Subtracted

1
Measurement of low-mass vector mesons via
di-electron decay in vsNN 200 GeV AuAu
collisions at RHIC-PHENIX
Yoshihide Nakamiya (Hiroshima Univ ) for the
PHENIX Collaboration
Physics Motivation
Invariant mass spectra
Low-mass vector mesons (?,?,f) life time
?23fm/c f46fm/c
Fig2. ??ee- in QGP

• Physics
• High energy heavy-ion collisions have
  capability creating Quark-Gluon Plasma (QGP)
• governed by the partonic degree of freedom at
  high energy density. Under extreme hot
• matter like QGP, the mass of vector mesons can
  change due to the partial restoration of
• chiral symmetry (Fig1).
• Targets
• According to hydrodynamics calculation, QGP
  duration time is expected to be about
• 10 fm/c. So that short lived vector mesons are
  desirable for this study. Low-mass vector
• mesons are suitable for observation of mass
  modification because their lifetime is
• comparable to duration time (Fig2).
• Probes
• Electromagnetic probes such as leptons and
  photons are clean keys to survey the
• property of QGP directly because they penetrate
  in medium with less strong interaction.
• ?Measurement of low-mass vector mesons via
  di-electron decay are suitable for this
• research.

Red point foreground Blue point background
Subtracted background
Subtracted background
Result
Fig6. Invariant mass spectra of di-electrons

• Invariant mass spectra and normalization
• The signal of low-mass vector mesons is
  extracted from the invariant mass spectrum
• after subtracting the combinatorial background
  which is evaluated by the event mixing
• technique . The foreground and the mixed events
  divided by centrality class is shown
• in the top-left figure and by transverse momentum
  range in the top-right figure. The invariant
• mass spectra after subtracting background are
  shown in the bottom-left and the bottom-right
• figure. The mixed event pairs are made of tracks
  in different events with a same centrality
• class and a same vertex class. Normalization
  between the foreground and the background is
• calculated by using like-sign methods.
  Normalization factor a is given by
• Signal counting
• The signal is counted by fitting with a
  Gaussian convoluted relativistic Breit-Wigner
  function.
• (Mass centroid and width are fixed at the PDG
  values, experimental resolution is fixed 6.9
• MeV for ? mesons and 5.6 MeV for f mesons based
  on a GEANT simulation.)

Black line statistical error Box systematic
error
The invariant ? yield per unit rapidity is
shown in Fig7. This yield is scaled by 0.5the
number of participant nucleons. The ? yield is
found to scale with the number of participant
nucleons, though within large errors.
PHENIX preliminary
Fig7. The invariant ? yield per unit rapidity as
a function of number of participants nucleons.
Black line represents statistical error and box
represents systematic error.
The invariant pT spectrum for ? meson is
compared with hadronic decay channels and
radiative decay channels at various collision
systems. The result of ??ee- in AuAu
collisions at vsNN200 GeV is in a good
agreement with that of ??p0? in AuAu at vsNN200
GeV, ? ?p0? in pp at vsNN200 GeV and ??p0p-p
in pp at vsNN200 GeV (Fig8). All data are
scaled by the number of binary nucleon-nucleon
collisions and branching ratios.
Fig8. The invariant pT spectrum for ? mesons as
a function of pT .
Methods
Experimental Setup

• Dalitz and photon conversion pair rejection
• 99 of generated electrons come from Dalitz
  decay or photon conversion from the beam pipe.
  They made enormous
• combinatorial background in reconstructing
  invariant mass of electron-positron pair. Dalitz
  pairs make a correlated peak
• near 0 GeV/c2 in the region of invariant mass and
  lasting up to p0 and ? mass. Thus we rejected any
  track which makes
• a correlated peak in such range (Fig3). Electrons
  by pair-creation at the beam pipe make the
  correlated peak at around 0.02
• GeV/c2 (Fig3), because they have finite opening
  angle since reconstruction is performed based on
  collision vertex.
• Thus we reject any tracks which make a correlated
  peak in this region. Signal to background ratio
  make progress by a few
• after applying this cut. However we cannot
  reject all background electrons because we cannot
  tagged their pairs
• completely. For we cannot detect one of a pair
  due to the acceptance of PHENIX spectrometers,
  besides some electrons
• curled up and cannot go out of magnetic field. In
  order to improve it, Hadron Blind Detector (HBD)
  have already installed
• and was on line at present. Signal to background
  ratio will be expected to improve dramatically.
• Ring sharing pair rejection
• Charged particles generated at the collision
  vertex are bent by the magnet and enter the RICH
  plane. At this time, the
• vector of a charged track is projected to the
  RICH PMT plane and connected to a RICH ring. When
  two tracks are parallel
• with each other, projected positions of the two
  tracks are the same. This fact sometimes make a
  ghost electron associated
• with a real electron. This RICH ghosting
  phenomenon makes correlation on the invariant
  mass spectrum and make
• normalization between foreground and backgrounds
  difficult. We reject such tracks by using two
  parameters. One is the
• post field opening angle (PFOA) which is the
  angle between two tracks at Drift Chamber. The
  other is the position

Fig5. The PHENIX spectrometer (Beam View)
Fig3. Invariant mass of di-electrons

• RHIC accelerator
• The RHIC accelerator provides various
  collision systems from protonproton
• collisions to AuAu collisions at a broad range
  of c.m.s energies (vs 22.5,
• 62.5, 200, 500 GeV). Therefore RHIC has the
  capability of systematic
• measurement for various particles.
• PHENIX detector
• PHENIX spectrometers are versatile devices to
  measure electrons and
• photons as well as hadrons. PHENIX spectrometers
  consist of two central
• arms, which covers the pseudo-rapidity of
  0.35 and 90 degrees in
• azimuthal angle.

Fig4. Correlation between the post field opening
angle (PFOA) and the position difference (PD)
between the two RICH rings.