8,779 research outputs found
The Shear Viscosity to Entropy Density Ratio of Trapped Fermions in the Unitarity Limit
We extract the shear viscosity to entropy density ratio \eta/s of cold
fermionic atoms in the unitarity limit from experimental data on the damping of
collective excitations. We find that near the critical temperature \eta/s is
roughly equal to 1/2 in units of \hbar/k_B. With the possible exception of the
quark gluon plasma, this value is closer to the conjectured lower bound
1/(4\pi) than any other known liquid.Comment: published versio
Polarized fermions in the unitarity limit
We consider a polarized Fermi gas in the unitarity limit. Results are
calculated analytically up to next-to-leading order in an expansion about d=4
spatial dimensions. We find a first order transition from superfluid to normal
phase. The critical chemical potential asymmetry for this phase transition is
delta_mu_c= 2/(mu epsilon)*(1-0.467\epsilon), where epsilon=4-d is the
expansion parameter and 'mu' is the average chemical potential of the two
fermion species. Stability of the superfluid phase in the presence of
supercurrents is also studied.Comment: 5 pages, 5 figures, LaTeX2e; minor changes, note added at the end, to
be published in PR
Meson Supercurrent State in High Density QCD
We study the effect of a non-zero strange quark mass on the
color-flavor-locked (CFL) phase of high density quark matter. We have
previously shown that for a strange quark mass
the CFL state becomes unstable toward the formation of a neutral kaon
condensate. Recently, several authors discovered that for the CFL state contains gapless fermions, and that the gapless modes
lead to an instability in current-current correlation functions. Using an
effective theory of the CFL state we demonstrate that this instability is
resolved by the formation of an inhomogeneous meson condensate, analogous to
Migdal's p-wave pion condensate. This state has a non-zero meson current which
is canceled by a backflow of gapless fermions.Comment: 4 pages, one figure, revised version, to appear in PRL (title changed
in journal
CENP-C unwraps the human CENP-A nucleosome through the H2A C-terminal tail
Centromeres are defined epigenetically by nucleosomes containing the histone H3 variant CENP-A, upon which the constitutive centromere-associated network of proteins (CCAN) is built. CENP-C is considered to be a central organizer of the CCAN. We provide new molecular insights into the structure of human CENP-A nucleosomes, in isolation and in complex with the CENP-C central region (CENP-C-CR), the main CENP-A binding module of human CENP-C. We establish that the short alpha N helix of CENP-A promotes DNA flexibility at the nucleosome ends, independently of the sequence it wraps. Furthermore, we show that, in vitro, two regions of human CENP-C (CENP-C-CR and CENP-C-motif) both bind exclusively to the CENP-A nucleosome. We find CENP-C-CR to bind with high affinity due to an extended hydrophobic area made up of CENP-A(V)(532) and CENP-A(V)(533). Importantly, we identify two key conformational changes within the CENP-A nucleosome upon CENP-C binding. First, the loose DNA wrapping of CENP-A nucleosomes is further exacerbated, through destabilization of the H2A C-terminal tail. Second, CENP-C-CR rigidifies the N-terminal tail of H4 in the conformation favoring H4(K20) monomethylation, essential for a functional centromere
Multiscale image analysis applied to gamma/hadron discrimination for VHE gamma-ray astronomy with ARGO-YBJ
Intrinsic differences in the processes involved in the development of
electromagnetic and hadronic showers in the atmosphere have been evidenced by
means of a careful analysis of the event image given by the ARGO-YBJ detector.
The images have been analyzed at different length scales and their multifractal
nature has been studied. The use of the multiscale approach together with a
properly designed and trained Artificial Neural Network, allowed us to obtain a
good gamma/hadron discrimination power. If confirmed by further studies on
different event cathegories, this result would allow to nearly double the
detector sensitivity to gamma ray sources.Comment: 10 pages, 5 figures, Presented at the "Incontro Nazionale di
Astrofisica delle Alte Energie", Rome 15-16 May 200
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