Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userMeasurement of the Bjorken Sum at very low $Q^2$
249
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
We present new data on the Bjorken sum $overline Gamma_1^{p-n}(Q^2)$ at 4-momentum transfer $ 0.021 leq Q^2 leq 0.496$ GeV$^2$. The data were obtained in two experiments performed at Jefferson Lab: EG4 on polarized protons and deuterons, and E97110 on polarized $^3$He from which neutron data were extracted. The data cover the domain where chiral effective field theory ($chi$EFT), the leading effective theory of the Strong Force at large distances, is expected to be applicable. We find that our data and the predictions from $chi$EFT are only in marginal agreement. This is somewhat surprising as the contribution from the $Delta(1232)$ resonance is suppressed in this observable, which should make it more reliably predicted by $chi$EFT than quantities in which the $Delta$ contribution is important. The data are also compared to a number of phenomenological models with various degrees of agreement.
rate research
Read More
We report on a new experimental method based on initial-state radiation (ISR) in e-p scattering, in which the radiative tail of the elastic e-p peak contains information on the proton charge form factor ($G_E^p$) at extremely small $Q^2$. The ISR technique was validated in a dedicated experiment using the spectrometers of the A1-Collaboration at the Mainz Microtron (MAMI). This provided first measurements of $G_E^p$ for $0.001leq Q^2leq 0.004 (GeV/c)^2$.
We extract the Bjorken integral Gamma^{p-n}_1 in the range 0.17 < Q^2 < 1.10 GeV^2 from inclusive scattering of polarized electrons by polarized protons, deuterons and 3He, for the region in which the integral is dominated by nucleon resonances. These data bridge the domains of the hadronic and partonic descriptions of the nucleon. In combination with earlier measurements at higher Q^2, we extract the non-singlet twist-4 matrix element f_2.
We measured the $g_1$ spin structure function of the deuteron at low $Q^{2}$, where QCD can be approximated with chiral perturbation theory ($chi$PT). The data cover the resonance region, up to an invariant mass of $Wapprox1.9$~GeV. The generalized Gerasimov-Drell-Hearn sum, the moment $bar{Gamma}_{1}^{d}$ and the integral $bar{I}_gamma^d$ related to the spin polarizability $gamma_{0}^{d}$ are precisely determined down to a minimum $Q^2$ of 0.02~GeV$^2$ for the first time, about 2.5 times lower than that of previous data. We compare them to several $chi$PT calculations and models. These results are the first in a program of benchmark measurements of polarization observables in the $chi$PT domain.
The spin-structure functions $g_1$ and $g_2$, and the spin-dependent partial cross-section $sigma_mathrm{TT}$ have been extracted from the polarized cross-sections differences, $Delta sigma_{parallel}hspace{-0.06cm}left( u,Q^{2}right)$ and $Delta sigma_{perp}hspace{-0.06cm}left( u,Q^{2}right)$ measured for the $vec{^textrm{3}textrm{He}}(vec{textrm{e}},textrm{e})textrm{X}$ reaction, in the E97-110 experiment at Jefferson Lab. Polarized electrons with energies from 1.147 to 4.404 GeV were scattered at angles of 6$^{circ}$ and 9$^{circ}$ from a longitudinally or transversely polarized $^{3}$He target. The data cover the kinematic regions of the quasi-elastic, resonance production and beyond. From the extracted spin-structure functions, the first moments $overline{Gamma_1}hspace{-0.06cm}left(Q^{2}right)$, $Gamma_2hspace{-0.06cm}left(Q^{2}right)$ and $I_{mathrm{TT}}hspace{-0.06cm}left(Q^{2}right)$ are evaluated with high precision for the neutron in the $Q^2$ range from 0.035 to 0.24~GeV$^{2}$. The comparison of the data and the chiral effective field theory predictions reveals the importance of proper treatment of the $Delta$ degree of freedom for spin observables.
We report a new, high-precision measurement of the proton elastic form factor ratio mu_p G_E/G_M for the four-momentum transfer squared Q^2 = 0.3-0.7 (GeV/c)^2. The measurement was performed at Jefferson Lab (JLab) in Hall A using recoil polarimetry. With a total uncertainty of approximately 1%, the new data clearly show that the deviation of the ratio mu_p G_E/G_M from unity observed in previous polarization measurements at high Q^2 continues down to the lowest Q^2 value of this measurement. The updated global fit that includes the new results yields an electric (magnetic) form factor roughly 2% smaller (1% larger) than the previous global fit in this Q^2 range. We obtain new extractions of the proton electric and magnetic radii, which are <r^2_E>^(1/2)=0.875+/-0.010 fm and <r^2_M>^(1/2)=0.867+/-0.020 fm. The charge radius is consistent with other recent extractions based on the electron-proton interaction, including the atomic hydrogen Lamb shift measurements, which suggests a missing correction in the comparison of measurements of the proton charge radius using electron probes and the recent extraction from the muonic hydrogen Lamb shift.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
