13,429 research outputs found
Top-antitop pair hadroproduction in association with a heavy boson at the NLO QCD accuracy + Parton Shower
The PowHel framework allows to make predictions of total and differential
cross-sections of multiparticle hadroproduction processes at both NLO QCD
accuracy and NLO QCD matched to Parton Shower, on the basis of the interface
between the POWHEG-BOX and HELAC-NLO codes. It has already been applied to
study several processes involving a top-antitop pair in association with a
third particle or hadronic jet. Our most recent predictions concern
top-antitop-V hadroproduction (with V = W or Z), at both parton and hadron
level, by considering different decay channels (hadronic and leptonic) of the
heavy particles. In particular, we show the results of our phenomenological
analyses under the same system of cuts also recently adopted by the CMS
collaboration at LHC.Comment: 4 pages, 2 figures, Proceedings of TOP 2012 - 5th International
Workshop on Top Quark Physics, September 16 - 21 2012, Winchester, U
A theory of solar type 3 radio bursts
Energetic electrons propagating through the interplanetary medium are shown to excite the one dimensional oscillating two stream instability (OTSI). The OTSI is in turn stabilized by anomalous resistivity which completes the transfer of long wavelength Langmuir waves to short wavelengths, out of resonance with the electrons. The theory explains the small energy losses suffered by the electrons in propagating to 1 AU, the predominance of second harmonic radiation, and the observed correlation between radio and electron fluxes
Nonlinear stability of solar type 3 radio bursts. 1: Theory
A theory of the excitation of solar type 3 bursts is presented. Electrons initially unstable to the linear bump-in-tail instability are shown to rapidly amplify Langmuir waves to energy densities characteristic of strong turbulence. The three-dimensional equations which describe the strong coupling (wave-wave) interactions are derived. For parameters characteristic of the interplanetary medium the equations reduce to one dimension. In this case, the oscillating two stream instability (OTSI) is the dominant nonlinear instability, and is stablized through the production of nonlinear ion density fluctuations that efficiently scatter Langmuir waves out of resonance with the electron beam. An analytical model of the electron distribution function is also developed which is used to estimate the total energy losses suffered by the electron beam as it propagates from the solar corona to 1 A.U. and beyond
t tbar W and t tbar Z Hadroproduction at NLO accuracy in QCD with Parton Shower and Hadronization effects
We present theoretical predictions for the hadroproduction of t tbar W+, t
tbar W- and t tbar Z at LHC as obtained by matching numerical computations at
NLO accuracy in QCD with Shower Monte Carlo programs. The calculation is
performed by PowHel, relying on the POWHEG-BOX framework, that allows for the
matching between the fixed order computation, with input of matrix elements
produced by the HELAC-NLO collection of event generators, and the Parton Shower
evolution, followed by hadronization and hadron decays as described by PYTHIA
and HERWIG. We focus on the dilepton and trilepton decay channels, studied
recently by the CMS Collaboration.Comment: 21 pages 12 figure
Nonlinear stability of solar type 3 radio bursts. 2: Application to observations near 1 AU
A set of rate equations including strong turbulence effects and anomalous resitivity are solved using parmeters which model several solar type 3 bursts. Exciter distributions observed at 1 AU are excitation of the linear bump-in-tail instability, amplifying Langmuir waves above the threshold for the oscillating two stream instability (OTSI). The OTSI, and the attendant anomalous resistivity produce a rapid spectral transfer of Langmuir waves to short wavelengths, out of resonance with the electron exciter. Further energy loss of the beam is thus precluded. The various parameters needed to model the bursts are extrapolated inside 1 AU with similar results. Again, the OTSI is excited and decouples the electron beam from the Langmuir radiation. Reabsorption of the Langmuir waves by the beam is shown to be unimportant in all cases, even at 0.1 AU. The theory provides a natural explanation for the observed realationship between radio flux, and the electron flux
Z0 - boson production in association with a top anti-top pair at NLO accuracy with parton shower effects
We present predictions for the production cross section of a Standard Model
Z0-boson in association with a top-antitop pair at the next-to-leading order
accuracy in QCD, matched with shower Monte Carlo programs to evolve the system
down to the hadronization energy scale. We adopt a framework based on three
well established numerical codes, namely the POWHEG-BOX, used for computing the
cross section, HELAC-NLO, which generates all necessary input matrix elements,
and finally a parton shower program, such as PYTHIA or HERWIG, which allows for
including t-quark and Z0-boson decays at the leading order accuracy and
generates shower emissions, hadronization and hadron decays.Comment: 10 pages, 5 figures; found and corrected a bug in the
phenomenological analysis, just affecting Fig.4 - 5 that turn out to change
slightly with respect to our previous version and the cross-section values
after all cuts. Conclusions qualitatively unchange
Stabilization of electron streams in type 3 solar radio bursts
It is shown that the electron streams that give rise to Type 3 solar radio bursts are stable and will not be decelerated while propagating out of the solar corona. The stabilization mechanism depends on the parametric oscillating two stream instability. Radiation is produced near the fundamental and second harmonic of the local electron plasma frequency. Estimates of the emission at the second harmonic indicate that the wave spectra created by the oscillating two stream instability can account for the observed intensities of Type 3 bursts
(1,0) superconformal theories in six dimensions and Killing spinor equations
We solve the Killing spinor equations of 6-dimensional (1,0) superconformal
theories in all cases. In particular, we derive the conditions on the fields
imposed by the Killing spinor equations and demonstrate that these depend on
the isotropy group of the Killing spinors. We focus on the models proposed by
Samtleben et al in \cite{ssw} and find that there are solutions preserving 1,2,
4 and 8 supersymmetries. We also explore the solutions which preserve 4
supersymmetries and find that many models admit string and 3-brane solitons as
expected from the M-brane intersection rules. The string solitons are smooth
regulated by the moduli of instanton configurations.Comment: 26 page
Curved branes from string dualities
We describe a simple method for generating new string solutions for which the
brane worldvolume is a curved space. As a starting point we use solutions with
NS-NS charges combined with 2-d CFT's representing different parts of
space-time. We illustrate our method with many examples, some of which are
associated with conformally invariant sigma models. Using U-duality, we also
obtain supergravity solutions with RR charges which can be interpreted as
D-branes with non-trivial worldvolume geometry. In particular, we discuss the
case of a D5-brane wrapped on AdS_3 x S^3, a solution interpolating between
AdS_3 x S^3 x R^5 and AdS_3 x S^3 x S^3 x R, and a D3-brane wrapped over S^3 x
R or AdS_2 x S^2. Another class of solutions we discuss involves NS5-branes
intersecting over a 3-space and NS5-branes intersecting over a line. These
solutions are similar to D7-brane or cosmic string backgrounds.Comment: 21 pages, harvmac; misprint correcte
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