Photon Beam Asymmetry Research Articles

Photoproduction of γp→f0(980)p in an effective Lagrangian approach

The most recent data on differential cross sections and photon beam asymmetries Σ from the LEPS2/BGOegg Collaboration for the γp→f0(980)p reaction are analyzed within a tree-level effective Lagrangian approach. The t-channel ρ and ω exchanges, the u- and s-channel N exchanges, the interaction current, and the possible s-channel N* exchanges are taken into account in constructing the reaction amplitudes to reproduce the data. The results show that the contributions from either the N(2040)3/2+ or the N(2100)1/2+ resonance exchange in the s channel are necessarily required to describe the LEPS2/BGOegg data and they dominate the differential cross sections of γp→f0(980)p. Further analysis shows that the contributions from the t-channel ρ and ω exchanges and the interaction current are rather small to both differential cross sections and photo beam asymmetries, and the contributions from u-channel N exchange are considerable in the case of including N(2040)3/2+ in the model while negligible in the case of including N(2100)1/2+ in the model. Predictions of target asymmetries T for γp→f0(980)p are given, which can be examined by future experiments. Published by the American Physical Society 2024

Study of [formula omitted] reactions with polarized photons at energy 40-65 MeV

The photon beam asymmetry of the C12(γ→,p01)11B and C12(γ→,p2−6)11B reactions has been measured in the energy range 40–65 MeV, using a tagged, linearly-polarized photon beam at the MAX-lab facility in Sweden. The asymmetry of the C12(γ→,p01)11B reaction to ground and first excited state of B11 is Σ≈0.82 over the measured energy range. The main contribution to the C12(γ→,p2−6)11B reaction to higher excited states comes from processes in which the residual nucleus is in the 3/2−(5.02MeV) or 7/2−(6.74MeV) excited states. The asymmetry of this reaction is Σ≈0.6, close to the value for free deuteron photodisintegration.

First measurement of differential cross sections and photon beam asymmetries for photoproduction of the f0(980) meson decaying into π0π0 at Eγ<2.4 GeV

For the first time, differential cross sections and photon beam asymmetries were measured for the reaction γp→f0(980)p→π0π0p by using a linearly polarized photon beam up to 2.4GeV and a liquid hydrogen target. The f0(980) photoproduction signal was clearly observed owing to the absence of contributions from vector-meson photoproduction. The results indicate the t-channel exchange of vector mesons as a mechanism of the f0(980) photoproduction at Eγ≈2.1GeV, putting constraints on the nature of f0(980).Received 17 January 2023Accepted 28 March 2023DOI:https://doi.org/10.1103/PhysRevC.107.L042201©2023 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasPhotonuclear reactionsPolarization phenomenaQuark modelPhysical SystemsHadronsMesonsPropertiesParticle propertiesTechniquesCalorimetersNuclear PhysicsParticles & Fields

Differential cross sections and photon beam asymmetries of η photoproduction on the proton at Eγ=1.3–2.4 GeV

We have carried out exclusive measurements for the photoproduction of an $\eta$ meson from a proton target with an egg-shaped calorimeter made of BGO crystals (BGOegg) and forward charged-particle detectors at the SPring-8 LEPS2 beamline. The differential cross sections and photon beam asymmetries of the $\gamma p \to \eta p$ reaction are measured in a center-of-mass energy ($W$) range of $1.82$-$2.32$ GeV and a polar angle range of $-1.0 < \cos{\theta^{\eta}_{\mathrm{c.m.}}} < 0.6$. The reaction is identified by selecting a proton and two $\gamma$'s produced by an $\eta$-meson decay. The kinematic fit method was employed to select the reaction candidate with the confidence level larger than $1$\%. A bump structure at $W$ = $2.0$-$2.3$ GeV in the differential cross section is confirmed at extremely backward $\eta$ polar angles, where the existing data are inconsistent with each other. This bump structure is likely associated with high-spin resonances that couple with $s\bar{s}$ quarks. The results of the photon beam asymmetries in a wide $\eta$ polar angle range are new for the photon beam energies exceeding $2.1$ GeV. These results are not reproduced by the existing partial wave analyses. They provide an additional constraint to nucleon resonance studies at high energies.

Measurement of Compton Scattering at MAMI for the Extraction of the Electric and Magnetic Polarizabilities of the Proton

A precise measurement of the differential cross sections dσ/dΩ and the linearly polarized photon beam asymmetry Σ_{3} for Compton scattering on the proton below pion threshold has been performed with a tagged photon beam and almost 4π detector at the Mainz Microtron. The incident photons were produced by the recently upgraded Glasgow-Mainz photon tagging facility and impinged on a cryogenic liquid hydrogen target, with the scattered photons detected in the Crystal Ball/TAPS setup. Using the highest statistics Compton scattering data ever measured on the proton along with two effective field theories (both covariant baryon and heavy-baryon) and one fixed-t dispersion relation model, constraining the fits with the Baldin sum rule, we have obtained the proton electric and magnetic polarizabilities with unprecedented precision: α_{E1}=10.99±0.16±0.47±0.17±0.34, β_{M1}=3.14±0.21±0.24±0.20±0.35; in units of 10^{-4} fm^{3} where the errors are statistical, systematic, spin polarizability dependent, and model dependent.

K+Σ− photoproduction within an isobar model

We used an isobar model to study the K+Σ− photoproduction reaction on a neutron target with focus on the resonance region. In order to achieve a reasonable agreement with the data, we included spin-3/2 and spin-5/2 nucleon resonances in a consistent formalism together with a Δ resonance and two kaon resonances on top of the Born terms. The free parameters of the model were adjusted to the data from the CLAS and LEPS Collaborations on differential cross sections and photon beam asymmetry. The cornerstone of this analysis was an upgrade of the fitting method. Previously, we used only the plain χ2 minimization, which could not prevent us from overfitting the data. We, therefore, introduced a regularization method, the least absolute selection shrinkage operator (LASSO), which, together with information criteria, restricts the number of nonzero parameters and prevents us from overfitting the data. In our analysis, we arrived at two models, fit M, whose parameters were fitted with the MINUIT code only, and fit L, where we used the more advanced LASSO. Both models describe the data in a similar way and we observe only slight differences in the dσ/dΩ data description at very forward angles where the fit M is flat whereas the fit L produces two broad peaks, and in the photon beam asymmetries above 2 GeV at backward kaon angles where the fit M produces a bump. Surprisingly, no hyperon resonances are needed for the correct data description in these models. On the other hand, the N(1720)3/2+ nucleon resonance was found to be very important in both models.

Model selection for K+Σ− photoproduction within an isobar model

We utilize an isobar model to investigate the ${K}^{+}{\mathrm{\ensuremath{\Sigma}}}^{\ensuremath{-}}$ photoproduction off a neutron in the resonance region. Except for the Born terms, we include high-spin (spin-3/2 and spin-5/2) nucleon resonances in the consistent formalism together with a few $\mathrm{\ensuremath{\Delta}}$ and kaon resonances to achieve an acceptable agreement with data. Interestingly, we reveal that no hyperon resonances are needed to achieve a reasonable description of data. On the other hand the $N(1720)3/{2}^{+}$ resonance was found to be very important for correct description of data. The free parameters of the model were fitted to experimental data from the LEPS and CLAS Collaborations on either differential cross sections or photon beam asymmetry. The novel feature of the fitting procedure is the use of a regularization method, the least absolute shrinkage selection operator, and information criteria in order to choose the best fit.

Single pi ^0 production off neutrons bound in deuteron with linearly polarized photons

The quasifree overrightarrow{gamma } drightarrow pi ^0n(p) photon beam asymmetry, varSigma , has been measured at photon energies, E_gamma , from 390 to 610 MeV, corresponding to center of mass energy from 1.271 to 1.424 GeV, for the first time. The data were collected in the A2 hall of the MAMI electron beam facility with the Crystal Ball and TAPS calorimeters covering pion center-of-mass angles from 49^circ to 148^circ . In this kinematic region, polarization observables are sensitive to contributions from the varDelta (1232) and N(1440) resonances. The extracted values of varSigma have been compared to predictions based on partial-wave analyses (PWAs) of the existing pion photoproduction database. Our comparison includes the SAID, MAID and Bonn–Gatchina analyses; while a revised SAID fit, including the new varSigma measurements, has also been performed. In addition, isospin symmetry is examined as a way to predict pi ^0n photoproduction observables, based on fits to published data in the channels pi ^0p, pi ^+n and pi ^-p.

Analysis of the differential cross section and photon beam asymmetry data for γp→η′p

The photoproduction reaction of $\gamma p \to \eta^\prime p$ is investigated based on an effective Lagrangian approach in the tree-level approximation, with the purpose being to understand the reaction mechanisms and to extract the resonance contents and the associated resonance parameters in this reaction. Apart from the $t$-channel $\rho$ and $\omega$ exchanges, $s$- and $u$-channel nucleon exchanges, and generalized contact term, the exchanges of a minimum number of nucleon resonances in the $s$ channel are taken into account in constructing the reaction amplitudes to describe the experimental data. It is found that a satisfactory description of the available data on both differential cross sections and photon beam asymmetries can be obtained by including in the $s$ channel the exchanges of the $N(1875)3/2^-$ and $N(2040)3/2^+$ resonances. The reaction mechanisms of $\gamma p \to \eta^\prime p$ are discussed and a prediction for the target nucleon asymmetries is presented.

Photoproduction γp→K+Λ(1520) in an effective Lagrangian approach

The data on differential cross sections and photon-beam asymmetries for the $\gamma p \to K^+\Lambda(1520)$ reaction have been analyzed within a tree-level effective Lagrangian approach. In addition to the $t$-channel $K$ and $K^\ast$ exchanges, the $u$-channel $\Lambda$ exchange, the $s$-channel nucleon exchange, and the interaction current, a minimal number of nucleon resonances in the $s$ channel are introduced in constructing the reaction amplitudes to describe the data. The results show that the experimental data can be well reproduced by including either the $N(2060)5/2^-$ or the $N(2120)3/2^-$ resonance. In both cases, the contact term and the $K$ exchange are found to make significant contributions, while the contributions from the $K^\ast$ and $\Lambda$ exchanges are negligible in the former case and considerable in the latter case. Measurements of the data on target asymmetries are called on to further pin down the resonance contents and to clarify the roles of the $K^\ast$ and $\Lambda$ exchanges in this reaction.

Photoproduction of the Λ(1520) hyperon with a 9 GeV photon beam at GlueX

The GlueX experiment is located at the Thomas Jefferson National Accelerator Facility (JLab) in Newport News, VA, USA. It features a hermetic 4π detector with excellent tracking and calorimetry capabilities. Its 9 GeV linearly polarized photon beam is produced from the 12 GeV electron beam, delivered by JLab’s Continuous Electron Beam Accelerator Facility (CEBAF), via bremsstrahlung on a thin diamond and is incident on a LH2 target. GlueX recently finished its first data taking period and published first results.The main goal of GlueX is to measure gluonic excitations of mesons. These so-called hybrid or exotic mesons are predicted by Quantum Chromodynamics (QCD) but haven’t been experimentally confirmed yet. They can have quantum numbers not accessible by ordinary quark-antiquark pairs which helps in identifying them using partial wave analysis techniques. The search for exotic mesons requires a very good understanding of photoproduction processes in a wide range of final states, one of them being pK + K − which contains many meson and baryon reactions. The Λ(1520) is a prominent hyperon resonance in this final state and is the subject of this presentation.This talk will give an introduction to the GlueX experiment and show preliminary results for the photoproduction of the Λ(1520) hyperon. The measurement of important observables like the photon beam asymmetry and spin-density matrix elements will be discussed and an outlook to possible measurements of further hyperon states in the pK + K − final state will be given.

Differential cross sections, photon beam asymmetries, and spin density matrix elements of ω photoproduction off the proton at Eγ=1.3−2.4 GeV

For the reaction γp→ωp, differential cross sections, photon beam asymmetries, and spin density matrix elements have been measured in the BGOegg experiment at the SPring-8 LEPS2 beamline by using a large acceptance electromagnetic calorimeter. A 1.3–2.4 GeV photon beam with high linear polarization has enabled a precision measurement of photon beam asymmetries Σ and polarized spin density matrix elements ρ1−11 over a wide angular range, especially above the total energy of about 2 GeV for the first time. The measured ρ1−11 indicates the dominance of s-channel resonances even at higher energies. The Σ values at backward angles are not reproduced by the existing partial-wave analysis in the highest-energy region, providing new constraints to the contribution of nucleon resonances with high spins.Received 27 March 2020Accepted 2 July 2020DOI:https://doi.org/10.1103/PhysRevC.102.025201©2020 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasPhotonuclear reactionsQuark modelPhysical SystemsHadronsNuclear Physics

Measurement of the photon beam asymmetry in γ⃗p→K+Σ0 at Eγ=8.5 GeV

We report measurements of the photon beam asymmetry $\Sigma$ for the reaction $\vec{\gamma} p\to K^+\Sigma^0$(1193) using the GlueX spectrometer in Hall D at Jefferson Lab. Data were collected using a linearly polarized photon beam in the energy range of 8.2-8.8 GeV incident on a liquid hydrogen target. The beam asymmetry $\Sigma$ was measured as a function of the Mandelstam variable $t$, and a single value of $\Sigma$ was extracted for events produced in the $u$-channel. These are the first exclusive measurements of the photon beam asymmetry $\Sigma$ for the reaction in this energy range. For the $t$-channel, the measured beam asymmetry is close to unity over the $t$-range studied, $-t=(0.1-1.4)~$(GeV/$c$)$^{2}$, with an average value of $\Sigma = 1.00\pm 0.05$. This agrees with theoretical models that describe the reaction via the natural-parity exchange of the $K^{*}$(892) Regge trajectory. A value of $\Sigma = 0.41 \pm 0.09$ is obtained for the $u$-channel integrated up to $-u=2.0$~(GeV/$c$)$^{2}$.

Recent results of charged pion and kaon photoproduction on the proton at SPring-8/LEPS

We carry out photoproduction experiments by using linearly polarized photon beams with energies of 1.5-3 GeV at SPring-8/LEPS. One of our physics motivations is to understand reaction mechanisms of hadron photoproduction. We study (1) y→p→ π-Δ++, (2) y→p→ π+n, (3) y→p→ K+Λ(K+Σ0), and (4) y→p→ π+Δ0 reactions which produce (1) uu, (2) dd, (3) ss, and (4) dd quark-antiquark pairs in the final state. Differential cross sections and photon beam asymmetries have been measured at forward meson angles of 0.6 &lt; cos θcm &lt;1. Precise comparison between the π-Δ++ and π+Δ0 reactions plays an important role in the study of nucleon resonances. It enables to distinguish between N* and Δ* by taking the cross section ratios of the two reactions. We have measured preliminary cross section ratios dσ(π+Δ0)/dσ(π-Δ++). The ratios are found to be close to 1/3, which is expected from the exchange of isospin=1 in the t channel, for 0.966&lt; cos θcm &lt;1 and all the photon energies. The ratios become larger than 1/3 and close to 1 for larger angles and higher energy regions. The larger ratios might be due to A resonances, giving the ratio 4/3, excited in the intermediate state

Photoproduction of $${\varvec{\pi }}^{{\varvec{-}}}{\varvec{\varDelta }}^{{\varvec{++}}}$$ π - Δ + + , $${\varvec{\pi }}^{{\varvec{+}}}{\varvec{\varDelta }}^\mathbf{0 }$$ π + Δ 0 , and $${\varvec{\pi }}^{{\varvec{+}}}{} \mathbf{n}$$ π + n on the Proton at $${\varvec{E}}_{{\varvec{\gamma }}}=\mathbf{1.5}$$ E γ = 1.5 –3.0 GeV

We carry out photoproduction experiments by using linearly polarized photon beams with energies of 1.5–3.0 GeV at SPring-8/LEPS. There is no distinct peak structure due to nucleon or $$\varDelta $$ resonances in the preliminary cross sections for the $$\gamma p$$ $$\rightarrow $$ $$\pi ^{-}\varDelta ^{++}$$ , $$\pi ^{+}\varDelta ^{0}$$ , and $$\pi ^{+}n$$ reactions for 0.7 $$<\cos \theta _{\pi }<$$ 1. The cross section ratios $$\sigma $$ ( $$\pi ^{+}\varDelta ^{0}$$ )/ $$\sigma $$ ( $$\pi ^{-}\varDelta ^{++}$$ ) are close to 1/3, which is expected from the isospin = 1 exchange in the t-channel, at small $$\pi $$ angles and become larger than 1/3 at large $$\pi $$ angles. The photon beam asymmetries are small negative values, small positive values, and large positive values for the $$\pi ^{-}\varDelta ^{++}$$ , $$\pi ^{+}\varDelta ^{0}$$ , and $$\pi ^{+}n$$ reactions, respectively. The asymmetries are sensitive to reaction mechanisms and are found to be largely different among these reactions. The asymmetries are strong constraint in theoretical studies and expected to play an important role in searching for new nucleon resonances.

Differential Cross Section and Photon-Beam Asymmetry for the γ[over →]p → π^{-}Δ^{++}(1232) Reaction at Forward π^{-} Angles for E_{γ}=1.5-2.95 GeV.

Differential cross sections and photon-beam asymmetries for the γ[over →]p→π^{-}Δ^{++}(1232) reaction have been measured for 0.7<cosθ_{π}^{c.m.}<1 and E_{γ}=1.5-2.95 GeV at SPring-8/LEPS. The first-ever high statistics cross-section data are obtained in this kinematical region, and the asymmetry data for 1.5<E_{γ}(GeV)<2.8 are obtained for the first time. This reaction has a unique feature for studying the production mechanisms of a pure uu[over ¯] quark pair in the final state from the proton. Although there is no distinct peak structure in the cross sections, a non-negligible excess over the theoretical predictions is observed at E_{γ}=1.5-1.8 GeV. The asymmetries are found to be negative in most of the present kinematical regions, suggesting the dominance of π exchange in the t channel. The negative asymmetries at forward meson production angles are different from the asymmetries previously measured for the photoproduction reactions producing a dd[over ¯] or an ss[over ¯] quark pair in the final state. Advanced theoretical models introducing nucleon resonances and additional unnatural-parity exchanges are needed to reproduce the present data.

Measurement of the beam asymmetry Σ and the target asymmetry T in the photoproduction of ω mesons off the proton using CLAS at Jefferson Laboratory

The photoproduction of $\omega$ mesons off the proton has been studied in the reaction $\gamma p\to p\,\omega$ using the CEBAF Large Acceptance Spectrometer (CLAS) and the frozen-spin target (FROST) in Hall B at the Thomas Jefferson National Accelerator Facility. For the first time, the target asymmetry, $T$, has been measured in photoproduction from the decay $\omega\to\pi^+\pi^-\pi^0$, using a transversely-polarized target with energies ranging from just above the reaction threshold up to 2.8 GeV. Significant non-zero values are observed for these asymmetries, reaching about 30-40% in the third-resonance region. New measurements for the photon-beam asymmetry, $\Sigma$, are also presented, which agree well with previous CLAS results and extend the world database up to 2.1 GeV. These data and additional $\omega$-photoproduction observables from CLAS were included in a partial-wave analysis within the Bonn-Gatchina framework. Significant contributions from $s$-channel resonance production were found in addition to $t$-channel exchange processes.

Features of πΔ photoproduction at high energies

Hybrid/exotic meson spectroscopy searches at Jefferson Lab require the accurate theoretical description of the production mechanism in peripheral photoproduction. We develop a model for πΔ photoproduction at high energies (5≤Elab≤16 GeV) that incorporates both the absorbed pion and natural-parity cut contributions. We fit the available observables, providing a good description of the energy and angular dependencies of the experimental data. We also provide predictions for the photon beam asymmetry of charged pions at Elab=9 GeV which is expected to be measured by GlueX and CLAS12 experiments in the near future.

Photoproduction of Λ and Σ0 hyperons off protons with linearly polarized photons at Eγ=1.5–3.0 GeV

We report the measurement of the $\gamma p \rightarrow K^{+}\Lambda$ and $\gamma p \rightarrow K^{+}\Sigma^{0}$ reactions at SPring-8. The differential cross sections and photon-beam asymmetries are measured at forward $K^{+}$ production angles using linearly polarized tagged-photon beams in the range of $E_{\gamma}=1.5$--3.0 GeV. With increasing photon energy, the cross sections for both $\gamma p \rightarrow K^{+}\Lambda$ and $\gamma p \rightarrow K^{+}\Sigma^{0}$ reactions decrease slowly. Distinct narrow structures in the production cross section have not been found at $E_{\gamma}=1.5$--3.0 GeV. The forward peaking in the angular distributions of cross sections, a characteristic feature of $t$-channel exchange, is observed for the production of $\Lambda$ in the whole observed energy range. A lack of similar feature for $\Sigma^{0}$ production reflects a less dominant role of $t$-channel contribution in this channel. The photon-beam asymmetries remain positive for both reactions, suggesting the dominance of $K^{*}$ exchange in the $t$ channel. These asymmetries increase gradually with the photon energy, and have a maximum value of +0.6 for both reactions. Comparison with theoretical predictions based on the Regge trajectory in the $t$ channel and the contributions of nucleon resonances indicates the major role of $t$-channel contributions as well as non-negligible effects of nucleon resonances in accounting for the reaction mechanism of hyperon photoproduction in this photon energy regime.

Differential cross section and photon-beam asymmetry for the γ⃗p→π+n reaction at forward π+ angles at Eγ=1.5 –2.95 GeV

Differential cross sections and photon beam asymmetries for the gamma p -> pi+ n reaction have been measured for 0.6<cos(theta)<1 and Egamma=1.5-2.95 GeV at SPring-8/LEPS. The cross sections monotonically decrease as the photon beam energy increases for 0.6<cos(theta)<0.9. However, the energy dependence of the cross sections for 0.9<cos(theta)<1 and Egamma=1.5-2.2 GeV (W=1.9-2.2 GeV) is different, which may be due to a nucleon or Delta resonance. The present cross sections agree well with the previous cross sections measured by other groups and show forward peaking, suggesting significant t-channel contributions in this kinematical region. The asymmetries are found to be positive, which can be explained by rho-exchange in the t-channel. Large positive asymmetries in the small |t| region, where the rho-exchange contribution becomes small, could be explained by introducing pi-exchange interference with the s-channel.