A New Limit On Vµ -> Vtau Oscillations

Next, in chapter 6, we discuss our work on the tagging of muons with the Chorus detector. After explaining the various algorithms used we come to the efficiencies, understanding the found results through event kinematics. A study of non-tagged muons shows that the software efficiency is very close to maximal, and physics is the cause of nearly all the losses. The bulk of our study lies in the determination of the scanning efficiencies. Chapter 7 starts with the description of the interface emulsion sheets and the proof of a method we developed to extract the real scanning efficiency for these sheets from the data itself. We proceed to compare the found results with the results obtained by our simulation and comment on the possible causes for the observed discrepancy. Moving on, we explain how the search for the primary vertex and a possible decay kink is conducted and give the corresponding efficiencies we found using our Monte-Carlo. To close the chapter, we elaborate on the video image simulation, a tool we developed to help improve and determine the efficiency of video image analysis programs. These programs will be used intensively in the future to search for kinks. Last but not least, chapter 8 combines all these numbers with the present scanning status to put a new limit on Vµ -> Vtau oscillations. It also includes a few words on backgrounds and the results from a study of systematic uncertainties. We conclude with a new exclusion plot.

While confirmations of the validity of the Standard Model at low and high energies (reached by present accelerators) continue to be produced, the high-energy physics community is turning towards the search for physics beyond the Standard Model. In chapter 2 we review the evidence for the existence of three neutrino flavors. We proceed by quickly summarizing their properties in the Standard Model and examine the present limits set by experiments on each neutrino's mass. We then move on to some theoretical considerations on neutrino masses, mainly based on cosmology and Grand Unified Theories and the chapter closes by illustrating the remarkable possibility of neutrino oscillations. Interestingly, at the end of 1997 it is in the neutrino physics domain that most inconsistencies with the Standard Model are found. Not in experiments using neutrinos to study the weak and strong interactions, as we see at the start of chapter 3, but definitely when probing the solar and atmospheric neutrino fluxes. This is detailed later in that chapter, without forgetting the only neutrino oscillation appearance evidence found so far (LSND). From the results presented it seems to be clear that neutrinos have opened a window towards new physics. However, solving all the present problems (cosmology, solar, atmospheric, LSND) with our simple neutrino oscillation interpretation seems impossible or difficult and definitely not very elegant. So maybe this window we are opening is really much larger than we think and pushing more could yield some fascinating results. The existing constraints are definitely such that experiments should be built to search for appearance at specific oscillation parameter values. When Chorus was designed and started datataking, these constraints were quite a bit weaker. However, the parameter space area which is being explored is still very relevant in the light of a model solving the Hot Dark Matter and solar problems while satisfying the very elegant see-saw mechanism. We briefly describe the experiment in chapter 4, explaining the chosen tau neutrino detection strategy and the proposed sensitivity to V?? -> Vtau oscillations. In chapter 5 we start by describing the trackers: purpose, geometry and readout. This is followed by an explanation of the event reconstruction algorithm and its efficiency. Our main contribution to the reconstruction in the Target Trackers consists in their alignment: after introducing a few concepts we give the details of the alignment procedure, the trackers' degrees of freedom and the accuracy obtained for each of these. Two more paragraphs say a few more words on the problem of separation of variables for minimization and the detector stability over time.