Description
In recent years a large effort has gone into accurate higher order
calculations of the Higgs production and of various background cross
sections. Many reactions are now known to next-to-leading order
accuracy. For the dominant Higgs production mechanism, the gluon-
gluon fusion, even the next-to-next-to leading order calculations
have been performed. As a result, the cross section for this Higgs
production mechanism is found to be a factor of more than 2 larger if
these higher order corrections are included. However, so far most
simulations performed in CMS and ATLAS are done with LO parton shower
Monte Carlos. Corrections to signal and background cross sections are
thus either ignored or taken into account in a very simple approach.
Normally the LO results are scaled with a so-called inclusive K-
factor, which is just the ratio of higher order cross section over
leading order cross section. However, if the event kinematics have to
be exploited in order to separate the signal from the background, the
use of an inclusive K-factor leads to wrong shapes. A typical example
is the Higgs search in the mass range between 155 GeV and 180 GeV,
where H->WW->lvlv is expected to be the main discovery channel. For
this channel a jet veto is required in order to remove ttbar
background. In addition, the other cuts exploit the spin correlation
between the W bosons and the resulting pt spectra of the charged
leptons.
We show on the example of H->WW-lvlv, that a reweighting procedure of
the LO MC events with effective, pt-dependent K-factors allows an
accurate simulation of higher order QCD calculations. Thus, the
discovery potential is found to increase considerably.
Author
Giovanna Davatz
(CMS)