The strong nuclear interaction between nucleons (protons and neutrons) is the effective force that holds the atomic nucleus together. This force stems from fundamental interactions between quarks and ...gluons (the constituents of nucleons) that are described by the equations of quantum chromodynamics. However, as these equations cannot be solved directly, nuclear interactions are described using simplified models, which are well constrained at typical inter-nucleon distances
but not at shorter distances. This limits our ability to describe high-density nuclear matter such as that in the cores of neutron stars
. Here we use high-energy electron scattering measurements that isolate nucleon pairs in short-distance, high-momentum configurations
, accessing a kinematical regime that has not been previously explored by experiments, corresponding to relative momenta between the pair above 400 megaelectronvolts per c (c, speed of light in vacuum). As the relative momentum between two nucleons increases and their separation thereby decreases, we observe a transition from a spin-dependent tensor force to a predominantly spin-independent scalar force. These results demonstrate the usefulness of using such measurements to study the nuclear interaction at short distances and also support the use of point-like nucleon models with two- and three-body effective interactions to describe nuclear systems up to densities several times higher than the central density of the nucleus.
Deeply virtual Compton scattering (DVCS) allows one to probe generalized parton distributions describing the 3D structure of the nucleon. We report the first measurement of the DVCS beam-spin ...asymmetry using the CLAS12 spectrometer with a 10.2 and 10.6 GeV electron beam scattering from unpolarized protons. The results greatly extend the Q^{2} and Bjorken-x phase space beyond the existing data in the valence region and provide 1600 new data points measured with unprecedented statistical uncertainty, setting new, tight constraints for future phenomenological studies.
Here, the single-differential and fully integrated cross sections for quasi-free $π^+π^-$ electroproduction off protons bound in deuterium have been extracted for the first time. The experimental ...data were collected at Jefferson Laboratory with the CLAS detector. The measurements were performed in the kinematic region of the invariant mass W from 1.3 GeV to 1.825 GeV and photon virtuality Q2 from 0.4 GeV2 to 1.0 GeV2. Sufficient experimental statistics allow for narrow binning in all kinematic variables, while maintaining a small statistical uncertainty. The extracted cross sections were compared with the corresponding cross sections off free protons, which allowed us to obtain an estimate of the contribution from events in which interactions between the final-state hadrons and the spectator neutron took place.
Here, we present the first measurement of dihadron angular correlations in electron-nucleus scattering. The data were taken with the CLAS detector and a 5.0 GeV electron beam incident on deuterium, ...carbon, iron, and lead targets. Relative to deuterium, the nuclear yields of charged-pion pairs show a strong suppression for azimuthally opposite pairs, no suppression for azimuthally nearby pairs, and an enhancement of pairs with large invariant mass. These effects grow with increased nuclear size. The data are qualitatively described by the gibuu model, which suggests that hadrons form near the nuclear surface and undergo multiple scattering in nuclei.These results show that angular correlation studies can open a new way to elucidate how hadrons form and interact inside nuclei.
Measuring Deeply Virtual Compton Scattering on the neutron is one of the necessary steps to understand the structure of the nucleon in terms of Generalized Parton Distributions (GPDs). Neutron ...targets play a complementary role to transversely polarized proton targets in the determination of the GPD \(E\). This poorly known and poorly constrained GPD is essential to obtain the contribution of the quarks' angular momentum to the spin of the nucleon. DVCS on the neutron was measured for the first time selecting the exclusive final state by detecting the neutron, using the Jefferson Lab longitudinally polarized electron beam, with energies up to 10.6 GeV, and the CLAS12 detector. The extracted beam-spin asymmetries, combined with DVCS observables measured on the proton, allow a clean quark-flavor separation of the imaginary parts of the GPDs \(H\) and \(E\).
The polarized cross section ratio $\sigma_{LT'}/\sigma_{0}$ from hard
exclusive $\pi^{-} \Delta^{++}$ electroproduction off an unpolarized hydrogen
target has been extracted based on beam-spin ...asymmetry measurements using a
10.2 GeV / 10.6 GeV incident electron beam and the CLAS12 spectrometer at
Jefferson Lab. The study, which provides the first observation of this channel
in the deep-inelastic regime, focuses on very forward-pion kinematics in the
valence regime, and photon virtualities ranging from 1.5 GeV$^{2}$ up to 7
GeV$^{2}$. The reaction provides a novel access to the $d$-quark content of the
nucleon and to $p \rightarrow \Delta^{++}$ transition generalized parton
distributions. A comparison to existing results for hard exclusive $\pi^{+} n$
and $\pi^{0} p$ electroproduction is provided, which shows a clear impact of
the excitation mechanism, encoded in transition generalized parton
distributions, on the asymmetry.
We report results of $\Lambda$ hyperon production in semi-inclusive
deep-inelastic scattering off deuterium, carbon, iron, and lead targets
obtained with the CLAS detector and the Continuous Electron ...Beam Accelerator
Facility 5.014~GeV electron beam. These results represent the first
measurements of the $\Lambda$ multiplicity ratio and transverse momentum
broadening as a function of the energy fraction~($z$) in the current and target
fragmentation regions. The multiplicity ratio exhibits a strong suppression at
high~$z$~and~an enhancement at~low~$z$. The measured transverse momentum
broadening is an order of magnitude greater than that seen for light mesons.
This indicates that the propagating entity interacts very strongly with the
nuclear medium, which suggests that propagation of diquark configurations in
the nuclear medium takes place at least part of the time, even at high~$z$. The
trends of these results are qualitatively described by the Giessen
Boltzmann-Uehling-Uhlenbeck transport model, particularly for the multiplicity
ratios. These observations will potentially open a new era of studies of the
structure of the nucleon as well as of strange baryons.
A multidimensional extraction of the structure function ratio
$\sigma_{LT'}/\sigma_{0}$ from the hard exclusive $\vec{e} p \to e^\prime n
\pi^+$ reaction above the resonance region has been ...performed. The study was
done based on beam-spin asymmetry measurements using a 10.6 GeV incident
electron beam on a liquid-hydrogen target and the CLAS12 spectrometer at
Jefferson Lab. The measurements focus on the very forward regime ($t/Q^{2}$
$\ll$ 1) with a wide kinematic range of $x_{B}$ in the valence regime (0.17 $<$
$x_{B}$ $<$ 0.55), and virtualities $Q^{2}$ ranging from 1.5 GeV$^{2}$ up to 6
GeV$^{2}$. The results and their comparison to theoretical models based on
Generalized Parton Distributions demonstrate the sensitivity to chiral-odd GPDs
and the directly related tensor charge of the nucleon. In addition, the data is
compared to an extension of a Regge formalism at high photon virtualities. It
was found that the Regge model provides a better description at low $Q^{2}$,
while the GPD model is more appropriate at high $Q^{2}$.
Several factors can contribute to the difficulty of aligning the sensors of tracking detectors, including a large number of modules, multiple types of detector technologies, and non-linear strip ...patterns on the sensors. All three of these factors apply to the CLAS12 CVT, which is a hybrid detector consisting of planar silicon sensors with non-parallel strips, and cylindrical micromegas sensors with longitudinal and arc-shaped strips located within a 5~T superconducting solenoid. To align this detector, we used the Kalman Alignment Algorithm, which accounts for correlations between the alignment parameters without requiring the time-consuming inversion of large matrices. This is the first time that this algorithm has been adapted for use with hybrid technologies, non-parallel strips, and curved sensors. We present the results for the first alignment of the CLAS12 CVT using straight tracks from cosmic rays and from a target with the magnetic field turned off. After running this procedure, we achieved alignment at the level of 10~\(\mu\)m, and the widths of the residual spectra were greatly reduced. These results attest to the flexibility of this algorithm and its applicability to future use in the CLAS12 CVT and other hybrid or curved trackers, such as those proposed for the future Electron-Ion Collider.
Strange matter is believed to exist in the cores of neutron stars based on
simple kinematics. If this is true, then hyperon-nucleon interactions will play
a significant part in the neutron star ...equation of state (EOS). Yet, compared
to other elastic scattering processes, there is very little data on
$\Lambda$-$N$ scattering. This experiment utilized the CLAS detector to study
the $\Lambda p \rightarrow \Lambda p$ elastic scattering cross section in the
incident $\Lambda$ momentum range 0.9-2.0 GeV/c. This is the first data on this
reaction in several decades. The new cross sections have significantly better
accuracy and precision than the existing world data, and the techniques
developed here can also be used in future experiments.
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