1,068 research outputs found
Top quark production at future lepton colliders in the asymptotic regime
The production of a tt(bar) pair from lepton-antilepton annihilation is
considered for values of the center of mass energy much larger than the top
mass, typically of the few TeV size. In this regime a number of simplifications
occurs that allows to derive the leading asymptotic terms of various
observables using the same theoretical description that was used for light
quark production. Explicit examples are shown for the Standard Model and the
Minimal Supersymmetric Standard Model cases.Comment: 20 pages and 13 figures. e-mail: [email protected]
Update on the ICUD-SIU consultation on multi-parametric magnetic resonance imaging in localised prostate cancer
Introduction: Prostate cancer (PCa) imaging is a rapidly evolving field. Dramatic improvements in prostate MRI during the last decade will probably change the accuracy of diagnosis. This chapter reviews recent current evidence about MRI diagnostic performance and impact on PCa management. Materials and methods: The International Consultation on Urological Diseases nominated a committee to review the literature on prostate MRI. A search of the PubMed database was conducted to identify articles focussed on MP-MRI detection and staging protocols, reporting and scoring systems, the role of MP-MRI in diagnosing PCa prior to biopsy, in active surveillance, in focal therapy and in detecting local recurrence after treatment. Results: Differences in opinion were reported in the use of the strength of magnets [1.5 Tesla (T) vs. 3T] and coils. More agreement was found regarding the choice of pulse sequences; diffusion-weighted MRI (DW-MRI), dynamic contrast-enhanced MRI (DCE MRI), and/or MR spectroscopy imaging (MRSI) are recommended in addition to conventional T2-weighted anatomical sequences. In 2015, the Prostate Imaging Reporting and Data System (PI-RADS version 2) was described to standardize image acquisition and interpretation. MP-MRI improves detection of clinically significant PCa (csPCa) in the repeat biopsy setting or before the confirmatory biopsy in patients considering active surveillance. It is useful to guide focal treatment and to detect local recurrences after treatment. Its role in biopsy-naive patients or during the course of active surveillance remains debated. Conclusion: MP-MRI is increasingly used to improve detection of csPCa and for the selection of a suitable therapeutic approach
The role of universal and non universal Sudakov logarithms in four fermion processes at TeV energies: the one-loop approximation revisited
We consider the separate effects on four fermion processes, in the TeV energy
range, produced at one loop by Sudakov logarithms of universal and not
universal kind, working in the 't Hooft xi=1 gauge. Summing the various vertex
and box contributions allows to isolate two quite different terms.The first one
is a combination of vertex and box quadratic and linear logarithms that are
partially universal and partially not universal and independent of the
scattering angle theta. The second one is theta-dependent, not universal,
linearly logarithmic and only produced by weak boxes. We show that for several
observables, measurable at future linear e+e- colliders (LC, CLIC), the role of
the latter term is dominant and we discuss the implications of this fact for
what concerns the reliability of a one-loop approximation.Comment: 22 pages and 13 figures; version to appear in Phys.Rev.D. e-mail:
[email protected]
Equal Time Correlations in Haldane Gap Antiferromagnets
The antiferromagnetic Heisenberg chain both with and without single ion
anisotropy is studied. Using the recently proposed density matrix
renormalization group technique we calculate the energy gaps as well as several
different correlation functions. The two gaps, ,
along with associated correlation lengths and velocities are determined. The
numerical results are shown to be in good agreement with theoretical
predictions derived from the nonlinear sigma model and a free boson model. We
also study the excitations that occur at the ends of open chains; in
particular we study the behavior associated with open boundary conditions,
using a model of spins coupled to the free bosons.Comment: 32 pages, uufiles encoded REVTEX 3.0, 19 postscript figures included,
UBCTP-93-02
SUSY Scalar Production in the Electroweak Sudakov Regime of Lepton Colliders
We consider the production of SUSY scalar pairs at lepton colliders, for c.m.
energies much larger than the mass of the heaviest SUSY (real or virtual)
particle involved in the process. In that energy regime, we derive the leading
and subleading terms of the electroweak Sudakov logarithms in the MSSM, first
working at one loop with physical states and then resumming to all orders with
asymptotic expansions. We show that the first order of the resummed expression
reproduces the physical one loop approximation, and compute systematically the
possible effects on various observables both at one loop and to all orders. We
discuss the regimes and the processes where the one loop approximation can or
cannot be trusted, working in an energy range between 1 TeV and 4 TeV under a
"light" SUSY mass assumption. As a byproduct of our analysis, we propose a
determination of the MSSM parameter tan(beta) showing how a relative accuracy
of about 25 percent can be easily achieved in the region tan(beta)>14, under
reasonable experimental assumptions.Comment: 35 pages and 15 figures e-mail: [email protected]
S(k) for Haldane Gap Antiferromagnets: Large-scale Numerical Results vs. Field Theory and Experiment
The structure function, S(k), for the s=1, Haldane gap antiferromagnetic
chain, is measured accurately using the recent density matrix renormalization
group method, with chain-length 100. Excellent agreement with the nonlinear
model prediction is obtained, both at where a single
magnon process dominates and at where a two magnon process
dominates. We repeat our calculation with crystal field anisotropy chosen to
model NENP, obtaining good agreement with both field theory predictions and
recent experiments. Correlation lengths, gaps and velocities are determined for
both polarizations.Comment: 11 pages, 3 postscript figures included, REVTEX 3.0, UBCTP-93-02
The spin-Peierls instability in spin 1/2 XY chain in the non adiabatic limit
The spin-Peierls instability in spin 1/2 XY chain coupled to dispersionless
phonons of frequency has been studied in the nonadiabatic limit. We
have chosen the Lang-Firsov variational wave function for the phonon subsystem
to obtain an effective spin Hamiltonian. The effective spin Hamiltonian is then
solved in the framework of mean-field approximation. We observed a dimerized
phase when g is less than a critical value and an anti-ferromagnetic phase when
it is greater than a critical value . The variation of lattice distortion,
dimerized order parameter and energy gap with spin phonon coupling parameter
has also been investigated here.Comment: 15 pages (Revtex, including 5 .ps figures); Submitted to PR
On negative higher-order Kerr effect and filamentation
As a contribution to the ongoing controversy about the role of higher-order
Kerr effect (HOKE) in laser filamentation, we first provide thorough details
about the protocol that has been employed to infer the HOKE indices from the
experiment. Next, we discuss potential sources of artifact in the experimental
measurements of these terms and show that neither the value of the observed
birefringence, nor its inversion, nor the intensity at which it is observed,
appear to be flawed. Furthermore, we argue that, independently on our values,
the principle of including HOKE is straightforward. Due to the different
temporal and spectral dynamics, the respective efficiency of defocusing by the
plasma and by the HOKE is expected to depend substantially on both incident
wavelength and pulse duration. The discussion should therefore focus on
defining the conditions where each filamentation regime dominates.Comment: 22 pages, 11 figures. Submitted to Laser physics as proceedings of
the Laser Physics 2010 conferenc
Quantum corrections to the conductivity and Hall coefficient of a 2D electron gas in a dirty AlGaAs/GaAs/AlGaAs quantum well: transition from diffusive to ballistic regime
We report an experimental study of the quantum corrections to the
longitudinal conductivity and the Hall coefficient of a low mobility, high
density two-dimensional two-dimensional electron gas in a AlGaAs/GaAs/AlGaAs
quantum well in a wide temperature range (1.5 K - 110 K). This temperature
range covers both the diffusive and the ballistic interaction regimes for our
samples. It was therefore possible to study the crossover region for the
longitudinal conductivity and the Hall effect
NMR and dc-susceptibility studies of NaVGe2O6
We report the results of measurements of the dc magnetic susceptibility
chi(T) and of the 23Na nuclear magnetic resonance (NMR) response of NaVGe2O6, a
material in which the V ions form a network of interacting one-dimensional spin
S=1 chains. The experiments were made at temperatures between 2.5 and 300 K.
The chi(T) data suggest that the formation of the expected low-temperature
Haldane phase is intercepted by an antiferromagnetic phase transition at 18 K.
The transition is also reflected in the 23Na NMR spectra and the corresponding
spin-lattice relaxation rate 1/T1(T). In the ordered phase, 1/T1(T) decreases
by orders of magnitude with decreasing temperature, indicating the formation of
a gap of the order of 12 K in the magnetic excitation spectrum.Comment: 10 pages, 15 figures; v2 with minor revisions of the tex
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