Proton Electromagnetic Form Factors Research Articles

Testing electromagnetic proton form factors in the annihilation process [formula omitted

We study the annihilation channel in proton antiproton collisions with production of a single neutral pion and an electron-positron pair. It is shown that this reaction allows access to the proton electromagnetic form factors in the ‘unphysical’ region and to the difference between complex phases of the Dirac and Pauli proton form factors. The differential cross section is given in the laboratory frame in an experimental setup where all outgoing particles are fully detected.

Comment on “High-Precision Determination of the Electric and Magnetic Form Factors of the Proton”

In a recent Letter, Bernauer, et al. present fits to the proton electromagnetic form factors, GEp(Q^2) and GMp(Q^2), along with extracted proton charge and magnetization radii based on large set of new, high statistical precision (<0.2%) cross section measurements. The Coulomb corrections they apply differ dramatically from more modern and complete calculations, implying significant error in their final results.

Polarizede-pelastic scattering in the collider frame

Double polarization elastic e-p cross sections and asymmetries are considered in collider kinematics. Covariant expressions are derived for the general situation involving crossed beams; these are checked against the well-known results obtained when the proton is at rest. Results are given using modern models for the proton electromagnetic form factors for kinematics of interest in e-p colliders such as the EIC facility which is in its planning stage. In context, parity-violating elastic e-p scattering is compared and contrasted with these double-polarization (parity-conserving) results.

Medium Modifications of Hadron Properties and Partonic Processes

Chiral symmetry is one of the most fundamental symmetries in QCD. It is closely connected to hadron properties in the nuclear medium via the reduction of the quark condensate < q̄q >, manifesting the partial restoration of chiral symmetry. To better understand this important issue, a number of Jefferson Lab experiments over the past decade have focused on understanding properties of mesons and nucleons in the nuclear medium, often benefiting from the high polarization and luminosity of the CEBAF accelerator. In particular, a novel, accurate, polarization transfer measurement technique revealed for the first time a strong indication that the bound proton electromagnetic form factors in 4He may be modified compared to those in the vacuum. Second, the photoproduction of vector mesons on various nuclei has been measured via their decay to e+ e− to study possible in-medium effects on the properties of the ρ meson. In this experiment, no significant mass shift and some broadening consistent with expected collisional broadening for the ρ meson has been observed, providing tight constraints on model calculations. Finally, processes involving in-medium parton propagation have been studied. The medium modifications of the quark fragmentation functions have been extracted with much higher statistical accuracy than previously possible.

Recoil Polarization Measurements of the Proton Electromagnetic Form Factor Ratio toQ2=8.5 GeV2

Among the most fundamental observables of nucleon structure, electromagnetic form factors are a crucial benchmark for modern calculations describing the strong interaction dynamics of the nucleon's quark constituents; indeed, recent proton data have attracted intense theoretical interest. In this Letter, we report new measurements of the proton electromagnetic form factor ratio using the recoil polarization method, at momentum transfers Q2=5.2, 6.7, and 8.5 GeV2. By extending the range of Q2 for which G(E)(p) is accurately determined by more than 50%, these measurements will provide significant constraints on models of nucleon structure in the nonperturbative regime.

Feasibility studies of the time-like proton electromagnetic form factor measurements with $ \overline{{P}}$ANDA at FAIR

The possibility of measuring the proton electromagnetic form factors in the time-like region at FAIR with the \( \overline{{P}}\)ANDA detector is discussed. Detailed simulations on signal efficiency for the annihilation of \( \bar{{p}}\) + p into a lepton pair as well as for the most important background channels have been performed. It is shown that precise measurements of the differential cross-section of the reaction \( \bar{{p}}\) + p \( \rightarrow\) e - + e + can be obtained in a wide kinematical range. The determination of the ratio R of the moduli of the electric and magnetic proton form factors will be possible up to a value of momentum transfer squared of q 2 ≃ 14 (GeV/c)^2 with absolute precision from 0.01 to 0.5 (for R ∼ 1 . The total \( \bar{{p}}\) + p \( \rightarrow\) e - + e + cross-section will be measured up to q 2 ≃ 28 (GeV/c)^2. The results obtained from simulated events are compared to the existing data. Sensitivity to the two-photon exchange mechanism is also investigated.

Antiproton-nucleus electromagnetic annihilation as a way to access the proton timelike form factors

Contrary to the reaction \( \bar{{p}}\) p \( \rightarrow\) e + e - with a high-momentum incident antiproton on a free target proton at rest, in which the invariant mass M of the e + e - pair is necessarily much larger than the \( \bar{{p}}\) p mass 2m , in the reaction \( \bar{{p}}\) d \( \rightarrow\) e + e - n the value of M can take values near or below the \( \bar{{p}}\) p mass. In the antiproton-deuteron electromagnetic annihilation, this allows to access the proton electromagnetic form factors in the timelike region of q2 near the \( \bar{{p}}\) p threshold. We estimate the cross-section \( d\sigma _{\bar pd \to e^ + e^ - n} /d\mathcal{M} \) for an antiproton beam momentum of 1.5GeV/c. We find that near the \( \bar{{p}}\) p threshold this cross-section is about 1pb/MeV. The case of heavy-nuclei target is also discussed. Elements of experimental feasibility are presented for the process \( \bar{{p}}\) d \( \rightarrow\) e + e - n in the context of the \( \overline{{{\rm P}}}\) ANDA project.

Electromagnetic form factors of the nucleon: New fit and analysis of uncertainties

Electromagnetic form factors of proton and neutron, obtained from a new fit of data, are presented. The proton form factors are obtained from a simultaneous fit to the ratio ${\ensuremath{\mu}}_{p}{G}_{\mathit{Ep}}/{G}_{\mathit{Mp}}$ determined from polarization transfer measurements and to $\mathit{ep}$ elastic cross section data. Phenomenological two-photon exchange corrections are taken into account. The present fit for protons was performed in the kinematical region ${Q}^{2}\ensuremath{\in}(0,6)$ ${\mathrm{GeV}}^{2}$. For both protons and neutrons we use the latest available data. For all form factors, the uncertainties and correlations of form factor parameters are investigated with the ${\ensuremath{\chi}}^{2}$ method.

Proton form factors at FAIR

The new FAIR accelerator complex, under construction as a major upgrade of the existing GSI laboratory of Darmstadt, will provide a new antiproton beam of unprecedent intensity and excellent momentum resolution. This could be a good tool to study proton electromagnetic form factors in the time-like energy domain.

Structure function method applied to polarized and unpolarized electron-proton scattering: A solution of theGE(p)/GM(p)discrepancy

The cross section for polarized and unpolarized electron-proton scattering is calculated taking into account radiative corrections in leading and next-to leading logarithmic approximation. The expression of the cross section is formally similar to the cross section of the Drell-Yan process, where the structure functions of the electron play the role of Drell-Yan probability distributions. The interference of the Born amplitude with the two photon exchange amplitude (box-type diagrams) is expressed as a contribution to K-factor and it is larger when the momentum is equally shared between the two photons, assuming that proton form factors decrease rapidly with the momentum transfer squared. The calculation of the box amplitude is done when the intermediate state is a proton or the Delta-resonance. The results of numerical estimations show that the present calculation of radiative corrections can bring into agreement the conflicting experimental results on proton electromagnetic form factors an that the two photon contribution is very small.

Extraction of proton form factors in the timelike region from single-polarizede+e−→p→p¯events

We have performed numerical simulations of the single-polarized ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}\stackrel{\ensuremath{\rightarrow}}{p}\overline{p}$ process in kinematic conditions under discussion for a possible upgrade of the existing DAFNE facility. By fitting the cross section and spin asymmetry angular distributions with typical Born expressions, we can study the conditions for extracting information on moduli and phases of the proton electromagnetic form factors in the timelike region, which are poorly known and whose preliminary data show puzzling features. We have explored also non-Born contributions by introducing a further component in the angular fit, which is related to two-photon exchange diagrams. Using a dipole parametrization, we show that these corrections can be identified if larger than 5% of the Born contribution; we also explore the conditions for extracting information on the phase and, consequently, on the relative weight between their real and imaginary parts, which are presently unknown.

On the strong energy dependence of the e+e−↔pp¯ amplitude near threshold

We study the energy dependence of the e+e−→pp¯ cross section close to the two-nucleon threshold, recently reported by the BaBar Collaboration. Our analysis also includes the p¯p→e+e− data collected by PS170 Collaboration and the e+e−→NN¯ data from the FENICE Collaboration. We show that the near-threshold enhancement in the e+e−→pp¯ cross section can be explained by the final-state interaction between proton and antiproton in the S13 partial wave, utilizing the Jülich nucleon–antinucleon model. As a consequence, the strong dependence of the proton electromagnetic form factors on the momentum transfer close to the two-nucleon threshold is then likewise driven by this final-state interaction effect. This result is in line with our previous studies of the near-threshold enhancement of the pp¯ invariant mass spectrum seen in the J/Ψ→γpp¯ decay by the BES Collaboration and in the B+→pp¯K+ decay by the BaBar Collaboration.

Extraction of proton form factors in the timelike region from unpolarizede+e−→pp¯events

We have performed numerical simulations of the unpolarized ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}p\overline{p}$ process in kinematic conditions under discussion for a possible upgrade of the existing DAFNE facility. By fitting the cross section angular distribution with a typical Born expression, we can extract information on the ratio $|{G}_{E}/{G}_{M}|$ of the proton electromagnetic form factors in the timelike region within a 5%--10% uncertainty. We have explored also non-Born contributions to the cross section by introducing a further component in the angular fit, which is related to two-photon exchange diagrams. We show that these corrections can be identified if larger than 5% of the Born contribution, and if relative phases of the complex form factors do not produce severe cancellations.

μpGEp/GMp and qF2p/F1p in a relativistic quark model

The possible zero of the proton electromagnetic form factor ratio G Ep/G Mp is studied within a light-front quark model. The position of the zero is determined by the presence of a hard constituent quark component in the nucleon wave function. The description of the data for the ratios μp G Ep/G Mp and qF 2p/F 1p is improved when the hard-scale component of the nucleon wave function is considered.

Proton electromagnetic form factors at large momentum transfer

We review the current experimental and theoretical status of the proton electromagnetic form factors.

Precision Measurements of the Timelike Electromagnetic Form Factors of Pion, Kaon, and Proton

Using 20.7 pb(-1) of e(+)e(-) annihilation data taken at sq.rt(r) = 3.671 GeV with the CLEO-c detector, precision measurements of the electromagnetic form factors of the charged pion, charged kaon, and proton have been made for timelike momentum transfer of |Q(2)| = 13.48 GeV(2) by the reaction e(+)e(-) --> h(+)h(-). The measurements are the first ever with identified pions and kaons of |Q(2)| > 4 GeV(2), with the results F(13.48 GeV(2)) = 0.075 +/- 0.008(stat) +/- 0.005(syst) and F(K)(13.48 GeV(2)) = 0.063 +/- 0.004(stat) +/- 0.001(syst). The result for the proton, assuming G(p)(E) = G(p)(M), is G(p)(M)(13.48 GeV(2)) = 0.014 +/- 0.002(stat) +/- 0.001(syst), which is in agreement with earlier results.

Proton polarizations in polarizedHe3studied with theHe3→(e→,e'p)dandHe3→(e→,e'p)pnprocesses

We study within the Faddeev framework the \vec{3He} (\vec{e},e' p)d as well as the \vec{3He} (\vec{e},e' p)pn and \vec{3He} (\vec{e},e' n)pp reactions in order to extract information on the proton and neutron polarization in polarized 3He. We achieve clear analytical insight for simplified dynamical assumptions and define conditions for experimental access to important 3He properties. In addition we point to the possibility to measure the electromagnetic proton form factors in the process \vec{3He} (\vec{e},e' p)d which would test the dynamical picture and put limits on medium corrections of the form factors.

Measurement of the cross section for [formula omitted] at center-of-mass energies from 2.0 to 3.07 GeV

Cross sections for e+e−→pp¯ have been measured at 10 center-of-mass energies from 2.0 to 3.07 GeV by the BESII experiment at the BEPC, and proton electromagnetic form factors in the time-like region have been determined.

The ratio of proton electromagnetic form factors via recoil polarimetry at [formula omitted

Recoil polarimetry was used to extract the ratio of the proton electromagnetic form factors, μ p G E p / G M p = 0.878 ± 0.064 (stat) ± 0.012 (sys) , at Q 2 = 1.13 ( GeV / c ) 2 from the reaction H 1 ( e → , e p → ) . This was an ancillary measurement in which the proton polarization was determined as part of a larger program utilizing a stand-alone polarimeter designed to measure μ n G E n / G M n . This measurement complements previous recoil polarimetry measurements of μ p G E p / G M p made at the Thomas Jefferson National Accelerator Facility.

Local Quark-Hadron Duality and Magnetic Form Factors of Bound Proton**Theproject supported by Natural Science Foundation ofHebei Province of China under Grant No. A2005000535

We discuss the consequence of local duality for elasticscattering, and derive a model-independent equation betweenstructure functions at x ∼ 1 and elastic electromagnetic formfactors. Then the electromagnetic form factors of proton arediscussed using the quark-hadron duality theory. We also debatethe form factor of proton in a bound state. It may be an effectiveapproach to study the form factor of proton in media.