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 21st,2006 by the CDF collaboration. The results reported in this document are slightly updated on April 10th, 2008. The update is about only a more precise measurement of the isolation efficiency ratio between B0 and B0s mesons.

In this analysis:

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 Bu and Bd mesons, produced in large quantities in Υ(4S) decays [1]. Measurements of similar modes predicted, but not yet observed, for the Bs 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 B0s→K+K- observables has been proposed as a way to directly determine the phase of the Vub 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 B0s→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 B0s→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].

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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:
  1. 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 ACP(B0→K+π-) and the second one was optimized to observe and measure the BR(B0s →K-π+).
  2. 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.
  3. 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 ACP(B0→K+π-))

Using the set of (looser) cuts optimized to measure the direct CP-asymmetry ACP(B0→K+π-) we measured all physical observables related to the modes which have a large yield: B0→π+π-, B0→K+π- and B0s→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, fs = (10.4 ± 1.4)% and fd = (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(B0s→K+K-) is in agreement with the old CDF measurement (PRL 97, 211802 2006).
The ratio BR(B0s→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(B0s→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(B0s →K-π+))


We observed for the first time three new rare charmless modes: B0s→K-π+, Λ0b→pπ- and Λ0b→pK-.





The significance for these rare modes is: 8σ for B0s→K-π+, 6σ for Λ0b→pπ- and 11σ for Λ0b→pK-.

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We measured:


Using the (HFAG 2008) input we obtained:


The theoretical expectations for this BR are:
BR(B0s→K-π+) ≈ [7-10]⋅10-6 Beneke and Neubert [Nucl.Phys. B 675,333(2003)],
BR(B0s→K-π+) ≈ [6-10]⋅10-6 Yu and Li [Phys. Rev.D 71,074026(2005)],
BR(B0s→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 ACP(B0s→K-π+)):


This is the first measurement of CP violation in the Bs 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(B0s→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 B0s→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 B0s→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(B0s→K+K-).

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We observed for the first time also two new baryon charmless modes 0b→pπ- and Λ0b→pK-. The measurements of the BR and of the direct CP asymmetries can be found here. In this analysis we only measured:


The result is in agreement with the theoretical expectation [R.Mohanta et al., Phys. Rev. D 63, 074001(2001)].

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Figures


Below the .eps and .gif versions of all figures available using the looser cuts optimized to measure ACP(B0→K+π-).

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Below the .eps and .gif versions of all figures available using the tighter cuts optimized to measure BR(B0s→K-π+).

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Below the .eps and .gif versions of probability ratio plots.

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Below the .eps and .gif versions of MC plots B0(s)→h+h- and Λ0b→ph-

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Below the .eps and .gif versions of DATA and MC plots of D0→K- π + (control data sample)

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