1. PHENIX Fig1. Phase diagram Subtracted background Subtracted background Red point : foreground Blue point : background Low-mass vector mesons (ω,ρ,φ) ～ life time ω ： 23fm/c φ ： 46fm/c QGP Duration time τ~10fm/c ω e+e+ 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 E3 W3 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 spectra Red : ω→e + e - AuAu 200 GeV Blue : ω→π 0 γ AuAu 200 GeV Light blue : ω→π 0 γ pp 200 GeV Yellow : ω→π 0 π + π - pp 200 GeV PHENIX preliminary Result Methods Experimental Setup Physics Motivation Measurement of low-mass vector mesons via di-electron decay in √s NN = 200 GeV Au+Au collisions at RHIC-PHENIX Yoshihide Nakamiya (Hiroshima Univ ) for the PHENIX Collaboration Yoshihide Nakamiya (Hiroshima Univ ) for the PHENIX Collaboration 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/c 2 in the region of invariant mass and lasting up to π 0 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/c 2 (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 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 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 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 rapidity is shown in Fig7. This yield is scaled by 0.5×the number of participant nucleons. The ω yield is found to scale with the number of participant nucleons, though within large errors. Fig6. Invariant mass spectra of di-electrons Fig5. The PHENIX spectrometer (Beam View) The invariant p T spectrum for ω meson is compared with hadronic decay channels and radiative decay channels at various collision systems. The result of ω→e + e - in Au+Au collisions at √s NN =200 GeV is in a good agreement with that of ω→π 0 γ in Au+Au at √s NN =200 GeV, ω →π 0 γ in p+p at √s NN =200 GeV and ω→π 0 π - π + in p+p at √s NN =200 GeV (Fig8). All data are scaled by the number of binary nucleon-nucleon collisions and branching ratios. PHENIX preliminary Fig8. The invariant p T spectrum for ω mesons as a function of p T. 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 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. Black line : statistical error Box : systematic error