5,105 research outputs found
Hypervelocity particle capture: Some considerations regarding suitable target media
Hypervelocity particles colliding with passive capture media will be traversed by shock waves; depending on the stress amplitude, the particle may remain solid or it may melt or vaporize. Any capture mechanism considered for cosmic dust collection in low Earth-orbit must be designed such that sample alteration and hence loss of scientific information is minimized. Capture of pristine particles is fundamentally difficult, because the specific heat of melting and even vaporization is exceeded upon impact at typical, geocentric encounter velocities. From the results of calculated and observed melting behaviors it is concluded that shock stresses in excess of 50 GPA should be avoided during hypervelocity particle capture on board Space Station and that stresses 20 GPa, even at 15 km/s collision velocities, should constitute desirable instrument design goals. Some principal characteristics of the capture medium that may satisfy these requirements are identified
Impact cratering in reduced-gravity environments: Early experiments on the NASA KC-135 aircraft
Impact experimentation on the NASA KC-135 Reduced-Gravity Aircraft was shown to be possible, practical, and of considerable potential use in examining the role of gravity on various impact phenomena. With a minimal facility, crater dimensional and growth-times were measured, and have demonstrated both agreement and disagreement with predictions. A larger facility with vacuum capability and a high-velocity gun would permit a much wider range of experimentation
Time-scales of close-in exoplanet radio emission variability
We investigate the variability of exoplanetary radio emission using stellar
magnetic maps and 3D field extrapolation techniques. We use a sample of hot
Jupiter hosting stars, focusing on the HD 179949, HD 189733 and tau Boo
systems. Our results indicate two time-scales over which radio emission
variability may occur at magnetised hot Jupiters. The first is the synodic
period of the star-planet system. The origin of variability on this time-scale
is the relative motion between the planet and the interplanetary plasma that is
co-rotating with the host star. The second time-scale is the length of the
magnetic cycle. Variability on this time-scale is caused by evolution of the
stellar field. At these systems, the magnitude of planetary radio emission is
anticorrelated with the angular separation between the subplanetary point and
the nearest magnetic pole. For the special case of tau Boo b, whose orbital
period is tidally locked to the rotation period of its host star, variability
only occurs on the time-scale of the magnetic cycle. The lack of radio
variability on the synodic period at tau Boo b is not predicted by previous
radio emission models, which do not account for the co-rotation of the
interplanetary plasma at small distances from the star.Comment: 10 pages, 7 figures, 2 tables, accepted in MNRA
Higgs mediated Double Flavor Violating top decays in Effective Theories
The possibility of detecting double flavor violating top quark transitions at
future colliders is explored in a model-independent manner using the effective
Lagrangian approach through the () decays. A
Yukawa sector that contemplates invariants of up to
dimension six is proposed and used to derive the most general flavor violating
and CP violating and vertices of renormalizable type.
Low-energy data, on high precision measurements, and experimental limits are
used to constraint the and vertices and then used to
predict the branching ratios for the decays. It is found
that this branching ratios may be of the order of , for a
relative light Higgs boson with mass lower than , which could be more
important than those typical values found in theories beyond the standard model
for the rare top quark decays () or . %% LHC experiments, by using a total integrated luminosity of of data, will be able to rule out, at 95% C.L., DFV top quark
decays up to a Higgs mass of 155 GeV/ or discover such a process up to a
Higgs mass of 147 GeV/.Comment: 24 pages, 11 figure
Cognition-Enhancing Drugs: Can We Say No?
Normative analysis of cognition-enhancing drugs frequently weighs the liberty interests of drug users against egalitarian commitments to a level playing field. Yet those who would refuse to engage in neuroenhancement may well find their liberty to do so limited in a society where such drugs are widespread. To the extent that unvarnished emotional responses are world-disclosive, neurocosmetic practices also threaten to provide a form of faulty data to their users. This essay examines underappreciated liberty-based and epistemic rationales for regulating cognition-enhancing drugs
The Chagos Islands cases: the empire strikes back
Good governance requires the accommodation of multiple interests in the cause of decision making. However, undue regard for particular sectional interests can take their toll upon public faith in government administration. Historically, broad conceptions of the good of the commonwealth were employed to outweigh the interests of groups that resisted colonisation. In the decision making of the British Empire, the standard approach for justifying the marginalisation of the interests of colonised groups was that they were uncivilised and that particular hardships were the price to be paid for bringing to them the imperial dividend of industrial society. It is widely assumed that with the dismantling of the British Empire, such impulses and their accompanying jurisprudence became a thing of the past. Even as decolonisation proceeded apace after the Second World War, however, the United Kingdom maintained control of strategically important islands with a view towards sustaining its global role. In an infamous example from this twilight period of empire, in the 1960s imperial interests were used to justify the expulsion of the Chagos islanders from the British Indian Ocean Territory (BIOT). Into the twenty-first century, this forced elision of the UKâs interests with the imperial âcommon goodâ continues to take centre stage in courtroom battles over the islandersâ rights, being cited before domestic and international tribunals in order to maintain the Chagossiansâ exclusion from their homeland. This article considers the new jurisprudence of imperialism which has emerged in a string of decisions which have continued to marginalise the Chagossiansâ interests
Langevin dynamics of the Lebowitz-Percus model
We revisit the hard-spheres lattice gas model in the spherical approximation
proposed by Lebowitz and Percus (J. L. Lebowitz, J. K. Percus, Phys. Rev.{\
144} (1966) 251). Although no disorder is present in the model, we find that
the short-range dynamical restrictions in the model induce glassy behavior. We
examine the off-equilibrium Langevin dynamics of this model and study the
relaxation of the density as well as the correlation, response and overlap
two-time functions. We find that the relaxation proceeds in two steps as well
as absence of anomaly in the response function. By studying the violation of
the fluctuation-dissipation ratio we conclude that the glassy scenario of this
model corresponds to the dynamics of domain growth in phase ordering kinetics.Comment: 21 pages, RevTeX, 14 PS figure
One-loop Beta Functions for the Orientable Non-commutative Gross-Neveu Model
We compute at the one-loop order the beta-functions for a renormalisable
non-commutative analog of the Gross Neveu model defined on the Moyal plane. The
calculation is performed within the so called x-space formalism. We find that
this non-commutative field theory exhibits asymptotic freedom for any number of
colors. The beta-function for the non-commutative counterpart of the Thirring
model is found to be non vanishing.Comment: 16 pages, 9 figure
Montana v. Wyoming: Sprinklers, Irrigation Water Use Efficiency and the Doctrine of Recapture
A Search for Fluctuation-Dissipation Theorem Violations in Spin-Glasses from Susceptibility Data
We propose an indirect way of studying the fluctuation-dissipation relation
in spin-glasses that only uses available susceptibility data. It is based on a
dynamic extension of the Parisi-Toulouse approximation and a Curie-Weiss
treatment of the average magnetic couplings. We present the results of the
analysis of several sets of experimental data obtained from various samples.Comment: 7 pages, 4 figure
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