Precision measurement of the ${\Lambda }_c^{+}$ baryon mass

The ${\Lambda }_c^{+}$ baryon mass is measured using ${\Lambda }_c^{+}\to \Lambda K_S^0{K}^{+}$ and ${\Lambda }_c^{+}\to {\Sigma }^0{K}_S^0{K}^{+}$ decays reconstructed in $232{\mathrm{fb}}^{-1}$ of data collected with the BABAR detector at the PEP-II asymmetric-energy $e^{+}{e}^{-}$ storage ring. The ${\Lambda }_c^{+}$ mass is measured to be $2286.46\pm 0.14\mathrm{MeV}/c^2$. The dominant systematic uncertainties arise from the amount of material in the tracking volume and from the magnetic field strength.

Figure 1

Invariant-mass distribution for ${\Lambda }_c^{+}\to \Lambda K_S^0{K}^{+}$ candidates. The lower part of the figure shows the normalized fit residuals. The dashed line indicates the present PDG value.

Figure 2

Invariant mass of reconstructed ${\Lambda }_c^{+}\to {\Sigma }^0{K}_S^0{K}^{+}$ candidates. The hashed histogram is the expected contribution from signal events with a wrongly assigned photon. The lower part of the figure shows the normalized fit residuals. The dashed line indicates the present PDG value.

Figure 3

Invariant-mass distribution of candidates used in cross-checks of systematic uncertainty. (a) $\Lambda \to p{\pi }^{-}$ decays. (b) $K_S^0\to {\pi }^{+}{\pi }^{-}$ decays. (c) ${\Lambda }_c^{+}\to p{K}^{-}{\pi }^{+}$ decays. (d) ${\Lambda }_c^{+}\to p{K}_S^0$ decays. The dashed lines show the PDG mass values for these hadrons. A correction to the mass distributions for underestimated energy loss has not been applied.

Figure 4

Fitted mass values for $\Lambda \to p{\pi }^{-}$ (left) and $K_S^0\to {\pi }^{+}{\pi }^{-}$ (right) candidates minus the PDG value as a function of decay vertex radius. The solid circles are for track reconstruction with the normal amount of detector material. The open squares are for track reconstruction assuming 20% more material in the SVT. The dashed lines show the $\pm 1\sigma$ uncertainty on the PDG value for the masses.

Figure 5

Fitted mass value for ${\Lambda }_c^{+}\to p{K}^{-}{\pi }^{+}$ (left) and ${\Lambda }_c^{+}\to p{K}_S^0$ (right) candidates minus the PDG value as a function of ${\Lambda }_c^{+}$ laboratory momentum. The solid circles are for track reconstruction with the normal amount of detector material. The open squares are for track reconstruction assuming 20% more material in the SVT. The dashed line shows the $+1\sigma$ uncertainty on the PDG value of the ${\Lambda }_c^{+}$ mass.

Figure 6

Fitted mass of ${\Lambda }_c^{+}\to p{K}^{-}{\pi }^{+}$ candidates minus the PDG value as a function of ${\Lambda }_c^{+}$ laboratory momentum. The additional energy-loss correction has not been applied. The solid circles are for track reconstruction with the normal magnetic field. The open squares are for track reconstruction assuming a 0.02% higher solenoid field. The open triangles are for track reconstruction assuming 20% higher magnetization. The dashed line shows the $+1\sigma$ uncertainty on the PDG value of the ${\Lambda }_c^{+}$ mass.

Figure 7

Fitted mass value of $K_S^0\to {\pi }^{+}{\pi }^{-}$ (left) and ${\Lambda }_c^{+}\to p{K}^{-}{\pi }^{+}$ (right) candidates minus the PDG value as a function of the azimuthal direction. The additional energy-loss correction has not been applied. The dashed lines show the $\pm 1\sigma$ uncertainty on the PDG value of the masses.