02-07-2012: PhD defense Dimitris Palioselitis @ UvA

02 - 07 - 2012

Nikhef, 19-06-2012

Measurement of the atmospheric neutrino energy spectrum 

Dimitris Palioselitis, promovendus aan het Nikhef, verdedigt zijn proefschrift maandag 2 juli 2012 aan de Universiteit van Amsterdam.

Cosmic rays are highly energetic particles that arrive at our Earth from various astrophysical sources. The identification of these sources as well as the mechanisms that accelerate particles to such high energies are topics of great interest in modern day research. The topic of this work is the measurement of the atmospheric neutrino energy spectrum. Atmospheric neutrinos are produced in the interactions of cosmic ray particles with nuclei in the Earth’s atmosphere. The study of the atmospheric neutrino energy distribution can provide insight and valuable information on the high energy collision processes that take place in the atmosphere and are outside the energy reach of present day accelerators, as well as on the intensity of the cosmic ray spectrum. 

The ANTARES detector is a neutrino telescope, located at a depth of 2475 m on the bottom of the Mediterranean sea. It consists of 12 vertically positioned flexible strings, equipped with a total of 885 photomultiplier tubes that detect light produced by muons traversing the instrumented volume. These muons can be products of collisions of cosmic rays in the atmosphere, but more importantly, they can be the products of charged current neutrino interactions in the vicinity of the detector. The time, position and pulse height of the hits recorded on the photomultipliers provide the necessary information to reconstruct the direction of the muons with high precision. The muon direction is very close to the neutrino direction, therefore the ANTARES detector is primarily used as a telescope. 

Another important quantity characterizing each track, other than its direction, is its energy. This is especially relevant for the measurement described in this work.  A maximum likelihood based method was developed for the reconstruction of the muon energy.  A resolution better than 0.4 – 0.3 in the logarithm of the energy is achieved throughout the whole energy range we consider, namely from 100 GeV to a few hundreds of TeV.

The vast majority of detected events in ANTARES are due to atmospheric muons. For the work presented here, this is a background that needs to be suppressed. Since muons, in contrast to neutrinos, cannot traverse the Earth, a directional cut where only upward going events are selected reduces this background significantly.  A cut on the quality of the track-reconstruction suppresses the muon background to levels below 1%. 

The shape of the reconstructed muon energy distribution is different from the shape of the atmospheric neutrino energy spectrum at the surface of the Earth, which is the goal of this measurement. The reason for that is the limited energy resolution as well as the overall acceptance of the ANTARES detector. The transformation between the reconstructed muon energy distribution and the atmospheric neutrino energy distribution is described by a response matrix, determined from Monte Carlo simulations. 

A measurement of the atmospheric neutrino energy spectrum in the range of 102.5 GeV to 105.3 GeV has been performed. The result obtained is compatible with the theoretical predictions and the measurements performed by the AMANDA and IceCube collaborations. The systematic uncertainties of the measurement have been estimated and the largest contributions arise from uncertainties in light propagation in water and the efficiency of the photomultiplier tubes. The limited statistics at the highest energies also affect the final result uncertainties. The next generation of neutrino telescopes will provide tighter constraints in these uncertainties and a more precise determination of the atmospheric neutrino energy spectrum reaching much higher energies. 

Dimitris Palioselitis:
Measurement of the atmospheric neutrino energy spectrum (pdf)

De promotie vindt plaats op maandag 2 juli, om 14.00 in de Agnietenkapel van de Universiteit van Amsterdam, Oudezijds Voorburgwal 231.

Promotor: prof. dr. Paul M. Kooijman
Co-Promoter: dr. M. Patrick Decowski

Contact: Dimitris Palioselitis