? ? e e - measurement in vs_NN = 200 GeV AuAu collisions

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? ? ee - measurement in vs_NN 200 GeV AuAu
collisions Improvement of signal/noise ratio
for studying low-mass vector mesons via
di-electron decay channel in PHENIX
Yoshihide Nakamiya (Hiroshima Univ )
for the PHENIX collaboration
Physics motivation
dN/dy pT slope
The past experiment
dN/dy(0.5Npart) as a function of Npart
dN/dy for ? mesons is scaled by number of
participant within The margin of error.
invariant pT slope for ? mesons Invariant pT
spectrum for ? mesons are compared with other
decay channels and other collision system.
KEK E325
Invariant mass spectra
G.Agakichiev et al. Low-mass ee- pair production
in 158A GeV Pb-Au collisions at the CERN SPS, its
dependence on multiplicity and transverse
Momentum (Phys.Lett.B422405-412,1998)
K.Ozawa et al. Observation of r/v Meson
Modification in Nuclear Matter (Phys.Rev.Lett
86-22)
Some evidence of mass shift for low-mass vector
mesons was observed in the past experiment,
especially ?/?.
It is interesting what happened in hot matter at
RHIC ?
Improvement
-ring sharing rejection Tracks share the same
ring when they are parallel to each other.
Because the mirror of RICH is ideally spherical.
This RICH ghosting phenomenon made correlation in
the mass range about 0.5 GeV/2 and it affect on
normalization between foreground and mixing. We
rejected such tracks by using two parameters,
One is post field opening angle (PFOA), the
other is dCROSS. PFOA is is the angle between the
two tracks vectors after they have been deflected
by the PHENIX magnet. dCROSS is similar to
radius of ellipse on RICH z-phi plane.
-Dalitz photon conversion pair rejection 99
of generated electrons come from Dalitz decay or
photon conversion from beam pipe. They made
enormous combinatorial background in
making invariant mass of electron-positron pair.
While pairs from Dalitz decay and photon
conversion made invariant mass in low-mass
region. So that we reject Dalitz and photon
conversion component by low-mass cut. However the
effect of low-mass cut is a few because we
cannot tagged their pairs completely. Since most
of electrons from them curled up and cannot go
out of magnetic field. It is expected to be
improved for Dalitz rejection after installation
of Hadron Blind Detector (HBD).
Experimental Setup
-RHIC accelerator RHIC provides collisions by
multiple species (pp,dAu,AuAu) and at some c.m.s
energy (vs 22.5,62,5,200,500 GeV) Therefore
systematic measurement for various particles
become possible. -PHENIX detector PHENIX
detector has the high capabilities to measure
electrons, photons and hadrons. PHENIX Central
arm has 180azimuthal angle and also
pseudo-rapidity coverage of 0.35.