The neutron is a cornerstone in our depiction of the visible universe. Despite the neutron zero-net electric charge, the asymmetric distribution of the positively- (up) and negatively-charged (down) ...quarks, a result of the complex quark-gluon dynamics, lead to a negative value for its squared charge radius, Formula: see text. The precise measurement of the neutron's charge radius thus emerges as an essential part of unraveling its structure. Here we report on a Formula: see text measurement, based on the extraction of the neutron electric form factor, Formula: see text, at low four-momentum transfer squared (Q
) by exploiting the long known connection between the N → Δ quadrupole transitions and the neutron electric form factor. Our result, Formula: see text, addresses long standing unresolved discrepancies in the Formula: see text determination. The dynamics of the strong nuclear force can be viewed through the precise picture of the neutron's constituent distributions that result into the non-zero Formula: see text value.
We have determined the proton and the neutron charge radii from a global analysis of the proton and the neutron elastic form factors, after first performing a flavor decomposition of these form ...factors under charge symmetry in the light cone frame formulation. We then extracted the transverse mean-square radii of the flavor dependent quark distributions. In turn, these are related in a model-independent way to the proton and neutron charge radii but allow us to take into account motion effects of the recoiling nucleon for data at finite but high momentum transfer. In the proton case we find
⟨
r
p
⟩
=
0.852
±
0
.
002
(
stat
.
)
±
0
.
009
(
syst
.
)
(
fm
)
, consistent with the proton charge radius obtained from muonic hydrogen spectroscopy
1
,
2
. The current method improves on the precision of the
⟨
r
p
⟩
extraction based on the form factor measurements. Furthermore, we find no discrepancy in the
⟨
r
p
⟩
determination among the different electron scattering measurements, all of which, utilizing the current method of extraction, result in a value that is consistent with the smallest
⟨
r
p
⟩
extraction from the electron scattering measurements
3
. Concerning the neutron case, past results relied solely on the neutron-electron scattering length measurements, which suffer from an underestimation of underlying systematic uncertainties inherent to the extraction technique. Utilizing the present method we have performed the first extraction of the neutron charge radius based on nucleon form factor data, and we find
⟨
r
n
2
⟩
=
-
0.122
±
0
.
004
(
stat
.
)
±
0
.
010
(
syst
.
)
(
fm
2
)
.
Double Deeply Virtual Compton Scattering (DDVCS) is the only experimental channel for the determination of the dependence of the Generalized Parton Distributions (GPDs) on both the average and the ...transferred momentum independently. The physics observables of the electron induced di-muon production reaction
e
→
±
p
→
e
±
p
μ
+
μ
-
off unpolarized hydrogen are discussed. Their measurement with the high luminosity and large acceptance SoLID spectrometer at the Thomas Jefferson National Accelerator Facility, using polarized and unpolarized positron and electron beams at 11 GeV is investigated. This experimental configuration is shown to provide unprecedented access to the GPDs with the determination of the real and imaginary parts of the Compton Form Factor
H
in an unexplored phase space, and to enable an exploratory investigation of higher twist effects.
Electromagnetic polarizabilities are fundamental properties of the proton that characterize its response to an external electromagnetic (EM) field. The generalization of the EM polarizabilities to ...non-zero four-momentum transfer opens up a powerful path to study the internal structure of the proton. They map out the spatial distribution of the polarization densities in the proton, provide access to key dynamical mechanisms that contribute to the electric and magnetic polarizability effects, and allow for the determination of fundamental characteristics of the system, such as the electric and magnetic polarizability radii. This article reviews our knowledge about proton EM generalized polarizabilities (GPs). An introduction is given to the basic concepts and the theoretical framework, which is then followed by a discussion that emphasizes the recent developments and findings of the virtual Compton scattering (VCS) experiments and future perspectives on the topic.
We report new precise H(e,e(')p)pi(0) measurements at the Delta(1232) resonance at Q(2)=0.127 (GeV/c)(2) obtained at the MIT-Bates out-of-plane scattering facility which are particularly sensitive to ...the transverse electric amplitude (E2) of the gamma(*)N-->Delta transition. The new data have been analyzed together with those of earlier measurements to yield precise quadrupole to dipole amplitude ratios: Re(E(3/2)(1+)/M(3/2)(1+))=(-2.3+/-0.3(stat+syst)+/-0.6(model))% and Re(S(3/2)(1+)/M(3/2)(1+))=(-6.1+/-0.2(stat+syst)+/-0.5(model))% for M(3/2)(1+)=(41.4+/-0.3(stat+syst)+/-0.4(model))(10(-3)/m(pi(+))). The derived amplitudes give credence to the conjecture of deformation in hadrons favoring, at low Q2, the dominance of mesonic effects.
.
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.
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