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تسجيل مستخدم جديدDetermination of the Charged Pion Form Factor at Q2=1.60 and 2.45 (GeV/c)2
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The H(e,epi+)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.
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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,epi+)n reaction, and thus is inherently model dep
endent. 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.
The data analysis for the reaction H(e,e pi^+)n, which was used to determine values for the charged pion form factor Fpi for values of Q^2=0.6-1.6 GeV^2, has been repeated with careful inspection of all steps and special attention to systematic uncer
tainties. Also the method used to extract Fpi from the measured longitudinal cross section was critically reconsidered. Final values for the separated longitudinal and transverse cross sections and the extracted values of Fpi are presented.
Cross sections for the reaction ${^1}$H($e,epi^+$)$n$ 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 we
re 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($e,epi^+$)$n$.
The electric form factor of the neutron, G_En, has been measured at the Mainz Microtron by recoil polarimetry in the quasielastic D(e_pol,en_pol)p reaction. Three data points have been extracted at squared four-momentum transfers Q^2 = 0.3, 0.6 and 0
.8 (GeV/c)^2. Corrections for nuclear binding effects have been applied.
New data are presented on the p(e,ep)pi^0 reaction at threshold at a four-momentum transfer of Q^2=0.05 GeV^2/c^2. The data were taken with the three-spectrometer setup of the A1 Collaboration at the Mainz Microtron MAMI. The complete center of mass
solid angle was covered up to a center of mass energy of 4 MeV above threshold. Combined with measurements at three different values of the virtual photon polarization epsilon, the structure functions sigma_T, sigma_L, sigma_{TT}, and sigma_{TL} are determined. The results are compared with calculations in Heavy Baryon Chiral Perturbation Theory and with a phenomenological model. The measured cross section is significantly smaller than both predictions.
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