Target Nucleons Research Articles

Studies of Final-State Interactions via Helicity Asymmetries in Exclusive Pseudoscalar Meson Photoproduction off Deuteron

Exclusive meson photoproduction off deuteron is a great tool to study final-state interactions (FSI) between the hadrons produced in the scattering of the incident photon off one of the target nucleons and the spectator nucleon. FSI in the reaction \(\gamma d\rightarrow K^+\varLambda n\) allow to access the dynamics of \(\varLambda n\) elastic scattering and provide a method of studying the hyperon-nucleon (YN) interaction. Also, as deuteron is often used as a neutron target, FSI contribute to the reaction dynamics in addition to the quasi-free mechanism and their understanding is critical for the estimate of observables off the free neutron. Here we show preliminary results for beam-helicity asymmetries for the reactions \(\mathbf {\gamma } d\rightarrow K^+\varLambda n\) and \(\mathbf {\gamma } d\rightarrow p\pi ^+\pi ^- n\) and their evolution over the spectator-nucleon momentum. The data were obtained with the CEBAF Large Acceptance Spectrometer at Thomas Jefferson National Accelerator Facility (JLab). We show how to kinematically select an event sample dominated by a specific FSI mechanism. We also discuss a method to extract polarization observables for photoproduction off the free nucleon using the helicity asymmetries for reactions off the deuteron.

Effects of diffraction in pp and pA collisions

Diffractive excitation is a large fraction of the pp cross section, also at high energies. Diffraction has been described by multi-Regge diagrams, or in the Good-Walker formalism as a result of fluctuations. The two formalisms are, however, just different sides of the same phenomenon. The dipole cascade formalism in impact parameter space is well suited to describe diffractive excitation including effects of saturation. Diffractive excitation is also an important effect in pA scattering, where the Glauber formalism has been used to estimate the number of NN subcollisions and of “wounded” nucleons. Diffractive excitation has here been either neglected or included in a simplified way, not including excitation of target nucleons. In this talk we discuss how these effects can be included with the help of the dipole cascade model DIPSY.

Near-threshold J/ψ-meson photoproduction on nuclei

On the basis of the first-collision model that relies on the nuclear spectral function and which includes incoherent processes involving charmonium production in proton–nucleon collisions, the photoproduction of J/ψ mesons on nuclei is considered at energies close to the threshold for their production on a nucleon. The absorption of final J/ψ mesons, their formation length, and the binding and Fermi motion of target nucleons are taken into account in this model along with the effect of the nuclear potential on these processes. The A dependences of the absolute and relative charmonium yields are calculated together with absolute and relative excitation functions under various assumptions on the magnitude of the cross section for J/ψN absorption, the J/ψ-meson formation length, and their inmedium modification. It is shown that, at energies above the threshold, these features are virtually independent of the formation length and the change in the J/ψ-meson mass in nuclear matter but are rather highly sensitive to the cross section for J/ψN interaction. The calculations performed in the present study can be used to determine the unknown cross section for J/ψ-meson absorption in nuclei from a comparison of their results with data expected from experiments in the Hall C of the CEBAF (USA) facility upgraded to the energy of 12 GeV. It is also shown that the absolute and relative excitation functions for J/ψ mesons in photon–nucleus reactions at subthreshold energies are sensitive to the change in the meson mass and, hence, carry information about the properties of charmonium in nuclear matter.

Quasielastic production of polarized hyperons in antineutrino-nucleon reactions

We have studied the differential cross section as well as the longitudinal and perpendicular components of polarization of final hyperon($\Lambda$,$\Sigma$) produced in the antineutrino induced quasielastic charged current reactions on nucleon and nuclear targets. The nucleon-hyperon transition form factors are determined from the experimental data on quasielastic $(\Delta S =0)$ charged current (anti)neutrino--nucleon scattering and the semileptonic decay of neutron and hyperons assuming G--invariance, T--invariance and SU(3) symmetry. The vector transition form factors are obtained in terms of nucleon electromagnetic form factors for which various parameterizations available in literature have been used. A dipole parameterization for the axial vector form factor and the pseudoscalar transition form factor derived in terms of axial vector form factor assuming PCAC and GT relation extended to strangeness sector have been used in numerical evaluations. The flux averaged cross section and polarization observables corresponding to CERN Gargamelle experiment have been calculated for quasielastic hyperon production and found to be in reasonable agreement with the experimental observations. The numerical results for the flux averaged differential cross section$\frac{d\sigma}{dQ^2}$ and longitudinal(perpendicular) polarization $P_L(Q^2)(P_P(Q^2))$ relevant for the antineutrino fluxes of MINER$\nu$A, MicroBooNE, and T2K experiments have been presented. This will be useful in interpreting future experimental results on production cross sections and polarization observables from the experiments on the quasielastic production of hyperons induced by antineutrinos and explore the possibility of determining the axial vector and pseudoscalar form factors in the strangeness sector.

Inclusive single-spin asymmetries, quark-photon, and quark-quark correlations

Generalized Parton Distributions (GPDs) provide information on the distribution of quarks in impact paarmeter space. For transversely polarized nucleons, these impact parameter distributions are transversely distorted and this deviation from axial symmetry leads on average to a net transverse force from the spectators on the active quark in a DIS experiment. %The strength of that force can be related to twist-3 PDFs. This force when acting along the whole trajectory of the active quark leads to a transverse single-spin asymmetries. For a longitudinally polarized nucleon target, the transverse force implies a torque acting on the quark Orbital Angular Momentum (OAM). The resulting change in OAM as the quark leaves the target equals the difference between the Jaffe-Manohar and Ji OAMs.

Transfer reaction code with nonlocal interactions

We present a suite of codes (NLAT for nonlocal adiabatic transfer) to calculate the transfer cross section for single-nucleon transfer reactions, (d,N) or (N,d), including nonlocal nucleon–target interactions, within the adiabatic distorted wave approximation. For this purpose, we implement an iterative method for solving the second order nonlocal differential equation, for both scattering and bound states. The final observables that can be obtained with NLAT are differential angular distributions for the cross sections of A(d,N)B or B(N,d)A. Details on the implementation of the T-matrix to obtain the final cross sections within the adiabatic distorted wave approximation method are also provided. This code is suitable to be applied for deuteron induced reactions in the range of Ed=10–70MeV, and provides cross sections with 4% accuracy. Program summaryProgram title: NLATCatalogue identifier: AFAY_v1_0Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AFAY_v1_0.htmlProgram obtainable from: CPC Program Library, Queen’s University, Belfast, N. IrelandLicensing provisions: GNU General Public License, version 3No. of lines in distributed program, including test data, etc.: 662136No. of bytes in distributed program, including test data, etc.: 15253066Distribution format: tar.gzProgramming language: Fortran 90.Computer: Dell Poweredge R620 (Intel XEON E5-2650v2) 2.9 GHz Intel Core i5.Operating system: Linux (Debian 7) Mac OSX.RAM: Around 3 GBClassification: 17.8, 17.9, 17.11, 17.16.Nature of problem:Calculates cross sections for deuteron induced single-nucleon transfer reactions using nonlocal potentials within the adiabatic distorted wave approximation.Solution method:See ‘Nature of problem’.Running time:Less than 2 hours. See sample output files without extension for indication of time taken.

Di-hadron fragmentation and mapping of the nucleon structure

The fragmentation of a colored parton directly into a pair of colorless hadrons is a non-perturbative mechanism that offers important insights into the nucleon structure. Di-hadron fragmentation functions can be extracted from semi-inclusive electron-positron annihilation data. They also appear in observables describing the semi-inclusive production of two hadrons in deep-inelastic scattering of leptons off nucleons or in hadron-hadron collisions. When a target nucleon is transversely polarized, a specific chiral-odd di-hadron fragmentation function can be used as the analyzer of the net density of transversely polarized quarks in a transversely polarized nucleon, the so-called transversity distribution. The latter can be extracted through suitable single-spin asymmetries in the framework of collinear factorization, thus in a much simpler framework with respect to the traditional one in single-hadron fragmentation. At subleading twist, the same chiral-odd di-hadron fragmentation function provides the cleanest access to the poorly known twist-3 parton distribution $e(x)$, which is intimately related to the mechanism of dynamical chiral symmetry breaking in QCD. When sensitive to details of transverse momentum dynamics of partons, the di-hadron fragmentation functions for a longitudinally polarized quark can be connected to the longitudinal jet handedness to explore possible effects due to $CP-$violation of the QCD vacuum. In this review, we outline the formalism of di-hadron fragmentation functions, we discuss different observables where they appear and we present measurements and future worldwide plans.

Aspects of QCD current algebra on a null plane

Consequences of QCD current algebra formulated on a light-like hyperplane are derived for the forward scattering of vector and axial–vector currents on an arbitrary hadronic target. It is shown that current algebra gives rise to a special class of sum rules that are direct consequences of the independent chiral symmetry that exists at every point on the two-dimensional transverse plane orthogonal to the lightlike direction. These sum rules are obtained by exploiting the closed, infinite-dimensional algebra satisfied by the transverse moments of null-plane axial–vector and vector charge distributions. In the special case of a nucleon target, this procedure leads to the Adler–Weisberger, Gerasimov–Drell–Hearn, Cabibbo–Radicati and Fubini–Furlan–Rossetti sum rules. Matching to the dispersion-theoretic language which is usually invoked in deriving these sum rules, the moment sum rules are shown to be equivalent to algebraic constraints on forward S-matrix elements in the Regge limit.

Extracting squared speed-of-sound parameter from asymmetric nuclear collisions at high energies

We apply the Landau hydrodynamic model in leading target/projectile nucleons and target/projectile cylinders in the multisource thermal model respectively, in asymmetric nuclear collisions. Each leading nucleon source or cylinder source is regarded as an energy reservoir to emit or produce particles and can be described by the Landau hydrodynamic model. The pseudorapidity distribution of final-state particles is in fact a sum of four Gaussian functions which correspond to the contributions of the leading target nucleons, target cylinder, projectile cylinder and leading projectile nucleons, respectively, in accordance with their central rapidities from low to high value. Based on the descriptions of the pseudorapidity distributions of final-state particles produced in proton-emulsion, deuteron-gold and gold-emulsion collisions, the squared speed-of-sound parameters for the four sources in asymmetric nuclear collisions have been extracted and shown to be approximately the same within the statistical error.

Quark Orbital Angular Momentum

Generalized parton distributions provide information on the distribution of quarks in impact parameter space. For transversely polarized nucleons, these impact parameter distributions are transversely distorted and this deviation from axial symmetry leads on average to a net transverse force from the spectators on the active quark in a DIS experiment. This force when acting along the whole trajectory of the active quark leads to transverse single-spin asymmetries. For a longitudinally polarized nucleon target, the transverse force implies a torque acting on the quark orbital angular momentum (OAM). The resulting change in OAM as the quark leaves the target equals the difference between the Jaffe–Manohar and Ji OAMs.

On the extraction of cross sections for π0 and η photoproduction off neutrons from deuteron data

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, \theta^*, where they happen to be quite similar. The corrections for both target nucleons are practically identical for $\pi_0$ production in the energy range of the Delta(1232)3/2+ resonance due to the specific isospin structure of this excitation. Also above the $\Delta$-isobar range large differences between proton and neutron correction factors are only predicted for extreme forward angles ($\theta^*$ < 20 deg), but the results are similar for larger angles. Numerical results for the gp-->pi0p and gn-->pi0n correction factors are discussed. Also the model description for the available data on the differential gd-->pi0pn cross sections are given.

Experimental Facility at A2 Collaboration (MAINZ) for Photoproduction reaction

&lt;p&gt;Since 20 years, a very successful experimental program with real photons has been achieved at the Mainz Microtron (MAMI) facility. The A2 Collaboration at MAINZ is a unique place for the experimental facility of photoproduction reaction. The combination of highly sensitive photo spectrometers Crystal Ball and TAPS as forward wall detector along with particle identification detector and Multiwire proportional chambers as inner wall detectors provide the high precision of the measurement. MAMI-C, which consists of there Race Track Microtrons (RTMs) with the Harmonic Double -Sided Microtron (HDSM) is an intense, stable and continuous-wave accelerator to accelerate electron to 1.5 GeV. This electron beam is responsible to produce real photons via bremsstrahlung process at Glasgow Photon Tagger. A major part of the ongoing and future programme at MAMI will exploit polarised nucleon targets.&lt;/p&gt;&lt;p&gt;Journal of Nepal Physical Society Vol.3(1) 2015: 73-81&lt;/p&gt;

Inelastic electromagnetic production of J/ψ in p–p ultra-peripheral collisions

J/ψ production in p–p ultra-peripheral collisions through the inelastic electromagnetic process, where the virtual photons emitted from the incoming nucleon interact with the partons in the target nucleon, is studied. The comparisons between the results of the equivalent photon approximation approach and the exact treatment ones are presented. Based on the method of Martin and Ryskin, the coherent and incoherent contributions are considered simultaneously. The distributions of Q2 (virtuality of the photon) and the total cross sections are calculated. The numerical results show that, the equivalent photon approximation approach is only effective in the Q2→∞ region where Qmax2 is small enough. It can be seen that an improper choice of Qmax2 will cause obvious errors in the equivalent photon approximation approach (the total cross sections are more than twice larger than the exact ones), and the exact treatment needs to be adopted to dealing with the widely kinematics region of Q2.

Spin-Directed Momentum Transfers in SIDIS Baryon Production

The measurement of transverse single-spin asymmetries for baryon production in the target fragmentation region of semi-inclusive deep-inelastic scattering (SIDIS), can produce important insight into those nonperturbative aspects of QCD directly associated with confinement and with the dynamical breaking of chiral symmetry. We discuss here, interns of spin-directed momentum transfers, the powerful quantum field-theoretical constraints on the spin-orbit dynamics underlying these transverse spin observables. The spin-directed momentum shifts, originating either in the target nucleon or in the QCD jets produced in the deep inelastic scattering process, represent significant quantum entanglement effects connecting information from current fragmentation with observables in target fragmentation.

Single-Spin Asymmetries AULsinϕh in Semi-Inclusive Pions Production

The single-spin asymmetry [Formula: see text] of charged and neutral pion production in semi-inclusive deep-inelastic scattering on longitudinally polarized nucleon targets is studied. We particularly consider the effects of the twist-3 transverse-momentum dependent distribution functions [Formula: see text] and [Formula: see text], which are calculated in two different spectator-diquark models. We estimate the asymmetry for [Formula: see text], [Formula: see text] and [Formula: see text] produced off the proton target at HERMES and compare the results with the HERMES measurements. We also predict the same asymmetric moment for different pions at the kinematics of CLAS 5.5 GeV on a proton target, as well as at COMPASS on a deuteron target for comparison.

Overview of HERMES results

The HERMES experiment has collected a wealth of deep-inelastic scattering data using the 27.6 GeV polarized lepton beam at HERA and various pure gas targets, both unpolarized and polarized. This allowed for a series of diverse and unique measurements. Among them are measurements that provide information on the threedimensional structure of the nucleon, both in momentum space and in position space. Results of measurements of exclusive ω production on an unpolarized and transversely polarized nucleon target, sensitive to the distribution in transverse-position and longitudinalmomentum space, are discussed as well as the three-dimensional extraction of azimuthal asymmetries measured in semi-inclusive deep-inelastic scattering, sensitive to twist-2 and twist-3 distributions in three-dimensional momentum space.

Experimental studies of Generalized Parton Distributions

Generalized Parton Distributions (GPDs) are nowadays the object of an intense effort of research, in the perspective of understanding nucleon structure. They describe the correlations between the longitudinal momentum and the transverse spatial position of the partons inside the nucleon and they can give access to the contribution of the orbital momentum of the quarks to the nucleon spin. Deeply Virtual Compton scattering (DVCS), the electroproduction on the nucleon, at the quark level, of a real photon, is the process more directly interpretable in terms of GPDs of the nucleon. Depending on the target nucleon (proton or neutron) and on the DVCS observable extracted (cross-sections, target- or beam-spin asymmetries, etc.), different sensitivity to the various GPDs for each quark flavor can be exploited. This article is focused on recent promising results, obtained at Jefferson Lab, on cross-sections and asymmetries for DVCS, and their link to GPDs. These data open the way to a “tomographic” representation of the structure of the nucleon, allowing the extraction of transverse-space densities of the quarks at fixed longitudinal momentum. The extensive experimental program to measure GPDs at Jefferson Lab with the 12 GeV-upgraded electron accelerator and the complementary detectors that will be housed in three experimental Halls (A, B and C), will also be presented.

Transverse Spin Structure of the Nucleon

We review the current status and future perspectives of theory and experiments of transverse spin phenomena in high-energy scattering processes off nucleon targets and related issues in nucleon structure and QCD. Systematic exploration of transverse spin effects requires measurements in polarized deep-inelastic scattering, polarized pp collisions, and e+e− annihilations. Sophisticated QCD-based techniques are also needed to analyze the experimental data sets.

Non-resonant kaon pair production and medium effects in proton–nucleus collisions

We study the non-resonant (non-ϕ) production of pairs by protons of 2.83 GeV kinetic energy on C, Cu, Ag and Au targets within the collision model, based on the nuclear spectral function, for incoherent primary proton–nucleon and secondary pion–nucleon creation processes. The model takes into account the initial proton and final kaon absorption, target nucleon binding and Fermi motion as well as nuclear mean-field potential effects on these processes. We calculate the antikaon momentum dependences of the exclusive absolute and relative pair yields in the acceptance window of the ANKE magnetic spectrometer, used in a recent experiment performed at COoler SYnchrotron (COSY), within the different scenarios for the antikaon–nucleus optical potential. We demonstrate that the above observables are strongly sensitive to this potential. Therefore, they can be useful to help determine the optical potential from the direct comparison of the results of our calculations with the data from the respective ANKE-at-COSY experiment. We also show that the pion–nucleon production channels dominate in the low-momentum , production in the considered kinematics and, hence, they have to be accounted for in the analysis of these data.

Simulation of dependence of the cross section of deuterons beam fragmentation into cumulative pions and protons on the mass of the target nucleus

We have studied the mechanisms influencing production of cumulative pions and protons in the fragmentation of the incident deuterons into cumulative pions and protons emitted at zero angle. We argue that the peripheral dependence on the atomic mass of the target nucleus, which was obtained in the experiments for medium and heavy nuclei, can be explained by scattering on target nucleons without introducing additional parameters.