Measurement of CP Violation in D0 → π+π- and
D0 → K+K- Decays at CDF
This webpage provides a concise summary of the analysis. Details can be found in
CDF Public Note 10296
Slides of the Joint Experimental Theoretical Physics seminar (Wine and Cheese) held in Fermilab, October 1 2010, are
Introduction and motivation
Time-integrated CP-violating asymmetries of singly-Cabibbo suppressed transitions as D0 → π+π- and
D0 → K+K- are powerful probes of new physics (NP). Contributions from "penguin" amplitudes are
negligible in the Standard Model , but as in D0-D0
oscillations, NP particles could play a role enhancing the size of CP-violation with respect to the CKM hierarchy expectation. Any asymmetry
significantly larger than 1% would unambiguously indicate non-SM physics .
We present a measurement of time-integrated CP asymmetry in the Cabibbo-suppressed D0 → h+h-
decays (where h=π,K):
Both direct and mixing-induced CP violation contribute to the asymmetry. The latter source produces a time-dependent asymmetry, whose expression when
neutral charmed mesons decay into CP eigenstates is 
that persists when integrated over time. In eq. (1) ηCP is the CP-parity of the decay final state, x,
y, p and q are the usual parameters used to describe flavored mesons mixing, φ is the CP violating phase and
t/τ the proper decay time in units of D0 lifetime. The measured integrated asymmetry, owing to the slow mixing rate of charm
mesons, is then at first order the sum of two terms:
The first term arises from direct and the second one from mixing-induced CP violation. The integration is performed over the observed ditribution of
proper decay time, D(t).
We updated and improved an early Run II analysis , using an event sample collected with the displaced-track trigger from
March 2001 to January 2010 that corresponds at about 5.94 fb-1 of integrated luminosity. The trigger requires presence of two charged
particles with transverse momenta greater than 2 GeV/c, impact parameters greater than 100 microns and basic cuts on azimuthal separation and scalar
sum of momenta.
The channels used in the analysis are D*-tagged D0 → h+h- and D0 →
π+K- decays, and D0 → π+K- decays were no D* tag is required (charge conjugate
states are implied). The reconstruction uses only tracking information without any attempt at identifying final state particles. We first reconstruct
a signal consistent with a D0 → h+h- or π+K- decay. Then we associate a
low-momentum charged track to the meson candidate to construct a D*+ candidate. The offline selection relies on confirmation of trigger
requirements and basic additional requirements on track and vertex quality.
The flavor of the charmed meson is unambiguously determined from the charge of the pion in the strong D*+ →
D0π+ decay. Knowing that primary D0 and D0 mesons are
produced in equal number in strong pp interactions, any asymmetry between the number of D0
and D0 decays is due to either CP non-conservation or detector-induced reconstruction
The main challenge is that the layout of the main tracker detector, the drift chamber, is intrinsically charge asymmetric due to a 35 degrees tilt
angle of the cells from the radial direction, thus different detection efficiencies for positive and negative low-momentum tracks induce an
instrumental asymmetry in the number of reconstructed D*-tagged D0 and D0
mesons. Other possible asymmetries may originate in slightly different performance of pattern-reconstruction and track-fitting algorithms for
negative and positive particles. The combined effect of these is a net asymmetry in the range of a few percents. This must be corrected to better
than one permille to match the expected statistical precision of the present measurement.
We exploit a fully data-driven method that uses higher statistic samples of D*-tagged (indicated with an asterisk) and untagged Cabibbo-favored
D0 → K-π+ decays to correct for all detector effects and aims at suppressing systematic uncertainties to
below the statistical ones. After equalization of kinematic distributions across samples, the physical asymmetry is extracted by subtracting the
instrumental effects through a combination of uncorrected "raw" asymmetries measured in these three samples:
Raw asymmetries are extracted from charm and anticharm signal-yields measured by detailed binned χ2 fits of mass distributions.
The full analysis procedure has been extensively tested to work with the desired accuracy on ensembles of simulated events.
Using 215K D*-tagged D0 → π-π+ decays, 476K D*-tagged D0 →
K-K+ decays, 5M D*-tagged D0 → K-π+
decays and 29M D0 → K-π+ decays where no D* tag was required, we obtain:
which are consistent with CP conservation and also with the SM predictions.
From eq. (2) the observed CP asymmetry describes a straight line in the plane
(aCPind,aCPdir) with angular coefficient -<t>/τ. The displaced-track trigger requirements
enrich the sample with high-valued proper decay time, with a mean value of 2.4τ (2.65τ) for the D0 →
π-π+ (D0 → K-K+) case. This complements measurements from B-factories, where
the unbiased acceptance in decay time limits <t>/&tau to be ≈1 . The figures show the plane
(aCPind,aCPdir) with the combination of three recent measurements assuming Gaussian uncertainties; the
bands cover 1σ intervals and the red contours represent the 68% and 95% CL limits of the combined result.
If we assume no direct CP violation in the charm sector the first term in eq. (2) disappears and our measurements traslate
Because <t>/τ in our sample is greater than in B-factories ones, the allowed range is more than five times narrower than the ones obtained using B-factories
measurements. Conversely, assuming that aCPind is zero, the CDF result directly compares to other measurements in different experimental
configurations with reduced uncertainties.
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List of figures and tables
All approved material can be found here.