Observation of the FCNC Decay Λb0→Λμ+μ- and Angular Distributions in B→K(*)μ+μ- Decays

Primary authors: Hideki Miyake, Shinhong Kim, Fumihiko Ukegawa

Introduction

Rare decays of bottom hadrons mediated by the flavor-changing neutral current (FCNC) process b→sμ+μ- occur in the standard model (SM) through higher order amplitudes. A variety of beyond-the-standard-model (BSM) theories, on the other hand, favor enhanced rates for these FCNC decays. One can obtain rich information about the b→sμ+μ- dynamics by the measurements of the branching ratio, their dependence on the di-lepton mass distributions, and the angular distributions of the decay products.
This page summarizes analysis results for the decays governed by the b→sμ+μ- transition; Λb0→Λμ+μ-, Bs0→φμ+μ-, B+→K+μ+μ-, B0→K*0(892)μ+μ-, B0→KS0μ+μ-, and B+→K*+(892)μ+μ-. In addition to BR and differential BR of the decays above, we measure the angular distributions in B→K(*)μ+μ- decays.
This analysis is based on a dataset of 6.8 fb-1.
The measurements of BR and differential BR are described in Phys. Rev. Lett. 107, 201802 (2011). The measurements of the angular distribution are described in arXiv:1108.0695.
Previous iterations used 4.4 fb-1 ( Phys. Rev. Lett. 106, 161801 (2011), web page) ) and 924 pb-1( Phys. Rev. D79, 011104(R) (2009), web page).

Update: In Nov. 8th, 2011, isospin asymmetry measurements in B→K(*)μ+μ- decays have been added.

Overview of the analysis

We study the following FCNC decays:
Λb0→Λμ+μ-,
Bs0→φμ+μ-,
B+→K+μ+μ-,
B0→K*0(892)μ+μ-,
B0→KS0μ+μ-, and
B+→K*+(892)μ+μ-
in 6.8 fb-1 of data collected by the CDF detector with a dimuon trigger.
After trigger and loose offline selection, the final selection is obtained using an artificial neural network discriminator optimized in an unbiased way to yield the best expected average resolution on the quantities to be measured. Various physics backgrounds are reduced using mass vetoes. Signal yields are obtained by an unbinned maximum log-likelihood fit to Hb mass distribution.
The result accuracy is limited by the statistical uncertainty. The results include the first observation of the baryonic FCNC decay Λb0→Λμ+μ- and a measurement of its BR, the first measurement of the differential BR (as a function of squared dimuon mass) of Bs0→φμ+μ- and Λb0→Λμ+μ- decays, the first measurements of new angular observables, AT(2), Aim, sensitive to right-handed currents, and a measurement of forward-backward asymmetry and polarization in B→K(*)μ+μ- decays that is competitive with world best results. No evidence for non-SM physics is found.

Branching ratios (Total and Differential)

To cancel dominant systematic uncertainties, the branching ratio of each rare decay Hb→hμ+μ- is measured relative to the corresponding resonant channel Hb→J/ψh, used as a normalization and a cross-check of the whole analysis. Hb stands for Λb, Bs, B+, and B0, and h stands for Λ, φ, K+, KS0, K*0, and K*+. We also measure the differential branching ratio with respect to the squared dimuon mass. Events populating the signal region in mass are divided into six exclusive and two additional q2 bins, where q2=Mμμc2. Since each q2 bin has a different amount of signal and background, we fit for the signal in each q2 bin with the same procedure used for the global fits. During the fit, we fix the mean of the Hb mass and BG slope to the number obtained from the global fit, therefore only the signal fraction is floated. Combined BRs between B0 and B+ are obtained by isospin symmetry assumption using B mass lifetime ratio. Isospin asymmetry is also obtained from the asymmetry of differential BRs between B0 and B+ decays.

Angular analysis

The full differential decay distribution for the decay B→K*μ+μ- is described by four independent kinematic variables: the di-muon invariant mass squared q2, the angle θμ between the μ++) direction and the direction opposite to the B (Bbar) meson in the di-muon rest frame, the angle θK between the kaon direction and the direction opposite to the B meson in the K* rest frame, and the angle φ between the two planes formed by the di-muon and the K-π systems. The distributions of θμ, θK, and φ are projected from the full differential decay distribution and can be parametrized with four angular observables, AFB, FL, AT(2) and Aim:

1/Γ×dΓ/dcosθK = 3/2 FL cos2θK + 3/4 (1-FL) (1-cos2θK),

1/Γ×dΓ/dcosθμ = 3/4 FL (1-cos2θμ) + 3/8 (1-FL) (1+cos2θμ) + AFBcosθμ,

1/Γ×dΓ/dcosφ = 1/2π[ 1+1/2(1-FL)AT(2) cos2φ + Aim sin2φ ],

where Γ = Γ (B→K*μ+μ-), AFB is the muon forward-backward asymmetry, FL is the K* longitudinal polarization fraction, AT(2) is the transverse polarization asymmetry, and Aim is the T-odd CP asymmetry of the transverse polarizations.

Results

Hb mass distributions


Λb0→Λμ+μ- yield: (png) (eps)
Bs0→φμ+μ- yield: (png) (eps)
B+→K+μ+μ- yield: (png) (eps)
B0→K*0μ+μ- yield: (png) (eps)
B0→KS0μ+μ- yield: (png) (eps)
B+→K*+μ+μ- yield: (png) (eps)

Differential BR




Λb0→Λμ+μ- differential BR: (png) (eps)
Bs0→φμ+μ- differential BR: (png) (eps)
B+→K+μ+μ- differential BR: (png) (eps)
B0→K*0μ+μ- differential BR: (png) (eps)
B0→KS0μ+μ- differential BR: (png) (eps)
B+→K*+μ+μ- differential BR: (png) (eps)
B→Kμ+μ- combined differential BR: (png) (eps)
B→K*μ+μ- combined differential BR: (png) (eps)

Isospin Asymmetry


B→K*μ+μ- and B→Kμ+μ- isospin asymmetry
(png) (eps)

Angular Analysis






B0→K*0(K+π-+μ- (single channel)
- FL (png) (eps)
- AFB: (png) (eps)
- AT(2): (png) (eps)
- Aim: (png) (eps)

B→K*μ+μ- (simultaneous fit of K*0 and K*+ channels)
- FL (png) (eps)
- AFB: (png) (eps)
- AT(2): (png) (eps)
- Aim: (png) (eps)

B+→K+μ+μ-
- AFB: (png) (eps)

Angular Distributions

B0→K*0μ+μ-
- kaon angular distributions: (png) (eps)
- muon angular distributions: (png) (eps)
- phi angular distributions: (png) (eps)
B→K*μ+μ-
- kaon angular distributions: (png) (eps)
- muon angular distributions: (png) (eps)
- phi angular distributions: (png) (eps)

Signal yields and significances

where the significance s is determined from the likelihood ratio to a null signal hypothesis.

Relative BR

Absolute BR

Yield

Differential BR


Isospin Asymmetry


q2max=19.30 for K*μ+μ- (20.30 for Kμ+μ-)

FL, AFB, AT(2), and Aim