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Roper resonances in chiral quark models Golli, B.; Širca, S.
The European physical journal. A, Hadrons and nuclei,
12/2008, Volume:
38, Issue:
3
Journal Article
Peer reviewed
Open access
We derive a method to calculate the multi-channel
K
-matrix applicable to a broad class of models in which mesons linearly couple to the quark core. The method is used to calculate pion scattering ...amplitudes in the energy region of low-lying P11 and P33 resonances. A good agreement with experimental data is achieved if in addition to the elastic channel we include the
and
N
(
channels where the
-meson models the correlated two-pion decay. We solve the integral equation for the
K
-matrix in the approximation of separable kernels; it yields a sizable increase of the widths of the
(1232) and the
N
(1440) resonances compared to the bare quark values.
We present a method to calculate pion electro-production amplitudes in a coupled-channel framework incorporating quasi-bound quark-model states. The method offers a clear prescription how to extract ...the resonant part of the amplitudes, even in the presence of different decay channels and a strong mixing of neighbouring resonances. The method is applied to the calculation of the M
1-
and the S
1-
amplitudes in the
P
11 partial wave in a simple chiral quark model. A good agreement with the observed M
1-
amplitude is found with a significant contribution from the pion cloud. The same effect is also prominent in the S
1-
amplitude but a rather uncertain data prevent us to draw a definitive conclusion.
.
We report on new measurements of the electric Generalized Polarizability (GP) of the proton
α
E
in a kinematic region where a puzzling dependence on momentum transfer has been observed, and we have ...found that
α
E
=
(
5
.
3
±
0
.
6
s
t
a
t
±
1
.
3
s
y
s
)
10
-
4
fm
3
at
Q
2
=
0
.
20
(GeV/
c
)
2
. The new measurements, when considered along with the rest of the world data, suggest that
α
E
can be described by either a local plateau or by an enhancement in the region
Q
2
=
0
.
20
(GeV/
c
)
2
to 0.33 (GeV/
c
)
2
. The experiment also provides the first measurement of the Coulomb quadrupole amplitude in the
N
→
Δ
transition through the exploration of the
p
(
e
,
e
p
)
γ
reaction. The new measurement gives
C
M
R
=
(
-
4
.
4
±
0
.
8
s
t
a
t
±
0
.
6
s
y
s
)
%
at
Q
2
=
0
.
20
(GeV/
c
)
2
and is consistent with the results from the pion electroproduction world data. It has been obtained using a completely different extraction method, and therefore represents a strong validation test of the world data model uncertainties.
We report on the first Q^{2}-dependent measurement of the beam-normal single spin asymmetry A_{n} in the elastic scattering of 570 MeV vertically polarized electrons off ^{12}C. We cover the Q^{2} ...range between 0.02 and 0.05 GeV^{2}/c^{2} and determine A_{n} at four different Q^{2} values. The experimental results are compared to a theoretical calculation that relates A_{n} to the imaginary part of the two-photon exchange amplitude. The result emphasizes that the Q^{2} behavior of A_{n} given by the ratio of the Compton to charge form factors cannot be treated independently of the target nucleus.
New precise results of a measurement of the elastic electron-proton scattering cross section performed at the Mainz Microtron MAMI are presented. About 1400 cross sections were measured with negative ...four-momentum transfers squared up to Q² = 1 (GeV/c)² with statistical errors below 0.2%. The electric and magnetic form factors of the proton were extracted by fits of a large variety of form factor models directly to the cross sections. The form factors show some features at the scale of the pion cloud. The charge and magnetic radii are determined to be <r²E>½ = 0.879(5)stat(4)syst(2)model(4)group fm and <r²M>½ = 0.777(13)stat(9)syst(5)model(2)group fm.
The helicity-dependent recoil proton polarizations P_{x}^{'} and P_{z}^{'} as well as the helicity-independent component P_{y} have been measured in the p(eover →,e^{'}pover →)π^{0} reaction at ...four-momentum transfer Q^{2}≃0.1 GeV^{2}, center-of-mass proton emission angle θ_{p}^{*}≃90°, and invariant mass W≃1440 MeV. This first precise measurement of double-polarization observables in the energy domain of the Roper resonance P_{11}(1440) by exploiting recoil polarimetry has allowed for the extraction of its scalar electroexcitation amplitude at an unprecedentedly low value of Q^{2}, establishing a powerful instrument for probing the interplay of quark and meson degrees of freedom in the nucleon.
A massive, but light, Abelian U(1) gauge boson is a well-motivated possible signature of physics beyond the standard model of particle physics. In this Letter, the search for the signal of such a ...U(1) gauge boson in electron-positron pair production at the spectrometer setup of the A1 Collaboration at the Mainz Microtron is described. Exclusion limits in the mass range of 40 MeV/c^{2} to 300 MeV/c^{2}, with a sensitivity in the squared mixing parameter of as little as ε^{2}=8×10^{-7} are presented. A large fraction of the parameter space has been excluded where the discrepancy of the measured anomalous magnetic moment of the muon with theory might be explained by an additional U(1) gauge boson.
The mechanism for the formation of the
Λ
(
1405
)
resonance is studied in a chiral quark model that includes quark-meson as well as contact (four point) interactions. The negative-parity
S
-wave ...scattering amplitudes for strangeness
-
1
and 1 are calculated within a unified coupled-channel framework that includes the
KN
,
K
¯
N
,
π
Σ
,
η
Λ
,
K
Ξ
,
π
Λ
, and
η
Σ
channels and possible genuine three-quark bare singlet and octet states corresponding to
1
2
-
resonances. It is found that in order to reproduce the scattering amplitudes in the
S
01
partial wave it is important to include the pertinent three-quark octet states as well as the singlet state, while the inclusion of the contact term is not mandatory. The Laurent-Pietarinen expansion is used to determine the
S
-matrix poles. Following their evolution as a function of increasing interaction strength, the mass of the singlet state is strongly reduced due to the attractive self-energy in the
π
Σ
and
K
¯
N
channels; when it drops below the
KN
threshold, the state acquires a dominant
K
¯
N
component which can be identified with a molecular state. The attraction between the kaon and the nucleon is generated through the
K
¯
N
Λ
∗
interaction rather than by meson-nucleon forces.
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