Noises: a Nuisance and a Resource. Development of New Decomposition Methods of Noisy Data

Noise is constant presence in measurements. Its origin is related to the microscopic properties of matter. Since the seminal work of Brown in 1828, the study of stochastic processes has gained an increasing interest with the development of new mathematical and analytical tools. In the last decades, the central role that noise plays in chemical and physiological processes has become recognized. The dual role of noise as nuisance/resource pushes towards the development of new decomposition techniques that divide a signal into its deterministic and stochastic components. In this thesis I show how methods based on Singular Spectrum Analysis have the right properties to fulfil the previously mentioned requirement. During my work I applied SSA to different signals of interest in chemistry: I developed a novel iterative procedure for the denoising of powder X-ray diffractograms; I “denoised” bi-dimensional images from experiments of electrochemiluminescence imaging of micro-beads obtaining new insight on ECL mechanism. I also used Principal Component Analysis to investigate the relationship between brain electrophysiological signals and voice emission.

Search for direct CP violation in charm neutral meson decays at LHCb

A search for time-integrated violation of the CP symmetry, ACP(K−K+), in the Cabibbo-suppressed D0 → K−K+ decays is performed at the LHCb detector using proton- proton collisions recorded from 2015 to 2018 at the centre of mass energy of 13 TeV. The data used corresponds to an integrated luminosity of 5.7 fb−1. The flavour of the charm mesons is defined from the charge of the pion in D∗+ → D0π+ and D∗− → D0π− decays. Nuisance asymmetries are constrained from D∗+ → D0(→ K−π+)π+, D+ → KS0π+, D+ → K−π+π+, Ds+ → KS0K+ and Ds+ → φπ+ decays. The ACP(K−K+) asymme- try is measured to be ACP (K−K+) = (6.8 ± 5.4 (stat) ± 1.6 (syst)) · 10−4, in agreement with the previous LHCb results and the current world average. This represents the world’s most precise measurement of this quantity to date. Combining ACP (K−K+) with the time-integrated CP asymmetry difference, ∆ACP = ACP (K−K+)− ACP (π−π+), and the time-dependent CP asymmetry, ∆Y , measured with D0 → K−K+ and D0 → π−π+ decays, the direct CP asymmetries in D0 → K−K+ and D0 → π−π+ decays, adKK and adππ, result to be adKK =(7.7±5.7)·10−4, adππ =(23.2±6.1)·10−4, where the errors include systematic and statistical uncertainties and the correlation be- tween the two values is ρ(adKK,adππ) = 0.88. The values differ from zero for 1.4 and 3.8 standard deviations, respectively. In particular, adππ shows an evidence for direct CP violation in D0 → π−π+ decays.