334 research outputs found
Friction force on a vortex due to the scattering of superfluid excitations in helium II
The longitudinal friction acting on a vortex line in superfluid He is
investigated within a simple model based on the analogy between such vortex
dynamics and that of the quantal Brownian motion of a charged point particle in
a uniform magnetic field. The scattering of superfluid quasiparticle
excitations by the vortex stems from a translationally invariant interaction
potential which, expanded to first order in the vortex velocity operator, gives
rise to vortex transitions between nearest Landau levels. The corresponding
friction coefficient is shown to be, in the limit of elastic scattering
(vanishing cyclotron frequency), equivalent to that arising from the Iordanskii
formula. Proposing a simple functional form for the scattering amplitude, with
only one adjustable parameter whose value is set in order to get agreement to
the Iordanskii result for phonons, an excellent agreement is also found with
the values derived from experimental data up to temperatures about 1.5 K.
Finite values of the cyclotron frequency arising from recent theories are shown
to yield similar results. The incidence of vortex-induced quasiparticle
transitions on the friction process is estimated to be, in the roton dominated
regime, about 50 % of the value of the friction coefficient, 8 % of which
corresponds to roton-phonon transitions and 42 % to roton
ones.Comment: 15 pages, 4 figures; typos corrected, to be published in PR
Mutual friction in helium II: a microscopic approach
We develop a microscopic model of mutual friction represented by the dissipative dynamics of a normal fluid flow which interacts with the helical normal modes of vortices comprising a lattice in thermal equilibrium. Such vortices are assumed to interact with the quasiparticles forming the normal fluid through a pseudomomentum-conserving scattering Hamiltonian. We study the approach to equilibrium of the normal fluid flow for temperatures below 1 K, deriving an equation of motion for the quasiparticle pseudomomentum which leads to the expected form predicted by the Hall–Vinen–Bekharevich–Khalatnikov equations. We obtain an expression for the mutual friction coefficient in terms of microscopic parameters, which turns out to be practically independent of the vortex mass for values arising from diverse theories. By comparing our expression of B with previous theoretical estimates, we deduce interesting qualitative features about the excitation of Kelvin modes by the quasiparticle scattering
Interacting Kasner-type cosmologies
It is well known that Kasner-type cosmologies provide a useful framework for
analyzing the three-dimensional anisotropic expansion because of the
simplification of the anisotropic dynamics. In this paper relativistic
multi-fluid Kasner-type scenarios are studied. We first consider the general
case of a superposition of two ideal cosmic fluids, as well as the particular
cases of non-interacting and interacting ones, by introducing a
phenomenological coupling function . For two-fluid cosmological scenarios
there exist only cosmological scaling solutions, while for three-fluid
configurations there exist not only cosmological scaling ones, but also more
general solutions. In the case of triply interacting cosmic fluids we can have
energy transfer from two fluids to a third one, or energy transfer from one
cosmic fluid to the other two. It is shown that by requiring the positivity of
energy densities there always is a matter component which violates the dominant
energy condition in this kind of anisotropic cosmological scenarios.Comment: Accepted for publication in Astrophysics &Space Science, 8 page
Postoperative complications after procedure for prolapsed hemorrhoids (PPH) and stapled transanal rectal resection (STARR) procedures
Procedure for prolapsing hemorrhoids (PPH) and stapled transanal rectal resection for obstructed defecation (STARR) carry low postoperative pain, but may be followed by unusual and severe postoperative complications. This review deals with the pathogenesis, prevention and treatment of adverse events that may occasionally be life threatening. PPH and STARR carry the expected morbidity following anorectal surgery, such as bleeding, strictures and fecal incontinence. Complications that are particular to these stapled procedures are rectovaginal fistula, chronic proctalgia, total rectal obliteration, rectal wall hematoma and perforation with pelvic sepsis often requiring a diverting stoma. A higher complication rate and worse results are expected after PPH for fourth-degree piles. Enterocele and anismus are contraindications to PPH and STARR and both operations should be used with caution in patients with weak sphincters. In conclusion, complications after PPH and STARR are not infrequent and may be difficult to manage. However, if performed in selected cases by skilled specialists aware of the risks and associated diseases, some complications may be prevented
ϒ production in p–Pb collisions at √sNN=8.16 TeV
ϒ production in p–Pb interactions is studied at the centre-of-mass energy per nucleon–nucleon collision √sNN = 8.16 TeV with the ALICE detector at the CERN LHC. The measurement is performed reconstructing bottomonium resonances via their dimuon decay channel, in the centre-of-mass rapidity intervals 2.03 < ycms < 3.53 and −4.46 < ycms < −2.96, down to zero transverse momentum. In this work, results on the ϒ(1S) production cross section as a function of rapidity and transverse momentum are presented. The corresponding nuclear modification factor shows a suppression of the ϒ(1S) yields with respect to pp collisions, both at forward and backward rapidity. This suppression is stronger in the low transverse momentum region and shows no significant dependence on the centrality of the interactions. Furthermore, the ϒ(2S) nuclear modification factor is evaluated, suggesting a suppression similar to that of the ϒ(1S). A first measurement of the ϒ(3S) has also been performed. Finally, results are compared with previous ALICE measurements in p–Pb collisions at √sNN = 5.02 TeV and with theoretical calculations.publishedVersio
Long-range Angular Correlations On The Near And Away Side In P-pb Collisions At √snn=5.02 Tev
7191/Mar294
(Anti-)deuteron production in pp collisions at 1as=13TeV
The study of (anti-)deuteron production in pp collisions has proven to be a powerful tool to investigate the formation mechanism of loosely bound states in high-energy hadronic collisions. In this paper the production of (anti-)deuterons is studied as a function of the charged particle multiplicity in inelastic pp collisions at s=13 TeV using the ALICE experiment. Thanks to the large number of accumulated minimum bias events, it has been possible to measure (anti-)deuteron production in pp collisions up to the same charged particle multiplicity (d Nch/ d \u3b7 3c 26) as measured in p\u2013Pb collisions at similar centre-of-mass energies. Within the uncertainties, the deuteron yield in pp collisions resembles the one in p\u2013Pb interactions, suggesting a common formation mechanism behind the production of light nuclei in hadronic interactions. In this context the measurements are compared with the expectations of coalescence and statistical hadronisation models (SHM)
Measurement of jet suppression in central Pb-Pb collisions at root s(NN)=2.76 TeV
The transverse momentum(p(T)) spectrum and nuclear modification factor (R-AA) of reconstructed jets in 0-10% and 10-30% central Pb-Pb collisions at root s(NN) = 2.76 TeV were measured. Jets were reconstructed using the anti-k(T) jet algorithm with a resolution parameter of R = 0.2 from charged and neutral particles, utilizing the ALICE tracking detectors and Electromagnetic Calorimeter (EMCal). The jet p(T) spectra are reported in the pseudorapidity interval of \eta(jet)\ 5 GeV/c to suppress jets constructed from the combinatorial background in Pb-Pb collisions. The leading charged particle requirement applied to jet spectra both in pp and Pb-Pb collisions had a negligible effect on the R-AA. The nuclear modification factor R-AA was found to be 0.28 +/- 0.04 in 0-10% and 0.35 +/- 0.04 in 10-30% collisions, independent of p(T), jet within the uncertainties of the measurement. The observed suppression is in fair agreement with expectations from two model calculations with different approaches to jet quenching. (C) 2015 CERN for the benefit of the ALICE Collaboration. Published by Elsevier B.V.Peer reviewe
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