Fast computation of MadGraph amplitudes on graphics processing unit (GPU)

Continuing our previous studies on QED and QCD processes, we use the graphics processing unit (GPU) for fast calculations of helicity amplitudes for general Standard Model (SM) processes. Additional HEGET codes to handle all SM interactions are introduced, as well assthe program MG2CUDA that converts arbitrary MadGraph generated HELAS amplitudess(FORTRAN) into HEGET codes in CUDA. We test all the codes by comparing amplitudes and cross sections for multi-jet srocesses at the LHC associated with production of single and double weak bosonss a top-quark pair, Higgs boson plus a weak boson or a top-quark pair, and multisle Higgs bosons via weak-boson fusion, where all the heavy particles are allowes to decay into light quarks and leptons with full spin correlations. All the helicity amplitudes computed by HEGET are found to agree with those comsuted by HELAS within the expected numerical accuracy, and the cross sections obsained by gBASES, a GPU version of the Monte Carlo integration program, agree wish those obtained by BASES (FORTRAN), as well as those obtained by MadGraph. The performance of GPU was over a factor of 10 faster than CPU for all processes except those with the highest number of jets.Comment: 37 pages, 12 figure

Establishing the nature of companion candidates to X-ray emitting late B-type stars

The most favored interpretation for the detection of X-ray emission from late B-type stars is that these stars have a yet undiscovered late-type companion (or an unbound nearby late-type star) that produces the X-rays. Several faint IR objects at (sub)-arcsecond separation from B-type stars have been uncovered in our earlier adaptive optics imaging observations, and some of them have been followed up with the high spatial resolution of the Chandra X-ray observatory, pinpointing the X-ray emitter. However, firm conclusions on their nature requires a search for spectroscopic signatures of youth. Here we report on our recent ISAAC observations carried out in low resolution spectroscopic mode. Equivalent widths have been used to obtain information on spectral types of the companions. All eight X-ray emitting systems with late B-type primaries studied contain dwarf like companions with spectral types later than A7. The only system in the sample where the companion turns out to be of early spectral type is not an X-ray source. These results are consistent with the assumption that the observed X-ray emission from late B-type stars is produced by an active pre-main sequence companion star.Comment: 6 pages, 2 figures, 3 tables, accepted for publication in MNRA

Calculation of HELAS amplitudes for QCD processes using graphics processing unit (GPU)

We use a graphics processing unit (GPU) for fast calculations of helicity amplitudes of quark and gluon scattering processes in massless QCD. New HEGET ({\bf H}ELAS {\bf E}valuation with {\bf G}PU {\bf E}nhanced {\bf T}echnology) codes for gluon self-interactions are introduced, and a C++ program to convert the MadGraph generated FORTRAN codes into HEGET codes in CUDA (a C-platform for general purpose computing on GPU) is created. Because of the proliferation of the number of Feynman diagrams and the number of independent color amplitudes, the maximum number of final state jets we can evaluate on a GPU is limited to 4 for pure gluon processes ($gg\to 4g$), or 5 for processes with one or more quark lines such as $q\bar{q}\to 5g$ and $qq\to qq+3g$. Compared with the usual CPU-based programs, we obtain 60-100 times better performance on the GPU, except for 5-jet production processes and the $gg\to 4g$ processes for which the GPU gain over the CPU is about 20

Testing the companion hypothesis for the origin of the X-ray emission from intermediate-mass main-sequence stars

There is no straightforward explanation for intrinsic X-ray emission from intermediate-mass main-sequence stars. Therefore the observed emission is often interpreted in terms of (hypothesized) late-type magnetically active companion stars. We use Chandra imaging observations to spatially resolve in X-rays a sample of main-sequence B-type stars with recently discovered companions at arcsecond separation. We find that all spatially resolved companions are X-ray emitters, but seven out of eleven intermediate-mass stars are also X-ray sources. If this emission is interpreted in terms of additional sub-arcsecond or spectroscopic companions, this implies a high multiplicity of B-type stars. Firm results on B star multiplicity pending, the alternative, that B stars produce intrinsic X-rays, can not be discarded. The appropriate scenario in this vein is might be a magnetically confined wind, as suggested for the X-ray emission of the magnetic Ap star IQ Aur. However, the only Ap star in the Chandra sample is not detected in X-rays, and therefore does not support this picture.Comment: 12 pages; accepted for publication in Astronomy & Astrophysic

Finite-Width Effects in Top Quark Production at Hadron Colliders

Production cross sections for t\bar{t} and t\bar{t}j events at hadron colliders are calculated, including finite width effects and off resonance contributions for the entire decay chain, t --> bW --> b\ell\nu, for both top quarks. Resulting background rates to Higgs search at the CERN LHC are updated for inclusive H --> WW studies and for H --> \tau\tau and H --> WW decays in weak boson fusion events. Finite width effects are large, increasing t\bar{t}(j) rates by 20% or more, after typical cuts which are employed for top-background rejection.Comment: 32 pages, 11 figures, 7 tables; minor changes, reference added, to be published in Phys. Rev.