Albrecht Karle: IceCube Results
Can neutrinos reveal the origin of cosmic rays ? The origin is still not understood a 100 years after their discovery. Primary candidates are e.g. SN remnants and gamma-ray bursts.
The galactic neutrino flux is two orders of magnitude below the atmospheric neutrino component. The cosmogenic neutrino flux is at very high energy. ICECUBE can study the range of energy between 10^4 and 10^9 GeV.
IceCube is composed by a total of 86 strings and 162 ice-top tanks. The
completion of the strings was done in deceomber 2010, and the full operation
is ongoing since May 2011. It includes Amanda and DeepCore as smaller-scale
projects, the latter aiming at the low-energy part of the spectrum.
There have been 20 years of construction at the south pole from Amanda to IceCube, with a break in 2001-2003. The number of neutrinos per day is now of over 200. The resolution in angle has dropped from 4 to 0.4 degrees for 100 TeV neutrinos.
Two scientists stay over the winter. The up-time of the experiment is quite high, above 99%.
Detection methods in IceCube depend on the topology: up-going muons, cascades, and down-going muons. The noise rate is of about 500 Hz.
They are working in order to understand the detector. The ice works well, but it is important to model precisely the material in order to reduce the systematic errors. Absorption depends on depth due to difefrent ice transparency.
The speaker showed the reconstructed moon shadow, from a deficit of 8700 events, showing a 0.7 degree resolution in angle. The absolute pointing resolution, accounting for magnetic deflection effects, is below a tenth of a degree. The energy resolution is limited by fluctuation in the energy deposition.
For Supernova detection the principle is to monitor the PMT count rate. Bursts from SN appear as a spike in the time profile. Their baseline method is to use the count rates binned in 2ms intervals. They keep all recorded PMT signals in a memory buffer for several hours, to search for SN signals and keep all the relevant information until screened.
They have a measurement of the flux of atmospheric neutrinos, a point source search performed with >100,000 upgoing events. They also searched for gamma-ray bursts, finding no signal and significantly constraining theoretical models. This was published in Nature earlier this year.
Different searches are going on at high energy. A diffuse neutrino flux is searched in up-going events. This is consistent with zero events from this component.
THe brightest events give information on a possible cosmogenic neutrino source. These are background-free. Two events are found above 1 PeV. These have tens of thousands of photoelectrons detected. The energy resolution at this energy is of about 10%. These events can be compared to different models. Backgrounds are expected to amount to 0.082+-0.004. The cosmogenic signal would give more events at still higher energy. The situation is not clear, these are not background events because their signature is quite clean. The speaker discussed some parallel analysis to study muon events with contained vertices,
A strategy suggested years ago was to search for muons together with the high-energy deposit in the detector, because atmospheric neutrinos must be accompanied by a muon. So by vetoing events with a contained vertex at high energy one can reduce backgrounds from above.
In summary, IceCube set competitive constraints on wimps, did not find anything significant in point source searches, and found two PeV events as a background to GZK searches, and it is not clear what these are, but they do not look like atmospheric neutrinos. Veto techniques are becoming important in many of the searches.