  Higgs pair production is crucial for measuring the Higgs boson self-coupling.
The dominant channel at hadron colliders is gluon fusion via heavy-quark loops.
We present the results of a fully exclusive simulation of gluon fusion Higgs
pair production based on the matrix elements for hh + 0, 1 partons including
full heavy-quark loop dependence, matched to a parton shower. We examine and
validate this new description by comparing it with (a) Higgs Effective Theory
predictions, (b) exact hh + 0-parton sample showered by pythia, and (c) exact
hh+1-parton distributions, by looking at the most relevant kinematic
distributions, such as PTh, PThh, Mhh spectra, and jet rate as well. We find
that matched samples provide an state-of-the-art accurate exclusive description
of the final state. The relevant LHE files for Higgs pair productions at the
LHC can be accessed via http://hepfarm02.phy.pku.edu.cn/foswiki/CMS/HH, which
can be used for relevant experimental analysis.
