Kyoto Univ a KEKb RIKENc CNS Univ of

有限密度における ベクター中間子の質量変化の検証 Kyoto Univ. a , KEKb, RIKENc, CNS Univ. of Tokyod, Megumi Naruki, KEK, 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, R. Mutob, T. Nakuraa, K. Ozawad, F. Sakumaa, O. Sasakib, M. Sekimotob, T. Tabaruc, K. H. Tanakab, M. Togawaa, S. Yamadaa, S. Yokkaichic, Y. Yoshimuraa (KEK-PS E 325 Collaboration) Introduction n Experimental Setup n Results n Future Plan n 1

Hadron Mass Hot/Dense Matter QCD Vaccum u d u Spontaneous Breaking of Chiral Symmetry u d Restoration hadrons constituent quarks ~300 Me. V/c 2 ~Ge. V/c 2 qq vacuum u current quarks ~Me. V/c 2 higgs vacuum 2

Mass modification at finite density dropping mass Hatsuda & Lee PRC 46(1992)R 34 • Brown-Rho scaling (’ 91) – m*/m = 0. 8 at r = r 0 • QCD Sum Rule by Hatsuda & Lee (’ 92) – m*/m = 1 - 0. 16 r/r 0 for r/w – m*/m = 1 - 0. 03 r/r 0 for f • Lattice Calc. by Muroya, Nakamura & Nonaka(’ 03) r w width broadening (at r 0) • Klingl, Kaiser, Weise (’ 97 -8) G*/G~10 for r/w/f • Rapp & Wambach (’ 99) : G*r/Gr ~2 • Oset & Ramos (’ 01) : DGf = 22 Me. V • Cabrera & Vicente (’ 03) : DGf = 33 Me. V f 3

KEK-PS E 325 experiment measures Invariant Mass of e+e-, K+Kin 12 Ge. V p + A → r , w , f + X reactions r/w e e f • low energy : the mass modification at the normal nuclear density • dilepton measurement : free from final state interactions Expected Invariant Mass distribution of r and w Decay in vacuum w mass modified by the formula : m*/m=1 -0. 16 r/r 0 Prog. Theor. Phys. 95(19 96)1009 r In Copper Nuclei

KEK-PS E 325 experiment We measure Invariant Mass of e+e-, K+Kin 12 Ge. V p + A r, w, f + X slowly moving r, w, f (plab~2 Ge. V/c） larger probability to decay inside nucleus Beam primary proton beam (~109/spill/1. 8 s) Target Øinteraction length 0. 2%/0. 05% (C/Cu) Øradiation length: 0. 4/0. 5%(C/Cu) History ’ 93 proposed ’ 96 construction start ü NIM, A 457, 581 (2001) ü NIM, A 516, 390 (2004) ’ 97 first K+K- data ’ 98 first e+e- data ü r/w: PRL, 86, 5019 (2001) ’ 99~’ 02 x 100 statistics in e+eü r/w: PRL, 96, 092301 (’ 06) ü f ee: PRL, 98, 042501 (‘ 07) ü a: PRC, 75, 025201 (‘ 06) x 6 statistics in K+Kü f KK: PRL, 98, 152302 (’ 07) 5

Detector Setup M. Sekimoto et al. , NIM, A 516, 390 (2004). Start Timing Counter Forward LG Calorimeter Hodoscope Aerogel Cherenkov Rear LG Calorimeter Forward TOF Side LG Calorimeter Barrel DC B 0. 81 Tm Cylindrical DC m ea b n 12 V Ge 1 m o t o pr Rear Gas Cherenkov Vertex DC Front Gas Cherenkov 6

Spectrometer Performance Ks M =496. 8± 0. 3(MC 496. 9± 0. 1) Me. V/c 2 s= L p p- p+ p- 3. 9± 0. 4(MC 3. 5± 0. 1) M =1115. 71± 0. 02(MC 1115. 53± 0. 01) Me. V/c 2 (s = 1. 73± 0. 02(MC 1. 62± 0. 01) Me. V/c ) by the simulation Mass spectra are well reproduced Expected mass resolution for f = 10. 7 Me. V/c 2 2 8

Invariant Mass Spectrum of e+e. C w e+e- f e+e- Cu w e+e- f e+e- we examine how well the data are reproduced with known hadronic sources & combinatorial background 9

Invariant Mass Spectrum of e+e- C w e+e- c 2/dof=161/140 f e+ e- Cu w e+e- c 2/dof=154/140 f e+ e- the excess over the known hadronic sources on the low mass side of w peak has been observed. 10

Invariant Mass Spectrum of e+e(background subtracted) C Cu r/w ratio is consistent with zero. 95%C. L. allowed regions: Nr/Nw<0. 04(stat. )+0. 09(sys. ) Nr/Nw<0. 10(stat. )+0. 21(sys. ) most of r decay in nucleus due to their short lifetime; t ~ 1. 3 fm 11

Invariant Mass Spectrum of e+ew e+e- Cu f e+e- we examine how well the data are reproduced with known hadronic sources & combinatorial background 12

e+e- Invariant Mass Distributions f C [Ge. V/c 2] f Cu [Ge. V/c 2] • fit with MC shape & quadratic curve • a hint on the spectrum of Cu data. • longer lifetime; t~50 fm kinematical dependence 13

To see bg dependence Slowly moving f mesons have a larger probability to decay inside the target nucleus. We divided the data into three by bg ( = p/m ); bg<1. 25, 1. 25<bg<1. 75 and 1. 75<bg. bg distribution 14

Invariant mass spectra of f e+e 1. 25<bg<1. 75<bg (Fast) Large Nucleus Small Nucleus bg<1. 25 (Slow) Rejected at 99% confidence level PRL 98(2007)042501 15

e r/w Model Calculation w/ medium modification e f • dropping mass: M(r)/M(0) = 1 – k 1 (r/r 0) (Hatsuda & Lee) • width broadening: G(r)/G(0) = 1 + k 2 (r/r 0) (k 2: 5~10) r, w f m*/m 1 – k 1 r/wr/r 0 1 – k 1 f r/r 0 G*/G 1 1 + k 2 r/r 0 surface uniform a=0. 710± 0. 021 a=0. 937± 0. 049 generation point momentum dist. density distribution measured Woods-Saxon, radius: C: 2. 3 fm/Cu: 4. 1 fm 16

Fit Results of Model Calculation m*/m = 1 - 0. 092 r/r 0 C Cu [Ge. V/c 2] the excesses for both C and Cu are well reproduced by the model including the 9% mass decrease at r 0. 17

Width Broadening events[/10 Me. V/c 2] k 1 = 0. 08 k 2 = 1 events[/10 Me. V/c 2] C Cu 「Ge. V/c 2] the best fit values are; k 1 = 9. 2 ± 0. 6% k 2 <0. 32(90%C. L. ) （Preliminary) 18

Invariant spectra of f e+efit with modified M. C. ( k 1=0. 034, k 2=2. 6 ) 1. 25<bg<1. 75<bg (Fast) Large Nucleus Small Nucleus bg<1. 25 (Slow) 19

Fit Results of model calculation m*/m = 1 – k 1 r/r 0, G*/G = 1 + k 2 r/r 0 Contours for k and r/w Contours for k 1 and 1 k 2 of f e+e- Best Fit Values f k 1 9. 2 ± 0. 2% 3. 4+0. 6 -0. 7% k 2 0 (fixed) 2. 6+1. 8 -1. 2 r/w 0. 7 ± 0. 1 (C) 0. 9 ± 0. 2 (Cu) - The data were well reproduced with the model; mr/w decreases by 9%, mf decreases by 3% and Gf increases by 3. 6 at r 0 m(r)/m(0) r, w 1 0. 9 fit result f fit result r/w 0. 8 prediction 0. 7 0 0. 5 1 r/r 0 syst. error is not included 20

J-PARC E 16 Electron pair spectrometer to explore the chiral symmetry in QCD high momentum beam line Hadron Hall 50 -Ge. V PS Split Point Switch Yard T 0 Target A-Line T 1 Target E 16 Beam Dump

J-PARC E 16 Electron pair spectrometer to explore the chiral symmetry in QCD primary proton beam at high momentum beam line 　　　　　　　+ large acceptance electron spectrometer 107 interaction (10 X E 325) 1010 protons/spill with 0. 1% interaction length target GEM Tracker e. ID : Gas Cherenkov 　 　 + Lead Glass Large Acceptance (5 X E 325) velocity dependence nuclear number dependence (p Pb) centrality dependence 　 systematic study of mass modification

Summary We have observed the excess over the known hadronic sources at the low-mass side of w. Obtained r / w ratio indicates that the excess is mainly due to the modification of r. We also observed the excess at the low-mass side of f, only at the low bg region of Cu data. The data were well reproduced by the model calculation based on the mass modification. The fit results show that; ü r/w : the mass decreases by 9% at r 0. ü f : the mass decreases by 3%, and the width increases by a factor of 3. 6 at r 0. The mass modification is not statistically significant for the K+Kinvariant mass distributions. The observed nuclear mass-number dependences of f e+e - and f K+K- are consistent. – We have obtained limits on the in-medium decay width broadenings for both the f e+e- and f K+K- decay channels. 23