64,029 research outputs found
A consistent approximation scheme beyond RPA for bosons
In this paper, we develop a consistent extension of RPA for bosonic systems.
In order to illustrate the method, we consider the case of the anharmonic
oscillator. We compare our results with those obtained in mean-field and
standard RPA approaches, with the exact ones and show that they are very close
to the exact ones.Comment: 19 pages, Latex, 1 figure, accepted version in EPJ
Mode expansion for the density profile of crystal-fluid interfaces: Hard spheres as a test case
We present a technique for analyzing the full three-dimensional density
profiles of a planar crystal-fluid interface in terms of density modes. These
density modes can also be related to crystallinity order parameter profiles
which are used in coarse-grained, phase field type models of the statics and
dynamics of crystal-fluid interfaces and are an alternative to crystallinity
order parameters extracted from simulations using local crystallinity criteria.
We illustrate our results for the hard sphere system using finely-resolved,
three-dimensional density profiles from density functional theory of
fundamental measure type.Comment: submitted for the special issue of the CODEF III conferenc
Observational constraint on the fourth derivative of the inflaton potential
We consider the flow-equations for the 3 slow-roll parameters n_S (scalar
spectral index), r (tensor to scalar ratio), and dn_S/dlnk (running of the
spectral index). We show that the combination of these flow-equations with the
observational bounds from cosmic microwave background and large scale structure
allows one to put a lower bound on the fourth derivative of the inflationary
potential, M_P^4(V''''/V) > -0.02.Comment: 3 pages, 3 figure
Shear Viscosity of Quark Matter
We consider the shear viscosity of a system of quarks and its ratio to the
entropy density above the critical temperature for deconfinement. Both
quantities are derived and computed for different modeling of the quark
self-energy, also allowing for a temperature dependence of the effective mass
and width. The behaviour of the viscosity and the entropy density is argued in
terms of the strength of the coupling and of the main characteristics of the
quark self-energy. A comparison with existing results is also discussed.Comment: 15 pages, 4 figure
Point-Source Power in 3 Year Wilkinson Microwave Anisotropy Probe Data
Using a set of multifrequency cross spectra computed from the 3 year WMAP sky maps, we fit for the unresolved point-source contribution. For a white-noise power spectrum, we find a Q-band amplitude of A = 0.011 ± 0.001 ÎŒK^2 sr (antenna temperature), significantly smaller than the value of 0.017 ± 0.002 ÎŒK^2 sr used to correct the spectra in the WMAP release. Modifying the point-source correction in this way largely resolves the discrepancy that Eriksen et al. found between the WMAP V- and W-band power spectra. Correcting the co-added WMAP spectrum for both the low-l power excess due to a suboptimal likelihood approximationâalso reported by Eriksen et al.âand the high-l power deficit due to oversubtracted point sourcesâpresented in this Letterâwe find that the net effect in terms of cosmological parameters is an ~0.7 Ï shift in n_s to larger values. For the combination of WMAP, BOOMERANG, and ACBAR data, we find ns = 0.969 ± 0.016, lowering the significance of n_s â 1 from ~2.7 Ï to ~2.0 Ï
Alloreactive cytotoxic T lymphocytes generated in the presence of viral- derived peptides show exquisite peptide and MHC specificity
The nature of alloreactivity to MHC molecules has been enigmatic, primarily because of the observation that allogeneic responses are considerably stronger than syngeneic responses. To better determine the specificity potential of allogeneic responses, we have generated alloreactive CTL specific for exogenous, viral-derived peptide ligands. This approach allowed us to critically evaluate both the peptide- and MHC-specificity of these alloreactive T cells. Exploiting the accessibility of the H-2Ld class I molecule for exogenous peptide ligands, alloreactive CTL were generated that are specific for either murine cytomegalovirus (MCMV) or lymphocytic choriomeningitis virus (LCMV) peptides bound by Ld alloantigens. Peptide specificity was initially observed in bulk cultures of alloreactive CTL only when tested on peptide-sensitized T2.Ld target cells that have defective presentation of endogenous peptides. Subsequent cloning of bulk alloreactive CTL lines generated to MCMV yielded CTL clones that had exquisitely specific MCMV peptide recognition requirement. All of the MCMV/Ld alloreactive CTL clones were also exquisitely MHC-specific in that none of the CTL clones lysed targets expressing MCMV/Lq complexes, even though Lq differs from Ld by only six amino acid residues and Lq also binds the MCMV peptide. This observation clearly demonstrates that alloreactive CTL are capable of the same degree of specificity for target cell recognition as are syngeneic CTL in MHC-restricted responses
Why does the Jeans Swindle work?
When measuring the mass profile of any given cosmological structure through
internal kinematics, the distant background density is always ignored. This
trick is often refereed to as the "Jeans Swindle". Without this trick a
divergent term from the background density renders the mass profile undefined,
however, this trick has no formal justification. We show that when one includes
the expansion of the Universe in the Jeans equation, a term appears which
exactly cancels the divergent term from the background. We thereby establish a
formal justification for using the Jeans Swindle.Comment: 5 pages, 2 figures, Accepted for publication in MNRAS Letter
Kinetic theory of cluster impingement in the framework of statistical mechanics of rigid disks
The paper centres on the evaluation of the function n(theta)=N(theta)/N0,
that is the normalized number of islands as a function of coverage 0<theta<1,
given N0 initial nucleation centres (dots) having any degree of spatial
correlation. A mean field approach has been employed: the islands have the same
size at any coverage. In particular, as far as the random distribution of dots
is concerned, the problem has been solved by considering the contribution of
binary collisions between islands only. With regard to correlated dots, we
generalize a method previously applied to the random case only. In passing, we
have made use of the exclusion probability reported in [S. Torquato, B. Lu, J.
Rubinstein, Phys.Rev.A 41, 2059 (1990)], for determining the kinetics of
surface coverage in the case of correlated dots, improving our previous
calculation [M. Tomellini, M. Fanfoni, M. Volpe Phys. Rev.B 62, 11300, (2000)].Comment: 10 pages, 3 figure
Mesonic correlation functions at finite temperature and density in the Nambu-Jona-Lasinio model with a Polyakov loop
We investigate the properties of scalar and pseudo-scalar mesons at finite
temperature and quark chemical potential in the framework of the
Nambu-Jona-Lasinio (NJL) model coupled to the Polyakov loop (PNJL model) with
the aim of taking into account features of both chiral symmetry breaking and
deconfinement. The mesonic correlators are obtained by solving the
Schwinger-Dyson equation in the RPA approximation with the Hartree (mean field)
quark propagator at finite temperature and density. In the phase of broken
chiral symmetry a narrower width for the sigma meson is obtained with respect
to the NJL case; on the other hand, the pion still behaves as a Goldstone
boson. When chiral symmetry is restored, the pion and sigma spectral functions
tend to merge. The Mott temperature for the pion is also computed.Comment: 24 pages, 9 figures, version to appear in Phys. Rev.
The QCD Critical End Point in the Context of the Polyakov--Nambu--Jona-Lasinio Model
We investigate the phase diagram of the so-called
Polyakov--Nambu--Jona-Lasinio model at finite temperature and nonzero chemical
potential with three quark flavors. Chiral and deconfinement phase transitions
are discussed, and the relevant order-like parameters are analyzed. A special
attention is payed to the critical end point (CEP): the influence of the
strangeness on the location of the CEP is studied; also the strength of the
flavor-mixing interaction alters the CEP location, once when it becomes weaker
the CEP moves to low temperatures and can even disappear.Comment: Prepared for Strangeness in Quark Matter 2011, Sept. 18--24, Cracow,
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