Measurement of the parity-violating asymmetry parameter ${\alpha }_b$ and the helicity amplitudes for the decay ${\mathrm{\Lambda }}_b^0\to J/\psi {\mathrm{\Lambda }}^0$ with the ATLAS detector

A measurement of the parity-violating decay asymmetry parameter, ${\alpha }_b$, and the helicity amplitudes for the decay ${\mathrm{\Lambda }}_b^0\to J/\psi ({\mu }^{+}{\mu }^{-}){\mathrm{\Lambda }}^0(p{\pi }^{-})$ is reported. The analysis is based on 1400 ${\mathrm{\Lambda }}_b^0$ and ${\overline{\mathrm{\Lambda }}}_b^0$ baryons selected in $4.6{\mathrm{fb}}^{-1}$ of proton–proton collision data with a center-of-mass energy of 7 TeV recorded by the ATLAS experiment at the LHC. By combining the ${\mathrm{\Lambda }}_b^0$ and ${\overline{\mathrm{\Lambda }}}_b^0$ samples under the assumption of $CP$ conservation, the value of ${\alpha }_b$ is measured to be $0.30\pm 0.16(\text{stat})\pm 0.06(\text{syst})$. This measurement provides a test of theoretical models based on perturbative QCD or heavy-quark effective theory.

Figure 1

The decay angles, as defined in the text.

Figure 2

The reconstructed mass of ${\mathrm{\Lambda }}_b^0$ and ${\overline{\mathrm{\Lambda }}}_b^0$ candidates, fitted with a three-component PDF (blue solid curve) consisting of signal (blue dashed curve), combinatorial (magenta long-dashed straight line), and $B_d^0$ background (red dot-dashed curve, bottom).

Figure 3

The $F_i$ ($i=2,4,6,18,19$) distribution for events in the sidebands (red open circles for the left sideband and blue open triangles for the right sideband), together with the distribution for events in the signal region (black filled circles).

Figure 4

Event distribution for each angular variable in simulated data after acceptance, efficiency, and resolution effects are taken into account. The red filled points show the distributions in the default MC sample, where the generated distributions are uniform in all angular variables. For illustration of the sensitivity, the default MC events weighted using PDFs with ${\alpha }_b=\pm 1$ (green filled down triangles and blue open squares) and the measured value ${\alpha }_b=0.3$ (open up triangles) are also shown. Other parameters are set to $k_{+}=0.21$ and $k_{-}=0.13$ (measured values), and ${\mathrm{\Delta }}_{+}={\mathrm{\Delta }}_{-}=0$.

Figure 5

The conditional ${\chi }_{\min }^2$ as a function of ${\alpha }_b$.

Figure 6

The predicted distributions of $F_i$ variables from the sum of the weighted MC events (red line) and the background (blue area) are compared with data (black points). The background is estimated by adding the left and right sidebands and scaling by 0.5. The ${\chi }^2$-test probability of each comparison is shown in the top right corner of the plot. The predictions of the unweighted MC events (black dashed line) are also shown.