Results of KEK-PS E325 experiment

• Introduction• E325 Experiment• Results of data analysisr/w e+e- spectra f e+e- spectra f K+K- spectranuclear mass-number

dependences of fe+e- & fK+K-

• Summary

F.Sakuma, RIKEN

2

effective mass in QCD vacuum

　 mu m≒ d 300MeV/c≒ 2

ms 500MeV/c≒ 2

chiral symmetry breaking

Quark Mass

bare massmu m≒ d 5MeV/c≒ 2

ms 150MeV/c≒ 2

How we can detect such a quark mass change?

Physics Motivationchiral symmetry

restoration

even at normal nuclear density, the chiral symmetry is

expected to restore partially

at very high temperature or density, the chiral symmetry is

expected to restore

W.WeiseNPA553,59 (1993)

3

Vector Meson Modificationdropping mass

width broadening

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 fMuroya, Nakamura & Nonaka (’03) Lattice Calc.

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

r/w meson

Vector Meson, r/w/f

T.Hatsuda, S.H.Lee,Phys. Rev.

C46(1992)R34.

mass decreases16% 130MeV/c2

large production cross-sectioncannot distinguish r & w

f mesonmass decreases

2~4% 20-40MeV/c2

small production cross-sectionnarrow decay width

(G=4.3MeV/c2), no other resonance nearby 　⇒ sensitive to the mass spectrum change

5

Expected Modified Mass Spectra in e+e-

pe

e pe

e

r/w/fr/w/f

outside decay inside decay

r+wm*/m=1-0.16r/r0

small FSI in e+e- decay channeldouble peak (or tail-like) structure

fm*/m=1-0.02r/r0

bglab~1

second peak is caused by inside-nucleus decay

depends on the nuclear size & meson velocity

enhanced for larger nuclei & slower meson

w

r

　 Invariant mass Attenuation

　 KEK-PS E325

Jlab CLAS g7 CBELSA/TAPS Jlab

CLAS g7Spring8

LEPSReaction pA 12 GeV gA 0.6 - 3.8

GeV gA 0.7 - 2.5 GeV gA 0.6 - 3.8 GeV

gA 1.5 - 2.5 GeV

Momentum

0 > 0.5 GeV/c

p > 0.8 GeV/c

p < 0.5 GeV/c

0.4 < p < 1.7 GeV/c

p > 0.8 GeV/c

1.1 < p < 2.2 GeV/c

r　 Dm ~ 0 　 　

Dm(r0)/m = -9% broadening 　 　 　 　

w

no broadening

Dm ~ 0?

Gw(r0) = 130-

150MeV/c2　

　 　→ swN ~ 70mb 　 　

f

Dm(r0)/m = -3.4% 　

sfN ~ 20mb

sfN ~ 35mb

Gf(r0)/G ~ 15MeV/c2 　 　 　

→ Gf(r0) ~ 45 MeV/c2

→ Gf(r0) ~ 80 MeV/c2 6

Experimental Results on the In-medium Mass and Width of the r,

w and f mesonR.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

88

KEK-PS E325 Experiment

Very thin targets(X/lI=0.2/0.05%, X/X0=0.4/0.5% for C/Cu)

History of E325’93 proposed’96 construction startNIM,A457 581(’01).NIM,A516 390(’04).

’97 first K+K- data’98 first e+e- datar/ w: PRL,86 5019(’01).

’99~’02 x100 statistics in e+e- r/w: PRL,96 092301('06).fee: PRL,98 042501(’07).a: PR,C75 025201(’06).

x6 statistics in K+K-

fKK: PRL,98 152302(’07).’02 completed

Primary proton beam (~109/spill/1.8s)

Measurements

Beam

Target

Invariant Mass of e+e-, K+K-

in 12GeV p+Ar,w,f+X reactions

slowly moving vector mesons (plab~2GeV/c)

large probability to decay inside a nucleus

9

Detector Setup

12GeV proton

beam

Forward LG Calorimeter

Rear LG CalorimeterSide LG

Calorimeter

Front Gas Cherenkov

Rear Gas Cherenkov

Barrel Drift Chamber

Cylindrical DC

Vertex DC

B0.81Tm

Hodoscope

Aerogel Cherenkov

Forward TOF

Start Timing Counter

1m

M.Sekimoto et al., NIM, A516, 390 (2004).

10

E325 Spectrometerfront view

top viewbeam

beam

11

M = 496.8±0.2 (MC 496.9) MeV/c2

σ = 3.9±0.4 (MC 3.5) MeV/c2

K0s

p+p- L pp-

－ Data－ MC

M = 1115.71±0.02 (MC 1115.52) MeV/c2

σ = 1.73±0.04 (MC 1.63) MeV/c2

[cou

nts

/ 2M

eV/c

2 ]

[cou

nts

/ 0.5

MeV

/c2 ]

Spectrometer Performance

－ Data－ MC

mass resolution for f-meson decaysfe+e- : 10.7 MeV/c2

fK+K- : 2.1 MeV/c2

12

C Cu

f(1020)

w(783) co

unts

/10M

eV/c

2

coun

ts/1

0MeV

/c2

Observed Invariant Mass Spectra

C Cu

e+e-

K+K- coun

ts/4

MeV

/c2

coun

ts/4

MeV

/c2

K+K- threshold

f(1020)

w(783)

f(1020) f(1020)

13

Result of r/we+e-

M.Naruki et al., PRL, 96 092301 (2006).

14

Ccounts/10MeV/c2

e+e- Invariant Mass Spectra

from 2002 run data (~70% of total data)

C & Cu targets

acceptance uncorrected

M<0.2GeV/c2 is suppressed by the detector acceptance

fit the spectra with known sources

f(1020)

w(783)

resonance– r/w/fe+e-, wp0e+e-, hge+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

Fitting with known sources

estimated spectrum using

GEANT4

experimental effects

+internal radiative

correction

fe+e-

relativisticBreit-

Wigner

16

the excess over the known hadronic sources on the low mass side of w peak has been observed.

C Cu

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..

c2/dof=159/140 c2/dof=150/140

Fitting Results

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

Fitting Results (BG subtracted)events[/10MeV/c2] events[/10MeV/c2]

C Cu

18

• 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

pr/w

ee

C Cur 52% 66%w 5% 10%

Cu Cr=4.1fm r=2.3fm

Simple Model Calculation

19

the excesses for C and Cu are well reproduced by the model including the mass modification.

m*/m = 1 - 0.092 r/r0

C Cu

Fitting Results by the Simple Model

r/w = 0.7±0.1 r/w = 0.9±0.2

20

mass of r/w meson decreases by 9% at normal nuclear density.

Contours for r/w and k

Best-Fit values are k = 0.092±0.002 r/w = 0.7±0.1 (C) 0.9±0.2 (Cu)

C and Cu data are simultaneously fitted.

free parameters– production ratio r/w– shift parameter k

21

Result of fe+e-

R.Muto et al., PRL, 98 042501 (2007).

22

fe+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

examine the mass shape as a function of bg (=p/m)(anomaly could be enhanced for slowly moving mesons)

f(1020)

23

bg<1.25 (Slow) 1.25<bg<1.75 1.75<bg (Fast)La

rge

Nuc

leus

Smal

l Nuc

leus

Rejected at 99% confidence level

Fitting Results

24

Amount of Excess

excluded from the fitting

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.

25

• 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):

C Cuf 3% 6%

Simple Model Calculation

f p

Simple model like r/w case, except for

to increase the decay probability

in a nucleus

26

bg<1.25 (Slow) 1.25<bg<1.75 1.75<bg (Fast)

Larg

e N

ucle

usSm

all N

ucle

usFitting Results by the Simple

Model

well reproduce the data, even slow/Cu

m*/m = 1 - 0.034 r/r0, G*/G = 1 + 2.6 r/r0

27

Pole Mass Shift M*/M = 1–k1r/r0

Width Broadening G*/G = 1+k2r/r0

Best-Fit values are

C and Cu data are simultaneously fitted. free parameters

– parameter k1 & k2

Contours for k1 and k2 of fe+e-

0.0060.007

1.81

t t2

1

2o

.

0.034

2.6k

k +

+

mass of f meson decreases by 3.4%width of f meson increases by a factor of

3.6at normal nuclear density.

28

Result of fK+K-

F.Sakuma et al., PRL, 98 152302 (2007).

29

fK+K- Invariant Mass Spectra

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

examine the mass shape as a function of bg

f(1020)

coun

ts/4

MeV

/c2

C

Fitting Resultsbg<1.7 (Slow) 1.7<bg<2.2 2.2<bg (Fast)

Larg

e N

ucle

usSm

all N

ucle

us

Mass-spectrum changes are NOT statistically significantHowever, impossible to compare fe+e- with fK+K-, directly

31

Kinematical Distributions of observed f

the detector acceptance is different between e+e- and K+K-

very limited statistics for fK+K- in bg<1.25 where the modification is observed in fe+e-

the histograms for fK+K- are scaled by a factor ~3

32

Summary ofshape analysis

Summary of Shape Analysis

Best Fit Valuesr, w f

k1 9.2 ± 0.2% 3.4+0.6-0.7%

k2 0 (fixed) 2.6+1.8-1.2

r/w 0.7 ± 0.1 (C)0.9 ± 0.2 (Cu)

-

syst. error is not included

prediction

1

fit result r/w

fit result fm(r)/m(0)

m*/m = 1 – k1 r/r0, G*/G = 1 + k2 r/r0

0.9

0.8

0.70 0.5 1 r/r0

33

34

Result of nuclear mass-number

dependences of fe+e- & fK+K-

F.Sakuma et al., PRL, 98 152302 (2007).

35

Vector Meson, f

f : T.Hatsuda, S.H.Lee,Phys. Rev. C46(1992)R34.

r0:normal nuclear density

mass decreases2~4% 20-40MeV/c2

narrow decay width (G=4.3MeV/c2) 　⇒ sensitive to the mass spectrum changesmall decay Q value (QK+K-=32MeV/c2)

⇒ the branching ratio is sensitive to f or K modification

K+K-

threshold

simple examplef mass decreases

GfK+K- becomes smallK mass decreases

GfK+K- becomes large

K : H.Fujii, T.Tatsumi,PTPS 120(1995)289.

f mass

36

GfK+K-/Gfe+e- and Nuclear Mass-Number Dependence a

GfK+K-/Gfe+e- changes in a nucleus NfK+K- /Nfe+e- changes alsoThe lager modification is expected in the

larger nucleus

difference between afK+K- and afe+e-

could be founddifference of a is expected to be

enhanced in slowly moving f mesons

( )( )

( )( ) ( )1 2

1 21 2

ln ln

K K e e

K K K K

e e e e

N A N AA A

N A N A

f f

f f

f f

a a a+ +

+ +

+ +

D

(A1!=A2)

( ) ( )1A A Aas s

37

Results of Nuclear Mass-Number Dependence a

averaged(0.13+/-0.12)

afK+K- and afe+e- are consistent

bg

bg

rapidity pT

possible modification of the decay widths is

discussed

Da= -K+K- e+e-

ae+e- with corrected for the K+K- acceptance

=

38

We attempt to obtain the upper limit of the GfK+K- and Gfe+e- modification

Discussion on GfK+K- and Gfe+e-

Theoretical predictionswidth broadening is expected to be

up to ~x10 (Klingl, Kaiser & Weise, etc.)

① The measured Da provides constraints on the GfK+K- and Gfe+e- modification by comparing with the values of expected Da obtained from the MC.

② The constraint on the GfK+K- modification is obtained from the K+K- spectra by comparing with the MC calculation.

39

( )( )( )

* 0tot 0

* 00

* 00

1 ,

1 ,

1

KK K K K

ee e e e

k

k

k

f f

f f

f f

r r

r r

r r

+ +

+ +

G G +

G G +

G G +

Discussion on GfK+K- and Gfe+e-

tot KWe expect k k since the meson mainly decays into KK as long as such decays are kinematically allowed.

f

the first experimental limits assigned to thein-medium broadening of the partial decay widths

broadening of GfK+K-br

oade

ning

of G

fe+

e-

40

SummaryKEK 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 we+e- and fe+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 fe+e- and fK+K- are consistent.

→ We have obtained limits on the in-medium decay width broadenings for both the fe+e- and fK+K- decay channels.

41

KEK-PS E325 Collaboration (2007)RIKEN Nishina Center H.Enyo(*), F. Sakuma, T. Tabaru, S. Yokkaichi KEKJ.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