We compute dilepton invariant mass spectra from the decays of rho mesons produced by photon reactions off nuclei. Our calculations employ a realistic model for the rho photoproduction amplitude on the nucleon which provides fair agreement with measured cross sections. Medium effects are implemented via an earlier constructed rho propagator based on hadronic many-body theory. At incoming photon energies of 1.5 -3 GeV as used by the CLAS experiment at JLAB, the average density probed for iron targets is estimated at about half saturation density. At the pertinent rho-meson 3-momenta the predicted medium effects on the rho propagator are rather moderate. The resulting dilepton spectra approximately agree with recent CLAS data.
We extend our recent study of dilepton invariant-mass spectra from the decays of rho mesons produced by photon reactions off nuclei. We specifically focus on experimental spectra as recently measured by the CLAS Collaboration at the Thomas Jefferson National Accelerator Facility using carbon and iron nuclei. Building on our earlier work we broaden our description to a larger set of observables in order to identify sensitivities to the medium effects predicted by microscopic calculations of the rho spectral function. We compute mass spectra for several target nuclei and study the spectral shape…
One-loop corrections due to the intermediate $πN$ and $ρN$ states are studied in $ω$ photoproduction near threshold. Our results show that the coupled-channel effects should be taken into account in extracting reliable nucleon resonance parameters from the forthcoming vector meson photoproduction data in the resonance region.
In this contribution I discuss prospects for photoproducing hybrid mesons at CEBAF, based on recent model results and experimental indications of possible hybrids. One excellent opportunity appears to be a search for the I=1, JPC=2+-, neutral "(b2)o" hybrid in (a2 pi)o through diffractive photoproduction. Other notable possibilities accessible through pi+ or pio exchange photoproduction are I=1, JPC=1-+, charged "pi1+" in f1 pi+, (b1 pi)+ and (rho pi)+; piJ(1770)+ in f2 pi+ and (b1 pi)+; pi(1800)+ in f0 pi+, f2 pi+, omega rho+ and (rho pi)+; a1 in f1 pi+ and f2 pi+; and omega in (rho pi)o, ome…
There exist model calculations showing the modification of the hadronic parameters of $ρ$ meson in the nuclear environment. From these parameters, we extract the $ρ$ meson nucleus optical potential and show the medium effect due to this potential on the $ρ$ meson mass distribution spectra in the photonuclear reaction. The calculated results reproduced reasonably the measured $e^+e^-$ invariant mass, i.e., $ρ$ meson mass, distribution spectra in the $( γ, ρ^0 \to e^+e^- )$ reaction on nuclei.
We make predictions for the cross sections of coherent photoproduction of $ρ$, $ϕ$, $J/ψ$, $ψ(2S)$, and $Υ(1S)$ mesons in Pb-Pb ultraperipheral collisions (UPCs) at $\sqrt{s_{NN}}=5.02$ TeV in the kinematics of run 2 at the Large Hadron Collider extending the approaches successfully describing the available Pb-Pb UPC data at $\sqrt{s_{NN}}=2.76$ TeV. Our results illustrate the important roles of hadronic fluctuations of the photon and inelastic nuclear shadowing in photoproduction of light vector mesons on nuclei and the large leading twist nuclear gluon shadowing in photoproduction of quarkon…
We study tensor meson photoproduction outside of the resonance region, at beam energies of few GeVs. We build a model based on Regge theory that includes the leading vector and axial exchanges. We consider two determinations of the unknown helicity couplings, and fit to the recent a2 photoproduction data from CLAS. Both choices give a similar description of the a2 cross section, but result in different predictions for the parity asymmetries and the f2 photoproduction cross section. We conclude that new measurements of f2 photoproduction in the forward region are needed to pin down the correct …
We propose that the scalar $κ(800)$ meson may play an important role in $K^*$ photoproduction. In the reactions of $γp \to K^{*+} Λ$ and $γp \to K^{*0} Σ^+$, we consider the production mechanisms including $t$-channel $K^*$, $K$, $κ$ exchanges, s-channel $N$, $Δ$ diagrams, and $u$-channel $Λ$, $Σ$, $Σ^*$ diagrams within the tree level approximation, and find that the $κ$-meson exchange may contribute significantly to $K^*Σ$ photoproduction, while it is rather supplementary in $K^*Λ$ photoproduction. We demonstrate how the observables of $K^*$ photoproduction can be used to constrain the $κ$ me…
An analysis of all possible polarization observables for the case of vector meson photoproduction from a nucleon target is presented. The question of which observables are needed to determine completely the basic photoproduction amplitudes and the relationships between spin observables are addressed.
The $σ$ meson exchange model for $ρ$ photoproduction at low energies is re-examined and a new model is developed by considering explicit two-pion exchange and the $f_2$ tensor meson exchange. The $f_2$ exchange model, which is motivated by the low energy proton-proton elastic scattering, is constructed by fully taking into account of the tensor structure of the $f_2$ meson interactions. Phenomenological informations together with tensor meson dominance and vector meson dominance assumptions are used to estimate the $f_2$ meson's coupling constants. For $2π$ exchange, the loop terms including i…
We evaluated recent CLAS Collaboration measurements for the $90^\circ$ meson photoproduction off the nucleon using a tagged photon beam spanning the energy interval $s = 3 - 11$ GeV$^2$. The results are compared with the "Quark Counting Rules" predictions.
Exclusive photoproduction of vector mesons in the perturbative two-gluon exchange formalism depends significantly on nucleon and nuclear gluon distributions. In the present study we calculate total cross sections and rapidity distributions of $J/ψ(1s)$, $ψ(2s)$, $Υ(1s)$, $Υ(2s)$, and $Υ(3s)$ in ultraperipheral proton-lead (pPb) and lead-lead (PbPb) collisions at the CERN Large Hadron Collider (LHC) at $\sqrt{s_{_{NN}}}=5$ TeV and $\sqrt{s_{_{NN}}}=2.76$ TeV respectively. Effects of gluon shadowing are investigated and potentials for constraining nuclear gluon modifications are discussed.
New high-precision total and differential cross sections for $η$ and $η'$ photoproduction on the proton obtained by the A2 Collaboration at the Mainz Microtron are presented. The data for $η$ photoproduction demonstrate a cusp at the energy W$\sim$1.9~GeV. Furthermore, we present a new version of the $η$MAID model for $η$ and $η'$ photoproduction. The model includes 23 nucleon resonances parametrized with Breit-Wigner shapes. The background is described by vector and axial-vector meson exchanges in the $t$ channel using the Regge phenomenology. Parameters of the resonances were obtained from a…
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 the photoproduction of scalar mesons close to the threshold of f_0(980) and a_0(980) using a unitary chiral model. Peaks for both resonances show up in the invariant mass distributions of pairs of pseudoscalar mesons. A discussion is made on the photoproduction of these resonances in nuclei, which can shed light on their nature, a subject of continuous debate.
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…
The comparison of photoproduction cross sections for $c\bar{c}$ and b-b(bar) in PbPb and pPb collisions can give sensitivity to nuclear shadowing effects. The photoproduction of vector mesons is even more sensitive to the underlying gluon distributions. In this study we present the cross sections and rapidity dependence of the photoproduction of heavy quarks and exclusive production of vector mesons in ultraperipheral pPb and PbPb collisions at the Large Hadron Collider at sqrt(s_NN)=5 TeV and sqrt(s_NN)=2.76$ TeV, respectively. The potentials of using these processes for constraining nuclear …
Enlarging the set of hard exclusive reactions to be studied in the framework of QCD collinear factorization opens new possibilities to access generalized parton distributions (GPDs). We studied the photoproduction of a large invariant mass photon-photon or photon-meson pair in the generalized Bjorken regime which may be accessible both at JLab and at the EIC.
Positivity constraints have proved to be important for spin observables of exclusive reactions involving polarized initial and final particles. Attention is focused in this note on the photoproduction of pseudoscalar mesons from spin 1/2 baryons, more specifically gamma+ N -> K+ Lambda, gamma+ N -> K Sigma, for which new experimental data are becoming available.
We calculate the centrality dependence of inclusive cross section of large-pT charmed-meson ($D^{0}$, $D^{*}$, $D^{*+}$, and $D_{s}^{+}$) from heavy quark fragmentation by the hard photoproduction processes for the nucleus-nucleus collisions in ultrarelativistic heavy ion collisions. The numerical results indicate that the modification of the hard photoproduction processes cannot be negligible for the charmed-meson production in Au-Au collisions at Relativistic Heavy Ion Collider (RHIC) and Pb-Pb collisions at Large Hadron Collider (LHC).
We make predictions for cross sections of $ρ$ and $ϕ$ vector meson photoproduction in ultraperipheral Xe-Xe collisions at $\sqrt{s_{NN}}=5.44$ TeV. Analyzing the momentum transfer distribution of $ρ$ mesons in this process, we explore the feasibility of extracting the nuclear density of $^{129}$Xe, which is needed in searches for dark matter with Xenon-based detectors.
We analyze the double polarization asymmetry of the omega-meson photoproduction in the vector-meson-dominance model of diffractive production and the one-pion exchange model. We find that the longitudinal beam-target asymmetry is very sensitive to the real part of the diffractive photoproduction amplitude and to the sign of the omega-pi-gamma coupling because of the different spin structures of the amplitudes associated with the different mechanisms.
Inclusive heavy meson photoproduction in $pPb$ and $PbPb$ collisions
Federal University of Pelotas（佩洛塔斯联邦大学）；The Henryk Niewodniczański Institute of Nuclear Physics Polish Academy of Sciences (PAN)（波兰科学院亨利克·涅沃德尼茨基核物理研究所）
The polarization observables in phi photoproduction are suggested for probing the nucleon strangeness. Based on models for phi photoproduction, some double polarization observables are shown to be very sensitive to the strangeness content of the proton because of the different spin structures of the amplitudes associated with different mechanisms.
We have evaluated pseudoscalar and scalar neutral pion photoproduction in $\vecγp\toπ^0p$ and $\vecγp\to a_0^0p$ above the resonance region and within Regge phenomenology. Our fit, including GlueX $Σ$ pseudoscalar photoproduction data, shows that previous SLAC $Σ$ measurements for $\vecγp \to π^0p$ above $E_γ= 4~\mathrm{GeV}$ are at variance with SLAC data with more recent measurements made by GlueX in vicinity of $E_γ= 9~\mathrm{GeV}$. The Regge model predicts that the beam polarization asymmetry $Σ$ of the scalar meson is opposite to that of pseudoscalar meson photoproduction, however, the c…
In a transport model that combines initial state interactions of the photon with final state interactions of the produced particles we present a calculation of inclusive photoproduction of mesons in nuclei in the energy range from 1 to 7 GeV. We give predictions for the photoproduction cross sections of pions, etas, kaons, antikaons, and $π^+π^-$ invariant mass spectra in ^{12}C and ^{208}Pb. The effects of nuclear shadowing and final state interaction of the produced particles are discussed in detail.
It is shown that within a truncated partial wave analysis for photoproduction of two pseudoscalar mesons, it is possible to halve the number of independent partial amplitudes by taking into account parity conservation. In addition, within the framework of the proposed formalism, one can rather easily resolve the double discrete ambiguity arising from the symmetry under complex conjugation of the helicity amplitudes. This leads to essential simplifications of the complete experiment problem, at least as far as mathematical aspects are concerned.
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…
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.
The $σ$-exchange and $f_2$-exchange mechanisms for $ρ$ meson photoproduction are re-examined. Then the commonly employed $σ$-exchange amplitude is revised by using the recent information from the analyses on the $ρ\to π^0π^0γ$ decay and the $σNN$ coupling constant from Bonn potential. Instead of relying on the Pomeron-$f$ proportionality assumption, the $f_2$ meson exchange amplitude is established from an effective Lagrangian which is constructed from the tensor structure of the $f_2$ meson. Phenomenological information together with tensor meson dominance and vector meson dominance assumptio…
We confront saturation-based results for diffractive $ψ(2s)$ and $ρ$ production at HERA and $J/ψ$ photoproduction with all available data including recent ones from HERA, ALICE and LHCb, finding a good agreement. We show that the $t$-distribution of differential cross-section of photoproduction of vector mesons offers a unique opportunity to discriminate among saturation and non-saturation models. This is due to emergence of a pronounced dip (or multiple dips) in the $t$-distribution of diffractive photoproduction of vector mesons at relatively large, but potentially accessible $|t|$ that can …
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…
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.
We calculate the nuclear cross section for vector meson exclusive photoproduction within the QCD color dipole picture and in the Regge approach. For the former approach, we have considered the phenomenological saturation model, whereas for the latter we use a model based on the dipole Pomeron framework. Theoretical estimates for scattering on both light and heavy nuclei are given for a large range on energy.
The exclusive $ρ$, $ω$ and $J/Ψ$ photoproduction in fixed - target collisions at the LHC is investigated. We estimate, for the first time, the rapidity and transverse momentum distributions of the vector meson photoproduction in $p He$, $p Ar$, $Pb He$ and $Pb Ar$ fixed - target collisions at the LHC using the STARlight Monte Carlo and present our results for the total cross sections. Predictions for the kinematical range probed by the LHCb detector are also presented. Our results indicate that the experimental analysis of this process in fixed - target collisions at the LHC is feasible. Such …
Photoproduction of a pair of particles with large invariant mass is a natural extension of collinear QCD factorization theorems which have been much studied for deeply virtual Compton scattering and deeply virtual meson production. We discuss the production of a photon and a meson, where the wide angle Compton scattering on a meson subprocess factorizes from generalized parton distribution. We calculate at dominant twist and leading order in $α_s$, the production cross-section of a transversely polarized $ρ$ meson which is sensitive to chiral-odd GPDs, and show that it may be measurable in nea…
The integrated cross section for the incoherent photoproduction of vector mesons on nuclei $γ^* A \to VX$, $X\not=A$, is calculated within Glauber theory and as a function of the photon energy. The inverse of the longitudinal momentum transfer is called coherence length $l_c$ and depends on the virtuality and the energy of the photon. Nuclear transmission factors strongly depend on $l_c/R_A$ ($R_A$ is the nuclear radius) and this effect may interfere with the search for color transparency effects.
This dissertation treats the problem of complete experiments for pseudoscalar meson photoproduction, in the context of a truncated partial-wave analysis (TPWA). The work contains algebraic and numerical considerations. The influence of measurement uncertainties is examined using bootstrap methods. The thesis was originally published online by the University of Bonn in the 1st quarter of 2019 (links are given in the comments).
Recent information on invariant mass distributions of the $γp\toπ^+π^0 n$ reaction, where previous theoretical models had shown deficiencies, have made more evident the need for new mechanisms, so far neglected or inaccurately included. We have updated a previous model to include new necessary mechanisms. We find that the production of the $ρ$ meson, and the $Δ(1700)$ excitation, through interference with the dominant terms, are important mechanisms that solve the puzzle of the $γp\toπ^+π^0 n$ reaction without spoiling the early agreement with the $γp\toπ^+π^- p$ and $γp\toπ^0π^0 p$ reactions.
The meson photoproductions off nucleons in the chiral quark model are described. The role of the S-wave resonances in the second resonance region is discussed, and it is particularly important for the Kaon, $η$ and $η'$ photoproductions.
A multipole analysis of vector meson photoproduction is formulated as a generalization of the pseudoscalar meson case. Expansion of spin observables in the multipole basis and behavior of these observables near threshold and resonances are examined.
We investigate $ρ$-meson photoproduction in a coupled-channels formulation. It is shown that channel coupling effects are large and account for discrepancies observed in several single-channel treatments.
For coherent photoproduction of phi and omega mesons from deuteron at forward angles, the isovector pi-exchange amplitude is strongly suppressed. We show a possibility to study the non-diffractive channels associated with the unnatural parity exchange in phi-photoproduction and with the baryon resonance excitations in omega-photoproduction by measuring the spin observables.
A quark model approach to the photoproduction of vector mesons off nucleons is proposed. Its starting point is an effective Lagrangian of the interaction between the vector meson and the quarks inside the baryon. This paper develops the formalism and highlights the dynamical roles of s-channel resonances. It is demonstrated that the proposed approach is ideal to support searching for the missing resonances in the vector meson photoproduction experiments proposed at TJNAF.
We consider the problem of determining amplitudes from observables for the case of pseudoscalar meson photoproduction. We find a number of surprisingly simple constraints which give necessary conditions for a complete set of measurements. These results contradict one of the selection rules derived previously.
We study coherent and incoherent phi-meson photoproduction from nuclei. The available data are analyzed in terms of single and coupled channel photoproduction. It is found that the data on coherent photoproduction can be well reproduced within a single channel optical model and show only little room for omega-phi mixing. These data indicate a normal distortion of the phi-meson in nuclei, which is compatible with the results obtained through the vector meson dominance model. The data on incoherent phi-meson photoproduction show an anomalous A-dependence resulting in a very strong phi-meson dist…
The diffractive $ϕ$-meson photoproduction amplitude is dominated by the Pomeron exchange process and contains the terms that govern the spin-spin and spin-orbital interactions. We show that these terms are responsible for the spin-flip transitions at forward photoproduction angles and appear in the angular distributions of $ϕ\to K^+K^-$ -decay in reactions with unpolarized and polarized photon beams. At large momentum transfers, the main contribution to the $ϕ$-meson photoproduction is found to be due to the excitation of nucleon resonances. Combined analysis of $ω$ and $ϕ$ photoproduction ind…
Using vector meson dominance (VMD), tensor glueball photoproduction cross sections, asymmetries and widths are calculated. The predicted hadronic VV' decays are comparable for different vector meson (V = rho, omega and phi) channels with the omega phi width the largest but the radiative omega gamma and phi gamma decays are suppressed relative to rho gamma by over a factor of 2. This decay profile is distinct from typical meson branching rates and may be a useful glueball detection signature. Results are compared to a previous VMD scalar glueball study.
An improved version of a recently developed quark model approach to vector meson photoproduction is applied to the investigation of contributions of vector meson photoproduction to the Gerasimov-Drell-Hearn (GDH) sum rule. We find that the sum rule converges at a few GeV's. Contributions to the proton channel are found to be small while to the neutron are relatively large.
Amplitude and partial wave analyses for pion, eta or kaon photoproduction are discussed in the context of `complete experiments'. It is shown that the model-independent helicity amplitudes obtained from at least 8 polarization observables including beam, target and recoil polarization can not be used to determine underlying resonance parameters. However, a truncated partial wave analysis, which theoretically requires only 5 observables will be possible with minimal model input.
We present a quark model study of the $ω$ meson photoproduction near threshold. With a limited number of parameters, all the data in history are reproduced. The roles played by the {\it s}- and {\it u}-channel processes (resonance excitations and nucleon pole terms), as well as the {\it t}-channel {\it natural} (Pomeron) and {\it unnatural} parity (pion) exchanges are clarified. This approach provides a framework for systematic study of vector meson photoproduction near threshold.
Within the Regge approach, we calculated the exclusive $f_1(1285)$ meson photoproduction cross section at energy of few GeV. The calculation shows that the cross section is sizable, being in the range of 100 nb, and much larger than the the expected cross section of the $η(1295)$ meson photoproduction at the same energy. These two facts make possible to use this reaction to study the poor known properties of the $f_1(1285)$ meson in the JLab kinematics.
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.
Photoproduction of omega-meson is analyzed within meson exchange model and Regge model and compared to rho-meson photoproduction. An interplay between two models and uncertainties in data reproduction are discussed.
Ambiguities in pseudoscalar-meson photoproduction, arising from incomplete experimental data, have analogs in pion-nucleon scattering. Amplitude ambiguities have important implications for the problems of amplitude extraction and resonance identification in partial-wave analysis. The effect of these ambiguities on observables is described. We compare our results with those found in earlier studies.
We discuss the photoproduction of omega- and rho0-mesons off protons in the particular channel where the target proton is excited to a Roper resonance N*(1440). We propose a simple meson-exchange model for these processes in order to evaluate their cross sections near threshold and at low momentum transfers. It is suggested in particular that the differential cross section for the associated photoproduction of a rho0-meson and a Roper resonance in these kinematic conditions could provide direct information on the strength of the scalar-isoscalar excitation of the N*(1440) and hence on the magn…
The complete experiment problem for photoproduction of single pseudoscalar mesons is reviewed briefly. If this problem is investigated in the context of a truncated partial wave analysis, the chance emerges to obtain a unique multipole solution measuring less polarization observables than demanded by the classical solution of the problem that is formulated in terms of spin amplitudes.
An investigation of $γp\to ωp$ is presented in a constituent quark model approach. The sparse data in the large $t$ region where the resonances dominate is well described within the model, while the diffractive behavior in the small $t$ region requires an additional t-channel exchange. Taking into account the t-channel $π^0$ exchange, we find a good overall agreement with the available data with only 3 free parameters. Our study shows that the differential cross section is not sensitive to s-channel resonances, however, the polarization observables are demonstrated to be very sensitive. Thus, …
In this talk we report on two physical processes, the photoproduction of $ρ$ mesons in nuclei and the $π^0 π^0$ photoproduction in nuclei at low energies. In the first case the aim is to observe experimentally the theoretically predicted changes on the $ρ$ properties in the nuclear medium. In the second case one wishes to investigate the modifications of the $ππ$ interaction in the nuclear medium in the region of the $σ$ meson. In the first case we observe that it is quite difficult to see large effects due to the $ρ$ modification in the medium because the detection of the $ρ$ is done through …
