402 research outputs found
Electroweak 2 -> 2 amplitudes for electron-positron annihilation at TeV energies
The non-radiative scattering amplitudes for electron-positron annihilation
into quark and lepton pairs in the TeV energy range are calculated in the
double-logarithmic approximation. The expressions for the amplitudes are
obtained using infrared evolution equations with different cut-offs for virtual
photons and for W and Z bosons, and compared with previous results obtained
with an universal cut-off.Comment: Revtex4, 17 pages, 7 figures. Some minor changes made, more refs
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Factorization and infrared properties of non-perturbative contributions to DIS structure functions
In this paper we present a new derivation of the QCD factorization. We deduce
the k_T- and collinear factorizations for the DIS structure functions by
consecutive reductions of a more general theoretical construction. We begin by
studying the amplitude of the forward Compton scattering off a hadron target,
representing this amplitude as a set of convolutions of two blobs connected by
the simplest, two-parton intermediate states. Each blob in the convolutions can
contain both the perturbative and non-perturbative contributions. We formulate
conditions for separating the perturbative and non-perturbative contributions
and attributing them to the different blobs. After that the convolutions
correspond to the QCD factorization. Then we reduce this totally unintegrated
(basic) factorization first to the k_T- factorization and finally to the
collinear factorization. In order to yield a finite expression for the Compton
amplitude, the integration over the loop momentum in the basic factorization
must be free of both ultraviolet and infrared singularities. This obvious
mathematical requirement leads to theoretical restrictions on the
non-perturbative contributions (parton distributions) to the Compton amplitude
and the DIS structure functions related to the Compton amplitude through the
Optical theorem. In particular, our analysis excludes the use of the singular
factors x^{-a} (with a > 0) in the fits for the quark and gluon distributions
because such factors contradict to the integrability of the basic convolutions
for the Compton amplitude. This restriction is valid for all DIS structure
functions in the framework of both the k_T- factorization and the collinear
factorization if we attribute the perturbative contributions only to the upper
blob.Comment: 19 pages, 6 figure
Asymptotic high energy behavior of the deeply virtual Compton scattering amplitude
We compute the deeply virtual Compton scattering (DVCS) amplitude for forward
and backward scattering in the asymptotic limit. Since this calculation does
not assume ordering of the transverse momenta, it includes important
logarithmic contributions that are beyond those summed by the DGLAP evolution.
These contributions lead to a power-like behavior for the forward DVCS
amplitude.Comment: Latex, 22 pages, 5 Figures; references enhanced; typos correcte
Matching NLO parton shower matrix element with exact phase space: case of W -> l nu (gamma) and gamma^* -> pi^+pi^-(gamma)
The PHOTOS Monte Carlo is often used for simulation of QED effects in decay
of intermediate particles and resonances. Momenta are generated in such a way
that samples of events cover the whole bremsstrahlung phase space. With the
help of selection cuts, experimental acceptance can be then taken into account.
The program is based on an exact multiphoton phase space. Crude matrix element
is obtained by iteration of a universal multidimensional kernel. It ensures
exact distribution in the soft photon region. Algorithm is compatible with
exclusive exponentiation. To evaluate the program's precision, it is necessary
to control the kernel with the help of perturbative results. If available,
kernel is constructed from the exact first order matrix element. This ensures
that all terms necessary for non-leading logarithms are taken into account. In
the present paper we will focus on the W -> l nu and gamma^* -> pi^+ pi^-
decays. The Born level cross sections for both processes approach zero in some
points of the phase space. A process dependent compensating weight is
constructed to incorporate the exact matrix element, but is recommended for use
in tests only. In the hard photon region, where scalar QED is not expected to
be reliable, the compensating weight for gamma^* decay can be large. With
respect to the total rate, the effect remains at the permille level. It is
nonetheless of interest. The terms leading to the effect are analogous to some
terms appearing in QCD. The present paper can be understood either as a
contribution to discussion on how to match two collinear emission chains
resulting from charged sources in a way compatible with the exact and complete
phase space, exclusive exponentiation and the first order matrix element of QED
(scalar QED), or as the practical study of predictions for accelerator
experiments.Comment: 24 page
Features of modern electronic trading in international financial markets
The subject of research is the Internet trading in the American and European financial markets for individuals from the Russian Federation. The relevance of the topic is due to the huge influx of Russian private investors into the global financial market over the past few years, caused by the simplification of the process of opening brokerage accounts, automation of taxation, the possibility of trading via web terminals, as well as significantly changed conditions of online trading in 2022. The whole work is the development of a report on investing in the financial markets of the United States of America and the European Union for citizens of the Russian Federation. The study was conducted using the methods of systematization, grouping and comparison, with their help, analysis and generalization of the results obtained were carried out, conclusions were drawn. It has been established that in the current geopolitical situation and the restrictive measures taken by many multinational banks and companies, private investors of Russian origin have risks of freezing their own assets on brokerage accounts, which can be reduced by opening a brokerage account in companies registered in the territory of countries friendly to the Russian Federation. The article suggests two least risky options for investing in securities of issuers from Europe and USA
The heat capacity of nitrogen chains in grooves of single-walled carbon nanotube bundles
The heat capacity of bundles of closed-cap single-walled carbon nanotubes
(SWNT) with one-dimensional chains of nitrogen molecules adsorbed in the
grooves has been first experimentally studied at temperatures from 2K to 40K
using an adiabatic calorimeter. The contribution of nitrogen C(T) to the total
heat capacity has been separated. In the region 2-8K the behaviour of the curve
C(T) is qualitatively similar to the theoretical prediction of the phonon heat
capacity of 1D chains of krypton (Kr) atoms localized in the grooves of SWNT
bundles. Below 3K the dependence C(T) is linear. Above 8K the dependence C(T)
becomes steeper in comparison with the case of Kr atoms. This behaviour of the
heat capacity C(T) is due to the contribution of the rotational degrees of
freedom of the nitrogen molecules.Comment: 15 pages, 4 figure
Chemi-ionization in Solar Photosphere: Influence on the Hydrogen Atom excited States Population
In this paper, the influence of chemi-ionization processes in collisions, as well as the influence of inverse chemi-recombination
processes on hydrogen atom excited-state populations in solar photosphere, are
compared with the influence of concurrent electron-atom and electron-ion
ionization and recombination processes. It has been found that the considered
chemi-ionization/recombination processes dominate over the relevant concurrent
processes in almost the whole solar photosphere. Thus, it is shown that these
processes and their importance for the non-LTE modeling of the solar atmosphere
should be investigated further
Resummation of double logarithms in electroweak high energy processes
At future linear collider experiments in the TeV range, Sudakov
double logarithms originating from massive boson exchange can lead to
significant corrections to the cross sections of the observable processes.
These effects are important for the high precision objectives of the Next
Linear Collider. We use the infrared evolution equation, based on a gauge
invariant dispersive method, to obtain double logarithmic asymptotics of
scattering amplitudes and discuss how it can be applied, in the case of broken
gauge symmetry, to the Standard Model of electroweak processes. We discuss the
double logarithmic effects to both non-radiative processes and to processes
accompanied by soft gauge boson emission. In all cases the Sudakov double
logarithms are found to exponentiate. We also discuss double logarithmic
effects of a non-Sudakov type which appear in Regge-like processes.Comment: 26 pages, 3 figures, Latex2
Wave packet evolution approach to ionization of hydrogen molecular ion by fast electrons
The multiply differential cross section of the ionization of hydrogen
molecular ion by fast electron impact is calculated by a direct approach, which
involves the reduction of the initial 6D Schr\"{o}dinger equation to a 3D
evolution problem followed by the modeling of the wave packet dynamics. This
approach avoids the use of stationary Coulomb two-centre functions of the
continuous spectrum of the ejected electron which demands cumbersome
calculations. The results obtained, after verification of the procedure in the
case atomic hydrogen, reveal interesting mechanisms in the case of small
scattering angles.Comment: 7 pages, 8 Postscript figure
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