Measurement of CP Violation in D0 → π+π- and D0 → K+K- Decays at CDF

Primary authors: A. Di Canto, M. J. Morello, G. Punzi, L. Ristori, D. Tonelli
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 here.

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 [1], 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 [1].

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 [1]


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).

Analysis overview

We updated and improved an early Run II analysis [2], 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 asymmetries.

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 [3]. 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 into

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.


  1. D.-S. Du, "CP violation for neutral charmed meson decays into CP eigenstates", Eur. Phys. J. C 50 (2007) 579; Y. Grossman, A. Kagan, and Y. Nir, "New Physics and CP Violation in Singly Cabibbo Suppressed D Decays", Phys. Rev. D75, 036008 (2006); I. I. Y. Bigi, "Could Charm 'Third-time' Be the Real Charm ? A Manifesto", arXiv:0902.3048 [hep-ph]; S. Bianco et al. "A Cicerone for the physics of charm" Riv. Nuovo Cim. 26 N7, 1 (2003); Z.-Z. Xing, "D0-D0 mixing and CP violation in neutral D-meson decays", Phys. Rev. D55, 196 (1997).
  2. D. E. Acosta et al. [CDF Collaboration], "Measurement of Partial Widths and Search for direct CP Violation in D0 Meson Decays to K+K- and π+π-.", Phys. Rev. Lett. 94, 122001 (2005).
  3. B. Aubert et al. [BaBar Collaboration], "Search for CP violation in the decays D0 → K+K- and D0 → π+π-", Phys. Rev. Lett. 100 (2008) 061803; M. Staric et al. [Belle Collaboration], "Measurement of CP asymmetry in Cabibbo suppressed D0 decays", Phys. Lett. B 670 (200) 190.

List of figures and tables

All approved material can be found here.