Large Momentum Transfer Region Research Articles

Coherent photoproduction of a [formula omitted]-meson on a tensor-polarized deuteron at large momentum transfer

Measured values of the tensor analyzing power T20 for the coherent photoproduction of a π0-meson on a deuteron are analyzed in the region of large momentum transfer. The incident photons of the photoproduction reaction γd→dπ0 are the quasi-real virtual photons arising from the electron scattering in the electroproduction reaction ed→edπ0. The kinematic region of large momentum transfer is of particular interest, as it corresponds to small inter-nucleon distances in the deuteron. In this case, the amplitude of the process under study is noticeably sensitive to both the deuteron wave function at small inter-nucleon distances and the various model uncertainties arising from the description of the process for large momentum transfer. A comparison shows a significant discrepancy between the experimental data and the predictions from a number of theoretical models.

“K−pp”, a [formula omitted]-meson nuclear bound state, observed in 3He(K−,Λp)n reactions

We observed a distinct peak in the Λp invariant mass spectrum of He3(K−,Λp)n, well below mK+2mp, i.e., the mass threshold of the K− to be bound to two protons. By selecting a relatively large momentum-transfer region q=350∼650 MeV/c, one can kinematically separate the peak from the quasi-free process, K‾N→K‾N followed by the non-resonant absorption by the two spectator-nucleons K‾NN→ΛN. We found that the simplest fit to the observed peak gives us a Breit–Wigner pole position at BKpp=47±3(stat.)−6+3(sys.) MeV having a width ΓKpp=115±7(stat.)−20+10(sys.) MeV, and the S-wave Gaussian reaction form-factor parameter QKpp=381±14(stat.)−0+57(sys.) MeV/c, as a new form of the nuclear bound system with strangeness – “K−pp”.

The Role of the Two Relativistic Spin Amplitudes in the Description of the Experimental Data of Elastic p–p Scattering at Energies in the Range 2–7000 GeV in the Mathematical Eikonal Method

Results of a theoretical calculation of the differential cross sections of elastic proton-proton scattering at energies in the range 2–7000 GeV with two relativistic spin amplitudes are analyzed within the framework of the mathematical eikonal method. It is shown that taking account the contribution of heavy meson resonances improves the agreement between the theoretical calculations and the experimental data somewhat in the region of large momentum transfers.

Transverse charge and magnetization densities based on Regge parameterization

In this paper, we discuss the charge and magnetization densities for proton and neutron in the transverse plane, whereas there are links between the generalized parton distributions (GPDs) and elastic form factors by means of sum rules. We use the extended Regge parameterization for large momentum transfer region for calculating Dirac and Pauli form factors which have been described in the electromagnetic form factors data, so we want to calculate the charge and magnetization densities for proton and neutron in the transverse plane and we also calculate [Formula: see text] and [Formula: see text] quark charge and magnetization densities. The extracted results are compared with other previous parameterizations.

Extrapolation and unitarity bounds for the B → π form factor

We address the problem of extrapolating the vector form factor $f_{B\pi}^+$, which is relevant to $B\to \pi\ell \nu_\ell$ decays, from the region of small to the region of large momentum transfer. As input, we use the QCD light-cone sum rule at small momentum transfer. We carry out a comprehensive Bayesian uncertainty analysis and obtain correlated uncertainties for the normalization and shape parameters of the form factor. The $z$-series parametrization for $f_{B\pi}^+$ is employed to extrapolate our results to large momentum transfer, and to compare with the lattice QCD results. To test the validity of our extrapolation we use the upper and lower bounds from the unitarity and positivity of the two-point correlator of heavy-light quark currents. This correlator is updated by including the NNLO perturbative term and the NLO correction to the quark condensate contribution. We demonstrate that an additional input including the form factor, its first and second derivative calculated at one value of momentum transfer from the light-cone sum rules, considerably improves the bounds. This only holds when the correlations between the form factor parameters are taken into account. We further combine our results with the latest experimental measurements of $B\to \pi \ell \nu_\ell$ by the BaBar and Belle collaborations, and obtain $|V_{ub}|= (3.32^{+0.26}_{-0.22}) \cdot 10^{-3}$ from a Bayesian analysis.

Inelastic x-ray scattering study of the state-resolved differential cross section of Compton excitations in helium atoms

The state-resolved differential cross sections for both the $1{s}^{2}$ ${}^{1}{S}_{0}\ensuremath{\rightarrow}1s2s$ ${}^{1}{S}_{0}$ monopolar transition and the $1{s}^{2}$ ${}^{1}{S}_{0}\ensuremath{\rightarrow}1s2p$ ${}^{1}{P}_{1}$ dipolar transition of atomic helium had been measured over a large momentum transfer region by high-resolution inelastic x-ray scattering (IXS). The almost-perfect match of the present measurement with the theoretical calculations gives a stringent test of the theoretical method and the calculated wave functions. Our results demonstrate that high-resolution IXS is a powerful tool for studying the excitations in atoms and molecules.

Single-crystal lattice dynamics derived from polycrystalline inelastic x-ray scattering spectra

We present a methodology to extract the single-crystal phonon dispersion from polycrystalline materials. The approach consists of recording inelastic x-ray scattering (IXS) spectra over a large momentum transfer region (typically $2--80\text{ }{\text{nm}}^{\ensuremath{-}1}$), and confront them with a model calculation which properly takes into account the polycrystalline state of the material and the IXS cross section. A least-squares refinement of the model spectra then provides the single-crystal dispersion scheme. For the benchmark cases beryllium and graphite we demonstrate that very good agreement with available single-crystal phonon dispersions is obtained. Furthermore, we show for the case of graphite that a simple texture can be properly taken into account. The proposed method promises to be a valuable tool in cases where single-crystalline materials are not available.

Effects of Velocity-Dependent Force on the Magnetic Form Factors of Odd-Z Nuclei

We investigate the effects of the velocity-dependent force on the magnetic form factors and magnetic moments of odd-Z nuclei. The form factors are calculated with the harmonic-oscillator wavefunctions. It is found that the contributions of the velocity-dependent force manifest themselves in the very large momentum transfer region (q  4fm-1). In the low and medium q region the contributions of the velocity-dependent force are very small compared with those without this force. However, in the high-q region the contributions of the velocity-dependent force are larger than the normal form factors. The diffraction structures beyond the existing experimental data are found after the contributions of the velocity-dependent force are included. The formula of the correction to the single particle magnetic moment due to the velocity-dependent force is reproduced exactly in the long-wavelength limit (q = 0) of the M1 form factor.

Inelastic X-Ray Scattering Study of Exciton Properties in an Organic Molecular Crystal

Excitons in a complex organic molecular crystal were studied by inelastic x-ray scattering (IXS) for the first time. The dynamic dielectric response function is measured over a large momentum transfer region, from which an exciton dispersion of 130 meV is observed. Semiempirical quantum chemical calculations reproduce well the momentum dependence of the measured dynamic dielectric responses, and thus unambiguously indicate that the lowest Frenkel exciton is confined within a fraction of the complex molecule. Our results demonstrate that IXS is a powerful tool for studying excitons in complex organic molecular systems. Besides the energy position, the IXS spectra provide a stringent test on the validity of the theoretically calculated exciton wave functions.

Quark-gluon vertex with an off-shellO(a)-improved chiral fermion action

We perform a study the quark-gluon vertex function with a quenched Wilson gauge action and a variety of fermion actions. These include the domain wall fermion action (with exponentially accurate chiral symmetry) and the Wilson clover action both with the non-perturbatively improved clover coefficient as well as with a number of different values for this coefficient. We find that the domain wall vertex function behaves very well in the large momentum transfer region. The off-shell vertex function for the on-shell improved clover class of actions does not behave as well as the domain wall case and, surprisingly, shows only a weak dependence on the clover coefficient $c_{SW}$ for all components of its Dirac decomposition and across all momenta. Including off-shell improvement rotations for the clover fields can make this action yield results consistent with those from the domain wall approach, as well as helping to determine the off-shell improved coefficient $c_q^\prime$.

Nucleon form factors from generalized parton distributions

We discuss the links between Generalized Parton Distributions (GPDs) and elastic nucleon form factors. These links, in the form of sum rules, represent powerful constraints on parametrizations of GPDs. A Regge parametrization for GPDs at small momentum transfer, is extended to the large momentum transfer region and it is found to describe the basic features of proton and neutron electromagnetic form factor data. This parametrization is used to estimate the quark contribution to the nucleon spin.

Interpretation ofyScaling of the Nuclear Response

The behavior of the nuclear matter response in the region of large momentum transfer, in which plane wave impulse approximation predicts the onset of y-scaling, is discussed. The theoretical analysis shows that scaling violations produced by final state interactions are driven by the momentum dependence of the nucleon-nucleon scattering cross section. Their study may provide valuable information on possible modifications of nucleon-nucleon scattering in the nuclear medium.

Diffractive light vector meson production at large momentum transfers.

The diffractive process $\gamma^*(Q^2) p\to V+X$ (where $V= \rho^0, \omega , \phi$ are the vector mesons, consisted of light quarks, $X$ represents the hadrons to that a proton dissociates) is studied. We consider the region of large momentum transfers, $|t|>>\Lambda^2_{QCD}$, and large energies, s. In the leading log approximation of perturbative QCD ( using BFKL equation ) the asymptotic behaviour of the cross section in the limit $s\to\infty , s>>|t|, Q^2$ is obtained. We compare the results derived from BFKL equation with that obtained in the lowest order of QCD (two--gluon exchange in the $t$- channel). The possibility to investigate these reactions at HERA is discussed.

Diffractive vector meson production at large momentum transfer

The diffractive process γp→V+X (whereV is a vector meson andX results from the dissociation of the proton) is studied. In particular, we consider the region of large momentum transfer (i.e. |t| ≫Λ QCD 2 ) and large centre-of-mass (CM) energy,s. The asymptotic (s→∞,s/|t|≫1) behaviour is derived from the BFKL equation and compared to that which is obtained in the Born approximation (two-gluon exchange). We also calculate the corrections to the Born graphs by iterating the BFKL kernel numerically. Improved convergence of the BFKL series is found by summing the logarithms which occur when an exchanged gluon goes nearly on shell. Importantly, we find evidence that the asymptotic solution to the BFKL equation is inappropriate over most of the HERA range and we provide more realistic predictions for the cross section. The predicted cross section is not too small and can be measured at HERA, up to momentum transfers |t|≈10 GeV2.

Multiquark configuration effect on nuclear charge form factors in hadron-quark hybrid model

High-energy electron scattering from nuclear target has revealed features that were not known before. A number of nuclear theoretical approaches have been made to explain the elastic electron scattering from helium isotopes in the region of large momentum transfers. A multiquark clustering effect is invoked to supplement the ordinary nuclear theoretical approach in explaining the nuclear form factors of light nuclei, namely3He,4He, and6Li. In all these three nuclei it is observed that ordinary nuclear-physics component dominates in the region of low momentum transfer but decreases with increasing the momentum transfer. The multiquark components become dominant in the higher-momentum-transfer regions.

Isoscalar spin response functions in the quasielastic scattering region

The role of particle-hole correlations in isoscalar spin response functions in the quasielastic scattering region is examined within the continuum RPA. Both spin-longitudinal and spin-transverse response functions are appreciably modified from the free response function without correlations, due to the repulsive central force and the exchange term of the tensor force in the particle-hole interaction. The latter force also induces an appreciable difference between the two isoscalar spin response functions. It is shown that the ground-state correlation (the backward-going contribution) due to the tensor force is important in the large momentum transfer region and is not weakened, even at large energy transfer. The experimental ratio between the two spin response functions at q=1.75fm −1 observed in (p, p') reactions is not largely modified around the quasielastic scattering peak by including particle-hole correlations in the spin-isoscalar mode. On the other hand, it is somewhat reduced in the large energy transfer region above the quasielastic scattering peak.

INCLUSIVE REACTIONS WITH THREE POLARIZED PARTICLES

In the reaction p+p→Λ+X with up to all three known particles polarized (which will be studied in Fermilab’s upcoming polarization facility) the relationship of the various polarization observables to the 20 three-in-three-out amplitudes is simply given by two 4-by-4 and 12 2-by-2 submatrices. The QCD predictions are said to be the firmest in the large momentum transfer region. There, in the lowest order perturbative calculations only two amplitudes are nonzero, thus yielding only six nonzero polarization quantities with four relations among them. For smaller momentum transfers, Regge model predictions for double correlation observables may be tested even though such models have failed to predict the large single Λ polarization which has been the subject of so much speculation in recent years. The three models which attempt to explain this large polarization can also be tested in these reactions with up to three polarized particles since they should yield small asymmetries for the incident particles. All in all, such polarization experiments will significantly strengthen the testing of the various proposed theoretical models.

Yscaling in a simple quark model

A simple quark model is used to define a nuclear pair model, that is, two composite hadrons interacting only through quark interchange and bound in an overall potential. An equivalent hadron model is developed, displaying an effective hadron-hadron interaction which is strongly repulsive. We compare the effective hadron model results with the exact quark model observables in the kinematic region of large momentum transfer, small energy transfer. The nucleon response function in this y-scaling region is, within the traditional framework, sensitive to the nucleon momentum distribution at large momentum. We find a surprisingly small effect of hadron substructure. Furthermore, we find in our model that a simple parametrization of modified hadron size in the bound state, motivated by the bound quark momentum distribution, is not a useful way to correlate different observables.

HARD QUARK-QUARK SCATTERING WITH EXCLUSIVE REACTIONS*

We have begun a program designed to study hard quark-quark scattering with exclusive reactions, focusing on quasi-elastic two-body reactions with all possible quark flavor exchanges. Examples are ..pi../sup -/p ..-->.. ..pi../sup -/p, rho/sup -/p, ..pi../sup +/..delta../sup -/, K/sup +/..sigma../sup -/, or K..lambda... Of the two-body exclusives, only elastic scattering had been measured at such large t previous to our experiment. By comparing the relative importance of different final states, the energy dependence of the production ratios of these states, the prominence of resonances such as rho/sup -/ over background in this region, and measuring polarizations where accessible, we have collected a large body of data on hard scattering in a completely new domain. Previously, essential all short distance QCD tests have been for inclusive processes. We have taken data with both negative and positive incident beam at 10 GeV/c on a hydrogen target and will present the first results, for ..pi../sup -/p ..-->.. ..pi../sup -/p and rho/sup -/p at THETA/sub cm/ = 90/sup 0/, -t = 9 GeV/sup 2//c/sup 2/. The apparatus consists of a magnetic spectrometer, with Cerenkov particle identification, which selects stable charged particles (protons in this case) at high momentum near 90/sup 0/ in the center-of-mass. Amore » large aperture array of PWCs observes the recoil particle or charged decay products. Cross sections are extremely low, approximately a 1 nb/(GeV/c)/sup 2/ for elastic scattering. We will report on a sample of more than 1000 ..pi../sup -/p elastic events, and on rho/sup -/p, where the rho/sup -/ decay distribution was observed. We find a surprisingly large rho/sup -/p cross section in this large momentum transfer region, with rho/sup -/p about half the elastic cross section, and a striking spin alignment of the rho/sup -/.« less

On the six-quark structure in the deuteron form factor

The charge and quadrupole deuteron form factors are investigated in a wide region of momentum transfer taking into account the deuteron six quark structure. It is shown that the contribution due to the antisymmetrization of the wave function with respect to quark variables is small and the contribution of the “true” six quark admixtures plays the leading role in the region of large momentum transfer.