Double Polarization Observables Research Articles

Neutron polarisation transfer, [formula omitted], in π+ photoproduction off the proton, [formula omitted

We report a first measurement of the double-polarisation observable, Cx′, in π+ photoproduction off the proton. The Cx′ double-polarisation observable represents the transfer of polarisation from a circularly polarised photon beam to the recoiling neutron. The MAMI circularly polarised photon beam impinged on a liquid deuterium target cell, with reaction products detected in the Crystal Ball calorimeter. Ancillary apparatus surrounding the target provided tracking, particle identification and determination of recoil nucleon polarisation. The Cx′ observable is determined for photon energies 800-1400 MeV, providing new constraints on models aiming to elucidate the spectrum and properties of nucleon resonances. This is the first determination of any polarisation observable from the beam-recoil group of observables for this reaction. Inclusion of the new data in the database of the SAID partial wave analysis shifted the solution to a new global minima which, not only gives better agreement with the current data, but also improves the description of a range of other single and double polarisation observables for charged pion photoproduction.

Measurement of polarization observables in multi-meson photoproduction off the proton with the CBELSA/TAPS experiment

The reaction γp → pπ 0π 0 was analyzed using data of the CBELSA/TAPS experiment where linearly polarized photons impinged on a transversely polarized butanol target. Single and double polarization observables were extracted. Within the BnGa-PWA resonance parameters were determined. N∗ and ∆∗ resonances demonstrated systematic differences in their decay branching ratios via excited hadrons which hint at the internal structure of these states.

The electromagnetic form factors of $$\Lambda _c$$ hyperon in the vector meson dominance model

We apply a modified vector meson dominance (VMD) model to analyze the electromagnetic form factors of the $\Lambda_c$ hyperon in the time-like reaction $e^+e^-\rightarrow \Lambda_c^+ \bar{\Lambda}_c^-$. In the model, we include the contributions from the vector charmed mesons and their excitations $\psi(1S)$, $\psi(2S)$, $\psi(3770)$, $\psi(4040)$, $\psi(4160)$ and $\psi(4415)$. We perform a combined fit to the available data on the Born cross section in reaction $e^+e^-\rightarrow \Lambda_c^+ \bar{\Lambda}_c^-$ and the ratio of the electromagnetic form factors $|G_E/G_M|$ to obtain the values of the model parameters. Our results show that the VMD model can simultaneously describe the data of electromagnetic form factors from the Belle and BESIII Collaborations, and the behavior of $|G_E/G_M|$ for the BESIII data can be qualitatively reproduced by the VMD model prediction at the threshold region. Moreover, we predict the single and double polarization observables in $e^+e^-\rightarrow \Lambda_c^+ \bar{\Lambda}_c^-$ reactions, which are experimentally accessible in the polarized process. We also obtain the form factors of the $\Lambda_c$ hyperon in the space-like region via analytic continuing the time-like form factors.

Double polarisation observables G and E and helicity dependent cross section for single π 0 photoproduction off proton and neutron at MAMI

Meson photoproduction, like other photon-induced reactions, allow to study the excitation spectra of the nucleons and, in combination with the use of polarised beam and/or target, allow to determine the properties of the nucleon resonances by accessing many different polarisation observables with great accuracy. The A2 Collaboration, located at the MAMI facility in Mainz, used elliptically polarised photons on longitudinally polarised proton and deuteron targets, for energies up to 1.6 GeV. From the experimental data, it was possible to extract the double polarisation observables E and G on single π 0 photoproduction off the proton and the neutron. New precise results on the helicity-dependent total and differential cross sections on the deuteron were collected too.

Extracting the spin polarizabilities of the proton by measurement of Compton double-polarization observables

The Compton double-polarization observable $\Sigma_{2z}$ has been measured for the first time in the $\Delta(1232)$ resonance region using a circularly polarized photon beam incident on a longitudinally polarized target at the Mainz Microtron. This paper reports these results, together with the model-dependent extraction of four proton spin polarizabilities from fits to additional asymmetry data using dispersion relation and chiral perturbation theory calculations, with the former resulting in: $\gamma_{E1E1} = -3.18 \pm 0.52$, $\gamma_{M1M1} = 2.98 \pm 0.43$, $\gamma_{E1M2} = -0.44 \pm 0.67$ and $\gamma_{M1E2} = 1.58 \pm 0.43$, in units of $10^{-4}~\mathrm{fm}^{4}$.

Recent results from the CBELSA/TAPS experiment at ELSA

In order to gain a better understanding of the dynamics inside the nucleon and of the non-perturbative regime of QCD, the nucleon excitation spectra and the properties of nucleon resonances are investigated. An essential experimental tool to achieve this goal is the study of different photoproduction reactions. Partial wave analyses are performed in order to obtain information about the contributing resonances. A complete experiment is needed to extract the underlying amplitudes unambiguously, which requires the measurement of carefully chosen single and double polarization observables in addition to the unpolarized cross section. The CBELSA/TAPS experiment in Bonn offers the possibility to measure several polarization observables using a linearly or circularly polarized photon beam and with a longitudinally or transversely polarized target. This contribution gives an overview of recently measured polarization observables in different final states. The impact of the new data is discussed.

Measurement of polarization observables in neutral double pion photoproduction off the proton with the CBELSA/TAPS experiment

Data on the reaction γp → pπ0π0 were taken with the CBELSA/TAPS experiment at ELSA where linearly polarized photons impinged on a transversely polarized butanol target. Target asymmetries and double polarization observables were extracted and entered the BnGa-PWA. Systematic differences in the branching ratios of N* and Δ* resonances hint at the internal structure of these states

Electromagnetic form factors of Λ hyperon in the vector meson dominance model

We perform an analysis on the electromagnetic form factors of the $\Lambda$ hyperon in the time-like reaction $e^+e^-\rightarrow \Lambda\bar\Lambda$ by using a modified vector meson dominance model. We consider both the intrinsic structure components and the meson clouds components. For the latter one, we not only include the contributions from the $\phi$ and $\omega$ mesons, but also take into account the contributions from the resonance states $\omega(1420)$, $\omega(1650)$, $\phi(1680)$ and $\phi(2170)$. We extract the model parameters by combined fit to the time-like effective form factor $|G_{\rm{eff}}|$, the electromagnetic form factor ratio $|G_E/G_M|$ and the relative phase $\Delta\Phi$ of the $\Lambda$ hyperon from the BaBar and BESIII Collaborations. We find that the vector meson dominance model can simultaneously describe these observables. Particularly, the inclusion of the resonance states in the model is necessary for explaining the ratio $|G_E/G_M|$ in a wide range of $\sqrt{s}$ as well as the large phase angle. With the fitted parameters, we predict the single and double polarization observables, which could be measured in polarized annihilation reactions. Moreover, we analytically continue the expression of the form factors to space-like region and estimate the space-like form factors of $\Lambda$ hyperon.

Partial-wave analyses of γp→ηp and γn→ηn using a multichannel framework

This paper presents results from partial-wave analyses of the photoproduction reactions $\gamma p \rightarrow \eta p$ and $\gamma n \rightarrow \eta n$. World data for the observables \DSG, $\Sigma$, $T$, $P$, $F$, and $E$ were analyzed as part of this work. The dominant amplitude in the fitting range from threshold to a c.m.\ energy of 1900 MeV was found to be $S_{11}$ in both reactions, consistent with results of other groups. At c.m.\ energies above 1600 MeV, our solution deviates from published results, with this work finding higher-order partial waves becoming significant. Data off the proton suggest that the higher-order terms contributing to the reaction include $P_{11}$, $P_{13}$, and $F_{15}$. The final results also hint that $F_{17}$ is needed to fit double-polarization observables above 1900 MeV. Data off the neutron show a contribution from $P_{13}$, as well as strong contributions from $D_{13}$ and $D_{15}$.

First Measurements of the Double-Polarization Observables F, P, and H in ω Photoproduction off Transversely Polarized Protons in the N^{*} Resonance Region.

First measurements of double-polarization observables in ω photoproduction off the proton are presented using transverse target polarization and data from the CEBAF Large Acceptance Spectrometer (CLAS) FROST experiment at Jefferson Lab. The beam-target asymmetry F has been measured using circularly polarized, tagged photons in the energy range 1200-2700MeV, and the beam-target asymmetries H and P have been measured using linearly polarized, tagged photons in the energy range 1200-2000MeV. These measurements significantly increase the database on polarization observables. The results are included in two partial-wave analyses and reveal significant contributions from several nucleon (N^{*}) resonances. In particular, contributions from new N^{*} resonances listed in the Review of Particle Properties are observed, which aid in reaching the goal of mapping out the nucleon resonance spectrum.

The electromagnetic form factors of Λ hyperon in e+e−→Λ̄Λ

We study the electromagnetic form factors of [Formula: see text] hyperon in the time-like region using the experimental data in the exclusive production of [Formula: see text] pair in electron–positron annihilation. We present a pQCD inspired parametrization of [Formula: see text] and [Formula: see text] with only two parameters, and we consider a suppression mechanism of the electric form factor [Formula: see text] compared to the magnetic form factor [Formula: see text]. The parameters are determined through fitting our parametrization to the data of the effective form factor [Formula: see text] from the DM2, BaBar and BESIII Collaborations in the reaction [Formula: see text]. We use the parametrizations for [Formula: see text] and [Formula: see text] to calculate the Born cross-sections as well as the ratio [Formula: see text]. Except the threshold region, our parametrization can describe the known behavior of the existing data of effective form factor, Born cross-section and the ratio [Formula: see text] from the BaBar, DM2 and BESIII Collaborations. We also predict the double spin polarization observables [Formula: see text], [Formula: see text] and [Formula: see text] in [Formula: see text] which could provide more information on the size of the lambda EMFFs as well as their phase angles.

Dispersion relation formalism for the two-photon exchange correction to elastic muon\u2013proton scattering: elastic intermediate state

We evaluate the two-photon exchange correction to the unpolarized cross section in the elastic muon–proton scattering within dispersion relations. One of the six independent invariant amplitudes requires a subtraction. We fix the subtraction function to the model estimate of the full two-photon exchange at one of three MUSE beam energies and make a prediction for the two other energies. Additionally, we present single and double polarization observables accounting for the lepton mass.

The impact of $K^{+}\Lambda$ K + Λ photoproduction on the resonance spectrum

The J\"ulich-Bonn coupled-channel framework is extended to $K^+\Lambda$ photoproduction. The spectrum of nucleon and $\Delta$ resonances is extracted from simultaneous fits to several pion-induced reactions in addition to pion, eta and $K^+\Lambda$ photoproduction off the proton. More than 40,000 data points up to a center-of-mass energy of E$\sim$2.3 GeV including recently measured double-polarization observables are analyzed. The influence of the $\gamma p\to K^+\Lambda$ channel on the extracted resonance parameters and the appearance of states not seen in other channels is investigated. The J\"ulich-Bonn model includes effective three-body channels and guarantees unitarity and analyticity, which is a prerequisite for a reliable determination of the resonance spectrum in terms of poles and residues.

Predicting K0Λ photoproduction observables by using the multipole approach

We present an isobar model for kaon photoproduction on the proton $$\gamma p\to K^+\Lambda$$ that can nicely reproduce the available experimental data from threshold up to $$W=2.0$$ GeV. The background amplitude of the model is constructed from a covariant Feynman diagrammatic method, whereas the resonance one is formulated by using the multipole approach. All unknown parameters in both background and resonance amplitudes are extracted by adjusting the calculated observables to experimental data. With the help of SU(3) isospin symmetry and some information obtained from the Particle Data Group we estimate the cross section and polarization observables for the neutral kaon photoproduction on the neutron $$\gamma n\to K^0\Lambda$$. The result indicates no sharp peak in the $$K^0\Lambda$$ total cross section. The predicted differential cross section exhibits resonance structures only at $$\cos\theta=-1$$. To obtain sizable observables the present work recommends measurement of the $$K^0\Lambda$$ cross section with $$W\gtrsim 1.70$$ GeV, whereas for the recoiled $$\Lambda$$ polarization measurement with $$W\approx 1.65$$–1.90 GeV would be advised, since the predictions of existing models show a large variance at this kinematics. The predicted electric and magnetic multipoles are found to be mostly different from those obtained in previous works. For $$W=1.75$$ and 1.95 GeV it is found that most of the single and double polarization observables demonstrate large asymmetries.

Isobar model for kaon photoproduction with spin- 7/2 and - 9/2 nucleon resonances

We have investigated the effects of spin-7/2 and -9/2 nucleon resonances in the kaon photoproduction process γp→K+Λ. To this end, the corresponding propagators were derived from the generalized spin projection operators. To remove the lower spin backgrounds in the scattering amplitude, we used the vertex factors obtained from the consistent interaction Lagrangians inspired by Pascalutsa and Vrancx et al. The scattering amplitude was included in our previous isobar model, and the effects of four nucleon resonances with spins 7/2 and 9/2 listed by the Particle Data Group were investigated by making use of all available kaon photoproduction data. A significant improvement to our previous model has been observed in all observables, especially in the beam-recoil double-polarization observables Cx, Cz, Ox′, and Oz′.

Beam-Recoil Polarization Measurement of π^{0} Electroproduction on the Proton in the Region of the Roper Resonance.

The helicity-dependent recoil proton polarizations P_{x}^{'} and P_{z}^{'} as well as the helicity-independent component P_{y} have been measured in the p(e[over →],e^{'}p[over →])π^{0} reaction at four-momentum transfer Q^{2}≃0.1 GeV^{2}, center-of-mass proton emission angle θ_{p}^{*}≃90°, and invariant mass W≃1440 MeV. This first precise measurement of double-polarization observables in the energy domain of the Roper resonance P_{11}(1440) by exploiting recoil polarimetry has allowed for the extraction of its scalar electroexcitation amplitude at an unprecedentedly low value of Q^{2}, establishing a powerful instrument for probing the interplay of quark and meson degrees of freedom in the nucleon.

Determining the dominant partial wave contributions from angular distributions of single- and double-polarization observables in pseudoscalar meson photoproduction

This work presents a simple method to determine the significant partial wave contributions to experimentally determined observables in pseudoscalar meson photoproduction. First, fits to angular distributions are presented and the maximum orbital angular momentum $\text{L}_{\mathrm{max}}$ needed to achieve a good fit is determined. Then, recent polarization measurements for $\gamma p \rightarrow \pi^{0} p$ from ELSA, GRAAL, JLab and MAMI are investigated according to the proposed method. This method allows us to project high-spin partial wave contributions to any observable as long as the measurement has the necessary statistical accuracy. We show, that high precision and large angular coverage in the polarization data are needed in order to be sensitive to high-spin resonance-states and thereby also for the finding of small resonance contributions. This task can be achieved via interference of these resonances with the well-known states. For the channel $\gamma p \rightarrow \pi^{0} p$, those are the $N(1680)\frac{5}{2}^{+}$ and $\Delta(1950)\frac{7}{2}^{+}$, contributing to the $F$-waves.

Multipoles model for K+Λ photoproduction on the nucleon reexamined

We have updated our previous multipoles model for the kaon photoproduction process $\ensuremath{\gamma}p\ensuremath{\rightarrow}{K}^{+}\mathrm{\ensuremath{\Lambda}}$ by using the recently available experimental data, including the new CLAS 2016 data, and up-to-date information on the nucleon resonance properties provided by the Particle Data Group (PDG). The background and resonance parameters are extracted by fitting the calculated observables to nearly 7400 experimental data points and constraining the resonance parameters within the PDG error bars. The model can nicely reproduce the experimental data with ${\ensuremath{\chi}}^{2}/{N}_{\mathrm{dof}}=1.63$. Different from the previous result, the present analysis finds the $N(1650){S}_{11}, N(1720){P}_{13}$, and $N(1900){P}_{13}$ states to be the most important resonances in the process. Excluding these states in the model increases the value of ${\ensuremath{\chi}}^{2}$ tremendously. As in our previous model, however, the contribution of the $N(1710){P}_{11}$ state in minimizing ${\ensuremath{\chi}}^{2}$ is found to be less significant. By including the new CLAS 2016 data in the fitting database and refitting the calculated observables to nearly 9000 data points, the ${\ensuremath{\chi}}^{2}/{N}_{\mathrm{dof}}$ increases to 2.88. In spite of the increase of ${\ensuremath{\chi}}^{2}$, the agreement of model calculations with the new data is improved and the conclusion on the most important resonances in the process remains the same. An extensive comparison between the result of model calculations and experimental data on differential cross section, single polarization observables $P, T$, and $\mathrm{\ensuremath{\Sigma}}$, as well as double polarization observables ${C}_{x}, {C}_{z}, {O}_{{x}^{\ensuremath{'}}}, {O}_{{z}^{\ensuremath{'}}}, {O}_{x}$, and ${O}_{z}$, is presented in this paper.

Polarized Solid State Target

The polarized solid state target is an indispensable experimental tool to study single and double polarization observables at low intensity particle beams like tagged photons. It was one of the major components of the Crystal-Barrel experiment at ELSA. Besides the operation of the ’CB frozen spin target’ within the experimental program of the Crystal-Barrel collaboration both collaborative groups of the D1 project, the polarized target group of the Ruhr Universitat Bochum and the Bonn polarized target group, have made significant developments in the field of polarized targets within the CRC16. The Bonn polarized target group has focused its work on the development of technically challenging polarized solid target systems towards the so called ’4π continuous mode polarized target’ to operate them in combination with 4π -particle detection systems. In parallel, the Bochum group has developed various highly polarized deuterated target materials and high precision NMR-systems, in the meantime used for polarization experiments at CERN, JLAB and MAMI, too.

Recent results on doubly-polarised pion photoproduction and the GDH sum rule on the nucleon at MAMI

New measurements of the helicity dependence of the total inclusive photoabsorption cross section and of the partial cross sections for several reaction channels on the proton and on the neutron were carried out at MAMI (Mainz) in the energy region 200 He targets.These new doubly-polarised pion-photoproduction data sets give new input to the partial wave analyses and allow to constrain the multipole solution of the different analyses; they also allow to study the double-polarisation observables G and E. Furthermore, the results obtained give information on the well-known Gerasimov-Drell-Hearn sum rule.