231 research outputs found
WIMP Dark Matter and the QCD Equation of State
Weakly Interacting Massive Particles (WIMPs) of mass m freeze out at a
temperature T_f ~ m/25, i.e. in the range 400 MeV -- 40 GeV for a particle in
the typical mass range 10 -- 1000 GeV. The WIMP relic density, which depends on
the effective number of relativistic degrees of freedom at T_f, may be measured
to better than 1% by Planck, warranting comparable theoretical precision.
Recent theoretical and experimental advances in the understanding of high
temperature QCD show that the quark gluon plasma departs significantly from
ideal behaviour up to temperatures of several GeV, necessitating an improvement
of the cosmological equation of state over those currently used. We discuss how
this increases the relic density by approximately 1.5 -- 3.5% in benchmark
mSUGRA models, with an uncertainly in the QCD corrections of 0.5 -- 1 %. We
point out what further work is required to achieve a theoretical accuracy
comparable with the expected observational precision, and speculate that the
effective number of degrees of freedom at T_f may become measurable in the
foreseeable future.Comment: 4pp, 2figs. More info including Matlab scripts used to generate
equation of state curves at
http://www.pact.cpes.sussex.ac.uk/arXiv/hep-ph/0501232
Constraining the QCD phase diagram by tricritical lines at imaginary chemical potential
We present unambiguous evidence from lattice simulations of QCD with three
degenerate quark species for two tricritical points in the (T,m) phase diagram
at fixed imaginary \mu/T=i\pi/3 mod 2\pi/3, one in the light and one in the
heavy mass regime. These represent the boundaries of the chiral and
deconfinement critical lines continued to imaginary chemical potential,
respectively. It is demonstrated that the shape of the deconfinement critical
line for real chemical potentials is dictated by tricritical scaling and
implies the weakening of the deconfinement transition with real chemical
potential. The generalization to non-degenerate and light quark masses is
discussed.Comment: 4 pages, 5 figure
Space-based passive microwave soil moisture retrievals and the correction for a dynamic open water fraction
The large observation footprint of low-frequency satellite microwave emissions complicates the interpretation of near-surface soil moisture retrievals. While the effect of sub-footprint lateral heterogeneity is relatively limited under unsaturated conditions, open water bodies (if not accounted for) cause a strong positive bias in the satellite-derived soil moisture retrieval. This bias is generally assumed static and associated with large, continental lakes and coastal areas. Temporal changes in the extent of smaller water bodies as small as a few percent of the sensor footprint size, however, can cause significant and dynamic biases. We analysed the influence of such small open water bodies on near-surface soil moisture products derived from actual (non-synthetic) data from the Advanced Microwave Scanning Radiometer for the Earth Observing System (AMSR-E) for three areas in Oklahoma, USA. Differences between on-ground observations, model estimates and AMSR-E retrievals were related to dynamic estimates of open water fraction, one retrieved from a global daily record based on higher frequency AMSR-E data, a second derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) and a third through inversion of the radiative transfer model, used to retrieve soil moisture. The comparison demonstrates the presence of relatively small areas (<0.05) of open water in or near the sensor footprint, possibly in combination with increased, below-critical vegetation density conditions (optical density <0.8), which contribute to seasonally varying biases in excess of 0.2 (m<sup>3</sup> m<sup>−3</sup>) soil water content. These errors need to be addressed, either through elimination or accurate characterisation, if the soil moisture retrievals are to be used effectively in a data assimilation scheme
L band push broom microwave radiometer: Soil moisture verification and time series experiment Delmarva Peninsula
The verification of a multi-sensor aircraft system developed to study soil moisture applications is discussed. This system consisted of a three beam push broom L band microwave radiometer, a thermal infrared scanner, a multispectral scanner, video and photographic cameras and an onboard navigational instrument. Ten flights were made of agricultural sites in Maryland and Delaware with little or no vegetation cover. Comparisons of aircraft and ground measurements showed that the system was reliable and consistent. Time series analysis of microwave and evaporation data showed a strong similarity that indicates a potential direction for future research
The pressure of strong coupling lattice QCD with heavy quarks, the hadron resonance gas model and the large N limit
In this paper we calculate the pressure of pure lattice Yang-Mills theories
and lattice QCD with heavy quarks by means of strong coupling expansions.
Dynamical fermions are introduced with a hopping parameter expansion, which
also allows for the incorporation of finite quark chemical potential. We show
that in leading orders the results are in full agreement with expectations from
the hadron resonance gas model, thus validating it with a first principles
calculation. For pure Yang-Mills theories we obtain the corresponding ideal
glueball gas, in QCD with heavy quarks our result equals that of an ideal gas
of mesons and baryons. Another finding is that the Yang-Mills pressure in the
large N limit is of order to the calculated orders, when the inverse
't Hooft coupling is used as expansion parameter. This property is expected in
the confined phase, where our calculations take place.Comment: 12 pages, 4 figure
The thermal QCD transition with two flavours of twisted mass fermions
We investigate the thermal QCD transition with two flavors of maximally
twisted mass fermions for a set of pion masses, 300 MeV \textless
\textless 500 MeV, and lattice spacings \textless 0.09 fm. We determine the
pseudo-critical temperatures and discuss their extrapolation to the chiral
limit using scaling forms for different universality classes, as well as the
scaling form for the magnetic equation of state. For all pion masses considered
we find resonable consistency with O(4) scaling plus leading corrections.
However, a true distinction between the O(4) scenario and a first order
scenario in the chiral limit requires lighter pions than are currently in use
in simulations of Wilson fermions.Comment: 11 pages, 11 figure
Resummation scheme for 3d Yang-Mills and the two-loop magnetic mass for hot gauge theories
Perturbation theory for non-Abelian gauge theories at finite temperature is
plagued by infrared divergences caused by magnetic soft modes ,
which correspond to the fields of a 3d Yang-Mills theory. We revisit a gauge
invariant resummation scheme to solve this problem by self-consistent mass
generation using an auxiliary scalar field, improving over previous attempts in
two respects. First, we generalise earlier SU(2) treatments to SU(N). Second,
we obtain a gauge independent two-loop gap equation, correcting an error in the
literature. The resulting two-loop approximation to the magnetic mass
represents a correction to the leading one-loop value, indicating a
reasonable convergence of the resummation.Comment: 16 pages, 3 figure
Real-time static potential in hot QCD
We derive a static potential for a heavy quark-antiquark pair propagating in
Minkowski time at finite temperature, by defining a suitable gauge-invariant
Green's function and computing it to first non-trivial order in Hard Thermal
Loop resummed perturbation theory. The resulting Debye-screened potential could
be used in models that attempt to describe the ``melting'' of heavy quarkonium
at high temperatures. We show, in particular, that the potential develops an
imaginary part, implying that thermal effects generate a finite width for the
quarkonium peak in the dilepton production rate. For quarkonium with a very
heavy constituent mass M, the width can be ignored for T \lsim g^2 M/12\pi,
where g^2 is the strong gauge coupling; for a physical case like bottomonium,
it could become important at temperatures as low as 250 MeV. Finally, we point
out that the physics related to the finite width originates from the
Landau-damping of low-frequency gauge fields, and could be studied
non-perturbatively by making use of the classical approximation.Comment: 20 pages. v2: a number of clarifications and a few references added;
published versio
Stress Transmission through Three-Dimensional Ordered Granular Arrays
We measure the local contact forces at both the top and bottom boundaries of
three-dimensional face-centered-cubic and hexagonal-close-packed granular
crystals in response to an external force applied to a small area at the top
surface. Depending on the crystal structure, we find markedly different results
which can be understood in terms of force balance considerations in the
specific geometry of the crystal. Small amounts of disorder are found to create
additional structure at both the top and bottom surfaces.Comment: 9 pages including 9 figures (many in color) submitted to PR
The deconfinement transition of finite density QCD with heavy quarks from strong coupling series
Starting from Wilson's action, we calculate strong coupling series for the
Polyakov loop susceptibility in lattice gauge theories for various small N_\tau
in the thermodynamic limit. Analysing the series with Pad\'e approximants, we
estimate critical couplings and exponents for the deconfinement phase
transition. For SU(2) pure gauge theory our results agree with those from
Monte-Carlo simulations within errors, which for the coarser N_\tau=1,2
lattices are at the percent level. For QCD we include dynamical fermions via a
hopping parameter expansion. On a N_\tau=1 lattice with N_f=1,2,3, we locate
the second order critical point where the deconfinement transition turns into a
crossover. We furthermore determine the behaviour of the critical parameters
with finite chemical potential and find the first order region to shrink with
growing \mu. Our series moreover correctly reflects the known Z(N) transition
at imaginary chemical potential.Comment: 18 pages, 7 figures, typos corrected, version published in JHE
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