407 research outputs found
Screening of a Moving Parton in the Quark-Gluon Plasma
The screening potential of a parton moving through a quark-gluon plasma is
calculated using the semi-classical transport theory. An anisotropic potential
showing a minimum in the direction of the parton velocity is found. As
consequences possible new bound states and J/psi dissociation are discussed.Comment: 4 pages, 2 figures, final, extended version, to be published in
Phys.Rev.
Field Theoretic Description of Ultrarelativistic Electron-Positron Plasmas
Ultrarelativistic electron-positron plasmas can be produced in high-intensity
laser fields and play a role in various astrophysical situations. Their
properties can be calculated using QED at finite temperature. Here we will use
perturbative QED at finite temperature for calculating various important
properties, such as the equation of state, dispersion relations of collective
plasma modes of photons and electrons, Debye screening, damping rates, mean
free paths, collision times, transport coefficients, and particle production
rates, of ultrarelativistic electron-positron plasmas. In particular, we will
focus on electron-positron plasmas produced with ultra-strong lasers.Comment: 13 pages, 7 figures, 1 table, published versio
Can Van Hove singularities be observed in relativistic heavy-ion collisions ?
Based on general arguments the in-medium quark propagator in a quark-gluon
plasma leads to a quark dispersion relation consisting of two branches, of
which one exhibits a minimum at some finite momentum. This results in a
vanishing group velocity for collective quark modes. Important quantities such
as the production rate of low mass lepton pairs and mesonic correlators depend
inversely on this group velocity. Therefore these quantities, which follow from
self energy diagrams containing a quark loop, are strongly affected by Van Hove
singularities (peaks and gaps). If these sharp structures could be observed in
relativistic heavy-ion collisions it would reveal the physical picture of the
QGP as a gas of quasiparticles.Comment: 12 pages including nine figures and style files, invited talk given
at the ICPAQGP-2001, November 26-30, 2001, Jaipur, Indi
Hard Thermal Loops, Static Response and the Composite Effective Action
First, we investigate the static non-Abelian Kubo equation. We prove that it
does not possess finite energy solutions; thereby we establish that gauge
theories do not support hard thermal solitons. A similar argument shows that
"static" instantons are absent. In addition, we note that the static equations
reproduce the expected screening of the non-Abelian electric field by a gauge
invariant Debye mass m=gT sqrt((N+N_F/2)/3). Second, we derive the non-Abelian
Kubo equation from the composite effective action. This is achieved by showing
that the requirement of stationarity of the composite effective action is
equivalent, within a kinematical approximation scheme, to the condition of
gauge invariance for the generating functional of hard thermal loops.Comment: 17 pages, MIT preprint CTP#2261. An Appendix [including one
(appended) PS figure] presenting a numerical analysis of the static solutions
has been included. A note relating our approach to alternative ones has been
added. We have also added references and comments in Section II
Shielding of a moving test charge in a quantum plasma
The linearized potential of a moving test charge in a one-component fully
degenerate fermion plasma is studied using the Lindhard dielectric function.
The motion is found to greatly enhance the Friedel oscillations behind the
charge, especially for velocities larger than a half of the Fermi velocity, in
which case the asymptotic behavior of their amplitude changes from 1/r^3 to
1/r^2.5. In the absence of the quantum recoil (tunneling) the potential reduces
to a form similar to that in a classical Maxwellian plasma, with a difference
being that the plasma oscillations behind the charge at velocities larger than
the Fermi velocity are not Landau-damped.Comment: 9 pages, 11 figures. v3: Fixed typo, updated abstrac
Electroweak pinch technique to all orders
The generalization of the pinch technique to all orders in the electroweak
sector of the Standard Model within the class of the renormalizable 't Hooft
gauges, is presented. In particular, both the all-order PT gauge-boson-- and
scalar--fermions vertices, as well as the diagonal and mixed gauge-boson and
scalar self-energies are explicitly constructed. This is achieved through the
generalization to the Standard Model of the procedure recently applied to the
QCD case, which consist of two steps: (i) the identification of special Green's
functions, which serve as a common kernel to all self-energy and vertex
diagrams, and (ii) the study of the (on-shell) Slavnov-Taylor identities they
satisfy. It is then shown that the ghost, scalar and scalar--gauge-boson
Green's functions appearing in these identities capture precisely the result of
the pinching action at arbitrary order. It turns out that the aforementioned
Green's functions play a crucial role, their net effect being the non-trivial
modification of the ghost, scalar and scalar--gauge-boson diagrams of the
gauge-boson-- or scalar--fermions vertex we have started from, in such a way as
to dynamically generate the characteristic ghost and scalar sector of the
background field method. The pinch technique gauge-boson and scalar
self-energies are also explicitly constructed by resorting to the method of the
background-quantum identities.Comment: 48 pages, 8 figures; v2: typos correcte
Kramers-Kronig Relations For The Dielectric Function And The Static Conductivity Of Coulomb Systems
The mutual influence of singularities of the dielectric permittivity e(q,w)
in a Coulomb system in two limiting cases w tends to zero, q tends to zero, and
opposite q tends to zero, w tends to zero is established. It is shown that the
dielectric permittivity e(q,w) satisfies the Kramers-Kronig relations, which
possesses the singularity due to a finite value of the static conductivity.
This singularity is associated with the long "tails" of the time correlation
functions.Comment: 9 pages, 0 figure
Enhanced inverse bremsstrahlung heating rates in a strong laser field
Test particle studies of electron scattering on ions, in an oscillatory
electromagnetic field have shown that standard theoretical assumptions of small
angle collisions and phase independent orbits are incorrect for electron
trajectories with drift velocities smaller than quiver velocity amplitude. This
leads to significant enhancement of the electron energy gain and the inverse
bremsstrahlung heating rate in strong laser fields. Nonlinear processes such as
Coulomb focusing and correlated collisions of electrons being brought back to
the same ion by the oscillatory field are responsible for large angle, head-on
scattering processes. The statistical importance of these trajectories has been
examined for mono-energetic beam-like, Maxwellian and highly anisotropic
electron distribution functions. A new scaling of the inverse bremsstrahlung
heating rate with drift velocity and laser intensity is discussed.Comment: 12 pages, 12 figure
Kinetic Equations for Longwavelength Excitations of the Quark-Gluon Plasma
We show that longwavelength excitations of the quark-gluon plasma are
described by simple kinetic equations which represent the exact equations of
motion at leading order in . Properties of the so-called ``hard thermal
loops'', i.e. the dominant contributions to amplitudes with soft external
lines, find in this approach a natural explanation. In particular, their
generating functional appears here as the effective action describing long
wavelength excitations of the plasma.Comment: January 8, 1993; 8 pages; SPhT/93-
Wearing a gay slogan t-shirt in the higher education classroom: A cautionary tale
© 2015, © The Author(s) 2015. Evidence from numerous studies suggests that homophobia and heterosexism remain common on university campuses. Since the 1970s LGBT academics have been encouraged to ‘put themselves on the line’ and ‘come out’ in the classroom, and in so doing empower LGBT students and provide them with positive role models. Wearing gay pride badges and t-shirts has been discussed as one way in which gay lecturers can come out and challenge homophobia. This paper explores psychology students’ reactions to a gay slogan t-shirt I wore in an undergraduate lecture, and considers whether wearing such t-shirts is an effective and productive way of challenging heterosexism and coming out in the classroom
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