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 (QCD). However, as these equations cannot be solved directly, physicists resort to describing nuclear interactions using effective models that are well constrained at typical inter-nucleon distances in nuclei but not at shorter distances. This limits our ability to describe high-density nuclear matter such as in the cores of neutron stars. Here we use high-energy electron scattering measurements that isolate nucleon pairs in short-distance, high-momentum configurations thereby accessing a kinematical regime that has not been previously explored by experiments, corresponding to relative momenta above 400 MeV/c. 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 power of using such measurements to study the nuclear interaction at short-distances and also support the use of point-like nucleons with two- and three-body effective interactions to describe nuclear systems up to densities several times higher than the central density of atomic nuclei.
Phys. Rev. Lett. 126, 082002 (2021) The quark structure of the $f_2(1270)$ meson has, for many years, been
assumed to be a pure quark-antiquark ($q\bar{q}$) resonance with quantum
numbers $J^{PC} = ...2^{++}$. Recently, it was proposed that the $f_2(1270)$ is a
molecular state made from the attractive interaction of two $\rho$-mesons. Such
a state would be expected to decay strongly to final states with charged pions,
due to the dominant decay $\rho \to \pi^+ \pi^-$, whereas decay to two neutral
pions would likely be suppressed. Here, we measure for the first time the
reaction $\gamma p \to \pi^0 \pi^0 p$, using the CLAS detector at Jefferson Lab
for incident beam energies between 3.6-5.4~GeV. Differential cross sections,
$d\sigma / dt$, for $f_2(1270)$ photoproduction are extracted with good
precision, due to low backgrounds, and are compared with theoretical
calculations.
A first measurement of the longitudinal beam spin asymmetry ALU in the semi-inclusive electroproduction of pairs of charged pions is reported. ALU is a higher-twist observable and offers the cleanest ...access to the nucleon twist-3 parton distribution function e(x). Data have been collected in the Hall-B at Jefferson Lab by impinging a 5.498 GeV electron beam on a liquid-hydrogen target, and reconstructing the scattered electron and the pion pair with the CLAS detector. One-dimensional projections of the sin(phiR) moments of ALU are extracted for the kinematic variables of interest in the valence quark region. The understanding of di-hadron production is essential for the interpretation of observables in single hadron production in semi-inclusive DIS, and pioneering measurements of single spin asymmetries in di-hadron production open a new avenue in studies of QCD dynamics.
We present a search for ten baryon number violating decay modes of Λ hyperons using the CLAS detector at Jefferson Laboratory. Nine of these decay modes result in a single meson and single lepton in ...the final state (Λ→mℓ) and conserve either the sum or the difference of baryon and lepton number (B±L). The tenth decay mode (Λ→p¯π+) represents a difference in baryon number of two units and no difference in lepton number. We observe no significant signal and set upper limits on the branching fractions of these reactions in the range (4–200)×10−7 at the 90% confidence level.
We present the first measurement of the exclusive reaction \(\gamma p \rightarrow a_2(1320)^0 \, p\) in the photon energy range \(3.5\)-\(5.5\) GeV and four-momentum transfer squared \(0.2<-t<2.0\) ...GeV\(^2\). Data were collected with the CEBAF Large Acceptance Spectrometer at the Thomas Jefferson National Accelerator Facility. The neutral \(a_2\) resonance was detected by measuring the reaction \(\gamma p \rightarrow \pi^0 \eta p\) and reconstructing the \(\pi^0 \eta\) invariant mass. The differential cross section \(d\sigma/dt\) was extracted at different beam energies in each \(-t\) bin. The most prominent feature of the differential cross section is a dip at \(-t\simeq 0.55\) GeV\(^2\). This can be well described in the framework of Regge phenomenology, where the exchange degeneracy hypothesis predicts a zero in the reaction amplitude for this value of the four-momentum transfer.
Phys. Rev. Lett. 125, 182001 (2020) We have measured beam-spin asymmetries to extract the $\sin\phi$ moment
$A_{LU}^{\sin\phi}$ from the hard exclusive $\vec{e} p \to e^\prime n \pi^+$
reaction above ...the resonance region, for the first time with nearly full
coverage from forward to backward angles in the center-of-mass. The
$A_{LU}^{\sin\phi}$ moment has been measured up to 6.6 GeV$^{2}$ in $-t$,
covering the kinematic regimes of Generalized Parton Distributions (GPD) and
baryon-to-meson Transition Distribution Amplitudes (TDA) at the same time. The
experimental results in very forward kinematics demonstrate the sensitivity to
chiral-odd and chiral-even GPDs. In very backward kinematics where the TDA
framework is applicable, we found $A_{LU}^{\sin\phi}$ to be negative, while a
sign change was observed near 90$^\circ$ in the center-of-mass. The unique
results presented in this paper will provide critical constraints to establish
reaction mechanisms that can help to further develop the GPD and TDA
frameworks.
Single spin azimuthal asymmetries
A
L
T
′
were measured at Jefferson Lab using 2.2 and 4.4 GeV longitudinally polarised electrons incident on
4He and
12C targets in the CLAS detector.
A
L
T
′
is ...related to the imaginary part of the longitudinal-transverse interference and in quasifree nucleon knockout it provides an unambiguous signature for final state interactions (FSI). Experimental values of
A
L
T
′
were found to be below 5%, typically
|
A
L
T
′
|
⩽
3
%
for data with good statistical precision. Optical model in eikonal approximation (OMEA) and relativistic multiple-scattering Glauber approximation (RMSGA) calculations are shown to be consistent with the measured asymmetries.
Single spin azimuthal asymmetries ALT′ were measured at Jefferson Lab using 2.2 and 4.4 GeV longitudinally polarised electrons incident on 4He and 12C targets in the CLAS detector. ALT′ is related to ...the imaginary part of the longitudinal-transverse interference and in quasifree nucleon knockout it provides an unambiguous signature for final state interactions (FSI). Experimental values of ALT′ were found to be below 5%, typically |ALT′|less-than-or-equals, slant3% for data with good statistical precision. Optical model in eikonal approximation (OMEA) and relativistic multiple-scattering Glauber approximation (RMSGA) calculations are shown to be consistent with the measured asymmetries.
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