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STUDY OF B0- >D(*)0H0 DECAY AND MEASUREMENT OF SIN (2BETA)
Studio del decadimento B0->D(*)h0 e misura di sin(2beta)
BOMBEN, MARCO
2007-05-29T09:34:36Z
Abstract
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.
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