First observation of f meson mass-modification in nuclear medium (KEK-PS E325)

First observation of f meson mass-modification in nuclear medium (KEK-PS E325) RIKEN, Kyoto Univ.a , KEKb, CNS Univ. of Tokyoc, ICEPP Univ. of Tokyod,Tohoku Univ.e Ryotaro Muto, RIKEN, Japan J. Chibab, H. En’yo, Y. Fukaoa, H. Funahashia, H. Hamagakic, M. Ieirib,M. Ishinod, H. Kandae, M. Kitaguchia, S. Miharad, K. Miwaa, T. Miyashitaa, T. Murakamia, T. Nakuraa, M. Naruki, M. Nomachib, K. Ozawac, F. Sakumaa, O. Sasakib, H.D. Satoa, M. Sekimotob, T. Tabaru, K.H. Tanakab, M. Togawaa, S. Yamadaa, S. Yokkaichi, Y. Yoshimuraa (KEK-PSE325 Collaboration)

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

Physics Motivation u u d In Hot/Dense Matter In Vacuum u Spontaneous Breaking of Chiral Symmetry u d Hadrons ~1GeV/c2 Current quarks ~5MeV/c2 Constituent quarks ~300MeV/c2 How to detect? Vector meson mass at normal nuclear density

Vector Mesons predictions of vector meson modification in medium Brown, Rho(1991), Hatsuda, Lee(1992), Klingl, Kaiser, Weise(1997), etc. r, w Large mass modification ~120MeV/c2 at r = r0 Large cross section Difficult to separate r and w on e+e- mass spectrum  Poster No.194 by M. Naruki f Mass modification 20 ~ 40MeV/c2 : relatively small Small decay width (4.4MeV/c2), no other resonance nearby :sensitive to small mass modification Hatsuda & Lee PRC 46, R34(1992)

E325 Experiment measures Invariant Mass ofe+e-, K+K- in 12GeV p + A r, w, f + X reactions Measurement of e+e- almost free from FSI Mass Resolution for e+e- : 8.0MeV/c2 for r / w, 10.7MeV/c2 for f Beam Primary proton beam (~109/spill/1.8s) Target Five targets; Carbon x 1 and Copper x 4 aligned in line Very thin targets to suppress g conversion Slowly moving r w f(plab~2GeV/c）  Large Acceptance Spectrometer

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

e+e- Invariant Mass Distributions f f C Cu r / w r / w

e+e- Invariant Mass Distributions C Cu f f

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

Fit Results C Cu ??

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; bg<1.25, 1.25<bg<1.75 and 1.75<bg. bg distribution

Fit Results (divided by bg) 1.75<bg (Fast) bg<1.25 (Slow) 1.25<bg<1.75 Small Nucleus Large 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/c2 • Integrate the amount of the excess in the above region (0.95~1.01 GeV/c2)  Nexcess excluded from the fitting

Model Calc. including Mass Modification • fmesons are generated uniformly in target nucleus • momentum distribution: measured • pole mass: m*/m = 1 – k1 r/r0 (from Hatsuda, Lee)  We set k1 = 0.04 • decay width: G*/G = 1 + k2r/r0  We set k2 = 10 (at r=r0, G*~48MeV (from Klingl et.al )) • density distribution • Woods-Saxon • radius: C:2.3fm/Cu:4.1fm e+ f e- f e+ e-

Results of Model Calc.with k1=0.04, k2=10 bg<1.25 (Slow) Amount of Excess by Model Calc.

Summary 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 fmeson peak in the Cu data, in bg < 1.25 region. • This observation is comparable to the model calculation including in-medium mass modification.

Summary 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 fmeson peak in the Cu data, in bg < 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 f and r / w mesons. (r / w results were presented in the poster session 1 by M. Naruki) Result of r/w e+e- Cu

Backup slides

f Kinematical Distributions bg y pTvs y pT

Check on Detector Simulation L p p- Ks p+ p- M =496.8±0.3(MC 496.9±0.1)MeV/c2 s = 3.9±0.4(MC 3.5±0.1)MeV/c2 M =1115.71±0.02(MC 1115.53±0.01)MeV/c2 s = 1.73±0.02(MC 1.62±0.01)MeV/c2 Mass spectra are well reproduced by the simulation

Results of Model Calc. 1.75<bg (Fast) bg<1.25 (Slow) 1.25<bg<1.75 Small Nucleus Large Nucleus

Target Configuration Very thin target with clean and high intensity beam Cu C Beam 23mm Vertex Distribution

Results of Model Calc.with k1=0.02&0.04, k2=10

Invariant Mass Spectrum of e+e- (background subtracted) the region 0.60-0.76 GeV/c2 is excluded from the fit. C Cu r/w ratio is consistent with zero Nr/Nw=0.0±0.02(stat.)±0.2(sys.) 0.0±0.04(stat.)±0.3(sys.) It is pretty much surprising because the r/w is known to be unity in pp reactions

First observation of f meson mass-modification in nuclear medium (KEK-PS E325)