Science Results
AMANDA has been operational since January 1997.  We summarize a few important published results (along with a few preliminary and status results from the 10 string and AMANDA-II detectors) for the science topics listed below:
  Note: by clicking on the figures, you can get higher resolution images.

Ice Properties
The optical properties of in situ ice beneath the south pole are measured by a combination of in situ lasers and light sources, 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 next figure shows the average scattering length is shown 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 wavelength of light used for this study was 532nm (the wavelength of light from a frequency doubled YAG).   On the left side of the same figure, the horizontal projection of the OM locations for the various strings are shown. The OMs in the yellow and  green color bands are in the best ice for our purpose.
Optical scattering properties versus depth, at 532nm


Atmospheric Neutrinos
The atmospheric neutrino analysis concentrated on signal purity. The distribution show on the right is from our 1997 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 inspect, the events topology is consistent with upgoing events.
 
Atm nu sky map

Here is the RA-Dec plot of atm nu events for B10 data .  There are ~300 events in the sample

The zenith angle distribution is shown next.  Horizontal events have cos(zenith)=0, and upward vertical events have cos(zenith)= -1. Note that AMANDA has very little sensitivity to neutrino oscillations due to its relatively large energy threshold, although we have included such effects in the predicted flux.zenith angle distribution

 
 

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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 soft 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.  The AMANDA-B10 detector achieves 10,000 m2 for declinations greater than 30 degrees if the muon energy at the detector is greater than 1 TeV.
 
 
This figure shows the space angle resolution and the deduced point spread function. The angular resolution and absolute pointing were checked by using events that struck both the SPASE air shower array and AMANDA-B10 array.  The point spread function is fit well by a function composed of the sum of two gaussians.  The tail is partially due to high energy muons that are not well reconstructed.angular resolution of AMANDA-B10 Effective area of Am-B10 detector
This is the effective area of the detector for muons that have energies of 1, 10, and 100 TeV.  This shows that AMANDA B10 reaches an effective area of 10,000 m2 for 1 TeV muons. 

 
 
flux limits of muons induced by neutrinos using pt source analysis

This figure provides the average muon flux limit as a function of declination.  The solid black line includes the impact of systematic uncertainty in the calculations (as illustrated by the various symbols in the legend), and represents the final result of the B10 analysis using data from 1997.

comparison of B10 results with other detectors
This figure compares the B10 muon flux limits as a function of declination (based on only 1 year of data collected during 1997, which corresponds to less than 1/3 of a year of live time due to delays associated with commissioning) with limits presented by MACRO and Super Kamiokanda collaborations.  The solid red line is the same as the solid black line of the left figure.  The dashed curves correspond azimuthal variation in the flux limits due to the bin-to-bin statistical variation for the same declination band.

 

The next figure shows the neutrino flux limits from the 1997 data analysis and compares to a representative set of models (selected to illustrate the variety of spectral shapes).  Also, the expected sensitivity of AMANDA-II and IceCube are shown.
neutrino flux limits for point sources
The AMANDA-B10 result is true for declinations greater than +40 degrees and includes the impact of systematic uncertainty.  The expected sensitivity for AMANDA-II is shown assuming that the complete data on tape is analyzed.
 
 
AMANDA-II Point Search:   The figure 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 is still in progress so we remain "blind" to the true azimuthal (or right ascension, RA) distribution of events to insure that human expectation does not bias the analysis.  So the events times are ignored, which effectively scrambles the right ascension of the event.

A total of 1129 events are shown in the figure of equatorial coordinates.  The RA coordinates are scrambled.  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.

AMANDA II sky map


GRBs

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.  AMANDA-B10 has reported limits for GRBs that were observed in 1997.
 
Simulation of GRB explosion
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.
B10 GRB flux limit


Diffuse Flux

The most sensitive mode to search for diffuse flux of HE neutrinos uses the muon signals.
 

The current (July 2002) limits are shown in the next figure for AMANDA-B10 and Baikal. Note that the AMANDA limits are not yet published and should be considered preliminary.  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.
Summary of diffuse flux predictions and experimental limits
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 therefore prompt MUONs become a significant background. The importance of the background depends on the magnitude of the flux and detector energy and angular resolution. From the figure, the minimum flux levels of AMANDA-II are not impacted by charm contributions for the most of the current models of charm production in the atmosphere.


Cascades
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-B10 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 are 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.  The search for a diffuse astronomical source was performed using data from 1997 collected by AMANDA-B10.
 
Cascade limit from AMANDA-B10
Figure:  Diffuse flux limits on the sum of all neutrino flavors as derived from AMANDA-B10 using muons (lower blue) and cascades (lower red).  These limits were derived assuming neutrinos oscillate so the flux of all flavors is equal. 


EHE Physics

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.

Experimentally, neutrinos 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.  The effective detection area for muons at these energies is very large, typically 0.2 km2 for AMANDA-B10!  We show preliminary results from our study of AMANDA-B10 data from 1997.
 
EHE diffuse flux limit This limit was derived using only 3 months of data from 1997 (the remaining data 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 maximum energy is about 1018 eV, but since few EHE models predict E-2, the horizontal extent of the red line indicates the complete interval of sensitivity that was studied.  It indicates that the lower energy threshold with this technique is about 1 PeV. The maximum energy is a consequence of limitations in the muon propagation programs (now corrected), not a limitation with AMANDA. 

The effective detection area of AMANDA-II is even larger than B10.  In addition,
we are upgrading the data acquisition system of AMANDA-II.  The system will record the complete waveform from all the OM 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 limit figure.


WIMPs
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 with each other.  Their annihilation produces high energy neutrinos (among others).  This
 
Indirect detection of Solar WIMPs
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 detector.
Earth WIMP limits by AMANDA-B10
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.


Supernova Monitor

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 OM.
 
 
SNa reach of AMANDA 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. 


Monopoles
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 is that the mass of the monopole must be large to possess enough kinetic energy to pass through the earth.
limit on flux of magnetic monopoles


Miscellaneous
Nothing yet.


last modified July 25, 2002
Steve Barwick