Hadrons in the nuclear medium -recent experimental results

Volker MetagII. Physikalisches Institut

Universität GiessenGermany

5th. International Conference onPerspectives in Hadron PhysicsTrieste, Italy, May 22-26, 2006

• motivation• first observation of medium modifications of the meson: a.) mass shift b.) in-medium width• first observation of an -nucleus bound state: • summary and outlook

B11ω

Motivation

widespread experimental activities to search for in-medium modifications of hadrons

Mas

s [G

eV]

• hadrons = excitations of the QCD vacuum

T.Hatsuda and S. Lee,PRC 46 (1992) R34

G.E.Brown and M. Rho,PRL 66 (1991) 2720

0

**

8.0qq

qq

mm

18.0;1mm

0

B

V

*V

• QCD-vacuum: complicated structure characterized by condensates

• in the nuclear medium: condensates are changed

change of the hadronic excitation energy spectrum

model predictions for in-medium masses of mesonsV. Bernard and U.-G. Meißner

NPA 489 (1988) 647NJL-model

-mass roughly constant

K. Saito, K. Tushima, and A.W. ThomasPRC 55 (1997) 2637

Quark-meson coupling model (QMC)

decrease of -mass by 15%at normal nuclear matter density

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

Model predictions for spectral functions of and mesons

F. Klingl et al. NPA 610 (1997) 297 NPA 650 (1999) 299

- meson

00.25

0.5

0.75

100.2

0.40.6

0.811.20

0.5

1

1.5

2

00.25

0.5

0.75

1

m [GeV]q [GeV]

AT [GeV-2]

M. Post et al., nucl-th/0309085

-meson

M. Lutz et al. , Nucl. Phys. A 706 (2002) 431

structure in spectral functiondue to coupling to baryon resonances

S. Zchocke et al. , Phys. Lett. B 562 (2003) 562

variation in -mass due todensity dependence of 4-quark condensate

Model predictions for spectral functions of and mesons

• CLAS (Jlab): C. Tur et al., A , , +X

• HADES (GSI): planned experiment - p n on bound proton

essential advantage: no final state interactions !!

221 ppm

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

experimental approach: dilepton spectroscopy: , , e+e-

• KEK-E325: M. Naruki et al., PRL 96 (2006) 092301:

;092.01mm0

0

no broadening in the medium!!-meson:

p (12 GeV) A , +X

• NA60: R. Arnaldi et al., PRL 96 (2006) 162302: In + In (158 AGeV)-spectral function shows strong broadening but no shift in mass

-mass in nuclei from photonuclear reactions

disadvantage:

• 0-rescattering

advantage:

• 0 large branching ratio (8 %)

• no -contribution ( 0 : 7 10-4)

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

p

A + X

0

2ppm

fraction of -decays in the medium( 0.1 0) : 35%

M. Effenberger et al.

rescattering of pions in nucleipredominantly proceeds through (1232) excitation:scattered pions have Ekin150 MeV

Expected in-medium signal

no distortion by pion rescattering expected in mass range of interest

detector system CB/TAPS @ ELSA

front view of TAPS

= 00 to 3600

= 50 to 300

side view

Crystal Barrel 1290 CsI

= 00 to 3600 = 300 to 1680

E=900-2200 MeV

TAPS 528 BaF2

comparison of meson masses and lineshapesfor LH2 and nuclear targets

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

0

inclusive 0 signal for LH2 and Nb target

difference in line shape of signal for proton and nuclear target

D. Trnka et al., PRL 94 (2005)192303after background subtraction

m = m0 (1 - /0) for = 0.13mNb = 763 MeV; 0.110 consistent with

decomposition of signal into in-medium and vacuum decay contributions

lineshape of vacuum contribution taken from LH2 experimentshape of in-medium contribution taken from BUU simulation (P. Mühlich and U. Mosel, NPA (2006)), assuming m = m0(1 - 0.16 /0)

Nb: in-medium: 45% C: in-medium: 40%

c/MeV400p c/MeV400p

vacuum contributionin-medium contribution

access to in-medium widthin-medium width proportional to absorption: vabs

normalization to C!!

P. Mühlich and U. MoselNPA (2006)

74 MeV

37 MeV

= 19 MeV

M. Kaskulov and E. Osetpriv. communication

= 34 MeV

94 MeV

transparency ratio:XN

XAA A

T

access to in-medium widthin-medium width proportional to absorption: vabs

transparency ratio:XN

XAA A

T

normalization to C!!

37 MeV

74 MeV

= 19 MeV= 34 MeV

94 MeV

Comparison to data (D.Trnka et al.) (0) 45 – 95 MeV

momentum dependence of signal (Nb-target)

mass modification only for p 0.5 GeV/c

determination of momentum dependence of - nucleus potential requires finer momentum bins improved 2nd. generation experiment

Nb

LH2

-, -meson-nucleus potential

K. Saito, K. Tsushima, A.W. Thomas, hep-ph/0506314

: E(1s) = -39 MeV; = 29 MeV: E(1s) = -100 MeV; = 31 MeV

: E(1s) = -56 MeV; = 33 MeV: E(1s) = -118 MeV; = 33 MeV

predictions within the quark meson coupling model (QMC)

The population of meson-nucleus bound states in recoil-free kinematics

forward going nucleon takes over photon momentum

magic incident energies : E 930 MeV : E 2750 MeV

(ELSA)

quasifree

-mesic states

T. Nagahiro et al. N. Phys. A 761 (2005) 92

quasifree

E. Marco and W. Weise, PLB 502 (2001) 59

attractive potential

signature for - mesic states

repulsivepotential

no intensity for negative energies

5°

15°

740 MeV/c

240 MeV/c

p

E = 2750 MeV

correlation between momentum and proton angle

simulation

E = 1.5 –2.5 GeV

background

bound states expected for p < 140 |p| < 500 MeV/c for p < 140

candidates

comparison of data on LH2 and Clarge proton angles

180 < p< 280

• for large proton angles: similar background distributions for C and LH2 data

C

LH2

small proton angles70 < p< 140

LH2

• for small proton angles: difference between C and LH2 data for negative energies

Cquasi-free

carbon data after background subtraction

70 < p< 140

quasifree

Evidence for B11ω

first evidence for the existenceof an -nucleus bound state:here: B11

ω

srnb550

dd

theo

E.Marco and W.WeisePLB 502 (2001) 59

theoretical prediction mesic states

srb4.1

dd

exp

Summary and outlook

• remaining open questions momentum dependence of -nucleus potential? structure in strength function? confirm existence of mesic states in heavier nuclei

major step forward towards understanding the origin of hadron masses

higher statistics needed !! improved experiments planned at MAMI and ELSA

• first information on in-medium width: 0ω ρρatMeV10040Γ

• first evidence for mesic 11B

• An in-medium dropping of the meson mass has been observed

consistent with

0

0 14.01mm

CBELSA/TAPS collaboration