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					Experimental Signature of in-medium
mass modification of vector mesons
      at normal nuclear density

Kyoto Univ.a , KEKb, RIKENc, CNS Univ. of Tokyod,
ICEPP Univ. of Tokyoe,Tohoku Univ.f

Ryotaro Muto, RIKEN, Japan
J. Chibab, H. En’yoc, Y. Fukaoa, H. Funahashia, H. Hamagakid, M. Ieirib,
M. Ishinoe, H. Kandaf , M. Kitaguchia, S. Miharae, K. Miwaa, T. Miyashitaa,
T. Murakamia, T. Nakuraa, M. Narukia, M. Nomachib, K. Ozawad, F. Sakumaa,
O. Sasakib, H.D. Satoa, M. Sekimotob, T. Tabaruc, K.H. Tanakab,
M. Togawaa, S. Yamadaa, S. Yokkaichic, Y. Yoshimuraa
(KEK-PS E325 Collaboration)
Abstract (KEK-PS E325)
We have measured e+e– and K+K– invariant mass spectra
to investigate in-medium mass modification of vector
mesons in 12GeV p+A r,w,f + X reactions.
   e+e- invariant mass
              e+e-
                                Physics Motivation
                                Experimental Setup

                                Preliminary Result of
               f e+e-
                               2002 data analysis
Physics Motivation
          Effective Mass of Quarks
    In Vacuum                                         At High r / T
mu  md  300MeV                                      mu  md  5MeV
    ms  500MeV                Spontaneous Breaking     ms  150MeV
                                 of Chiral Symmetry
                   qq   r ,T

                                             How to measure 

                                              Using Vector Mesons
 Vector Meson
Mass of Vector Meson r, w, f
                                     Hatsuda & Lee P.R.C 1992
 = 2 x Mq + small interaction term

             r, w
  •large mass modification
  150MeV at r = r0

  •large cross section
  •mass modification 20 ~ 40MeV
  •small decay width(4.4MeV/c2)
  sensitive to mass modification
Expected Signal
In 12GeV p + A  r, w, f+ X       mass modified by the formula
  Invariant Mass of e+e-, K+K-               m*/m=1-0.16r/r0

      Expected Invariant Mass distribution of ρandω


          Decay in vacuum          In Copper Nuclei

                         Slowly moving r, w, f (plab~2GeV/c)
                         → Large Acceptance Spectrometer
                                                       see poster Instr.3
Experimental Setup                                       by F. Sakuma
   Forward LG Calorimeter

                                                            Aerogel Cherenkov
  Rear LG Calorimeter
                                                                Forward TOF

Side LG Calorimeter

Barrel Drift Chamber

   Cylindrical DC

                                                    Rear Gas Cherenkov
                              Front Gas Cherenkov
       Vertex Drift chamber
   •very thin target with clean and high intensity beam
                      material beam      Interaction radiation
                               intensity length(%) length(%)
                      C         ~1x109        0.2%       0.4%
                      CuX4      ~1x109    0.05%X4      0.5%X4

                              Vertex Distribution
Spectrometer Performance
               Λ→pπ-                          K0→π+π-

ML= 1115.5MeV/c2 (PDG1115.7MeV/c2)   MK= 497.6MeV/c2 (PDG 497.7MeV/c2)
sL = 1.8MeV/c2  (Sim. 1.9MeV)        sK = 3.8MeV/c2  (Sim. 4.1MeV )

          Mass and Width are well reproduced by MC.
Invariant Mass Spectrum of e+e- (2002 data)

         w   e+e-              w   e+e-

  C                      Cu

             f   e+e-              f e+e-
On the Fit
   Resonance
     Breit-Wigner shape

     experimental effect estimated by Geant4
      simulation – energy loss, mass resolution,
      mass acceptance etc.
   Background
     combinatorial background obtained by mixed
   Relative abundances of mesons (r,w,f)
    and background are obtained by the fitting.
Invariant Mass Spectrum of e+e- (2002 data)

           w   e+e-                     w   e+e-

  C                            Cu

  excess                       excess

               f   e+e-                     f e+e-

  the excess over the known hadronic sources on the
     low mass side of w peak has been observed.
Invariant Mass Spectrum of e+e-
        (after subtracting background)

  C                                Cu

              r / w ratio is consistent with zero
   0.0A0.01(stat.)A0.2(sys.)                0.0A0.05A0.5

      The excess can be understood as modified r mesons.
          Model Calculation
 With the formula : m*/m=1-0.16r/r0

  C                               Cu

•generate on surface of forward hemisphere of the nucleus
•spectral function : Breit-Wigner + mass modification.
Invariant Mass Spectrum of
           f e+e-

           C                      Cu

~1000f                  ~1000f

         Mass(GeV/c2)            Mass(GeV/c2)
  KEK PS-E325 experiment measured e+e- and
  K+K- pairs to investigate invariant mass of vector
  mesons decaying in nuclear matter.
  In 2002 e+e- data, we have observed the excess
  over the known hadronic sources below the w
  peak. Obtained r / w ratio indicates that this
  excess is mainly due to the modification of r
  Model calculation well reproduced the tendency
  of data.
  Analysis on phi meson is now in progress.
w kinematical distribution

Energy Loss

•black line – MC
•red line – Breit-Wigner(gaussian
convoluted) fit result
•green line – same as red line but
excluded by fit
        Model Calculation
With the formula : m*/m=1-0.16r/r0
   Combinatorial background

                     Major background souces are
                     • p0→gg (g→ee)
                     • p0→ eeg

                      + p- invariant mass is well
                     described with the mixed
                      + p- correlation is only
                     significant for K0s

Spectrum of p pair   It is reasonable to use ee mixed event
                        for the combinatorial background

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