3,120 research outputs found
On the dependence of the wave function of a bound nucleon on its momentum and the EMC effect
It is widely discussed in the literature that the wave function of the
nucleon bound in a nucleus is modified due to the interaction with the
surrounding medium. We argue that the modification should strongly depend on
the momentum of the nucleon. We study such an effect in the case of the
point-like configuration component of the wave function of a nucleon bound in a
nucleus A, considering the case of arbitrary final state of the spectator A-1
system. We show that for non relativistic values of the nucleon momentum, the
momentum dependence of the nucleon deformation appears to follow from rather
general considerations and discuss the implications of our theoretical
observation for two different phenomena: i) the search for medium induced
modifications of the nucleon radius of a bound nucleon through the measurement
of the electromagnetic nucleon form factors via the A(e,e'p)X process, and ii)
the A-dependence of the EMC effect; in this latter case we also present a new
method of estimating the fraction of the nucleus light-cone momentum carried by
the photons and find that in a heavy nuclei protons loose about 2% of their
momentum.Comment: 38 pages, 1 figure; changed references and text in Section I
(Introduction
Application of the Principle of Maximum Conformality to Top-Pair Production
A major contribution to the uncertainty of finite-order perturbative QCD
predictions is the perceived ambiguity in setting the renormalization scale
. For example, by using the conventional way of setting , one obtains the total production cross-section
with the uncertainty \Delta \sigma_{t \bar{t}}/\sigma_{t
\bar{t}}\sim ({}^{+3%}_{-4%}) at the Tevatron and LHC even for the present
NNLO level. The Principle of Maximum Conformality (PMC) eliminates the
renormalization scale ambiguity in precision tests of Abelian QED and
non-Abelian QCD theories. In this paper we apply PMC scale-setting to predict
the cross-section at the Tevatron and LHC
colliders. It is found that remains almost unchanged by
varying within the region of . The convergence
of the expansion series is greatly improved. For the -channel,
which is dominant at the Tevatron, its NLO PMC scale is much smaller than the
top-quark mass in the small -region, and thus its NLO cross-section is
increased by about a factor of two. In the case of the -channel, which is
dominant at the LHC, its NLO PMC scale slightly increases with the subprocess
collision energy , but it is still smaller than for
TeV, and the resulting NLO cross-section is increased by
. As a result, a larger is obtained in comparison
to the conventional scale-setting method, which agrees well with the present
Tevatron and LHC data. More explicitly, by setting GeV, we
predict pb,
pb and pb. [full abstract can be found in the
paper.]Comment: 15 pages, 11 figures, 5 tables. Fig.(9) is correcte
Light-Front Quantization and AdS/QCD: An Overview
We give an overview of the light-front holographic approach to strongly
coupled QCD, whereby a confining gauge theory, quantized on the light front, is
mapped to a higher-dimensional anti de Sitter (AdS) space. The framework is
guided by the AdS/CFT correspondence incorporating a gravitational background
asymptotic to AdS space which encodes the salient properties of QCD, such as
the ultraviolet conformal limit at the AdS boundary at , as well as
modifications of the geometry in the large infrared region to describe
confinement and linear Regge behavior. There are two equivalent procedures for
deriving the AdS/QCD equations of motion: one can start from the Hamiltonian
equation of motion in physical space time by studying the off-shell dynamics of
the bound state wavefunctions as a function of the invariant mass of the
constituents. To a first semiclassical approximation, where quantum loops and
quark masses are not included, this leads to a light-front Hamiltonian equation
which describes the bound state dynamics of light hadrons in terms of an
invariant impact variable which measures the separation of the partons
within the hadron at equal light-front time. Alternatively, one can start from
the gravity side by studying the propagation of hadronic modes in a fixed
effective gravitational background. Both approaches are equivalent in the
semiclassical approximation. This allows us to identify the holographic
variable in AdS space with the impact variable . Light-front
holography thus allows a precise mapping of transition amplitudes from AdS to
physical space-time. The internal structure of hadrons is explicitly introduced
and the angular momentum of the constituents plays a key role.Comment: Invited talk presented by GdT at the XIV School of Particles and
Fields, Morelia, Mexico, November 8-12, 201
QCD Rescattering and High Energy Two-Body Photodisintegration of the Deuteron
Photon absorption by a quark in one nucleon followed by its high momentum
transfer interaction with a quark in the other may produce two final-state
nucleons with high relative momentum. We sum the relevant quark rescattering
diagrams, to show that the scattering amplitude depends on a convolution
between the large angle scattering amplitude, the hard photon-quark
interaction vertex and the low-momentum deuteron wave function. The computed
absolute values of the cross section are in reasonable agreement with the data.Comment: 4 pages, revised version to be published in Phys. Rev. Let
Systematics of Heavy Quark Production at HERA
We discuss heavy quark and quarkonium production in various kinematic regions
at the HERA ep collider. In contrast to fixed target experiments, collider
kinematics allows the possibility of detailed measurements of particle
production in the proton fragmentation region. One thus can study parton
correlations in the proton Fock states materialized by the virtual photon
probe. We discuss various configurations of inelastic electron-proton
scattering, including peripheral, diffractive, and deep inelastic processes. In
particular, we show that intrinsic heavy quark Fock states can be identified by
the observation of quarkonium production at large and a low mean
transverse momentum which is insensitive to the virtuality of the photon.Comment: 17 pages, postscript. To obtain a copy of this paper send e-mail to
[email protected]
Light-cone QCD predictions for elastic ed-scattering in the intermediate energy region
The contributions of helicity-flip matrix elements to the deuteron form
factors are discussed in the light-cone frame. Normalized , ,
and are obtained in a simple QCD-inspired model. We find
that plays an important role in . Our numerical results
are consistent with the data in the intermediate energy region.Comment: 9 pages, REVTeX file, 5 figure
Self-Consistency Requirements of the Renormalization Group for Setting the Renormalization Scale
In conventional treatments, predictions from fixed-order perturbative QCD
calculations cannot be fixed with certainty due to ambiguities in the choice of
the renormalization scale as well as the renormalization scheme. In this paper
we present a general discussion of the constraints of the renormalization group
(RG) invariance on the choice of the renormalization scale. We adopt the RG
based equations, which incorporate the scheme parameters, for a general
exposition of RG invariance, since they simultaneously express the invariance
of physical observables under both the variation of the renormalization scale
and the renormalization scheme parameters. We then discuss the self-consistency
requirements of the RG, such as reflexivity, symmetry, and transitivity, which
must be satisfied by the scale-setting method. The Principle of Minimal
Sensitivity (PMS) requires the slope of the approximant of an observable to
vanish at the renormalization point. This criterion provides a
scheme-independent estimation, but it violates the symmetry and transitivity
properties of the RG and does not reproduce the Gell-Mann-Low scale for QED
observables. The Principle of Maximum Conformality (PMC) satisfies all of the
deductions of the RG invariance - reflectivity, symmetry, and transitivity.
Using the PMC, all non-conformal -terms (
stands for an arbitrary renormalization scheme) in the perturbative expansion
series are summed into the running coupling, and one obtains a unique,
scale-fixed, scheme-independent prediction at any finite order. The PMC scales
and the resulting finite-order PMC predictions are both to high accuracy
independent of the choice of initial renormalization scale, consistent with RG
invariance. [...More in the text...]Comment: 15 pages, 4 figures. References updated. To be published in
Phys.Rev.
Interaction of small size wave packet with hadron target
We calculate in QCD the cross section for the scattering of an energetic
small-size wave packet off a hadron target. We use our results to study the
small- behaviour of , the distribution over cross
section for the pion, in the leading -order.Comment: Revised version of the report CEBAF-TH-96-0
Gluon Virtuality and Heavy Sea Quark Contributions to the Spin-Dependent g_1 Structure Function
We analyze the quark mass dependence of photon gluon fusion in polarized deep
inelastic scattering for both the intrinsic and extrinsic gluon distributions
of the nucleon. We calculate the effective number of flavors for each of the
heavy and light quark photon gluon fusion contributions to the first moment of
the spin-dependent structure function .Comment: LaTex, 19 page
Frame Dependence of Spin-One Angular Conditions in Light Front Dynamics
We elaborate the frame dependence of the angular conditions for spin-1 form
factors. An extra angular condition is found in addition to the usual angular
condition relating the four helicity amplitudes. Investigating the
frame-dependence of the angular conditions, we find that the extra angular
condition is in general as complicated as the usual one, although it becomes
very simple in the frame involving only two helicity amplitudes. It
is confirmed that the angular conditions are identical in frames that are
connected by kinematical transformations. The high behaviors of the
physical form factors and the limiting behaviors in special reference frames
are also discussed.Comment: 29 pages RevTeX. submitted to Phys. Rev.
- …