Region Of Small Momentum Transfer Research Articles

Erratum: Nucleon form factors and root-mean-square radii on a (10.8 fm)4 lattice at the physical point [Phys. Rev. D 99 , 014510 (2019)

We present the nucleon form factors and root-mean-square (RMS) radii measured on a (10.8 fm$)^4$ lattice at the physical point. We compute the form factors at small momentum transfer region in $q^2\le 0.102$ GeV$^2$ with the standard plateau method choosing four source-sink separation times $t_{\rm sep}$ from 0.84 to 1.35 fm to examine the possible excited state contamination. We obtain the electric and magnetic form factors and their RMS radii for not only the isovector channel but also the proton and neutron ones without the disconnected diagram. We also obtain the axial-vector coupling and the axial radius from the axial-vector form factor. We find that these three form factors do not show large $t_{\rm sep}$ dependence in our lattice setup. On the other hand, the induced pseudoscalar and pseudoscalar form factors show the clear effects of the excited state contamination, which affect the generalized Goldberger-Treiman relation.

Electrical conductivity of hot Abelian plasma with scalar charge carriers

We study the electrical conductivity of hot Abelian plasma containing scalar charge carriers in the leading logarithmic order in coupling constant $\alpha$ using the Boltzmann kinetic equation. The leading contribution to the collision integral is due to the M{\o}ller and Bhabha scattering of scalar particles with a singular cross section in the region of small momentum transfer. Regularizing this singularity by taking into account the hard thermal loop corrections to the propagators of intermediate particles, we derive the second order differential equation which determines the kinetic function. We solve this equation numerically and also use a variational approach in order to find a simple analytical formula for the conductivity. It has the standard parametric dependence on the coupling constant $\sigma\approx 2.38\, T/(\alpha \log\alpha^{-1})$ with the prefactor taking a somewhat lower value compared to the fermionic case. Finally, we consider the general case of hot Abelian plasma with an arbitrary number of scalar and fermionic particle species and derive the simple analytical formula for its conductivity.

Nucleon form factors and root-mean-square radii on a (10.8 fm)4 lattice at the physical point

We present the nucleon form factors and root-mean-square (RMS) radii measured on a ${(10.8\text{ }\text{ }\mathrm{fm})}^{4}$ lattice at the physical point. We compute the form factors at small momentum transfer region in ${q}^{2}\ensuremath{\le}0.102\text{ }\text{ }{\mathrm{GeV}}^{2}$ with the standard plateau method choosing four source-sink separation times ${t}_{\mathrm{sep}}$ from 0.84 to 1.35 fm to examine the possible excited state contamination. We obtain the electric and magnetic form factors and their RMS radii for not only the isovector channel but also the proton and neutron ones without the disconnected diagram. We also obtain the axial-vector coupling and the axial radius from the axial-vector form factor. We find that these three form factors do not show large ${t}_{\mathrm{sep}}$ dependence in our lattice setup, and those RMS radii are consistent with the experimental values. On the other hand, the induced pseudoscalar and pseudoscalar form factors show the clear effects of the excited state contamination, which affect the generalized Goldberger-Treiman relation.

Nucleon form factors on a large volume lattice near the physical point in 2+1 flavor QCD

We present results for the isovector nucleon form factors measured on a $96^4$ lattice at almost the physical pion mass with a lattice spacing of 0.085 fm in 2+1 flavor QCD. The configurations are generated with the stout-smeared $O(a)$-improved Wilson quark action and the Iwasaki gauge action at $\beta$=1.82. The pion mass at the simulation point is about 146 MeV. A large spatial volume of $(8.1~{\rm fm})^3$ allows us to investigate the form factors in the small momentum transfer region. We determine the isovector electric radius and magnetic moment from nucleon electric ($G_E$) and magnetic ($G_M$) form factors as well as the axial-vector coupling $g_A$. We also report on the results of the axial-vector ($F_A$), induced pseudoscalar ($F_P$) and pseudoscalar ($G_P$) form factors in order to verify the axial Ward-Takahashi identity in terms of the nucleon matrix elements, which may be called the generalized Goldberger-Treiman relation.

Semileptonic decays of Bc meson to P -wave charmonium states

Inspired by a series of unexpected measurements of semileptonic decays mediated via $b\rightarrow c $ charged current interactions, we explore semileptonic $B_c$ decays to the four lightest $P$-wave charmonium states, $\chi_{c0}, \chi_{c1}, \chi_{c2}, h_c$, by the recently developed improved perturbative QCD formalism, in which the charm quark mass effect is included both in the Sudakov factor and the hard kernels. We first directly evaluate the concerned transition form factors with vector and axial-vector currents in the region of small momentum transfer squared, and then recast them to the full kinematical region by adopting the exponential parametrization. The obtained form factors are used to evaluate the semileptonic decay branching ratios, which can reach the order of $10^{-3}$, letting the corresponding measurement appear feasible. For a better analysis, a comparison of our results with the predictions of other models is provided. We also present the ratios between the tau and light lepton branching ratios and the polarization contributions in the relevant processes, which still need experimental tests in the ongoing and forthcoming experiments. Any significant deviations from the Standard Model results may provide some hints of new physics effects.

Two-Photon Exchange Correction in Elastic Lepton–Proton Scattering

We present the dispersion relation approach based on unitarity and analyticity to evaluate the two-photon exchange contribution to elastic electron-proton scattering. The leading elastic and first inelastic $\pi N$ intermediate state contributions are accounted for in the region of small momentum transfer $Q^2 < 1~\mathrm{GeV}^2$ based on the available data input. The novel methods of analytical continuation allow us to exploit the MAMI form factor data and the MAID parameterization for the pion electroproduction amplitudes as input in the calculation. The results are compared to the recent CLAS, VEPP-3 and OLYMPUS data as well as to the full two-photon exchange correction in the near-forward approximation, which is based on the Christy and Bosted unpolarized structure functions fit. Additionally, predictions are given for a forthcoming muon-proton scattering experiment.

Elastic squared form factor and binding effect of carbon dioxide studied by the high resolution X-ray scattering

The elastic squared form factor of gaseous carbon dioxide was measured for the first time by high resolution X-ray scattering to separate the elastic and inelastic scattering. Bond lengths of rCO = 1.172 ± 0.008 Å and rOO = 2.344 ± 0.016 Å of CO2 were determined from the measured elastic squared form factors, which are in good agreement with the NIST benchmark data of rCO = 1.1621 Å and rOO = 2.3242 Å. Moreover, the obtained elastic squared form factor difference shows a clear binding effect in the region of small momentum transfers. The present work shows that high resolution X-ray scattering is a powerful tool to study the binding effect of molecules.

Transverse response of bound single-nucleon Dirac-4 spinors of spin-orbit partners to elastic electron scattering

Transverse responses of the upper components, lower components, and crossed term of the bound single-nucleon Dirac-4 spinors of spin-orbit partners to elastic magnetic electron scattering are investigated. For the nuclear spin-orbit partners with $l\ensuremath{\le}3$ or in the $s$-d $f$shell or lower, results show that in the elastic magnetic electron scattering process the lower component of the $j=(l\ensuremath{-}\frac{1}{2})$ partner may play a relatively more important role than that of the $j=(l+\frac{1}{2})$ partner, while the upper component of the $j=(l+\frac{1}{2})$ partner may play a relatively more important role than that of $j=(l\ensuremath{-}\frac{1}{2})$ partner. The results also show that for the response of the crossed term the $j=(l+\frac{1}{2})$ partner may play a relatively more important role than the $j=(l\ensuremath{-}\frac{1}{2})$ partner. In addition, it is also found that the dominant $M1$ multipole of the magnetic form factors in the small momentum transfer region may mainly originate from the convection current transition density.

Rapidity and momentum transfer distributions of coherent J/ψ photoproduction in ultraperipheral pPb collisions at the LHC

Based on accurate calculations of the flux of equivalent photons of the proton and heavy nuclei and the pQCD framework for the gluon distribution in the proton and nuclei, we analyze the rapidity and momentum transfer distributions of coherent J/ψ photoproduction in ultraperipheral proton-Pb collisions at the LHC. We demonstrate that unlike the case of proton-proton UPCs marred by certain theoretical uncertainties and experimental limitations, after a cut excluding the region of small momentum transfers, ultraperipheral proton-Pb collisions offer a clean way to study the gluon distribution in the proton down to x ≈ 10−5. Our analysis of the momentum transfer distributions shows that an interplay of J/ψ production by low-energy photons on the nucleus and by high-energy photons on the proton in proton-Pb UPCs can result in some excess of events at small p t in a definite region of the rapidity y.

Hadronic Reactions and the Rearrangement of Sakatons

From the viewpoint of the composite theory of hadron, various kinds of hadronic reactions are discussed in terms of a model of the sakaton rearrangements (SR). It is shown that the SR diagrams are closely related with the phenomenological characteristics of hadronic reactions such as the existence or absence of resonant states in the s-, t- and u-channels, etc., real to imaginary ratios of scattering amplitudes, dip structure of differential cross sections in the small momentum transfer region, the energy dependence of the cross sections for the two-body and the multiple-production processes at high energies and so on.

B→D (*) form factors from QCD light-cone sum rules

We derive new QCD sum rules for B→D and B→D * form factors. The underlying correlation functions are expanded near the light-cone in terms of B-meson distribution amplitudes defined in HQET, whereas the c-quark mass is kept finite. The leading-order contributions of two- and three-particle distribution amplitudes are taken into account. From the resulting light-cone sum rules we calculate all B→D (*) form factors in the region of small momentum transfer (maximal recoil). In the infinite heavy-quark mass limit the sum rules reduce to a single expression for the Isgur–Wise function. We compare our predictions with the form factors extracted from experimental B→(*) l ν l decay rates fitted to dispersive parameterizations.

Neutron diffraction data and molecular dynamics simulations of the molten mixture Ag(Br0.7I0.3)

The structure factors of the ionic liquid mixture Ag(Br(0.7)I(0.3)) at three temperatures, 723, 923, and 1023 K, as well as of the pure molten AgI at 923 K and the pure molten AgBr at 773 and 923 K, were studied experimentally and by means of molecular dynamics simulations. The experiments were carried out using the high intensity total scattering time-of-flight spectrometer, HIT-II, at the KENS spallation neutron source in Japan. The experimental data are very reliable, with the possible exception of the small momentum transfer region, whose accessibility is limited by neutron energy and detector positions. The simulations made use of the semiempirical rigid ion potentials of the Vashishta-Rahman [Phys. Rev. Lett. 40, 1337 (1978)] type using a new set of parameters appropriate for the mixture. Within the known constraints of the pairwise rigid ion potentials, the simulated structure factors are in fair agreement with experiment. The results for the pair distribution functions suggest that the molten mixture retains the superionic character found in previous calculations of both the AgI and AgBr melts. This suggestion is confirmed by the results for the self-diffusion coefficients. Values obtained for the ionic conductivities are also presented.

BFKL pomeron in string models

We consider scattering amplitudes in string models in the Regge limit of high energies and fixed momentum transfers with the use of the unitarity in direct channels. Intermediate states are taken in the multi-Regge kinematics corresponding to the production of resonances with fixed invariant masses and large relative rapidities. In QCD such kinematics leads to the BFKL equation for the pomeron wave function in the leading logarithmic approximation. We derive a similar equation in the string theory and discuss its properties. The purpose of this investigation is to find a generalization of the BFKL approach to the region of small momentum transfers where non-perturbative corrections to the gluon Regge trajectory and reggeon couplings are essential. The BFKL equation in the string theory contains additional contributions coming from a linear part of the Regge trajectory and from the soft pomeron singularity appearing already in the tree approximation. In higher dimensions in addition, a non-multi-Regge kinematics corresponding to production of particles with large masses is important. We solve the equation for the pomeron wave function in the string theory for D = 4 and discuss integrability properties of analogous equations for composite states of several reggeized gluons in the multi-colour limit.maren.stein@desy.de.

ϕ Electro-production in Pomeron Exchange Model**Theproject supported in part by National NaturalScience Foundation of China under Grant No. 10247004,the Natural Science Foundation of Guangxi Provinceunder Grant No. 0339008, and Guangxi University underContract No. 022014

Based on the Pomeron exchange model, elastic production of ϕmeson in electron-proton interaction is investigated with bothlinear and non-linear Pomeron trajectories. The numericalcalculations of the differential cross section for e + p→ e' + p + ϕ are performed. The theoreticalpredictions show that the dependence of the differential crosssection on virtual photon virtuality, Q2, is of moderation,the change of the energy scale parameter s0 causes moderateeffect on the differential cross section, and the lineartrajectory is a good approximation to non-linearity of the Pomerontrajectory, in particular, at small momentum transfer region ∣ t ∣ ⩽ 0.2 GeV2.

Skewed quark distribution of the pion in the light-front quark model

We calculate the skewed quark distributions(SQDs) of the pion in the light-front quark model, and discuss the calculation of the nonvalence contribution to the SQDs in this model. The frame-independence of our model calculation is guaranteed by the constraint of the sum rule between the SQDs and form factor. Our numerical results show large nonvalence contributions to the SQDs at small momentum transfer region as the skewedness increases.

Role of charge-density fluctuations and many-particle Coulomb correlations in the mechanism of high-temperature superconductivity of cuprate metal-oxide compounds

The charge-fluctuation (plasmon) mechanism of d-wave Cooper pairing in high-Tc superconductors (HTSCs) is considered. This mechanism arises from the interaction of current carriers with collective low-frequency electron-density excitations. It is shown that for layered crystals of cuprate metal-oxide compounds the existence of a one-electron spectrum of anisotropic extended saddles (flat bands) with an anomalously high density of states can give rise to damped long-wavelength charge-density fluctuations that lead to suppression of the static screened Coulomb repulsion in the region of small momentum transfers. As a result, an effective attraction between electrons arises in the d-wave Cooper channel; this attraction is substantially enhanced by many-particle Coulomb correlations of the local-field-effects type, described by a Coulomb vertex Γc. It leads to Cooper pairing with dx2−y2 symmetry of the superconducting order parameter and can make for rather high maximum values of the critical temperature, Tc∼100 K, at the optimum doping level for cuprate metal-oxide compounds. Taking the anisotropy of the electron–phonon interaction into account makes it possible to describe the weak oxygen isotope effect in HTSC compounds.

Differential dispersion relations with an arbitrary number of subtractions: a recursive approach

Making use of a recursive approach, derivative dispersion relations are generalized for an arbitrary number of subtractions. The results for both cross even and odd amplitudes are theoretically consistent at sufficiently high energies and in the region of small momentum transfer.

The soft pomeron and non-perturbative corrections in quark-quark scattering

We consider elastic quark-quark scattering at high energy with fixed momentum transfer t and perform factorization of soft-gluon exchanges into a vacuum expectation value of Wilson lines. Taking into account non-perturbative corrections whose structure is predicted from an infrared renormalon analysis, we represent the scattering amplitude as an asymptotic series. In the region of small momentum transfer, where the non-perturbative corrections are dominant, the scattering amplitude is a Gaussian distribution in t with a slope depending on a non-perturbative scale. A non-perturbative origin of the soft pomeron is thus identified.

Finite 3 pi cut approximation for the pi NN-bar form factor.

Assuming the length of the 3[pi] cut to be finite and approximating the integrated 4[pi] amplitude by a constant, we derive an expression for the [pi][ital N[bar N]] form factor which is very close to that given by a simple pole. The specific predictions of the obtained form factor for the region of small momentum transfer are compared with existing effective pole formulas and discussed along the lines of the Goldberger-Treiman relation.

Electroweak vector boson production in high-energy ep collisions

We present the results of a comprehensive study of W and Z production in high-energy ep collisions. The processes ep → eW ± X, ep → νW su− X, ep → eZ X and ep → νZ X are investigated. The region of small momentum transfer in eW and eZ production, with a fermion exchanged in the u-channel, is treated using the photon structure function approach, and carefully matched to the deep inelastic region. Low momentum photon exchange contributions to νW and eZ production, where the proton either stays intact or is transformed into a nucleon resonance, are calculated using form factors functions fitted directly to experimental data. These processes will be the source of rare but spectacular events. Total cross sections for W and Z production at HERA and at an ep collider in the LEP tunnel (LEP/LHC), and detailed results for the characteristics of eW, νW, eZ and νZ events are presented. In ep→eW X, eZ X the uncertainties of the quark densities in the proton and photon allow the total cross section predictions to vary by ±40% at HERA and ±15% at LEP/LHC.