to the most vertical point (smallest depth) of the Monte Carlo
The depth dependence also agrees with the predictions of
Bugaev at al. and previous experimental results.
AMANDA: 2nd Generation DAQ
Last season (January, 2003), the AMANDA collaboration
initiated a major upgrade to the data acquisition system. The new
system records the complete waveform produced by every OM in AMANDA-II.
The figure on the left shows the electronics and the figure on the
right provides one example of a waveform. We expect that the new capabilities
will greatly improve the sensitivity of AMANDA, especially to neutrinos
at the highest energies.
Summary of new capabilities and features of the TWR
Full waveform records of
Deadtimeless operation up
to trigger rates of ~175 Hz ( contrast to 15% deadtime
at 90 Hz with 1st generation DAQ)
dynamic range to 5000 photoelectrons (nearly
a factor of 100 improvement)
for multi-level triggering, including software triggers
joint trigger with initial deployment phase of IceCube strings
The waveform above
shows several distinct pulses. To reduce the data traffic, the
AMANDA team had to develop specialized compression procedures.
The horizontal flat lines indicate regions where the waveform
has been cut out, leaving only the waveform fragments of interest.
|The figure on the right indicates that the new electronics
can measure the number of photons at each OM with much better
accuracy than before. The good agreement between the light blue
squares and dark blue stars indicates that AMANDA can measure
as many as 5000 photoelectrons in each module, nearly a factor
of 100 better than achieved with the original DAQ system.