12,551 research outputs found
Type II superconductivity and magnetic flux transport in neutrons stars
The transition to a type II proton superconductor which is believed to occur
in a cooling neutron star is accompanied by changes in the equation of
hydrostatic equilibrium and by the formation of proton vortices with quantized
magnetic flux. Analysis of the electron Boltzmann equation for this system and
of the proton supercurrent distribution formed at the transition leads to the
derivation of a simple expression for the transport velocity of magnetic flux
in the liquid interior of a neutron star. This shows that flux moves easily as
a consequence of the interaction between neutron and proton superfluid vortices
during intervals of spin-down or spin-up in binary systems. The differences
between the present analysis and those of previous workers are reviewed and an
error in the paper of Jones (1991) is corrected.Comment: 7 pages, no figures. To be published in Monthly Notices of the Royal
Astronomical Societ
Flux Expulsion - Field Evolution in Neutron Stars
Models for the evolution of magnetic fields of neutron stars are constructed,
assuming the field is embedded in the proton superconducting core of the star.
The rate of expulsion of the magnetic flux out of the core, or equivalently the
velocity of outward motion of flux-carrying proton-vortices is determined from
a solution of the Magnus equation of motion for these vortices. A force due to
the pinning interaction between the proton-vortices and the neutron-superfluid
vortices is also taken into account in addition to the other more conventional
forces acting on the proton-vortices. Alternative models for the field
evolution are considered based on the different possibilities discussed for the
effective values of the various forces. The coupled spin and magnetic evolution
of single pulsars as well as those processed in low-mass binary systems are
computed, for each of the models. The predicted lifetimes of active pulsars,
field strengths of the very old neutron stars, and distribution of the magnetic
fields versus orbital periods in low-mass binary pulsars are used to test the
adopted field decay models. Contrary to the earlier claims, the buoyancy is
argued to be the dominant driving cause of the flux expulsion, for the single
as well as the binary neutron stars. However, the pinning is also found to play
a crucial role which is necessary to account for the observed low field binary
and millisecond pulsars.Comment: 23 pages, + 7 figures, accepted for publication in Ap
Measurement of the electroweak production of dijets in association with a Z-boson and distributions sensitive to vector boson fusion in proton-proton collisions at ās= 8 TeV using the ATLAS detector
Measurements of fiducial cross sections for the electroweak production of two jets in association with a Z-boson are presented. The measurements are performed using 20.3 fbā1 of proton-proton collision data collected at a centre-of-mass energy of sā = 8 TeV by the ATLAS experiment at the Large Hadron Collider. The electroweak component is extracted by a fit to the dijet invariant mass distribution in a fiducial region chosen to enhance the electroweak contribution over the dominant background in which the jets are produced via the strong interaction. The electroweak cross sections measured in two fiducial regions are in good agreement with the Standard Model expectations and the background-only hypothesis is rejected with significance above the 5Ļ level. The electroweak process includes the vector boson fusion production of a Z-boson and the data are used to place limits on anomalous triple gauge boson couplings. In addition, measurements of cross sections and differential distributions for inclusive Z-boson-plus-dijet production are performed in five fiducial regions, each with different sensitivity to the electroweak contribution. The results are corrected for detector effects and compared to predictions from the Sherpa and Powheg event generators
Search for dark matter in events with a hadronically decaying W or Z boson and missing transverse momentum in pp collisions at ās=8āāTeV with the ATLAS detector
A search is presented for dark matter pair production in association with a W or Z boson in pp collisions representing 20.3āāfbā1 of integrated luminosity at sā=8āāTeV using data recorded with the ATLAS detector at the Large Hadron Collider. Events with a hadronic jet with the jet mass consistent with a W or Z boson, and with large missing transverse momentum are analyzed. The data are consistent with the standard model expectations. Limits are set on the mass scale in effective field theories that describe the interaction of dark matter and standard model particles, and on the cross section of Higgs production and decay to invisible particles. In addition, cross section limits on the anomalous production of W or Z bosons with large missing transverse momentum are set in two fiducial regions
Search for new particles in events with one lepton and missing transverse momentum in pp collisions at sā = 8 TeV with the ATLAS detector
This paper presents a search for new particles in events with one lepton (electron or muon) and missing transverse momentum using 20.3 fbā1 of proton-proton collision data at sā = 8 TeV recorded by the ATLAS experiment at the Large Hadron Collider. No significant excess beyond Standard Model expectations is observed. A W ā² with Sequential Standard Model couplings is excluded at the 95% confidence level for masses up to 3.24 TeV. Excited chiral bosons (W *) with equivalent coupling strengths are excluded for masses up to 3.21 TeV. In the framework of an effective field theory limits are also set on the dark matter-nucleon scattering cross-section as well as the mass scale M * of the unknown mediating interaction for dark matter pair production in association with a leptonically decaying W
Search for supersymmetry at ās= 8 TeV in final states with jets and two same-sign leptons or three leptons with the ATLAS detector
A search for strongly produced supersymmetric particles is conducted using signatures involving multiple energetic jets and either two isolated leptons (e or Ī¼) with the same electric charge, or at least three isolated leptons. The search also utilises jets originating from b-quarks, missing transverse momentum and other observables to extend its sensitivity. The analysis uses a data sample corresponding to a total integrated luminosity of 20.3 fb-1 of ās = 8 TeV proton-proton collisions recorded with the ATLAS detector at the Large Hadron Collider in 2012. No deviation from the Standard Model expectation is observed. New or significantly improved exclusion limits are set on a wide variety of supersymmetric models in which the lightest squark can be of the first, second or third generations, and in which R-parity can be conserved or violated
Measurement of the production of a W boson in association with a charm quark in pp collisions at sā = 7 TeV with the ATLAS detector
The production of a W boson in association with a single charm quark is studied using 4.6 fbā1 of pp collision data at sā = 7 TeV collected with the ATLAS detector at the Large Hadron Collider. In events in which a W boson decays to an electron or muon, the charm quark is tagged either by its semileptonic decay to a muon or by the presence of a charmed meson. The integrated and differential cross sections as a function of the pseudorapidity of the lepton from the W-boson decay are measured. Results are compared to the predictions of next-to-leading-order QCD calculations obtained from various parton distribution function parameterisations. The ratio of the strange-to-down sea-quark distributions is determined to be 0.96+0.26ā0.30 at Q 2 = 1.9 GeV2, which supports the hypothesis of an SU(3)-symmetric composition of the light-quark sea. Additionally, the cross-section ratio Ļ(W + + cĀÆ)/Ļ(W ā + c) is compared to the predictions obtained using parton distribution function parameterisations with different assumptions about the sāsĀÆ quark asymmetry
Search for quantum black hole production in high-invariant-mass lepton + jet final states using pp collisions at ās = 8 TeV and the ATLAS detector
A search is reported for a neutral Higgs boson in the decay channel HāZĪ³, Zāā +ā - (ā = e, Ī¼), using 4.5 fb-1 of pp collisions at s=7 TeV and 20.3 fb-1 of pp collisions at s=8 TeV, recorded by the ATLAS detector at the CERN Large Hadron Collider. The observed distribution of the invariant mass of the three final-state particles, māāĪ³, is consistent with the Standard Model hypothesis in the investigated mass range of 120-150 GeV. For a Higgs boson with a mass of 125.5 GeV, the observed upper limit at the 95% confidence level is 11 times the Standard Model expectation. Upper limits are set on the cross section times branching ratio of a neutral Higgs boson with mass in the range 120-150 GeV between 0.13 and 0.5 pb for s=8 TeV at 95% confidence level
Search for direct pair production of the top squark in all-hadronic final states in proton-proton collisions at ās=8 TeV with the ATLAS detector
The results of a search for direct pair production of the scalar partner to the top quark using an integrated luminosity of 20.1 fbā1 of proton-proton collision data at sā = 8 TeV recorded with the ATLAS detector at the LHC are reported. The top squark is assumed to decay via tĀÆ ātĻĀÆ01 or tĀÆ ābĻĀÆĀ±1 ābW(ā)ĻĀÆ01, where ĻĀÆ01 (ĻĀÆĀ±1) denotes the lightest neutralino (chargino) in supersymmetric models. The search targets a fully-hadronic final state in events with four or more jets and large missing transverse momentum. No significant excess over the Standard Model background prediction is observed, and exclusion limits are reported in terms of the top squark and neutralino masses and as a function of the branching fraction of tĀÆ ātĻĀÆ01. For a branching fraction of 100%, top squark masses in the range 270ā645 GeV are excluded for ĻĀÆ01 masses below 30 GeV. For a branching fraction of 50% to either tĀÆ ātĻĀÆ01 or tĀÆ ābĻĀÆĀ±1, and assuming the ĻĀÆĀ±1 mass to be twice the ĻĀÆ01 mass, top squark masses in the range 250ā550 GeV are excluded for ĻĀÆ01 masses below 60 GeV
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