Neutral Pion Photoproduction Research Articles

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.

Chiral dynamics in the [formula omitted] reaction

We investigate the neutral pion photoproduction on the proton near threshold in covariant chiral perturbation theory with the explicit inclusion of Δ degrees of freedom. This channel is specially sensitive to chiral dynamics and the advent of very precise data from the Mainz microtron has shown the limits of the convergence of the chiral series for both the heavy baryon and the covariant approaches. We show that the inclusion of the Δ resonance substantially improves the convergence leading to a good agreement with data for a wider range of energies.

Coherent Photoproduction of Pairs of Neutral Pions on Nuclei

On the basis of an analysis of the available experimental data for double neutral pion photoproduction on nucleons, estimates have been obtained of the relative contribution of isoscalar and isovector transitions to the amplitude of this process. The obtained results are used to calculate the cross section of coherent photoproduction of pairs of neutral pions on a deuteron and on the three-nucleon isotope of helium. A strong sensitivity of the cross section to peculiarities of the isotopic structure of the one-nucleon amplitude is demonstrated.

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.

Neutral pion photoproduction on tensor-polarized deuterium on the VEPP-3 storage ring

An experiment to measure component T 20 of the tensor analyzing power for the coherent photo-production of neutral pions on polarized deuterons is described. The measurements cover the photon energies in the ranges E γ = 200–500 MeV and E γ = 300–600 MeV, and the emission angles of neutral pions in the ranges = \(\theta _{\pi ^0 }^{c.m.} \) = 100°–140° and = \(\theta _{\pi ^0 }^{c.m.} \) = 60°–65°, respectively. The experiment uses a hyperfine internal gas polarized target. Detectors of neutral pions and deuterons register these particles in coincidence. The target’s degree of polarization and luminosity is measured by detecting elastic electron-deuteron scattering at low momentum transfer. A selection of the preliminary results is presented.

Cosmic-ray calibration of lead scintillation monitor with internal trigger

Cosmic-ray calibration of the 4-channel lead scintillation monitor with internal trigger shows that the consideration of only coinciding signals of counters placed in the monitor depth and detecting the final part of the high-energy photon shower, with signals of any other counters at the monitor top, detecting both the initial part of the high-energy photon shower and low-energy background photons, decreases the number of detected coincidences by a factor of 2–4. An increase in the number of such coincidences when using the monitor in the setup for studying neutral pion photoproduction on nuclei in the photon beam of the labeling system of the “PAKHRA” accelerator of the Lebedev Physical Institute makes it possible to separate the high-energy electromagnetic shower from the low-energy background and to monitor the beam photon energy.

New analysis of threshold photoproduction data from MAMI

In this talk I will review the recently published results by the A2 and CB-TAPS Collaborations at MAMI on neutral pion photoproduction in the near-threshold region. The combined measurement of the differential cross section and the photon beam asymmetry with low statistical errors allowed for a precise determination of the energy dependence of the real parts of the S- and P-wave amplitudes for the first time, providing the most stringent test to date of the predictions of Chiral Perturbation Theory and its energy region of agreement with experiment.

MEASUREMENT OF THE DOUBLE POLARIZATION OBSERVABLE E IN π0 AND η PHOTOPRODUCTION OFF PROTONS WITH THE CBELSA/TAPS EXPERIMENT

Double polarization experiments using a longitudinally or transversely polarized frozen-spin-butanol target and a linearly or circularly polarized photon beam were performed with the CBELSA/TAPS experiment at the electron accelerator ELSA. With its nearly 4π angular coverage, this setup is very well suited to study neutral meson photoproduction off the nucleon up to beam energies of 3.2 GeV. Results obtained for the double polarization observable E in neutral pion and eta photoproduction show the large sensitivity of the data on the contributing resonances. If the data are compared to the predictions of state of the art partial wave analyses, large discrepancies are observed.

Upper energy limit of heavy baryon chiral perturbation theory in neutral pion photoproduction

With the availability of the new neutral pion photoproduction from the proton data from the A2 and CB-TAPS Collaborations at Mainz it is mandatory to revisit Heavy Baryon Chiral Perturbation Theory (HBChPT) and address the extraction of the partial waves as well as other issues such as the value of the low-energy constants, the energy range where the calculation provides a good agreement with the data and the impact of unitarity. We find that, within the current experimental status, HBChPT with the fitted LECs gives a good agreement with the existing neutral pion photoproduction data up to ∼170 MeV and that imposing unitarity does not improve this picture. Above this energy the data call for further improvement in the theory such as the explicit inclusion of the Δ(1232). We also find that data and multipoles can be well described up to ∼185 MeV with Taylor expansions in the partial waves up to first order in pion energy.

Well-Established Nucleon Resonances Revisited by Double-Polarization Measurements

The first measurement is reported of the double-polarization observable G in the photoproduction of neutral pions off protons, covering the photon energy range from 620 to 1120 MeV and the full solid angle. G describes the correlation between the photon polarization plane and the scattering plane for protons polarized along the direction of the incoming photon. The observable is highly sensitive to contributions from baryon resonances. The new results are compared to the predictions from SAID, MAID, and BnGa partial wave analyses. In spite of the long-lasting efforts to understand γp→pπ(0) as the simplest photoproduction reaction, surprisingly large differences between the new data and the latest predictions are observed which are traced to different contributions of the N(1535) resonance with spin parity J(P)=1/2(-) and N(1520) with J(P)=3/2(-). In the third resonance region, where N(1680) with J(P)=5/2(+) production dominates, the new data are reasonably close to the predictions.

Polarization Components inπ0Photoproduction at Photon Energies up to 5.6 GeV

We present new data for the polarization observables of the final state proton in the (1)H(γ,p)π(0) reaction. These data can be used to test predictions based on hadron helicity conservation and perturbative QCD. These data have both small statistical and systematic uncertainties and were obtained with beam energies between 1.8 and 5.6 GeV and for π(0) scattering angles larger than 75° in the center-of-mass frame. The data extend the polarization measurements database for neutral pion photoproduction up to E(γ)=5.6 GeV. The results show a nonzero induced polarization above the resonance region. The polarization transfer components vary rapidly with the photon energy and π(0) scattering angle in the center-of-mass frame. This indicates that hadron helicity conservation does not hold and that the perturbative QCD limit is still not reached in the energy regime of this experiment.

Pion photoproduction in a dynamical coupled-channels model

The charged and neutral pion photoproduction reactions are investigated in a dynamical coupled-channels approach based on the formulation of Haberzettl, Huang, and Nakayama [Phys. Rev. C 83, 065502 (2011)]. The hadronic final-state interaction is provided by the J\"ulich $\ensuremath{\pi}N$ model, which includes the channels $\ensuremath{\pi}N$ and $\ensuremath{\eta}N$ comprising stable hadrons as well as the effective $\ensuremath{\pi}\ensuremath{\pi}N$ channels $\ensuremath{\pi}\ensuremath{\Delta}$, $\ensuremath{\sigma}N$, and $\ensuremath{\rho}N$. This hadronic model has been quite successful in describing $\ensuremath{\pi}N\ensuremath{\rightarrow}\ensuremath{\pi}N$ scattering for center-of-mass energies up to $1.9$ GeV. By construction, the full pion photoproduction current satisfies the generalized Ward-Takahashi identity and thus is gauge invariant as a matter of course. The calculated differential cross sections and photon spin asymmetries up to 1.65 GeV center-of-mass energy for the reactions $\ensuremath{\gamma}p\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}n$, $\ensuremath{\gamma}p\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}p$, $\ensuremath{\gamma}n\ensuremath{\rightarrow}{\ensuremath{\pi}}^{\ensuremath{-}}p$, and $\ensuremath{\gamma}n\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}n$ are in good agreement with the experimental data.

John Riley Holt. 15 February 1918 — 6 January 2009

John Riley Holt was an experimental physicist who dedicated his working life to research in nuclear and particle physics at the University of Liverpool. He was born in 1918 in Runcorn, Cheshire, and in 1938 was awarded a first-class honours degree in physics at Liverpool University. He obtained his PhD in 1941 and became a member of Sir James Chadwick’s team working on the UK atomic weapon project. After the war he developed several new experimental techniques, which he used to make a systematic study of the deuteron stripping process with the use of the Liverpool cyclotron. After the Liverpool synchro-cyclotron became operational in 1954 he initiated a programme of precision measurements of cross-sections for proton–proton and pion–proton scattering. After the first observation of parity violation in 1957, his group completed an important experiment that observed parity violation in muon decay. When the construction of an electron synchrotron (NIN A, at the Daresbury Laboratory) was proposed, he became leader of the magnet design team. As NIN A became operational in 1966, John established a group to measure the cross-sections for the photoproduction of neutral and charged pions. The group then developed a collaboration with colleagues in other universities to measure the spin dependence of cross-sections for meson photoproduction by using a polarized photon beam and a polarized proton target. Before his retirement he contributed to the design of the experiment to determine the spin structure of the proton, performed by the European Muon Collaboration.

Meson production in hadron- and photon-induced reactions

The meson production and associated excited-baryon-analysis program by the Julich-Athens/GA-Washington/DC Collaboration is presented. The program’s analysis is based on a dynamical coupled-channels approach developed by the Collaboration [1-4], where the basic symmetries, such as the two-body unitarity, analyticity, and gauge invariance are respected. In particular, gauge invariance is enforced as dictated by the generalized Ward-Takahashi identity. In the hadronic reactions sector, the πN , ηN , KΛ , and K∑ channels are included, in addition to the effective ππN channels σN , ρN , and πΔ . Energies up to √s = 2 GeV are considered. In the photon-induced reactions sector, the neutral and charged pion photoproduction processes are considered up to √s = 1.65 GeV so far [4]. These are currently being extended to higher energies including the ηN , KΛ , and K∑ channels.

Photoproduction of neutral pions off protons

Photoproduction of neutral pions has been studied with the CBELSA/TAPS detector in the reaction $\gamma p\to p\pi^0$ for photon energies between 0.85 and 2.50 GeV. The $\pi^0$ mesons are observed in their dominant neutral decay mode: $\pi^0\to\gamma\gamma$. For the first time, the differential cross sections cover the very forward region, $\theta_{\rm c.m.}<60^\circ$. A partial-wave analysis of these data within the Bonn-Gatchina framework observes the high-mass resonances $G_{17}$(2190), $D_{13}$(2080), and $D_{15}$(2070).

Neutral pion production in the threshold region

We give an overview of the physics motivation and evolution of the neutral pion photoproduction measurements in the threshold region conducted in the A2 collaboration at MAMI. The latest two experiments have been performed with the almost 4π Crystal Ball detector. The first was with a linearly polarized photon beam and unpolarized liquid-hydrogen target. The data analysis is now complete and the linearly polarized beam asymmetry along with differential cross sections provide the most stringent test to date of the predictions of Chiral Perturbation Theory and its energy region of convergence. More recently a measurement was performed using both circularly polarized photons and a transversely polarized butanol frozen-spin target, with the goal of extracting both the target and beam-target asymmetries. From these we intend to extract πN scattering sensitive information for the first time in photo-pion reactions. This will be used to test isospin conservation and further test dynamics of chiral symmetry breaking in QCD as calculated at low energies by Chiral Perturbation Theory.

Neutral pion photoproduction off 3H and 3He in chiral perturbation theory

We calculate electromagnetic neutral pion production off three-nucleon bound states (3H, 3He) at threshold to leading one-loop order in the framework of chiral nuclear effective field theory. In addition, we analyze the dependence of the nuclear S-wave amplitude E0+ on the elementary neutron amplitude E0+π0n which in the case of 3He provides a stringent test of the prediction based on chiral perturbation theory. Uncertainties from higher order corrections are estimated.

Measurement of the beam asymmetryΣin the forward direction forγ⃗p→pπ0

Photoproduction of neutral pions has been studied with the CBELSA/TAPS detector for photon energies between 0.92 and 1.68~GeV at the electron accelerator ELSA. The beam asymmetry~$\Sigma$ has been extracted for $115^\circ < \theta_{\rm c.m.} < 155^\circ$ of the $\pi^0$~meson and for $\theta_{\rm c.m.} < 60^\circ$. The new beam asymmetry data improve the world database for photon energies above 1.5~GeV and, by covering the very forward region, extend previously published data for the same reaction by our collaboration. The angular dependence of $\Sigma$ shows overall good agreement with the SAID parameterization.

Unexpected impact ofDwaves in low-energy neutral pion photoproduction from the proton and the extraction of multipoles

Contributions of $D$ waves to physical observables for neutral pion photoproduction from the proton in the near-threshold region are studied and means to isolate them are proposed. Various approaches to describe the multipoles are employed---a phenomenological one, a unitary one, and heavy baryon chiral perturbation theory. The results of these approaches are compared and found to yield essentially the same answers. $D$ waves are seen to enter together with $S$ waves in a way that any means which attempt to obtain the ${E}_{0+}$ multipole accurately must rely on knowledge of $D$ waves and that consequently the latter cannot be dismissed in analyses of low-energy pion photoproduction. It is shown that $D$ waves have a significant impact on double-polarization observables that can be measured. This importance of $D$ waves is due to the soft nature of the $S$ wave and is a direct consequence of chiral symmetry and the Nambu-Goldstone nature of the pion. $F$-wave contributions are shown to be negligible in the near-threshold region.

Measurement of $ \Sigma$ beam asymmetry in $ \pi^{0}_{}$ photoproduction off the neutron in the second and third resonances region⋆

The \( \Sigma\) beam asymmetry in the photoproduction of neutral pions from quasi-free nucleons in a deuteron target was measured for the first time between 0.60 and 1.50GeV, with the GRAAL polarized and tagged photon beam. The asymmetry values from the quasi-free proton were found equal to the ones extracted from a pure proton target. The asymmetries from quasi-free proton and quasi-free neutron were found equal up to 0.82GeV and substantially different at higher energies. The results are compared with recent partial-wave analyses.