298 research outputs found
Nucleon Form Factors and Distribution Amplitudes in QCD
We derive light-cone sum rules for the electromagnetic nucleon form factors
including the next-to-leading-order corrections for the contribution of
twist-three and twist-four operators and a consistent treatment of the nucleon
mass corrections. The essence of this approach is that soft Feynman
contributions are calculated in terms of small transverse distance quantities
using dispersion relations and duality. The form factors are thus expressed in
terms of nucleon wave functions at small transverse separations, called
distribution amplitudes, without any additional parameters. The distribution
amplitudes, therefore, can be extracted from the comparison with the
experimental data on form factors and compared to the results of lattice QCD
simulations. A selfconsistent picture emerges, with the three valence quarks
carrying 40%:30%:30% of the proton momentum.Comment: 27 pages, 7 figures uses revte
Axial form factor of the nucleon at large momentum transfers
Motivated by the emerging possibilities to study threshold pion
electroproduction at large momentum transfers at Jefferson Laboratory following
the 12 GeV upgrade, we provide a short theory summary and an estimate of the
nucleon axial form factor for large virtualities in the range using next-to-leading order light-cone sum rules.Comment: A comparison to the new neutrino data analysis and several references
added. Final version to appear in Phys.Rev.
Magnetocaloric properties of Gd(Co1-xFex)2 compounds, with x ≤ 0.60
In this paper the results of specific magnetization and magnetocaloric effect (MCE) measurements for Gd(Co1-xFex)2 system upon the Co substitution by Fe for the x = 0 ÷ 0.60 range are presented. Phase composition was controlled by X-ray diffraction analysis. MCE has been studied within the temperature range of 300-850 K in magnetic fields up to 17 kOe by the magnetic entropy change calculation (ΔSm). It was found that in contrast to the previously studied R(Co-Fe)2 compounds where R = Dy, Ho, Er, an ordinary symmetrical peak of ΔSm(T) in the vicinity of TC is observed for presented samples. Additionally, the MCE comparison of Gd(Co0.88Fe0.12)2 with that for the isostructural Gd(Ni0.88Fe0.12)2 compound having a plateau-like ΔSm temperature dependence is given. The obtained results are discussed. © 2018 The Authors, published by EDP Sciences.This work has been supported by the State contracts No. 3.6121.2017/8.9 between U FU and the Ministry of Education and Science of Russian Federation
Kinematical twist-3 effects in DVCS as a quark spin rotation
We point out that the kinematical twist-3 contributions to the DVCS
amplitude, required to restore electromagnetic gauge invariance of the twist-2
amplitude up to O(t/q^2), can be understood as a spin rotation applied to the
twist-2 quark density matrix in the target. This allows for a compact
representation of the twist-3 effects, as well as for a simple physical
interpretation.Comment: 4 pages, revtex, 3 eps figures included using eps
Effect of alloying elements (Zr, Hf, Co), heat and mechanical treatment conditions on the phase composition and magnetic properties of SmFe11Ti compounds with ThMn12 structure
The results of thermomagnetic, metallographic and X-ray diffraction phase analysis as well as the measurements of specific magnetization (σs), Curie temperature (TC), coercive force (HC) of (Sm,M)(Fe,M)12-xTix alloys samples, where M = Zr, Hf, Co with the ThMn12 main phase structure (1-12) are presented. The effect of the annealing temperature and the cooling rate on the formation of 1-12 phase and its magnetic properties, including the effect of high-energy milling on the magnetic hysteresis properties and alloys structure are described. It was found that the highest magnetic characteristics such as σs = 112.6 emu/g and TC = 600°C are attained in the (Sm0.8Zr0.2)(Fe0.75Co0.25)11.4Ti0.6 alloy after its annealing at 1050 °C and rapid cooling. It is noted that a mechanical milling of the alloy leads to 1-12 phase amorphization which accompanied by an α-(Fe) or metal Co phases impurity formation. © 2018 The Authors, published by EDP Sciences.The work was supported by the State contracts No. 3.6121.2017/ 8.9 between UrFU and the Ministry of Education and Science of Russian Federation and by the Fund of assistance to development of small enterprises in scientific-technical sphere No. 11996GU / 017
Infrared enhanced analytic coupling and chiral symmetry breaking in QCD
We study the impact on chiral symmetry breaking of a recently developed model
for the QCD analytic invariant charge. This charge contains no adjustable
parameters, other than the QCD mass scale , and embodies asymptotic
freedom and infrared enhancement into a single expression. Its incorporation
into the standard form of the quark gap equation gives rise to solutions for
the dynamically generated mass that display a singular confining behaviour at
the origin. Using the Pagels-Stokar method we relate the obtained solutions to
the pion decay constant , and estimate the scale parameter ,
in the presence of four active quarks, to be about 880 MeV.Comment: 14 pages, 3 figures; to appear in J. Phys.
Conceptual Design of Experimental Facility for Large-Diameter NTD-Si at the IRT-T Reactor
The IRT-T reactor has been conducting research in the field of irradiation of ingots of single-crystal semiconductor materials since 1987. The article describes the existing silicon doping facility. The results of studies on the possibility of creating an additional irradiation channel for neutron-transmutation doping of silicon are presented. It is shown that the use of a graphite reflector and a thermal neutron filter based on boron makes it possible to achieve non-uniformity of irradiation up to 5 %. The principal possibility of irradiating single-crystal silicon ingots with a diameter of up to 203 mm and a length of up to 500 mm is shown. The questions of optimizing the configuration of the core and the regime of reactors operation for increasing the neutron flux in the irradiation channels are discussed. In addition, applying the facility to produce base materials for neutron dosimeter in neutron capture therapy studies is proposed
The massive analytic invariant charge in QCD
The low energy behavior of a recently proposed model for the massive analytic
running coupling of QCD is studied. This running coupling has no unphysical
singularities, and in the absence of masses displays infrared enhancement. The
inclusion of the effects due to the mass of the lightest hadron is accomplished
by employing the dispersion relation for the Adler D function. The presence of
the nonvanishing pion mass tames the aforementioned enhancement, giving rise to
a finite value for the running coupling at the origin. In addition, the
effective charge acquires a "plateau-like" behavior in the low energy region of
the timelike domain. This plateau is found to be in agreement with a number of
phenomenological models for the strong running coupling. The developed
invariant charge is applied in the processing of experimental data on the
inclusive lepton decay. The effects due to the pion mass play an
essential role here as well, affecting the value of the QCD scale parameter
extracted from these data. Finally, the massive analytic running
coupling is compared with the effective coupling arising from the study of
Schwinger-Dyson equations, whose infrared finiteness is due to a dynamically
generated gluon mass. A qualitative picture of the possible impact of the
former coupling on the chiral symmetry breaking is presented.Comment: 13 pages, 7 figures, revtex
The gluon content of the and mesons and the , electromagnetic transition form factors
We compute power-suppressed corrections to the \eta\gamma and
\eta^{\prime}\gamma transition form factors
Q^2F_{\eta(\eta^{\prime})\gamma}(Q^2) arising from the end point regions x \to
0,1 by employing the infrared-renormalon approach. The contribution to the form
factors from the quark and gluon content of the \eta,\eta^{\prime} mesons is
taken into account using for the \eta-\eta^{\prime} mixing the SU_f(3) singlet
\eta_1 and octet \eta_8 basis. The theoretical predictions obtained this way
are compared with the corresponding CLEO data and restrictions on the input
parameters (Gegenbauer coefficients) B_2^q(\eta_1), B_2^g(\eta_1), and
B_2^q(\eta_8) in the distribution amplitudes for the \eta_1,\eta_8 states with
one nonasymptotic term are deduced. Comparison is made with the results from
QCD perturbation theory.Comment: 25 pages, RevTeX4 used. 9 figures as EPS files. Text significantly
changed to include variation of theoretical parameters. Figures modified.
Corrected typo in equation (34) and trivial mistake in -coefficient.
References added. Conclusions unchange
- …