V0 decays

The appearance of the decay of an unobserved neutral strange particle into two observed charged daughter particles gives rise to the terminology 'V0' to describe the decay topology. The following neutral strange species have been analysed:

Species	Decay channel	Branching ratio
K⁰_S	π⁺ + π^-	0.692
Λ	p + π^-	0.639
anti-Λ	anti-p + π⁺	0.639

Candidate V0s are formed by combining together all possible pairs of opposite charge-sign tracks in an event. The invariant mass of the V0 candidate under different decay hypotheses can then be determined from the track momenta and the daughter masses (e.g. for Λ the positive daughter is assumed to be a proton, the negative daughter a π-minus). Raw invariant mass spectra are shown below. The spectra contain three contributions: real particles of the species of interest; neutral strange particles of a different species; combinatorial background from chance positive/negative track crossings.

Invariant mass spectrum for V0 candidates under K0s decay hypothesis

Figure 1: Invariant mass spectrum under K0s hypothesis

Invariant mass spectrum for V0 candidates under Lambda decay hypothesis

Figure 2: Invariant mass spectrum under Λ hypothesis

Invariant mass spectrum for V0 candidates under anti-Lambda decay hypothesis

Figure 3: Invariant mass spectrum under anti-Λ hypothesis

Selection cuts are applied to the candidates to suppress the background whilst maintaining as much signal as possible. There are two methods for reducing background; energy-loss particle identification and geometrical cuts on the V0 candidates.