ETD

Archivio digitale delle tesi discusse presso l'Università di Pisa

Tesi etd-07022015-003536


Tipo di tesi
Tesi di laurea magistrale
Autore
MOCCI, MARIA PAOLA
URN
etd-07022015-003536
Titolo
Measurement of CP asymmetries of charm hadron decays to eta mesons.
Dipartimento
FISICA
Corso di studi
FISICA
Relatori
relatore Prof. Punzi, Giovanni
relatore Dott. Stracka, Simone
Parole chiave
  • CP-asymmetry
  • hadron collider
Data inizio appello
23/07/2015
Consultabilità
Completa
Riassunto
The charm quark plays an important role in testing the Standard-Model (SM) because it represents a unique opportunity to study the possible coupling of non-SM particles with the up-type quark sector.
In this respect, CP violation is a powerful tool for investigating possible New Physics effects. This field is only partially explored to date both theoretically and experimentally;
in fact the first evidence for the D^0-bar(D^0) mixing was obtained only a few years ago. A theoretical description is not straightforward, since the masses of charmed hadrons, O(2 GeV), belong to a range where non-perturbative hadronic physics is operative and the phenomenological approximations commonly used in the strange and bottom sectors are of little help. Current theoretical approachesto charm dynamics are based on establishing relations among decay rates in different modes through amplitudes and phases, allowing to extract the theoretical parameters by fitting the measured branching-fractions in several decay channels, therefore collecting as many measurements as possible in different decay modes is crucial in reducing theoretical uncertainties and advancing our limited understanding of the physics of the charm sector. In this context, the LHCb experiment at CERN with its potential of collecting large samples of charm-decays represents an interesting opportunity.
The decay modes of charm mesons with an eta meson (eta/eta') in the final state, are an interesting area in need of better measurements. They amount to a relevant fraction of the hadronic decays (~30%), and are still incompletely known: the D_s branching fractions have significant uncertainties, and the measurements of CP asymmetries have uncertainties of 1-4 %, which is insufficient to test the expected CPV effects ( <1%). This is due to the experimental difficulty of dealing with the photons in the final state coming from the eta meson decays, making for lower resolutions and larger backgrounds. As a consequence, these channels were never studied at hadronic colliders, due to the copious background of neutral particles produced in those collisions - all current data coming from e^+e^- experiments BELLE and CLEO.
In this thesis, we perform the first analysis of D^+/D_s^+ -> eta^(') pi^+ (charge conjugation is implied throughout this summary) channels at a hadronic machine. We show that it is possible to perform a good reconstruction at LHCb by using a eta^(') decay channel with at least two charged particles in the final state (e.g. pi^+pi^-gamma). The two pion-tracks, which are reconstructed with high accuracy by the LHCb tracking system, combined with the pion
from the D decay track and the calorimeter information allowto reconstruct the decay vertex and the photon-momentum with good precision. An additional improvement is provided by constraining the eta^(')-daughter-particles to the eta^(') mass.
Due to their experimental difficulty, the investigation of these decay modes was not taken into consideration at LHCb during Run I. This has important implications on the data available for our analysis. LHCb data is collected by selecting only the small fraction of events passing some strict selection criteria (``trigger lines"), aimed at well-defined decay channels, and none of these trigger lines had been foreseen for the decays of our interest. We have therefore exploited for our study the data selected by a trigger line devised with a different aim: the reconstruction of D^+-> h^+h^+h^- decays. We determined that a sufficient number of decays of our interest are selected within that line as a side-effect. A selection optimized to improve the signal/background (S/B) ratio was performed. As a result the S/B is improved from 0.08(0.3) to 1(4) for the D(D_s) in the eta channel, and from 0.4(1.5) to 3(14) for the D(D_s) in the eta' channel. The resulting number of signal events, 9000,16900 for the eta and 63900,183500 for the eta' channel, is the largest available to date.
Following these studies, a preliminary measurement of the difference between D and D_s CP-asymmetries in the eta' channel was performed, for both 2011 and 2012 data-samples. The central value of this measurement is currently being kept ``blind" to avoid any experimenter bias until the final approval of all details of the measurement by the collaboration, but its resolution is already determined. The statistical accuracy of Delta A_(CP) is on the
order of 10^-2 and 10^-3 for 2011 and 2012 data-samples, respectively. An upper bound to the systematics uncertainties was determined to be less than the statistical accuracy, on the order of few per mille.
The accuracy of this measurement is limited by the D CP-asymmetry, since the statistics is lower compared to the D_s. We envision the accuracy over the CP-asymmetry measurement for the D_s sample to be a sensible improvement over the existing result. The next step in the analysis would be the measurement of CP-asymmetries difference between the D_s decay in the eta and eta' channels, which would allow to cancel the bias deriving from production-asymmetry effects.

In conclusion, this work demonstrates that it is possible to reconstruct large and clean samples of charm decays into eta final states, and to measure CP-asymmetries in the D^+/D_s^+ -> eta^(') pi^+ channels at LHCb. The sample collected during LHCb-Run I already represents the largest sample of these reconstructed decays to date, yielding competitive measurement with respect to the existing results from B-factories. This analysis represents a valid starting-point for further accurate measurements of CP-violation and branching-fractions
of these decay modes in a hadronic environment. A larger sample is expected from Run II, which
includes dedicated trigger selections for the D/D_s^+ -> eta^(') h^+ channels, introduced as an outcome of this feasibility study.


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