Measurement of branching fractions and direct CP-asymmetries of B⁰_(s)->h⁺h'^- decays in 1 fb^-1.

Primary authors: S. Donati, M. Morello, G. Punzi, D. Tonelli

This webpage collects the links to the CDF public note and to the relevant supporting plots for the measurement of the branching fractions and the direct CP-asymmetries in the B⁰_(s) →h⁺h'^- decays. These preliminary results were approved on September 21^st,2006 by the CDF collaboration. The results reported in this document are slightly updated on April 10^th, 2008. The update is about only a more precise measurement of the isolation efficiency ratio between B⁰ and B⁰_s mesons.

In this analysis:

first observation of three new rare charmless decays: B⁰_s→K^-π⁺, Λ⁰_b→pπ^- and Λ⁰_b→pK^-; (here)
measurement of the BR(B⁰_s→K^-π⁺); (here)
first measurement a of a direct CP asymmetry A_CP(B⁰_s→K^-π⁺) in a B⁰_s decay; (here)
update of the upper limit on the rare annihilation modes: BR(B⁰_s→π⁺π^-) (world's best) and BR(B⁰→K⁺K^-) (same precision of current measurements); (here)
high precision measurement of the direct CP asymmetry A_CP(B⁰→K⁺π^-); (here)
high precision measurement of the BR(B⁰→π⁺π^-); (here)
high precision measurement of the BR(B⁰_s→K⁺K^-); (here)
first measurement of the ratio BR(&Lambda⁰_b→pπ^-)/BR(&Lambda⁰_b→pK^-). (here)

A complete description of the analysis is available here: note-8579v1.

Plots and figures available. (here)

Motivations

The decay modes of B mesons into pairs of charmless pseudoscalar mesons are effective probes of the quark-mixing (CKM) matrix and are sensitive to potential new physics effects. Their branching fractions and CP asymmetries can be predicted with good accuracy and compared to the rich experimental data available for B_u and B_d mesons, produced in large quantities in Υ(4S) decays [1]. Measurements of similar modes predicted, but not yet observed, for the B_s meson are important to supplement our understanding of B meson decays. The measurement of observables from both strange and non-strange B mesons allows a cancellation of hadronic uncertainties, thus enhancing the precision of the extraction of physics parameters from experimental data [2,3,4,5].
A combination of B⁰→π⁺π^- and B⁰_s→K⁺K^- observables has been proposed as a way to directly determine the phase of the V_ub element of the CKM matrix (angle γ), or alternatively as a test of our understanding of the dynamics of B hadron decays, when compared with other determinations of γ [6]. The B⁰_s→K^-π⁺ mode can also be used in measuring γ [3], and its CP asymmetry is a powerful model-independent test [7] of the source of the direct CP asymmetry recently observed in the B⁰→K⁺π^- mode [8]. The B⁰_s→π⁺π^- mode proceeds only through annihilation diagrams, which are currently poorly known and a source of significant uncertainty in many theoretical calculations [9]. Its features are similar to the as yet unobserved B⁰→K⁺K^- mode, but it has a larger predicted branching fraction [10]; a measurement of both modes would allow a determination of the strength of penguin-annihilation [4].

[1] - B.Aubert et al. (Babar Collaboration), Phys. Rev. Lett. 89, 281802 (2002);
Babar Collaboration hep-ex/0608003 submitted to Phys Rev. D;
Y.Chao et al. (Belle Collaboration); Phys Rev. D 69, 111102 (2004);
Belle Collaboration, (K. Abe etal) hep-ex/0609015
A.Bornheim et al. (Cleo Collaboration), Phys. Rev. D 68, 052002 (2003);
B.Aubert et al. (Babar Collaboration),arXiv:hep-ex/0508046.
[2] - M.Gronau, Phys.Lett. B 492, 297 (2000).
[3] - M.Gronau and J.L.Rosner, Phys.Lett. B 482, 71 (2000).
[4] - A.J.Buras et al.,Nucl.Phys. B 697, 133 (2004).
[5] - J.F.Sun, G.H.Zhu, and D.S.Du, Phys. Rev. D 68, 054003 (2003).
[6] - R.Fleischer, Phys. Lett. B 459, 306 (1999);
D.London and J.Matias, Phys.Rev. D 70, 031502 (2004).
[7] - H.J.Lipkin, Phys. Lett. B 621, 126 (2005) .
[8] - B.Aubert et al. (Babar Collaboration), Phys. Rev. Lett. 93, 131801 (2004);
Babar Collaboration hep-ex/0607106;
Y.Chao et al. (Belle Collaboration), Phys. Rev. Lett. 93, 191802 (2004);
Y. Unno (for the Belle Collaboration), talk presented at Moscow ICHEP 2006;
[9] - M.Beneke and M.Neubert, Nucl. Phys. B 675, 333 (2003).
[10] - Y.D.H.Yang, F.Su, G.R.Lu, and H.J.Hao,arXiv:hep-ph/0507326, submitted to Eur. Phys. J. C.

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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 1 fb^-1 of integrated luminosity. This is an improvement of the measurement performed on the first 180 pb^-1(BR/ACP on 180pb^-1) and on the intermediate data sample of 355 pb^-1 (ACP(B0→Kπ) on 355pb^-1). Besides the larger statistics, and an higher-resolution tracking, the analysis includes many improvements of the selection of the sample and of the fitting technique:

New cut optimization - We optimized the resolution individually for each of the physics observable to be measured. In particular in this analysis we used two different sets of cuts. The first one was optimized to measure the direct CP-asymmetry A_CP(B⁰→K⁺π^-) and the second one was optimized to observe and measure the BR(B⁰_s →K^-π⁺).
More accurate mass resolution model - The mass line shape has been determined using the full CDF simulation and the correct QED calculation (Phys. Lett.B 633,309-313,2006) to keep into account all non Gaussian known effects.
We measured the efficiency ratio ε(K^-π⁺)/ε(K⁺π^-) from the data using a huge sample of prompt D⁰→K^-π⁺ decays.

A complete description of the analysis is available here: note-8579v1.

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Results (looser cuts optimized to measure A_CP(B⁰→K⁺π^-))

Using the set of (looser) cuts optimized to measure the direct CP-asymmetry A_CP(B⁰→K⁺π^-) we measured all physical observables related to the modes which have a large yield: B⁰→π⁺π^-, B⁰→K⁺π^- and B⁰_s→K⁺K^-.

We measured:

.

This result can be compared with latest B-Factories measurements (HFAG 2008).

We measured:

.
Using the (HFAG 2008) averaged values for the BR(B⁰→K⁺π^-)= (19.4 ± 0.6)⋅10^-6, f_s = (10.4 ± 1.4)% and f_d = (39.8 ± 1.0)% it is possible to extract the absolute values of the branching fractions:

The BR(B⁰→π⁺π^-) is in agreement with the current B-Factories measurement (HFAG 2008), while the BR(B⁰_s→K⁺K^-) is in agreement with the old CDF measurement (PRL 97, 211802 2006).
The ratio BR(B⁰_s→K⁺K^-)/BR(B⁰→K⁺π^-) is an important parameter to measure the SU(3) symmetry breaking, in fact the two diagrams differ only for the exchange of the spectator quark s ↔ d. The theoretical expectations for this BR ratio are not completely in agreement, Matias et al. [PRL97, 061801(2006)] and hep-ph/0610109 predict small SU(3) breaking (BR(B⁰_s→K⁺K^-)/BR(B⁰→K⁺π^-)≈1) while Khodjamirian et al.[PRD68,114007(2003)] predict large SU(3) breaking ( ≈2). The CDF measurement disfavors prediction of large breaking.

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Results (tighter cuts optimized to measure BR(B⁰_s →K^-π⁺))

We observed for the first time three new rare charmless modes: B⁰_s→K^-π⁺, Λ⁰_b→pπ^- and Λ⁰_b→pK^-.

The significance for these rare modes is: 8σ for B⁰_s→K^-π⁺, 6σ for Λ⁰_b→pπ^- and 11σ for Λ⁰_b→pK^-.

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We measured:

Using the (HFAG 2008) input we obtained:

The theoretical expectations for this BR are:
BR(B⁰_s→K^-π⁺) ≈ [7-10]⋅10^-6 Beneke and Neubert [Nucl.Phys. B 675,333(2003)],
BR(B⁰_s→K^-π⁺) ≈ [6-10]⋅10^-6 Yu and Li [Phys. Rev.D 71,074026(2005)],
BR(B⁰_s→K^-π⁺) ≈ 4.9⋅10^-6 Williamson and Zupan [Phys. Rev. D 74, 014003(2006)].

This mode is a self tagging mode , then we measured its direct CP asymmetry A_CP(B⁰_s→K^-π⁺)):

This is the first measurement of CP violation in the B_s mesons system and it is different from 0 by 2.5 σ. The measurement is in agreement with the Standard Model expectation [H.J.Lipkin, Phys. Lett. B 621, 126 (2005)]. In fact using the BR(B⁰_s→K^-π⁺) just measured we can calculate the expected value for this asymmetry (≈0.37).
In the same paper Lipkin suggests a robust test of the Standard Model vs New Physics by comparison of the direct CP asymmetries in the B⁰_s→K^-π⁺ and B⁰→K⁺π^-. In particular the Standard Model with some minimal assumption predicts that:

.

Then we measured:

Using the (HFAG 2008) input we obtained:

The measurement is in agreement with the Standard Model expectation.

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We measured:

Using the (HFAG 2008) input we obtained:

We updated the upper limit on B⁰→K⁺K^- (same uncertainty of the current measurements) and B⁰_s→π⁺π^- (world's best upper limit). Both modes are annihilation dominated decays and they have not been observed yet. The measurement of these BRs is crucial because the related Feynman diagrams are hard to calculate and contribute to many relevant processes like the BR(B⁰_s→K⁺K^-).

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We observed for the first time also two new baryon charmless modes :Λ⁰_b→pπ^- and Λ⁰_b→pK^-. The measurements of the BR and of the direct CP asymmetries can be found here. In this analysis we only measured: