Reaction Γp Research Articles

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

Measurement of the cross-section ratio σψ(2S)/σJ/ψ(1S) in exclusive photoproduction at HERA

The exclusive photoproduction reactions γp → J/ψ(1S)p and γp → ψ(2S)p have been measured at an ep centre-of-mass energy of 318 GeV with the ZEUS detector at HERA using an integrated luminosity of 373 pb−1. The measurement was made in the kinematic range 30 < W < 180 GeV, Q2< 1 GeV2 and |t| < 1 GeV2, where W is the photon-proton centre-of-mass energy, Q2 is the photon virtuality and t is the squared four-momentum transfer at the proton vertex. The decay channels used were J/ψ(1S) → μ+μ−, ψ(2S) → μ+μ− and ψ(2S) → J/ψ(1S)π+π− with subsequent decay J/ψ(1S) → μ+μ−. The ratio of the production cross sections, R = σψ(2S)/σJ/ψ(1S), has been measured as a function of W and |t| and compared to previous data in photoproduction and deep inelastic scattering and with predictions of QCD-inspired models of exclusive vector-meson production, which are in reasonable agreement with the data.

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.

Decay of the Λ(1405) hyperon to Σ0π0 measured at GlueX

Among the light baryons, the Jπ = 1/2− Λ(1405) hyperon is an important special case as it sits just below the N K̅ threshold and decays almost exclusively to Σπ. Some long-standing hypotheses for Λ(1405) are that it could be a N K̅ bound state or a Σπ continuum resonance. It may also be considered as a simple quark-model resonance, the P-wave companion of the Λ(1520). In recent years chiral unitary models have suggested that there are two isospin zero poles present in this mass region, and that the “line shape" of the Λ(1405) depends to what extent each of the two poles are stimulated in a given reaction. Below the N K̅ threshold, the Λ(1405) decays to the three Σπ charge combinations. The Σ0π0 mode is purely I = 0 and it is uncontaminated by complications arising from I = 1 scattering processes contributing to the reaction mechanism in the Σ+π− and Σ−π+ decays. It is also not affected from production and decay of the nearby Σ0(1385) hyperon. The GlueX experiment at Jefferson Lab has been used to study the Λ(1405) → Σ0π0 decay mode. We focus on the preliminary results of dσ/dMΣ0π0 and fits to the line shape of the Λ(1405) region in the −(t − tmin) range 0 - 1.5 GeV2 from analyzing the reaction γp → K+Λ* using the data collected during the first phase of the GlueX program.

Beam Asymmetries from Light Scalar Meson Production on the Proton at GlueX

The GlueX facility, featuring a linearly polarised 9 GeV real photon beam delivered to a large-acceptance detector system, has recently completed its first phase of running, and analysis efforts of this dataset are well underway. It has been suggested that at GlueX energies, quark systems beyond the three quark and quark-antiquark systems of baryons and mesons, such as hybrid mesons, tetraquarks and glueballs, should exist, and studies of these systems could shed new light on how quarks combine under the strong force, particularly the role played by gluons.Meticulous study of the spectrum of hadronic states is required to understand the strong force in the non-perturbative energy regime, and the light scalar meson sector is an area that remains poorly understood. GlueX data encompasses final states at energies where photoproduction of the a0 (980) and f 0 (980) mesons can provide discriminatory evidence between various models, manifested in experimental observables such as the cross section and beam asymmetry, and performing detailed measurements of these quantities is considered a priority of the ongoing research program.The work presented showcases efforts to measure the beam asymmetry of the reaction γp→pηπ whose mass spectrum encompasses several mesons, including the a 0(980) light scalar, and the a 2(1320) tensor. Future prospects for related analyses in the light scalar meson sector, informed by this measurement, will also be discussed.

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 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

η’ beam asymmetry at threshold using the BGO-OD experiment

The unexpected nodal structure of the beam asymmetry recently reported by the GRAAL collaboration in η′ photoproduction very close to threshold could be explained by a previously unobserved narrow resonance. The BGO-OD experiment is ideally suited to verify this measurement via the detection of forward going charged particles which in the threshold region of interest allows the identification of the reaction γp → η′ p solely based on the proton going in the forward direction. This yields unprecedented statistics if, in the missing mass analysis of the η′ meson, the background can be sufficiently well controlled. Preliminary results using a linearly polarised photon beam are shown. The reaction γp → η′ p was identified in the BGO forward spectrometer, with simulated data used to seperate signal and background.

First measurement of Ξ− polarization in photoproduction

Despite decades of studies of the photoproduction of hyperons, both their production mechanisms and their spectra of excited states are still largely unknown. While the parity-violating weak decay of hyperons offers a means of measuring their polarization, which could help discern their production mechanisms and identify their excitation spectra, no such study has been possible for doubly strange baryons in photoproduction, due to low production cross sections. However, by making use of the reaction γp→K+K+Ξ−, we have measured, for the first time, the induced polarization, P, and the transferred polarization from circularly polarized real photons, characterized by Cx and Cz, to recoiling Ξ−s. The data were obtained using the CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Lab for photon energies from just over threshold (2.4 GeV) to 5.45 GeV. These first-time measurements are compared, and are shown to broadly agree, with model predictions in which cascade photoproduction proceeds through the decay of intermediate hyperon resonances that are produced via relativistic meson exchange, offering a new step forward in the understanding of the production and polarization of doubly-strange baryons.

Effective and Definite Resonances in the Photoproduction of η′ Mesons on Protons

A qualitative description of the most accurate experimental differential cross sections of the reaction γp→η′p from the CLAS 2009 Collaboration and unprecedentedly accurate total cross sections of this reaction, measured by the A1 Mainz 2015 Collaboration in the isobaric model of photoproduction has been obtained. Masses and widths of four resonances, extracted from such fits, lay claim to a real existence and lie in the usual mass and width intervals of nucleon resonances. But in order for our model not to contradict the results of measurements of the Σ-asymmetry of this reaction made by the GRAAL 2013 Collaboration, the need arose for an additional resonance with mass equal to the threshold of the reaction under discussion, and width Γ = 110 MeV.

Differential cross sections and polarization observables from CLAS K⁎ photoproduction and the search for new N⁎ states

The reaction γp→K⁎+Λ was measured using the CLAS detector for photon energies between the threshold and 3.9 GeV at the Thomas Jefferson National Accelerator Facility. For the first time, spin-density matrix elements have been extracted for this reaction. Differential cross sections, spin density matrix elements, and the Λ recoil polarization are compared with theoretical predictions using the BnGa partial wave analysis. The main result is the evidence for significant contributions from N(1895)1/2− and N(2100)1/2+ to the reaction. Branching ratios for decays into K⁎Λ for these resonances and further resonances are reported.

Investigating convergence of the reaction γp → π±Δ and tensor meson a2 exchange at high energy

A Regge approach to the reaction processes γp→π−Δ++ and γp→π+Δ0 is presented for the description of existing data up to Eγ=16 GeV. The model consists of the t-channel π(139)+ρ(775)+a2(1320) exchanges which are reggeized from the relevant Born amplitude. Discussion is given on the minimal gauge prescription for the π exchange to render convergent the divergence of the u-channel Δ-pole in the former process. A new Lagrangian is constructed for the a2NΔ coupling in this work and applied to the process for the first time with the coupling constant deduced from the duality plus vector dominance. It is shown that, while the π exchange dominates over the process, the role of the a2 exchange is crucial rather than the ρ in reproducing the cross sections for total, differential, and photon polarization asymmetry to agree with data at high energy.

Threshold [formula omitted] photoproduction on transverse polarised protons at MAMI

Polarisation-dependent differential cross sections σT associated with the target asymmetry T have been measured for the reaction γp→→pπ0 with transverse target polarisation from π0 threshold to photon energies of 190 MeV. The data were obtained using a frozen-spin butanol target with the Crystal Ball / TAPS detector set-up and the Glasgow photon tagging system at the Mainz Microtron MAMI. Results for σT have been used in combination with our previous measurements of the unpolarised cross section σ0 and the beam asymmetry Σ for a model-independent determination of S- and P-wave multipoles in the π0 threshold region, which includes for the first time a direct determination of the imaginary part of the E0+ multipole.

Photoproduction of ω mesons off the proton

The differential cross sections and unpolarized spin-density matrix elements for the reaction γp→pω were measured using the CBELSA/TAPS experiment for initial photon energies ranging from the reaction threshold to 2.5 GeV. These observables were measured from the radiative decay of the ω meson, ω→π0γ. The cross sections cover the full angular range and show the full extent of the t-channel forward rise. The overall shape of the angular distributions in the differential cross sections and unpolarized spin-density matrix elements are in fair agreement with previous data. In addition, for the first time, a beam of linearly-polarized tagged photons in the energy range from 1150 MeV to 1650 MeV was used to extract polarized spin-density matrix elements.These data were included in the Bonn–Gatchina partial wave analysis (PWA). The dominant contribution to ω photoproduction near threshold was found to be the 3/2+ partial wave, which is primarily due to the sub-threshold N(1720)3/2+ resonance. At higher energies, pomeron-exchange was found to dominate whereas π-exchange remained small. These t-channel contributions as well as further contributions from nucleon resonances were necessary to describe the entire dataset: the 1/2−, 3/2−, and 5/2+ partial waves were also found to contribute significantly.

High-statistics study of the reaction γp → p2π0

The photoproduction of 2$\pi^0$ mesons off protons was studied with the Crystal Barrel/TAPS experiment at the electron accelerator ELSA in Bonn. The energy of photons produced in a radiator was tagged in the energy range from 600\,MeV to 2.5\,GeV. Differential and total cross sections and $p\pi^0\pi^0$ Dalitz plots are presented. Part of the data was taken with a diamond radiator producing linearly polarized photons, and beam asymmetries were derived. Properties of nucleon and $\Delta$ resonances contributing to the $p\pi^0\pi^0$ final state were determined within the BnGa partial wave analysis. The data presented here allow us to determine branching ratios of nucleon and $\Delta$ resonances for their decays into $p\pi^0\pi^0$ via several intermediate states. Most prominent are decays proceeding via $\Delta(1232)\pi$, $N(1440)1/2^+\pi$, $N(1520)3/2^-\pi$, $N(1680)5/2^+\pi$, but also $pf_0(500)$, $pf_0(980)$, and $pf_2(1270)$ contribute to the reaction.

The polarization observables T, P, and H and their impact on [formula omitted] multipoles

Data on the polarization observables T, P, and H for the reaction γp→pπ0 are reported. Compared to earlier data from other experiments, our data are more precise and extend the covered range in energy and angle substantially. The results were extracted from azimuthal asymmetries measured using a transversely polarized target and linearly polarized photons. The data were taken at the Bonn electron stretcher accelerator ELSA with the CBELSA/TAPS detector. Within the Bonn-Gatchina partial wave analysis, the new polarization data lead to a significant narrowing of the error band for the multipoles for neutral-pion photoproduction.

The isospin structure of photoproduction of πη pairs from the nucleon in the threshold region

Photoproduction of πη-pairs from nucleons has been investigated from threshold up to incident photon energies of ≈ 1.4 GeV. The quasi-free reactions γp→pπ0η, γn→nπ0η, γp→nπ+η, and γn→pπ−η were for the first time measured from nucleons bound in the deuteron. The corresponding reactions from a free-proton target were also studied to investigate final-state interaction effects (for neutral pions the free-proton results could be compared to previous measurements; the γp→nπ+η reaction was measured for the first time). For the π0η final state coherent production via the γd→dπ0η reaction was also investigated. The experiments were performed at the tagged photon beam of the Mainz MAMI accelerator using an almost 4π coverage electromagnetic calorimeter composed of the Crystal Ball and TAPS detectors. The total cross sections for the four different final states obey the relation σ(pπ0η)≈σ(nπ0η)≈2σ(pπ−η)≈2σ(nπ+η) as expected for a dominant contribution from a Δ⋆→ηΔ(1232)→πηN reaction chain, which is also supported by the shapes of the invariant-mass distributions of nucleon–meson and π–η pairs. The experimental results are compared to the predictions from an isobar reaction model.

Measurement of polarisation observables in [formula omitted] photoproduction off the proton

The reaction γp→KS0Σ+ is studied in the photon energy range from threshold. Linearly polarised photon beams from coherent bremsstrahlung enabled the first measurement of photon beam asymmetries in this reaction up to Eγ=1650MeV. In addition, the recoil hyperon polarisation was determined through the asymmetry in the weak decay Σ+→pπ0 up to Eγ=2250MeV. The data are compared to partial wave analyses, and the possible impact on the interpretation of a recently observed prominent structure in the cross section near the K⁎ thresholds is discussed.

N(1520) 3/2(-) helicity amplitudes from an energy-independent multipole analysis based on new polarization data on photoproduction of neutral pions.

New data on the polarization observables T, P, and H for the reaction γp→pπ(0) are reported. The results are extracted from azimuthal asymmetries when a transversely polarized butanol target and a linearly polarized photon beam are used. The data were taken at the Bonn electron stretcher accelerator ELSA using the CBELSA/TAPS detector. These and earlier data are used to perform a truncated energy-independent partial wave analysis in sliced-energy bins. This energy-independent analysis is compared to the results from energy-dependent partial wave analyses.

Energy-independent PWA of the reaction γp → K+Λ

Using all recent data on the differential cross sections and spin observables for the reaction γp → K + Λ, an energy-independent partial-wave analysis is performed for center-of-mass energies from 1.462 to 1.662 GeV. The analysis requires multipoles up to L = 2; there is no evidence that the fit requires multipoles with L = 3. At present the available data allow us to extract the dominant multipoles only. These are compatible with the multipoles obtained in the energy-dependent fit. This result supports the reliability of the Bonn-Gatchina energy-dependent results.