Monte Carlo Simulation
8B Monte Carlo simulation plays a crucial role in the solar
neutrino analysis, because it is used to compute the standard
solar model neutrino flux prediction as a function of recoil
electron energy and zenith angle (angle of the sun with respect
to the vertical), taking into account reconstruction resolution
and analysis cut efficiencies. Since the new cuts are tighter,
cut efficiencies are smaller and a good Monte Carlo simulation
especially of the cut efficiencies becomes more important.
The various cut efficiencies are studied with calibration
data and Monte Carlo simulation of calibration data. The
discrepancy between calibration Monte Carlo and data efficiency
dominates the systematic error below 6.5 MeV.
The GRINGO cut efficiency is quite sensitive to the amount of light
scattering.
Light scattering produces hit PMTs which have a later time. These
`late hits' can pull the vertex fit and degrade the stability of
the fit. The light scattering and absorption model originally used in the
Monte Carlo assumes that the amount of Mie scattering, Rayleigh
scattering and light absorption is proportional to one single
water quality parameter, which depends only on the photon
attenuation length (water transparency). An
alternate model which assumes, that only the absorption depends
on the water transparency while Mie and Rayleigh scattering
coefficients remain aproximately constant in the region of
interest. This model drastically improves the agreement of
calibration Monte Carlo and data cut efficiency. Scattering
measurements done with a laser seem to favor this alternate
model.
However, there are other causes of late hits. Reflection, pre-
and afterpulses as well as non-gaussian tails in the PMT timing
response also cause late hits.
The detector simulation was retuned to apropriately describe
the late hits. As a consequence, the GRINGO cut efficiency
shows no discrepancy between calibration MC and data, which
significantly reduces the systematic error.
Since late hits will also affect the energy scale, the MC
energy scale and resolution as well as vertex and angular resolution
was retuned using LINAC calibration data.
PMT timing of LINAC calibration events. The time-of-flight
of the photon (from the vertex to the pmt) is subtracted. The
two boxes show the early and late hits in a different scale.