STUDY OF B0- >D(*)0H0 DECAY AND MEASUREMENT OF SIN (2BETA)

This thesis describes a measurement of a CP violating asymmetry in neutral B meson decays, B0->D(*)0h0, where D(*)0 is a charmed meson and h0 is a light meson such like a pi0, eta, eta' or omega, performed on data collected by the BABAR experiment at the Stanford Linear Accelerator Center (SLAC) asymmetric-energy electron-positron collider PEP-II. In this accelerator electrons and positrons collide at an energy in the center of mass frame of s = (10.58 GeV)^2, equal to the Upsilon(4S) resonance mass. The analysis reported in this thesis is the first measurement of CP-violation in B0->D(*)0h0 decays. A possible manifestation of CP violation in neutral B meson decays appears in final states accessible to both B0 and B0bar; a B0 can mix into a B0bar and this gives an extra phase that translates into a CP violating effect. If the D0 (D0bar) meson decays into a CP eigenstate, then both B0 and B0bar can contribute to the final state, realizing the condition for possible CP violation. The interesting point in measuring CP violation using B0->D(*)0h0 modes is that theoretical uncertainties are well under control since SM contributions other than leading amplitude are highly suppressed. Observation of a sizeable difference from the SM expectation in CP violating asymmetries for this decay would be an evidence for New Physics contributions. CP-violation was well established in B meson physics looking at b->ccs transitions; the predicted theoretical uncertainties for these modes are relatively small. Once the CP-violation is established, it is crucial to test its mechanism and its agreement with SM expectations, measuring other decays. The b->s channels are interesting because they have only penguin diagrams contribuiting to decay amplitude. On the contray, the B0->D(*)0h0 amplitude receive no contribution from any penguin diagram; measuring CP-asymmetry in B0->D(*)0h0 decays is therefore an independent test of the avor sector of the Standard Model. The results of measured CP-asymmetry using b->ccs, b->sqq and B0->D(*)0h0 decays could indicate a pattern that allows to determine which New Physics is likely to be correct. We analyzed about 384x10^6 BBbar pairs (corresponding to a luminosity of 349 fb^(-1)). Useful decay chains, and background sources that could mimic our signal were identifed, together with discriminant variables enhancing signal significance over background. The analysis selected B0B0bar pairs in which one neutral B meson was reconstructed in our decay modes and the other one was tagged as B0 or B0bar in an inclusive way. We eventually selected roughly 1100 events, with an estimated signal yield of 340+-32 signal events. The fit to time-dependent CP-asymmetry indicated that: C = -0.23 +- 0.16 +- 0.04 S = -0.56 +- 0.23 +- 0.05 where the first error is statistical and the second is systematic. Assuming S = -sin2(beta), then sin2beta = 0.56 +- 0.23 +- 0.05. This is the first measurement for these decay modes. The result is consistent with the Standard Model expectation and is 2.5 standard deviation away from CP-conserved hypothesis C = S = 0. This result is consistent with world average ("WA"): sin2beta = 0.675 +- 0.026. The results presented in this thesis are dominated by statistical uncertainties, therefore there is room to improve the analysis using more data. BABAR data taking will continue until the end of 2008 with the plans to increase peak luminosity and to almost triplicate the dataset used in this analysis. On the other hand one can also envisage the possibility of including more channels. For example B0->D0h0 with D0->KSpi0 might be added, trading the large D0->K0Spi0 branching ratio against the poorer primary vertex information. Overall, a decrease of the statistical uncertainty by a factor between 1.5 and 2. can be envisaged in the near future. Measurements at the proposed future "SuperB factory", with very large luminosity should be able to push the uncertainty to the systematic limit. In any case, a comparison of statistical uncertainties with the other measurements using b->c transitions shows that this channel is one of the most promising in this domain. The B0->D(*)0h0 decays can shed some light into the present situation for the flavor sector of the Standard Model, adding an independent measurement of the mixing phase beta that will contribute to constrain contributions to physics beyond the Standard Model.