The Bethe-Bloch equation can be used to predict charged particle energy loss. Hans Bichsel's model adds to this and the Bichsel function predictions for particle energy loss are compared with measured values. Tracks with dE/dx sufficiently far from the predicted value are rejected. e.g. when selecting for Λ hyperons, the positive track is required to have dE/dx consistent with that of a proton, and the negative track consistent with that of a π-minus.

The quantity σ = sqrt(N) x log( measured dE/dx - model dE/dx ) / R is used to quantify the deviation of the measured dE/dx from the model value. N is the number of track hits used in dE/dx determination and R is a resolution factor. A cut of |σ| < 3 applied to both V0 daughter tracks was found to significantly reduce the background with no loss of signal. Figures one to three below show the invariant mass distriubtions of the V0 candidates accepted and rejected and table one summarises the results of the cut. Background rejection is more successful for (anti-)Λ than for K^{0}_{S} because most background tracks are pions; the selection of an (anti-)proton daughter rejects the majority of the background tracks.

Figure 1a: Invariant mass spectrum of V0 candidates under K0s hypothesis passing dE/dx cut |

Figure 1b: Invariant mass spectrum of V0 candidates under K0s hypothesis failing dE/dx cut |

Figure 2a: Invariant mass spectrum of V0 candidates under Λ hypothesis passing dE/dx cut |

Figure 2b: Invariant mass spectrum of V0 candidates under Λ hypothesis failing dE/dx cut |

Figure 3a: Invariant mass spectrum of V0 candidates under anti-Λ hypothesis passing dE/dx cut |

Figure 3b: Invariant mass spectrum of V0 candidates under anti-Λ hypothesis failing dE/dx cut |

Species | Pass (millions) | Fail (millions) | % pass |
---|---|---|---|

K^{0}_{S} |
95.5 | 48.9 | 66.2 % |

Λ | 32.5 | 111.9 | 22.5 % |

anti-Λ | 11.8 | 132.5 | 8.2 % |

*Table 1*