Vector Mesons in the Nuclear Medium
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Vector Mesons in the Nuclear Medium. Volker Metag II. Physikalisches Institut, Universität Gießen. introduction. medium modifications of vector-mesons: 1) -meson: CERES, KEK-PS325 2) -meson: CB/TAPS@ELSA. outlook: medium modification in the charm sector: D-mesons: PANDA@FAIR. summary.

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Vector Mesons in the Nuclear Medium

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Vector Mesons in the Nuclear Medium

Volker Metag

II. Physikalisches Institut, Universität Gießen

introduction

medium modifications of vector-mesons:

1) -meson: CERES, KEK-PS325

2) -meson: CB/TAPS@ELSA

outlook:

medium modification in the charm sector:

D-mesons: PANDA@FAIR

summary

invited talk given at CHIRAL-05,

RIKEN, Tokyo, Japan, 15. –17. February 2005


Mass [GeV]

Motivation

Hadrons = excitations of the QCD vacuum

QCD-vacuum: complicated structure

characterized by condensates

  • In the nuclear medium:

  • condensates are changed

  • change of the hadronic excitation

    energy spectrum

  • widespread experimental activities to search for

    in-medium modifications of hadrons


1.) lowering of in-medium mass 2.) broadening of resonance

for rB, T 

theoretical predictions for mass changes of vector mesons in the nuclear medium

T. Renk et al.,

PRC 66 (2002) 014902

experimental approach:

dilepton spectroscopy: r, w, f e+e-

reconstruction of invariant mass from 4-momenta of decay products:

essential advantage: no final state interaction


-mass in nuclei

M. Effenberger et al.

W. Schön et al.Act. Phys. Pol. B 27 (96) 2959

PRC 60 (1999) 044614

expected signal: -A at p = 1.3 GeV/c

-pbound  nat rest  ne+e-

“at rest”: | pw |  | pF |

exp: detect e+e-

HADES


gA   + X

p

g

p0g

g



w

p0

g

g

-mass in nuclei from photonuclear reactions

J.G.Messchendorp et al., Eur. Phys. J. A 11 (2001) 95

advantage:

• p0g large branching ratio (8 %)

• no -contribution (  0 : 7  10-4)

disadvantage:

• p0-rescattering


expected mass distribution for p, Nb

(including detector resolution and 2p0 background)

after cut on p0kinetic energy

within 0.6 < Mp0g< 0.8 :


The accelerator facility ELSA at Bonn

ELSA = Elektron Strecher Anlage


The Crystal Barrel/ TAPS detector @ELSA

tagging spectrometer:

tagging range: 31% -94% of Ebeam

CB: 1280 CsI modules

TAPS: 540 BaF2 modules

E = Ebeam - Ee


4 p detector system CB/TAPS @ ELSA

Crystal BarrelTAPS

1290 CsI 528 BaF2

front view of TAPS

side view

g

  • = 00 to 3600

     = 300 to 1680

  • = 00 to 3600

     = 50 to 300


comparison of meson masses and lineshapes

for LH2 and nuclear targets

0



No change of mass and lineshape for longlived mesons (0, , ) decaying outside nuclei


inclusive 0 signal for LH2 and Nb target

D. Trnka (Giessen)

after background subtraction

difference in line shape of  signal for proton and nuclear target


contribution from  in-medium decays

  • decays in vacuum

    removed bysubtracting 

    mass distribution measured

    with LH2 target (64%)

  • strength of in-medium

  • decays concentrated

    around masses of 725 MeV

  • mass drop by about 7%

    at estimated baryon

    density of about 0.55 0

    consistent with

    m =m0 (1 -  /0);

    for  = 0.15


momentum dependence of  signal

  • mass modification only

    for p 0.5 GeV/c

first indication for an

in-medium modification

of the  meson mass !!


photoproduction of  mesons off nuclei

  • A   A at E  1.2 GeV (CB/TAPS@ELSA)

in-medium  mass distribution (0)

exp: D. Trnka (Giessen)

theo: P. Mühlich et al., nucl-th/0310067

m =m0 (1 -  /0); for  = 0.16

comparison to BUU calculation assuming

first experimental indication for a dropping  mass in the nuclear medium


CB/TAPS @ MAMI C

g-Beam

TAPS forward wall

528 BaF2 - modules

Crystal Ball

672 NaI - modules


Charmed mesons in the medium @ PANDA

in-medium spectral function of charmonia:

consequence of dropping D-mass (cu) in the medium

Ye.S.Golubeva et al.,

nucl-th/0212074

 (3770)

A. Hayashigaki, PLB487 (2000) 96


FAIR

Facility for Antiproton

and Ion Research

SIS 100

SIS 300

GSI as of today

CBM

HESR

Super-FRS

PP

Elec.Cooler

PANDA

Atom. Phys.

CR+RESR

FLAIR

NESR


summary

vector mesons:

:  first evidence for medium modifications from e+e- in

ultra-relativistic nucleus-nucleus-collisions (CERES/CERN);

 indications for medium modifications at normal nuclear matter

density from e+e- in pA collisions at 12 GeV (KEK);

 :•first evidence for medium-modifications at normal nuclear matter density from 0 by comparison of p(,) and A(,) (CB/TAPS @ ELSA);

• -pn on bound protons, planned at HADES/GSI;

  • mesons with charm:

  • D:planned experiments at FAIR


conclusion

Experiments so far are in accordance with theoretical scenarios for

changes of hadron properties in the nuclear medium. Studying the

in-medium behaviour of hadrons is a promising approach to learn

more about the origin of their mass.

We have come a little closer to understanding the mass of matter

Thanks:CBELSA/TAPS collaboration (Bonn)

A2-collaboration (Mainz)

Giessen-team: in particular

Johan Messchendorp, Susan Schadmand, David Trnka

Giessen-Theory:

W. Cassing, St. Leupold, U. Mosel, P. Mühlich


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