4,503 research outputs found
Semi-exclusive DVCS
We consider Semi-Exclusive Deeply Virtual Compton Scattering, gamma* p \to
gamma Y (SECS), where Y is an inclusive state of intermediate mass, \Lambda_QCD
<< m_Y << Q. When the photon is produced with a large transverse momentum k_T
\sim m_Y the subprocess is hard and the struck quark fragments independently of
the target spectators. Using completeness this allows to express the SECS cross
section in terms of ordinary parton distributions. Apart from direct
comparisons with data (yet to come) new information on Bloom-Gilman duality may
be obtained through comparisons of resonance production via DVCS (gamma* p \to
gamma N*) with the SECS scaling distribution in m_Y.Comment: 6 pages, 2 figure
Comover Enhancement of Quarkonium Production
Quarkonium data suggest an enhancement of the hadroproduction rate from
interactions of the heavy quark pair with a comoving color field generated in
the hard gg -> Q\bar{Q} subprocess. We review the motivations and principal
consequences of this comover enhancement scenario (CES).Comment: 7 pages, 1 eps figure, to appear in the proceedings of the CERN
2001-2002 workshop `Hard Probes in Heavy Ion Collisions at the LHC
Turbulence, Complexity, and Solar Flares
The issue of predicting solar flares is one of the most fundamental in
physics, addressing issues of plasma physics, high-energy physics, and
modelling of complex systems. It also poses societal consequences, with our
ever-increasing need for accurate space weather forecasts. Solar flares arise
naturally as a competition between an input (flux emergence and rearrangement)
in the photosphere and an output (electrical current build up and resistive
dissipation) in the corona. Although initially localised, this redistribution
affects neighbouring regions and an avalanche occurs resulting in large scale
eruptions of plasma, particles, and magnetic field. As flares are powered from
the stressed field rooted in the photosphere, a study of the photospheric
magnetic complexity can be used to both predict activity and understand the
physics of the magnetic field. The magnetic energy spectrum and multifractal
spectrum are highlighted as two possible approaches to this.Comment: 2 figure
Many-Body Dynamics and Exciton Formation Studied by Time-Resolved Photoluminescence
The dynamics of exciton and electron-hole plasma populations is studied via
time-resolved photoluminescence after nonresonant excitation. By comparing the
peak emission at the exciton resonance with the emission of the continuum, it
is possible to experimentally identify regimes where the emission originates
predominantly from exciton and/or plasma populations. The results are supported
by a microscopic theory which allows one to extract the fraction of bright
excitons as a function of time.Comment: 11 pages, 5 figure
The hbar Expansion in Quantum Field Theory
We show how expansions in powers of Planck's constant hbar = h/2\pi can give
new insights into perturbative and nonperturbative properties of quantum field
theories. Since hbar is a fundamental parameter, exact Lorentz invariance and
gauge invariance are maintained at each order of the expansion. The physics of
the hbar expansion depends on the scheme; i.e., different expansions are
obtained depending on which quantities (momenta, couplings and masses) are
assumed to be independent of hbar. We show that if the coupling and mass
parameters appearing in the Lagrangian density are taken to be independent of
hbar, then each loop in perturbation theory brings a factor of hbar. In the
case of quantum electrodynamics, this scheme implies that the classical charge
e, as well as the fine structure constant are linear in hbar. The connection
between the number of loops and factors of hbar is more subtle for bound states
since the binding energies and bound-state momenta themselves scale with hbar.
The hbar expansion allows one to identify equal-time relativistic bound states
in QED and QCD which are of lowest order in hbar and transform dynamically
under Lorentz boosts. The possibility to use retarded propagators at the Born
level gives valence-like wave-functions which implicitly describe the sea
constituents of the bound states normally present in its Fock state
representation.Comment: 8 pages, 1 figure. Version to be published in Phys. Rev.
Polarization as a Probe to the Production Mechanisms of Charmonium in Collisions
Measurements of the polarization of \jp produced in pion-nucleus collisions
are in disagreement with leading twist QCD prediction where \jp is observed
to have negligible polarization whereas theory predicts substantial
polarization. We argue that this discrepancy cannot be due to poorly known
structure functions nor the relative production rates of \jp and .
The disagreement between theory and experiment suggests important higher twist
corrections, as has earlier been surmised from the anomalous non-factorized
nuclear -dependence of the \jp cross section.Comment: 8 page
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