Meson-nucleus interactions studied inphoto-nuclear experiments
Volker Metag
II. Physikalisches Institut

Outline:◆ theoretical predictions for meson-nucleus optical potentials◆ exp. approaches and results on the imaginary part of the ω, η’- nucleus potential◆ exp. approaches and results on the real part of the ω, η’- nucleus potential◆ search for meson-nucleus bound states◆ summary & outlook*funded by the DFG within SFB/TR16

Hadrons and Hadron Interactions in QCD
Symposium
Kyoto, Japan, March 2-6, 20151

bound systems

earth ↔ moonbound by gravitation

2

bound systems

earth ↔ moonbound by gravitation

π- ↔ nucleus

bound by superpositionof attractive Coulomband repulsive strong
interaction
talk by Kenta Itahashi

2bound systems

earth ↔ moonbound by gravitation

π- ↔ nucleus

ω, η’ ↔ nucleus

bound by superpositionof attractive Coulomband repulsive strong
interaction

bound solely by
the strong interaction

talk by Kenta Itahashi

talks by Kenta Itahashi
and Yoshiki Tanaka?
2meson-nucleon/nucleus interaction
I. meson-nucleon interaction:
meson

⬄

nucleonfor short-lived mesons (η, ω, η’, Φ) no beams available;
study of meson-nucleon interaction by final state interactions
in elementary reactions, e.g. p + p
p + p + η’scattering length (talk by Pawel Moskal)- interaction attractive or repulsive ?

3

meson-nucleon/nucleus interaction
I. meson-nucleon interaction:
meson

⬄

nucleonfor short-lived mesons (η, ω, η’, Φ) no beams available;
study of meson-nucleon interaction by final state interactions
in elementary reactions, e.g. p + p
p + p + η’scattering length (talk by Pawel Moskal)- interaction attractive or repulsive ?

II. meson-nucleus interaction:
meson

⬄

nucleus- if attractive, interaction strong enough to form meson-nucleus bound states ?

3

symmetry breaking in the hadronic sector
nonet of pseudoscalar mesons
M=958 MeV/c2

MeV/c2
1000
750η

M=548 MeV/c2

K M=498 MeV/c2
π

500
250

M=140 MeV/c2

04symmetry breaking in the hadronic sector
nonet of pseudoscalar mesons

MeV/c2
1000

M=958 MeV/c2750η

M=548 MeV/c2

500

K M=498 MeV/c2π250
M=140 MeV/c2

Pseudoscalar Meson Spectrum
S. Klimt et al., Nucl. Phys. A 516 (1990) 429
Mass [GeV]

mass as a result of
symmetry breaking

The NJL Model

1000

η’

!

η0 !

800
600
400
200
0

0

π, K, η0, η8

spontaneous!
U(3)L x U(3)R!
breaking!
m0i = 0!

η!
SU(3)L x SU(3)R!
!
Goldstone!

Kbosons!

ππ, K, η! 8!
U(1)A!
!
breaking
0

mi = 0SU(3)F!
breaking!
0
mud ≈ 5 MeV !
m0s ≈ 130 MeV

4symmetry breaking in the hadronic sector
nonet of pseudoscalar mesons

MeV/c2
1000

M=958 MeV/c2750η

M=548 MeV/c2

500

K M=498 MeV/c2π250
M=140 MeV/c2

Pseudoscalar Meson Spectrum
S. Klimt et al., Nucl. Phys. A 516 (1990) 429
Mass [GeV]

mass as a result of
symmetry breaking
partial restoration ofchiral symmetry
predicted in a nucleus→ impact on in-medium
meson masses ??

The NJL Model

1000

η’

!

η0 !

800
600
400
200
0

0

π, K, η0, η8

spontaneous!
U(3)L x U(3)R!
breaking!
m0i = 0!

η!
SU(3)L x SU(3)R!
!
Goldstone!

Kbosons!

ππ, K, η! 8!
U(1)A!
!
breaking
0

mi = 0SU(3)F!
breaking!
0
mud ≈ 5 MeV !
m0s ≈ 130 MeV

4meson-nucleus optical potential
U (r) = V (r) + iW (r)5meson-nucleus optical potential
U (r) = V (r) + iW (r)V (r) =m( 0 ) ·(r)
0⬄real partin-medium mass modification5

meson-nucleus optical potential
U (r) = V (r) + iW (r)V (r) =m( 0 ) ·(r)0

W (r) =
0 /2 ·
0= 12 · ~c · ⇥(r) · ⇤inel ·⬄imaginary part⬄

real part(r)

in-medium mass modification

in-medium width
inelastic cross section

5

model predictions for the in-medium mass of the η’ meson
V. Bernard und U.-G. Meiβner,
Phys. Rev. D 38 (1988) 1551

almost no density dependence
of in-medium η’ mass

P. Costa et al.,PLB 560 (2003) 171

Δmη’ (ρ=ρ0; T=0)≈-150 MeV
6

model predictions for the in-medium mass of the η’ meson
H. Nagahiro, M. Takizawa and S. Hirenzaki,
Phys. Rev. C 74 (2006) 045203

S. Sakai and D. Jido
PRC 88 (2013) 064906

SU(3)
SU(2)

Δmη’ (ρ0)≈-80 MeV

Δmη’ (ρ0)≈-150 MeV
Δmη (ρ0)≈+20 MeV

S. Bass and A. Thomas,
PLB 634 (2006) 368

Δmη’ (ρ0)≈-40 MeV for θηη’ = -200
7

model predictions for in-medium mass/width of the ω meson
F. Klingl et al.,
NPA 610 (1997) 297;
NPA 650 (1999) 299

M. Lutz et al.,
NPA 706 (2002) 437

P. Mühlich et al., NPA 780 (2006) 187

splitting into ω-like• lowering of in-medium massand N*N-1 mode
• broadening of resonanceto coupling toedium modificatios through coupling to duewith increasing nuclear density baryon resonances:
Friman , H.J.Pirner, NPA 617 (1997) 496
nucleonresonancesN*(1710), N*(1520)
N*ωρ"
edium effects calculable fromρ"ωementary γ-,π- induced reactions:ploit information from coupledspectral function for ω mesonat rest:almost no mass shift;strong in-medium broadening
Re(U) ≈0; Im(U) large

+ ...

N-1

8

experimental approaches to determine
the meson-nucleus optical potential
U (r) = V (r) + iW (r)real partV (r) =m( 0 ) ·(r)0

⦁ line shape analysis
⦁ excitation function
⦁ momentum distribution
⦁ meson-nucleus bound states

TA = A·A!⇥ 0 XN !⇥ 0 X

9experimental approaches to determine
the meson-nucleus optical potential
U (r) = V (r) + iW (r)real partV (r) =m( 0 ) ·imaginary part(r)0

⦁ line shape analysis
⦁ excitation function
⦁ momentum distribution
⦁ meson-nucleus bound states(r)W (r) =
0 /2 ·
0= 12 · ~c · ⇥(r) · ⇤inel ·⦁ transparency ratio measurement
0X
A!⇥
0
A!⌘ X

TATA
= = A· N !⇥0 X
A · N !⌘0 X

9

The imaginary part W of themeson-nucleus optical potential

10

Photoproduction of ω and η’ mesons on nuclei
experiments performed with the CBELSA/TAPS detector (Bonn)
M. Kotulla et al, PRL 100 (2008) 19230

M. Nanova et al., PLB 710 (2012) 600
2

M = (956.1 ± 0.9) MeV/c
σ = (8.8 ± 1.0) MeV
384 counts

350

counts / 10 MeV/ccounts / 10 MeV/c

400 C

2

η’→π0π0η→6γ
2

ω→π0γ→3γ

300
250
200

A·

N !⌘ 0 X

125

75

50

25

Nb

350
300
250
200

1000
1100
1200
0 0
2
M(π π η) [MeV/c ]
2

900

2

M = (955.6 ± 1.0) MeV/c
σ = (8.9 ± 0.6) MeV
385 counts

counts / 10 MeV/c

2

TA =

A!⌘ 0 X2

M = (955.3 ± 1.6) MeV/c
σ = (9.0 ± 0.9) MeV
208 counts

150

50

400

Ca

175

100

0

transparency ratio

200

η’

100

150

counts / 10 MeV/c

ω

0
100

900

Pb

1000
1100
1200
0 0
2
M(π π η) [MeV/c ]2

M = (956.4 ± 1.6) MeV/c
σ = (9.0 ± 0.6) MeV
115 counts

80
60
40

150
100

20

50
0

900

1000
1100
1200
0 0
2
M(π π η) [MeV/c ]

0

900

1000
1100
1200
0 0
2
M(π π η) [MeV/c ]11transparency ratio for ω and η’ mesons for different nuclei
σγA→ωX

TAD=

TA

D

TA

D

Zeff ·σ(γ pbound→ω p) + Neff ·σ(γ nbound→ω n)data on photo production cross sections off bound proton and neutron fromω: F. Dietz et al., EPJA 51 (2015) 6
η’: I. Jaegle et al., EPJA 47 (2011) 11

1
1

d’
CLAS
10-1

CBELSA/TAPSGiBUU: K (l=l0) = 140 MeV
10

102

10-1

Aeff

10

2

10

Aeff

Γω(<pω> =1.1 GeV/c; ρ=ρ0) ≈ 140 MeV Γη’(<pη’> =1.05 GeV/c; ρ=ρ0) ≈ 20 MeV
low density approximation: Γ(ρ0)= hc·β·ρ0·σinel
inel = (65±25) mbσ ωN

ω: W(ρ=ρ0)= -Γ0/2 ≈ - 70 MeV

inel = (10.3±1.4) mb
σ η’N

η’: W(ρ=ρ0)= -Γ0/2 = - (10±2.5) MeV

12

comparison of transparency ratios for ω and η’ mesons
Zeff ·σ(γ pbound→ω p) + Neff ·σ(γ nbound→ω n)

TA

D

TAD=

σγA→ωX

1

GiBUU

Y
R
A
N
I
ω LIM
E
PR
η'

-1

10

10

2

10

Aeff

inelastic interactions with nuclear medium much weaker for η’ mesonω: one-body absorption (e.g., ωN→πN) dominant (A. Ramos, EPJA 49 (2013) 148 )η’: additional absorption mechanism not present in D; 2-body absorption ??

13momentum dependence of transparency ratios for ω and η’
M. Nanova et al., PLB 710 (2012) 600

η’

C

C

ω M. Kotulla et al., PRL 100 (2008) 192302

no strong variation of transparency ratio with meson momentum;14momentum dependence of σinel and Γ for ω and η’ mesons

ω(⇥0 ) = ~c ·M. Nanova et al., PLB 710 (2012) 600

M. Kotulla et al., PRL 100 (2008) 192302

Γ[MeV]

200

Ramos et al.

250

150

GiBUU

Cabrera et al.,100

curves: GiBUU

5000ση’N ≈ 10 mb

Γ[MeV]K [MeV]300

inel

σinel[mb]σinel[mb]inelσωN ≈ 60 mb

η’· ⇥0 · ⇤inel

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8
p [GeV/c]p[GeV/c]p[GeV/c]solid curves: A. Ramos et al., EPJA 49 (2013) 148

curve: E. Oset and A. Ramos, PLB 704 (2011) 334

dotted curves: D. Cabrera, R. Rapp, PLB 729 (2014) 67
15

The real part V of themeson-nucleus optical potential

16

The real part of the ω-nucleus optical potential
M. Thiel et al., EPJA 49 (2013) 132• line shape analysis:counts / [9MeV/c2]

comparison with GiBUU calculations
for different in-medium scenarios
x 103
γ Nb → ω + X

4

0γ
ω→π
0
ω→π γ

30 000 ω

32
1
0600 650 700 750 800 850 900
mπ0γ [MeV/c2]

scenarios difficult to distinguish

17The real part of the ω-nucleus optical potential
M. Thiel et al., EPJA 49 (2013) 132• line shape analysis:counts / [9MeV/c2]

comparison with GiBUU calculations
for different in-medium scenarios
x 103
γ Nb → ω + X

4

0γ
ω→π
0
ω→π γ

30 000 ω

32
1
0600 650 700 750 800 850 900
mπ0γ [MeV/c2]

scenarios difficult to distinguishsensitivity limited by 4 effects:
1) mass resolution σ ≈ 3%;
only mass shifts ≫ 3% observable2) only 30% of all ω→π0γ decays occur
within the Nb nucleus3) ω decays occur over a wide range
of densities, thereby smearing out
any density-dependent signal4) ω→π0γ signal smeared out and reduced
due to large in-medium width (Γmed ≈16·Γvac );ω mesons removed in nuclear medium
via inelastic channels (like ωN→πN)

17

The real part of the ω-nucleus optical potential
M. Thiel et al., EPJA 49 (2013) 132• line shape analysis:counts / [9MeV/c2]

comparison with GiBUU calculations
for different in-medium scenarios
x 103
γ Nb → ω + X

4

0γ
ω→π
0
ω→π γ

30 000 ω

sensitivity limited by 4 effects:
1) mass resolution σ ≈ 3%;
only mass shifts ≫ 3% observable2) only 30% of all ω→π0γ decays occur
within the Nb nucleus33) ω decays occur over a wide range
of densities, thereby smearing out
any density-dependent signal

2
1
0
600 650 700 750 800 850 900
mπ0γ [MeV/c2]

scenarios difficult to distinguish4) ω→π0γ signal smeared out and reduced
due to large in-medium width (Γmed ≈16·Γvac );ω mesons removed in nuclear medium
via inelastic channels (like ωN→πN)

KEK E325; M. Naruki et al., PRL96 (2006) 092301: Δm(ρ0) = -9.2% ; no broadening
talk by S.Yokkaichi, J-PARC E16

17

The real part of the ω-nucleus potential
J. Weil, U. Mosel and V. Metag, PLB 723 (2013 ) 120

ω→π0γ

sensitive to nuclear density at production point● measurement of the excitation functionof the meson
in case of dropping mass higher meson yield for given √sbecause of increased phase space
due to lowering of the production threshold➯ cross section enhancement
π0γ excitation function

Eγthr

18

The real part of the ω-nucleus potential
J. Weil, U. Mosel and V. Metag, PLB 723 (2013 ) 120

ω→π0γ

sensitive to nuclear density at production point● measurement of the excitation functionof the meson
in case of dropping mass higher meson yield for given √sbecause of increased phase space
due to lowering of the production threshold➯ cross section enhancement
π0γ excitation function● momentum distribution of the meson:
in case of dropping mass - when leaving thenucleus hadron has to become on-shell;mass generated at the expense of kinetic
energy➯ downward shift of momentum distribution
π0γ momentum distribution
γ+93Nb→π0γ+XEγ=0.9-1.3 GeV

Eγthr

18

excitation function for ω and η’ photoproduction off Cωη’

CB/TAPS @ MAMI

data: M. Nanova et al., PLB 727 (2013) 417
calc.: E. Paryev, J. Phys. G 40 (2013) 025201

Carbonω→π0γ→3γ

-1

10

10

ση’ [µb]

σ/A [µb]

V. Metag et al., PPNP, 67 (2012) 530.

C dataσtot
σdiff

η’→π0π0η→6γ★CB/TAPS@MAMI● CBELSA/TAPS

1

V(ρ=ρ0) = 0 MeV

GiBUU
collisional broad.
and mass shift
V = 0 MeV
V = -20 MeV
V = -40 MeV
V = -55 MeV
V = -94 MeV
Eγthr V = -125 MeV

-2

10

0.9

1

1.1

1.2

1.3

1.4
Eγ [GeV]V(ρ=ρ0) = -25 MeV
V(ρ=ρ0) = -50 MeV
V(ρ=ρ0) = -75 MeV
V(ρ=ρ0) = -100 MeV
V(ρ=ρ0) = -150 MeV

10

-1

Eγthr

1000

1500

2000

2500
Eγ[MeV]Vω(ρ=ρ0) = -(42±17(stat)±20(syst)) MeV

Vη’(ρ=ρ0) = -(40±6(stat)±10(syst)) MeV

data disfavour scenario:„broadening and -125 MeV mass shift“

data disfavour strong mass shifts

19momentum distribution for ω and η’ photoproductionω

CB/TAPS @ MAMI
M. Thiel et al., EPJA 49 (2013) 132
Eγ=900-1300 MeVη’

dση’/dpη’ [µb/GeV/c]

data: M. Nanova et al., PLB 727 (2013) 417
calc.: E. Paryev, J. Phys. G 40 (2013) 025201

vacuumcollisional broadening(CB)CB+mass shift (-16%)
mass shift (-16%)

C data1

10data not consistent with strong mass shift
scenario (Δm/m≈-16%)

Eγ=1500-2200 MeV-1

V(ρ=ρ0) = 0 MeV
V(ρ=ρ0) = -25 MeV
V(ρ=ρ0) = -50 MeV
V(ρ=ρ0) = -75 MeV
V(ρ=ρ0) = -100 MeV
V(ρ=ρ0) = -150 MeV

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2
pη’ [GeV/c ]Vη’(pη’≈1.1 GeV/c;ρ=ρ0) = -(32±11) MeV
20

real part of ω-nucleus potential from ω kinetic energy
CBELSA/TAPS @ ELSAγωEγ=1.25-3.1 GeV

p 10≤θp≤110
the higher the attraction the lower the kinetic energy of the ω meson

21

real part of ω-nucleus potential from ω kinetic energy
CBELSA/TAPS @ ELSAγωEγ=1.25-3.1 GeV

p 10≤θp≤110
the higher the attraction the lower the kinetic energy of the ω mesond σπ0γ/dEkin dΩ [nb/MeV/sr]

H. Nagahiro, priv. com.
2.2
2.1
2

1.9

2

1.8
1.7
(V0 , W0)
- (156,70) MeV- (100,70) MeV- ( 50,70) MeV- ( 0,70) MeV- (-20,70) MeV- (-50,70) MeV

1.6
1.5
1.4
20

30

40

50

60

70

80
90
Eπ0γ -782 [MeV]21

real part of ω-nucleus potential from ω kinetic energy
CBELSA/TAPS @ ELSAγωEγ=1.25-3.1 GeV

p 10≤θp≤110
the higher the attraction the lower the kinetic energy of the ω meson
S. Friedrich, PLB 736 (2014) 26
d m/0a/dE kin d1 [nb/MeV/sr]d σπ0γ/dEkin dΩ [nb/MeV/sr]

H. Nagahiro, priv. com.
2.2

1.5

2.1
2

1

2

1.9

Carbon

1.8

2

0.5

1.7
(V0 , W0)
- (156,70) MeV- (100,70) MeV- ( 50,70) MeV- ( 0,70) MeV- (-20,70) MeV- (-50,70) MeV

1.6
1.5
1.4
20

30

40

50

60

70

80
90
Eπ0γ -782 [MeV]

0

-0.5

Ekin=(60.5±7)MeV-300 -200 -100

0100

200 300 400
E/0a -782 [MeV]21

real part of ω-nucleus potential from ω kinetic energy
ω

CBELSA/TAPS @ ELSAγ

Eγ=1.25-3.1 GeV

p 10≤θp≤110
the higher the attraction the lower the kinetic energy of the ω meson

2.1
2

1.9
1.8

90

1.5

80

d)

70

Carbon1

2

peak position [MeV]d σπ0γ/dEkin dΩ [nb/MeV/sr]

2.2

d m/0a/dE kin d1 [nb/MeV/sr]

S. Friedrich, PLB 736 (2014) 26

H. Nagahiro, priv. com.

60

2

0.5

1.7

50
(V0 , W0)
- (156,70) MeV- (100,70) MeV- ( 50,70) MeV- ( 0,70) MeV- (-20,70) MeV- (-50,70) MeV

1.6
1.5
1.4
20

30

40

50

60

70

0

40
30

80
90
Eπ0γ -782 [MeV]

-150

-100

-50
0
50
potential depth [MeV]-0.5

Ekin=(60.5±7)MeV-300 -200 -100Vω(pω≈300 MeV/c; ρ=ρ0) = -(15±35) MeV

0100

200 300 400
E/0a -782 [MeV]21ωcompilation of results for real and imaginary partof the ω, η’ -nucleus optical potential

η’imaginary part:WωA(ρ=ρ0)= -Γ0/2 = - (65-75) MeV

Wη’A(ρ=ρ0) = -Γ0/2 = - (7.5-12.5) MeV

real part:CNb
excitation function

excitation function
mom. distribution

peak E kinpeak Ekin-60 -50 -40 -30 -20 -10 0 10
VtC [MeV]

cross section fit-60

VωA(ρ=ρ0) =-(29±19(stat)±20(syst))MeV

-50

-40

-30

-20

-10
0
Vη’A [MeV]Vη’A(ρ=ρ0) =-(37±5(stat)±10(syst))MeV

22

compilation of results for real and imaginary partof the ω, η’ -nucleus optical potential
UωA(ρ=ρ0)=-(29±19(stat)±20(syst) + i(70±10) MeV

Uη’A(ρ=ρ0)=-(37±5(stat)±10(syst) + i(10±3) MeV

imaginary part [MeV]100⎮Im U ⎮>⎮Re U⎮
80

t

60

40

⎮Re U ⎮>>⎮Im U⎮
20

0

d’
20

40

60
80
100
potential depth [MeV]⎮Im U ⎮>⎮Re U⎮; ➯ ω not a good candidate⎮Re U⎮ >>⎮Im U⎮; ➯ η’ promising

to search for meson-nucleus bound states!

candidate to search for mesic states

first (indirect) observation of in-medium mass shift of η’ at ρ=ρ0 and T=0
in good agreement with QMC model predictions (S. Bass et al. , PLB 634 (2006) 368)

23

search for η’-mesic states
BGO-OD@ELSA
12p

C(γ,p) η’X @ 2.8 GeV

η’γ

formation and decay of η’-mesic stateΔp/p ≈ 1-2 %BGO-OD ideally suited for exclusive measurement

approved proposal: ELSA/3-2012-BGO

24

search for η’-mesic states
BGO-OD@ELSA
12

C(γ,p) η’X @ 2.8 GeV

p

η’ N

γ

η
formation and decay of η’-mesic stateΔp/p ≈ 1-2 %BGO-OD ideally suited for exclusive measurement

approved proposal: ELSA/3-2012-BGO

24

search for η’-mesic states

FRS@GSIFRS@GSI

BGO-OD@ELSA
12

12C( 12C(γ,p) η’X @ 2.8 GeV

p

η’ N

C( ,

,

)

)

K. Itahashi
et al., Prog.
Phys. 128(2012
K. Itahashi) 601 ) 601
et al., Theo.
Prog. Theo.
Phys. 128(2012γηformation and decay of η’-mesic stateΔp/p ≈ 1-2 %

2.5 G

eV p
roto
ns

12C target

2.7-

aerogelS2 Cerenkov

2. 9

GeV
(pro /c deut
tons eron
s
)

S4

MWDC
p/d separation by
aerogel Cerenkov
TOF S2-S4 (diff.≈20 ns)

plastic
scintillators

aerogelCerenkov

BGO-OD ideally suited for exclusive measurement

missing mass spectrometry: Δm=1.6 MeV/c2

approved proposal: ELSA/3-2012-BGO

status report by K. Itahashi and Y.Tanaka

24The BGO-OD experimentγ-beamBGO-ballscintillatingfibersopen dipole MWPC
magnetTOF

25

BGO-OD: results of test experimentseparation and identificationof charged particles in open dipole

clone1rb
Entries

Counts/5MeV

momentum resolution: Δp/p = 1%104

1.991986e+07

Mean

43232

569.1
72.93

RMS
χ2 / ndf

58.94 / 49

p03064 ± 69.0

p1

581.9 ± 0.5

p228.14 ± 0.77

p3

-3.877e-09 ± 4.177e-03

p4

13.42 ± 0.14

p5

-0.006899 ± 0.000499

p6-3.675e-08 ± 4.258e-07

η →2γ

BGO: π0,η→2γ

run for η’- mesic states has started !!!

500

550

600

650
700
750
Invariant mass [MeV/c2]

26

summary⦁ real and imaginary part of the ω and η’-nucleus potential have been determinedω:⎮Im U ⎮>⎮Re U⎮ → not a good candidate for the search for mesic states
η’:⎮Re U⎮ >>⎮Im U⎮ → good candidate for the search for mesic states
first results on momentum dependence of the ω- and η’-nucleus optical potential

⦁ The run for η’ mesic states has started:
photo-nuclear experiments: LEPS2, BGO-OD: 12C(γ, p) η’⦻11B
T. Nakanohadronic pick-up reaction: FRS@GSI: 12C(p,d) η’⦻11C
K.Itahashi,Y. Tanaka
27