Angular analysis of the rare decay $$ {\Lambda}_b\to \Lambda (1520)\left(\to N\overline{K}\right){\ell}^{+}{\ell}^{-} $$

Abstract

We study the differential decay rate for the rare decay Λ b → Λ(1520)(→ NK) + − where is a light lepton and NK = pK − , nK 0 , as this decay mode can provide new and complementary constraints on the Wilson coefficients in b → s + − transitions compared to other modes. We provide a determination of the complete angular distribution, assuming unpolarised Λ b baryons and neglecting the lepton mass. The resulting angular observables are expressed in terms of helicity amplitudes involving hadronic form factors within the Standard Model and New Physics models with chirality-flipped operators. We study these observables at low and large Λ recoils, using effective theories to determine relations among the hadronic form factors involved. As there is currently no determination of the form factors available from lattice simulations or light-cone sum rules, we perform a first illustration of the sensitivity of some observables to New Physics contributions using hadronic inputs from quark models.

Test of lepton universality with $$ {\Lambda}_b^0\to {pK}^{-}{\mathrm{\ell}}^{+}{\mathrm{\ell}}^{-} $$ decays

“…The first measurement of the branching fraction of the rare muonic decay mode Λ 0 b → pK − µ + µ − is also performed and its value is found to be B(Λ 0 b → pK − µ + µ − ) 0.1<q 2 <6 GeV 2 /c 4 = 2.65 ± 0.14 ± 0.12 ± 0.29 + 0.38 − 0.23 × 10 −7 , where the uncertainty is dominated by the limited knowledge of the Λ 0 b → pK − J/ψ normalisation mode. This result is obtained in the range m(pK − ) < 2600 MeV/c 2 , which includes several resonant structures, and thus cannot be directly compared to the recent predictions computed for the exclusive decay Λ 0 b → Λ(1520) + − [29].…”

“…Due to the lack of information on the exact resonant content in the pK − spectrum, it is challenging to compute the expected branching fraction of these decays in the SM, for which no prediction has been found in the literature. Predictions for specific excited Λ resonances, Λ * , in the decays Λ 0 b → Λ * + − with Λ * → pK − , have been computed [28,29] but cannot be directly compared to this result.…”

The $$ {\Lambda}_b\to {\Lambda}^{\ast }(1520)\left(\to N\overline{K}\right){\mathrm{\ell}}^{+}{\mathrm{\ell}}^{-} $$ decay at low-recoil in HQET

“…Theoretical studies of Λ b decay through a b → s + − transition have received considerable attention in the recent years [10][11][12][13][15][16][17][18]. Among the different semileptonic modes of Λ b decays to hadrons, the decay to Λ * ≡ Λ * (1520) has the dominant contribution [19].…”

“…In order to calculate the amplitudes, we need to know the form factor parametrizations of the Λ b → Λ * hadronic matrix elements. We follow the helicity parametrizations [17] (see appendix D) where the Λ * |sγ µ b|Λ b ( Λ * |sγ µ γ 5 b|Λ b ) transition is parametrized in terms of four q 2 -dependent form factors…”

“…The Λ b → Λ * transition matrix element can be parametrized using the Λ b and Λ * spinors in terms of form factors. For the transitions through vector, and axial vector currents, the JHEP07(2020)002 matrix elements are parametrized in terms of eight form factors [17]…”