3,318 research outputs found

### QCD Green functions in a gluon field

We formulate a dressed perturbative expansion of QCD, where the standard
diagrams are evaluated in the presence of a constant external gluon field whose
magnitude is gaussian distributed. The approach is Poincar{\'e} and gauge
invariant, and modifies the usual results for hard processes only by power
suppressed contributions. Long distance propagation of quarks and gluons turns
out to be inhibited due to a branch point singularity instead of a pole at
$p^2=0$ in the quark and gluon propagators. The dressing keeps the (massless)
quarks in q qbar fluctuations of the photon at a finite distance from each
other.Comment: 21 pages, 7 figures. Minor modifications in text. Version to be
published in JHE

### Single Spin Asymmetry at Large x_F and k_T

The large single spin asymmetries observed at high momentum fractions x_F and
transverse momenta k_T in p^\uparrow p -> \pi(x_F,k_T)+X as well as in pp ->
\Lambda^\uparrow(x_F,k_T)+X suggest that soft helicity flip processes are
coherent with hard scattering. Such coherence can be maintained if x_F -> 1 as
k_T -> \infty, while k_T^2(1-x_F) \sim \Lambda_QCD^2 stays fixed. The entire
hadron wave function, rather than a single quark, then contributes to the
scattering process. Analogous coherence effects have been seen experimentally
in the Drell-Yan process at high x_F. We find that the p^\uparrow p ->
\pi(x_F,k_T)+X production amplitudes have large dynamic phases and that
helicity flip contributions are unsuppressed in this limit, giving rise to
potentially large single spin asymmetries.Comment: 11 pages, 2 figures. v2: References and a preprint number added.
Calculation of section 4 modified. v3: Minor changes in text. Version to be
published in JHE

### Spin asymmetry at large x_F and k_T

We suggest that the large single spin asymmetries observed at high momentum
fractions x_F and transverse momenta k_T of the pion in p^\uparrow p ->
\pi(x_F,k_T)+X arise from the coherence of the soft interactions with the hard
parton scattering process. Such coherence can be maintained if x_F -> 1 as k_T
-> \infty, while k_T^2(1-x_F) ~ \Lambda_{QCD}^2 stays fixed. Analogous
coherence effects have been seen experimentally in the Drell-Yan process at
high x_F. We find that the p^\uparrow p -> \pi X production amplitudes have
large dynamic phases and that helicity flip contributions are unsuppressed in
this limit, giving rise to potentially large single spin asymmetries.Comment: 3 pages, 1 figure. Talk given at EPS HEP 2007, Mancheste

### Quantum computers can search rapidly by using almost any selective transformations

The search problem is to find a state satisfying certain properties out of a
given set. Grover's algorithm drives a quantum computer from a prepared initial
state to the target state and solves the problem quadratically faster than a
classical computer. The algorithm uses selective transformations to distinguish
the initial state and target state from other states. It does not succeed
unless the selective transformations are very close to phase-inversions. Here
we show a way to go beyond this limitation. An important application lies in
quantum error-correction, where the errors can cause the selective
transformations to deviate from phase-inversions. The algorithms presented here
are robust to errors as long as the errors are reproducible and reversible.
This particular class of systematic errors arise often from imperfections in
apparatus setup. Hence our algorithms offer a significant flexibility in the
physical implementation of quantum search.Comment: 8 pages, Accepted for publication in PR

### 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

### 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.

### Interpretation of High Energy String Scattering in terms of String Configurations

High energy string scattering at fixed momentum transfer, known to be
dominated by Regge trajectory exchange, is interpreted by identifying families
of string states which induce each type of trajectory exchange. These include
the usual leading trajectory $\alpha(t)=\alpha^\prime t+1$ and its daughters as
well as the ``sister'' trajectories $\alpha_m(t)=\alpha(t)/m-(m-1)/2$ and their
daughters. The contribution of the sister $\alpha_m$ to high energy scattering
is dominated by string excitations in the $m^{th}$ mode. Thus, at large $-t$,
string scattering is dominated by wee partons, consistently with a picture of
string as an infinitely composite system of ``constituents'' which carry zero
energy and momentum.Comment: 14 pages, phyzzx, psfig required, Florida Preprint UFIFT-94-

### Stellar laboratories. V. The Xe VI ultraviolet spectrum and the xenon abundance in the hot DO-type white dwarf RE0503-289

For the spectral analysis of spectra of hot stars with a high resolution and
high signal-to-noise ratio (S/N), advanced non-local thermodynamic equilibrium
(NLTE) model atmospheres are mandatory. These are strongly dependent on the
reliability of the atomic data that are used for their calculation.
Reliable Xe VI oscillator strengths are used to identify Xe lines in the
ultraviolet spectrum of the DO-type white dwarf RE0503-289 and to determine its
photospheric Xe abundance.
We publish newly calculated oscillator strengths that are based on a recently
measured Xe VI laboratory line spectrum. These strengths were used to consider
their radiative and collisional bound-bound transitions in detail in our NLTE
stellar-atmosphere models to analyze Xe VI lines exhibited in high-resolution
and high S/N UV observations of RE0503-289.
We identify three hitherto unknown Xe VI lines in the ultraviolet spectrum of
RE0503-289 and confirm the previously measured photospheric Xe abundance of
this white dwarf (log Xe = -4.2 +/- 0.6).
Reliable measurements and calculations of atomic data are prerequisite for
stellar-atmosphere modeling. Observed Xe VI line profiles in the ultraviolet
spectrum of the white dwarf RE0503-289 were well reproduced with the newly
calculated Xe VI oscillator strengths.Comment: 3 pages, 4 figure

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