The experimental angular distributions for 20Ne+16O elastic transfer are reanalyzed using different forms of potential both phenomenological and semi-microscopic. The significant increase in cross sections at backward hemisphere due to the contribution of alphacluster transfer is investigated using the distorted wave Born approximation (DWBA) method. The spectroscopic amplitude (SA) for the configuration 20Ne as consisting of 16O (core) and an alpha-particle orbiting this core at the different concerned energies is extracted. The agreement between the experimental data and theoretical calculations using the two considered approaches is reasonably good.
Deuteron two-body photodisintegration is analysed within the framework of the Quark-Gluon Strings Model. It is found that the forward-backward angle asymmetry predicted by the model is confirmed by the recent data at different photon energies from Jlab. New calculations for polarization observables, the cross section asymmetry $Σ$ and the polarization transfer $C_{z^{\prime}}$, for photon energies $(1.2\div6)$ GeV are presented and compared with the data available up to 2 GeV.
The elastic backward proton-deuteron scattering is analyzed within a covariant approach based on the Bethe-Salpeter equation with realistic meson-exchange interaction. Contributions of the one-nucleon and one-pion exchange mechanisms to the cross section and polarization observables are investigated in explicit form. Results of numerical calculations for the cross section, tensor analyzing power and spin transfers are presented. The one-pion exchange contribution is essential for describing the spin averaged cross section, while in polarization observables it is found to be less important.
The so called "barrier distribution" derived from quasi-elastic backscattering of heavy ions gives us, in fact, an information about the "reaction threshold distribution". For light nuclear systems these distributions are quite close, but for heavy ones (when interaction of nuclei becomes more and more complicated and the potential barrier can disappear at all) they may significantly differ. The same should hold also for weakly bound neutron rich projectiles, for which the neutron transfer and break-up reaction channels play an important role at near-barrier energies. This is not a terminology…
Backward angle (u-channel) scattering provides complementary information for studies of hadron spectroscopy and structure, but has been less comprehensively studied than the corresponding forward angle case. As a result, the physics of u-channel scattering poses a range of new experimental and theoretical opportunities and questions. We summarize recent progress in measuring and understanding high energy reactions with baryon charge exchange in the u-channel, as discussed in the first backward angle (u-channel) Physics Workshop. In particular, we discuss backward angle measurements and their t…
We report on the first measurement of cross sections for exclusive deeply virtual pion electroproduction off the proton, $e p \to e^\prime n π^+$, above the resonance region at backward pion center-of-mass angles. The $φ^*_π$-dependent cross sections were measured, from which we extracted three combinations of structure functions of the proton. Our results are compatible with calculations based on nucleon-to-pion transition distribution amplitudes (TDAs) and shed new light on nucleon structure.
The threshold electrodisintegration of the deuteron at backward angles is studied with a relativistic Hamiltonian, including a relativistic one-pion-exchange potential (OPEP) with off-shell terms as predicted by pseudovector coupling of pions to nucleons. The bound and scattering states are obtained in the center-of-mass frame, and then boosted from it to the Breit frame, where the evaluation of the relevant matrix elements of the electromagnetic current operator is carried out. The latter includes, in addition to one-body, also two-body terms due to pion exchange, as obtained, consistently wi…
The article is devoted to the relativistic study of elastic $pD$ backward scattering based on the one-nucleon exchange diagram. Calculations were performed using relativistic deuteron wave functions obtained by solving the Bethe-Salpeter equation in the Minkowski space with relativistic separable potentials. The unpolarized differential cross section as well as some polarization observables of the reaction for initial proton momentum up to 7.3 GeV were calculated. The obtained results are compared with the calculations of other authors.
We consider backward vector meson exclusive electroproduction off nucleons in the framework of collinear QCD factorization. Nucleon to vector meson transition distribution amplitudes arise as building blocks for the corresponding factorized amplitudes. In the near-backward kinematics, the suggested factorization mechanism results in the dominance of the transverse cross section of vector meson production ($σ_T \gg σ_L$) and in the characteristic $1/Q^8$-scaling behavior of the cross section. We evaluate nucleon to vector meson TDAs in the cross-channel nucleon exchange model and present estima…
We provide systematic analysis of the differential cross section of the proton-$^3{\rm He}$ elastic scattering at $θ_{\rm cm}=180^{\circ}$ and at $T_p<700$ MeV. Three mechanisms are discussed: 2N pair exchange in the triplet and singlet spin states, pion exchange and direct mechanism. It is shown that t Tp>150 MeV tree-body structure, including triplet and singlet states of the 2N pair, becomes of great importance for understanding energy dependence of the reaction observables. Prediction for the differential cross section and the polarization correlation $C_{00nn} which can be studied now in …
The elastic proton-deuteron backward reaction is analyzed within a covariant approach based on the Bethe-Salpeter equation with realistic meson-exchange interaction. Lorentz boost and other relativistic effects in the cross section and spin correlation observables, like tensor analyzing power and polarization transfer etc., are investigated in explicit form. Results of numerical calculations for a complete set of polarization observables are presented.
Nucleon to meson transition distribution amplitudes (TDAs), non-diagonal matrix elements of nonlocal three quark operators between a nucleon and a meson states, arise within the collinear factorized description of hard exclusive electroproduction of mesons off nucleons in the backward direction. Below we address the problem of modelling pion to nucleon TDAs. We suggest a factorized Ansatz for quadruple distributions with input from the soft pion theorem for pi N TDAs. In order to satisfy the polynomiality property in its full form the spectral representation is complemented with a D-term like …
The mechanism of $p^3$He backward elastic scattering is studied. It is found that the triangle diagrams with the subprocesses $pd\to ^3$He$ π^0$, $pd^*\to ^3$He$ π^0$ and $p(pp)\to^3$He$ π^+$, where $d^*$ and $pp$ denote the singlet deuteron and diproton pair in the $^1S_0$ state, respectively, dominate in the cross section at 0.3-0.8 GeV, and their contribution is comparable with that for a sequential transfer of a $np$ pair at 1-1.5 GeV. The contribution of the $d^*+pp$, estimated on the basis of the spectator mechanism of the $p(NN)\to ^3$He$ π$ reaction, increases the $p^3$He$\to ^3$He$ p$…
The elastic backward proton-deuteron scattering is analyzed including relativistic effects in the deuteron and the mechanism of this reaction which includes the graphs corresponding to the emission, rescattering and absorption of the virtual pion by a deuteron nucleon in addition to the one-nucleon exchange graph. It allows one to obtain a rather satisfactory description of all the experimental data on the differential cross section, tensor analyzing power of the deuteron and transfer polarization in this reaction.
Important $E2$ contributions to the $(e,e^{\prime})$ cross sections of low-lying orbital $M1$ excitations are found in heavy deformed nuclei, arising from the small energy separation between the two excitations with $I^πK = 2^+1$ and 1$^+1$, respectively. They are studied microscopically in QRPA using DWBA. The accompanying $E2$ response is negligible at small momentum transfer $q$ but contributes substantially to the cross sections measured at $θ= 165 ^{\circ}$ for $0.6 < q_{\rm eff} < 0.9$ fm$^{-1}$ ($40 \le E_i \le 70$ MeV) and leads to a very good agreement with experiment. The electric re…
The recent exclusive backward-angle electroproduction of omega from Jefferson Lab Hall C electron-proton fixed-target scattering experiments above the resonance region hints at a new domain of applicability of QCD factorization in a unique u-channel kinematics regime. Thanks to this effort, the interest in studying nucleon structure through u-channel meson production observables has grown significantly. In the fixed target configuration, the u-channel meson electroproduction observables feature a unique interaction picture: the target proton absorbs nearly all momentum induced by virtual photo…
It is shown that owing to a dominance of $np-$pair transfer mechanism of backward elastic $p^3He$- scattering for incident proton kinetic energies $T_p>1$ GeV the cross section of this process is defined mainly by the values of the Faddeev component of the wave function of $^3He$ nucleus, $ϕ^{23}({\bf q}_{23}, {\bf p}_1)$, at high relative momenta $q_{23}> 0.6 GeV/c$ of the NN-pair in the $^1S_0-$ state and at low spectator momenta $p_1\sim 0-0.2$ GeV/c.
Backward-angle meson electroproduction above the resonance region, which was previously ignored, is anticipated to offer unique access to the three quark plus sea component of the nucleon wave function. In this letter, we present the first complete separation of the four electromagnetic structure functions above the resonance region in exclusive omega electroproduction off the proton, e + p -> e' + p + omega, at central Q^2 values of 1.60, 2.45 GeV^2 , at W = 2.21 GeV. The results of our pioneering -u ~ -u min study demonstrate the existence of a unanticipated backward-angle cross section peak…
Backward photoproduction of $ω$ off a proton target is investigated in a Reggeized model where the $u$-channel nucleon Reggeon is constructed from the nucleon Born terms in a gauge invariant way. The $t$-channel meson exchanges are considered as a background. While the $N_α$ trajectory of the nucleon Reggeon reproduces the overall shape of NINA data measured at the Daresbury Laboratory in the range of $u$-channel momentum transfer squared $-1.7<u<0.02$ GeV$^2$ and energies at $E_γ=2.8$, 3.5 and 4.7 GeV, a possibility of parton contributions is searched for through the nucleon isoscalar form fa…
The elastic backward proton-deuteron scattering is analyzed including both relativistic effects in the deuteron and the reaction mechanism. It is shown that inclusion of the graphs corresponding to the emission, rescattering and absorption of the virtual pion by a deuteron nucleon in addition to the one-nucleon exchange graph allows a rather satisfactory description of all the experimental data on the differential cross section, tensor analyzing power of the deuteron and transfer polarization in this reaction.
We argue that the transverse shape of the fireball created in heavy ion collision is controlled by event-by-event fluctuations of the eccentricity vectors for the forward-going and backward-going wounded nucleons: $\vecε_n^{\mathrm{F}}\equiv ε_n^{\mathrm{F}} e^{i nΦ_n^{\mathrm{*F}}}$ and $\vecε_n^{\mathrm{B}}\equiv ε_n^{\mathrm{B}} e^{i nΦ_n^{\mathrm{*B}}}$. Due to the asymmetric energy deposition of each wounded nucleon along its direction of motion, the eccentricity vector of the produced fireball is expected to interpolate between $\vecε_n^{\mathrm{F}}$ and $\vecε_n^{\mathrm{B}}$ along the …
The main features of the $(e,e^{\prime})$ cross sections of low-lying orbital excitations with $K^π = 1^+$ in heavy deformed nuclei are studied in RPA on the example of $^{156}$Gd. The dependence of the DWBA E2 and M1 cross sections on the scattering angle $0^{\circ} < θ< 180 ^{\circ}$ and incident electron energy $E_i < 210$ MeV is analyzed in PWBA. The cross section is larger for M1 than for E2 transitions at any angle if $E_i < 30$ MeV. The longitudinal (Coulomb) C2 excitation dominates the E2 response for $5^{\circ} < θ< 170 ^{\circ}$. Only transverse M1 and E2 excitations compete for $θ> …
The relationship between the backward elastic scattering probabilities and reaction cross sections is derived. This is a a very simple and useful method to extract reaction cross sections for heavy ion systems. We compare the results of our method with those using the traditional full elastic scattering angular distributions, for several systems, at energies near and above the Coulomb barrier. From the calculated reaction and capture cross sections using the present method, we derive the cross sections of other mechanisms for nearly spherical systems.
It is shown that the shoulder observed in the cross section of $p^3He$ backward elastic scattering at proton beam energies of 0.4 -- 0.7 GeV can be explained by the mechanism of virtual $π$-meson production with a dominant contribution from the subprocesses $pd^*\to ^3$He$ π^0$ and $p(pp)\to^3$He$ π^+$, where $d^*$ denotes the singlet deuteron and the $pp$ pair is in the $^1S_0$ state. At high energies, 1-3 GeV, the mechanism of $np$-pair transfer dominates and probes high-momentum $NN(^1S_0$) correlations in $^3$He.
The two-body transfer amplitude for the rearrangement process i+(jkl) - j+(ikl) is constructed on the basis of technique of 4-dimensional covariant nonrelativistic graphs. The developed formalism is applied to describing backward elastic $p^3He$ scattering in the energy range 0.5 - 1.7 GeV. Numerical calculations performed using the 5- channel wave function of the $^3He$ nucleus show that the transfer of a noninteracting np- pair dominates and explains satisfactorily the energy and angular dependence of the differential cross section at energies 0.9 - 1.7$ GeV. A weak sensitivity to high momen…
The elastic backward proton-deuteron scattering is analyzed within a fully covariant approach based on the invariant expansion of the reaction amplitude. The method of calculation of any observables for any reaction mechanism is developed. The cross section and spin observables, like tensor analyzing power and polarization transfer coefficient etc., are investigated in explicit form within framework of the impulse approximation using different kinds of the relativistic deuteron wave function (DWF) with P-wave components. Our results of numerical calculation for complete set of polarization obs…
In this article we propose a simple model which can provide a combined explanation of the $Z\to b\bar b$ forward-backward asymmetry, the Cabibbo Angle Anomaly (CAA), $τ\toμνν$ and $b\to s\ell^+\ell^-$ data. This model is obtained by extending the Standard Model (SM) by two heavy vector-like quarks (an $SU(2)_L$ doublet (singlet) with hypercharge $-5/6$ (-1/3)), two new scalars (a neutral and a singly charged one) and a gauged $L_μ-L_τ$ symmetry. The mixing of the new quarks with the SM ones, after electroweak symmetry breaking, does not only explain $Z\to b\bar b$ data but also generates a lep…
We have measured the beam-normal single-spin asymmetries in elastic scattering of transversely polarized electrons from the proton, and performed the first measurement in quasi-elastic scattering on the deuteron, at backward angles (lab scattering angle of 108 degrees) for Q2 = 0.22 GeV^2/c^2 and 0.63 GeV^2/c^2 at beam energies of 362 MeV and 687 MeV, respectively. The asymmetry arises due to the imaginary part of the interference of the two-photon exchange amplitude with that of single photon exchange. Results for the proton are consistent with a model calculation which includes inelastic int…
We show analytically that in the cumulative particles production off nuclei multiple interactions lead to a glory-like backward focusing effect. Employing the small phase space method we arrived at a characteristic angular dependence of the production cross section $dσ\sim 1/ \sqrt {π- θ}$ near the strictly backward direction. This effect takes place for any number $n\geq 3 $ of interactions of rescattered particle, either elastic or inelastic (with resonance excitations in intermediate states), when the final particle is produced near corresponding kinematical boundary. Such a behaviour of th…
We investigate photoproduction of $ω$ off a nucleon target $γN\toωN$ by using a Reggeized model where $π(135)+σ(500)+f_1(1285)+f_2(1270)$+Pomeron exchanges are included in the $t$-channel for the reaction at forward angles. The reaction mechanism at low energy is featured by the dominance of the $π$ exchange with the absorptive cuts introduced to modulate the pion contribution to both $γp\to ωp$ and $γn\to ωn$ reactions. Necessity of $σ$ exchange is illustrated in the analysis of the cross section from natural parity exchanges. Cross sections for differential, total, and spin density matrix wi…
Baryon to meson transition distribution amplitudes (TDAs), non-diagonal matrix elements of the nonlocal three quark operator between a nucleon and a meson state, extend the concept of generalized parton distributions. These non-perturbative objects which encode the information on three quark correlations inside the nucleon may be accessed experimentally in backward meson electroproduction reactions. We suggest a general framework for modelling nucleon to pion (pi N) TDAs employing the spectral representation for pi N TDAs in terms of quadruple distributions. The factorized Ansatz for quadruple…
Systematic errors in the database of $np$ differential cross sections below 350 MeV are studied. By applying angle-dependent normalizations with the help of the energy-dependent Nijmegen partial-wave analysis PWA93 the $χ^2$-values of some seriously flawed data sets can be reduced significantly at the expense of a few degrees of freedom. It turns out that in these special cases the renormalized data sets can be made statistically acceptable such that they do not have to be discarded any longer in partial-wave analyses of the two-nucleon scattering data.
We study the scaling regime of pion electroproduction in the backward region, eN->e'N'pi. We compute the leading-twist amplitude in the kinematical region, where it factorises into a short-distance matrix element and long-distance dominated nucleon Distribution Amplitudes and nucleon to pion Transition Distribution Amplitudes. Using the chiral limit of the latter, we obtain a first estimate of the cross section, which may be experimentally studied at JLab or Hermes.
