E781 Results of Sigma0 Reconstruction
1.0 The first obvious test of lambda reconstruction in V0 package
is the reconstruction of neutral Sigma Hyperon decay. Indeed,
we do a test for V0 and Photon packages.
First for Selex, the Sigma0 decay in Vertex spectrometer was
analyzed by Victor Kurshetsov (see his home page).
He suggested sigma0 to lambda rate equal to 1:2 and calculated
single photon efficiency to be about 40%.
I did Sigma0 reconstruction in order to compare different types
of lambdas we do have in V0 package and to obtain the acceptance
to reconstruct Sigma0 decay.
We have 3 types of lambdas:
- lambda in Vertex;
- lambda in M1 spectrometer (2p reconstruction);
- lambda in M1 spectrometer (1p reconstruction);
We have 2 types of photon hits:
- all photon hits (any hit in adcgam_bk)
- identified gammas (id_adcgam==1)
The second sample gives us more than less natural gammas which
do have expected energy distribution in led glass cluster, do
not have second maximum or charged particle entry in corresponding
cluster etc. But we have lost something (e.g. due to overlapping
photon and charged particle clusters).
I do not intend to suppress any background here using pi0
reconstruction, sigma0 kinematics etc, but this should be done
for real physics.
I demand a save-full energy cut: E gamma > 2 GeV.
All the PAW fits to Sigma0 mass were done by Grisha Davidenko.
Thank you, Grisha.
1.1 Lambda in Vertex.
The mass distributions for all photon hits and identified gammas
combinations with lambda in vertex selected in soft cut mode and
in mass interval +-5 MeV/cc (3000 lambdas) are shown in fig.1a
and in fig.1b
respectively.
one million filtered interactions
---------------------------------
Sigma0 events mass mean value GeV/cc rms
all photon hits 443 +- 92 1.193 +- 0.0016 0.0081
identified gammas 324 +- 73 1.192 +- 0.0016 0.0075
We see that we have right peak at right place, we lost 25% of
sigma(lambda,gamma) decays due to cut on gamma ID. The number
443 is in an agreement with sigma/lambda = 1:2 rate and single
photon efficiency of 44%.
1.2 Lambda in M1 spectrometer, 2p topology.
First, I took the lambdas selected with default V0 setup in mass
range +- 25 MeV/cc and any photon hits. The mass distribution
of Lambda+gamma combinations is shown in fig.2 .
Next, I did Lambda+gamma with identified gammas and lambdas
selected in soft cut mode (see fig.3 ).
One can see a nice peaks at sigma0 mass.
Sigma0 events mass mean value GeV/cc rms
default setup 2439 +- 187 1.191 +- 0.0009 0.0114
soft cut mode 1346 +- 112 1.192 +- 0.0010 0.0105
Applying the cuts we win about 10% in the mass resolution and
loose 45% in statistics. The default V0 setup seems to be good
to extract sigma0.
1.3 Lambda in M1 spectrometer, 1p topology.
I used soft cut mode, identified gammas and in addition,
uncoleaniarity cut of 2 mr (maximal angle between momentum vector
and geometrical vector of primary vertex and lambda decay vertex).
This works against helps cascade lambdas and artificial vertices.
The mass distributions for 1p topology is presented in fig.4a
and that for 2p topology in fig.4b .
(unco cut = 2) Sigma0 events mass mean value GeV/cc rms
1p topology 376 +- 67.3 1.193 +- 0.0020 0.0101
1p topology 1168 +- 106.2 1.192 +- 0.0010 0.0102
We may expect 442 sigma0 in vertex, 2439 in M1 with 2p topology and
376 in M1 with unmeasured pion momentum.
all together:
0.33% sigmas per 1 filtered interaction or 18% per lambda
=========================================================
(the multiple lambda entries might be in the sigma0 peak)
vmatveev@fnal.gov