The search for a Higgs boson decaying to a b¯b pair is one of the
key analyses ongoing at the ATLAS experiment. Despite being the
largest branching ratio decay for a Standard Model Higgs boson, a large
dataset is necessary to perform this analysis because of the very large
backgrounds affecting the measurement. To discriminate the electroweak
H → b¯b signal from the large QCD backgrounds, the associated production
of the Higgs with a W or a Z boson decaying leptonically is
used.
Different techniques have been proposed to enhance the signal over
background ratio in the VH(b¯b) channel, from dedicated kinematic cuts,
to a single large radius jet to identify the two collimated b’s in the Higgs
high transverse momentum regime, to multivariate techniques. The
high-pT approach, using a large radius jet to identify the b’s coming
from the Higgs decay, has been tested against an analysis based on
kinematic cuts for a dataset of 4.7 fb−1 luminosity at √s = 7 TeV, and
compatible results were found for the same transverse momentum range.
Using a kinematic cut based approach the VH(b¯b) signal search has
been performed for the full LHC Run 1 dataset: 4.7 fb−1 at √s = 7 TeV
and 20.7 fb−1 at √s = 8 TeV. Several backgrounds to this analysis, such
as Wb¯b have not been measured in data yet, and an accurate study of the
theoretical description has been performed, comparing the predictions
of various Monte Carlo generators at different orders.
The complexity of the analysis requires a profile likelihood fit with
several categories and almost 200 parameters, taking into account all
the systematics coming from experimental or modelling limitations, to
extract the result. To validate the fit model, a test of the ability to
extract the signal is performed on the resonant V Z(b¯b) background. A
4.8σ excess compatible with the Standard Model rate expectation has
been measured, with a best fit value μVZ = 0.93+0.22−0.21.
The full LHC Run1 dataset result for the VH(b¯b) process is a
limit of (1.3)1.4 x SM (expected) observed, with a best fit value
of 0.2±0.5(stat)±0.4(sys) for a Higgs boson of 125 GeV mass
