First measurement of CP-violating Asymmetries and Branching Fractions of Λb0 Charmless Two-Body Decays

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

For more information: Public note

Introduction and motivation

This page documents the preliminary CDF Run 2 results on the analysis of charmless two-body decays of Λb0 baryons in and pK final states. These result include the first measurement of a CP-violating asymmetry in hadronic Λb0 decays, and the first measurement of branching fractions of charmless Λb0 decays.

Two-body charmless Λb0 decays into self-tagging final states with a proton and a charged pion (or kaon) are promising for studying CP-violating asymmetries, which may reach significant size, O(0.3), in the Standard Model, thus being experimentally (relatively) accessible. This is particularly promising since measurements of asymmetries and of (as yet unknown) branching fractions, are believed to be potentially useful to rule out, or allow, some extensions of the Standard Model. For instance, some Minimal Supersymmetric Extensions of the Standard Model in which the R-parity is violated could both strongly suppress asymmetries in charmless Λb0 decays and enhance by factors O(100) their branching fractions, reaching rates of up to 10-4. This makes the measurement of these quantities particularly suited to increase our information on possible new physics scenarios before the LHC will be taking data. However, the branching ratios for these modes are very suppressed in the SM, being order 10-6, and the first observation of these decays was only recently obtained in the search for rare two-body charmless B-meson decays. We describe here a dedicated analysis aimed at the first measurement of CP-violating asymmetries and branching fraction in such decays.

Analysis overview

The analysis uses 1 fb-1 of data collected by the CDF trigger on impact parameters (two-track-trigger); it is based on the same general framework used for the measurement of two body charmless decays of the B mesons (page link). The offline selection is optimized for the observation of rare channels by maximizing the expected statistical resolution on the quantity one wishes to measure as predicted by repeating the actual measurements on pseudoexperiments. This selection allows extracting a visible signal of Λb0 decays over a background composed of combinatorics and the more abundant two-body charmless B meson decays. Then a multivariate unbinned Maximum Likelihood fit is used to further discriminate these backgrounds and discriminate also the different components to the actual Λb0 signal, coming from proton-kaon and proton-pion final states. The fit uses kinematics information - deriving from the correlations between ratios of momenta of final state particles and their invariant mass - combined with PID information, extracted from the 1.5σ kaon-pion separation available from the ionization energy-loss measurement in the CDF II drift chamber. These variables allows also a separation between Λb0 and anti-Λb0 decays, thus permitting the measurement of the direct CP-violating asymmetry. Fit results are then corrected for detector acceptance, and trigger and selection efficiencies to extract the final branching fractions. The correction factors are extracted from control samples of real data and from simulated samples.

Final results.

Mass pojection, whole mass region Mass projection, Lambda/b(0) mass region

Derived results

Using the previous results related with the Λb0 is possible to evaluate the ratio between charmless and charmed decays of the Λb0 and the absolute branching fractions.

The derivation of the absolute BR is not straight forward because to extract it we need to take into account the relative production fraction of Λb0 respect B0-->Kπ BR, using the value from PDG[3]. For this value we can use the CDF II value for fΛb0 /fd=0.56+0.28-0.21[4], obtaining a BR value of:

Otherwise we can use the PDG value fΛb0 /fd=0.25±0.04 [3], obtaining a BR value of:

These values should be compared with the theoretical prediction in [1] of Br(Λb0--> p π)=1·10-6 and Br(Λb0--> p K)=2·10-6.

Using the CDF II result on the Λb0-> Λc+π (public page)[5] and the PDG[4] for B0->Kπ decay mode is possible to extract the ratio:

List of Figures.

Fit projections.

Whole mass region Whole mass region (log scale) Λb0 mass region

Analysis templates


  1. Charmless two-body hadronic decays of Lambda(b) baryon. R. Mohanta, A.K. Giri, M.P. Khanna (Panjab U.) . Jun 2000. 16pp. Published in hys.Rev.D63:074001,2001. e-Print: hep-ph/0006109
  2. Effects of R-parity violation on CP asymmetries in Lambda(b) --> p pi decay, Rukmani Mohanta (Panjab U.) . May 2000. 8pp. Published in Phys.Rev.D63:056006,2001. e-Print: hep-ph/0005240
  3. Review of Particle Physics. By Particle Data Group (W.-M. Yao et al.). 2006. Published in J.Phys.G33:1-1232,2006
  4. Measurement of B Hadron Relative Fragmentation Fractions, CDF Collaboration, March 28 2006. CDF Note 8140
  5. Measurement of $\sigma(\Lambda_b)/\sigma(B^0) \times BR(\Lambda_b\to\Lambda_c\pi^-) / BR(B^0\to D^+\pi^-)$ in $p\bar{p}$ Collisions at $\sqrt{s}=1.96$ TeV, CDF Collaboration, 2006. PRL 98, 061802 2007, hep-ex/0601003.
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