Optimization of the reconstruction of neutrino interaction vertices in OPERA experiment.

Bachelor Degree Thesis (2009).

Neutrino oscillations have been one of the major fields of interest for particle physics, since they are the first and only (thus far) evidence of physics beyond the Standard Model. Some experiments, such as SuperKamiokande, MACRO and Soudan2, confirmed the disappearance of neutrinos of a certain flavor. Yet, the picture cannot be completed without the evidence of the oscillation phenomenon in the appearance channels. The final aim of the OPERA experiment is the detection of the appearance of a ντ in a pure νμ beam. Tau neutrinos in OPERA are searched by the observation of the typical topology of the events: a kink in one of the charge tracks escaping from the neutrino vertex. OPERA detection technology is Emulsion Cloud Chamber (ECC). Optical microscopes analyze the emulsions and register the data. The events are then reconstructed off-line. The OPERA Collaboration exploits two different automatic off-line reconstruction frameworks, FEDRA and EasyReconstruct. My thesis work has been a cross-check of the results given by the two frameworks in the case of real νμ-events, in order to evaluate the quality of the frameworks. I also compared the results given by the two automatic frameworks with the a set of  events reconstructed manually by a human operator.

My thesis describes the off-line elaboration techniques in detail and mainly it focuses on the comparison of the vertexing algorithms. The reconstruction of the vertices positions in the transverse plan is consistent in both the algorithm and in the manual check within the errors due to the alignment. Thus both the algorithms can be use to localize the interaction vertices. FEDRA underestimates in some cases the vertex multiplicity; since the acceptances of the algorithms are identical, I indicate that the cause of the underestimation is to be found into the Pair Based Vertexing method that only FEDRA uses in the interaction vertex reconstruction.

The slopes of the tracks reconstructed by FEDRA and EasyReconstruct are in perfect agreement with the manual check, proving that these two frameworks are both valid in the angular reconstruction of the track and so in the identification of the so called “long-type” decays. My analysis underline that a systematic uncertainty in the tracks impact parameter is introduced by both the algorithms, due to the fact that they both use Kalman filter. Thus, the event reconstruction based only on the automatic reconstruction frameworks is not enough accurate to identify the τ in the so called “short-type” decays: in this case, the information obtained by the manual check must be provided in order to correctly match the tracks to the interaction vertex.

Elena Gramellini

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