In the SM, all the particles acquire
its masses from its interaction with the Higgs field through the
exchange of the Higgs boson. The Higgs boson has not been
observed yet experimentally. However some of its properties can be
inferred from Electroweak measurements. As the Higgs boson prefers to
couple to heavy particles, this means that one can learn about the
Higgs boson via the measurement of the masses of the Top quark and W
boson. Figure below shows the one sigma contour of the Higgs mass
obtained from the latest world average measurements of the Top quark
mass and the W boson mass. It seems to indicate that a lighter Higgs is
preferred.
The one sigma contour of the Higgs mass extracted from
Electroweak fit, and using the latest world average measurements of the
Top quark and W boson masses.
Direct
Searches for the SM Higgs Boson
At Tevatron
SM Higgs boson can be produced in several processes, and the channels
that the Higgs boson decays depends very much on its mass. At CDF
many direct searches for the SM Higgs boson are being performed.
Academia Sinica has performed searches for the Higgs boson in a few
final state signatures :
Missing Transverse Energy + Bjets
All-hadronic (multi-jet)
Missing Transverse Energy + BJets
Academia Sinica collaborates with Purdue University and the
CIEMAT Institute in Madrid, to search for the Higgs boson in the
missing energy and b-jets channel. This channel contains the
contributions from ZH→ννbb and from WH→lνbb (charged
lepton not
identified). The missing energy comes from the neutrinos in the W/Z
decays, and from the un-identified charged lepton in the W decay. This
channel has good sensitivity due to its relativity large predicted
production cross section, and the large decay branching fraction of
Z→νν. The analysis was performed on a data sample with
integrated luminosity of 1 fb-1. We find 268 (16) single
(double) b-tagged candidate events, where 248 +- 43 (14.4 +-
2.7) are expected from standard model background processes. We observe
no significant excess over the expected background and thus set 95%
confidence level upp limit on the Higgs boson production cross section
for Higgs boson masses ranging from 110 to 140 GeV/c2. For a mass of
115 GeV/c2, the observed (expected) limit is 20.4 (14.2) times the
standard model prediction. These results are now published in Physical
Review Letters journal ( PRL 100, 211801 (2008) ).
This result is then combined with results from other CDF searchres. The
results from individual channels, and combined results are shown in
Figure below.
Feynman diagrams for Higgs boson production in association with a W or
Z boson.
Reconstructed di-jet invariant mass (in GeV unit). (LEFT single b-tagged, (RIGHT) double b-tagged.
Event display of a p-pbar collision event with 2 b-tagged jets and
large missing transverse energy.
Summer 2007 results on the search for SM Higgs from CDF.
All-Hadronic (Multi-jets)
In this analysis Academia Sinica searched for the Higgs boson that is
produced in association with a W or Z boson (associated production), or
produced through the vector boson fusion process. In the associated
production process, we consider the channels where the W or Z boson
decays hadronically.
For both associated production and vector boson fusion channels, we
consider the Higgs boson decays into a pair of b-quarks. Thus the final
state contains at least four jets, with at least two b-jets. The
dominant background comes from QCD multi-jet production, with a small
fraction of background from ttbar, single-top, W/Z+jets and diboson.
The analysis is performed on 4 fb-1 of data sample. An innovative
technique is used to estimate the background and several methods are
used to increase the Higgs search sensitivity :
Data driven method to estimate QCD background
Use two types of b-tagging algorithms to increase b-tagging efficiency
re-design multi-jet trigger to increase trigger efficiency for low mass Higgs boson
Study jet width to discriminate Higgs signal from QCD background
Train neural network algorithm to separate Higgs signal from QCD background
The latest analysis is performed with the full CDF run II data sample
with L~10 fb-1. The observed (expected) upper limit at 95%
C.L. on the
Higgs production cross section times branching ratio varies between 7.0
(7.6) and 26.2 (25.8) times the SM prediction for Higgs boson
masses between 110 and 140 GeV/c2.
The results are published in JHEP 02 (2013) 004 and is also shown in Fermilab Today: Result of the Week .
Feynman diagrams for production and decay channels considered in the
all-hadronic channel search. (LEFT, MIDDLE) Associated production,
(RIGHT) Vector Boson Fusion.
(LEFT) Neural Network output distribution for the SS channel , (RIGHT)
Results of the observed and expected limit of the Higgs search in the all-hadronic channel using 10 fb-1 data sample.
