Angular analysis of the decay Λ b → Λ(→ Nπ)ℓ + ℓ −

Abstract

We study the differential decay rate for the rare Λ b → Λ(→ N π) + − transition, including a determination of the complete angular distribution, assuming unpolarized Λ b baryons. On the basis of a properly chosen parametrization of the various helicity amplitudes, we provide expressions for the angular observables within the Standard Model and a subset of new physics models with chirality-flipped operators. Hadronic effects at low recoil are estimated by combining information from lattice QCD with (improved) formfactor relations in Heavy Quark Effective Theory. Our estimates for large hadronic recoil -at this stage -are still rather uncertain because the baryonic input functions are not so well known, and non-factorizable spectator effects have not been worked out systematically so far. Still, our phenomenological analysis of decay asymmetries and angular observables for Λ b → Λ(→ N π) + − reveals that this decay mode can provide new and complementary constraints on the Wilson coefficients in radiative and semileptonic b → s transitions compared to the corresponding mesonic modes.

Differential branching fraction and angular analysis of Λ0 b → Λμ+μ− decays

“…Secondly, as the Λ 0 b baryon may be considered as consisting of a heavy quark combined with a light diquark system, the hadronic physics differs significantly from that of the B meson decay. A further motivation specific to the Λ 0 b → Λµ + µ − channel is that the polarisation of the Λ baryon is preserved in the Λ → pπ − decay, 1 giving access to complementary information to that available from meson decays [3].…”

“…Theoretical aspects of the Λ 0 b → Λµ + µ − decay have been considered both in the SM and in some of its extensions [3][4][5][6][7][8][9][10][11][12][13][14][15][16]. Although based on the same effective Hamiltonian as that for the corresponding mesonic transitions, the hadronic form factors for the Λ 0 b baryon case are less well-known due to the less stringent experimental constraints.…”

“…The decay has a non-trivial angular structure which, in the case of unpolarised Λ 0 b production, is described by the helicity angles of the muon and proton, the angle between the planes defined by the Λ decay products and the two muons, and the square of the dimuon invariant mass, q 2 . In theoretical investigations, the differential branching fraction, and forward-backward asymmetries for both the dilepton and the hadron systems of the decay, have received particular attention [3,11,[15][16][17]. Different treatments of form factors are used depending on the q 2 region and can be tested by comparing predictions with data as a function of q 2 .…”

Perturbative corrections to Λ b → Λ form factors from QCD light-cone sum rules

“…First, the polarization asymmetry of the Λ-baryon in the decay products allows a "clean" extraction of the helicity structure of the weak effective Hamiltonian in the factorization limit [4,5]. Second, the angular distribution for the fourbody decays Λ b → Λ(→ N π) + − offers additional information on the Wilson coefficients of effective weak operators [6,7], due to the fact that the cascade weak decay Λ → N π is parity violating. Third, the systematic uncertainty entering the computation of the Λ b → Λ + − amplitude, induced by the Λ-baryon decay width, is negligible compared with the counterpart B → K * + − channels.…”

“…We also present the experimental data bins from LHCb [48] for a comparison, where various experimental uncertainties are added in quadrature. 6) where the definition of A FB (q 2 ) differs from [48] due to the distinct convention of the θ angle. We plot the q 2 dependence of the differential forward-backward asymmetry and the longitudinal polarization fraction in figure 12, and collect the theory predictions for the binned distributions of these two observables in table 4.…”