The proton, one of the components of atomic nuclei, is composed of fundamental particles called quarks and gluons. Gluons are the carriers of the force that binds quarks together, and free quarks are ...never found in isolation-that is, they are confined within the composite particles in which they reside. The origin of quark confinement is one of the most important questions in modern particle and nuclear physics because confinement is at the core of what makes the proton a stable particle and thus provides stability to the Universe. The internal quark structure of the proton is revealed by deeply virtual Compton scattering
, a process in which electrons are scattered off quarks inside the protons, which subsequently emit high-energy photons, which are detected in coincidence with the scattered electrons and recoil protons. Here we report a measurement of the pressure distribution experienced by the quarks in the proton. We find a strong repulsive pressure near the centre of the proton (up to 0.6 femtometres) and a binding pressure at greater distances. The average peak pressure near the centre is about 10
pascals, which exceeds the pressure estimated for the most densely packed known objects in the Universe, neutron stars
. This work opens up a new area of research on the fundamental gravitational properties of protons, neutrons and nuclei, which can provide access to their physical radii, the internal shear forces acting on the quarks and their pressure distributions.
The physics of the gravitational form factors of the proton, as well as their understanding within quantum chromodynamics, has advanced significantly in the past two decades through both theory and ...experiment. This Colloquium provides an overview of this progress, highlights the physical insights unveiled by studies of gravitational form factors, and reviews their interpretation in terms of the mechanical properties of the proton.
I give an overview on experimental studies of the spectrum and the structure of the excited states of the nucleon and what we can learn about their in ternal structure. One focus is on the efforts to ...obtain a more complete picture of the light-quark baryon exci tation spectrum employing electromagnetic beams that will allow us to draw some conclusions on the symme tries underlying the spectrum. For the higher mass ex citations, the full employment of coupled channel ap proaches is essential when searching for new excited states in the large amounts of data already accumulated in different channels involving a variety of polarization observables. The other focus is on the study of transition form factors and helicity amplitudes and their de pendences on
Q
2
, especially on some of the more promi nent resonances, especially Δ(1232)3/2
+
,
N
(1440)1/2
+
, and negative parity states
N
(1535)1/2
-
, and
N
(1675)5/2
-
.These were obtained in pion and eta electroproduction experi ments off proton targets and have already led to further insights in the active degrees-of-freedom as a function of the distance scale involved.
The CLAS12 Geant4 simulation Ungaro, M.; Angelini, G.; Battaglieri, M. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
04/2020, Letnik:
959, Številka:
C
Journal Article
Recenzirano
Odprti dostop
The Geant4 Monte-Carlo (GEMC) package is used to simulate the passage of particles through the various CLAS12 detectors. The geometry is implemented through a database of Geant4 volumes created ...either through the GEMC native API, by the CLAS12 geometry service, or imported from the CAD engineering model. The truth information is digitized with a plugin mechanism by routines specific to each detector and includes the use of the CLAS12 calibration database constants to produce both ADC and TDC response functions. Theoretical models that produce the generated events interface with GEMC through the LUND data format. The merging of simulated data with real random trigger data provides a mechanism to include both beam and electronic background into the simulation of generated events to accurately model beam data from the CLAS12 detector. The performance of simulation is demonstrated by comparison with the experimental data.
Electroexcitation of nucleon resonances Aznauryan, I.G.; Burkert, V.D.
Progress in particle and nuclear physics,
2012, 2012-1-00, 2012-01-01, Letnik:
67, Številka:
1
Journal Article
Recenzirano
Odprti dostop
We review recent progress in the investigation of the electroexcitation of nucleon resonances, both in experiment and in theory. We describe current experimental facilities, the experiments performed ...on
π
and
η
electroproduction off protons, and theoretical approaches used for the extraction of resonance contributions from the experimental data. The status of
2
π
,
K
Λ
, and
K
Σ
electroproduction is also presented. The most accurate results have been obtained for the electroexcitation amplitudes of the four lowest excited states, which have been measured in a range of
Q
2
up to
8
and 4.5 GeV
2 for
Δ
(
1232
)
P
33
,
N
(
1535
)
S
11
and
N
(
1440
)
P
11
,
N
(
1520
)
D
13
, respectively. These results have been confronted with calculations based on lattice QCD, large-
N
c
relations, perturbative QCD (pQCD), and QCD-inspired models. The amplitudes for the
Δ
(
1232
)
indicate large pion-cloud contributions at low
Q
2
and do not show any sign of approaching the pQCD regime for
Q
2
<
7
GeV
2
. Measured for the first time, the electroexcitation amplitudes of the Roper resonance,
N
(
1440
)
P
11
, provide strong evidence for this state as a predominantly radial excitation of a three-quark (3
q
) ground state, with additional non-3-quark contributions needed to describe the low
Q
2
behavior of the amplitudes. The longitudinal transition amplitude for the
N
(
1535
)
S
11
was determined and has become a challenge for quark models. Explanations may require large meson-cloud contributions or alternative representations of this state. The
N
(
1520
)
D
13
clearly shows the rapid changeover from helicity-3/2 dominance at the real photon point to helicity-1/2 dominance at
Q
2
>
0.5
GeV
2
, confirming a long-standing prediction of the constituent quark model. The interpretation of the moments of resonance transition form factors in terms of transition transverse charge distributions in infinite momentum frame is presented.
We report the predictions for the 3
q
core contributions to the electroexcitation of the resonances
Δ
(
1232
)
3
2
+
,
N
(
1440
)
1
2
+
,
N
(
1520
)
3
2
-
,
N
(
1535
)
1
2
-
, and
N
(
1675
)
5
2
-
on ...the proton obtained in the light-front relativistic quark model (LF RQM). For these states, experimental data on the electroexcitation transition amplitudes allow us to make comparison between the experiment and LF RQM predictions in wide range of
Q
2
and also to quantify the expected meson-baryon contributions as a function of
Q
2
.
The CLAS detector at Jefferson Lab has provided the dominant part of all available worldwide data on exclusive meson electroproduction off protons in the resonance region. New results on the γυpN* ...transition amplitudes (electrocouplings) are available from analyses of the CLAS data and will be presented. Their impact on understanding of hadron structure will be discussed emphasizing the credible access to the dressed quark mass function that has been achieved for the first time by a combined analysis of the experimental results on the electromagnetic nucleon elastic and N → N* transition form factors. We will also discuss further convincing evidences for a new baryon state N′ (1720)3/2+ found in a combined analysis of charged double pion photo- and electroproduction cross sections off the protons.
PDF
