We present deeply virtual pi super(0) electroproduction cross-section measurements at x sub(B)0.36 and three different Q super(2) values ranging from 1.5 to 2GeV super(2), 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 sigma sub(L)T 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-Q super(2) regime.
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Here, experimental cross sections for the $^4He(e,e'p)X$ reactions in the missing energy range from 0.017 to 0.022 GeV and up to a missing momentum of 0.632 GeV/$c$ at $x_B=1.24$ and ...$Q^2$=2(GeV/$c$)$^2$ are reported. The data are compared to relativistic distorted-wave impulse approximation calculations for the $^4He(e,e'p)^3H$ channel. Significantly more events are observed for $p_{m}$$≥$0.45 GeV/$c$ than are predicted by the theoretical model, and striking fluctuations in the ratio of data to the theoretical model around $p_{m}$$= $0.3 GeV/$c$ are possible signals of initial-state multinucleon correlations.
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We report the first longitudinal-transverse separation of the deeply virtual exclusive π0 electroproduction cross section off the neutron and coherent deuteron. The corresponding four structure ...functions dσL/dt, dσT/dt, dσLT/dt, and dσTT/dt are extracted as a function of the momentum transfer to the recoil system at Q2=1.75 GeV2 and xB=0.36. The ed→edπ0 cross sections are found compatible with the small values expected from theoretical models. The en→enπ0 cross sections show a dominance from the response to transversely polarized photons, and are in good agreement with calculations based on the transversity generalized parton distributions of the nucleon. By combining these results with previous measurements of π0 electroproduction off the proton, we present a flavor decomposition of the u and d quark contributions to the cross section
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We report measurements of the exclusive neutral pion electroproduction cross section off protons at large values of xB (0.36, 0.48, and 0.60) and Q2 (3.1 to 8.4 GeV2) obtained from Jefferson Lab ...Hall A experiment E12-06-014. The corresponding structure functions dσT/ d t + ε dσL/ d t , dσTT / d t , d σ L T / d t , and d σ L T ′ / d t are extracted as a function of the proton momentum transfer t−tmin. The results suggest the amplitude for transversely polarized virtual photons continues to dominate the cross section throughout this kinematic range. The data are well described by calculations based on transversity generalized parton distributions coupled to a helicity flip distribution amplitude of the pion, thus providing a unique way to probe the structure of the nucleon.
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We report on new p(e,e′p)π∘ measurements at the Δ+(1232) resonance at the low momentum transfer region, where the mesonic cloud dynamics is predicted to be dominant and rapidly changing, offering a ...test bed for chiral effective field theory calculations. The new data explore the Q2 dependence of the resonant quadrupole amplitudes and for the first time indicate that the Electric and the Coulomb quadrupole amplitudes converge as Q2→0. The measurements of the Coulomb quadrupole amplitude have been extended to the lowest momentum transfer ever reached, and suggest that more than half of its magnitude is attributed to the mesonic cloud in this region. The new data disagree with predictions of constituent quark models and are in reasonable agreement with dynamical calculations that include pion cloud effects, chiral effective field theory and lattice calculations. The measurements indicate that improvement is required to the theoretical calculations and provide valuable input that will allow their refinements.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
We report a precision measurement of the parity-violating asymmetry A PV in the elastic scattering of longitudinally polarized electrons from 208 Pb . We measure APV=550±16(stat)±8(syst) parts per ...billion, leading to an extraction of the neutral weak form factor FW(Q2=0.00616 GeV2)=0.368±0.013 . Combined with our previous measurement, the extracted neutron skin thickness is Rn−Rp=0.283±0.071 fm. The result also yields the first significant direct measurement of the interior weak density of 208 Pb: ρW0=− 0.0796 ± 0.0036 ( exp ) ± 0.0013 ( theo ) fm−3 leading to the interior baryon density ρb0 = 0.1480 ± 0.0036 ( exp ) ± 0.0013 ( theo ) fm −3. The measurement accurately constrains the density dependence of the symmetry energy of nuclear matter near saturation density, with implications for the size and composition of neutron stars.
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We have measured the beam-normal single-spin asymmetry $A_n$ in the elastic scattering of 1-3 GeV transversely polarized electrons from $^1$H and for the first time from $^4$He, $^{12}$C, and ...$^{208}$Pb. For $^1$H, $^4$He and $^{12}$C, the measurements are in agreement with calculations that relate $A_n$ to the imaginary part of the two-photon exchange amplitude including inelastic intermediate states. Surprisingly, the $^{208}$Pb result is significantly smaller than the corresponding prediction using the same formalism. These results suggest that a systematic set of new $A_n$ measurements might emerge as a new and sensitive probe of the structure of heavy nuclei.
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Here, we report the first measurement of the (e, e' p) three-body breakup reaction cross sections in helium-3 (3He) and tritium (3H) at large momentum transfer ($\langle{Q^2}\rangle$ ≈ 1.9 (GeV/c)2) ...and xB > 1 kinematics, where the cross section should be sensitive to quasielastic (QE) scattering from single nucleons. The data cover missing momenta 40 ≤ pmiss ≤ 500 MeV/c that, in the QE limit with no rescattering, equals the initial momentum of the probed nucleon. The measured cross sections are compared with state-of-the-art ab-initio calculations. Overall good agreement, within ±20%, is observed between data and calculations for the full pmiss range for 3H and for 100 ≤ pmiss ≤ 350 MeV/c for 3He. Including the effects of rescattering of the outgoing nucleon improves agreement with the data at pmiss > 250 MeV/c and suggests contributions from charge-exchange (SCX) rescattering. The isoscalar sum of 3He plus 3H, which is largely insensitive to SCX, is described by calculations to within the accuracy of the data over the entire pmiss range. This validates current models of the ground state of the three-nucleon system up to very high initial nucleon momenta of 500 MeV/c.
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We report the first measurement of the
(
e
,
e
′
p
)
three-body breakup reaction cross sections in helium-3 (
3
He
) and tritium (
3
H
) at large momentum transfer
⟨
Q
2
⟩
≈
1.9
(
GeV
/
c
)
2
...and
x
B
>
1
kinematics, where the cross section should be sensitive to quasielastic (QE) scattering from single nucleons. The data cover missing momenta
40
≤
p
miss
≤
500
MeV
/
c
that, in the QE limit with no rescattering, equals the initial momentum of the probed nucleon. The measured cross sections are compared with state-of-the-art ab initio calculations. Overall good agreement, within
±
20
%
, is observed between data and calculations for the full
p
miss
range for
3
H
and for
100
≤
p
miss
≤
350
MeV
/
c
for
3
He
. Including the effects of rescattering of the outgoing nucleon improves agreement with the data at
p
miss
>
250
MeV
/
c
and suggests contributions from charge-exchange (SCX) rescattering. The isoscalar sum of
3
He
plus
3
H
, which is largely insensitive to SCX, is described by calculations to within the accuracy of the data over the entire
p
miss
range. This validates current models of the ground state of the three-nucleon system up to very high initial nucleon momenta of
500
MeV
/
c
.
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