Recent data from the PIONS$@$MAX-lab Collaboration, measuring the total cross section of the pion incoherent photoproduction $γd\toπ^-pp$ near threshold, have been used to extract the E$_{0+}$ multipole and total cross section of the reaction $γn\!\to\!π^-p$, also near threshold. These are the first measurements of the reaction $γd\toπ^-pp$ in the threshold region. The value of E$_{0+}$ is extracted through a fit to the deuteron data in a photoproduction model accounting for final-state interactions. The model takes an $S$-wave approximation for the elementary reaction $γn\!\to\!π^-p$ with E$_{0+}\! = $ const in the threshold region. The obtained value E$_{0+} = -31.86\pm 0.8$ (in $10^{-3}/m_{π^+}$ units) is in agreement with other existing results. Model predictions for the total cross section $σ(γn\!\to\!π^-p)$ are also given.
We investigate the influence of new GRAAL Sigma-beam asymmetry measurements on the neutron in multipole fits to the single-pion photoproduction database. Results are compared to those found with the addition of a double-polarization quantity associated with the sum rule.
Perturbative QCD is applied to investigate the near threshold heavy quarkonium photoproduction at large momentum transfer. We take into account the contributions from the leading three-quark Fock states of the nucleon. The dominant contribution comes from the three-quark Fock state with one unit quark orbital angular momentum (OAM) whereas that from zero quark OAM is suppressed at the threshold. From our analysis, we also show that there is no direct connection between the near threshold heavy quarkonium photoproduction and the gluonic gravitational form factors of the nucleon. Based on the co…
We present an overview of the GW SAID group effort to analyze on new pion photoproduction on both proton- and neutron-targets. The main database contribution came from the recent CLAS and MAMI unpolarized and polarized measurements. The differential cross section for the processes gamma n --> pi- p was extracted from new measurements accounting for Fermi motion effects in the impulse approximation (IA) as well as NN and piN effects beyond the IA. The EM coupling results are compared to other recent studies.
We argue that the reaction mechanism for the coherent pion production is not known with sufficient accuracy to determine the neutron radius of 208Pb to the claimed precision of 0.03 fm.
We apply perturbative QCD to investigate the near threshold heavy quarkonium photoproduction at large momentum transfer. From an explicit calculation, we show that the conventional power counting method will be modified and the three quark Fock state with nonzero orbital angular momentum dominates the near threshold production. It carries a power behavior of $1/(-t)^5$ for the differential cross section. We further comment on the impact of our results on the interpretation of the experiment measurement in terms of the gluonic gravitational form factors of the proton.
We investigate kaon photoproduction off the neutron target, i.e., $γn \to K^0 Λ$, focusing on the role of nucleon resonances given in the Review of Particle Data Group in the range of $\sqrt{s} \approx 1600 - 2200$ MeV. We employ an effective Lagrangian method and a Regge approach. The strong couplings of nucleon resonances with $KΛ$ vertices are constrained by quark model predictions. The numerical results of the total and differential cross sections are found to be in qualitative agreement with the recent CLAS and FOREST experimental data. We discuss the effects of the narrow nucleon resonan…
We investigate $η$ photoproduction off the neutron target, i.e., $γn \to ηn$, employing an effective Lagrangian method combining with a Regge approach. As a background, we consider nucleon exchange in the $s$-channel diagram and $ρ$- and $ω$-meson Regge trajectories in the $t$ channel. The role of nucleon resonances given in the Review of Particle Data Group in the range of $W \approx 1500 - 2100$ MeV and the narrow nucleon resonance $N(1685,1/2^+)$ is extensively studied. The numerical results of the total and differential cross sections, double polarization observable $E$, and helicity-depen…
High-accuracy $Υ$-meson photoproduction data from EIC and EicC experiments will allow the measurement of the near-threshold total cross section of the reaction $γp\toΥp$, from which the absolute value of the $Υp$ scattering length, $|α_{Υp}|$, can be extracted using a Vector-Meson Dominance model. For this evaluation, we used $Υ$-meson photoproduction quasi-data from the QCD approach (the production amplitude can be factorized in terms of gluonic generalized parton distributions and the quarkonium distribution amplitude). A comparative analysis of $|α_{Υp}|$ with the recently determined scatte…
Strangeness photoproduction in the elementary reactions $γN\rightarrow KY$ offers the possibility to study nucleon resonances in the mass region above 1.7 GeV, where the number of states and their properties are still under debate. These reactions could allow the first 'complete' experiment in pseudoscalar meson photoproduction due to the easier access to recoil polarization observables in experiments. In addition to measurements on proton targets, a full isospin decomposition of the $I=1/2$ electromagnetic amplitudes requires also data obtained from quasifree neutron targets. This contributio…
A first measurement is presented of exclusive photoproduction of $ρ^0$ mesons associated with leading neutrons at HERA. The data were taken with the H1 detector in the years $2006$ and $2007$ at a centre-of-mass energy of $\sqrt{s}=319$ GeV and correspond to an integrated luminosity of $1.16$ pb$^{-1}$. The $ρ^0$ mesons with transverse momenta $p_T<1$ GeV are reconstructed from their decays to charged pions, while leading neutrons carrying a large fraction of the incoming proton momentum, $x_L>0.35$, are detected in the Forward Neutron Calorimeter. The phase space of the measurement is defined…
We discuss the procedure of extracting the photoproduction cross section for neutral pseudoscalar mesons off neutrons from deuteron data. The main statement is that the final-state interaction (FSI) corrections for the proton and neutron target are in general not equal, but for pi0 production there are special cases were they have to be identical and there are large regions in the parameter space of incident photon energy and pion polar angle, θ^*, where they happen to be quite similar. The corrections for both target nucleons are practically identical for $π_0$ production in the energy range …
Inspired by the preliminary experimental data by the CLAS Collaboration, the $K^{\ast 0}Λ$ photoproduction off a neutron is studied within an effective Lagrangian approach. The contributions from the Born terms including $s$, $u$, and $t$ channels are considered to calculate the amplitude, with which the cross sections are calculated and compared with preliminary CLAS data. The theoretical results indicate that the contribution from the $K$-exchange $t$ channel plays a dominant role for the $K^{\ast 0}Λ$ photoproduction. The contribution from the $κ$ exchange is found one order of magnitude sm…
In Skyrme-type models, the leading term of the low-energy photoproduction amplitude is identical to the standard expression and independent of the number of flavors considered, but subleading terms are not.
We study photoproduction reactions for Theta^+, which was recently observed as a candidate of an exotic five quark state uudd(bar s). We employ effective lagrangian interactions in the linear and non-linear formalism of chiral symmetry and investigate two cases of spin and parity of Theta^+, J^P = 1/2^+ and 1/2^-. We calculate cross sections and discuss possibilities to discriminate one from the other.
The long-standing discrepancy between the Gerasimov-Drell-Hearn sum rule and the analysis of pion photoproduction multipoles is greatly diminished by use of s-wave multipoles that are in accord with the predictions of chiral perturbation theory and describe the experimental data in the threshold region. The remaining difference may be due to contributions of channels with more pions and/or heavier mesons whose contributions to the sum rule remain to be investigated by a direct measurement of the photoabsorption cross sections.
We extract a parameter $ξ$, a characteristic product of the strong and electromagnetic amplitudes for the N^*(1535), to be (2.20+-0.15) in unit of 10^(-4)MeV^(-1) from the new data on photoproduction of etas off proton.
We consider the chiral expansion of the threshold amplitude for the reaction $γp \to π^0 π^0 p$ to order ${\cal O}(M_π^2)$. We substantiate a claim that this photoproduction channel is significantly enhanced close to threshold due to pion loops. A precise measurement of the corresponding cross sections is called for which allows to test chiral perturbation theory.
We present a calculation of $π^0$ photoproduction on the proton in manifestly Lorentz-invariant baryon chiral perturbation theory up to and including chiral order $q^4$. With the results we analyze the latest $π^0$ photoproduction data in the threshold region obtained at the Mainz Microtron. In the calculation of observables and the fit of the low-energy constants, we take $S$, $P$, and $D$ waves into account. We compare the results for the multipoles with the corresponding single-energy analysis. Furthermore, we also fit the $O(q^4)$ heavy-baryon chiral perturbation theory calculation and com…
{ {\bf Background:} Knowing the difference between the neutron and proton densities of nuclei is a significant topic because of its importance for understanding neutron star structures and cooling mechanisms. The coherent-nuclear photoproduction of pions, $({\g,π^0})$, combined with elastic electron scattering, has been suggested to be a very accurate probe of density differences. {\bf Purpose:} Study the ${(\g,π^0})$ reaction mechanism so as to better access the uncertainties involved in extracting the neutron density. {\bf Methods:} Include the effects of final-state pion-nucleus charge-exch…
The recently discovered enhancement of eta photoproduction on the quasi-free neutron at energies around s^1/2 ~ 1.67 GeV is addressed within an SU(3) coupled channels model. The K Sigma threshold plays a significant role and the quasi-free cross sections on proton and neutron, sigma_n and sigma_p, can be quantitatively explained. For the reaction gamma p --> pi^0 eta p, we evaluate the polarization observables I^S and I^C from a chiral unitary amplitude developed earlier. The I^S and I^C observables have been recently measured for the first time by the CBELSA/TAPS collaboration. We show the si…
Based on recent experimental data obtained by the CLAS Collaboration, the $Σ(1385)$ photoproduction off a neutron target at laboratory photon energies $E_γ$ up to 2.5\,GeV is investigated in an effective Lagrangian approach including $s$-, $u$-, and $t$-channel Born-term contributions. The present calculation does not take into account any explicit $s$-channel baryon-resonance contributions, however, in the spirit of duality, we include $t$-channel exchanges of mesonic Regge trajectories. The onset of the Regge regime is controlled by smoothly interpolating between Feynman-type single-meson ex…
Near-threshold heavy quarkonium photoproduction in a light-front spectator model
Institute of Modern Physics, Chinese Academy of Sciences（中国科学院近代物理研究所）；School of Nuclear Science and Technology, University of Chinese Academy of Sciences（中国科学院大学核科学与技术学院）；CAS Key Laboratory of High Precision Nuclear Spectroscopy, Institute of Modern Physics, Chinese Academy of Sciences（中国科学院近代物理研究所高精度核谱学重点实验室）；Department of Physics, Indian Institute of Technology Kanpur（坎普尔印度理工学院物理系）；University of Science and Technology of China（中国科学技术大学）
The $J/ψ$-$p$ scattering length, $α_{J/ψp}$, can be extracted from the $J/ψ$ photoproduction cross section near threshold using the Vector Meson Dominance (VMD) model to relate the reaction $γp \to J/ψp$ to $J/ψp \to J/ψp$. Such estimates based on experimental data result in values for $|α_{J/ψp}|$, which are much lower than most of the theoretical predictions. In this work, we study the relations between the different results, depending on the use of the total or the differential cross sections, and the method of extrapolating the data to threshold in the case of a low-statistics data sample,…
The importance of Born terms and resonance exchange for eta and eta-prime photoproduction off both the proton and neutron within U(3) baryon chiral perturbation theory is investigated. Low-lying resonances such as the vector mesons and J^P= 1/2^+, 1/2^- baryon resonances are included explicitly and their contributions together with the Born terms are calculated. The coupling constants of the resonances are determined from strong and radiative decays. We obtain reasonable agreement with experimental data near threshold.
We apply the nuclear model with explicit mesons to photoproduction of neutral pions off protons at the threshold. In this model the nucleons do not interact with each other via a potential but rather emit and absorb mesons that are treated explicitly on equal footing with the nucleons. We calculate the total cross section of the reaction for energies close to threshold and compare the calculations with available experimental data. We show that the model is able to reproduce the experimental data and determine the range of the parameters where the model is compatible with the experiment.
The incoherent pion photoproduction reaction gd-->pi-pp is considered theoretically in a wide energy region Eth =< Eg =< 2700 MeV. The model applied contains the impulse approximation as well as the NN- and piN-FSI amplitudes. The aim of the paper is to study a reliable way for getting the information on elementary gn-->pi-p reaction cross section beyond the impulse approximation for gd-->pi-pp. For the elementary gN-->piN, NN-->NN, and piN-->piN amplitudes, the results of the GW DAC are used. There are no additional theoretical constraints. The calculated cross section dSigma/dOmega(gd->pi-pp…
We investigate near threshold neutral pion photoproduction off protons to fourth order in heavy baryon chiral perturbation theory in the light of the new data from MAMI. We show that the unitarity cusp at the secondary π^+ n threshold is in agreement with expectations from the final state theorem. We also analyze the fourth order corrections to the P-wave low-energy theorems and show that potentially large Delta-isobar contributions are cancelled by sizeable pion loop effects. This solidifies the parameter free third order predictions, which are in excellent agreement with the data.
The photoproduction of the $J/ψ$ off the proton is believed to deepen our understanding of various physics issues. On the one hand, it is proposed to provide access to the origin of the proton mass, based on the QCD multipole expansion. On the other hand, it can be employed in a study of pentaquark states. The process is usually assumed to proceed through vector-meson dominance, that is the photon couples to a $J/ψ$ which rescatters with the proton to give the $J/ψp$ final state. In this Letter, we provide a compelling hint for and propose measurements necessary to confirm a novel production m…
We examine the photoproduction of the light scalar mesons $f_0$-$a_0 (980)$ with emphasis on isospin-violating $f_0$ and $a_0$ mixing due to the mass difference between neutral and charged kaons. General forms for the invariant amplitude for scalar meson photoproduction are derived, which yield expressions for the invariant mass distribution of the final meson pairs. A final state interaction form factor is obtained which incorporates $f_0$ and $a_0$ mesons, $K\bar K$ threshold effects, and $f_0-a_0$ mixing. This form factor is applied to predict the effective mass distribution of the $ππ$ and…
Probing the neutron-skin thickness through $J/ψ$ photoproduction in ultra-peripheral collisions
Shanghai Institute of Applied Physics, Chinese Academy of Sciences（中国科学院上海应用物理研究所）；University of Chinese Academy of Sciences（中国科学院大学）；Fudan University（复旦大学）；East China Normal University（华东师范大学）
We discuss the photoproduction processes of light vector mesons ($ρ$, $ω$, and $ϕ$) from the nucleon near threshold. We first develop a simple model based on meson exchanges which is modified by the nucleon pole terms. We then extend this model to study other physically interesting topics. As examples, we discuss the missing nucleon resonances problem in $ω$ photoproduction and the direct $ϕNN$ coupling constant in $ϕ$ photoproduction. The calculated cross sections are compared with the recent experimental data. Various spin observables are discussed, which may be measured at current photon/el…
One-loop corrections to $ω$ photoproduction near threshold have been investigated by using the approximation that all relevant transition amplitudes are calculated from the tree diagrams of effective Lagrangians. With the parameters constrained by the data of $γN \to πN$, $γN \to ρN$, and $πN \to ωN$ reactions, it is found that the one-loop effects due to the intermediate $πN$ and $ρN$ states can significantly change the differential cross sections and spin observables. The results from this exploratory investigation suggest strongly that the coupled-channel effects should be taken into accoun…
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.
Using the recent precise measurements of eta photopro- duction in proton and deuteron targets, we extract the ratio of the helicity amplitudes A^n_1/2 / A^p_1/2, for the excitation of N^*(1535), in the effective Lagrangian approach, It is fairly model-independent, free from the final-state interaction effects, and negative as predicted by the quark models. We stress the importance of polarization observables in further elucidation of the N^*(1520) photoexcitation amplitudes.
We study the inclusive photoproduction of $T_{{\bar c}{\bar c}}(3875)^-$ mesons (which are the antiparticles of the doubly charmed tetraquarks $T_{cc}(3875)^+$ discovered recently by the LHCb Collaboration) from nuclei in the near-threshold energy region within the nuclear spectral function approach by considering incoherent direct ($γp(n) \to D^+(D^0){T_{{\bar c}{\bar c}}(3875)^-}Λ^+_c$) photon--nucleon $T_{{\bar c}{\bar c}}(3875)^-$ creation processes as well as five possible different scenarios for their internal structure with the main goal of clarifying the possibility to decipher this st…
Vector meson photoproduction and electroproduction have been suggested as a tool to find or confirm the nucleon resonances. In order to extract more reliable informations on the nucleon resonances, understanding the non-resonant background is indispensable. We consider final state interactions in omega photoproduction as a background production mechanism. For the intermediate states, we consider nucleon--vector-meson and nucleon-pion channels. The role of the final state interactions is discussed in omega meson photoproduction near threshold.
Background: Coherent π0 photoproduction on heavy nuclei has been suggested as a reliable tool to infer neutron skin thicknesses. To this aim, various experiments have been performed, especially on 208Pb. Purpose: We analyze the sensitivity of that reaction process to the nucleonic density, and especially to the neutron skin thickness, for 12C, 40Ca and 208Pb, for which reliable data exist, and on 116,124Sn, for which measurements have been performed in Mainz. We study also the role played by the π0-nucleus final-state interaction. Method: A model of the reaction is developed at the impulse app…
A microscopic few-body description of near-threshold coherent photoproduction of the eta meson on tritium and 3He targets is given. The photoproduction cross-section is calculated using the Finite Rank Approximation (FRA) of the nuclear Hamiltonian. The results indicate a strong final state interaction of the eta meson with the residual nucleus. Sensitivity of the results to the choice of the eta N T-matrix is investigated.
We analyze the electromagnetic production of K^+Lambda and K^0Lambda near their production thresholds by using isobar models. In the K^+Lambda channel we show that the model can nicely describe the available experimental data. In the K^0Lambda channel we demonstrate that the K^0 charge form factor has sizable effects on the longitudinal cross section. By extending the model up to W = 1730 MeV, we are able to observe the existence of the narrow P_{11} (J^p = 1/2^+) resonance in the kaon photoproduction process. It is found that the most convincing mass (width) of this resonance is 1650 MeV (5 M…
We discuss the possibility of determining the parity of the $Θ^+$ baryon from photoproduction $γN\to KΘ^+$ process near threshold. We utilize the conservation laws of parity and angular momentum for the analysis of angular distributions and spin observables near threshold. Since the discussion is in essence a partial wave analysis of the production mechanism the result should be less dependent on the model parameters. Our analysis shows that the angular distribution and photon polarization asymmetry for the process of neutron target are sensitive to the parity of the $Θ^+$, but not for the cas…
Incoherent photoproduction of the $η$-meson on the deuteron is studied for photon energies from threshold to 800 MeV. The dominant contribution, the $γ$N-$η$N amplitude, is described within an isobar model. The final state interaction derived from the CD-Bonn potential is included and found to be important for the description of the production cross section close to threshold. Possible effects from the $ηN$ final state interaction are discussed.
We study the $Υ(1S)$ meson photoproduction on protons and nuclei at the near-threshold center-of-mass energies below 11.4 GeV (or at the corresponding photon laboratory energies $E_γ$ below 68.8 GeV). We calculate the absolute excitation functions for the non-resonant and resonant photoproduction of $Υ(1S)$ mesons off protons at incident photon laboratory energies of 63--68 GeV by accounting for direct ($γp \to {Υ(1S)}p$) and two-step ($γp \to P^+_b(11080,11125,11130) \to {Υ(1S)}p$) $Υ(1S)$ production channels within different scenarios for the non-resonant total cross section of elementary re…
We shall review here our current knowledge and new frontiers of the pion photoproduction study, from threshold through the Delta resonance region, emphasizing the hadron physics program, of interest to a 250 MeV, high duty factor electron facility, such as the upgraded MAX-lab at the Lund University, Sweden, where it would be possible to produce tagged photons of high quality, with energy below approximately 200 MeV.
Observed for the processes gamma+p arrow eta+p, gamma+n arrow eta+n are calculated throughout the resonance energy region using the single isobar model without a background. The connection between invariant photoexcitation amplitudes of the resonances on neutrons and protons are calculated. The results are discussed in terms of a comparison with experimental data and calculations performed by other authors.
We discuss the possibility of extracting the neutron-neutron scattering length $a_{nn}$ and effective range $r_{nn}$ from cross section data ($d^2σ/dM_{nn}/dΩ_π$), as a function of the $nn$ invariant mass $M_{nn}$, for $π^+$ photoproduction on the deuteron ($γd\to π^+nn$). The analysis is based on a $γd\to π^+nn$ reaction model in which realistic elementary amplitudes for $γp\to π^+n$, $NN\to NN$, and $πN\to πN$ are built in. We show that $M_{nn}$ dependence (lineshape) of a ratio $R_{\rm th}$, $d^2σ/dM_{nn}/dΩ_π$ normalized by $dσ/dΩ_π$ for $γp\toπ^+ n$ and the nucleon momentum distribution i…
We present a theoretical investigation of $Λ(1520)$ attenuation in $γA$ reactions near the threshold. It is performed in the framework of a collision model based on the nuclear spectral function. The model accounts for both primary photon-nucleon $γN \to KΛ(1520)$ and secondary pion-nucleon $πN \to KΛ(1520)$ production processes. We calculate the target mass and momentum dependences of the forward $Λ(1520)$ hyperon production from nuclei at photon energy of 2 GeV as well as for two options for its in-medium width. We find that the considered dependences are markedly sensitive to this width. Ou…
Inclusive $K^+$ and exclusive $K^+Y$ photoproduction on the deuteron are investigated theoretically. Modern hyperon-nucleon forces and a recently updated kaon photoproduction operator for the $γ+N\to K^++Y$ process are used. Sizable effects of the hyperon-nucleon final state interaction are found near the $K^+ΛN$ and $K^+ΣN$ thresholds in the inclusive reaction. Angular distributions for the exclusive process show clear $YN$ final state interaction effects in certain kinematic regions. Precise data especially for the inclusive process around the $K^+ΣN$ threshold would help to clarify the stre…
We apply the effective Lagrangian approach to investigate the role of the nucleon resonances in $ω$ meson photoproduction at energies near the threshold. The resonant amplitudes are calculated from effective Lagrangians with the gamma NN* and omega NN* coupling constants fixed by the empirical helicity amplitudes and the vector meson dominance model. The contributions from the nucleon resonances are found to be significant in changing the differential cross sections in a wide interval of t and various spin observables.
The effects of final state interaction in incoherent $η$ photoproduction on deuteron are studied within a three-body approach including a realistic $NN$ potential. The results are compared with available data, and differences with other theoretical predictions are analyzed. The role of the $ηN$ interaction and the possibility of extracting the $ηN$ scattering parameters from this reaction are discussed.
We calculate electromagnetic neutral pion production off tri-nucleon bound states (3H, 3He) at threshold in chiral nuclear effective field theory to fourth order in the standard heavy baryon counting. We show that the fourth order two-nucleon corrections to the S-wave multipoles at threshold are very small. This implies that a precise measurement of the S-wave cross section for neutral pion production off 3He allows for a stringent test of the chiral perturbation theory prediction for the S-wave electric multipole E_{0+}^{pi0 n}.
Coherent $η$ meson photoproduction on $^4$He and $^{12}$C is considered in the near-threshold region. The elementary $η$ photoproduction operator includes contributions from the $S_{11}(1535)$ and $D_{13}(1520)$ resonances as well as $t$-channel vector meson exchange and the nucleon pole terms. Due to the suppression of the dominant $S_{11}(1535)$ resonance for spin and isospin saturated nuclei, the reaction is mainly governed by $ω$ exchange. Furthermore, the influence of Fermi motion and of different prescriptions for the choice of the invariant reaction energy $W_{γN}$ in the elementary amp…
Incoherent $η$-photoproduction on the deuteron is studied for photon energies from threshold up to 720 MeV. The elementary $γN \to ηN$ amplitude is constructed in the frame of a conventional isobar model. Effects of final state interaction are investigated and their role in the cross section as well as in polarization observables is found to be important. Inclusion of such effects leads to a satisfactory agreement with experimental data in the near threshold region. In addition, recent experimental results on the coherent reaction $d(γ,η)d$ are discussed.
We study, within phenomenological Lagrangian approach, the possibility that the near-threshold peak found in the phi-photoproduction cross section is caused by a resonance. We show that, by employing a J^P=3/2^- resonance with a mass of 2.08 +/- 0.05 GeV and a width of 0.570 +/- 0.169 GeV, the LEPS data which include new data on nine spin-density matrix elements can indeed be described reasonably well. We also find that the ratio of helicity amplitudes A_1/2/A_3/2 calculated from the resulting coupling constants differs in sign from that of D13(2080). The resonance is further found to be able …
