Sigma-b0 studies

The decay chain

     Sigma-b0 -> lambda-b0  + pi0

                 lambda-c+    pi-

                 p k- pi+
has been studied for evidence of sigma-b0 production.

The ntuples supplied had a reconstructed lambda-b0 plus gamma-pairs within a mass range 105 - 165 MeV.

This figure, Fig. 1 (37 plots), has a set of plots used to study the above described decay chain.

Plots 1 and 2 show the pi0 mass and energy, respectively, for all pi0s on the ntuple. There is evidence for a pi0 peak over a large combinatorial backgound; the majority of pi0s are soft.

Plots 3 and 4 give the pi0 mass distribution for different pi0 energy selections. The pi0 peak is enhanced relative to the combinatorial background with increasing energy.

Plots 5 and 6 show the lambda-b0 mass and energy for all events on the ntuple. In the absence of a fit, the lambda-b0 has a width ~ 25 MeV; the mean energy is ~100 GeV.

In plots 7 - 13, the lambda-b0 + pi0 mass range 5750 - 5850 MeV has been selected in order to study the kinematics of the lambda-b0 and pi0 within the region in which the sigma-b0 is expected to lie. The plots show that, other things being equal, the lambda-b (pi0) momentum is ~100GeV ( 5 GeV). Plot 13 does not show a clear pi0 signal.

Plots 14 - 31 show lambda-b0 + pi0 mass distributions covering the sigma-b mass range for a set of pi0 selections. Here an attempt has been made to get a clean set of pi0s by selecting the pi0 region (125-145 MeV) and subtracting as background sidebins, 115-125, and 145-155 for the 'narrow pi0' case. It will be seen that there is no evidence for an enhancement in the region 5800-5850 MeV where the sigma-b0 and its excited state may be expected to lie.


In the absence of Monte Carlo simulation for sigma-b0 production an attempt has been made to study the effect of gamma measurement errors on the sigma-b0 mass using the data. This study is shown in plots 32- 37 and repeated here in Fig. 2 (6 plots).

The nominal gamma measurement error, deltaE/E = 0.1 / sqrt(E(GeV)), has been used to smear the gammas from pi0 decay and the resulting smeared energy, momenta have been used to estimate the errors on the pi0 and sigma-b0 mass. The corresponding resolutions are shown in plots 1 and 2 of Fig 2.

The pi0 mass resolution has also been obtained from fits of the gamma-gamma mass spectra. These fits are presented in Figs 3 and 4 where the mass spectrum has been fitted to a gaussian distribution plus a linear backgound. From these fits the pi0 mass resolution is ~ 10 MeV, about twice that obtained using the nominal gamma measurement errors.

When the nominal gamma resolution is doubled to deltaE/E = 0.2 / sqrt(E(GeV)) the resolutions for the pi0 and sigma-b0 are as shown in plots 5 and 6. Here the pi0 resolution is closer to that found experimentally and the sigma-b0 resolution is ~ 20 Mev; this latter resolution does not take account of the effect of lambda-b uncertainties.

Comments and conclusions

The poor signal/background for the pi0, and ~20 MeV resolution for the sigma-b0, make it unlikely that, with the present statistics, the sigma-b0 and its neighbouring excited states can be separated and identified. It would be useful to have Monte Carlo simulation to confirm this.