Analysis of D0 -> K+ pi- pi0 Decays: Search for D0-D0bar Mixing, and Measurements of the Doubly Cabibbo-Suppressed Decay Rate and Resonance Contributions Page: 20 of 187
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channels chosen in an analysis. As explained below, an observation of CP violation
would be the most suggestive sign of new physics in this system.
1.2 Theoretical Predictions
In the Standard Model, the mixing rate in the D0-D0 system is expected on general
grounds to be very small, but it is difficult to calculate how small because of significant
contributions from long-distance effects. This theoretical uncertainty limits, but does
not eliminate, the potential for identifying signs of new physics.
At small distances, neutral-meson mixing proceeds via flavor-changing neutral
currents (FCNC). Since there are no tree-level FCNC contributions in the Standard
Model, processes such as mixing occur at the quark level primarily via box diagrams,
as shown in Figure 1.1. In the B0-B0 or K0-K systems, the mesons comprise down-
type quarks. In contrast, the D0-D0 system comprises up-type quarks. D mixing
is therefore sensitive to the contributions of virtual down-type quarks coupled to the
Weak Interaction, making it a process that might reveal physics not seen in the B or
K systems. One immediate result of this difference is that there is no contribution
from heavy quarks in the D-mixing box diagram. This particular type of contribution
breaks the Glashow-Iliopoulos-Maiani (GIM) cancellation in the B or K systems and
allows sizable mixing rates. The heaviest quark in the D-mixing box diagram is the
1In K mixing, it is m - mU that is relevant; in B mixing, it is mt - m .3
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Wilson, Michael Galante. Analysis of D0 -> K+ pi- pi0 Decays: Search for D0-D0bar Mixing, and Measurements of the Doubly Cabibbo-Suppressed Decay Rate and Resonance Contributions, report, December 13, 2005; [Menlo Park, California]. (https://digital.library.unt.edu/ark:/67531/metadc878913/m1/20/: accessed July 18, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.