# Presentation: 801058

## Experimental Signature of in-medium mass modification of vector mesons at normal nuclear density

** ****ICEPP Univ. of Tokyo****e****,Tohoku Univ.****f**
**Ryotaro Muto, RIKEN, Japan**
- J. Chibab, H. En’yoc, Y. Fukaoa, H. Funahashia, H. Hamagakid, M. Ieirib,
- M. Ishinoe, H. Kandaf* *, M. Kitaguchia, S. Miharae, K. Miwaa, T. Miyashitaa,
- T. Murakamia,  T. Nakuraa, M. Narukia, M. Nomachib, K. Ozawad, F. Sakumaa,
- O. Sasakib, H.D. Satoa, M. Sekimotob,  T. Tabaruc, K.H. Tanakab,
- M. Togawaa, S. Yamadaa, S. Yokkaichic, Y. Yoshimuraa
- (KEK-PS* **E325* Collaboration)

**Experimental Signature of in-medium mass modification of vector mesons**** at normal nuclear density**

## Abstract (KEK-PS E325)

-  Physics Motivation
-  Experimental Setup
-  Preliminary Result of  2002 data analysis

**-Contents-**

**Abstract (KEK-PS E325)**

**e****+****e****-**** invariant mass**

**Cu**

***ω→******e******+******e******­***

***→******e******+******e******­***

## Physics Motivation

**In Vacuum**

**Using Vector Mesons**

*Spontaneous Breaking*
* **of Chiral Symmetry*

**Effective Mass of Quarks**

**How to measure ******

**At High**** ********** ****/ T**

## Vector Meson

**Mass of Vector Meson ******
** ******** ****2 x Mq**** + **small interaction term

- mass modification 20 ~ 40MeV
- small decay width(4.4MeV/c2)sensitive to mass modification

_**Hatsuda & Lee P.R.C 1992**_

- large mass modification     150MeV at = 0
- large cross section

****

****

## Expected Signal

**Slowly moving ********(p****lab****~****2GeV/c****)**
**→ ****Large Acceptance Spectrometer**

**In 12GeV p + A ****→****+ X**
** ****Invariant Mass of**** ****e****+****e****-****, K****+****K****- **

**Expected Invariant Mass distribution of ****ρ****and****ω**

**Decay**** in vacuum**

**In ****Copper**** Nuclei**

- ω

- ρ

- mass modified by the formula
- m*/m=1-0.16
- Prog.Theor.Phys.95(1996)1009

## Experimental Setup

**Front Gas Cherenkov**

**Rear Gas Cherenkov**

**Hodoscope**

**Aerogel Cherenkov**

**Forward TOF**

**B**

**Experimental Setup**

**Vertex Drift chamber**

**1m**

- see _poster Instr.3 _by F. Sakuma

## Slide 7

## Target

**Target**

| material | beam intensity(p/spill) | Interaction  length(%) | radiation length(%) |
| --- | --- | --- | --- |
| C | ~1x109 | 0.2% | 0.4% |
| CuX4 | ~1x109 | 0.05%X4 | 0.5%X4 |

***23mm***

**C**

**Cu**

**very thin target with clean and high intensity beam**

**Vertex Distribution**

## Spectrometer Performance

- MK= 497.6MeV/c2    (PDG 497.7MeV/c2)
- K = 3.8MeV/c2      (Sim. 4.1MeV )

**Λ****→pπ****-**

**K****0****→π****+****π****-**

- M= 1115.5MeV/c2    (PDG1115.7MeV/c2)
-   = 1.8MeV/c2       (Sim. 1.9MeV)

**Mass and Width are well reproduced by MC.**

## Invariant Mass Spectrum of e+e- (2002 data)

## On the Fit

- Resonance
- Breit-Wigner shape
- experimental effect estimated by Geant4 simulation – energy loss, mass resolution, mass acceptance etc.
- Background
- combinatorial background obtained by mixed events
- Relative abundances of mesons  and background are obtained by the fitting.

## Invariant Mass Spectrum of e+e- (2002 data)

**Invariant Mass Spectrum of e+e- (2002 data)**

***excess***

***excess***

***C***

***Cu***

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

## Invariant Mass Spectrum of e+e-             (after subtracting background)

****** ****ratio is consistent with zero**

- 0.00.01(stat.)0.2(sys.)

- 0.00.050.5

***C***

- The excess can be understood as modified mesons.

## Model CalculationWith the formula : m*/m=1-0.16

- generate on surface of forward hemisphere of the nucleus
- spectral function : Breit-Wigner + mass modification.

***C***

***Cu***

## Invariant Mass Spectrum of e+e-

- C

- Cu

*Work in progress*

- Mass(GeV/c2)

- Mass(GeV/c2)

**~1000******

**~1000******

## Summary

- KEK PS-E325 experiment measured e+e- and K+K- pairs to investigate invariant mass of vector mesons decaying in nuclear matter.
- In 2002 e+e- data, we have observed the excess over the known hadronic sources below the  peak. Obtained  ratio indicates that this excess is mainly due to the modification of  mesons.
- Model calculation well reproduced the tendency of data.
- Analysis on phi meson is now in progress.

##  kinematical distribution

- ycm(1.66)

## Energy Loss

**black** line – MC
**red** line – Breit-Wigner(gaussian convoluted) fit result
**green** line – same as red line but excluded by fit

## Model CalculationWith the formula : m*/m=1-0.16

## Combinatorial background

-   0→  (→ee)
-     0→ ee
- + - invariant mass is well described with the mixed events.
- + - correlation is only significant for K0s

- Spectrum of  pair

- K0s

- It is reasonable to use ee mixed event
- for the combinatorial background