Amplitude analysis of the ${\mathrm{\Lambda }}_c^{+}\to p{K}^{-}{\pi }^{+}$ decay and ${\mathrm{\Lambda }}_c^{+}$ baryon polarization measurement in semileptonic beauty hadron decays

An amplitude analysis of ${\mathrm{\Lambda }}_c^{+}\to p{K}^{-}{\pi }^{+}$ decays together with a measurement of the ${\mathrm{\Lambda }}_c^{+}$ polarization vector in semiōleptonic beauty hadron decays is presented. A sample of 400 000 candidates is selected from proton-proton collisions recorded by the LHCb detector at a center-of-mass energy of 13 TeV. An amplitude model is developed and the resonance fractions as well as two- and three-body decay parameters are reported. The mass and width of the $\mathrm{\Lambda }(2000)$ state are also determined. A significant ${\mathrm{\Lambda }}_c^{+}$ polarization is found. A large sensitivity of the ${\mathrm{\Lambda }}_c^{+}\to p{K}^{-}{\pi }^{+}$ decay to the polarization is seen, making the amplitude model suitable for ${\mathrm{\Lambda }}_c^{+}$ polarization measurements in other systems.

Figure 1

Invariant mass distribution of selected $p{K}^{-}{\pi }^{+}$ candidates from the total dataset. The results from the fit described in the text are also shown.

Figure 2

Dalitz plot for the total sample of ${\mathrm{\Lambda }}_c^{+}\to p{K}^{-}{\pi }^{+}$ candidates.

Figure 3

Distributions for selected candidates together with amplitude fit projections in the lab system for (top row) invariant mass squared projections; (bottom row) decay orientation angle projections.

Figure 4

Distributions for selected candidates together with amplitude fit projections in the $\tilde{B}$ system for (top row) invariant mass squared projections; (bottom row) decay orientation angle projections.

Figure 5

Definition of the ${\mathrm{\Lambda }}_c^{+}$ polarization system. The ${\widehat{\mathit{y}}}_{{\mathrm{\Lambda }}_c^{+}}$ axis is orthogonal to the page, towards the reader.

Figure 6

Definition of the Euler angles describing the rotation from the ${\mathrm{\Lambda }}_c^{+}$ polarization system to the decay plane reference system: (left) proton polar and azimuthal angles and (right) $\chi$ angle.