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

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 $T/T_c=0.156, 0.8, 1.0, 1.25$ and $1.67$ with $T_c\simeq203$MeV for two flavors of Wilson-Clover fermions with a zero-temperature pion mass of $\simeq270$MeV. Exploiting exact sum rules and applying a phenomenologically motivated ansatz allows us to determine the spectral function $\rho(\omega,T)$ 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 $\rho$ 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

Stellar Clusters in NGC 1313: Evidence for Infant Mortality

We present evidence that infant mortality of stellar clusters is likely to be a major and very efficient process for the dissolution of young clusters in the spiral galaxy NGC 1313. Performing stellar PSF photometry on archival HST/ACS images of the galaxy, we find that a large fraction of early B-type stars are seen outside of star clusters and well spread within the galactic disk, consistent with the scenario of infant mortality. We also calculate the UV flux produced by the stars in and out the clusters and find that 75 to 90% of the UV flux in NGC 1313 is produced by stars outside the clusters. These results suggest that the infant mortality of star clusters is probably the underlying cause of the diffuse UV emission in starburst galaxies. Infant mortality would also explain the numerous B-type stars observed in the background field of our Galaxy as well. We exclude the possibility that unresolved low-mass star clusters and scaled OB associations might be the main source for the diffuse UV emission.Comment: 5 pages, 3 figures, accepted for publication in the Astrophysical Journal Letter

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

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 $(T, m_q = 0)$ 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 $(T = 0, m_q = 0)$ 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 $\leq \bar{m}^{\bar{\text{MS}}} \leq$ 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 $O(a)$ improved Wilson quarks. All simulations are done on lattices with a temporal extent of $N_t=16$ and spatial extent $L=32,48$ 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 $m_\pi=200$ 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 $U_A(1)$ 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 $N_{\mathrm f}$ = 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

A Universal Stellar Initial Mass Function? A Critical Look at Variations

Few topics in astronomy initiate such vigorous discussion as whether or not the initial mass function (IMF) of stars is universal, or instead sensitive to the initial conditions of star formation. The distinction is of critical importance: the IMF influences most of the observable properties of stellar populations and galaxies, and detecting variations in the IMF could provide deep insights into the process by which stars form. In this review, we take a critical look at the case for IMF variations, with a view towards whether other explanations are sufficient given the evidence. Studies of the field, local young clusters and associations, and old globular clusters suggest that the vast majority were drawn from a "universal" IMF: a power-law of Salpeter index ($\Gamma=1.35$) above a few solar masses, and a log normal or shallower power-law ($\Gamma \sim 0-0.25$) between a few tenths and a few solar masses (ignoring the effects of unresolved binaries). The shape and universality of the IMF at the stellar-substellar boundary is still under investigation and uncertainties remain large, but most observations are consistent with a IMF that declines ($\Gamma < -0.5$) well below the hydrogen burning limit. Observations of resolved stellar populations and the integrated properties of most galaxies are also consistent with a "universal IMF", suggesting no gross variations in the IMF over much of cosmic time. There are indications of "non-standard" IMFs in specific local and extragalactic environments, which clearly warrant further study. Nonetheless, there is no clear evidence that the IMF varies strongly and systematically as a function of initial conditions after the first few generations of stars.Comment: 49 pages, 5 figures, to appear in Annual Reviews of Astronomy and Astrophysics (2010, volume 48