34 research outputs found
NLO automated tools for QCD and beyond
Theoretical predictions for scattering processes with multi-particle final
states at next-to-leading order (NLO) in perturbative QCD are essential to
fully exploit the physics potential of present and future high-energy
colliders. The status of NLO QCD calculations and tools is reviewed.Comment: 13 pages, 2 tables, to appear in the proceedings of Linear Collider
2011 (Understanding QCD at linear colliders in searching for old and new
physics), 12-16 September 2011, ECT*, Trento, Italy; added reference
Inadequacy of zero-width approximation for a light Higgs boson signal
The zero-width approximation (ZWA) restricts the intermediate unstable
particle state to the mass shell and, when combined with the decorrelation
approximation, fully factorizes the production and decay of unstable particles.
The ZWA uncertainty is expected to be of O(Gamma/M), where M and Gamma are the
mass and width of the unstable particle. We review the ZWA and demonstrate that
errors can be much larger than expected if a significant modification of the
Breit-Wigner lineshape occurs. A thorough examination of the recently
discovered candidate Standard Model Higgs boson is in progress. For M_H ~ 125
GeV, one has Gamma_H/M_H < 10^(-4), which suggests an excellent accuracy of the
ZWA. We show that this is not always the case. The inclusion of off-shell
contributions is essential to obtain an accurate Higgs signal normalization at
the 1% precision level. For gg -> H -> VV, V = W,Z, O(5-10%) corrections occur
due to an enhanced Higgs signal in the region M_VV > 2 M_V, where also sizable
Higgs-continuum interference occurs. We discuss how experimental selection cuts
can be used to suppress this region in search channels where the Higgs mass
cannot be reconstructed. We note that H -> VV decay modes in non-gluon-fusion
channels are similarly affected.Comment: Brief Review for MPLA, 19 pages, 6 figures, 2 tables; added 2
reference
Gluon-induced WW background to Higgs boson searches at the LHC
Vector-boson pair production is an important background for Higgs boson and new physics searches at the Large Hadron Collider LHC. We have calculated the loop-induced gluon-fusion process gg -> WW -> leptons, allowing for arbitrary invariant masses of the intermediate W bosons. This process contributes at O(alpha_s^2) relative to quark-antiquark annihilation, but its importance is enhanced by the large gluon flux at the LHC and by experimental cuts employed in Higgs boson searches. We find that gg -> WW provides only a moderate correction (ca. 5%) to the inclusive W-pair production cross section at the LHC. However, after taking into account realistic experimental cuts, the gluon-fusion process becomes significant and increases the theoretical WW background estimate for Higgs searches in the pp -> H -> WW -> leptons channel by approximately 30%
NLO QCD predictions for internal jet shapes in DIS at HERA
The transverse momentum flow inside jets is a sensitive measure of internal
jet structure. For the current jets in deep inelastic scattering this jet shape
measure is determined at order alpha_s^2, i.e. with up to three partons inside
a single jet. The scale dependence of jet shapes in various jet algorithms is
discussed. Results agree well with recent measurements by the ZEUS
Collaboration, without introducing the hadronization parameter R_sep.Comment: 10 pages, LaTeX2.09, REVTeX3.1, 9 postscript figure
Gluon-induced QCD corrections to pp --> ZZ --> l anti-l l' anti-l'
A calculation of the loop-induced gluon-fusion process gg --> Z(photon)Z(photon) --> l anti-l l' anti-l' is presented, which provides an important background for Higgs boson searches in the H --> ZZ channel at the LHC. We find that the photon contribution is important for Higgs masses below the Z-pair threshold and that the gg-induced process yields a correction of about 15% relative to the NLO QCD prediction for the q anti-q-induced process when only a M(l anti-l), M(l' anti-l') > 5 GeV cut is applied