32,671,973 research outputs found
Generalised Thurston-Bennequin invariants for real algebraic surface singularities
A generalised Thurston-Bennequin invariant for a Q-singularity of a real
algebraic variety is defined as a linking form on the homologies of the real
link of the singularity. The main goal of this paper is to present a method to
calculate the linking form in terms of the very good resolution graph of a real
normal unibranch surface singularity. For such singularities, the value of the
linking form is the Thurston-Bennequin number of the real link of the
singularity. As a special case of unibranch surface singularities, the
behaviour of the linking form is investigated on the Brieskorn double points
x^m+y^n\pm z^2=0.Comment: 22 pages, TeX, 12 figure
Many-body effects in nuclear structure
We calculate, for the first time, the state-dependent pairing gap of a finite
nucleus (120Sn) diagonalizing the bare nucleon-nucleon potential (Argonne v14)
in a Hartree-Fock basis (with effective k-mass m_k eqult to 0.7 m), within the
framework of the BCS approximation including scattering states up to 800 MeV
above the Fermi energy to achieve convergence. The resulting gap accounts for
about half of the experimental gap. We find that a consistent description of
the low-energy nuclear spectrum requires, aside from the bare nucleon-nucleon
interaction, not only the dressing of single-particle motion through the
coupling to the nuclear surface, to give the right density of levels close to
the Fermi energy (and thus an effective mass m* approximately equal to m), but
also the renormalization of collective vibrational modes through vertex and
self-energy processes, processes which are also found to play an essential role
in the pairing channel, leading to a long range, state dependent component of
the pairing interaction. The combined effect of the bare nucleon-nucleon
potential and of the induced pairing interaction arising from the exchange of
low-lying surface vibrations between nucleons moving in time reversal states
close to the Fermi energy accounts for the experimental gap.Comment: 5 pages, 4 figures; author list correcte
Analysis of the Lambda p Final State Interaction in the Reaction p+p->K^+(Lambda p)
The missing mass spectrum measured in high resolution studies of the reaction
pp -> K^+X is analyzed with respect to the strong final state interaction near
the Lambda p production threshold. The observed spectrum can be described by
factorizing the reaction amplitude in terms of a production amplitude and a
final state scattering amplitude. Parametrizing the Lambda p final state
interaction in terms of the inverse Jost function allows a direct extraction of
the low-energy phase-equivalent potential parameters. Constraints on the
singlet and triplet scattering lengths and effective ranges are deduced in a
simultaneous fit of the Lambda p invariant mass spectrum and the total cross
section data of the free Lambda p scattering using the effective range
approximation.Comment: 9 pages,6 figures, 2 table
Anisotropic field dependence of the magnetic transition in Cu2Te2O5Br2
We present the results of measurements of the thermal conductivity of
Cu2Te2O5Br2, a compound where tetrahedra of Cu^{2+} ions carrying S=1/2 spins
form chains along the c-axis of the tetragonal crystal structure. The thermal
conductivity kappa was measured along both the c- and the a-direction as a
function of temperature between 3 and 300 K and in external magnetic fields H
up to 69 kOe, oriented both parallel and perpendicular to the c-axis. Distinct
features of kappa(T) were observed in the vicinity of T_N=11.4 K in zero
magnetic field. These features are unaltered in external fields which are
parallel to the c-axis, but are more pronounced when a field is applied
perpendicularly to the c-axis. The transition temperature increases upon
enhancing the external field, but only if the field is oriented along the
a-axis.Comment: 5 pages, 3 figure
Observation of the TeV gamma-ray source MGRO J1908+06 with ARGO-YBJ
The extended gamma ray source MGRO J1908+06, discovered by the Milagro air
shower detector in 2007, has been observed for about 4 years by the ARGO-YBJ
experiment at TeV energies, with a statistical significance of 6.2 standard
deviations. The peak of the signal is found at a position consistent with the
pulsar PSR J1907+0602. Parametrizing the source shape with a two-dimensional
Gauss function we estimate an extension \sigma = 0.49 \pm 0.22 degrees,
consistent with a previous measurement by the Cherenkov Array H.E.S.S.. The
observed energy spectrum is dN/dE = 6.1 \pm 1.4 \times 10^-13 (E/4 TeV)^{-2.54
\pm 0.36} photons cm^-2 s^-1 TeV^-1, in the energy range 1-20 TeV. The measured
gamma ray flux is consistent with the results of the Milagro detector, but is
2-3 times larger than the flux previously derived by H.E.S.S. at energies of a
few TeV. The continuity of the Milagro and ARGO-YBJ observations and the stable
excess rate observed by ARGO-YBJ along 4 years of data taking support the
identification of MGRO J1908+06 as the steady powerful TeV pulsar wind nebula
of PSR J1907+0602, with an integrated luminosity above 1 TeV about 1.8 times
the Crab Nebula luminosity.Comment: 6 pages, accepted for pubblication by ApJ. Replaced to correct the
author lis
Suppression of charged particle production at large transverse momentum in central Pb-Pb collisions at TeV
Inclusive transverse momentum spectra of primary charged particles in Pb-Pb
collisions at = 2.76 TeV have been measured by the ALICE
Collaboration at the LHC. The data are presented for central and peripheral
collisions, corresponding to 0-5% and 70-80% of the hadronic Pb-Pb cross
section. The measured charged particle spectra in and GeV/ are compared to the expectation in pp collisions at the same
, scaled by the number of underlying nucleon-nucleon
collisions. The comparison is expressed in terms of the nuclear modification
factor . The result indicates only weak medium effects ( 0.7) in peripheral collisions. In central collisions,
reaches a minimum of about 0.14 at -7GeV/ and increases
significantly at larger . The measured suppression of high- particles is stronger than that observed at lower collision energies,
indicating that a very dense medium is formed in central Pb-Pb collisions at
the LHC.Comment: 15 pages, 5 captioned figures, 3 tables, authors from page 10,
published version, figures at http://aliceinfo.cern.ch/ArtSubmission/node/98
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