Excitation of nucleon resonances in
charge-exchange reactions
J. Benlliure
Universidad de Santiago de Compostela
Spain
Motivation
Charge-exchange reactions are unique probes to investigate spin-isopin properties of nuclei.
- Contrary to b-decay, these reactions may excite spin-isospin modes over a broad range in energy.
- Spin-isospin excitations manifest in two different energy domains
 at low energies: particle-hole excitations (Gamow-
Teller, spin-dipole, spin- quadrupole or quasi-elastic).
- Gamow-Teller: BGT transition strengths
- spin-dipole: radial distributions of protons and neutrons
27A(20Ne,20Na)
@ 900 MeV/u
A. Krasznahorkay et al. NPA 731 (2004) 224
 at high energies: excitation of nucleon resonances (e.g. D resonance)
- In-medium properties of baryon resonances (downward shift of the D mass
when excited in nuclei) .
C. Bachelier et al. PLB 172 (1986) 23
Energy(MeV)
José Benlliure, ARIS 2014
Tokyo, Japan, June 2014
Motivation
Subnucleonic degrees of freedom play a role in ground-state and structural properties of nuclei.
 Gamow-Teller strength quenching.
role of D-h excitations
 Three-body nuclear forces.
role of D(1232) and Roper(1440)
The relative probability for inelastic (n,p) and (p,n ) reactions can be used to prove the abundance
of protons and neutrons at the nuclear periphery.
 Because of the strong absorption isobar chargeexchange reactions are peripheral processes.
n
p-
po
p
José Benlliure, ARIS 2014
p
Do
p
Tokyo, Japan, June 2014
Motivation
To investigate spin-isospin excitations at nuclear and subnucleonic level in asymmetric nuclear
matter inducing isobar charge-exchange reactions, (p,n) and (n,p), on radioactive relativistic projectiles
208Pb(1000
124Sb
124Sn
---- p(208Pb,208Bi)
---- d(208Pb,208Bi)
124In
n
p
p
n
p+
p
n
p
p+,-
p,n
N
MeV/u)+p,d
n
n
D
N
José Benlliure, ARIS 2014
p
Do
p-
po
N
p
A. Kelic et al., PR.C 70 (2004) 64608
p
The momentum recoil induced by the pion
emission proves the excitation of the resonance.
Tokyo, Japan, June 2014
Experimental requirements
Quasi-elastic and inelastic isobar charge-exchange reactions, (p,n)
and (n,p), in asymmetric nuclear matter:
 relativistic heavy-ion collisions induced by exotic projectiles (isospin
asymmetry and radial distributions of protons and neutrons)
124Sb
124Sn
 isobar charge-exchange (clean reaction channel)
124In
Observables:
 cross sections for both charge-exchange reactions and channels
 missing-energy spectra of the projectile residues
Requirements for the setup:
 isotopic identification of relativistic projectile residues
 separation of elastic and resonant charge-exchange channels
(magnetic analysis of the projectile residues)
José Benlliure, ARIS 2014
Tokyo, Japan, June 2014
The experiment
Inverse kinematics
GSI
DB
 2 10 4
B
DE ~ 25 MeV (124Sn @ 1000 MeV/u)
DZ/Z ~ 7 10-3
DA/A ~ 2.4 10-3
José Benlliure, ARIS 2014
Tokyo, Japan, June 2014
The experiment
Te
Te
Te
Te
Te
Te
Te
120
121
122
123
124
125
126
Sb
Sb
Sb
123
123
123
Sn
Sn
Sn
Sn
Sn
Sn
Sn
Sn
Sn
Sn
Sn
Sn
Sn
Sn
Sn
Sn
Sn
Sn
Sn
Sn
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
In
115
Reactions investigated:
 124Sn+CH2,C, Cu, Pb 124Sb,124In @ 1000 A MeV
 124Sn+Be  120Sn+CH2,C  120Sb,120In @ 1000 A MeV
 112Sn+CH2,C,Cu,Pb 112Sb,112In @ 400, 700, 1000 A MeV
 112Sn+Be  110Sn+CH2,C  110Sb,110In @ 1000 A MeV
José Benlliure, ARIS 2014
Tokyo, Japan, June 2014
Results
Isotopic identification of isobaric charge-exchange residues
José Benlliure, ARIS 2014
Tokyo, Japan, June 2014
Results
Missing-energy spectra in isobar charge-changing reactions induced by 112Sn
(p,n)
José Benlliure, ARIS 2014
(n,p)
Tokyo, Japan, June 2014
Results
Missing-energy spectra in isobar charge-changing reactions induced by 112Sn
C. Bachelier et al. PLB 172 (1986) 23
 The observed spectra are in good qualitative agreement with previous measurements at Saturne
(France)
José Benlliure, ARIS 2014
Tokyo, Japan, June 2014
Results
Unfolding the missing-energy spectrum with the experimental response function
Y: measured spectrum
H: experimental response function
X: observable
The primary beam centred through the FRS
constitutes our response function H
J. Vargas, J.B. and M. Caamaño NIMA 707 (2013) 16
José Benlliure, ARIS 2014
Tokyo, Japan, June 2014
Results
Unfolding the missing-energy with the experimental response function
112Sn(63Cu,X)112Sb
 The unfolding procedure improves the resolution of the final spectra
José Benlliure, ARIS 2014
Tokyo, Japan, June 2014
Results
Excitation of nucleon resonances
a(p,X)a’ @ 4.2 GeV
112Sn(63Cu,X)112Sb
D(1232)
N*(1440)
D(1600) ?
H.P. Morsch et al., PRL 69, 1336 (1992)
 Unfolded spectra show additional structures in the inelastic charge-exchange peak that can be
associated to the excitation of different nucleon resonances as the D and Roper resonances.
José Benlliure, ARIS 2014
Tokyo, Japan, June 2014
Recent measurements with the FRS
Resolving nucleon resonances
José Benlliure, ARIS 2014
Tokyo, Japan, June 2014
Results
Energy and target dependence of the resonance excitation
112Sn(12C,12B)112Sb
112Sn(C,X)112Sb+p-
1000 A MeV
700 A MeV
400 A MeV
In nucleus-nucleus collisions D-resonance excitation
may occurs below the pion production threshold:
-
Fermi momentum of the nucleons
José Benlliure, ARIS 2014
Tokyo, Japan, June 2014
Results
Energy and target dependence of the resonance excitation
112Sn(12C,12B)112Sb
112Sn(C,C)112Sb+p-
1000 A MeV
700 A MeV
400 A MeV
112Sn(p,n)112Sb+p112Sn(C,B)112Sb+p+
In nucleus-nucleus collisions D-resonance excitation
occurs below the pion production threshold:
- Fermi momentum of the nucleons
- Lower energy of the D-resonance
José Benlliure, ARIS 2014
Tokyo, Japan, June 2014
Results
Charge-exchange cross sections
 Both cross sections are sensitive to the neutron excess at projectile periphery s(n,p) > s(p,n)
and the projectile size s(124Sb) > s(112Sb)
 Elastic (n,p) reactions in the projectile are inhibited in heavy targets by charge conservation
 Simple targets such as protons or carbons seem better suited to link these cross sections to the
relative abundance of protons and neutrons at the projectile periphery but model calculations are
required
José Benlliure, ARIS 2014
Tokyo, Japan, June 2014
Model calculations
Differential cross sections:
Isaac Vidaña, U. Coimbra
Free NN cross sections:
Scattering amplitudes:
+ target excitations …..
Vertices:
José Benlliure, ARIS 2014
Tokyo, Japan, June 2014
Model calculations
Isaac Vidaña, U. Coimbra Preliminary calculations
 Preliminary model calculations provide a good qualitative description of the measured distributions
 The calculations clearly show the contribution of different resonances but also their excitation in the
projectile but also in the target nucleus
 Measurements with proton and carbon targets will be used to extract the properties of the
observed resonances
José Benlliure, ARIS 2014
Tokyo, Japan, June 2014
Future plans at
Experimental programme at the Super-FRS
Projectile excitations
Exclusive measurements:
Pion detection:
- Projectile and target excitations
Target excitations
- Identification of resonance isobars
Measurements with secondary beams:
 Higher secondary beam intensities
 Contaminants suppresion
 Thinner reaction targets
José Benlliure, ARIS 2014
Tokyo, Japan, June 2014
Conclusions
 A high-resolving power magnetic spectrometer has been proven to be an excellent tool to identify
nucleonic excitations in heavy-ion charge-exchange reactions:
- several baryon resonances were identified in the projectile missing energy spectra
- cross sections for the elastic and inelastic (p,n) and (n,p) channels where also measured
- in-medium properties of these baryons and relative abundance of protons and neutrons at the
nuclear periphery are investigated using reliable model calculations
 The Super-FRS at FAIR will offer unique opportunities for these investigations
- extremely asymmetric nuclear matter
- improved separation capabilities (pre-separator) and resolution
- exclusive measurements detecting pions in coincidence
Collaborators:
U. Santiago de Compostela: J. Vargas, Y. Ayyad, S. Beceiro, D. Cortina, P. Díaz, M. Mostazo, C. Paradela
GSI: T. Aumann, J. Atkinson, K. Boretzky, A. Estrade, H. Geissel, A. Kelic, Y. Litvinov, S. Pietri, A. Prochazka,
M. Takechi, J. Winfield
CEA/DAM: A. Chatillon, J. Taieb
José Benlliure, ARIS 2014
U. Coimbra: I. Vidaña
U. Giessen: H. Lenske
Tokyo, Japan, June 2014
Recent measurements with the FRS
Missing-energy spectra with secondary beams
112Sn
110Sn
110Sn(C,X)110Sb+p110Sb
Main difficulties:
- Low statistics
- Thick S2 target (1 g/cm2 C)
- Important background at S2
José Benlliure
SuperFRS collaboration meeting, October 2013
Recent measurements with the FRS
Unfolding and regularization of the missing-energy spectrum
The most reliable unfolding techniques are based on
iterative procedures (Richardson-Lucy)
136I
(p,n)
D++D++
Regularization methods are needed to determine the
optimum number of iterations
J. Vargas, J.B. and M. Caamaño NIMA 707 (2013) 16
José Benlliure
SuperFRS collaboration meeting, October 2013
Proposal
To investigate the isospin dependence of spin-isospin excitations at low and
high momentum transfer for both isobar charge-exchange channels (p,n)
and (n,p) using relativistic exotic projectiles.
 In-medium properties of baryon resonance in isospin asymmetric
nuclear matter. (mean energy and width of the resonance)
132Sb
132Sn
132In
 Gamow-Teller transition strengths
 Radial distributions of neutrons and protons.
 Nuclear matrix elements for inelastic neutrino interactions.
José Benlliure
SuperFRS collaboration meeting, February 2014