Speaker
Yosuke Watanabe
(Tokyo University)
Description
The PHENIX experiment observed a large enhancement of electron-positron
pairs in the invariant mass range of 0.2 to 0.5 GeV/c^2 in Au+Au collisions
at sqrt(s_NN)=200 GeV. However, it is difficult to draw a firm physics
conclusion from the measurement, since the measurement still has large
statistical and systematic uncertainties.The main uncertainty comes from
the small signal-to-background ratio of about 1/200 in minimum bias
collisions. The electron decay branching ratios of light vector mesons
are very small (~10^{-4}), while there are many background electrons mainly
originating from pi^0 Dalitz decays and gamma conversions.
A Hadron Blind Detector,(HBD) has been installed in the PHENIX to reject
such background electrons using the opening angle information of electron
positron pairs, since the opening angle of electron positron pairs from
those decays is small compared to the pairs from light vector mesons.
PHENIX has successfully collected p+p and Au+Au data with the HBD in 2009 and
2010.
In Au+Au central collisions, high occupancy of HBD readout pads needs to be
handled. The high occupancy is mainly due to the scintillation light emitted
by CF4, which is the Cherenkov radiator gas of the HBD. In Au+Au peripheral
collisions or p+p collisions, the effect of scintillation light is negligible
compared to the Cherenkov light by electrons. However, in Au+Au central
collisions, the number of charged particles is so large that the effect of
scintillation lights becomes significant.
In this poster, details of the analysis scheme to handle this situation
will be presented and current performance of HBD in di-electron measurements
will be reported.
Primary author
Yosuke Watanabe
(Tokyo University)
