175 research outputs found
A glimpse of gluons through deeply virtual compton scattering on the proton
No full textThe internal structure of nucleons (protons and neutrons) remains one of the greatest outstanding problems in modern nuclear physics. By scattering high-energy electrons off a proton we are able to resolve its fundamental constituents and probe their momenta and positions. Here we investigate the dynamics of quarks and gluons inside nucleons using deeply virtual Compton scattering (DVCS)-a highly virtual photon scatters off the proton, which subsequently radiates a photon. DVCS interferes with the Bethe-Heitler (BH) process, where the photon is emitted by the electron rather than the proton. We report herein the full determination of the BH-DVCS interference by exploiting the distinct energy dependences of the DVCS and BH amplitudes. In the regime where the scattering is expected to occur off a single quark, measurements show an intriguing sensitivity to gluons, the carriers of the strong interaction.Physic
E00-110 experiment at Jefferson Lab Hall A: Deeply virtual Compton scattering off the proton at 6 GeV
No full textWe present final results on the photon electroproduction ((e) over right arrowp - > ep gamma) cross section in the deeply virtual Compton scattering (DVCS) regime and the valence quark region from Jefferson Lab experiment E00-110. Results from an analysis of a subset of these data were published before, but the analysis has been improved, which is described here at length, together with details on the experimental setup. Furthermore, additional data have been analyzed, resulting in photon electroproduction cross sections at new kinematic settings for a total of 588 experimental bins. Results of the Q(2) and x(B) dependencies of both the helicity-dependent and the helicity-independent cross sections are discussed. The Q(2) dependence illustrates the dominance of the twist-2 handbag amplitude in the kinematics of the experiment, as previously noted. Thanks to the excellent accuracy of this high-luminosity experiment, it becomes clear that the unpolarized cross section shows a significant deviation from the Bethe-Heitler process in our kinematics, compatible with a large contribution from the leading twist-2Physic
Rosenbluth Separation of the πᵒ Electroproduction Cross Section
No full textWe present deeply virtual πᵒ electroproduction cross-section measurements at xB = 0.36 and three different Q2 values ranging from 1.5 to 2 GeV2, obtained from Jefferson Lab Hall A experiment E07-007. The Rosenbluth technique is used to separate the longitudinal and transverse responses. Results demonstrate that the cross section is dominated by its transverse component and, thus, is far from the asymptotic limit predicted by perturbative quantum chromodynamics. Nonetheless, an indication of a nonzero longitudinal contribution is provided by the measured interference term σLT. Results are compared with several models based on the leading-twist approach of generalized parton distributions (GPDs). In particular, a fair agreement is obtained with models in which the scattering amplitude includes convolution terms of chiral-odd (transversity) GPDs of the nucleon with the twist-3 pion distribution amplitude. This experiment, together with previous extensive unseparated measurements, provides strong support to the exciting idea that transversity GPDs can be accessed via neutral pion electroproduction in the high-Q2 regime
The Recoil Proton Polarization: a new discriminative DVCS observable
No full textGeneralized parton distributions describe the correlations between the longitudinal momentum and the transverse position of quarks and gluons in a nucleon. They can be constrained by measuring photon leptoproduction observables, arising from the interference between Bethe-Heitler and Deeply virtual Compton scattering processes. At leading-twist/leading-order, the amplitude of the latter is parameterized by complex integrals of the GPDs {H, E, H̃, Ẽ} . As data collected on an unpolarized or longitudinally polarized target constrains H and H̃, E is poorly known as it requires data collected with a transversely polarized target, which is very challenging to implement in fixed target experiments. The only alternative considered so far has been DVCS on a neutron with a deuterium target, while assuming isospin symmetry and absence of final-state interactions. Today, we introduce the polarization of the recoil proton as a new DVCS observable, highly sensitive to E, which appears feasible for an experimental study at a high-luminosity facility such as Jefferson Lab
Exclusive electroproduction off protons in the resonance region at photon virtualities 0.4 GeV GeV
No full textInternational audienceThe exclusive electroproduction process ep→e′p′π0 was measured in the range of photon virtualities Q2=0.4–1.0GeV2 and the invariant mass range of the pπ0 system of W=1.1–1.8 GeV. These kinematics are covered in exclusive π0 electroproduction off the proton with nearly complete angular coverage in the pπ0 center-of-mass system and with high statistical accuracy. Nearly 36 000 cross-section points were measured, and the structure functions σT+εσL,σLT, and σTT, were extracted via fitting the ϕπ0 dependence of the cross section. A Legendre polynomial expansion analysis demonstrates the sensitivity of our data to high-lying N* and Δ* resonances with M>1.6 GeV. As part of a broad effort to determine the electrocouplings of the N* and Δ* resonances using both single- and double-pion electroproduction, this dataset is crucial for the reliable extraction of the high-lying resonance electrocouplings from the combined isospin analysis of the Nπ and π+π−p channels
Generalized Parton Distributions of Pions at the Forthcoming Electron-Ion Collider
No full textInternational audienceWe analyze deeply virtual Compton scattering on pions projected for a future electron-ion collider and conveyed in the Sullivan process. The relevant amplitude is known to be parametrized by generalized parton distributions. Hence taking advantage of state-of-the-art models for them, supplemented with effective leading-order scale evolution, we evaluate the amplitude for the process to occur and examine the pion’s structure from the perspective of electron-ion colliders. We estimate the expected event-rates for the Sullivan process showing: first, that deeply virtual Compton scattering on pions may be measurable at forthcoming experimental facilities. Second, that gluons may be decisive in the description of pions, driving the behavior of the relevant amplitudes and modulating the expected event-rates
Differential cross section measurements for γn → π−p above the first nucleon resonance region
No full textThe quasifree
γ
d
→
π
−
p
(
p
)
differential cross section has been measured with CLAS at photon beam energies
E
γ
from 0.445 to 2.510 GeV (corresponding to
W
from 1.311 to 2.366 GeV) for pion center-of-mass angles
cos
θ
c
.
m
.
π
from
−
0.72
to 0.92. A correction for final state interactions has been applied to these data to extract the
γ
n
→
π
−
p
differential cross sections. These cross sections are quoted in 8428
(
E
γ
,
cos
θ
c
.
m
.
π
)
bins, a factor of nearly 3 increase in the world statistics for this channel in this kinematic range. These new data help to constrain coupled-channel analysis fits used to disentangle the spectrum of
N
∗
resonances and extract their properties. Selected photon decay amplitudes
N
∗
→
γ
n
at the resonance poles are determined for the first time and are reported here
Exclusive Electroproduction off the Neutron in Deuterium in the Resonance Region
No full textNew results for the exclusive and quasi-free cross sections off neutrons
bound in deuterium are presented
over a wide final state hadron angle range with a kinematic coverage of the
invariant mass () up to 1.825 GeV and the virtual photon four-momentum
transfer squared () from 0.4 to 1.0 GeV. The exclusive structure
functions were extracted and their Legendre moments were obtained.
Final-state-interaction contributions have been kinematically separated from
the extracted quasi-free cross sections off bound neutrons solely based on the
analysis of the experimental data. These new results will serve as long-awaited
input for phenomenological analyses to extract the evolution of
previously unavailable electroexcitation amplitudes and to
improve state-of-the-art models of neutrino scattering off nuclei by augmenting
the already available results from free protons.Comment: The author list has been update
The 3D structure of pions at future electron-ion colliders
No full textDIS2022 conference presentatio
The CLAS12 Micromegas Vertex Tracker
No full textInternational audienceThe Micromegas Vertex Tracker was designed to improve upon the tracking capabilities of the baseline design of the CLAS12 spectrometer in Hall B at Jefferson Laboratory. A Barrel Micromegas Tracker made with six concentric cylinders, each made of three 120 ∘ -sector tiles, surrounds the Silicon Vertex Tracker, and a Forward Micromegas Tracker composed of 6 disks is placed 30 cm downstream of the liquid-hydrogen target. Both trackers sit in a 5 T solenoid magnetic field. All Micromegas elements are based on resistive technology to withstand luminosities up to 1035 cm −2 s −1 , as well as on bulk technology to enforce gain uniformity and mechanical robustness. Due to the high magnetic field, dedicated electronics have been designed and displaced ∼ 2 m away from the detectors. The electronics readout is based on the DREAM ASICs that allow sustained operation up to 20 kHz trigger rate at the maximum luminosity
- …
