250 research outputs found
QCD and Hadron Dynamics
Perturbative QCD predicts and describes various features of multihadron
production. An amazing similarity between observable hadron systems and
calculable underlying parton ensembles justifies the attempts to use the
language of quarks and gluons down to small momentum scales, to approach the
profound problems that are commonly viewed as being entirely non-perturbative.Comment: Talk at the Royal Society meeting "Structure of Matter", London, May
200
Particle Multiplicity in Jets and Sub-jets with Jet Axis from Color Current
We study the particle multiplicity in a jet or sub-jet as derived from an
energy-multiplicity 2-particle correlation. This definition avoids the notion
of a globally fixed jet axis and allows for the study of smaller jet cone
openings in a more stable way. The results are sensitive to the mean color
current in the jet from primary parton which takes into
account intermediate partonic processes in the sub-jet production where at high energies. We generalize previous calculations in
Leading Logarithmic Approximation (LLA). The size of the effects related to
this jet axis definition are computed for multiplicities in sub-jets with
different opening angles and energies by including contributions from the
Modified LLA (MLLA) and Next-to-MLLA to the leading order QCD results
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
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
Photon bremsstrahlung and diffusive broadening of a hard jet
The photon bremsstrahlung rate from a quark jet produced in deep-inelastic
scattering (DIS) off a large nucleus is studied in the collinear limit. The
leading medium-enhanced higher twist corrections which describe the multiple
scattering of the jet in the nucleus are re-summed to all orders of twist. The
propagation of the jet in the absence of further radiative energy loss is shown
to be governed by a transverse momentum diffusion equation. We compute the
final photon spectrum in the limit of soft photons, taking into account the
leading and next-to-leading terms in the photon momentum fraction y. In this
limit, the photon spectrum in a physical gauge is shown to arise from two
interfering sources: one where the initial hard scattering produces an
off-shell quark which immediately radiates the photon and then undergoes
subsequent soft re-scattering; alternatively the quark is produced on-shell and
propagates through the medium until it is driven off-shell by re-scattering and
radiates the photon. Our result has a simple formal structure as a product of
the photon splitting function, the quark transverse momentum distribution
coming from a diffusion equation and a dimensionless factor which encodes the
effect of the interferences encountered by the propagating quark over the
length of the medium. The destructive nature of such interferences in the small
y limit are responsible for the origin of the Landau-Pomeranchuck-Migdal (LPM)
effect. Along the way we also discuss possible implications for quark jets in
hot nuclear matter.Comment: 24 pages, 3 figures, Revtex
Soft and hard processes in QCD
QCD equations for the generating functions are applied to separate soft and
hard jets in -processes of multiparticle production. The dependence of
average multiplicities and higher moments of multiplicity distributions of
particles created in a "newly born" soft subjets on the share of energy devoted
to them is calculated in fixed coupling gluodynamics. This dependence is the
same as for the total multiplicity up to a constant factor if soft jets are
defined as those carrying out a fixed share of initial energy at all energies.
The constant factor depends on this share in a non-trivial way. Other
definitions are also proposed. The relation between these quantities for soft
and hard processes is discussed.Comment: 6 pages, no Figures, LATEX, more detailed discussion of definitions
and proposals for experiment is give
QCD and hybrid NBD on oscillating moments of multiplicity distributions in lepton- and hadron-initiated reactions
QCD predictions for moments of multiplicity distributions are compared with
experimental data on e+e- collisions and their two-NBD fits. Moments of the
multiplicity distribution in a two-NBD model for 1.8 TeV pp collisions are
considered. Three-NBD model predictions and fits for pp at LHC energies are
also discussed. Analytic expressions for moments of hybrid NBD are derived and
used to get insight into jet parameters and multicomponent structure of the
processes. Interpretation of observed correlations is proposed.Comment: 8 pages, no figures, Phys. Lett.
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
Relativistic effects in the processes of heavy quark fragmentation
In the framework based on the quasipotential method and relativistic quark
model a new covariant expression for the heavy quark fragmentation amplitude to
fragment into the pseudoscalar and vector S-wave heavy mesons is obtained. It
contains all possible relativistic corrections including the terms connected
with the transformation law of the bound state wave function to the reference
frame of the moving meson. Relativistic corrections of order {\bf p}^2/m^2 to
the heavy quark fragmentation distributions into (\bar c c), (\bar b c) and
(\bar b b) states are calculated as functions of the longitudinal momentum
fraction z and the transverse momentum p_T relative to the jet axis.Comment: 23 pages, 6 figure
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
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