QCD, monopoles on the Lattice and gauge invariance

The number and the location of the monopoles observed on the lattice in QCD configurations happens to depend strongly on the choice of the gauge used to expose them, in contrast to the physical expectation that monopoles be gauge invariant objects. It is proved by use of the non abelian Bianchi identities (NABI) that monopoles are indeed gauge invariant, but the method used to detect them depends, in a controllable way, on the choice of the abelian projection. Numerical checks are presented.Comment: 3 pages, 1 figure. Presented at the Conference QUARK CONFINEMENT AND THE HADRON SPECTRUM IX, Madrid Aug.30-Sept.3 201

On the phase diagram of the Higgs SU(2) model

The Higgs SU(2) model with fixed Higgs length is usually believed to have two different phases at high gauge coupling (\beta), separated by a line of first order transitions but not distinuguished by any typical symmetry associated with a local order parameter, as first proved by Fradkin and Shenker. We show that in regions of the parameter space where it is usually supposed to be a first order phase transition only a smooth crossover is in fact present.Comment: 6 pages, 6 figures. Talk presented at The XXVI International Symposium on Lattice Field Theory, July 14 - 19, 2008 - Williamsburg, Virginia, US

A test of first order scaling in Nf =2 QCD: a progress report

We present the status of our analysis on the order of the finite temperature transition in QCD with two flavors of degenerate fermions. Our new simulations on large lattices support the hypothesis of the first order nature of the transition, showing a preliminary two state signal. We will discuss the implications and the next steps in our analysis.Comment: 6 pages, 4 figures. Talk presented at The XXVI International Symposium on Lattice Field Theory, July 14 - 19, 2008 - Williamsburg, Virginia, US

Phase diagram of the lattice SU(2) Higgs model

We perform a detailed study of the phase diagram of the lattice Higgs SU(2) model with fixed Higgs field length. Consistently with previsions based on the Fradkin Shenker theorem we find a first order transition line with an endpoint whose position we determined. The diagram also shows cross-over lines: the cross-over corresponding to the pure SU(2) bulk is also present at nonzero coupling with the Higgs field and merges with the one that continues the line of first order transition beyond the critical endpoint. At high temperature the first order line becomes a crossover, whose position moves by varying the temperature.Comment: 18 pages, 15 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 $m_\pi$ \textless 500 MeV, and lattice spacings $a$ \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

Monopoles, abelian projection, and gauge invariance

A direct connection is proved between the Non-Abelian Bianchi Identities(NABI), and the abelian Bianchi identities for the 't Hooft tensor. As a consequence the existence of a non-zero magnetic current is related to the violation of the NABI's and is a gauge-invariant property. The construction allows to show that not all abelian projections can be used to expose monopoles in lattice configurations: each field configuration with non-zero magnetic charge identifies its natural projection, up to gauge transformations which tend to unity at large distances. It is shown that the so-called maximal-abelian gauge is a legitimate choice. It is also proved, starting from the NABI, that monopole condensation is a physical gauge invariant phenomenon, independent of the choice of the abelian projection.Comment: 9 pages, no figur

A versatile wavelength-dependent spectrophotometer for efficiency measurements of CCD and cryogenic gratings

A user-friendly and automatic illuminator with adjustable wavelength and optical power has been developed to obtain precision quantum efficiency (QE) curves of astronomical CCD as well as optical transmission measurements for cryogenic holographic gratings and other optical components. Integrating commercial components with custom mechanical parts and control software, this equipment is able to illuminate a target with light of controlled intensity and wavelength. This facility is primarily intended for testing of Volume Phase Holographic (VPH) gratings at low temperature as well as obtaining CCD quantum efficiencies. A Labview control application runs on a desktop computer allowing full automation of the spectrophotometer. The apparatus includes a Quartz-Tungsten light source, neutral density filters, a monochromator, visible and near-infrared power meters, as well as collimating and focusing optics. Rotation mechanisms allow the characterization of gratings for all angles of diffractions. For CCD testing, network commands allow the facility to get raw images, compute and record QE curves for further detector characterization

Magnetic properties of the strongly interacting matter

We study the magnetic properties of the strongly interacting matter using Lattice QCD simulations. The QCD medium shows a paramagnetic behavior in the range of temperatures 100–400 MeV, with a sharp increase of the magnetic susceptibility above the deconfinement temperature. We expect a significant magnetic contribution to the pressure of the system in non-central heavy-ion collisions

A theory for image persistence in HgCdTe photodiodes

Image persistence can produce systematic errors, which remain significant in some applications even when buried in noise. Ideally the image persistence amplitude, linearity and decay over time could be calibrated independently for each pixel to levels well below the noise floor, however averaging multiple measurements to characterize persistence to this accuracy is impractical due to the long time scales for the decay and the need to emulate the exposure and readout timing of the observations to be calibrated. We examine a compromise where the initial persistence response is characterized independently for each pixel but the latter parts of the decay are assumed to follow the mean decay curve. When averaged spatially, persistence increases monotonically with stimulus amplitude until the photodiodes approach forward bias. For several Teledyne 1.7 μm cutoff HgCdTe detectors tested, persistence is linear over most of the normal signal range. We characterize the temporal response, and examine the dependence of charge emission time constants on total stimulus duration. We describe the suppression of persistence by signal in the current frame and begin to examine the superposition of the decay curves from multiple stimuli