I looked at the Lambda decay into proton and pion- channel using FHC and FMS, and try to recreate the Lambda decay vertex. The .g and .kumac is similar to Qinghua's simulation setting except the Lambda decay channel.

     The Lambda particle is shot from (0,0,0) to (125,0,850) with energy is 50 GeV, and assume the branch ratio of Lambda -> Proton+pion- is 100%. The FHC and FMS positions are as follows,

     The number of event is 2k.

    1, as the proton and pion- pass through FMS will have energy loss. I exclude event which energy deposit in FMS is larger than 5 GeV. The following is the Lambda decay vertex in x and z component.

      2, the energy of proton and pion- decay from lambda.

     3,  energy deposit in FMS of each event.

    4,  energy deposit in FHC of each event.

   5, energy deposit in both FMS and FHC of each event.

  6,  I use one loop to find the possible Lambda vetex in the line between primary vetex and proton position in the surface of FHC. If the recreated Lambda mass is around 1.11568 GeV with error 0.0002 GeV, then I define this position is the recreated Lambda vertex.

     The recreated Lambda mass

    And I compare the the Lambda recreated vertex with the real Lambda decay vertex, (http://www.star.bnl.gov/protected/spin/xuanli/FHC/PPion/vertex_compare.list)

    the error is within 2cm.