84 research outputs found
Matter-induced vertices for photon splitting in a weakly magnetized plasma
We evaluate the three-photon vertex functions at order and in a
weak constant magnetic field at finite temperature and density with on shell
external lines. Their application to the study of the photon splitting process
leads to consider high energy photons whose dispersion relations are not
changed significantly by the plasma effects. The absorption coefficient is
computed and compared with the perturbative vacuum result. For the values of
temperature and density of some astrophysical objects with a weak magnetic
field, the matter effects are negligible.Comment: 14 pages, 1 figure. Accepted for publication in PR
First Direct Measurement of Jets in GeV Heavy Ion Collisions by STAR
We present the first measurement of reconstructed jets in ultra-relativistic
heavy ion collisions. Utilizing the large coverage of the STAR Time Projection
Chamber and Electromagnetic Calorimeter, we apply several modern jet
reconstruction algorithms and background subtraction techniques and explore
their systematic uncertainties in heavy ion events. The differential spectrum
for inclusive jet production in central Au+Au collisions at GeV is presented. In order to assess the jet reconstruction biases, this
spectrum is compared with the jet cross section measured in GeV
p+p collisions scaled by the number of binary N-N collisions to account for
nuclear geometric effects.Comment: Proceedings of the 3rd International Conference on Hard and Electro-
Magnetic Probes of High-Energy Nuclear Collisions 8-14 June 2008, Illa da
Toxa (Galicia-Spain
Photon Splitting in a Strong Magnetic Field: Recalculation and Comparison With Previous Calculations
We recalculate the amplitude for photon splitting in a strong magnetic field
below the pair production threshold, using the worldline path integral variant
of the Bern--Kosower formalism. Numerical comparison (using programs that we
have made available for public access on the Internet) shows that the results
of the recalculation are identical to the earlier calculations of Adler and
later of Stoneham, and to the recent recalculation by Baier, Milstein, and
Shaisultanov.Comment: Revtex, 9 pages, no figure
A Short Review on Jet Identification
Jets can be used to probe the physical properties of the high energy density
matter created in collisions at the Relativistic Heavy Ion Collider (RHIC).
Measurements of strong suppression of inclusive hadron distributions and
di-hadron correlations at high have already provided evidence for
partonic energy loss. However, these measurements suffer from well-known
geometric biases due to the competition of energy loss and fragmentation. These
biases can be avoided if the jets are reconstructed independently of their
fragmentation details - quenched or unquenched. In this paper, we discuss
modern jet reconstruction algorithms (cone and sequential recombination) and
their corresponding background subtraction techniques required by the high
multiplicities of heavy ion collisions. We review recent results from the STAR
experiment at RHIC on direct jet reconstruction in central Au+Au collisions at
GeV.Comment: Proceedings for the invited talk of Hot Quarks 2008, Estes Park, CO
18-23 August 200
Probing For New Physics and Detecting non linear vacuum QED effects using gravitational wave interferometer antennas
Low energy non linear QED effects in vacuum have been predicted since 1936
and have been subject of research for many decades. Two main schemes have been
proposed for such a 'first' detection: measurements of ellipticity acquired by
a linearly polarized beam of light passing through a magnetic field and direct
light-light scattering. The study of the propagation of light through an
external field can also be used to probe for new physics such as the existence
of axion-like particles and millicharged particles. Their existence in nature
would cause the index of refraction of vacuum to be different from unity in the
presence of an external field and dependent of the polarization direction of
the light propagating. The major achievement of reaching the project
sensitivities in gravitational wave interferometers such as LIGO an VIRGO has
opened the possibility of using such instruments for the detection of QED
corrections in electrodynamics and for probing new physics at very low
energies. In this paper we discuss the difference between direct birefringence
measurements and index of refraction measurements. We propose an almost
parasitic implementation of an external magnetic field along the arms of the
VIRGO interferometer and discuss the advantage of this choice in comparison to
a previously proposed configuration based on shorter prototype interferometers
which we believe is inadequate. Considering the design sensitivity in the
strain, for the near future VIRGO+ interferometer, of in the range 40 Hz Hz leads to a variable
dipole magnet configuration at a frequency above 20 Hz such that Tm/ for a `first' vacuum non linear QED detection
Jet Reconstruction in Heavy Ion Collisions
We examine the problem of jet reconstruction at heavy-ion colliders using
jet-area-based background subtraction tools as provided by FastJet. We use
Monte Carlo simulations with and without quenching to study the performance of
several jet algorithms, including the option of filtering, under conditions
corresponding to RHIC and LHC collisions. We find that most standard algorithms
perform well, though the anti-kt and filtered Cambridge/Aachen algorithms have
clear advantages in terms of the reconstructed transverse-momentum offset and
dispersion.Comment: 31 pages, 17 figure
Strong laser fields as a probe for fundamental physics
Upcoming high-intensity laser systems will be able to probe the
quantum-induced nonlinear regime of electrodynamics. So far unobserved QED
phenomena such as the discovery of a nonlinear response of the quantum vacuum
to macroscopic electromagnetic fields can become accessible. In addition, such
laser systems provide for a flexible tool for investigating fundamental
physics. Primary goals consist in verifying so far unobserved QED phenomena.
Moreover, strong-field experiments can search for new light but weakly
interacting degrees of freedom and are thus complementary to accelerator-driven
experiments. I review recent developments in this field, focusing on photon
experiments in strong electromagnetic fields. The interaction of
particle-physics candidates with photons and external fields can be
parameterized by low-energy effective actions and typically predict
characteristic optical signatures. I perform first estimates of the accessible
new-physics parameter space of high-intensity laser facilities such as POLARIS
and ELI.Comment: 7 pages, Key Lecture at the ELI Workshop and School on "Fundamental
Physics with Ultra-High Fields", 9 September - 2 October 2008 at Frauenworth
Monastery, German
Anomalous Pseudoscalar-Photon Vertex In and Out of Equilibrium
The anomalous pseudoscalar-photon vertex is studied in real time in and out
of equilibrium in a constituent quark model. The goal is to understand the
in-medium modifications of this vertex, exploring the possibility of enhanced
isospin breaking by electromagnetic effects as well as the formation of neutral
pion condensates in a rapid chiral phase transition in peripheral,
ultrarelativistic heavy-ion collisions. In equilibrium the effective vertex is
afflicted by infrared and collinear singularities that require hard thermal
loop (HTL) and width corrections of the quark propagator. The resummed
effective equilibrium vertex vanishes near the chiral transition in the chiral
limit. In a strongly out of equilibrium chiral phase transition we find that
the chiral condensate drastically modifies the quark propagators and the
effective vertex. The ensuing dynamics for the neutral pion results in a
potential enhancement of isospin breaking and the formation of
condensates. While the anomaly equation and the axial Ward identity are not
modified by the medium in or out of equilibrium, the effective real-time
pseudoscalar-photon vertex is sensitive to low energy physics.Comment: Revised version to appear in Phys. Rev. D. 42 pages, 4 figures, uses
Revte
Experimental and Theoretical Challenges in the Search for the Quark Gluon Plasma: The STAR Collaboration's Critical Assessment of the Evidence from RHIC Collisions
We review the most important experimental results from the first three years
of nucleus-nucleus collision studies at RHIC, with emphasis on results from the
STAR experiment, and we assess their interpretation and comparison to theory.
The theory-experiment comparison suggests that central Au+Au collisions at RHIC
produce dense, rapidly thermalizing matter characterized by: (1) initial energy
densities above the critical values predicted by lattice QCD for establishment
of a Quark-Gluon Plasma (QGP); (2) nearly ideal fluid flow, marked by
constituent interactions of very short mean free path, established most
probably at a stage preceding hadron formation; and (3) opacity to jets. Many
of the observations are consistent with models incorporating QGP formation in
the early collision stages, and have not found ready explanation in a hadronic
framework. However, the measurements themselves do not yet establish
unequivocal evidence for a transition to this new form of matter. The
theoretical treatment of the collision evolution, despite impressive successes,
invokes a suite of distinct models, degrees of freedom and assumptions of as
yet unknown quantitative consequence. We pose a set of important open
questions, and suggest additional measurements, at least some of which should
be addressed in order to establish a compelling basis to conclude definitively
that thermalized, deconfined quark-gluon matter has been produced at RHIC.Comment: 101 pages, 37 figures; revised version to Nucl. Phys.
Transport Properties of the Quark-Gluon Plasma -- A Lattice QCD Perspective
Transport properties of a thermal medium determine how its conserved charge
densities (for instance the electric charge, energy or momentum) evolve as a
function of time and eventually relax back to their equilibrium values. Here
the transport properties of the quark-gluon plasma are reviewed from a
theoretical perspective. The latter play a key role in the description of
heavy-ion collisions, and are an important ingredient in constraining particle
production processes in the early universe. We place particular emphasis on
lattice QCD calculations of conserved current correlators. These Euclidean
correlators are related by an integral transform to spectral functions, whose
small-frequency form determines the transport properties via Kubo formulae. The
universal hydrodynamic predictions for the small-frequency pole structure of
spectral functions are summarized. The viability of a quasiparticle description
implies the presence of additional characteristic features in the spectral
functions. These features are in stark contrast with the functional form that
is found in strongly coupled plasmas via the gauge/gravity duality. A central
goal is therefore to determine which of these dynamical regimes the quark-gluon
plasma is qualitatively closer to as a function of temperature. We review the
analysis of lattice correlators in relation to transport properties, and
tentatively estimate what computational effort is required to make decisive
progress in this field.Comment: 54 pages, 37 figures, review written for EPJA and APPN; one parag.
added end of section 3.4, and one at the end of section 3.2.2; some Refs.
added, and some other minor change
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