Combination of Direct and Indirect CP Violation
(updated 21 March 2013)


People working on this:   Marco Gersabeck

Notation: This combination uses measurements of direct and indirect CP violation to extract the level of agreement for a no-CP-violation hypothesis. The observables are:

A Γ ≡ [τ(D0 → h+ h) − τ(D0 → h+ h )] / [τ(D0 → h+ h) + τ(D0 → h+ h )]

where h+ h can be K+ K or π+ π, and

ΔACP ≡ ACP(KK) - ACP(ππ).

Following Gersabeck et al., J. Phys. G 39 (2012) 045005, the following relations between observables and underlying parameters are obtained:

AΓ = -aCPind - aCPdir yCP

which thus constrains mostly indirect CP violation (aCPind), and where the direct CP violation contribution (aCPdir) can differ for different final states. Further,

ΔACP = ΔaCPdir (1 + yCP 〈t〉/τ) - Δ〈t〉/τ AΓ,

where Δ〈X〉 is the difference between quantity X for the KK and ππ final state and 〈X〉 is their average. 〈t〉/τ is the mean decay time relative to the true lifetime of the D0 meson. The contribution of the decay-dependent direct CP violation to AΓ is known to be ≤10-4. Therefore, all AΓ measurements are combined, irrespective of whether they are based on KK or ππ decays, and thus aCPind = -AΓ is assumed, leading to

ΔACP = ΔaCPdir (1 + yCP 〈t〉/τ) + Δ〈t〉/τ aCPind,

A χ2 fit is performed in the plane ΔaCPdir vs. aCPind. For the BaBar result the difference of the quoted values for ACP(KK) and ACP(ππ) is calculated, adding all uncertainties in quadrature. This may overestimate the systematic uncertainty for the difference as it neglects correlated errors; however, the result is conservative and the effect is small as all measurements are statistically limited. For all measurements, statistical and systematic uncertainties are added in quadrature when calculating the χ2.

For world average values of mixing parameters, in particular yCP = (1.064 ± 0.209)% which is used in this fit, click here. For tables of measured CP asymmetries, click here.

Year Experiment Results Δ〈t〉/τ 〈t〉 Comment Reference
2012 Belle preliminary AΓ = (−0.03 ±0.20 (stat.) ±0.08 (syst.))% - - 976 fb−1 near Υ(4S) resonance M. Staric for the Belle Collab., arXiv:1212.3478.
2012 BaBar AΓ = (0.09 ±0.26 (stat.) ±0.06 (syst.))% - - 468 fb−1 near Υ(4S) resonance J.P. Lees et al. (BaBar Collab.), Phys.Rev. D87 (2013) 012004.
2011 LHCb AΓ = (−0.59 ±0.59 (stat.) ±0.21 (syst.))% - - 28 pb−1 s  = 7 TeV pp collisions R. Aaij et al. (LHCb Collab.), JHEP 1204 (2012) 129.
2008 BaBar ACP(KK) = (0.00 ±0.34 (stat.) ±0.13 (syst.))%
ACP(ππ) = (−0.24 ±0.52 (stat.) ±0.22 (syst.))%
0.00 1.00 385.8 fb−1 near Υ(4S) resonance B. Aubert et al. (BABAR Collab.), Phys. Rev. Lett. 100, 061803 (2008).
2012 Belle preliminary ΔACP = (−0.87 ±0.41 (stat.) ±0.06 (syst.))% 0.00 1.00 976 fb−1 near Υ(4S) resonance B.R. Ko for the Belle Collab., arXiv:1212.1975.
2012 CDF ΔACP = (−0.62 ±0.21 (stat.) ±0.10 (syst.))% 0.25 2.58 9.7 fb−1 s  = 1.96 TeV p p  collisions T. Aaltonen et al. (CDF Collab.), Phys.Rev.Lett. 109 (2012) 111801.
2013 LHCb preliminary ΔACP = (−0.34 ±0.15 (stat.) ±0.10 (syst.))% 0.11 2.10 1.0 fb−1 s  = 7 TeV pp collisions, prompt D* The LHCb Collaboration, LHCb-CONF-2013-003.
2013 LHCb ΔACP = (0.49 ±0.30 (stat.) ±0.14 (syst.))% 0.02 1.06 1.0 fb−1 s  = 7 TeV pp collisions, B→D0μX R. Aaij et al. (LHCb Collab.), arXiv:1303.2614.
Fit Result
Agreement with no CP violation
CL = 2.1x10−2


Combination Plot: The combination plot shows the measurements listed in the Table above for ΔACP and AΓ, where the bands represent ±1σ intervals. The point of no CP violation (0,0) is shown as a filled circle, and two-dimensional 68% CL, 95% CL, and 99.7% CL regions are plotted as ellipses with the best fit value as a cross indicating the one-dimensional uncertainties in their center.



From the fit, the change in χ2 from the minimum value for the no-CPV point (0,0) is 7.7; this corresponds to a CL of 2.1x10−2 for two degrees of freedom. Thus the data is consistent with no CP violation at 2.1% CL. The central values and ± 1σ errors for the individual parameters are:

aCPind = (-0.010 ± 0.162 )%

ΔaCPdir = (−0.329 ± 0.121 )%

This page is maintained by M. Gersabeck and A. Schwartz and was last updated