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. Ishino

d, 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-PS E325 Collaboration)

First observation of First observation of meson meson mass-modification in nuclear mass-modification in nuclear

medium (KEK-PS E325) medium (KEK-PS E325)

ContentsContents

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 MotivationPhysics Motivation

Spontaneous Breaking of Chiral Symmetry

Hadrons ~1GeV/c2

Constituent quarks ~300MeV/c2

Current quarks ~5MeV/c2

In Vacuum In Hot/Dense Matter

Tqq

,

How to detect?

Vector meson massat normal nuclear density

u

ud

u

ud

Vector MesonsVector Mesons

Mass modification 20 ~ 40MeV/c2 : relatively smallSmall decay width (4.4MeV/c2), 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/c2 at = 0

Large cross sectionDifficult to separate and on e+e- mass spectrum

Poster No.194 by M. Naruki

E325 ExperimentE325 Experiment

Slowly moving (plab~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/c2 for 10.7MeV/c2 for

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

Five targets; Carbon x 1 and Copper x 4 aligned in line Very thin targets to suppress conversion

Beam

Target

Targets C Cu

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 SetupDetector Setup

12GeV proto

n

beam

B

Hodoscope

Aerogel Cherenkov

Forward TOF

Start Timing Counter

ee++ee-- Invariant Mass Distributions Invariant Mass Distributions

C Cu

ee++ee-- Invariant Mass Distributions Invariant Mass Distributions

C Cu

On the FitOn the FitBack 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 ResultsFit Results

C Cu

??

To see To see dependence dependenceSlowly 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 Fit Results (divided by ))<1.25 (Slow) 1.25<<1.75 1.75< (Fast)

Lar

ge

Nu

cleu

sS

mal

l N

ucl

eus

Data cannot be reproduced (99% C.L.)

The amount of the ExcessThe 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

• mesons are generated uniformly in target nucleus

• momentum distribution: measured• pole mass: m*/m = 1 – k1 /0

(from Hatsuda, Lee)

We set k1 = 0.04 • decay width: */ = 1 + k2 /0

We set k2 = 10

(at =0, *~48MeV (from Klingl et.al ))• density distribution

– Woods-Saxon– radius: C:2.3fm/Cu:4.1fm

Model Calc. Model Calc. including Mass Modificationincluding Mass Modification

e+

e+

e-

e-

Results of Model Calc.Results of Model Calc.with kwith k11=0.04, k=0.04, k22=10=10

<1.25 (Slow)Amount of Excess

by Model Calc.

SummarySummary• 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

SummarySummary

Result of e+e-

Cu

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 DistributionsKinematical Distributions

y

pTpTvs y

Check on Detector SimulationCheck on Detector Simulation p -

Mass spectra are well reproduced by the simulation

Ks + -

M =496.8±0.3(MC 496.9±0.1) MeV/c2

= 3.9±0.4(MC 3.5±0.1) MeV/c2

M =1115.71±0.02(MC 1115.53±0.01) MeV/c2

= 1.73±0.02(MC 1.62±0.01) MeV/c2

Results of Model Calc.Results of Model Calc.<1.25 (Slow) 1.25<<1.75 1.75< (Fast)

Lar

ge

Nu

cleu

sS

mal

l N

ucl

eus

Target ConfigurationTarget Configuration

material beam intensity(p/spill)

Interaction length(%)

radiation length(%)

C ~1x109 0.2% 0.4%

CuX4 ~1x109 0.05%X4 0.5%X4

23mm

CCu

Very thin target with clean and high intensity beam

Vertex Distribution

Beam

Results of Model Calc.Results of Model Calc.with kwith k11=0.02&0.04, k=0.02&0.04, k22=10=10

Invariant Mass Spectrum of e+e- (background subtracted)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.)

C Cu

It is pretty much surprising because the is known to be unity in pp reactions

the region 0.60-0.76 GeV/c2 is excluded from the fit.