4,879 research outputs found
Hadron Optics: Diffraction Patterns in Deeply Virtual Compton Scattering
We show that the Fourier transform of the Deeply Virtual Compton Scattering
(DVCS) amplitude with respect to the skewness variable at fixed
invariant momentum transfer squared provides a unique way to visualize the
structure of the target hadron in the boost-invariant longitudinal coordinate
space. The results are analogous to the diffractive scattering of a wave in
optics. As a specific example, we utilize the quantum fluctuations of a fermion
state at one loop in QED to obtain the behavior of the DVCS amplitude for
electron-photon scattering. We then simulate the wavefunctions for a hadron by
differentiating the above LFWFs with respect to and study the
corresponding DVCS amplitudes in light-front longitudinal space. In both cases
we observe that the diffractive patterns in the longitudinal variable conjugate
to sharpen and the positions of the first minima move in with
increasing momentum transfer. For fixed , higher minima appear at positions
which are integral multiples of the lowest minimum. Both these observations
strongly support the analogy with diffraction in optics.Comment: Some plots modified, clarifications and references adde
Hadron Spin Dynamics
Spin effects in exclusive and inclusive reactions provide an essential new
dimension for testing QCD and unraveling hadron structure. Remarkable new
experiments from SLAC, HERMES (DESY), and the Jefferson Laboratory present many
challenges to theory, including measurements at HERMES and SMC of the single
spin asymmetries in pion electroproduction, where the proton is polarized
normal to the scattering plane. This type of single spin asymmetry may be due
to the effects of rescattering of the outgoing quark on the spectators of the
target proton, an effect usually neglected in conventional QCD analyses. Many
aspects of spin, such as single-spin asymmetries and baryon magnetic moments
are sensitive to the dynamics of hadrons at the amplitude level, rather than
probability distributions. I illustrate the novel features of spin dynamics for
relativistic systems by examining the explicit form of the light-front
wavefunctions for the two-particle Fock state of the electron in QED, thus
connecting the Schwinger anomalous magnetic moment to the spin and orbital
momentum carried by its Fock state constituents and providing a transparent
basis for understanding the structure of relativistic composite systems and
their matrix elements in hadronic physics. I also present a survey of
outstanding spin puzzles in QCD, particularly the double transverse spin
asymmetry A_{NN} in elastic proton-proton scattering, the J/psi to rho-pi
puzzle, and J/psi polarization at the Tevatron.Comment: Concluding theory talk presented at SPIN2001, the Third
Circum-Pan-Pacific Symposium on High Energy Physics, October, 2001, Beijin
Classical sum rules and spin correlations in photoabsorption and photoproduction processes
In this paper we study the possibility of generalizing the classical
photoabsorption () sum rules, to processes
and crossed helicity amplitudes. In the first case, using detailed balance, the
sum rule is written as where 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 and for
which the differential cross section for the process or in which the helicities of the photon and particle 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
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
The QCD Effective Charge to All Orders
Using the pinch technique, we show how to construct the QCD effective charge
to all orders.Comment: 4 pages, no figures. Contributed to QCD 02: High-Energy Physics
International Conference in Quantum Chromodynamics, Montpellier, France, 2-9
July 200
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
The Spin Structure of a Polarized Photon
We show that the first moment of the spin-dependent structure function
of a real photon vanishes independent of the momentum
transfer it is probed with. This result is non-perturbative: it holds to
all orders in perturbation theory in abelian and non-abelian gauge theory and
at every twist.Comment: LaTex, 13 page
Implication of the overlap representation for modelling generalized parton distributions
Based on a field theoretically inspired model of light-cone wave functions,
we derive valence-like generalized parton distributions and their double
distributions from the wave function overlap in the parton number conserved
s-channel. The parton number changing contributions in the t-channel are
restored from duality. In our construction constraints of positivity and
polynomiality are simultaneously satisfied and it also implies a model
dependent relation between generalized parton distributions and transverse
momentum dependent parton distribution functions. The model predicts that the
t-behavior of resulting hadronic amplitudes depends on the Bjorken variable
x_Bj. We also propose an improved ansatz for double distributions that embeds
this property.Comment: 15 pages, 8 eps figure
Possible multiparticle ridge-like correlations in very high multiplicity proton-proton collisions
The CMS collaboration at the LHC has reported a remarkable and unexpected
phenomenon in very high-multiplicity high energy proton-proton collisions: a
positive correlation between two particles produced at similar azimuthal
angles, spanning a large range in rapidity. We suggest that this "ridge"-like
correlation may be a reflection of the rare events generated by the collision
of aligned flux tubes connecting the valence quarks in the wave functions of
the colliding protons. The "spray" of particles resulting from the approximate
line source produced in such inelastic collisions then gives rise to events
with a strong correlation between particles produced over a large range of both
positive and negative rapidity. We suggest an additional variable that is
sensitive to such a line source which is related to a commonly used measure,
ellipticity.Comment: Updated figure. Version to be published in Physics Letters
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