Double Pion Photoproduction Research Articles

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

Polarization Observables in Double-Charged-Pion Photo-Production with Circularly-Polarized Photons Off Transversely-Polarized Protons

Double-pion photo-production represents one of the strongest contributions to the photonucleon total cross-section at high energies, and thus it plays an important role in probing the nucleon resonance spectrum. The polarization observables are useful because of the additional sensitivity they provide in unraveling the reaction dynamics, and so the goal of this study is to extract the specific polarization observables $$I^{\odot }$$ , $$P_{x}$$ , $$P_{y}$$ , $$P^{\odot }_{x}$$ , $$P^{\odot }_{y}$$ , for the $$\gamma p \rightarrow p \pi ^{+} \pi ^{-}$$ reaction. The data were taken as a part of the CLAS g9b (FROST) experiment at Jefferson Laboratory, which used a transversely-polarized target and a circularly-polarized photon beam with energies up to 3 GeV. The data will help deepen the current knowledge of resonance photocouplings and hadronic decays and possibly assist in identifying new baryon resonances via partial-wave analyses and in this way will contribute to a more comprehensive understanding of the strong interaction.

Event mixing constraints for Bose-Einstein correlations in reactions with three particles in the final state

Bose-Einstein correlations (BEC) are widely used to gain an insight into the spatiotemporal characteristics of boson emitters. It was used for the first time in the 1950s by R. Hanbury-Brown and R. Q. Twiss[Hanbury-Brown R, Twiss R Q 1954 Phil. Mag. 45 663] in astronomy to measure the dimension of distant astronomical objects emitting photons, and hence is also known as Hanbury-Brown-Twiss effect (HBT). In nuclear and particle physics field, BEC also has important applications in the investigation of the space-time properties of subatomic reaction region, especially in elementary-particle collisions and relativistic heavy-ion collisions with large multiplicity at high energies. Its potential application in exclusive reactions with low multiplicity in the non-perturbative QCD energy region may offer complementary information like duration and size of nucleon resonances, which are generally excited by hadronic or electromagnetic probes and usually decay into the ground states accompanied by emission of identical mesons. However, the event mixing technique, which is highly adopted for BEC observations in inclusive reactions at high energies with large multiplicity cannot be directly applied to the BEC measurement in exclusive reactions with very limited multiplicity at low energies. The event mixing method produces un-correlated samples from original sample through making mixed events by randomly selecting the momenta of two bosons from different original events. It works well for the high multiplicity case because the degree of freedom of final state particles is large compared with that of the low multiplicity case. In exclusive reactions with a very limited number of identical bosons in the final state, this method is however strongly interfered by non-BEC factors such as global conservation laws and decays of resonances. Appropriate constraints are required to control the event mixing process in order to eliminate the influence of those non-BEC factors. In this study, we are trying to develop an event mixing method for BEC measurement in reactions having only three final state particles and only two identical bosons among them. For this end, five constraint modes for the event mixing are proposed and investigated via Monte Carlo simulation. Each mode employs one or a combination of the following cut conditions:1) missing mass cut (MM) that requires the missing mass of the mixed event to be equal to that of the original event; 2) polar angle consistency cut (PAC) that requires that the swapping particles should come from the same polar angle bin; 3) azimuthal angle consistency cut (AAC); 4) momentum consistency cut (MC); 5) energy upper limit cut (EU) that requires that any boson energy should not exceed a given upper limit. The double neutral pion photoproduction on the proton around 1 GeV is taken for example to demonstrate the effects of these constraints on the event mixing. In the simulation, one event sample free of BEC effects and four samples in the presence of BEC effects are generated for testing the ability for these constraints to extract BEC parameters. It is found the constraint mode using the MM and PAC cuts, and the mode employing the MM and AAC cuts, and the mode adopting the MM and the EU cuts can be used to observe BEC effects and extract BEC parameters. Among them, optimum results can be achieved by the combination of the MM and EU cuts.

Double neutral pion photoproduction off the proton with FOREST at ELPH

Total cross section for the double neutral pion photoproduction off the proton is presented in the incident photon energy range of 0.58 to 1.15 GeV. The data were accumulated in the years 2009–2010, recorded by a 4π EM calorimeter at ELPH, named FOREST. The number of recorded events obtained during this period is 1.6×109 for a hydrogen target. Two neutral pions are reconstructed via detecting their decay products, four gammas (2π 0 → 4γ ). Compared to the previous results obtained by other groups, our data are in good agreement with theirs within error bars.

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.

Double pion photoproduction off nuclei – Are there effects beyond final-state interaction?

Photoproduction of π0π0 and π0π± pairs from nuclei has been measured over a wide mass range (2H, 7Li, 12C, 40Ca, and natPb) for photon energies from threshold to 600 MeV. The experiments were performed at the MAMI accelerator in Mainz, using the Glasgow photon tagging spectrometer and a 4π electromagnetic calorimeter consisting of the Crystal Ball and TAPS detectors. A shift of the pion–pion invariant-mass spectra for heavy nuclei to small invariant masses has been observed for π0 pairs but also for the mixed-charge pairs. The precise results allow for the first time a model-independent analysis of the influence of pion final-state interactions. The corresponding effects are found to be large and must be carefully considered in the search for possible in-medium modifications of the σ-meson. Results from a transport model calculation reproduce the shape of the invariant-mass distributions for the mixed-charge pairs better than for the neutral pairs, but also for the latter differences between model results and experiment are not large, leaving not much room for σ-in-medium modification.

Photoproduction of charged pions from the deuteron

The photoproduction of the charged pions from the deuteron has been studied with the Neutral Kaon Spectrometer (NKS) and the Neutral Kaon Spectrometer 2 (NKS2) at the Research Center for Electron Photon Science, Tohoku University (ELPH) which was previously known as the Laboratory of Nuclear Science (LNS-Tohoku) using the GeV-region tagged photon beam. The total cross section for the quasi-free (QF) π + π − photoproduction from the deuteron was measured with NKS in the energy region from 0.8 to 1.1 GeV. The cross section for the double Δ excitation in the intermediate state was derived in the analysis of the non-quasi-free (NQF) process, in which two nucleons of the deuteron share the energy of the incident photon. The prospect of the measurements of double pion photoproduction with NKS2 and upgraded NKS2 is discussed. The pion photoproduction from the nucleon has been investigated both experimentally and theoreti- cally for the study of the nucleon resonances for tens of years. The contribution of the double pion photoproduction from the proton is dominant in the energy region higher than 0.6 GeV. The π + π − photoproduction cross section accounts for approximately 30 percent of the total photoproduction in this energy region.

Chiral corrections to [formula omitted] processes at low energies

We calculate in chiral perturbation theory the double-pion photoproduction processes π − γ → π − π 0 π 0 and π − γ → π + π − π − at low energies. At leading order these reactions are governed by the chiral pion-pion interaction. The next-to-leading order corrections arise from pion-loop diagrams and chiral-invariant counterterms involving the low-energy constants ℓ ¯ 1 , ℓ ¯ 2 , ℓ ¯ 3 and ℓ ¯ 4 . The pertinent production amplitudes A 1 and A 2 depending on five kinematical variables are given in closed analytical form. We find that the total cross section for neutral pion-pair production π − γ → π − π 0 π 0 gets enhanced in the region s < 7 m π by a factor 1.5 – 1.8 by the next-to-leading order corrections. In contrast to this behavior the total cross section for charged pion-pair production π − γ → π + π − π − remains almost unchanged in the region s < 6 m π in comparison to its tree-level result. Although the dynamics of the pion-pair production reactions is much richer, this observed pattern can be understood from the different influence of the chiral corrections on the pion-pion final state interaction ( π + π − → π 0 π 0 versus π − π − → π − π − ). We present also results for the complete set of two-pion invariant mass spectra. The predictions of chiral perturbation theory for the π − γ → 3 π processes can be tested by the COMPASS experiment which uses Primakoff scattering of high-energy pions in the Coulomb field of a heavy nucleus to extract cross sections for π − γ reactions with various final states.

BEAM ASYMMETRY Σ OF THE π- PHOTOPRODUCTION OFF NEUTRON

We present the analysis of data performed in order to identify the events of the γ + n → π- + p reaction obtained by bombarding a liquid Deuterium target with a polarised γ beam of 0.55-1.5 GeV at the Graal-experiment. We show the effect of different kinematic and hardware constraints used to reduce the contamination coming from the concurrent reaction channels. By the simulation we estimate the contamination degree due to the other reaction channels so we can test the reliability of our method. We describe a new three-dimensional cut based on the Fermi momentum reconstruction and its effect on the suppression of the concurrent double charged pion photoproduction. We present the preliminary beam asymmetry Σ of the π- fotoproduction off quasi-free neutron up to about θc.m., π- = 165° together with some theoretical multipolar analysis. For a comparison we also report the data present in literature on the same reaction for Eγ =850-1740 MeV and θc.m., π- ≤105°.

Double and single pion photoproduction within a dynamical coupled-channels model

Within a dynamical coupled-channels model that has already been fixed by analyzing the data of the $\ensuremath{\pi}N\ensuremath{\rightarrow}\ensuremath{\pi}N$ and $\ensuremath{\gamma}N\ensuremath{\rightarrow}\ensuremath{\pi}N$ reactions, we present the predicted double pion photoproduction cross sections up to the second resonance region, $Wl1.7$ GeV. The roles played by the different mechanisms within our model in determining both the single and double pion photoproduction reactions are analyzed, focusing on the effects attributable to the direct $\ensuremath{\gamma}N\ensuremath{\rightarrow}\ensuremath{\pi}\ensuremath{\pi}N$ mechanism, the interplay between the resonant and nonresonant amplitudes, and the coupled-channels effects. The model parameters that can be determined most effectively in the combined studies of both the single and double pion photoproduction data are identified for future studies.

Photoproduction of η -mesons on the deuteron above S11(1535) in the presence of a narrow P11(1670) -resonance

Incoherent photoproduction of η -mesons on the deuteron is considered. The main attention is paid to the region above the S 11(1535) -resonance where a rather narrow resonance-like structure in the total cross-section extracted for γn → ηn has been reported. The corresponding experimental results are analyzed from the phenomenological standpoint within the model containing a baryon P11 with a mass of about 1670MeV and a width less than 30MeV. This resonance was suggested in some recent works as a nonstrange member of the pentaquark antidecuplet with J P = 1/2+ . The calculation is also performed for the polarized and unpolarized angular distributions of η -mesons. In addition, we present our predictions for the cross-sections of the neutral kaons and double pion photoproduction, where the same narrow P 11(1670) -resonance is assumed to contribute through the decay into K 0Λ and πΔ configuration.

Two Pion Production from Nuclei

Differences in the photoproduction of mesons on the free proton and on nuclei are expected to reveal changes in the properties of hadrons. Double pion photoproduction from nuclei has been used to investigate the in medium modification of meson-meson interactions with the TAPS photon spectrometer at MAMI-B. Recent developments are reported. Hadron-hadron interactions and meson production and decays involving light nuclei are also studied in hadron induced experiments. In particular, the physics program of the new WASA-at-COSY facility includes double pion production from light nuclei. Here, the efforts focus on the ABC effect, the puzzling low-mass enhancement in the pi-pi invariant mass spectrum, first observed in inclusive measurements of two-pion production in nuclear collisions.

Double-pion photoproduction from nuclei

Differences in the photoproduction of mesons on the free proton and on nuclei are expected to reveal changes in the properties of hadrons. Inclusive studies of nuclear photoabsorption have provided evidence of medium modifications. However, the results have not been explained in a model independent way. A deeper understanding of the situation is anticipated from a detailed experimental study of meson photoproduction from nuclei in exclusive reactions. In the energy regime above the Delta(1232) resonance, the dominant double pion production channels are of particular interest. Double pion photoproduction from nuclei is also used to investigate the in-medium modification of meson-meson interactions.

The chiral limit of QCD and above

I review aspects of chiral dynamics pertinent to the structure of baryons and few-nucleon systems, such as chiral extrapolations for the nucleon and the delta mass, double pion photoproduction off protons, single neutral pion electroproduction off the deuteron, pion photoproduction in the delta region, the quark mass dependence of the nuclear forces and the possibility of an infrared limit cycle in QCD.

Helicity dependence of pion photoproduction on the proton: new results from Mainz

The helicity dependence of all single- and double-pion photoproduction channels on the proton has been measured for the first time at incident photon energies from 200 to 800 MeV. The measurement at the tagged photon beam facility of the MAMI accelerator in Mainz used the large acceptance detector DAPHNE and a frozen-spin target. This completely new and up to now inaccessible observable represents a very sensitive tool to study baryon resonances since the smaller resonant multipoles can be accessed via the interference with the dominant ones. The most recent results from the Mainz experiment are presented, with a particular emphasis on the second resonance region, where there are clear systematic discrepancies with the existing multipole analyses. In this region, double polarization data are mostly sensitive to the properties of the D 13 (1520) resonance and can be used to determine its properties much more accurately than in previous measurements.

Two-pion production in photon-induced reactions

Differences in the photoproduction of mesons on the free proton and on nuclei are expected to reveal changes in the properties of hadrons. Inclusive studies of nuclear photoabsorption have provided evidence of medium modifications. However, the results have not been explained in a model independent way. A deeper understanding of the situation is anticipated from a detailed experimental study of meson photoproduction from nuclei in exclusive reactions. In the energy regime above the Delta(1232) resonance, the dominant double pion production channels are of particular interest. Double pion photoproduction from nuclei is also used to investigate the in-medium modification of meson-meson interactions.

Helicity asymmetries in double pion photoproduction on the proton

Based on a prior model on double pion photoproduction on the proton, successfully tested in total cross sections and invariant mass distributions, we make a theoretical study of the angular dependence of helicity asymmetries from the interaction of circularly polarized photons with unpolarized protons. We show that this observable is sensitive to details of the internal mechanisms and, thus, represents a complementary test of the theoretical model.

Transparency of nuclei and mean free path of protons and pions in nuclear matter at intermediate energies

In the second resonance region of photon energies, the differential yields of neutral and charged pions are measured in the (γ, πp) reactions on Li, C, and Al nuclei versus the proton energy and the azimuthal angle of pion emission. These experimental data are analyzed on the basis of a model that takes into account both single and double quasifree pion photoproduction. This results in deriving the mean free paths of protons and neutral and charged pions and in estimating the transparency of the participant nuclei.

Production of neutral and charged pions in the reaction 12C(γ, πp) in the second resonance region

The results are presented that were obtained by investigating the production of pion-proton pairs in the reaction 12C(γ, πp). The experiment in question was performed in the second resonance region of photon energy. The results of this measurement are analyzed on the basis of a model that takes into account the processes of single and double quasifree pion photoproduction.

Measurement of high lying nucleon resonances and search for a missing state in double charged pion electroproduction off the proton

We have analyzed data taken by the CLAS collaboration in double charged pion electroproduction off the proton. We have used an isobar model specifically developed for the analysis of the Nππ channel. This approach relates baryon resonance parameters to the experimental cross section, allowing the extraction of electromagnetic and some strong couplings from the fit to the data. A remarkable feature of the CLAS data is a resonance structure around 1.7 GeV which has not been seen in double pion photoproduction. The structure appears at all Q 2 values from 0.5 to 1.5 GeV 2 and it can not be reproduced with conventional N ∗ parameters.