Large Momentum Transfer Research Articles

A COMPREHENSIVE ANALYSIS OF THE PION–PHOTON TRANSITION FORM FACTOR IN THE LEADING ORDER APPROXIMATION

We perform a comprehensive analysis of the pion–photon transition form factor Fπγ(Q2) involving the transverse momentum corrections in the leading order approximation with the present CLEO experimental data, in which the contributions beyond the leading Fock state have been taken into consideration. As is well known, the leading Fock-state contribution dominates of Fπγ(Q2) at large momentum transfer (Q2) region. One should include the contributions beyond the leading Fock state in small Q2 region. In this paper, we construct a phenomenological expression to estimate the contributions beyond the leading Fock state based on its asymptotic behavior at Q2→0. Our present theoretical results agree well with the experimental data in the whole Q2 region. Then, we extract some useful information of the pionic leading twist-2 distribution amplitude (DA) by comparing our results of Fπγ(Q2) with the CLEO data. By taking best fit, we have the distribution amplitude moments, [Formula: see text] and all of higher moments, which are closed to the asymptotic-like behavior of the pion wave function.

Simulations of the temperature dependence of static friction at theN2/Pb interface

A molecular dynamics approach for studying the static friction between two bodies, an insulatorand a metal, as a function of the temperature is presented. The upper block is formed byN2 molecules and the lower block by Pb atoms. In both slabs the atoms are mobile.The interaction potential in each block describes properly the lattice dynamicsof the system. We show that the lattice vibrations and the structural disorderare responsible for the behaviour of the static friction as a function of thetemperature. We found that a large momentum transfer from the Pb atoms to theN2 moleculesmisplaces the N2 planes in the proximity of the interface. AroundT = 20 K this effect produces the formation of an hcp stacking at the interface. By increasing thetemperature, the hcp stacking propagates into the slab, toward the surface. AboveT = 25 K, our analysis shows a sharp, rapid drop of more than three order of magnitude inthe static friction force due to the misplacing of planes in the stacking of thefcc(111) layers, which are no longer in the minimum energy configuration. AboveT = 35 K, we also observe a tendency for the splitting of planes and the formation of steps near thesurface. By increasing the temperature we obtain the subsequent melting of theN2 slab interfacelayer at T = 50 K. The temperature behaviour of the calculated static friction is in good agreement withrecent measurements made with the quartz crystal microbalance (QCM) method on thesame system.

STAR results on high transverse momentum, heavy flavour and electromagnetic probes

We summarize here recent results from the STAR collaboration focusing on processes involving large momentum transfers. Measurements of angular correlations of di-hadrons are explored in both the pseudorapidity (η) and azimuthal (ϕ) projections. In central Au+Au, an elongated structure is found in the η projection which persists up to the highest measured p⊥. After quantifying the particle yield in this structure and subtracting it from the near-side yield, we observe that the remainder exhibits a behaviour strikingly similar to that of the near-side yield in d+Au. For heavy flavour production, using electron–hadron correlations in p + p collisions, we obtain an estimate of the b-quark contribution to the non-photonic electrons in the p⊥ region 3–6 GeV/c, and find it consistent with FONLL calculations. Together with the observed suppression of non-photonic electrons in Au+Au, this strongly suggests suppression of b-quark production in Au+Au collisions. We discuss results on the mid-rapidity ϒ cross-section in p + p collisions. Finally, we present a proof-of-principle measurement of photon–hadron (γ–h) correlations in p + p collisions, paving the way for the tomographic study of the matter produced in central Au+Au via γ-jet measurements.

Post-collisional effects on single ionization in 75 keV p + He collisions

We report fully differential cross-section calculations using distorted wave theories for helium single ionization by 75 keV p impact. Comparisons are made with absolute experimental data and we find that good results are obtained in the magnitude without the need for normalization factors. However, discrepancies are quite apparent in the position and shape of the peak structures in the fully differential angular distribution of the ejected electrons. We assess the influence of the internuclear interaction on low-energy electron emission in the scattering plane and in the perpendicular plane. Our continuum distorted wave-eikonal initial state calculations with (without) the internuclear interaction yield better results for the large (small) momentum transfer. We discuss this behaviour as a consequence of active electron screening for low-energy electron emission.

OPE and sum rules for correlators of pseudoscalar and axial currents

We study the operator product expansion (OPE) and quark-hadron duality for two-and three-point correlators of the axial (A) and pseudoscalar (P) currents of the light quarks. In the chiral limit, these correlators are often dominated by nonperturbative power corrections leading to subtleties of quark-hadron duality relations and of the extraction of properties of light pseudoscalars. For the two-point correlators, we show the sum rule for 〈PP〉 to be sensitive to the excited light pseudoscalar. For the three-point correlators, we derive the Ward identities which provide the normalization of the pion electromagnetic form factor at zero momentum transfer. For large momentum transfer, we demonstrate the way the correct behavior of the pion form factor in agreement with perturbative QCD emerges from condensate terms in the OPE for the 〈PV P〉 and 〈AV P〉 correlators. The local-duality sum rule for 〈AV A〉 is shown to lead to the pion form factor with the required properties for all values of the momentum transfer.

Electron–phonon interaction in the strongly correlated Hubbard model

We study numerically the renormalization of the on-site electron–phonon interaction or vertex as a function of temperature and hole doping density in the two-dimensional strongly correlated Hubbard model. In the underdoped region, we find that the forward scattering process increases monotonically with lowering temperature, whereas the backward scattering process shows little temperature dependence. With decreasing hole doping density, the electron–phonon vertex increases at small phonon momentum transfer, but decreases at large phonon momentum transfer, leading to a pronounced forward scattering peak in the small doping region. Our findings indicate that the electron–phonon interaction is strong in the high- T c superconductors and may play an important role in pairing and low-energy renormalization of quasiparticles.

Effects of the final-state electron-ion interactions on the fully differential cross sections for heavy-particle-impact ionization of helium

Three-dimensional fully differential cross sections for heavy-particle-impact ionization of helium are examined. Previously, the three-body distorted-wave (3DW) model has achieved good agreement with experiment in the scattering plane for small momentum transfers, but poor agreement for large momentum transfers. Poor agreement was also observed outside the scattering plane for all momentum transfers. In particular, the 3DW calculations predicted cross sections that were too small both perpendicular to the scattering plane and for large momentum transfers. The important unanswered question concerns the physical effects that cause the significant disagreement between experiment and theory. In previous works, the role of the projectile-ion interaction has been examined. Although the importance of exchange between the ejected electron and the residual bound electrons has been well established, and frequently studied, for electron-impact ionization, the importance of this effect has not been examined for heavy-particle scattering. In this paper we examine the role of this effect for heavy-particle scattering.

Superconductivity of repulsive carriers

We present the concept of Coulomb repulsion-induced superconducting pairing with large momentum and its application to a qualitative explanation of the underlying properties of cuprates. At low doping, the Fermi surface of the cuprates has the form of hole pockets with both mirror and conventional nesting promoting the superconducting and an insulating pairing, respectively. The kinematic constraint results in a cutoff of large momentum transfers under the Coulomb repulsion. The pairing potential turns out to be oscillating in the real space and ensures a rise of both bound and quasi-stationary states of two particles. The latter can be considered as developed fluctuations of the two-component superconducting order parameter apparent above the transition temperature. The relative phase of the components, as it follows from the Ginzburg–Landau phenomenology, can be associated with an orbital antiferromagnetic order. The competition between the superconducting and insulating orbital antiferromagnetic states leads to the natural explanation of the phase diagram of the cuprates. We explain an universal dependence of the superconducting transition temperature on the number of layers in the unit cell observed in homologous cuprate families.

Analysis of the vector form factors f+ Kπ(Q2) and f- Kπ(Q2) with light-cone QCD sum rules

In this article, we calculate the vector form factors f+ Kπ(Q2) and f- Kπ(Q2) within the framework of the light-cone QCD sum rule approach. The numerical values of f+ Kπ(Q2) are compatible with existing theoretical calculations, and the central value of f+ Kπ(0) (f+ Kπ(0)=0.97) is in excellent agreement with the values from chiral perturbation theory and lattice QCD. The values of |f- Kπ(0)| are very large compared to the theoretical calculations and experimental data, and they cannot give any reliable prediction. At large momentum transfer with Q2 > 5 GeV2, the form factors f+ Kπ(Q2) and |f- Kπ(Q2)| can either show the asymptotic behavior of \(\frac{1}{Q^2}\) or decrease more quickly than \(\frac{1}{Q^2}\); more experimental data are needed to select the ideal sum rules.

The response of many-fermion system in the framework of spectral function and momentum distribution

According to Benhar et al. (BFF), there are significant differences between the two alternative definitions of the incoherent dynamical response function of many-fermion system, i.e., (i) the spectral function and (ii) the momentum distribution function within the impulse approximation (IA). The aim of present article is to look into details of these differences. The coherent and incoherent response functions are calculated and it is shown that at a larger momentum transfer, the coherent part is negligible. By using the impulse approximation, we discuss the condition that both (i) the spectral function and (ii) momentum distribution reproduce identical response functions. In agreement with BFF, we show that the ambiguities arise as a result of two different implementations of IA assumptions which lead to the above alternative formalisms. Finally we present a few examples for the non-interacting and interacting nuclear matter and light nuclei, and we calculate the corresponding nuclear structure factors.

Electron impact ionization of atoms at large momentum transfer: Renormalized plane wave first-order models

We consider electron impact single ionization of an atom near the Bethe ridge in the regime of large energy and momentum transfer. For evaluation of the ionization amplitude, a plane wave Born series is employed. A regularization procedure is utilized in circumventing typical divergences of the higher-order Born terms. The regularized Born series for the ionization amplitude is derived and, on this basis, renormalized analogues of the traditional plane wave Born and impulse approximations are developed. Numerical results using different approximations are presented and analyzed for the case of electron-hydrogen ionization.

Fractional momentum correlations in multiparton collisions at LHC

Multiple parton collisions will represent a rather common feature in pp collisions at the LHC, where regimes with very large momentum transfer may be studied and events rare in lower energy accelerators might occur with a significant rate. A reason of interest in large p t regimes is that, differently from low p t , evolution will induce correlations in x in the multiparton structure functions. We have estimated the cross section of multiple production of W bosons with equal sign, where the correlations in x induced by evolution are particularly relevant, and the cross section of b b ¯ b b ¯ production, where the effects of evolution are much smaller.

Studying close proximity proton pairs in nuclei via large momentum transfer exclusive electron nucleus reactions

The biggest deficiency of the independent particle model of nuclei is in ignoring correlated pairs of nucleons, whose constituents are at short relative distances and have large relative momenta. The experimental exploration of these correlated nucleon pairs has proven to be difficult due to competing mechanisms such as final state rescattering and two-body currents. We recently performed an experiment at Jefferson Lab and measured the 12 C (e,e'p), 12 C (e,e'pp) reactions under conditions chosen to minimize the effects of competing mechanisms. This was achieved using kinematics with high momentum transfer and Bjorken scaling parameter larger than unity. Preliminary results of these measurements are presented.

Analysis of the Σ–n form-factors with light-cone QCD sum rules

In this work, we study the four form-factors f1(Q2), f2(Q2), g1(Q2) and g2(Q2) of the Σ → n in the framework of the light-cone QCD sum rules approach up to twist-6 three valence quark light-cone distribution amplitudes. The f1(0) is the basic input parameter in extracting the CKM matrix element |Vus| from the hyperon decays. The four form-factors f1(Q2), f2(Q2), g1(Q2) and g2(Q2) at intermediate and large momentum transfers with Q2 > 3 GeV2 have significant contributions from the end-point (soft) terms. The numerical values of the four form-factors f1(0), f2(0), g1(0) and g2(0) are compatible with the experimental data and theoretical calculations (in magnitude), although the uncertainties are large.

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.

Super strong nuclear force caused by migrating Kbar mesons - Revival of the Heitler-London-Heisenberg scheme in kaonic nuclear clusters

We have studied the structure of K (-) pp comprehensively by solving this threebody system in a variational method, starting from the Ansatz that the Λ(1405) resonance (≡Λ (*)) is a K (-) p bound state. The structure of K (-) pp reveals a molecular feature, namely, the K (-) in Λ (*) as an "atomic center" plays a key role in producing strong covalent bonding with the other proton. We point out that strongly bound K̄ nuclear systems are formed by "super strong" nuclear force due to migrating real bosonic particles K̄ a la Heitler-London-Heisenberg, whereas the normal nuclear force is caused by mediating virtual mesons. We have shown that the elementary process, p + p → K (+) + Λ (*) + p, which occurs in a short impact parameter and with a large momentum transfer, leads to unusually large self-trapping of Λ (*) by the involved proton, since the Λ (*)-p system exists as a compact doorway state propagating to K (-) pp.

The Fundamental Kaonic Nuclear ClusterK-pp

We have studied the structure of K − pp comprehensively by solving this three-body system exactly in a variational method, starting from the Ansatz that the Λ(1405) resonance (≡ Λ ∗ )i s aK − p bound state. We have found that our original prediction for the presence of K − pp as a compact bound system with M = 2322 MeV/c 2 , BK =4 8 MeV andΓ =6 1 MeV remains unchanged by varying the ¯ KN and NN interactions widely as far as they reproduce Λ(1405). The structure of K − pp reveals a molecular feature, namely, the K − in Λ ∗ as an “atomic center” plays a key role in producing strong covalent bonding with the other proton. We have shown that the elementary process, p+p → K + +Λ ∗ +p, which occurs in a short impact parameter and with a large momentum transfer, leads to unusually large self-trapping of Λ ∗ by the participating proton, since the Λ ∗ -p system exists as a compact doorway state propagating to K − pp.

Electron scattering off simple atoms for large momentum transfer collisions

In a previous paper this author examined the Born expansion and isolated those parts of the expansion that contribute most significantly to the scattering amplitude for large momentum transfer collisions in inelastic collisions from the ground state of both hydrogen and helium. It turned out that certain terms where the scattering electron interacts once with the nucleus and once with the other electron dominate. The physical reason is that large momentum transfer collisions require the nucleus to take the bulk of the incident momentum but require an interaction with the one of the bound electrons to change the state of the atom. The arguments are quite general and this paper will extend this analysis by comparing the inelastic results obtained by this method for hydrogen and helium to a close coupling calculation with many intermediate states. Further, we will extend this analysis to the correction to the 1st Born result for elastic electron–hydrogen and electron–helium collisions and provide some results for scattering from the initial metastable states of hydrogen for large momentum transfer collisions. A comparison of the results of this analytic approach will be made to the numerical close coupling approach and experiments where available. The agreement is remarkable.

An off-axis magnetron-sputtering system forin situstudies of artificial superlattices growth by synchrotron radiation scattering

A sputtering chamber for the growth of artificial superlattices of oxide-based materials is described. The chamber is designed to fit into a standard Huber eight-circle diffractometer. The chamber serves for investigation with synchrotron radiation of growth characteristics of oxide-based artificial superlattices in situ. Two Be windows of large area in the vacuum chamber enable measurement of reflections of X-rays at entrance and exit angles up to approximately 50 degrees . Large perpendicular momentum transfers are practical with this apparatus. The possibility of investigating X-ray scattering in situ is demonstrated by observation of the effects of the modulation length and the stacking period on the growth characteristics of BaTiO(3)/LaNiO(3) artificial superlattices.

X-ray optics for liquid surface/interface spectrometers

A new X-ray optics which enables precise structural investigations of liquid surfaces/interfaces is introduced. The new device is based on the use of high-energy microbeams and gives access to large momentum transfer values perpendicular to the liquid surface/interface. The performance of a prototype of this new optics, which has been constructed and implemented at the high-energy diffraction beamline ID15A at the European Synchrotron Radiation Source, is demonstrated.