Nationaal instituut voor subatomaire fysica


27-05-2011: PhD defense Eleonora Presani @ UvA

Nikhef, 18-05-2011

Neutrino Induced Showers From Gamma-Ray Bursts

Eleonora Presani, promovenda aan het Nikhef, verdedigt haar proefschrift vrijdag 27 mei 2011 aan de Universiteit van Amsterdam.

The seek for understanding of our Universe has brought humans, since their early history, to look at the sky and study the phenomena that take part in the Cosmo.
After studying for centuries the relative movements of the Sun and the stars we now want to learn about the physics of the very core of these objects. If much has been learned during the centuries, much is still to be understood.
For example, the origin of Cosmic Rays, charged particles that travel across the whole Universe to reach our planet with extremely high energies. Their composition and their acceleration mechanism is still to be completely explained. Among the candidates as sources of Cosmic Rays are the Active Galactic Nuclei (AGN) and the Gamma Ray Bursts (GRBs). AGNs are the central zone of some galaxies, extremely compact and actively emitting radiation. They are the brightest source of radiation at possibly any wavelength. This radiation is believed to be a result of accretion of mass by the supermassive black hole at the centre of the host galaxy.

Gamma-Ray Bursts are extremely intense and relatively short bursts of gamma radiation that occur a few times per day in the detectable Universe. They are the brightest source of gamma radiation in the Universe. GRBs are originated by the death of a very massive star, collapsing in a black hole, or from the merging of two compact objects, such as a black hole or a neutron star. The emission mechanism of Gamma-Ray Bursts is not yet completely understood. Although many models succeed in explaining part of the observations, none is by itself capable of explaining them all. A way to contribute significantly in the understanding of the gamma emission of GRBs, is to observe very high energy neutrinos generated during the Gamma Ray Burst explosion. This observation would help to constrain the hadronic component of the GRB. This could help confirming them as source for high energy Cosmic Rays.

The observation of high energy neutrinos is particularly challenging, as these particles are subject only to weak interaction and therefore interacts very little with matter. In order to be able to detect them, it is therefore necessary to have an extremely big volume detector. As neutrinos are neutral and weakly interacting, it is not possible to detect them directly. Rather, muons generated by the interaction of neutrinos with matter nuclei around the detector are used. Muons travelling faster than light in a transparent medium emit a visible light radiation called Cherenkov radiation. This allows for the reconstruction of the incoming neutrino, its direction and energy.

The Antares detector is a neutrino telescope placed in the Mediterranean sea, at 2475 m of depth. It consists of a three-dimensional array of photo sensitive devices (photomultipliers) that measure the Cherenkov signal photons, generated by muons travelling in the sea water around the detector.

Neutrinos, as all leptons, are divided in three families, electron neutrinos, muon neutrinos and tau neutrinos. They all interact via weak interaction, and this can take place with the exchange of a mediating particle, called boson, which can be charged (Charged Current Interaction) or neutral (Neutral Current Interaction). Each neutrino type, and each interaction, cause a different topological signature in the Antares detector. The analysis presented in this thesis is focused in detecting neutral current interactions and/or electron and tau neutrinos.

The observation of neutrinos of such flavours is foreseen by neutrino flavour oscillation. While at the production site the majority of neutrinos are of the muonic type, due to oscillation they will reach Earth in equal ratio of each flavour. Usually, neutrino detectors are focused in the observation of charged current muon neutrinos interaction because they leave a clear track-like signature in the detector, making them easily identifiable. Other flavours and interactions cause an hadronic and electromagnetic shower in the detector, identifiable as a temporally isotropic light emission. This signature is more difficult to distinguish from background. For this reason, in the analysis presented in this work, it has been chosen to look for shower signatures in time coincidence with Gamma Ray Bursts.

This thesis presents a reconstruction algorithm developed for the Antares detector able to reconstruct the time and position of an hadronic or electromagnetic shower happening in the detector and induced by a neutrino interaction. Standard Antares data taken during the year 2008 have been analysed and showers happening within a small time window from a satellite GRB trigger were sought. This has been done maximizing the discovery potential of the analysis, studying the background behaviour from Antares data. A set of ten Gamma-Ray Bursts happening during the year 2008 have then been analysed and an upper limit on neutrino emission has been set.

This analysis complements the track searches typically performed in neutrino telescope experiments. Although the sensitivity per burst is lower than that of the track search, it can detect neutrinos of any flavour, which are invisible for track based analysis. It represents a first step toward many possible studies on neutrino emission from Gamma Ray Bursts, many of which will be exploited with the detector that will succeed Antares, KM3NeT.KM3NeT will cover an instrumented volume between 5 and 8 cubic kilometres, while Antares is about $0.035 \km3$. It will employ a different structure and photomultiplier type, leading to a huge improvement in sensitivity. What now has been presented just as an upper limit will probably become a discovery in few years time.

Eleonora Presani: Neutrino Induced Showers From Gamma-Ray Bursts (pdf)

De promotie vindt plaats op vrijdag 27 mei, om 12.00 in de Agnietenkapel van de Universiteit van Amsterdam.

Promotor: Prof. dr. P. M. Kooijman
Co-Promoteres:Dr. Els de Wolf en Dr. C.J. Reed

Contact: Eleonora Presani