50 research outputs found
Development of a surgical stereo endoscopic image dataset for validating 3D stereo reconstruction algorithms
In the last decades, endoscopic stereo images have been exploited to retrieve tissue surface information of the surgical site using 3D reconstruction algorithms. The application of such algorithms in Computer Assisted Surgery (CAS) tools for Minimally Invasive Surgery (MIS) requires a robust validation process in order to guarantee reliability and safety. 3D reconstruction algorithms are commonly evaluated comparing their result with respect to a reference Ground Truth (GT). However, few datasets providing endoscopic images and GT are openly available.
Considering the increasing necessity of surgical datasets, the aim of this work is the generation of an Endoscopic Abdominal Stereo (EndoAbS) dataset composed of stereo-images with associated GT for 3D stereo-reconstruction algorithm validation. To recreate the surgical scenario, a polyurethane surgical phantom abdomen was built. Images were captured with a stereo-endoscope, while for acquiring the GT a laser scanner (calibrated with respect to the stereoendoscope) was used. This dataset is openly available on-line for the benefit of the CAS community
Modified structure of protons and neutrons in correlated pairs
The atomic nucleus is made of protons and neutrons (nucleons), which are themselves composed of quarks and gluons. Understanding how the quark–gluon structure of a nucleon bound in an atomic nucleus is modified by the surrounding nucleons is an outstanding challenge. Although evidence for such modification—known as the EMC effect—was first observed over 35 years ago, there is still no generally accepted explanation for its cause1,2,3. Recent observations suggest that the EMC effect is related to close-proximity short-range correlated (SRC) nucleon pairs in nuclei4,5. Here we report simultaneous, high-precision measurements of the EMC effect and SRC abundances. We show that EMC data can be explained by a universal modification of the structure of nucleons in neutron–proton SRC pairs and present a data-driven extraction of the corresponding universal modification function. This implies that in heavier nuclei with many more neutrons than protons, each proton is more likely than each neutron to belong to an SRC pair and hence to have distorted quark structure. This universal modification function will be useful for determining the structure of the free neutron and thereby testing quantum chromodynamics symmetry-breaking mechanisms and may help to discriminate between nuclear physics effects and beyond-the-standard-model effects in neutrino experiments
Measurement of nuclear transparency ratios for protons and neutrons
This paper presents, for the first time, measurements of neutron transparency ratios for nuclei relative to C measured using the (e,e′n) reaction, spanning measured neutron momenta of 1.4 to 2.4 GeV/c. The transparency ratios were extracted in two kinematical regions, corresponding to knockout of mean-field nucleons and to the breakup of Short-Range Correlated nucleon pairs. The extracted neutron transparency ratios are consistent with each other for the two measured kinematical regions and agree with the proton transparencies extracted from new and previous (e,e′p) measurements, including those from neutron-rich nuclei such as lead. The data also agree with and confirm the Glauber approximation that is commonly used to interpret experimental data. The nuclear-mass-dependence of the extracted transparencies scales as Aα with α=−0.289±0.007, which is consistent with nuclear-surface dominance of the reactions
First measurement of Xi(-) polarization in photoproduction
Despite decades of studies of the photoproduction of hyperons, both their production mechanisms and their spectra of excited states are still largely unknown. While the parity-violating weak decay of hyperons offers a means of measuring their polarization, which could help discern their production mechanisms and identify their excitation spectra, no such study has been possible for doubly strange baryons in photoproduction, due to low production cross sections. However, by making use of the reaction γp→K+K+Ξ−, we have measured, for the first time, the induced polarization, P, and the transferred polarization from circularly polarized real photons, characterized by Cx and Cz, to recoiling Ξ−s. The data were obtained using the CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Lab for photon energies from just over threshold (2.4 GeV) to 5.45 GeV. These first-time measurements are compared, and are shown to broadly agree, with model predictions in which cascade photoproduction proceeds through the decay of intermediate hyperon resonances that are produced via relativistic meson exchange, offering a new step forward in the understanding of the production and polarization of doubly-strange baryons
Differential cross section for γd → ωd using CLAS at Jefferson Lab
The cross section for coherent -meson photoproduction off the
deuteron has been measured for the first time as a function of the momentum
transfer and photon energy using
the CLAS detector at the Thomas Jefferson National Accelerator Facility. The
cross sections are measured in the energy range GeV. A
model based on rescattering is consistent with the data at low and
intermediate momentum transfer, . For GeV, the
total cross-section of scattering, based on fits within the
framework of the Vector Meson Dominance model, is in the range of 30-40 mb.Comment: 8 pages, 5 figure
Measurements of the γvp→p′π+π- cross section with the CLAS detector for 0.4 GeV2<Q2<1.0 GeV2 and 1.3 GeV<W<1.825 GeV
New results on the single-differential and fully integrated cross sections for the process γvp→p′π+π- are presented. The experimental data were collected with the CLAS detector at Jefferson Laboratory. Measurements were carried out in the kinematic region of the reaction invariant mass W from 1.3 to 1.825 GeV and the photon virtuality Q2 from 0.4 to 1.0 GeV2. The cross sections were obtained in narrow Q2 bins (0.05 GeV2) with the smallest statistical uncertainties achieved in double-pion electroproduction experiments to date. The results were found to be in agreement with previously available data where they overlap. A preliminary interpretation of the extracted cross sections, which was based on a phenomenological meson-baryon reaction model, revealed substantial relative contributions from nucleon resonances. The data offer promising prospects to improve knowledge on the Q2 evolution of the electrocouplings of most resonances with masses up to ∼1.8 GeV
Hard exclusive pion electroproduction at backward angles with CLAS
We report on the first measurement of cross sections for exclusive deeply
virtual pion electroproduction off the proton, ,
above the resonance region at backward pion center-of-mass angles. The
-dependent cross sections were measured, from which we
extracted three combinations of structure functions of the proton. Our results
are compatible with calculations based on nucleon-to-pion transition
distribution amplitudes (TDAs) and shed new light on nucleon structure.Comment: 7 pages, 6figure
Beam-target helicity asymmetry e in K0 Λ and K0 Σ0 photoproduction on the neutron
We report the first measurements of the E beam-target helicity asymmetry for the γ - n - →K0Λ and K0Σ0 channels in the energy range 1.70≤W≤2.34 GeV. The CLAS system at Jefferson Lab uses a circularly polarized photon beam and a target consisting of longitudinally polarized solid molecular hydrogen deuteride with low background contamination for the measurements. The multivariate analysis method boosted decision trees is used to isolate the reactions of interest. Comparisons with predictions from the KaonMAID, SAID, and Bonn-Gatchina models are presented. These results will help separate the isospin I=0 and I=1 photocoupling transition amplitudes in pseudoscalar meson photoproduction
Photon beam asymmetry Sigma for eta and eta ' photoproduction from the proton
Measurements of the linearly-polarized photon beam asymmetry for
photoproduction from the proton of and mesons are
reported. A linearly-polarized tagged photon beam produced by coherent
bremsstrahlung was incident on a cryogenic hydrogen target within the CEBAF
Large Acceptance Spectrometer. Results are presented for the reaction for incident photon energies from 1.070 to 1.876 GeV, and from
1.516 to 1.836 GeV for the reaction. For , the data reported here considerably extend the range of
measurements to higher energies, and are consistent with the few previously
published measurements for this observable near threshold. For , the results obtained are consistent with the few previously
published measurements for this observable near threshold, but also greatly
expand the incident photon energy coverage for that reaction. Initial analysis
of the data reported here with the Bonn-Gatchina model strengthens the evidence
for four nucleon resonances -- the , ,
and resonances -- which presently lack the
"four-star" status in the current Particle Data Group compilation, providing
examples of how these new measurements help refine models of the
photoproduction process.Comment: 10 pages, 3 figure
Semi-inclusive pi(0) target and beam-target asymmetries from 6 GeV electron scattering with CLAS
We present precision measurements of the target and beam-target spin
asymmetries from neutral pion electroproduction in deep-inelastic scattering
(DIS) using the CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Lab. We
scattered 6-GeV, longitudinally polarized electrons off longitudinally
polarized protons in a cryogenic NH target, and extracted double and
single target spin asymmetries for in
multidimensional bins in four-momentum transfer ( GeV),
Bjorken- (), hadron energy fraction (), transverse
pion momentum ( GeV), and azimuthal angle between the
lepton scattering and hadron production planes. We extracted asymmetries as a
function of both and , which provide access to transverse-momentum
distributions of longitudinally polarized quarks. The double spin asymmetries
depend weakly on . The moments are zero within
uncertainties, which is consistent with the expected suppression of the Collins
fragmentation function. The observed moments suggest that quark
gluon correlations are significant at large .Comment: 18 preprint pages, 3 figure