535 research outputs found
Ultrahigh energy neutrino scattering: an update
We update our estimates of charged and neutral current neutrino total cross
sections on isoscalar nucleons at ultrahigh energies using a global (x, Q^2)
fit, motivated by the Froissart bound, to the F_2 (electron-proton) structure
function utilizing the most recent analysis of the complete ZEUS and H1 data
sets from HERA I. Using the large Q^2, small Bjorken-x limits of the "wee"
parton model, we connect the ultrahigh energy neutrino cross sections directly
to the large Q^2, small-x extrapolation of our new fit, which we assume
saturates the Froissart bound. We compare both to our previous work, which
utilized only the smaller ZEUS data set, as well as to recent results of a
calculation using the ZEUS-S based global perturbative QCD parton distributions
using the combined HERA I results as input. Our new results substantiate our
previous conclusions, again predicting significantly smaller cross sections
than those predicted by extrapolating pQCD calculations to neutrino energies
above 10^9 GeV.Comment: 8 pages, 1 figure, 3 table
Hard collinear gluon radiation and multiple scattering in a medium
The energy loss of hard jets produced in the Deep-Inelastic scattering (DIS)
off a large nucleus is considered in the collinear limit. In particular, the
single gluon emission cross section due to multiple scattering in the medium is
calculated. Calculations are carried out in the higher-twist scheme, which is
extended to include contributions from multiple transverse scatterings on both
the produced quark and the radiated gluon. The leading length enhanced parts of
these power suppressed contributions are resummed. Various interferences
between such diagrams lead to the Landau-Pomeranchuk-Migdal (LPM) effect. We
resum the corrections from an arbitrary number of scatterings and isolate the
leading contributions which are suppressed by one extra power of the hard scale
. All powers of the emitted gluon forward momentum fraction are
retained. We compare our results with the previous calculation of single
scattering per emission in the higher-twist scheme as well as with multiple
scattering resummations in other schemes. It is found that the leading
() contribution to the double differential gluon production cross
section, in this approach, is equivalent to that obtained from the single
scattering calculation once the transverse momentum of the final quark is
integrated out. We comment on the generalization of this formalism to
Monte-Carlo routines.Comment: 30 pages, 7 figures, revtex4, typos correcte
Analytic Expression for the Joint x and Q^2 Dependences of the Structure Functions of Deep Inelastic Scattering
We obtain a good analytic fit to the joint Bjorken-x and Q^2 dependences of
ZEUS data on the deep inelastic structure function F_2(x, Q^2). At fixed
virtuality Q^2, as we showed previously, our expression is an expansion in
powers of log (1/x) that satisfies the Froissart bound. Here we show that for
each x, the Q^2 dependence of the data is well described by an expansion in
powers of log Q^2. The resulting analytic expression allows us to predict the
logarithmic derivatives {({\partial}^n F_2^p/{{(\partial\ln Q^2}})^n)}_x for n
= 1,2 and to compare the results successfully with other data. We extrapolate
the proton structure function F_2^p(x,Q^2) to the very large Q^2 and the very
small x regions that are inaccessible to present day experiments and contrast
our expectations with those of conventional global fits of parton distribution
functions.Comment: 4 pages, 3 figures, a few changes in the text. Version to be
published in Physical Review Letter
Higher twist jet broadening and classical propagation
The transverse broadening of jets produced in deep-inelastic scattering (DIS)
off a large nucleus is studied in the collinear limit. A class of medium
enhanced higher twist corrections are re-summed to calculate the transverse
momentum distribution of the produced collinear jet. In contrast to previous
approaches, resummation of the leading length enhanced higher twist corrections
is shown to lead to a two dimensional diffusion equation for the transverse
momentum of the propagating jet. Results for the average transverse momentum
obtained from this approach are then compared to the broadening expected from a
classical Langevin analysis for the propagation of the jet under the action of
the fluctuating color Lorentz force inside the nucleons. The set of
approximations that lead to identical results from the two approaches are
outlined. The relationship between the momentum diffusion constant and the
transport coefficient is explicitly derived.Comment: 17 pages, 6 figures, revtex4, references added, typos corrected,
discussion update
Ultra-high energy neutrino scattering
Estimates are made of the ultra-high energy neutrino cross sections based on
an extrapolation to very small Bjorken x of the logarithmic Froissart
dependence in x shown previously to provide an excellent fit to the measured
proton structure function F_2^p(x,Q^2) over a broad range of the virtuality
Q^2. Expressions are obtained for both the neutral current and the charged
current cross sections. Comparison with an extrapolation based on perturbative
QCD shows good agreement for energies where both fit data, but our rates are as
much as a factor of 10 smaller for neutrino energies above 10^9 GeV, with
important implications for experiments searching for extra-galactic neutrinos.Comment: 4 pages, 1 figure, 1 table; Title, abstract and text changed,
conclusions unchanged. Version accepted for publication in Physical Review
Asymptotic behavior of double parton distribution functions
The double parton distribution functions are investigated in the region of
small longitudinal momentum fractions in the leading logarithm approximation of
perturbative QCD. It is shown that these functions have the factorization
property in the case of one slow and one fast parton.Comment: 7 pages, revtex
A comparison study of medium-modified QCD shower evolution scenarios
The computation of hard processes in hadronic collisions is a major success
of perturbative Quantum Chromodynamics (pQCD). In such processes, pQCD not only
predicts the hard reaction itself, but also the subsequent evolution in terms
of parton branching and radiation, leading to a parton shower and ultimately to
an observable jet of hadrons. If the hard process occurs in a heavy-ion
collision, a large part of this evolution takes place in the soft medium
created along with the hard reaction. An observation of jets in heavy-ion
collision thus allows a study of medium-modified QCD shower evolution. In
vacuum, Monte-Carlo (MC) simulations are well established tools to describe
such showers. For jet studies in heavy-ion collisions, MC models for in-medium
showers are currently being developed. However, the shower-medium interaction
depends on the nature of the microscopic degrees of freedom of the medium
created in a heavy-ion collision which is the very object one would like to
investigate. This paper presents a study in comparison between three different
possible implementations for the shower-medium interaction, two of them based
on medium-induced pQCD radiation, one of them a medium-induced drag force, and
shows for which observables differences between the three scenarios become
visible. We find that while single hadron observables such as R_AA are
incapable of differentiating between the scenarios, jet observables such as the
longiudinal momentum spectrum of hadrons in the jet show the potential to do
so.Comment: 15 pages, 10 figures, minor change
Double parton scattering in double logarithm approximation of perturbative QCD
Using the explicit form of the known single distribution functions (the
Green's functions) in the double logarithm approximation of perturbative QCD,
we analyze the structure of splitting diagrams as a source of double parton
perturbative correlations in the proton. The related phenomenological effects
are discussed for the conditions of the LHC experiments.Comment: 8 pages, 1 figure, Refs. and explanations added, published version
(Phys. Rev. D
Large distance behaviour of light cone operator product in perturbative and nonperturbative QCD regimes
We evaluate the coordinate space dependence of the matrix elements of the
commutator of the electromagnetic and gluon currents in the vicinity of the
light-cone but at large distances within the parton model, DGLAP, the
resummation approaches to the small x behaviour of DIS processes, and for the
Unitarity Bound. We find that an increase of the commutator with relative
distance as is the generic property of QCD
at small but fixed space-time interval in perturbative and
nonperturbative QCD regimes. We explain that the factor follows within the
dipole model (QCD factorization theorem) from the properties of Lorents
transformation. The increase of disappeares at central impact parameters
if cross section of DIS may achieve the Unitarity Limit. We argue that such
long range forces are hardly consistent with thermodynamic equilibrium while a
Unitarity Limit may signal equilibration. Possible implications of this new
long range interaction are briefly discussed.Comment: 23 page
The casuality and/or energy-momentum conservation constraints on QCD amplitudes in small x regime
The causality and/or the energy-momentum constraints on the amplitudes of
high energy processes are generalized to QCD. The constraints imply that the
energetic parton may experience at most one inelastic collision only and that
the number of the constituents in the light cone wave function of the
projectile is increasing with the collision energy and the atomic number.Comment: 24 pages,8 figures. The paper is streamlined, some references are
changed and misprints are eliminate
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