CERN-THESIS-2012-153 26/07/2012 - CERN Document Server

2.5 Top Quark Physics Beyond the Standard Model
As mentioned at the beginning of this chapter, the SM has proven rather successful in the description of
experimental results. Figure 2.9 shows a summary of the SM production cross-section for different processes
measured by the ATLAS experiment and compared to the theoretical expectations. Similar measurements
can be found for the CMS experiment. There are, however, a number of shortcomings with the SM. For
instance, the SM does not account for the existence of dark matter and dark energy, the baryon asymmetry
of the universe, non-zero mass of the neutrinos, and furthermore it does not solve the hierarchy problem.
Another question, directly related to the top quark, can be raised: why are the masses of the charged leptons
as they are? Some of these questions might actually be answered, or hints given, at the LHC as more data
is being collected. However, as of now, divergences from the SM are yet to be found. Several extensions to
the SM have been formulated, in an attempt to answer some of those questions. One of the most common
examples of those theories is SUSY, which postulates a fermion-boson symmetry, and provides a solution to
the hierarchy problem.
[pb]
total
σ
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10
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10
10
1
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35 pb
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35 pb
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1.0 fb
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ATLAS Preliminary
LHC pp
Theory
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s = 7 TeV
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Data 2010 (L = 35 pb )
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Data 2011 (L = 1.0 ­ 4.7 fb )
LHC pp
Theory
s = 8 TeV
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Data 2012 (L = 5.8 fb )
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2.1 fb
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4.7 fb
W Z tt t WW WZ Wt ZZ
Figure 2.9: Summary of several Standard Model total production cross section measurements performed
by the ATLAS experiment compared to the corresponding theoretical expectations. The error bars include
the full uncertainty, both statistical and systematic. All theoretical expectations were calculated at NLO or
higher.
Top quark physics, because of its recent discovery, is a rather new field. Several analyses currently
on-going, will provide more precision to known quantities, and possibly hints of new physics will be found.
Some of those analysis specifically aim for physics BSM. Examples of these are analyses are: studies related
21
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5.8 fb