1,627 research outputs found
Chemical Equilibration in Hadronic Collisions
We study chemical equilibration in out-of-equilibrium Quark-Gluon Plasma
using the first principles method of QCD effective kinetic theory, accurate at
weak coupling. In longitudinally expanding systems--relevant for relativistic
nuclear collisions--we find that for realistic couplings chemical equilibration
takes place after hydrodynamization, but well before local thermalization. We
estimate that hadronic collisions with final state multiplicities
live long enough to reach approximate
chemical equilibrium, which is consistent with the saturation of strangeness
enhancement observed in proton-proton, proton-nucleus and nucleus-nucleus
collisions.Comment: 7 pages, 3 figures, see also our companion paper arXiv:1811.03068, v2
small changes, published versio
The Sphaleron Rate in SU(N) Gauge Theory
The sphaleron rate is defined as the diffusion constant for topological
number NCS = int g^2 F Fdual/32 pi^2. It establishes the rate of equilibration
of axial light quark number in QCD and is of interest both in electroweak
baryogenesis and possibly in heavy ion collisions. We calculate the
weak-coupling behavior of the SU(3) sphaleron rate, as well as making the most
sensible extrapolation towards intermediate coupling which we can. We also
study the behavior of the sphaleron rate at weak coupling at large Nc.Comment: 18 pages with 3 figure
Thermalization in Weakly Coupled Nonabelian Plasmas
We investigate how relativistic, nonabelian plasmas approach equilibrium in a
general context. Our treatment is entirely parametric and for small Yang-Mills
coupling . First we study isotropic systems with an initially
nonequilibrium momentum distribution. We consider both the case of initially
very high occupancy and initially very low occupancy. Then we consider systems
which are anisotropic. We consider both weak anisotropy and large anisotropy,
and allow the occupancy to be parametrically large or small. Writing the
typical momentum of an initial excitation as Q and the final temperature as T,
full equilibration occurs in a time t ~ \alpha^{-2}/T for T > Q, and t ~
\alpha^{-2} Q^{1/2} T^{-3/2} for T < Q, unless the initial system is
sufficiently anisotropic and T > \alpha^{2/3} Q, in which case equilibration
occurs somewhat faster, t ~ \alpha^{-13/7} Q^{5/7} T^{-12/7} (or \alpha^{-2}/T
if that is longer).Comment: 55 pages including many figures, but with a comprehensive review of
results in the first 6 pages
Bjorken Flow, Plasma Instabilities, and Thermalization
At asymptotically high energies, thermalization in heavy ion collisions can
be described via weak-coupling QCD. We present a complete treatment of how
thermalization proceeds, at the parametric weak-coupling level. We show that
plasma instabilities dominate the dynamics, from immediately after the
collision until well after the plasma becomes nearly in equilibrium. Initially
they drive the system close to isotropy, but Bjorken expansion and increasing
diluteness makes the system again become more anisotropic. At time \tau ~
\alpha^(-12/5) Q^(-1) the dynamics become dominated by a nearly-thermal bath;
and at time \tau ~ \alpha^(-5/2) Q^(-1)$ the bath comes to dominate the energy
density, completing thermalization. After this time there is a nearly isotropic
and thermal Quark-Gluon Plasma.Comment: 22 pages, 5 figure
Hard thermal loops for soft or collinear external momenta
We consider finite temperature 1-loop diagrams with hard loop momenta and an
arbitrary number of external gauge fields when the external momenta are either
soft, or near the light cone and nearly collinear with the loop momentum. We
obtain a recursion relation for these diagrams which we translate into an
equation for their generating functional. By integrating out the soft fields
while keeping two collinear ones we find an integral equation, originally due
to Arnold, Moore, and Yaffe, which sums the bremsstrahlung and pair
annihilation contribution to the thermal photon production rate.Comment: 17 pages, title corrected, clarifying paragraph added to the
appendix, version to appear in JHE
Plasma photoemission from string theory
Leading 't Hooft coupling corrections to the photoemission rate of the planar
limit of a strongly-coupled {\cal {N}}=4 SYM plasma are investigated using the
gauge/string duality. We consider the full order \alpha'^3 type IIB string
theory corrections to the supergravity action, including higher order terms
with the Ramond-Ramond five-form field strength. We extend our previous results
presented in arXiv:1110.0526. Photoemission rates depend on the 't Hooft
coupling, and their curves suggest an interpolating behaviour from strong
towards weak coupling regimes. Their slopes at zero light-like momentum give
the electrical conductivity as a function of the 't Hooft coupling, in full
agreement with our previous results of arXiv:1108.6306. Furthermore, we also
study the effect of corrections beyond the large N limit.Comment: 36 pages, 5 figures, paragraph added in the conclusions, references
added, typos correcte
Three-loop matching coefficients for hot QCD: Reduction and gauge independence
We perform an integral reduction for the 3-loop effective gauge coupling and
screening mass of QCD at high temperatures, defined as matching coefficients
appearing in the dimensionally reduced effective field theory (EQCD).
Expressing both parameters in terms of a set master (sum-) integrals, we show
explicit gauge parameter independence. The lack of suitable methods for solving
the comparatively large number of master integrals forbids the complete
evaluation at the moment. Taking one generic class of masters as an example, we
highlight the calculational techniques involved. The full result would allow to
improve on one of the classic probes for the convergence of the weak-coupling
expansion at high temperatures, namely the comparison of full and effective
theory determinations of the spatial string tension. Furthermore, the full
result would also allow to determine one new contribution of order O(g**7) to
the pressure of hot QCD.Comment: 19 pages, 2 figures. v2: new Section 6 discussing applications, to
match journal versio
Sphalerons and the Electroweak Phase Transition in Models with Higher Scalar Representations
In this work we investigate the sphaleron solution in a
gauge theory, which also encompasses the Standard Model, with higher scalar
representation(s) (). We show that the field profiles
describing the sphaleron in higher scalar multiplet, have similar trends like
the doublet case with respect to the radial distance. We compute the sphaleron
energy and find that it scales linearly with the vacuum expectation value of
the scalar field and its slope depends on the representation. We also
investigate the effect of gauge field and find that it is small for the
physical value of the mixing angle, and resembles the case for the
doublet. For higher representations, we show that the criterion for strong
first order phase transition, , is relaxed with respect to
the doublet case, i.e. .Comment: 20 pages, 5 figures & 1 table, published versio
Jet coherence in QCD media: the antenna radiation spectrum
We study the radiation of a highly energetic partonic antenna in a colored
state traversing a dense QCD medium. Resumming multiple scatterings of all
involved constituents with the medium we derive the general gluon spectrum
which encompasses both longitudinal color coherence between scattering centers
in the medium, responsible for the well known Landau-Pomeranchuk-Migdal (LPM)
effect, and transverse color coherence between partons inside a jet, leading,
in vacuum, to angular ordering of the parton shower. We discuss shortly the
onset of transverse decoherence which is reached in opaque media. In this
regime, the spectrum consists of independent radiation off the antenna
constituents.Comment: 15 pages, 2 figures, paper shortened and partly rewritten, references
added, results unchange
Intermediate distance correlators in hot Yang-Mills theory
Lattice measurements of spatial correlation functions of the operators FF and
FF-dual in thermal SU(3) gauge theory have revealed a clear difference between
the two channels at "intermediate" distances, x ~ 1/(pi T). This is at odds
with the AdS/CFT limit which predicts the results to coincide. On the other
hand, an OPE analysis at short distances (x << 1/(pi T)) as well as effective
theory methods at long distances (x >> 1/(pi T)) suggest differences. Here we
study the situation at intermediate distances by determining the time-averaged
spatial correlators through a 2-loop computation. We do find unequal results,
however the numerical disparity is small. Apart from theoretical issues, a
future comparison of our results with time-averaged lattice measurements might
also be of phenomenological interest in that understanding the convergence of
the weak-coupling series at intermediate distances may bear on studies of the
thermal broadening of heavy quarkonium resonances.Comment: 31 page
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