Search

Data on hyperon form factors recently obtained from annihilation reactions are compared each other, and to proton and neutron form factors, in terms of two kinematical variables: the transferred momentum square, $q^2$, and the modulus $P$ of the relative momentum between the outgoing baryons. They are critically discussed in terms of possible correlated structures. The present status of the time-like form factor data for baryons is described and suggestions are given on the reactions and the kinematical range where data are desirable in order to clarify the arisen questions., 20 pages, 10 figures

International audience; Recently, the BESIII experiment renewed the interest on baryon form factors by measuring the modulus and phase of the ratio GEΛ/GMΛ between the electric and the magnetic Λ form factors with unprecedented accuracy. The BESIII measurement together with older, less precise data, can be analyzed by means of a dispersive procedure based on analyticity and a set of first-principle constraints. Such a dispersive procedure shows the unique ability to determine, for the first time, the complex structure of the ratio knowing its modulus and phase measured by the BESIII Collaboration at only one energy point. Different classes of solutions are obtained, and in all cases, the timelike and spacelike behaviors show interesting properties; spacelike zeros or unexpected large determinations for the phase. More data at different energies would be crucial to enhance the predictive power of the dispersive procedure and to unravel further remarkable features of the Λ baryon.

The antiproton experiment PANDA at FAIR is designed to bring hadron physics to a new level in terms of scope, precision and accuracy. In this work, its unique capability for studies of hyperons is outlined. We discuss ground-state hyperons as diagnostic tools to study non-perturbative aspects of the strong interaction, and fundamental symmetries. New simulation studies have been carried out for two benchmark hyperon-antihyperon production channels: $\bar{p}p \to \bar{\Lambda}\Lambda$ and $\bar{p}p \to \bar{\Xi}^+\Xi^-$. The results, presented in detail in this paper, show that hyperon-antihyperon pairs from these reactions can be exclusively reconstructed with high efficiency and very low background contamination. In addition, the polarisation and spin correlations have been studied, exploiting the weak, self-analysing decay of hyperons and antihyperons. Two independent approaches to the finite efficiency have been applied and evaluated: one standard multidimensional efficiency correction approach, and one efficiency independent approach. The applicability of the latter was thoroughly evaluated for all channels, beam momenta and observables. The standard method yields good results in all cases, and shows that spin observables can be studied with high precision and accuracy already in the first phase of data taking with PANDA., Comment: 26 pages, 16 figures. Changes: Revised title and abstract and corrections/clarifications in the text according to suggestions by journal referees

The European physical journal / A 57(4), 149 (2021). doi:10.1140/epja/s10050-021-00444-5, Published by Springer, Heidelberg

The internal structure of composite particles is conveniently described in terms of form factors (FFs)—these are experimentally accessible in annihilation and scattering of elementary reactions, and are theoretically calculable by all models that describe the properties of particles. FFs depend only on one kinematical variable, q2. This is the four-momentum transferred by the virtual photon that carries the interaction. Important developments in accelerator and detector techniques have brought impressive advances, both by extending the kinematical region and by reaching a higher precision. A critical review on the underlying methods and findings in polarized and unpolarized experiments is presented. The unique role played by polarization in determining the ratio of electric to magnetic form factors in the space-like region, and the extraction of individual form factors in the whole kinematical region, are described. Recent results at electron accelerators and electron–positron colliders confirm the existence of periodical structure in the annihilation cross section. We suggest a global framework which describes the dynamical structure of charge distribution in baryons, in order to build a coherent view of the creation and annihilation of baryonic matter.

The Facility for Antiproton and Ion Research (FAIR) in Darmstadt, Germany, provides unique possibilities for a new generation of hadron-, nuclear- and atomic physics experiments. The future antiProton ANnihilations at DArmstadt (PANDA or $$\overline{\mathrm{P}}$$ P ¯ ANDA) experiment at FAIR will offer a broad physics programme, covering different aspects of the strong interaction. Understanding the latter in the non-perturbative regime remains one of the greatest challenges in contemporary physics. The antiproton–nucleon interaction studied with PANDA provides crucial tests in this area. Furthermore, the high-intensity, low-energy domain of PANDA allows for searches for physics beyond the Standard Model, e.g. through high precision symmetry tests. This paper takes into account a staged approach for the detector setup and for the delivered luminosity from the accelerator. The available detector setup at the time of the delivery of the first antiproton beams in the HESR storage ring is referred to as the Phase One setup. The physics programme that is achievable during Phase One is outlined in this paper.

The dimension of the proton, the basic building block of matter, is still object of controversy. The most precise electron-proton scattering data at low transferred momenta are re-analyzed and the extraction of the proton radius is discussed. A recent experiment from the JLAB-CLAS collaboration gives a small value for the radius (The symbol $$R_E^\alpha $$ R E α stands for the root-mean-square charge radius of the proton $$\sqrt{\langle r_E^2\rangle }$$ ⟨ r E 2 ⟩ , obtained by the experimental or theoretical Collaboration $$\alpha $$ α .) $$R_E^\mathrm{CLAS}= (0.831\pm 0.007_\mathrm{stat}\pm 0.012_\mathrm{syst})$$ R E CLAS = ( 0.831 ± 0 . 007 stat ± 0 . 012 syst ) fm (Xiong et al. in Nature 575:147, 2019), in contrast with previous electron scattering experiments, in particular with the MAINZ experiment (Bernauer et al. (A1 Collaboration), Phys. Rev. C 90:015206, 2014) that concluded $$R_E^\mathrm{MAINZ}= (0.879\pm 0.005_\mathrm{stat}\pm 0.004_\mathrm{syst}\pm 0.002_\mathrm{model}\pm 0.004_\mathrm{group})$$ R E MAINZ = ( 0.879 ± 0 . 005 stat ± 0 . 004 syst ± 0 . 002 model ± 0 . 004 group ) fm. The experimental results are re-analyzed in terms of different fits of the cross section and of its discrete derivative with analyticity constraints. The uncertainty on the derivative is two orders of magnitude larger than the error on the measured observable, i.e., the cross section. The systematic error associated with the radius is evaluated taking into account the uncertainties from different sources, as the extrapolation to the static point, the choice of the class of fitting functions, and the range of the data sample.

The data on the proton form factors in the time-like region from the BaBar, BESIII and CMD-3 Collaborations are examined to have coherent pieces of information on the proton structure. Oscillations in the annihilation cross section, previously observed, are determined with better precision. The moduli of the individual form factors, determined for the first time, their ratio and the angular asymmetry of the annihilation reaction $e^+e^-\to\bar p p$ are discussed. Fiits of the available data on the cross section, the effective form factor, and the form factor ratio, allow to propose a description of the electric and magnetic time-like form factors from the threshold up to the highest momenta., 20 pages, 7 figures

The tripletlike QED processes γ+ℓi→ℓj+ℓj−+ℓi with i≠j and i=e, μ, j=e, μ, τ has been investigated as the reactions where a dark photon A′ is produced as a virtual state with a subsequent decay into a ℓj+ℓj− pair. This effect arises due to the so-called kinetic mixing and is characterized by a small parameter ε describing the coupling strength relative to the electric charge e. In these processes, the dark photon appears as a resonance with a Breit-Wigner propagator in the ℓj+ℓj− system. The distributions over the invariant mass of the produced ℓj+ℓj− pair are calculated, and the value of the parameter ε, as a function of the dark photon mass, is estimated for a given number of measured events in the specific experimental conditions. Assuming a standard deviation of σ=2 (corresponding to ≈95% confidence limit) and a number of measured events equal to 104, we obtain the following limits for the parameter space of the dark photon: ε2≤10−7 (γ+μ−→e++e−+μ−) and ε2≤10−7÷6×10−8 (γ+e−→μ++μ−+e−) in the A′ mass region 520

The precise determination of the proton radius from recent elastic scattering electron-proton data is discussed. The necessary precision on the elastic cross section to discriminate among the values coming from atomic spectroscopy is scrutinized in terms of the relevant quantity, i.e., the derivative of the form factor. It is shown that such precision is two orders of magnitude higher than the precision on the cross section, that is the measured observable. Different fits on the available data and of their discrete derivative, with analytical constraints are shown. The systematic error associated to the radius is evaluated taking into account the uncertainties from different sources, as the extrapolation to the static point, the choice of the class of fitting functions and the range of the data sample. This error is shown to be even orders of magnitude larger than commonly assumed., 18 pages, 10 figures, 1 table

The analyzing powers for neutron charge exchange nA → pX reactions on nuclei have been measured on C, CH2 and Cu targets at incident neutron momenta 3.0 - 4.2 GeV/c by detecting one charged particle in forward direction. The polarized neutron measurements are the first of their kind. The experiment was performed using the Nuclotron accelerator in JINR Dubna, where polarized neutrons and protons were obtained from breakup of a polarized deuteron beam which has a maximum momentum of 13 GeV/c. The polarimeter ALPOM2 was used to obtain the analyzing power dependence on the transverse momentum of the final-state nucleon. These data have been used to estimate the figure of merit of a proposed experiment at Jefferson Laboratory to measure the recoiling neutron polarization in the quasi-elastic 2 H(e, e′n) reaction, which yields information on the charge and magnetic elastic form factors of the neutron.

We discuss recent experimental results concerning the cross section ratio of positron over electron elastic scattering on protons, and compare with the predictions of a pre-existent calculation. The deviation from unity of this ratio, $i.e.$, a charge asymmetry different from zero, is the signature of contributions beyond the Born approximation. After reviewing the published results, we compare the elastic data to a calculation which includes the diagram corresponding to two-photon exchange. It turns out that all the data on the cross section ratio, in the limit of their precision, do not show evidence of enhanced two-photon contribution beyond the expected percent level. Our results confirm that experimental evidence for a large contribution of two-photon exchange is not yet found., 23 pages, 7 figures

International audience; This contribution, in the field of hadron physics, highlights two aspects of the structure of the nucleon, in its ground and excited states: the sensitivity of the tensor analyzing power for the process d→+p→d+X to the longitudinal form factor of an excited state, the Roper resonance, calculated in the frame of the algebraic collective model for the electroexcitation of nucleon resonances; - the electromagnetic proton form factor, given by a model based on vector meson dominance. This model predicted the decrease of the electric to magnetic proton form factor ratio, well before precise data appeared and it contains the necessary analytical properties to be extended to the annihilation region, where theory and experiment know important developments today.

International audience; Recent data on the cross section ratio for electron and positron elastic scattering on protons are discussed. A deviation from unity of this ratio constitutes a model independent signature of charge-odd contributions that arise from mechanisms beyond the one-photon approximation, as the exchange of two photons. The relevance of this issue is related to that fact that the information on the proton structure from lepton elastic (and inelastic) scattering holds only within a formalism based on the one photon exchange approximation. The present analysis shows that the deviations of the data from unity and from a previously developed theoretical approach lie within the theoretical and experimental errors. The data suggest that other reasons for explanation of the discrepancy between the electromagnetic proton form factors extracted from the experiments according to the polarized and the unpolarized methods are more likely.

The annihilation of a lepton pair into a proton-antiproton pair, and the reverse process, allow for a measurement of the timelike proton form factors. This work aims at studying the corrections due to direct soft interactions between the proton and the antiproton before their annihilation or after their creation. The analysis is carried on in the spacetime formalism used by us in a series of recent works on timelike FFs. In particular the effect of $\bar{p}p$ annihilation into many-meson states is considered. We expect it to determine similar real and imaginary parts for the form factor near the $\bar{p}p$ channel threshold. We also discuss the possibility that the timelike FF differ by a phase in the hadron pair production and annihilation., 19 pages, 10 figures

After recalling the arguments for possible excess of two-photon contribution over $\alpha$-counting, model independent statements about the consequences on the observables will be given. The relevant experimental data are discussed: (polarized and unpolarized) electron and positron elastic scattering on the proton, as well as annihilation data. A reanalysis of unpolarized electron-proton elastic scattering data is presented in terms of the electric to magnetic form factor squared ratio. This observable is in principle more robust against experimental correlations and global normalizations. The present analysis shows indeed that it is a useful quantity that contains reliable and coherent information. The comparison with the ratio extracted from the measurement of the longitudinal to transverse polarization of the recoil proton in polarized electron-proton scattering shows that the results are compatible. These results bring a decisive piece of information in the controversy on the deviation of the proton form factors from the dipole dependence., Comment: 12 pages, 5 figures, contribution to NSTAR 2017, to appear in Few Body Systems

The exclusive charmonium production process in (P) over barp annihilation with an associated pi 0 meson (p) over barp -> J/psi pi(0) is studied in the framework of QCD collinear factorization. The feasibility of measuring this reaction through the J/psi -> e(+) e(-) decay channel with the AntiProton ANnihilation at DArmstadt ((P) over bar ANDA) experiment is investigated. Simulations on signal reconstruction efficiency as well as the background rejection from various sources including the (P) over barp -> pi(+)pi(-)pi(0) and (p) over barp -> J/psi pi(0)pi(0) reactions are performed with PANDAROOT, the simulation and analysis software framework of the (P) over bar ANDA experiment. It is shown that the measurement can be done at (P) over bar ANDA with significant constraining power under the assumption of an integrated luminosity attainable in four to five months of data taking at the maximum design luminosity.

As well known, spacelike proton form factors expressed in the Breit frame may be interpreted as the Fourier transform of static space distributions of electric charge and current. In particular, the electric form factor is simply the Fourier transform of the charge distribution $F(q)=\int e^{i\vec q \cdot \vec r} \rho(r)d^3r$. We don't have an intuitive interpretation of the same level of simplicity for the proton timelike form factor appearing in the reactions $e^+e^-\leftrightarrow \bar{p}p$. However, one may suggest that in the center of mass (CM) frame, where $q_\mu x^\mu =qt$, a timelike electric form factor is the Fourier transform $F(q) =\int e^{iqt} R(t)dt$ of a function $R(t)$ expressing how the electric properties of the forming (or annihilating) proton-antiproton pair evolve in time. Here we analyze in depth this idea, show that the functions $\rho(r)$ and $R(t)$ can be formally written as the time and space integrals of a unique correlation function depending on both time and space coordinates., Comment: 8 figures, 15 pages

International audience; In frame of a general view of proton electromagnetic form factors, two recent findings related to reanalyses of data are presented. Recent experiments in the scattering and in the annihilation region provided us with more precise data and/or extending the kinematical region, allowing a deeper analysis and a common view of these fundamental quantities. We will discuss two issues: the discrepancy between the form factors extracted from unpolarized and polarized $ep$ elastic scattering experiments, in connection with the commonly used dipole parametrization; peculiar oscillations in e^+e^- -> p̅p (gamma) annihilation cross section, that become periodical when plotted as a function of the 3-momentum of the relative motion of the final proton and antiproton, after subtraction of a smooth function.

Antiproton--proton annihilation into light neutral mesons in the few GeV energy domain is investigated in view of a global description of the existing data and predictions for future experiments at PANDA, FAIR. An effective meson model earlier developed, with mesonic and baryonic degrees of freedom in $s$, $t$, and $u$ channels, is applied here to $\pi^0\pi^0$ production. Form factors with logarithmic $s$ and $t(u)$ dependencies are applied. A fair agreement with the existing angular distributions is obtained. Applying SU(3) symmetry, it is straightforward to recover the angular distributions for $ \pi^0\eta$, and $\eta\eta$ production in the same energy range. A good agreement is generally obtained with all existing data., Comment: 20 pages, 9 Figures

The differential cross section for elastic scattering of protons on electrons at rest is calculated taking into account the QED radiative corrections to the leptonic part of interaction. These model-independent radiative corrections arise due to emission of the virtual and real soft and hard photons as well as to vacuum polarization. We analyze an experimental setup when both the final particles are recorded in coincidence and their energies are determined within some uncertainties. The kinematics, the cross section, and the radiative corrections are calculated and numerical results are presented., 27 pages, 10 figures. The title of the article has been changed. Improved the quality of figures. An article without an appendix is approved for publication in the Phys.Rev.C

A reanalysis of unpolarized electron-proton elastic scattering data is done in terms of the electric to magnetic form factor squared ratio. This observable is in principle more robust against experimental correlations and global normalizations. The present analysis shows indeed that it is a useful quantity that contains reliable and coherent information. The comparison with the ratio extracted from the measurement of the longitudinal to transverse polarization of the recoil proton in polarized electron-proton scattering shows that the results are compatible within the experimental errors. Limits are set on the kinematics where the physical information on the form factors can be safely extracted. The results presented in this work bring a decisive piece of information to the controversy on the deviation of the proton form factors from the dipole dependence., 18 pages 13 figures, 11 tables- Updated version for Phys. Rev. C

A statistical analysis of the elastic unpolarized electron proton scattering data shows that, at large momentum transfer, the size and the $\epsilon$ dependence of the radiative corrections, as traditionally calculated and applied, may induce large correlations of the parameters of the Rosenbluth fit, which prevent a correct extraction of the electric proton form factor. Using the electron QED structure (radiation) function approach the cross section of elastic electron-proton scattering in leading and next-to leading approximations is calculated and expressed as a correction to the Born cross section, which is different for the electric and the magnetic contribution. When properly applied to the data, it may give the solution to the problem of the discrepancy of the polarized and unpolarized results on electron proton scattering., Comment: 11 pagex, 5 figures

International audience; The reaction p + CH(2)→ forward charge particle + X is used for this aim traditionally. Analyzing power of this reaction falls off as 1/p, where p is the laboratory momentum. At the proton momenta of order 8 GeV/c, which are expected at the JLab experiment, the low analyzing power creates problems for off-line analysis of data. On the other hand, it is well known that the reaction p+p → p+p has the much more analyzing power. So, the calorimeter is predestinated for suppression of inelastic events in this reaction. In the report it is shown that the problem is solved quite well.

New data of narrow low mass unflavoured mesonic structures are presented. A table of these exotic masses is obtained adding previously published data. The mass sequence shows a significant coupling of some of these clusters with stable hadrons: pion, nucleon, and deuteron. Indeed this coupling allows to reproduce rather well the masses of exotic narrow baryons and dibaryons. A discussion is presented to suggest a possible interpretation of these exotic hadronic structures.

Model independent expressions for all polarization observables in $\mu+p\to \mu+p$ elastic scattering are obtained, taking into account the lepton mass and including the two-photon exchange contribution. The spin structure of the matrix element is parametrized in terms of six independent complex amplitudes, functions of two independent kinematical variables. General statements about the influence of the two--photon--exchange terms on the differential cross section and on polarization observables are given. Polarization effects have been investigated for the case of a longitudinally polarized lepton beam and polarized nucleon in the final state., Comment: 28 pages, 3 figures

An accurate data base for the analyzing power of thick CH2 analyzers for high energy protons has been at the basis of several research efforts in a number of laboratories. Starting in the late eighties such data were collected at SATURNE in Saclay, and with the Synchrophasotron in Dubna, and led to an extensive program of study of polarization phenomena in pd interaction, either backward elastic scattering or breakup [1–6]. At about the same time it became evident that measuring polarization observables in elastic ep was going to be the best approach to determine the form factors of the proton, GEp and GMp, as had been predicted by Akhiezer and Rekalo [7].

We report a new measurement of analyzing powers for the reaction p ⇒ + CH 2 → one charged particle +X, at proton momenta of 1.75, 3.8, 4.5 and 5.3 GeV / c . These results extend the existing data basis, necessary for proton polarimetry at intermediate energy, and confirm the feasibility of a large acceptance polarimeter based on this process. Particular attention was devoted to the investigation of the optimal target thickness and of the useful angular range. The experiment is performed at the accelerator complex of the JINR-VBLHE (Dubna).

We analyze polarization effects in associative photoproduction of pseudoscalar\((\bar D_c )\) and vector\((\bar D_c^ * )\) charmed mesons in exclusive processes\(\gamma + N \to Y_c + \bar D_c , Y_c = \Lambda _c^ + , \Sigma _c \). We calculate the differential cross section and all polarization observables in framework of au effective Lagrangian approach. In case of collinear kinematics it is possible to give model independent predictions for polarization observables in case of\(\bar D_c \) production, and the analysis for\(\bar D_c^ * \) is largely simplified.

Using the symmetry properties of the strong interaction, such as the Pauli principle, the P-invariance, the conservation of the total angular momentum and isotopic invariance, we establish the spin structure of the threshold matrix elements for the processes $p+p\to \Theta^+ +\Sigma^+$ and $n+p\to \Theta^+ +\Lambda^0$, in a model independent way, which applies to any spin and parity of the $\Theta^+$-hyperon in the near threshold region. We predict the double spin observables for these processes, such as the dependence of the differential cross section on the polarizations of the colliding nucleons, and the coefficients of polarization transfer from a nucleon beam or target to the produced $\Sigma^+$ or $\Lambda^0$ hyperon. We prove that these observables are sensitive to the P-parity of the $\Theta^+$ baryon, for any value of its spin. As an example of dynamical considerations, we analyzed these reactions in the framework of K-meson exchange.

The spin structure of the matrix element for the reactions $\gamma+ N\to Y_c +{D}^*$, where $Y_c=\Lambda_c^+(2285)$, $\Sigma_c(2455)$ are charmed baryons with spin 1/2, and $D^*(2010)$ is the vector charmed meson, can be parametrized, in collinear regime, in terms of three independent scalar amplitudes, which are functions of the photon energy $E_{\gamma}$, only. In framework of an effective Lagrangian approach generalized to charm photoproduction, we calculate the energy dependence of the differential cross section, the density matrix element of $\bar{D}^*$, $\rho_{11}$, the asymmetry $A_z$ in the collision of circularly polarized photons with polarized nucleons, and the polarization of the produced $Y_c$-hyperon, $P_z$, in the collision of circularly polarized photons with unpolarized target. All these polarization observables either vanish or are large, in absolute value, with a smooth $E_{\gamma}$-dependence, and differ for $\Lambda_c^+$ and $\Sigma_c$ --production.

Recent measurements of the branching ratios for the decays $\omega\to\pi^0\pi^0\gamma$ and $\rho^0\to\pi^0\pi^0\gamma$ lead to coupling constants of the $V\sigma\gamma$-interaction ($V$ is a vector meson) one order of magnitude smaller than previously assumed to describe the threshold cross section of $\gamma+p\to p+\rho^0$ and the HERMES effect. The new $g^2_{V\sigma\gamma}$ couplings are in contradiction with the predictions of the QCD sum rules, but are in good agreement with VDM-estimation of the $\sigma\to \gamma\gamma$-width.

We discuss the applicability of pQCD to the elastic scattering of electrons on protons and deuterons. We analyze the Q2-dependence of the reduced deuteron form factor, taking into account the recent data on the electric proton form factor and we find that the value of the QCD-scale parameter Λ differs essentially from the value Λ = 0.1 GeV, previously found using the dipole parametrization of the electromagnetic nucleon form factors GE and GM. Moreover, the predicted scaling behavior of the reduced deuteron form factor cannot be recovered in the Dirac and Pauli representations for the nucleon electromagnetic form factors.

We suggest a method to determine the product of the coupling constants $g_{\rho\sigma\gamma}g_ {\sigma NN}$, in absolute value and relative sign, through the $t$-dependence of the two possible T-even asymmetries for the photoproduction of vector mesons, $\gamma+p\to p+V^0$, $V^0=\rho$ or $\omega$, in the collision of circularly polarized photons with a polarized proton target. In the framework of a $\pi+\sigma$-model, these asymmetries are very sensitive to the relative role of $\pi$- and $\sigma$-exchanges, in the near threshold region.

We investigate the effect of high order radiative corrections in unpolarized electron proton elastic scattering and compare with the calculations at lowest order, which are usually applied to experimental data. Particular attention is devoted to the $\epsilon$ dependence of radiative corrections, which is directly related to the electric proton form factor. We consider in particular the effects of the interference terms for soft and hard photon emission. Both quadratic amplitude describing the collinear emission along the scattered electron as well as the interference with the amplitudes of emission from the initial electron and the emission from protons are important in leading and next to leading approximation and they may compensate in particular kinematical conditions., Comment: 16 pages, 11 figures, 1 table

We compare the existing data about electromagnetic form factors of the proton in the time-like region of momentum transfer (up to s≃ 14 GeV2), with the corresponding data in the space-like region. From the constrains given by the Phragmen–Lindelof theorem, it turns out that the asymptotic regime can not be reached simultaneously for both form factors (electric and magnetic) in the considered region of momentum transfer. The angular dependence for the annihilation processes, such as p +p↔e + +e − , is sensitive to the asymptotic properties of form factors.

We have studied the deuteron breakup in the p(d,2p)n reaction at T{sub d}=2.0 GeV. In this experiment, the vector analyzing power A{sub Y}, the tensor analyzing power A{sub YY}, and the polarization of the forward scattered proton are measured close to the quasifree pp scattering kinematics. These observables are presented as a function of the neutron momentum ranging from 0.04 to 0.45 GeV/c in the rest frame of the deuteron. Marked deviations from the impulse approximation using conventional deuteron wave functions are observed. Corrections due to the multiple scattering, the final state interaction, and the {Delta} excitation are calculated in a model where the spin-isospin structure of the elementary amplitudes is treated rigorously. With this model, a reasonable account of the measured polarization observables is obtained. The global agreement with all observables is, however, not good enough that we can reliably discriminate between conventional wave functions. {copyright} {ital 1997} {ital The American Physical Society}

It is suggested that proton elastic scattering on atomic electrons allows a precise measurement of the proton charge radius. Very small values of transferred momenta (up to four order of magnitude smaller than the ones presently available) can be reached with high probability., Comment: 4 pages, 4 figures

Angular distributions for the differential cross section and three deuteron analyzing powers {ital iT}{sub 11}, {ital T}{sub 20}, and {ital T}{sub 22} of the reaction {ital d}{searrow}{ital p}{r_arrow}{sup 3}He{pi}{sup 0} have been measured over the whole angular domain at 20 energies close to threshold (0.03{lt}{ital T}{sub {pi}}{sup c.m.}{lt}10.2 MeV). The differential cross section and tensor analyzing power {ital T}{sub 20} both show strong variation in energy and angle due to interference between {ital S}- and {ital P}-wave pion production, whereas {ital iT}{sub 11} and {ital T}{sub 22} remain consistent with zero over the whole experimental range. All the data at different energies and angles fall on universal curves when plotted as functions of the single variable {ital p}{sub {pi}}cos{Theta}, evaluated in the c.m. The broad features of the results are in line with theoretical expectations. {copyright} {ital 1996 The American Physical Society.}

A search for the lepton-family-violating decay {eta}{r_arrow}{mu}{ital e} and the rare decay {eta}{r_arrow}{ital e}{sup +}{ital e}{sup {minus}} yields the following branching ratio ({ital B}) upper limits at a 90{percent} confidence level: {ital B}({eta}{r_arrow}{mu}{ital e}){lt}6{times}10{sup {minus}6} and {ital B}({eta}{r_arrow}{ital e}{sup +}{ital e}{sup {minus}}){lt}2{times}10{sup {minus}4}. This is the first direct search for {eta}{r_arrow}{mu}{ital e}. The measurements were carried out at the SPES2 tagged {eta} facility at Laboratoire National Saturne in the course of a measurement of {ital B}({eta}{r_arrow}{mu}{sup +}{mu}{sup {minus}}). {copyright} {ital 1996 The American Physical Society.}

The reaction {ital pd}{r_arrow}{sup 3}He{eta} has been studied with the SPES2 spectrometer, for proton energies between 0.2 MeV and 11 MeV above threshold. The total cross section rises from 0.25 to 0.40 {mu}b in this region. The observed energy dependence can be attributed to the strong final state interaction. The angular distribution is nearly isotropic, consistent with {ital S} wave production. A measurement of the {ital pd}{r_arrow}{sup 3}He{pi}{sup +}{pi}{sup {minus}} cross section in the threshold region is also presented. The setup, which is capable of producing 10{sup 5} tagged {eta}/s with only a few percent background, is used to investigate rare {eta} decays. {copyright} {ital 1996 The American Physical Society.}

A new measurement of the branching ratio for the decay \ensuremath{\eta}\ensuremath{\rightarrow}${\mathrm{\ensuremath{\mu}}}^{+}$${\mathrm{\ensuremath{\mu}}}^{\mathrm{\ensuremath{-}}}$, made at the SATURNE II proton synchrotron, resulted in \ensuremath{\Gamma}(\ensuremath{\eta}\ensuremath{\rightarrow}${\mathrm{\ensuremath{\mu}}}^{+}$${\mathrm{\ensuremath{\mu}}}^{\mathrm{\ensuremath{-}}}$)/\ensuremath{\Gamma}(\ensuremath{\eta}\ensuremath{\rightarrow}all)= [5.7\ifmmode\pm\else\textpm\fi{}0.7(stat.)\ifmmode\pm\else\textpm\fi{}0.5(syst.)]\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}6}$. The reaction pd${\ensuremath{\rightarrow}}^{3}$He\ensuremath{\eta} close to threshold yielded 800 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ tagged \ensuremath{\eta}'s in a narrow momentum band around 257 MeV/c. Muon pairs were detected in two range telescopes. The data obtained consist of 114 events \ensuremath{\eta}\ensuremath{\rightarrow}${\mathrm{\ensuremath{\mu}}}^{+}$${\mathrm{\ensuremath{\mu}}}^{\mathrm{\ensuremath{-}}}$ on a background of 14 events. The new value for the branching ratio is 1.3\ifmmode\pm\else\textpm\fi{}0.2 times the unitarity lower limit, consistent with most quark and vector meson dominance models which describe the decay as an electromagnetic transition with a two-photon intermediate state. The result resolves the discrepancy between the two previous measurements of this branching ratio.

We study the effect of final state radiation in the process $e^++e^- \to \bar p+ p$, in the kinematical conditions of BaBar and BESIII experiment. We show that this effect could be large, in particular in the low $x$ region ($x$ is the photon energy fraction) and should be taken into account.

The possibility of measuring the proton electromagnetic form factors in the time-like region at FAIR with the \PANDA 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 precision measurements of the differential cross section of the reaction $\bar p +p \to e^++ e^-$ can be obtained in a wide angular and kinematical range. The individual determination of the moduli of the electric and magnetic proton form factors will be possible up to a value of momentum transfer squared of $q^2\simeq 14$ (GeV/c)$^2$. The total $\bar p +p\to e^++e^-$ cross section will be measured up to $q^2\simeq 28$ (GeV/c)$^2$. The results obtained from simulated events are compared to the existing data. Sensitivity to the two photons exchange mechanism is also investigated., 12 pages, 4 tables, 8 figures Revised, added details on simulations, 4 tables, 9 figures

The analyzing power {ital T}{sub 20} in the inclusive {sup 1}H({sup 6}Li,{ital d} or {alpha} or {ital t}){ital X} reaction with 4.5 GeV tensor polarized {sup 6}Li nuclei has been measured at an angle of 0.8{degree}. The kinematics chosen favor the detection of spectator fragments; in the impulse approximation the laboratory momentum of such a fragment is then the Lorentz boosted internal momentum. Nonzero {ital T}{sub 20} values have been observed, in agreement with the known nonsphericity of {sup 6}Li indicated by its quadrupole moment. The sign of {ital T}{sub 20} in the {ital d} channel suggests that near {ital q}=0 the {ital D} state in {sup 6}Li has the same sign as in the deuteron; an abrupt change of sign near {ital q}=0.12 GeV/{ital c} is in agreement with theoretical expectation of a node in the {alpha}{ital d} position wave function. The {alpha}-channel data show larger {ital T}{sub 20} values than the {ital d} channel; in this case the small-{ital q}-region has not been explored enough to establish a similar node. A few data points in the {ital t} channel might suggest that {ital T}{sub 20} becomes positive above {ital q}=0.4 GeV/{ital c} in this case.

In frame of a phenomenological approach based on Compton-like Feynman amplitudes, we study the annihilation channel in antiproton nucleon collisions with production of a single charged or neutral pion and a lepton-antilepton pair. These reactions allow to access nucleon and axial electromagnetic form factors in time-like region and offer a unique possibility to study the kinematical region below two nucleon threshold. The differential cross section in an experimental set-up where the pion is fully detected is given with explicit dependence on the relevant nucleon form factors. The possibility to measure heavy charged pion in the annihilation channel is also discussed.