The study of exclusive \(\pi^{\pm}\) electroproduction on the nucleon, including separation of the various structure functions, is of interest for a number of reasons. The ratio ...\(R_L=\sigma_L^{\pi^-}/\sigma_L^{\pi^+}\) is sensitive to isoscalar contamination to the dominant isovector pion exchange amplitude, which is the basis for the determination of the charged pion form factor from electroproduction data. A change in the value of \(R_T=\sigma_T^{\pi^-}/\sigma_T^{\pi^+}\) from unity at small \(-t\), to 1/4 at large \(-t\), would suggest a transition from coupling to a (virtual) pion to coupling to individual quarks. Furthermore, the mentioned ratios may show an earlier approach to pQCD than the individual cross sections. We have performed the first complete separation of the four unpolarized electromagnetic structure functions above the dominant resonances in forward, exclusive \(\pi^{\pm}\) electroproduction on the deuteron at central \(Q^2\) values of 0.6, 1.0, 1.6 GeV\(^2\) at \(W\)=1.95 GeV, and \(Q^2=2.45\) GeV\(^2\) at \(W\)=2.22 GeV. Here, we present the \(L\) and \(T\) cross sections, with emphasis on \(R_L\) and \(R_T\), and compare them with theoretical calculations. Results for the separated ratio \(R_L\) indicate dominance of the pion-pole diagram at low \(-t\), while results for \(R_T\) are consistent with a transition between pion knockout and quark knockout mechanisms.
We present experimental results of reflection and transmission coefficients for single- and double-layered liquid-containing flexible shields based on textile materials with gel-powder fillers.
Background: Measurements of forward exclusive meson production at different squared four-momenta of the exchanged virtual photon, \(Q^2\), and at different four-momentum transfer, t, can be used to ...probe QCD's transition from meson-nucleon degrees of freedom at long distances to quark-gluon degrees of freedom at short scales. Ratios of separated response functions in \(\pi^-\) and \(\pi^+\) electroproduction are particularly informative. The ratio for transverse photons may allow this transition to be more easily observed, while the ratio for longitudinal photons provides a crucial verification of the assumed pole dominance, needed for reliable extraction of the pion form factor from electroproduction data. Method: Data were acquired with 2.6-5.2 GeV electron beams and the HMS+SOS spectrometers in Jefferson Lab Hall C, at central \(Q^2\) values of 0.6, 1.0, 1.6 GeV\(^2\) at W=1.95 GeV, and \(Q^2\)=2.45 GeV\(^2\) at W=2.22 GeV. There was significant coverage in \(\phi\) and \(\epsilon\), which allowed separation of \(\sigma_{L,T,LT,TT}\). Results: \(\sigma_L\) shows a clear signature of the pion pole, with a sharp rise at small -t. In contrast, \(\sigma_T\) is much flatter versus t. The longitudinal/transverse ratios evolve with \(Q^2\) and t, and at the highest \(Q^2\)=2.45 GeV\(^2\) show a slight enhancement for \(\pi^-\) production compared to \(\pi^+\). The \(\pi^-/\pi^+\) ratio for transverse photons exhibits only a small \(Q^2\)-dependence, following a nearly universal curve with t, with a steep transition to a value of about 0.25, consistent with s-channel quark knockout. The \(\sigma_{TT}/\sigma_T\) ratio also drops rapidly with \(Q^2\), qualitatively consistent with s-channel helicity conservation. The \(\pi^-/\pi^+\) ratio for longitudinal photons indicates a small isoscalar contamination at W=1.95 GeV, consistent with what was observed in our earlier determination of the pion form factor at these kinematics.
The charged pion form factor, Fpi(Q^2), is an important quantity which can be used to advance our knowledge of hadronic structure. However, the extraction of Fpi from data requires a model of the ...1H(e,e'pi+)n reaction, and thus is inherently model dependent. Therefore, a detailed description of the extraction of the charged pion form factor from electroproduction data obtained recently at Jefferson Lab is presented, with particular focus given to the dominant uncertainties in this procedure. Results for Fpi are presented for Q^2=0.60-2.45 GeV^2. Above Q^2=1.5 GeV^2, the Fpi values are systematically below the monopole parameterization that describes the low Q^2 data used to determine the pion charge radius. The pion form factor can be calculated in a wide variety of theoretical approaches, and the experimental results are compared to a number of calculations. This comparison is helpful in understanding the role of soft versus hard contributions to hadronic structure in the intermediate Q^2 regime.
Phys.Rev.C78:045202,2008 Cross sections for the reaction ${^1}$H($e,e'\pi^+$)$n$ were measured in Hall
C at Thomas Jefferson National Accelerator Facility (JLab) using the CEBAF
high-intensity, ...continous electron beam in order to determine the charged pion
form factor. Data were taken for central four-momentum transfers ranging from
$Q^2$=0.60 to 2.45 GeV$^2$ at an invariant mass of the virtual photon-nucleon
system of $W$=1.95 and 2.22 GeV. The measured cross sections were separated
into the four structure functions $\sigma_L$, $\sigma_T$, $\sigma_{LT}$, and
$\sigma_{TT}$. The various parts of the experimental setup and the analysis
steps are described in detail, including the calibrations and systematic
studies, which were needed to obtain high precision results. The different
types of systematic uncertainties are also discussed. The results for the
separated cross sections as a function of the Mandelstam variable $t$ at the
different values of $Q^2$ are presented. Some global features of the data are
discussed, and the data are compared with the results of some model
calculations for the reaction ${^1}$H($e,e'\pi^+$)$n$.
Phys.Rev.Lett.97:192001,2006 The H(e,e'pi+)n cross section was measured at four-momentum transfers of
Q2=1.60 and 2.45 GeV2 at an invariant mass of the photon nucleon system of
W=2.22 GeV. The ...charged pion form factor (F_pi) was extracted from the data by
comparing the separated longitudinal pion electroproduction cross section to a
Regge model prediction in which F_pi is a free parameter. The results indicate
that the pion form factor deviates from the charge-radius constrained monopole
form at these values of Q2 by one sigma, but is still far from its perturbative
Quantum Chromo-Dynamics prediction.
Cross sections for the reaction \({^1}\)H(\(e,e'\pi^+\))\(n\) were measured in Hall C at Thomas Jefferson National Accelerator Facility (JLab) using the CEBAF high-intensity, continous electron beam ...in order to determine the charged pion form factor. Data were taken for central four-momentum transfers ranging from \(Q^2\)=0.60 to 2.45 GeV\(^2\) at an invariant mass of the virtual photon-nucleon system of \(W\)=1.95 and 2.22 GeV. The measured cross sections were separated into the four structure functions \(\sigma_L\), \(\sigma_T\), \(\sigma_{LT}\), and \(\sigma_{TT}\). The various parts of the experimental setup and the analysis steps are described in detail, including the calibrations and systematic studies, which were needed to obtain high precision results. The different types of systematic uncertainties are also discussed. The results for the separated cross sections as a function of the Mandelstam variable \(t\) at the different values of \(Q^2\) are presented. Some global features of the data are discussed, and the data are compared with the results of some model calculations for the reaction \({^1}\)H(\(e,e'\pi^+\))\(n\).
The H(e,e'pi+)n cross section was measured at four-momentum transfers of Q2=1.60 and 2.45 GeV2 at an invariant mass of the photon nucleon system of W=2.22 GeV. The charged pion form factor (F_pi) was ...extracted from the data by comparing the separated longitudinal pion electroproduction cross section to a Regge model prediction in which F_pi is a free parameter. The results indicate that the pion form factor deviates from the charge-radius constrained monopole form at these values of Q2 by one sigma, but is still far from its perturbative Quantum Chromo-Dynamics prediction.
PDF
