RIKEN Accel. Prog. Rep. 50 (2017)

Ⅱ-6. Particle Physics

Using Λb → Λμ+ μ− data within a Bayesian analysis of|ΔB| = |ΔS| = 1 decays†∗1,∗2∗3S. Meinel,and D.
D. vanvan Dyk
Dyk∗3Meinel∗1,∗2
andThe tensions between theory and experiment for
one of the angular observables in the kinematic distribution of the decay B̄ → K̄ ∗ (→ K̄π)μ+ μ− ,have sparked much interest in the determination of
the short-distance couplings in ﬂavor-changing neutral
currents of the form b → s+ − . Several competingglobal analyses that use the available data on suchrare decays of B̄ mesons to various degrees ﬁnd thata negative shift in one of the Wilson coeﬃcients, C9 ,improves the agreement with the data3) . However, itremains unclear whether this eﬀect is caused by physics
beyond the Standard Model, or merely by uncontrolled
hadronic contributions.
The baryonic rare decay Λb → Λ(→ p π − )μ+ μ−4) offers complementary constraints compared to the commonly used mesonic channels. A recent lattice QCD
calculation of the relevant Λb → Λ form factors5) enables us to evaluate the Λb → Λ(→ p π − )μ+ μ− observables with high precision. In this work, we studied theconstraining power of the b-baryon decay in a global
analysis of |ΔB| = |ΔS| = 1 decays. Our analysis
includes the following observables:−1P5 1,2) ,• For the baryonic decay Λb → Λ(→ p π − )μ+ μ−
we used the LHCb measurements4) of the differential branching fraction B15,20 , the fractionof longitudinally polarized dileptons FL 15,20 ,the lepton-side forward-backward asymmetry
AFB 15,20 , and the hadron-side forward-backward
2asymmetry AΛFB 15,20 , all in the bin 15 GeV ≤2.
q 2 ≤ 20 GeV2  qmax
• For the decay B̄s → μ+ μ− , we included the timeintegrated branching ratio from a combined analysis of the CMS and LHCb collaborations6) .
7)

8)• We use the Belle and Babar measurements ofthe branching ratio of the inclusive mode B̄ →Xs + − in the bin 1 GeV2 ≤ q 2 ≤ 6 GeV2 .
We performed a Bayesian analysis of several scenarios,including the Standard Model (where only nuisanceparameters are ﬁtted) and scenarios in which subsetsof the Wilson coeﬃcients are allowed to ﬂoat. The 2Dmarginalized posterior for the C9 -C10 scenario is shownin Fig. 1. The best-ﬁt point has C9 − C9SM = +1.6+0.7−0.9 ,†∗1∗2∗3Condensed from the article in Phys. Rev. D 94, 013007(2016)RIKEN Nishina Center
Department of Physics, University of Arizona
Physik-Institut, Universität Zürich- 136 -−2C10−3−4−5−6−7−4−202C9468Fig. 1. The 2D-marginalized posterior in the C9 -C10 plane.
To demonstrate the impact of the baryonic decay inthe analysis, we show the results from a ﬁt to the B̄ →+ −+ −Xs   and B̄s → μ μ branching ratios only (bluelines) and from the full ﬁt including also the Λb →−+ −Λ(→ p π )μ μ observables (orange-red areas). The
SM point is marked with a diamond, while the best-ﬁt
point from the full ﬁt is marked with a cross.

which is opposite in sign compared to the global ﬁtsof mesonic-only decays3) . Furthermore, the posterior
odds favor the Standard Model over the scenarios inwhich Wilson coeﬃcients are allowed to ﬂoat. The
uncertainties in our analysis of Λb → Λ(→ p π)+ −are currently dominated by experiment, and will be
reduced with higher-statistics results and a full angularanalysis expected in the near future.
References
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