3,838 research outputs found
Antiscreening of the Ampere force in QED and QCD plasmas
The static forces between electric charges and currents are modified at the
loop level by the presence of a plasma. While electric charges are screened,
currents are not. The effective coupling constant at long distances is enhanced
in both cases as compared to the vacuum, and by different amounts, a clear sign
that Lorentz symmetry is broken. We investigate these effects quantitatively,
first in a QED plasma and secondly using non-perturbative simulations of QCD
with two light degenerate flavors of quarks.Comment: 17 pages, 8 figure
The pion quasiparticle in the low-temperature phase of QCD
We investigate the properties of the pion quasiparticle in the
low-temperature phase of two-flavor QCD on the lattice with support from chiral
effective theory. We find that the pion quasiparticle mass is significantly
reduced compared to its value in the vacuum, by contrast with the static
screening mass, which increases with temperature. By a simple argument, near
the chiral limit the two masses are expected to determine the quasiparticle
dispersion relation. Analyzing two-point functions of the axial charge density
at non-vanishing spatial momentum, we find that the predicted dispersion
relation and the residue of the pion pole are simultaneously consistent with
the lattice data at low momentum. The test, based on fits to the correlation
functions, is confirmed by a second analysis using the Backus-Gilbert method.Comment: 22 pages, 8 figure
Charge transport and vector meson dissociation across the thermal phase transition in lattice QCD with two light quark flavors
We compute and analyze correlation functions in the isovector vector channel
at vanishing spatial momentum across the deconfinement phase transition in
lattice QCD. The simulations are carried out at temperatures and with MeV for two flavors of Wilson-Clover
fermions with a zero-temperature pion mass of MeV. Exploiting exact
sum rules and applying a phenomenologically motivated ansatz allows us to
determine the spectral function via a fit to the lattice
correlation function data. From these results we estimate the electrical
conductivity across the deconfinement phase transition via a Kubo formula and
find evidence for the dissociation of the meson by resolving its
spectral weight at the available temperatures. We also apply the Backus-Gilbert
method as a model-independent approach to this problem. At any given frequency,
it yields a local weighted average of the true spectral function. We use this
method to compare kinetic theory predictions and previously published
phenomenological spectral functions to our lattice study.Comment: 28 pages, 6 figure
Chiral dynamics in the low-temperature phase of QCD
We investigate the low-temperature phase of QCD and the crossover region with
two light flavors of quarks. The chiral expansion around the point in the temperature vs. quark-mass plane indicates that a sharp real-time
excitation exists with the quantum numbers of the pion. We determine its
dispersion relation and test the applicability of the chiral expansion. The
time-dependent correlators are also analyzed using the Maximum Entropy Method
(MEM), yielding consistent results. Finally, we test the predictions of
ordinary chiral perturbation theory around the point for the
temperature dependence of static observables. Around the crossover temperature,
we find that all quantities considered depend only mildly on the quark mass in
the considered range 8MeV 15MeV.Comment: 7 pages, 5 figures, talk presented at the 32nd International
Symposium on Lattice Field Theory (Lattice 2014), 23 - 28 June, 2014 Columbia
University New York, NY, US
QCD thermodynamics with O(a) improved Wilson fermions at Nf=2
We present an update of our study of the phase diagram of two-flavour QCD at
zero baryon density with dynamical improved Wilson quarks. All
simulations are done on lattices with a temporal extent of and spatial
extent and 64, ensuring that discretisation effects are small and
finite size effects can be controlled. In the approach to the chiral limit we
currently have three scans with pion masses between 540 and 200 MeV. In this
proceedings article the focus is on the new scan at MeV and the
measurement of screening masses. We also present first results concerning a
test of scaling in the approach to the chiral limit and the chiral
extrapolation of the difference of screening masses in scalar and pseudoscalar
channels, which provides a measure for the strength of the anomalous breaking
of the symmetry.Comment: 7 pages, 5 figures, talk presented at the 31st International
Symposium on Lattice Field Theory (Lattice 2013), 29 July - 3 August 2013,
Mainz, German
An estimate for the thermal photon rate from lattice QCD
We estimate the production rate of photons by the quark-gluon plasma in
lattice QCD. We propose a new correlation function which provides better
control over the systematic uncertainty in estimating the photon production
rate at photon momenta in the range {\pi}T/2 to 2{\pi}T. The relevant Euclidean
vector current correlation functions are computed with = 2
Wilson clover fermions in the chirally-symmetric phase. In order to estimate
the photon rate, an ill-posed problem for the vector-channel spectral function
must be regularized. We use both a direct model for the spectral function and a
model-independent estimate from the Backus-Gilbert method to give an estimate
for the photon rate.Comment: 15 pages, 11 figures, talk presented at 35th annual International
Symposium on Lattice Field Theory, 18-24 June 2017, Granada, Spai
Testing the strength of the anomaly at the chiral phase transition in two-flavour QCD
We study the thermal transition of QCD with two degenerate light flavours by
lattice simulations using -improved Wilson quarks. Particular
emphasis lies on the pattern of chiral symmetry restoration, which we probe via
the static screening correlators. On volumes we observe that the
screening masses in transverse iso-vector vector and axial-vector channels
become degenerate at the transition temperature. The splitting between the
screening masses in iso-vector scalar and pseudoscalar channels is strongly
reduced compared to the splitting at zero temperature and is actually
consistent with zero within uncertainties. In this proceedings article we
extend our studies to matrix elements and iso-singlet correlation functions.
Furthermore, we present results on larger volumes, including first results at
the physical pion mass.Comment: 10 pages, 9 figures, invited contribution to the 9th International
Workshop on Chiral Dynamics, Sept. 17-21, 2018, Duke University, Durham, NC,
US
Analysis of Weather Data Collected From Two Locations in a Small Urban Community
The heat island effect is a well known feature in the microclimate of urban areas, and is considered to be the difference between the urban area and its surroundings. While this study only employs two instruments, the authors are not aware of any studies which examine the differences in temperature between an instrument inside a town the size of Sedalia and its surroundings by collecting hourly information. We attempt to infer here the impact of Sedalia, Missouri, the State Fair Community College campus, and the state fairgrounds on the temperature patterns for a small region of west-central Missouri. The two stations, one on the grounds of State Fair Community College and the other at the Sedalia Airport were used. Temperature, precipitation, cloudiness, and wind information were gathered hourly between 1 February and 31 March, 2005. The weather station at the regional airport was located 11 km (7 miles) northeast of the campus instrument. Our results indicate that the city has no discernable impact on the distribution of monthly precipitation totals. We found a distinct difference between the local surface temperatures as recorded by each instrument. For the Sedalia area, the temperature differences between the town center and the outside location were approximately 2 - 6oF (1.0 - 3.3o C) warmer, typically, than the surrounding environment, as inferred by these instruments. This difference was as much as 11o F (6oC) when comparing hourly temperature information. Additionally, the difference was larger for clear days and days during which there was little wind
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