Studying medium modifications of mesons in elementary reactions

Studying medium modifications of mesonsin elementary reactions

Volker MetagII. Physikalisches Institut,

Universität Gießen,Germany

Lecture given at Berkeley School„Medium Properties, Chiral Symmetry and Astrophysical Phenomena“

May 21-25, 2007, Berkeley, USA

Energy/Mass Distribution in the Universe

accelerated expansion of universe dark energy (homogen. distributed) gravitational lensing dark matter stars visible (baryonic) matter

forc

e ca

rrie

rs

interactions

The Standard Model

c t

d s b

e µ

u

e

I II III

three generations of matter

lep

ton

s q

uar

ks

elementary particles

W±

weak interaction

W-u

d

e-

e

Z0strong interaction

q

q q

qgluon (g)

g

e-

electromagnetic interaction

e-

photon ()e-

e-

Leptons Quarkst

c

u

b

s

d

e

e

10-3

10-2

10-1

1

10

102

103

104

105

10-6

10-5

10-4

M l,q [MeV/c2]

masses of quarks and leptons

masses of elementary particles (quarks, leptons) generated by interaction with Higgs-field

search for Higgs-particle (LHC)

hadrons:strongly interacting composite particles

Baryons (qqq)

proton: (uud) J = ½+, neutron: (udd) J = ½+,

Mesons (qq)

2

2

vector mesons: J = 1-, +(ud), 0(uu-dd)/ , -(du)(uu-dd)/ , (ss)

2

pseudoscalar mesons: J = 0-, +(ud), 0(uu-dd)/ , -(du)

the mass of composite systems

nucleon: mass not determined by sum of constituent masses m = E/c2; „mass without mass“ (Wilczek) mass given by energy stored in motion of quarks and by energy in colour gluon fields

M mi

binding energyeffect 10-8

atom 10-10 m

M » mi

nucleon 10-15 m

atomic nucleus 10-14 m

M mi

binding energyeffect 10-3

mass split comparable to hadron masses !

the interaction among quarks has to become so strong that it overcomes their quantum mechanical resistance to localization (Wilczek)

mN = 938 MeV mq 5 – 10 MeV

how is the mass of the nucleon generated?

1260

770135

0: 600

≈ 47

0

0:

≈ 49

0

1:

1:a1

scalar mesons

vector mesons

21

21

23

21,2

11232

1535 1520

938≈

600 ≈

290

23

nucleonchiral symmetry

the role of chiral symmetry breaking

• chiral symmetry = fundamental symmetry of QCD for massless quarks

• chiral symmetry broken on hadron level

The Nobel Prize in Physics 2004

„for the discovery of asymptotic freedom in the theory of the strong interaction"

Gross Politzer Wilczek

phase transition: ferromagnetism paramagnetism

restoration of full rotational symmetry

TCurie

mag

net

isat

ion

M

ferro magnetic

para magnetic

paramagnetic:

full rotational symmetry

ferromagnetic:

rotational symmetry about 1 axis

temperature T

C. Ratti, M. Thaler, W. Weise, PRD73 (2006) 014019

242222

2

24

111

16

1

24Gqqm

QQs

sRds

Q sq

s

+ higher order terms

QCD sum rules: provide link between hadronic observables and condensates

222

2

ssMs

ss1F~sR

hadronic spectral function:

chiral condensate as function of baryon density B and temperature T

heavy ion reactions:A+AV+X

mV(>>0;T>>0)

elementary reaction:, V+X

mV(=0;T=0)

, . p - beams

SPS

LHC

RHIC

SIS18

SIS300/(FAIR)

qq is not an observable!!

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

hadron masses

model predictions for in-medium masses of mesons

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

- meson

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

for B:

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

smallfor)1(

00 mm

- meson

Model predictions for spectral functions of and mesons

P. Mühlich, priv. com.

spectral function(structure due to coupling to

S11,P13 resonances)

broadening and strength below vacuum pole mass

R. Rapp and J. Wambach, EPJA 6 (1999) 415

Test concepts for hadron mass generation by comparingpredictions based on these concepts with experimental observations how hadron properties are changed in a strongly interacting environment.

Motivation for studying in-medium modifications of hadrons:

• in-medium mass shift (partial restoration of chiral symmetry, meson-baryon coupling)

possible in-medium modifications of hadrons:

• in-medium broadening of hadron resonances (meson-baryon coupling, collisional broadening)

• hadron-nucleus bound states (meson-nucleus attractive potential)

221ω pppT,ρ,m

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

essential advantage: no final state interactions !!

dilepton spectroscopy: , , e+e-

experimental approach:

Information on medium modifications of mesons

advantage: sizable effects due to high densities and temperatures

disadvantage:any signal represents an integration over the full space-time history of the heavy-ion collision with strong variations in densities and temperatures

lectures by J. Stroth, G. Usai

from heavy-ion collisionsadvantage: well controlled conditions: important for theoretical interpretation no time dependence of baryon density: B B(t);T=0;

disadvantage: small medium effects since 0 and T=0

from elementary reactions

, -peaks observed;

additional yield above combinatorial background assigned to e+e-

CB

All targets

A e+e- + XJLAB-CLAS: G7

E = 0.6-3.8 GeV KEK-E325: p (12 GeV) A , +X

%.m

m 01

e+e- invariant mass spectra from photon and proton induced reactions

e+e- invariant mass spectra from photon and proton induced reactions

A e+e- + XJLAB-CLAS: G7

slightly broadened; no mass shift

w/o shift

No consistent picture!! lecture by Chaden Djalali

KEK-E325: p (12 GeV) A , +X

shifted in mass:no broadening!!

;092.01mm0

0

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

KEK-E325: p (12 GeV) A , +X; e+e-

- meson in the nuclear medium

mass shift of - meson for low recoil momenta in Cu: ;.mm

00 0401

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

-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

simulation: Nb +Xfm

ELSA@BONN

quasi-monochromatic photon beamvia electron bremsstrahlungs tagginglinearly and circularly polarized

DFG TR-16

Crystal Barrel and TAPS

photon beam

SciFi Detector

TAPS 528 BaF2Crystal Barrel 1290 CsI

Line Shape for long-lived Mesons

no deviation in the line shape observed for long-living mesons!

Strategy of the experiment compare line shape for vacuum (LH2 data) with nuclear target data if deviation in-medium modification

background subtracted line shapes of long-living mesons

0

c = 2.5 •107 fm

0 0 0 6

c = 1.5 •105 fm

0 0 6

c = 1 •103 fm

First Observation of mass modification in the medium

background subtracted signal for different 3-momenta line shape off Nb (nuclear medium) compared to lineshape off LH2 (vacuum)

enhancement on low mass side of signal for small momenta

sensitivity to low 3-momenta important for studies (p < 500 MeV)

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

decays outside and inside the nucleus

decays outside the nucleus

New Analysis of CBELSA/TAPS Data

fully inclusive 0 signal for 12C target

cut on momentum to enhance in-medium decays: p < 500 MeV new data analysis (fully inclusive event class: correlated and ): better statistics

new background analysis: event class: correlated and in one event mixing one and one from different events

mixed event background subtraction: shape model independent

Mixed Event Background subtracted: signal

fully inclusive 0 signal for 12C target

lowering of mass!

new signal off 12C p < 500 MeV

background shape model independent

comparison signal off 12C (medium) signal off LH2 (vacuum) response from simulation

(vacuum)

clear deviation in the line shape observed

decomposition line shape of vacuum contribution taken from LH2 experiment

shape of in-medium contribution taken from BUU simulation using linear scaling m = m0(1 - 0)

systematic study of medium and vacuum part under way (~16% fits)

In-medium and Vacuum Line Shape

P. Mühlich and U. Mosel, NPA (2006)

cMeVp /500

C: in-medium: 35%

0 -signal on Nb

after subtraction of mixed event background

decomposition in vacuumand in-medium decays

consistent with 0.16αwith)ρ

ρα(1mm

00ω

inclusive 0 -invariant mass spectrum

access to in-medium width

normalization to C!!

P. Mühlich and U. MoselNPA 773 (2006) 156

180 MeV

93 MeV

= 47 MeV

M. Kaskulov, E.Hernandez and E. OsetEPJ A 31 (2007) 245

= 34 MeV

94 MeV

transparency ratio:XN

XAA A

T

in-medium width proportional to absorption: (,|p|) vabs

access to in-medium width

transparency ratio:XN

XAA A

T

normalization to C!!

93 MeV

180 MeV

= 47 MeV

= 34 MeV

94 MeV

gets broadened in the medium by a factor 10!!comparison to data (D.Trnka et al.) (0,<|p|> 750 MeV/c) 95 MeV

in-medium width proportional to absorption: (,|p|) vabs

KEK Jlab CBELSA/TAPS CERES NA 60

mass shift: -9%

no broadening–

mass shift: -14% (=0) 90 MeV – –

mass shift: -9%

no broadeningno mass shift some broadening

–

broadening favored over density dependent mass shift

no mass shift strong broadening

mass shift: -4% (0)=47MeV – – – –

CBELSA/TAPS reaches the lowest 3-momentahigh sensitivity to decays inside the nucleus

M.Naruki et al.,PRL 96 (2006)R. Muto et al.,PRL 98 (2007)

D. Trnka et al,PRL 94 (2005)

Ch. Djalali et al.,priv. comm.

R. Arnaldi et al.,PRL 96 (2006)

D. Adamova et al.,subm. to PLB

Overall picture

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

magic incident energies : E 930 MeV : E 2750 MeV

forward going nucleon takes over photon momentum

A attraction strong enough to allow for bound states??

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

theoretically predicted signatures for bound states

comparison of carbon and LH2 data

D. Trnkapriv. communicationpreliminary!!!

excess yield relative to LH2 referencespectrum fornegative energies(bound state regime)

normalisation onquasi-elastic peak

evidence for 11B ?? ω

small proton angles: 60 < p< 100

MeV/c400pω

1200 MeV < E < 2200 MeV:

Carbon LH2

TAPS Crystal Barrel

Aerogel Cerenkov

Current experiment at ELSA: search for -mesic states

SFB/TR16

HADES

lecture by

Joachim Stroth

-pbound nat rest ne+e-

exp: detect e+e-

2i2

i pEm

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

“at rest”: | p| | pF |

-mass in nuclei: e+e-

M. Effenberger et al.PRC 60 (1999) 044614

expected signal: -A at p = 1.3

GeV/c

summary

• in-medium properties of the meson:

)/13.01( 00 mm

• evidence for dropping mass in the nuclear medium:

• in-medium width (=0, <|p|> 750 MeV/c) 95 MeV in-medium broadening by factor 10!

• evidence for mesic nuclei? improved experiment ongoing

• Further high resolution experiments (KEK, CLAS, NA60) needed to obtain an overall consistent picture of in-medium properties of light vector mesons (, , )

• in-medium properties of the meson from ,p A + X e+e- +X (HADES)

CBELSA/TAPS collaboration