The explicit breaking of the axial symmetry by quantum fluctuations gives rise to the so-called axial anomaly. This phenomenon is solely responsible for the decay of the neutral pion π
into two ...photons (γγ), leading to its unusually short lifetime. We precisely measured the decay width Γ of the Formula: see text process. The differential cross sections for π
photoproduction at forward angles were measured on two targets, carbon-12 and silicon-28, yielding Formula: see text, where stat. denotes the statistical uncertainty and syst. the systematic uncertainty. We combined the results of this and an earlier experiment to generate a weighted average of Formula: see text Our final result has a total uncertainty of 1.50% and confirms the prediction based on the chiral anomaly in quantum chromodynamics.
AI-supported algorithms, particularly generative models, have been successfully used in a variety of different contexts. This work employs a generative modeling approach to unfold detector effects ...specifically tailored for exclusive reactions that involve multiparticle final states. Our study demonstrates the preservation of correlations between kinematic variables in a multidimensional phase space. We perform a full closure test on two-pion photoproduction pseudodata generated with a realistic model in the kinematics of the Jefferson Lab CLAS g11 experiment. The overlap of different reaction mechanisms leading to the same final state associated with the CLAS detector’s nontrivial effects represents an ideal test case for AI-supported analysis. Uncertainty quantification performed via bootstrap provides an estimate of the systematic uncertainty associated with the procedure. The test demonstrates that GANs can reproduce highly correlated multidifferential cross sections even in the presence of detector-induced distortions in the training datasets, and provides a solid basis for applying the framework to real experimental data.
Background: The deuteron plays a pivotal role in nuclear and hadronic physics, as both the simplest bound multinucleon system and as an effective neutron target. Quasielastic electron scattering on ...the deuteron is a benchmark reaction to test our understanding of deuteron structure and the properties and interactions of the two nucleons bound in the deuteron. Purpose: The experimental data presented here can be used to test state-of-the-art models of the deuteron and the two-nucleon interaction in the final state after two-body breakup of the deuteron. Focusing on polarization degrees of freedom, we gain information on spin-momentum correlations in the deuteron ground state (due to the D-state admixture) and on the limits of the impulse approximation (IA) picture as it applies to measurements of spin-dependent observables like spin structure functions for bound nucleons. Information on this reaction can also be used to reduce systematic uncertainties on the determination of neutron form factors or deuteron polarization through quasielastic polarized electron scattering. Method: We measured the beam-target double-spin asymmetry (A(parallel to)) for quasielastic electron scattering off the deuteron at several beam energies (1.6-1.7, 2.5, 4.2, and 5.6-5.8 GeV), using the CEBAF Large Acceptance Spectrometer (CLAS) at the Thomas Jefferson National Accelerator Facility. The deuterons were polarized along (or opposite to) the beam direction. The double-spin asymmetries were measured as a function of photon virtuality Q(2) (0.13-3.17 (GeV/c)(2)), missing momentum (p(m) = 0.0-0.5 GeV/c), and the angle between the (inferred) spectator neutron and the momentum transfer direction (theta(nq)). Results: The results are compared with a recent model that includes final-state interactions (FSI) using a complete parametrization of nucleon-nucleon scattering, as well as a simplified model using the plane wave impulse approximation (PWIA). We find overall good agreement with both the PWIA and FSI expectations at low to medium missing momenta (p(m) <= 0.25 GeV/c), including the change of the asymmetry due to the contribution of the deuteron D state at higher momenta. At the highest missing momenta, our data clearly agree better with the calculations including FSI. Conclusions: Final-state interactions seem to play a lesser role for polarization observables in deuteron two-body electrodisintegration than for absolute cross sections. Our data, while limited in statistical power, indicate that PWIA models work reasonably well to understand the asymmetries at lower missing momenta. In turn, this information can be used to extract the product of beam and target polarization (PbPt) from quasielastic electron-deuteron scattering, which is useful for measurements of spin observables in electron-neutron inelastic scattering. However, at the highest missing (neutron) momenta, FSI effects become important and must be accounted for.
A procedure is described for finding nonpolarized transverse, longitudinal, and polarized longitudinal-transverse and longitudinal-transverse structure functions derived from data on the differential ...cross sections of exclusive channels of π
+
n
and π
0
p
electroproduction measured with the CLAS detector. The measured cross sections cover the wide kinematic range of W < 1.7 GeV in terms of invariant mass of a finite hadronic system and photon virtuality in the range of
Q
2
< 5.0 GeV
2
. The exclusive structure functions obtained from a single pion production channel play an important role in studying the structure of excited nucleon states.
The analysis of the nine 1-fold differential cross sections for the γr,vp→π+π−p photo- and electroproduction reactions obtained with the CLAS detector at Jefferson Laboratory was carried out with the ...goal to establish the contributing resonances in the mass range from 1.6 GeV to 1.8 GeV. In order to describe the photo- and electroproduction data with Q2-independent resonance masses and hadronic decay widths in the Q2 range below 1.5 GeV2, it was found that an N′(1720)3/2+ state is required in addition to the already well-established nucleon resonances. This work demonstrates that the combined studies of π+π−p photo- and electroproduction data are vital for the observation of this resonance. The contributions from the N′(1720)3/2+ state and the already established N(1720)3/2+ state with a mass of 1.745 GeV are well separated by their different hadronic decays to the πΔ and ρp final states and the different Q2-evolution of their photo-/electroexcitation amplitudes. The N′(1720)3/2+ state is the first recently established baryon resonance for which the results on the Q2-evolution of the photo-/electrocouplings have become available. These results are important for the exploration of the nature of the “missing” baryon resonances.
The integrated cross sections of the
ep
→
e
π
+
n
,
ep
→
e
π
0
p
,
ep
→
eK
+
Λ, and
ep
→
eK
+
Σ
0
reactions are evaluated in the energy range of nucleon resonance excitation at photon virtualities
Q
...2
from 5 to 12 GeV
2
. These exclusive channels will be explored at photon virtualities
Q
2
> 5 GeV
2
for the first time in future experiments with the CLAS12 detector. The cross-section evaluation is based on the extrapolation of exclusive contributions to the inclusive structure functions
F
1
and
F
2
from a region of
Q
2
< 5 GeV
2
, in which the experimental data are available, to the region of higher
Q
2
. This evaluation of cross sections is of particular importance in the development of the program of experiments with the CLAS12 detector for studying the structure of the ground and excited nucleon states, which may reveal the dynamics of strong interactions in the nonperturbative regime.
The electroexcitation amplitudes or $\gamma_vpN^*$ electrocouplings of the $N(1440)1/2^+$, $N(1520)3/2^-$, and $\Delta(1600)3/2^+$ resonances were obtained for the first time from the $ep \to ...e'\pi^+\pi^-p'$ differential cross sections measured with the CLAS detector at Jefferson Lab within the range of invariant mass $W$ of the final state hadrons from 1.4–1.7~GeV for photon virtualities $Q^2$ from 2.0--5.0~GeV$^2$. A good description of the nine independent one-fold differential $\gamma_v p\to \pi^+\pi^-p'$ cross sections achieved within the data-driven Jefferson Lab-Moscow State University (JM) meson-baryon reaction model in each bin of ($W$,$Q^2$) allows for separation of the resonant and non-resonant contributions. The electrocouplings were determined in the fits of the $\pi^+\pi^-p$ cross sections within three overlapping $W$ intervals with a substantial contribution from each of the three resonances listed above. Consistent results on the electrocouplings extracted from the data in these $W$ intervals provide evidence for their reliable extraction. These studies extend information on the electrocouplings of the $N(1440)1/2^+$ and $N(1520)3/2^-$ available from this channel over a broader range of $Q^2$. The electrocouplings of the $\Delta(1600)3/2^+$, which decays preferentially into $\pi\pi N$ final states, have been determined for the first time. Here, the reliable extraction of the electrocouplings for these states is also supported by the description of the $\pi^+\pi^-p$ differential cross sections with $Q^2$-independent masses and total/partial hadronic decay widths into the $\pi\Delta$ and $\rho p$ final states.
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.
Background: The electromagnetic form factors of the proton measured by unpolarized and polarized electron scattering experiments show a significant disagreement that grows with the squared four ...momentum transfer (Q2). Calculations have shown that the two measurements can be largely reconciled by accounting for the contributions of two-photon exchange (TPE). TPE effects are not typically included in the standard set of radiative corrections since theoretical calculations of the TPE effects are highly model dependent, and, until recently, no direct evidence of significant TPE effects has been observed. Purpose: We measured the ratio of positron-proton to electron-proton elastic-scattering cross sections in order to determine the TPE contribution to elastic electron-proton scattering and thereby resolve the proton electric form factor discrepancy. Methods: We produced a mixed simultaneous electron-positron beam in Jefferson Lab's Hall B by passing the 5.6 GeV primary electron beam through a radiator to produce a bremsstrahlung photon beam and then passing the photon beam through a convertor to produce electron/positron pairs. The mixed electron-positron (lepton) beam with useful energies from approximately 0.85 to 3.5 GeV then struck a 30-cm long liquid hydrogen (LH2) target located within the CEBAF Large Acceptance Spectrometer (CLAS). By detecting both the scattered leptons and the recoiling protons we identified and reconstructed elastic scattering events and determined the incident lepton energy. A detailed description of the experiment is presented. Results: We present previously unpublished results for the quantity R2γ, the TPE correction to the elastic- scattering cross section, at Q2 ≈ 0:85 and 1.45 GeV2 over a large range of virtual photon polarization ε. Conclusions: Our results, along with recently published results from VEPP-3, demonstrate a non-zero contribution from TPE effects and are in excellent agreement with the calculations that include TPE effects and largely reconcile the form-factor discrepancy up to Q2 ≈ 2 GeV2. These data are consistent with an increase in R2γ with decreasing " at Q2 ≈ 0:85 and 1.45 GeV2. There are indications of a slight increase in R2γ with Q2.
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