Measurement of Direct CP Violating Asymmetries in Charmless Decays of Strange Bottom Mesons and Bottom Baryons with 9.3 fb-1.

Primary authors: M.J. Morello, G. Punzi, F. Ruffini

This webpage collects the links to the CDF public note and to the relevant supporting plots for the measurement of the measurements of ACP (B0(s)->hh') and ACP0b->ph) These preliminary results were approved on June 28th, 2012 by the CDF collaboration.


Non invariance of the fundamental interactions under the combined symmetry transformation of charge conjugation and parity inversion (CP violation) is an established experimental fact. The vast majority of experimental data are well described by the standard model (SM), and have supported the success of the Cabibbo-Kobayashi-Maskawa (CKM)[1] theory of quark-flavor dynamics.
However, additional sources of CP violation are required to explain the matter-antimatter asymmetry of the Universe in standard big-bang cosmology. This would have profound consequences on our understanding of fundamental interactions.
Violation of CP is direct if the partial decay-width of a particle into a final state differs from the width of the corresponding antiparticle into the CP-conjugate final state. In recent times, the pattern of direct CP violation in charmless mesonic decays of B mesons has shown some unanticipated discrepancies from expectations. Under standard assumptions of isospin symmetry and smallness of contributions from higher-order processes, similar CP asymmetries are predicted for B0→K+ π- and B+→K+ π0 decays [2][3], However, experimental data show a significant discrepancy [4], which has prompted intense experimental and theoretical activity. Several simple extensions of the standard model could accommodate the discrepancy[5], but uncertainty on the contribution of higher-order SM amplitudes has prevented a firm conclusion[6].
Recently, it has been suggested a way [7] to accommodate this discrepancy within the SM, taking into account only constraints imposed by the Pauli principle. Nevertheless, high accuracy measurements of the violation of CP symmetry in charmless modes remains, therefore, a very interesting subject of study and may provide useful information to our comprehension of this discrepancy.
Rich samples of bottom-flavored hadrons of all types from the Tevatron offer the opportunity to explore new territory in the field of Bs mesons and b-flavored baryons. Additional information coming from different decays yields further constraints on the possible explanations of previous findings, and may possibly reveal new deviations from expectations. Specifically, measurements of direct CP violation in B0s→π+ K- decays have been proposed as a nearly model-independent test for the presence of non-SM physics [8][9].
The relationships between charged-current quark couplings in the SM predict a well-defined hierarchy between direct CP violation in B0→K+ π- and B0s→π+ K- decays, yielding a significant asymmetry for the latter, of about 30%. This large effect allows easier experimental investigation and any discrepancy may indicate contributions from non-SM amplitudes.
Supplementary information could come from CP\ violation in bottom baryons. Interest in charmless b-baryon decays is prompted by branching fractions recently observed being larger than expected [10]. Asymmetries up to about 10% are predicted for Λ0b->p π and Λ0b->pK decays in the SM [11] and are accessible with current CDF event samples.

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Data and analysis

The data have been collected by the hadronic B trigger designed to select events containing track pairs exiting from a common displaced vertex and correspond to about 9.3 fb-1 of integrated luminosity. This is an improvement of the measurement performed on 6 fb-1 (public web page published on Phys. Rev. Lett. 108, 211803 (2012)

A complete description of the analysis is available here: CDF Public note 10726.

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We measured:

These results can be compared with the B-Factories and LHCb measurements (HFAG 2011).
The measurement of CP violation in the B mesons system is compatible and of comparable accuracy with current results from B-Factories and LHCb.

We report an updated measurement of ACP in Bs->Kpi decays, with a significance of about 3 sigma. This result confirms the LHCb evidence with the same level of resolution.
The measurement of CP violation in the Bs mesons system is in agreement at 2 sigma level with the prediction within the SM [10].

The observed asymmetry in the Λ0b->ph decays are consistent with zero. However the limited experimental precision does not allow a conclusive discrimination between SM prediction (~8%), much suppressed values expected in supersymmetric scenarios (~0.3%), or prevision from pQCD approach [12], that are 31% for ACP(Λ0b->ppi) and 5% for ACP(Λ0b->pK).

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Tables and figures

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