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Results of KEK-PS E325 experiment

Results of KEK-PS E325 experiment. Introduction E325 Experiment Results of data analysis r/w  e + e - spectra f  e + e - spectra f  K + K - spectra nuclear mass-number dependences of f e + e - & f K + K - Summary. F.Sakuma, RIKEN. Physics Motivation. Quark Mass.

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Results of KEK-PS E325 experiment

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  1. Results of KEK-PS E325 experiment • Introduction • E325 Experiment • Results of data analysis • r/w  e+e- spectra • f  e+e- spectra • f  K+K- spectra • nuclear mass-number dependences of fe+e- & fK+K- • Summary F.Sakuma, RIKEN

  2. Physics Motivation Quark Mass chiral symmetry restoration effective mass in QCD vacuum mu≒md≒300MeV/c2 ms≒500MeV/c2 bare mass mu≒md≒5MeV/c2 ms≒150MeV/c2 chiral symmetry breaking How we can detect such a quark mass change? at very high temperature or density, the chiral symmetry is expected to restore W.Weise NPA553,59 (1993) even at normal nuclear density, the chiral symmetry is expected to restore partially

  3. Vector Meson Modification dropping mass • Brown & Rho (’91) m*/m=0.8 (r=r0) • Hatsuda & Lee (’92) m*/m=1-0.16r/r0 for r/w m*/m=1-0.03r/r0 for f • Muroya, Nakamura & Nonaka (’03) Lattice Calc. width broadening • Klingl, Kaiser & Weise (’97&98) 1GeV> for r, 45MeV for f (r=r0) • Oset & Ramos (’01) 22MeV for f (r=r0) • Cabrera & Vicente (’03) 33MeV for f (r=r0)

  4. Vector Meson, r/w/f r/w meson • mass decreases 16%  130MeV/c2 • large production cross-section • cannot distinguish r & w f meson • mass decreases 2~4%  20-40MeV/c2 • small production cross-section • narrow decay width (G=4.3MeV/c2), no other resonance nearby ⇒sensitive to the mass spectrum change T.Hatsuda, S.H.Lee, Phys. Rev. C46(1992)R34.

  5. Expected Modified Mass Spectra in e+e- bglab~1 e e e p p r/w/f r/w/f e outside decay inside decay • small FSI in e+e- decay channel • double peak (or tail-like) structure + m*/m=1-0.16r/r0 • second peak is caused by inside-nucleus decay w f m*/m=1-0.02r/r0 r • depends on the nuclear size & meson velocity • enhanced for larger nuclei & slower meson

  6. Experimental Results on the In-medium Mass and Width of the r, w and f meson R.S.Hayano and T.Hatsuda, arXiv:0812.1702 [nucl-ex] the majority of experiments does not find evidence for a mass shift. [CBELSA/TAPS, arXiv:1005.5694] our report is minority report!? confirmation @ J-PARC E16

  7. KEK-PS E325 Experiment

  8. KEK-PS E325 Experiment Measurements • History of E325 • ’93 proposed • ’96 construction start • NIM,A457 581(’01). • NIM,A516 390(’04). • ’97 first K+K- data • ’98 first e+e- data • r/ w: PRL,86 5019(’01). • ’99~’02 • x100 statistics in e+e- • r/w: PRL,96 092301('06). • fee: PRL,98 042501(’07). • a: PR,C75 025201(’06). x6 statistics in K+K- • fKK: PRL,98 152302(’07). • ’02 completed Invariant Mass ofe+e-, K+K- in 12GeV p+Ar,w,f+X reactions slowly moving vector mesons (plab~2GeV/c) large probability to decay inside a nucleus Beam Primary proton beam (~109/spill/1.8s) Target Very thin targets (X/lI=0.2/0.05%, X/X0=0.4/0.5% for C/Cu) 8

  9. Detector Setup Forward LG Calorimeter Start Timing Counter Hodoscope Rear LG Calorimeter Aerogel Cherenkov Side LG Calorimeter Forward TOF Barrel Drift Chamber B 0.81Tm Cylindrical DC Rear Gas Cherenkov Vertex DC Front Gas Cherenkov M.Sekimoto et al., NIM, A516, 390 (2004). 1m 12GeV proton beam

  10. E325 Spectrometer front view top view beam beam

  11. Spectrometer Performance M = 496.8±0.2 (MC 496.9) MeV/c2 σ = 3.9±0.4 (MC 3.5) MeV/c2 M = 1115.71±0.02 (MC 1115.52) MeV/c2 σ = 1.73±0.04 (MC 1.63) MeV/c2 K0sp+p- L pp- －Data －MC －Data －MC [counts / 2MeV/c2] [counts / 0.5MeV/c2] mass resolution for f-meson decays fe+e- : 10.7 MeV/c2 fK+K- : 2.1 MeV/c2

  12. Observed Invariant Mass Spectra C Cu w(783) w(783) counts/10MeV/c2 counts/10MeV/c2 e+e- f(1020) f(1020) C Cu f(1020) f(1020) counts/4MeV/c2 counts/4MeV/c2 K+K- K+K- threshold

  13. Result of r/we+e- M.Naruki et al., PRL, 96 092301 (2006).

  14. e+e- Invariant Mass Spectra counts/10MeV/c2 • from 2002 run data (~70% of total data) • C & Cu targets • acceptance uncorrected • M<0.2GeV/c2 is suppressed by the detector acceptance C w(783) f(1020)  fit the spectra with known sources

  15. Fitting with known sources • resonance • r/w/fe+e-, wp0e+e-, hge+e- • relativistic Breit-Wigner shape (with internal radiative corrections) • nuclear cascade code JAM gives momentum distributions • experimental effects are estimated through the Geant4 simulation (multiple scattering, energy loss, external bremsstrahlung, chamber resolution, detector acceptance, etc.) • background • combinatorial background obtained by the event mixing method • fit parameter • relative abundance of these components is determined by the fitting estimated spectrum using GEANT4 fe+e- experimental effects + internal radiative correction relativistic Breit-Wigner

  16. Fitting Results Cu C c2/dof=159/140 c2/dof=150/140 the excess over the known hadronic sources on the low mass side of w peak has been observed. the region 0.60-0.76GeV/c2 is excluded from the fit, because the fit including this region results in failure at 99.9% C.L..

  17. Fitting Results (BG subtracted) events[/10MeV/c2] events[/10MeV/c2] Cu C r/w ratios are consistent with zero ! r/w = 0.0±0.03(stat.)±0.09(sys.) 0.0±0.04(stat.)±0.21(sys.) r/w=1.0±0.2 in former experiment (p+p, 1974)  the origin of the excess is modified r mesons

  18. Simple Model Calculation e e p r/w Cu C r=4.1fm r=2.3fm • pole mass: m*/m = 1-kr/r0 (Hatsuda-Lee formula) • generated at surface of incident hemisphere of target nucleus • aw~2/3 [PR, C75 025201 (2006).] • decay inside a nucleus: • nuclear density distribution : Woods-Saxon • mass spectrum: relativistic Breit-Wigner Shape • no width modification

  19. Fitting Results by the Simple Model m*/m = 1 - 0.092r/r0 r/w = 0.7±0.1 r/w = 0.9±0.2 Cu C the excesses for C and Cu are well reproduced by the model including the mass modification.

  20. Contours for r/w and k • C and Cu data are simultaneously fitted. • free parameters • production ratio r/w • shift parameter k • Best-Fit values are k = 0.092±0.002 r/w = 0.7±0.1 (C) 0.9±0.2 (Cu) mass of r/w meson decreases by 9% at normal nuclear density.

  21. Result of fe+e- R.Muto et al., PRL, 98 042501 (2007).

  22. fe+e- Invariant Mass Spectra • from 2001 & 2002 run data • C & Cu targets • acceptance uncorrected • fit with • simulated mass shape of f (evaluated as same as r/w) • polynomial curve background f(1020)  examine the mass shape as a function of bg (=p/m) (anomaly could be enhanced for slowly moving mesons)

  23. Fitting Results 1.75<bg (Fast) bg<1.25 (Slow) 1.25<bg<1.75 Small Nucleus Large Nucleus Rejected at 99% confidence level

  24. Amount of Excess A significant enhancement is seen in the Cu data, in bg<1.25  the excess is attributed to the f mesons which decay inside a nucleus and are modified To evaluate the amount the excess Nexcess, fit again excluding the excess region (0.95~1.01GeV/c2) and integrate the excess area. excluded from the fitting

  25. Simple Model Calculation p f Simple model like r/w case, except for • pole mass: m*/m = 1-k1r/r0(Hatsuda-Lee formula) • width broadening: G*/G = 1+k2r/r0 (no theoretical basis) • e+e- branching ratio is not changed G*e+e-/G*tot=Ge+e-/Gtot • uniformly generated in target nucleus • af~1 [PR, C75 025201 (2006).] • decay inside a nucleus (for bg<1.25): to increase the decay probability in a nucleus

  26. Fitting Results by the Simple Model m*/m = 1 - 0.034r/r0, G*/G = 1 + 2.6r/r0 1.75<bg (Fast) bg<1.25 (Slow) 1.25<bg<1.75 Small Nucleus Large Nucleus well reproduce the data, even slow/Cu

  27. Contours for k1 and k2 of fe+e- Pole Mass Shift M*/M = 1–k1r/r0 Width Broadening G*/G = 1+k2r/r0 • C and Cu data are simultaneously fitted. • free parameters • parameter k1 & k2 • Best-Fit values are • mass of f meson decreases by 3.4% • width of f meson increases by a factor of 3.6 • at normal nuclear density.

  28. Result of fK+K- F.Sakuma et al., PRL, 98 152302 (2007).

  29. fK+K- Invariant Mass Spectra C • from 2001 run data • C & Cu targets • acceptance uncorrected • fit with • simulated mass shape of f (evaluated as same as r/w) • combinatorial background obtained by the event mixing method counts/4MeV/c2 f(1020)  examine the mass shape as a function of bg

  30. Fitting Results 2.2<bg (Fast) bg<1.7 (Slow) 1.7<bg<2.2 Small Nucleus Large Nucleus Mass-spectrum changes are NOT statistically significant However, impossible to compare fe+e- with fK+K-, directly

  31. Kinematical Distributions of observed f • the detector acceptance is different between e+e- and K+K- • very limited statistics for fK+K- in bg<1.25 where the modification is observed in fe+e- the histograms for fK+K- are scaled by a factor ~3

  32. Summary ofshape analysis

  33. Summary of Shape Analysis m*/m = 1 – k1r/r0, G*/G = 1 + k2r/r0 m(r)/m(0) fit result f 1 0.9 fit result r/w 0.8 prediction 0.7 0 0.5 1 r/r0 syst. error is not included

  34. Result of nuclear mass-number dependences offe+e- & fK+K- F.Sakuma et al., PRL, 98 152302 (2007).

  35. Vector Meson, f • mass decreases 2~4%  20-40MeV/c2 • narrow decay width (G=4.3MeV/c2) ⇒ sensitive to the mass spectrum change • small decay Q value (QK+K-=32MeV/c2) • ⇒ the branching ratio is • sensitive to f or K modification f mass K+K- threshold simple example • f mass decreases  GfK+K- becomes small • K mass decreases  GfK+K- becomes large r0:normal nuclear density f : T.Hatsuda, S.H.Lee, Phys. Rev. C46(1992)R34. K : H.Fujii, T.Tatsumi, PTPS 120(1995)289.

  36. GfK+K-/Gfe+e- and Nuclear Mass-Number Dependence a • GfK+K-/Gfe+e- changes in a nucleus  NfK+K- /Nfe+e- changes also • The lager modification is expected in the larger nucleus (A1!=A2) • difference between afK+K-and afe+e- • could be found • difference of a is expected to be enhanced in slowly moving f mesons

  37. - Da= e+e- K+K- ae+e- with corrected for the K+K- acceptance possible modification of the decay widths is discussed = Results of Nuclear Mass-Number Dependence a bg rapidity pT bg averaged (0.13+/-0.12) afK+K- and afe+e- are consistent

  38. Discussion onGfK+K- and Gfe+e- We attempt to obtain the upper limit of the GfK+K- and Gfe+e-modification • The measured Da provides constraints on the GfK+K- and Gfe+e-modification bycomparing with the values of expected Da obtained from the MC. The constraint on the GfK+K-modification is obtained from the K+K- spectra by comparing with the MC calculation. Theoretical predictions width broadening is expected to be up to ~x10 (Klingl, Kaiser & Weise, etc.)

  39. Discussion onGfK+K- and Gfe+e- broadening of Gfe+e- broadening of GfK+K- the first experimental limits assigned to the in-medium broadening of the partial decay widths

  40. Summary • KEK PS-E325 measured e+e- and K+K- invariant mass distributions in 12GeV p+A reactions. • The significant excesses at the low-mass side of we+e- and fe+e- peak have been observed. • These excesses are well reproduced by the simple model calculations which take Hatsuda-Lee prediction into account. • Mass spectrum changes are not statistically significant in the K+K- invariant mass distributions. • Our statistics in the K+K- decay mode are very limited in the bg region in which we see the excess in the e+e- mode. • 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.

  41. KEK-PS E325 Collaboration (2007) RIKEN Nishina Center H.Enyo(*), F. Sakuma, T. Tabaru, S. Yokkaichi KEK J.Chiba, M.Ieiri, R.Muto, M.Naruki, O.Sasaki, M.Sekimoto, K.H.Tanaka Kyoto-Univ. H.Funahashi, H. Fukao, M.Ishino, H.Kanda, M.Kitaguchi, S.Mihara, T.Miyashita, K. Miwa, T.Murakami, T.Nakura, M.Togawa, S.Yamada, Y.Yoshimura CNS,Univ.of Tokyo H.Hamagaki Univ.of Tokyo K. Ozawa (*) spokesperson

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