PHENIX

Fig1. Phase diagram

Subtracted background

Subtracted background

Red point : foregroundBlue point : background

Low-mass vector mesons(ω,ρ,φ)～ life time ω： 23fm/c φ： 46fm/c

QGPDuration time τ~10fm/c

ω

e+

e-

x

y

Φ= -3/16π (= -0.589)

Φ= 5/16π (= 0.981)

Φ=11/16π (= 2.159)

Φ= 19/16π (= 3.729)

E0

E1

E2

E3W3

W0

W1

W2

e-

-trigger, vertex, centrality (by BB)

-tracking, pT (by DC and PC)-electron ID(by RICH and EMCal ) RHIC accelerator

The RHIC accelerator provides various collision systems from proton+proton collisions to Au+Au collisions at a broad range of c.m.s energies (√s = 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.

Invariant mass spectraInvariant mass spectra

Red : ω→e+e- AuAu 200 GeV Blue : ω→π0γ AuAu 200 GeV Light blue : ω→π0γ pp 200 GeV Yellow : ω→π0π+π- 　 pp 200 GeV

PHENIX preliminary

ResultResult

MethodsMethodsExperimental SetupExperimental Setup

Physics Motivation Physics Motivation

Measurement of low-mass vector mesons via di-electron decay iMeasurement of low-mass vector mesons via di-electron decay in √sn √sNN NN = 200 GeV Au+Au collisions at RHIC-PHENIX= 200 GeV Au+Au collisions at RHIC-PHENIX　　　 　　　 Yoshihide NakamiyaYoshihide Nakamiya (Hiroshima Univ ) for the PHENIX Collaboratio (Hiroshima Univ ) for the PHENIX Collaborationn

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 π0 and η 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 parallelwith 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 difference between the two RICH rings defined by

When two tracks fulfill PFOA < 0.25 and PD < 3, these tracks share the same ring and both tracks are rejected (Fig4)

Fig3. Invariant mass of di-electrons

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

Invariant mass spectra and normalization

The signal of low-mass vector mesons is extracted from the invariant mass spectrumafter 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-rightfigure. 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 α 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 ω mesons and 5.6 MeV for φ mesons based on a GEANT simulation.)

The invariant yield The invariant yield is obtained after correcting the acceptance and efficiency for electrons in PHENIX spectrometers, and also corrected the multiplicity dependent efficiency.

The invariant ω yield per unit The invariant ω yield per unit rapidity is shown in Fig7. This rapidity is shown in Fig7. This yield is scaled by 0.5×the number yield is scaled by 0.5×the number of participant nucleons. of participant nucleons. The ω yield is found to scale The ω yield is found to scale with the number of participant with the number of participant nucleons, though within large nucleons, though within large errors.errors.

Fig6. Invariant mass spectra of di-electrons

Fig5. The PHENIX spectrometer(Beam View)

The invariant pThe invariant pTT spectrum for ω meson is compare spectrum for ω meson is compare

d with hadronic decay channels and radiative decay d with hadronic decay channels and radiative decay channels at various collision systems. The result of channels at various collision systems. The result of ω→eω→e++ee-- in Au+Au collisions at √s in Au+Au collisions at √sNNNN=200 GeV is in a =200 GeV is in a

good agreement with that of ω→πgood agreement with that of ω→π00γ in Au+Au at √sγ in Au+Au at √sNN

NN=200 GeV, ω →π=200 GeV, ω →π00γ in p+p at √sγ in p+p at √sNNNN=200 GeV and ω=200 GeV and ω

→π→π00ππ--ππ+ + in p+p at √sin p+p at √sNNNN=200 GeV (Fig8). All data are =200 GeV (Fig8). All data are

scaled by the number of binary nucleon-nucleon collscaled by the number of binary nucleon-nucleon collisions and branching ratiosisions and branching ratios..

PHENIX preliminary

Fig8. The invariant pT spectrum for ω mesons as a function of pT .

Fig7. The invariant ω yield per unit rapidity as a function of number of participants nucleons. Black line represents statistical error and box represents systematic error.

Fig2. ω→e+e- 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 about10 fm/c. So that short lived vector mesons are desirable for this study. Low-mass vectormesons 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.

Black line : statistical errorBox : systematic error