E781; neutral cascade particle

 1.0 The neutral cascade particle x0 (mass=1.315 GeV, ctau=8.71 cm)
     has 99.54% branching ratio for lambda and pi0 decay mode.
     It might be interesting object for our charmed baryon analysis.
     The background comes from the broad resonance sigma(1385) and
     non-resonant lambda and pi0 or sigma0 and gamma combinations.
     Sigma background can be suppressed with appropriate mass cut.

     The data presented here were extracted from our 1M sample.
     
     Lambda & pi0 background is suppressed using geometrical approach.
     I assume that x0 originates from primary interaction. It decays
     somewhere on the lambda path in between primary vertex and lambda
     decay point. Using Z coordinate of x0 decay is unknown parameter,
     we find X and Y from lambda vector, then calculate the gamma as
     they would originate from this point. They must make an effective
     mass in a given pi0 mass window. The Fit condition is a minimum
     of miss distance of full x0 momentum vector to primary vertex.
     The background solutions (at the edges of Z interval) were killed
     with the cut on minimal decay length of x0 and lambda and cut on
     the uncoleaniarity angle between momentum and geometrical vectors
     The ratio of neutral pion momentum to lambda momentum must be less
     than 0.35 (a safely kinematical cut for x0 and lambda decay). 

     Only lambdas detected in M1 spectrometer are used in the analysis.
     Lambda and x0 were required to have decay length > 10 cm.
     The uncoleaniarity cut was equal to 5 mrad.

     The pi0 mass window was chosen to be +- 20 MeV. 
     The hits in all 3 Photon calorimeters were accepted.
     At least 1 photon was required to be identified gamma (id=1).

     The results of first selection are presented in    fig.1 

     In second selection I used only Photon 2 and 3 to constrain pi0,
     because we might expect greater momentum vector errors for gammas
     detected in Photon 1 than in Photon 2,3 due to shorter distance
     from charm target (5m, 33m, 50m for  Photons 1,2,3 respectively)
     and large error in Z(x0) position of about 10-20 cm.
     The pi0 mass window was chosen to be +- 10 MeV. 
     Both gammas were required to be identified gammas (id=1).

     The results of second selection are presented in    fig.2 

     In third selection I did cut on sigma0 entries. Namely, lambdas
     from x0 candidates should have no one lambda, gamma combination
     in sigma0 mass window of +-8 MeV.
     
     The results of third selection are presented in    fig.3 

     The short outcome is given in the table below.


                            Photons 1+2+3
                   ---------------------------------
                     x0 events      mass mean value   GeV/cc     rms
  lambda (2p)       88.4  +- 34.5    1.327 +- 0.0024           0.0049
  anti-lambda (2p)  24.7  +-  5.2    1.319 +- 0.0004           0.0046
  

                            Photons 2+3
                   ---------------------------------
                     x0 events      mass mean value   GeV/cc     rms
  lambda (2p)       41.2  +- 12.5    1.329 +- 0.0011           0.0049
  lambda (1p)       16.8  +-  9.2    1.313 +- 0.0024           0.0044
  anti-lambda (2p)  10.4  +-  4.7    1.316 +- 0.0014           0.0048


                            Photons 2+3, sigma0 cut
                   ---------------------------------
                     x0 events      mass mean value   GeV/cc     rms
  lambda (2p)       49.8  +- 13.4    1.329 +- 0.0030           0.0099


     The conclusion:

     1. We do have about 25 anti-ksi-zero reconstructed decays per
        1 million filtered interactions with fully reconstructed lambda
        and about 35 reconstructions with all lambdas (1p+2p).
     
     2. We do have 3.5 sigmas peak in lambda and pi0 effective mass.
        It is shifted by 14 MeV relatedly to PDT x0 mass.
        We do not have such a shift if lambda was reconstructed
        with unknown pi- momentum and we do not have it for anti-x0,
        with anti-proton and pi+ calculated momenta (pfit).
     
     3. The cut of sigma0 + gamma combinatorial background does not
        makes influence on the peak position. The background goes down. 
        
     4. We maybe see a systematics between pi- and pi+ pfit results
        for soft pion in M1 spectrometer.
        
     5. We probably do have high ratio 1:6 of anti-x0 to x0 production.
        Then we should have about 150 x0 in Photon 1+2+3 spectra.
        
     The second conclusion:

       x0 is the interesting object for our charmed baryon analysis.
       We need to increase the statistics to understand the situation.

       The Photon block seems to be not responsible for the mass shift,
       but I shall take care to add a real gamma simulation to our
       Embed because it is time to make real physics with Photons.

       I request for  lambda strip file from 10M or 50M filtered data.
       A 50M strip will be of the size of current 1M sample.
       We could expect to have more than 1000 anti-ksi-zero events.
       We could have enough to understand a a systematics and 
       to see lambda_c => lambda, pi+, pi0 decay etc.