RIKEN, Kyoto Univ. a, KEK b, CNS Univ. of Tokyo c, ICEPP Univ. of Tokyo d,Tohoku Univ. e Ryotaro Muto, RIKEN, Japan J. Chiba b, H. En’yo, Y. Fukao a, H. Funahashi a, H. Hamagaki c, M. Ieiri b,M. Ishino d, H. Kanda e, M. Kitaguchi a, S. Mihara d, K. Miwa a, T. Miyashita a, T. Murakami a, T. Nakura a, M. Naruki, M. Nomachi b, K. Ozawa c, F. Sakuma a, O. Sasaki b, H.D. Sato a, M. Sekimoto b, T. Tabaru, K.H. Tanaka b, M. Togawa a, S. Yamada a, S. Yokkaichi, Y. Yoshimura a (KEK-PS E325 Collaboration) First observation of  meson mass-modification in nuclear medium (KEK-PS E325)

Contents Physics Motivation Experimental Setup Result of   e + e - analyses   e+e- Invariant Mass Distribution Our experiment, KEK-PS E325, measured e + e - and K + K - invariant mass spectgra to investigate possible in-medium mass modification of vector mesons.

Physics Motivation Spontaneous Breaking of Chiral Symmetry Hadrons ~1GeV/c 2 Constituent quarks ~300MeV/c 2 Current quarks ~5MeV/c 2 In Vacuum In Hot/Dense Matter How to detect? Vector meson mass at normal nuclear density u u d u u d

Vector Mesons Mass modification 20 ~ 40MeV/c 2 : relatively small Small decay width (4.4MeV/c 2 ), no other resonance nearby : sensitive to small mass modification Hatsuda & Lee PRC 46, R34(1992)   predictions of vector meson modification in medium Brown, Rho(1991), Hatsuda, Lee(1992), Klingl, Kaiser, Weise(1997), etc. Large mass modification ~120MeV/c 2 at  =  0 Large cross section Difficult to separate  and  on e + e - mass spectrum  Poster No.194 by M. Naruki

E325 Experiment Slowly moving  (p lab ~2GeV/c ）  Large Acceptance Spectrometer measures Invariant Mass of e + e -, K + K - in 12GeV p + A  , ,  + X reactions Measurement of e + e -  almost free from FSI Mass Resolution for e + e - : 8.0MeV/c 2 for  10.7MeV/c 2 for  Primary proton beam (~10 9 /spill/1.8s) Five targets; Carbon x 1 and Copper x 4 aligned in line Very thin targets to suppress  conversion Beam Target TargetsCCu Interaction length(%)0.2%0.05%X4 radiation length(%)0.4%0.5%X4

Forward LG Calorimeter Rear LG Calorimeter Side LG Calorimeter Front Gas Cherenkov Rear Gas Cherenkov Barrel Drift Chamber Cylindrical DC Vertex Drift chamber 1m Detector Setup 12GeV proton beam B Hodoscope Aerogel Cherenkov Forward TOF Start Timing Counter

e + e - Invariant Mass Distributions CCu   

e + e - Invariant Mass Distributions CCu  

On the Fit Back Ground : Quadratic curve  Resonance Shape : Breit-Wigner shape experimental effect estimated by Geant4 simulation – energy loss, mass resolution, mass acceptance etc. Blue histogram : Detector Simulation Red line : Breit-Wigner (gaussian convoluted) fitting result Experimental effects are fully taken into account detector simulation for 

Fit Results CCu ??

To see  dependence Slowly moving  mesons have a larger probability to decay inside the target nucleus. We divided the data into three by  ;  <1.25, 1.25<  <1.75 and 1.75< .  distribution

Fit Results (divided by  )  <1.25 (Slow)1.25<  < <  (Fast) Large Nucleus Small Nucleus Data cannot be reproduced (99% C.L.)

The amount of the Excess Fit again excluding the region where the excess was seen; 0.95~1.01GeV/c 2 Integrate the amount of the excess in the above region (0.95~1.01 GeV/c 2 )  N excess excluded from the fitting

 mesons are generated uniformly in target nucleus momentum distribution: measured pole mass: m*/m = 1 – k 1  /  0 (from Hatsuda, Lee)  We set k 1 = 0.04 decay width:  */  = 1 + k 2  /  0  We set k 2 = 10 (at  =  0,  *~48MeV (from Klingl et.al )) density distribution –Woods-Saxon –radius: C:2.3fm/Cu:4.1fm Model Calc. including Mass Modification e+e+ e+e+ e-e- e-e-  

Results of Model Calc. with k 1 =0.04, k 2 =10  <1.25 (Slow) Amount of Excess by Model Calc.

Summary KEK PS-E325 measured e + e - and K + K - invariant mass distributions in 12GeV p + A reactions. Significant enhancement was seen on the e + e - invariant mass distributions at the low-mass side of the  meson peak in the Cu data, in  < 1.25 region. This observation is comparable to the model calculation including in- medium mass modification. Cu

Summary Result of   e + e - Cu KEK PS-E325 measured e + e - and K + K - invariant mass distributions in 12GeV p + A reactions. Significant enhancement was seen on the e + e - invariant mass distributions at the low-mass side of the  meson peak in the Cu data, in  < 1.25 region. This observation is comparable to the model calculation including in- medium mass modification. We observed the signature of the in- medium mass modification both in  and  mesons. (  results were presented in the poster session 1 by M. Naruki)

Backup slides

 Kinematical Distributions  y pTpT p T  vs y

Check on Detector Simulation   p  - Mass spectra are well reproduced by the simulation K s   +  - M =496.8 ± 0.3(MC ± 0.1) MeV/c 2  = 3.9 ± 0.4(MC 3.5 ± 0.1) MeV/c 2 M = ± 0.02(MC ± 0.01) MeV/c 2  = 1.73 ± 0.02(MC 1.62 ± 0.01) MeV/c 2

Results of Model Calc.  <1.25 (Slow)1.25<  < <  (Fast) Large Nucleus Small Nucleus

Target Configuration materialbeam intensity (p/spill) Interaction length(%) radiation length(%) C~1x %0.4% CuX4~1x %X40.5%X4 23mm C Cu Very thin target with clean and high intensity beam Vertex Distribution Beam

Results of Model Calc. with k 1 =0.02&0.04, k 2 =10

Invariant Mass Spectrum of e+e- (background subtracted)  ratio is consistent with zero  =0.0±0.02(stat.)±0.2(sys.) 0.0±0.04(stat.)±0.3(sys.) CCu It is pretty much surprising because the  is known to be unity in pp reactions the region GeV/c 2 is excluded from the fit.