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
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