804 research outputs found

### Perturbative QCD and factorization of coherent pion photoproduction on the deuteron

We analyze the predictions of perturbative QCD for pion photoproduction on
the deuteron, gamma D -> pi^0 D, at large momentum transfer using the reduced
amplitude formalism. The cluster decomposition of the deuteron wave function at
small binding only allows the nuclear coherent process to proceed if each
nucleon absorbs an equal fraction of the overall momentum transfer.
Furthermore, each nucleon must scatter while remaining close to its mass shell.
Thus the nuclear photoproduction amplitude, M_{gamma D -> pi^0 D}(u,t),
factorizes as a product of three factors: (1) the nucleon photoproduction
amplitude, M_{gamma N_1 -> pi^0 N_1}(u/4,t/4), at half of the overall momentum
transfer, (2) a nucleon form factor, F_{N_2}(t/4), at half the overall momentum
transfer, and (3) the reduced deuteron form factor, f_d(t), which according to
perturbative QCD, has the same monopole falloff as a meson form factor. A
comparison with the recent JLAB data for gamma D -> pi^0 D of Meekins et al.
[Phys. Rev. C 60, 052201 (1999)] and the available gamma p -> pi^0 p data shows
good agreement between the perturbative QCD prediction and experiment over a
large range of momentum transfers and center of mass angles. The reduced
amplitude prediction is consistent with the constituent counting rule, p^11_T
M_{gamma D -> pi^0 D} -> F(theta_cm), at large momentum transfer. This is found
to be consistent with measurements for photon lab energies E_gamma > 3 GeV at
theta_cm=90 degrees and \elab > 10 GeV at 136 degrees.Comment: RevTeX 3.1, 17 pages, 6 figures; v2: incorporates minor changes as
version accepted by Phys Rev

### Deuteron Electromagnetic Form Factors in the Intermediate Energy Region

Based on a Perturbative QCD analysis of the deuteron form factor, a model for
the reduced form factor is suggested. The numerical result is consistent with
the data in the intermediate energy region.Comment: 9 pages, to appear in Phys.Rev.

### Classical sum rules and spin correlations in photoabsorption and photoproduction processes

In this paper we study the possibility of generalizing the classical
photoabsorption ($\gamma a \to b c$) sum rules, to processes $b c \to \gamma a$
and crossed helicity amplitudes. In the first case, using detailed balance, the
sum rule is written as $\int_{\nu_{th}}^\infty {\frac{{d\nu}}{\nu}} K\Delta
\sigma_{Born} (\nu)=0$ where $K$ is a kinematical constant which depends only
on the mass of the particles and the center of mass energy. For other crossed
helicity amplitudes, we show that there is a range of values of $s$ and $t$ for
which the differential cross section for the process $\gamma b \to a c$ or $a c
\to \gamma b$ in which the helicities of the photon and particle $a$ have
specific values, is equal to the differential cross section for the process in
which one of these two helicities is reversed (parallel-antiparallel spin
correlation).Comment: 9 pages, 2 figure

### Structure Functions are not Parton Probabilities

The common view that structure functions measured in deep inelastic lepton
scattering are determined by the probability of finding quarks and gluons in
the target is not correct in gauge theory. We show that gluon exchange between
the fast, outgoing partons and target spectators, which is usually assumed to
be an irrelevant gauge artifact, affects the leading twist structure functions
in a profound way. This observation removes the apparent contradiction between
the projectile (eikonal) and target (parton model) views of diffractive and
small x_{Bjorken} phenomena. The diffractive scattering of the fast outgoing
quarks on spectators in the target causes shadowing in the DIS cross section.
Thus the depletion of the nuclear structure functions is not intrinsic to the
wave function of the nucleus, but is a coherent effect arising from the
destructive interference of diffractive channels induced by final state
interactions. This is consistent with the Glauber-Gribov interpretation of
shadowing as a rescattering effect.Comment: 35 pages, 8 figures. Discussion of physical consequences of final
state interactions amplified. Material on light-cone gauge choices adde

### Light-Front Holography, Light-Front Wavefunctions, and Novel QCD Phenomena

Light-Front Holography, a remarkable feature of the AdS/CFT correspondence,
maps amplitudes in anti-de Sitter (AdS) space to frame-independent light-front
wavefunctions of hadrons in physical space-time. The model leads to an
effective confining light-front QCD Hamiltonian and a single-variable
light-front Schrodinger equation which determines the eigenspectrum and the
light-front wavefunctions of hadrons for general spin and orbital angular
momentum. The coordinate z in AdS space is identified with a Lorentz-invariant
coordinate zeta which measures the separation of the constituents within a
hadron at equal light-front time and determines the off-shell dynamics of the
bound-state wavefunctions and the fall-off in the invariant mass of the
constituents. The soft-wall holographic model, modified by a positive-sign
dilaton metric, leads to a remarkable one-parameter description of
nonperturbative hadron dynamics -- a semi-classical frame-independent first
approximation to the spectra and light-front wavefunctions of meson and
baryons. The model predicts a Regge spectrum of linear trajectories with the
same slope in the leading orbital angular momentum L of hadrons and the radial
quantum number n. The hadron eigensolutions projected on the free Fock basis
provides the complete set of valence and non-valence light-front Fock state
wavefunctions which describe the hadron's momentum and spin distributions
needed to compute measures of hadron structure at the quark and gluon level.
The effective confining potential also creates quark- antiquark pairs. The
AdS/QCD model can be systematically improved by using its complete orthonormal
solutions to diagonalize the full QCD light-front Hamiltonian or by applying
the Lippmann-Schwinger method to systematically include the QCD interaction
terms. A new perspective on quark and gluon condensates is also presented.Comment: Presented at LIGHTCONE 2011, 23 - 27 May, 2011, Dallas, T

### Orbital Angular Momentum in Scalar Diquark Model and QED

We compare the orbital angular momentum of the 'quark' in the scalar diquark
model as well as that of the electron in QED (to order {\alpha}) obtained from
the Jaffe-Manohar de- composition to that obtained from the Ji relation. We
estimate the importance of the vector potential in the definition of orbital
angular momentum

### Photoproduction of charm near threshold

Charm and bottom production near threshold is sensitive to the multi-quark,
gluonic, and hidden-color correlations of hadronic and nuclear wavefunctions in
QCD since all of the target's constituents must act coherently within the small
interaction volume of the heavy quark production subprocess. Although such
multi-parton subprocess cross sections are suppressed by powers of $1/m^2_Q$,
they have less phase-space suppression and can dominate the contributions of
the leading-twist single-gluon subprocesses in the threshold regime. The small
rates for open and hidden charm photoproduction at threshold call for a
dedicated facility.Comment: 5 pages 5 figures Changes: 1- Added refs 24,25; 2- Added two
sentences, top of column 2 of page 3, on the definition of x, its range and
the domain of validity of the mode

### Thermal Field Theory and Generalized Light Front Coordinates

The dependence of thermal field theory on the surface of quantization and on
the velocity of the heat bath is investigated by working in general coordinates
that are arbitrary linear combinations of the Minkowski coordinates. In the
general coordinates the metric tensor $g_{\bar{\mu\nu}}$ is non-diagonal. The
Kubo, Martin, Schwinger condition requires periodicity in thermal correlation
functions when the temporal variable changes by an amount
$-i\big/(T\sqrt{g_{\bar{00}}})$. Light front quantization fails since
$g_{\bar{00}}=0$, however various related quantizations are possible.Comment: 10 page

### Intrinsic Charm Contribution to Double Quarkonium Hadroproduction

Double $J/\psi$ production has been observed by the NA3 collaboration in $\pi
N$ and $p N$ collisions with a cross section of the order of 20-30 pb. The
$\psi \psi$ pairs measured in $\pi^-$ nucleus interactions at 150 and 280
GeV$/c$ are observed to carry an anomalously large fraction of the projectile
momentum in the laboratory frame, $x_{\psi \psi} \geq 0.6$ at 150 GeV$/c$ and
$\geq 0.4$ at 280 GeV$/c$. We postulate that these forward $\psi \psi$ pairs
are created by the materialization of Fock states in the projectile containing
two pairs of intrinsic $c \overline c$ quarks. We calculate the overlap of the
charmonium states with the $|\overline u d c \overline c c \overline c \rangle$
Fock state as described by the intrinsic charm model and find that the $\pi^- N
\rightarrow \psi \psi$ longitudinal momentum and invariant mass distributions
are both well reproduced. We also discuss double $J/\psi$ production in $pN$
interactions and the implications for other heavy quarkonium production
channels in QCD.Comment: Revtex, APS style, 7 pages, 3 figures in uuencoded fil

### A nonperturbative calculation of the electron's magnetic moment

In principle, the complete spectrum and bound-state wave functions of a
quantum field theory can be determined by finding the eigenvalues and
eigensolutions of its light-cone Hamiltonian. One of the challenges in
obtaining nonperturbative solutions for gauge theories such as QCD using
light-cone Hamiltonian methods is to renormalize the theory while preserving
Lorentz symmetries and gauge invariance. For example, the truncation of the
light-cone Fock space leads to uncompensated ultraviolet divergences. We
present two methods for consistently regularizing light-cone-quantized gauge
theories in Feynman and light-cone gauges: (1) the introduction of a spectrum
of Pauli-Villars fields which produces a finite theory while preserving Lorentz
invariance; (2) the augmentation of the gauge-theory Lagrangian with higher
derivatives. In the latter case, which is applicable to light-cone gauge (A^+ =
0), the A^- component of the gauge field is maintained as an independent degree
of freedom rather than a constraint. Finite-mass Pauli-Villars regulators can
also be used to compensate for neglected higher Fock states. As a test case, we
apply these regularization procedures to an approximate nonperturbative
computation of the anomalous magnetic moment of the electron in QED as a first
attempt to meet Feynman's famous challenge.Comment: 35 pages, elsart.cls, 3 figure

- …