The neutrino interaction analysis chain in OPERA

2009 - 2010The aim of the OPERA experiment is to provide a “smoking-gun” proof of neutrino oscillations, through the detection of the appearance signal of ντ’s in an initially pure νμ beam. The beam is produced at CERN, 732 Km far from the detector, which is located underground in the Gran Sasso laboratory. The evidence of the appearance signal will be provided by the detection of the daughter particles produced in the decay of the τ lepton. A micro-metric spatial resolution is needed in order to measure and study the topology of the ντ-induced events. With this goal, nuclear emulsions, the highest resolution tracking detector, were chosen to be the core of the OPERA apparatus. The analysis of the large amount of nuclear emulsions used in the OPERA experiment has required the development of a new generation of fast automatic microscopes, featuring a scanning speed more than one order of magnitude higher than in past emulsion-based experiments. The long R&D carried out by the Collaboration has given rise to two new systems: the European Scanning System (ESS) and the Japanese S-UTS. The work presented in this thesis has been carried out in one of the laboratories involved in the OPERA emulsion scanning, hosted at the University of Salerno, and during a 6 month’s stay at the IPNL (Institut de Physique Nucleaire de Lyon). As for emulsion data-taking, several bricks from the 2008, 2009 and 2010 OPERA runs were scanned in Salerno and about 250 νμ-induced events were located. For the events triggered in the 2008 run, a kinematical analysis was performed, by developing a new likelihood-based software, able to estimate the momentum of the particles traversing the emulsion sheets through multiple Coulomb scattering. The algorithm was also tested on a set of Monte-Carlo data and a set of pion tracks collected during the 2003, 2004 and 2007 test beam campaigns at CERN. The 2008 run sample was used also to perform the hadron interaction search and the data collected were merged with those from other laboratories to estimate the background to the hadronic decay channel. The kinematical analysis of the τh decay channel is the subject of the second part of this thesis, developed while staying at the IPNL. The study on the quantities used to discriminate the signal and the background was accomplished by using simulated data. The kinematical cuts suggested by the OPERA Proposal were reviewed and the efficiencies obtained by applying these cuts were re-computed. In addition, another set of discriminating variables are suggested and their background suppression power is explored. Such estimators are proposed to be the subject of further work in the years to come. [edited by author]IX n.s

Search for dark matter produced in association with bottom or top quarks in √s = 13 TeV pp collisions with the ATLAS detector

A search for weakly interacting massive particle dark matter produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and miss- ing transverse momentum are considered. The analysis uses 36.1 fb−1 of proton–proton collision data recorded by the ATLAS experiment at √s = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are in- terpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour- neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross- section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour- charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements