217 research outputs found
QCD factorization for forward hadron scattering at high energies
We consider the QCD factorization of DIS structure functions at small x and
amplitudes of 2->2 -hadronic forward scattering at high energy. We show that
both collinear and k_T-factorization for these processes can be obtained
approximately as reductions of a more general (totally unintegrated) form of
the factorization. The requirement of ultraviolet and infrared stability of the
factorization convolutions allows us to obtain restrictions on the fits for the
parton distributions in k_T- and collinear factorization.Comment: 18 pages, 10 figures In the present version misprints found in the
prevcious version are corrected and some more details are explaine
Collisions of low-energy antiprotons with molecular hydrogen: ionization, excitation and stopping power
A time-dependent coupled-channel approach was used to calculate ionization,
excitation, and energy-loss cross sections as well as energy spectra for
antiproton and proton collisions with molecular hydrogen for impact energies 8
keV < E < 4000 keV.Comment: 4 pages, 4 figures, conference LEAP0
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
Small-x Dipole Evolution Beyond the Large-N_c Limit
We present a method to include colour-suppressed effects in the Mueller
dipole picture. The model consistently includes saturation effects both in the
evolution of dipoles and in the interactions of dipoles with a target in a
frame-independent way.
When implemented in a Monte Carlo simulation together with our previous model
of energy--momentum conservation and a simple dipole description of initial
state protons and virtual photons, the model is able to reproduce to a
satisfactory degree both the gamma*-p cross sections as measured at HERA as
well as the total p-p cross section all the way from ISR energies to the
Tevatron and beyond
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
Markovian MC simulation of QCD evolution at NLO level with minimum k_T
We present two Monte Carlo algorithms of the Markovian type which solve the
modified QCD evolution equations at the NLO level. The modifications with
respect to the standard DGLAP evolution concern the argument of the strong
coupling constant alpha_S. We analyze the z - dependent argument and then the
k_T - dependent one. The evolution time variable is identified with the
rapidity. The two algorithms are tested to the 0.05% precision level. We find
that the NLO corrections in the evolution of parton momentum distributions with
k_T - dependent coupling constant are of the order of 10 to 20%, and in a small
x region even up to 30%, with respect to the LO contributions.Comment: 32 pages, 9 pdf figure
A Catching Trap for All Antiproton Seasons
We describe the origin, development, and status of the Los Alamos antiproton
catching trap. Originally designed for the antiproton gravity experiment, it
now is clear that this device can be a source of low-energy antiprotons for a
wide range of physics, both on site, at CERN, and also off site.Comment: 18 pages, LaTeX, 6 figures available upon request, In honor of
Herbert Walthe
SORL1 mutation in a Greek family with Parkinson's disease and dementia
Whole exome sequencing and linkage analysis were performed in a three generational pedigree of Greek origin with a broad phenotypic spectrum spanning from Parkinson’s disease and Parkinson’s disease dementia to dementia of mixed type (Alzheimer disease and vascular dementia). We identified a novel heterozygous c.G1135T (p.G379W) variant in SORL1 which segregated with the disease in the family. Mutation screening in sporadic Greek PD cases identified one additional individual with the mutation, sharing the same 12.8Mb haplotype. Our findings provide support for SORL1 mutations resulting in a broad range of additional phenotypes and warrants further studies in neurodegenerative diseases beyond AD
Small x behavior of parton distributions. A study of higher twist effects
Higher twist corrections to the structure function F_2 at small x are studied
for the case of a flat initial condition for the twist-two QCD evolution in the
next-to-leading order approximation. We present an analytical parameterization
of the contributions from the twist-two and higher twist operators of the
Wilson operator product expansion. Higher twist terms are evaluated using two
different approaches, one motivated by BFKL and the other motivated by the
renormalon formalism. The results of the latter approach are in very good
agreement with deep inelastic scattering data from HERA.Comment: 46 pages, 9 figures, typeset using REVTeX. Misprints corrected, new
results, discussions, references and acknowledgements added, 2 pictures
replaced. The conclusions remain unchange
Theoretical issues of small physics
The perturbative QCD predictions concerning deep inelastic scattering at low
are summarized. The theoretical framework based on the leading log
resummation and factorization theorem is described and some recent
developments concerning the BFKL equation and its generalization are discussed.
The QCD expectations concerning the small behaviour of the spin dependent
structure function are briefly summarized and the importance of
the double logarithmic terms which sum contributions containing the leading
powers of is emphasised. The role of studying final states
in deep inelastic scattering for revealing the details of the underlying
dynamics at low is pointed out and some dedicated measurements, like deep
inelastic scattering accompanied by an energetic jet, the measurement of the
transverse energy flow etc., are briefly discussed.Comment: 17 pages, LATEX, 7 uuencoded eps figures include
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