5,864 research outputs found
Periodically driven stochastic un- and refolding transitions of biopolymers
Mechanical single molecule experiments probe the energy profile of
biomolecules. We show that in the case of a profile with two minima (like
folded/unfolded) periodic driving leads to a stochastic resonance-like
phenomenon. We demonstrate that the analysis of such data can be used to
extract four basic parameters of such a transition and discuss the statistical
requirements of the data acquisition. As advantages of the proposed scheme, a
polymeric linker is explicitly included and thermal fluctuations within each
well need not to be resolved.Comment: 7 pages, 5 figures, submitted to EP
On the Mass of M31
Recent work by several groups has established the properties of the dwarf
satellites to M31. We reexamine the reported kinematics of this group employing
a fresh technique we have developed previously. By calculating the distribution
of a chi statistic (which we define in the paper) for the M31 system, we
conclude that the total mass (disk plus halo) of the primary is unlikely to be
as great as that of our own Milky Way. In fact the chi distribution for M31
indicates that, like NGC 3992, it does not have a massive halo. In contrast,
the analysis of the satellites of NGC 1961 and NGC 5084 provides strong
evidence for massive halos surrounding both spiral galaxies.Comment: To appear in MNRAS, 10 pages with 6 figure
Multi-strange baryon measurements at LHC energies, with the ALICE experiment
The status of the charged multi-strange baryon analysis (Xi-, anti-Xi+,
Omega-, anti-Omega+) at LHC energies is presented. This report is based on the
results obtained with ALICE (A Large Ion Collider Experiment), profiting from
the characteristic cascade-decay topology. A special attention is drawn to the
early pp data-taking period (2009-2010) and subsequently, on the uncorrected
pT-spectra extracted at mid-rapidity for centre of mass energies of 0.9 TeV and
7 TeV.Comment: 4 pages, 5 figures, Hot Quarks 2010 proceedings, La Londe Les Maures,
France, June 2010 (to be published in Journal of Physics: Conference Series
Flavor in Minimal Conformal Technicolor
We construct a complete, realistic, and natural UV completion of minimal
conformal technicolor that explains the origin of quark and lepton masses and
mixing angles. As in "bosonic technicolor", we embed conformal technicolor in a
supersymmetric theory, with supersymmetry broken at a high scale. The exchange
of heavy scalar doublets generates higher-dimension interactions between
technifermions and quarks and leptons that give rise to quark and lepton masses
at the TeV scale. Obtaining a sufficiently large top quark mass requires strong
dynamics at the supersymmetry breaking scale in both the top and technicolor
sectors. This is natural if the theory above the supersymmetry breaking also
has strong conformal dynamics. We present two models in which the strong top
dynamics is realized in different ways. In both models, constraints from
flavor-changing effects can be easily satisfied. The effective theory below the
supersymmetry breaking scale is minimal conformal technicolor with an
additional light technicolor gaugino. We argue that this light gaugino is a
general consequence of conformal technicolor embedded into a supersymmetric
theory. If the gaugino has mass below the TeV scale it will give rise to an
additional pseudo Nambu-Goldstone boson that is observable at the LHC.Comment: 37 pages; references adde
Minimal Conformal Technicolor and Precision Electroweak Tests
We study the minimal model of conformal technicolor, an SU(2) gauge theory
near a strongly coupled conformal fixed point, with conformal symmetry softly
broken by technifermion mass terms. Conformal symmetry breaking triggers chiral
symmetry breaking in the pattern SU(4) -> Sp(4), which gives rise to a
pseudo-Nambu-Goldstone boson that can act as a composite Higgs boson. The top
quark is elementary, and the top and electroweak gauge loop contributions to
the Higgs mass are cut off entirely by Higgs compositeness. In particular, the
model requires no top partners and no "little Higgs" mechanism. A nontrivial
vacuum alignment results from the interplay of the top loop and technifermion
mass terms. The composite Higgs mass is completely determined by the top loop,
in the sense that m_h/m_t is independent of the vacuum alignment and is
computable by a strong-coupling calculation. There is an additional composite
pseudoscalar A with mass larger than m_h and suppressed direct production at
LHC. We discuss the electroweak fit in this model in detail. Corrections to Z
-> bb and the T parameter from the top sector are suppressed by the enhanced
Sp(4) custodial symmetry. Even assuming that the strong contribution to the S
parameter is positive and usuppressed, a good electroweak fit can be obtained
for v/f ~ 0.25, where v and f are the electroweak and chiral symmetry breaking
scales respectively. This requires fine tuning at the 10% level.Comment: 34 pages, 4 figures; v2: updated precision electroweak fi
A giant, periodic flare from the soft gamma repeater SGR1900+14
Soft gamma repeaters are high-energy transient sources associated with
neutron stars in young supernova remnants. They emit sporadic, short (~ 0.1 s)
bursts with soft energy spectra during periods of intense activity. The event
of March 5, 1979 was the most intense and the only clearly periodic one to
date. Here we report on an even more intense burst on August 27, 1998, from a
different soft gamma repeater, which displayed a hard energy spectrum at its
peak, and was followed by a ~300 s long tail with a soft energy spectrum and a
dramatic 5.16 s period. Its peak and time integrated energy fluxes at Earth are
the largest yet observed from any cosmic source. This event was probably
initiated by a massive disruption of the neutron star crust, followed by an
outflow of energetic particles rotating with the period of the star. Comparison
of these two bursts supports the idea that magnetic energy plays an important
role, and that such giant flares, while rare, are not unique, and may occur at
any time in the neutron star's activity cycle.Comment: Accepted for publication in Natur
Entropy production for mechanically or chemically driven biomolecules
Entropy production along a single stochastic trajectory of a biomolecule is
discussed for two different sources of non-equilibrium. For a molecule
manipulated mechanically by an AFM or an optical tweezer, entropy production
(or annihilation) occurs in the molecular conformation proper or in the
surrounding medium. Within a Langevin dynamics, a unique identification of
these two contributions is possible. The total entropy change obeys an integral
fluctuation theorem and a class of further exact relations, which we prove for
arbitrarily coupled slow degrees of freedom including hydrodynamic
interactions. These theoretical results can therefore also be applied to driven
colloidal systems. For transitions between different internal conformations of
a biomolecule involving unbalanced chemical reactions, we provide a
thermodynamically consistent formulation and identify again the two sources of
entropy production, which obey similar exact relations. We clarify the
particular role degenerate states have in such a description
Observations of HI Absorbing Gas in Compact Radio Sources at Cosmological Redshifts
We present an overview of the occurrence and properties of atomic gas
associated with compact radio sources at redshifts up to z=0.85. Searches for
HI 21cm absorption were made with the Westerbork Synthesis Radio Telescope at
UHF-high frequencies (725-1200 MHz). Detections were obtained for 19 of the 57
sources with usable spectra (33%). We have found a large range in line depths,
from tau=0.16 to tau<=0.001. There is a substantial variety of line profiles,
including Gaussians of less than 10km/s, to more typically 150km/s, as well as
irregular and multi-peaked absorption profiles, sometimes spanning several
hundred km/s. Assuming uniform coverage of the entire radio source, we obtain
column depths of atomic gas between 1e19 and 3.3e21(Tsp/100K)(1/f)cm^(-2).
There is evidence for significant gas motions, but in contrast to earlier
results at low redshift, there are many sources in which the HI velocity is
substantially negative (up to v=-1420km/s) with respect to the optical
redshift, suggesting that in these sources the atomic gas, rather than falling
into the centre, may be be flowing out, interacting with the jets, or rotating
around the nucleus.Comment: 10 pages, accepted for publication in A&
Theories of Reference: What Was the Question?
The new theory of reference has won popularity. However, a number of noted philosophers have also attempted to reply to the critical arguments of Kripke and others, and aimed to vindicate the description theory of reference. Such responses are often based on ingenious novel kinds of descriptions, such as rigidified descriptions, causal descriptions, and metalinguistic descriptions. This prolonged debate raises the doubt whether different parties really have any shared understanding of what the central question of the philosophical theory of reference is: what is the main question to which descriptivism and the causal-historical theory have presented competing answers. One aim of the paper is to clarify this issue. The most influential objections to the new theory of reference are critically reviewed. Special attention is also paid to certain important later advances in the new theory of reference, due to Devitt and others
The build-up of nuclear stellar cusps in extreme starburst galaxies and major mergers
Nuclear stellar cusps are defined as central excess light component in the stellar light profiles of galaxies and are suggested to be stellar relics of intense compact starbursts in the central âŒ100â500âpc region of gas-rich major mergers. Here, we probe the build-up of nuclear cusps during the actual starburst phase for a complete sample of luminous infrared galaxy (LIRG) systems (85 LIRGs, with 11.4 < logâ[LIR/L_â] < 12.5) in the Great Observatories All-sky LIRG Survey sample. Cusp properties are derived via 2D fitting of the nuclear stellar light imaged in the near-infrared (NIR) by the Hubble Space Telescope and have been combined with mid-infrared (IR) diagnostics for active galactic nucleus (AGN)/starburst characterization. We find that nuclear stellar cusps are resolved in 76âperâcent of LIRGs (merger and non-interacting galaxies). The cusp strength and luminosity increase with far-IR luminosity (excluding AGN) and merger stage, confirming theoretical models that starburst activity is associated with the build-up of nuclear stellar cusps. Evidence for ultracompact nuclear starbursts is found in âŒ13âperâcent of LIRGs, which have a strong unresolved central NIR light component but no significant contribution of an AGN. The nuclear NIR surface density (measured within 1âkpc radius) increases by a factor of âŒ5 towards late merger stages. A careful comparison to local early-type galaxies with comparable masses reveals (a) that local (U)LIRGs have a significantly larger cusp fraction and (b) that the majority of the cusp LIRGs have host galaxy luminosities (H band) similar to core ellipticals which are roughly one order in magnitude larger than those for cusp ellipticals
- âŠ