On the Flavor Problem in Strongly Coupled Theories - THEP Mainz

4 The Asymmetry in Top
Pair Production
The top quark is the heaviest particle in the SM and explaining its large mass provides
a challenge for many theories which try to extend it. Especially technicolor like
models of physics beyond the SM have a problem to generate the top mass, because
typically the Yukawa interaction is an irrelevant operator in these theories. The RS
model with fermions in the bulk avoids this problem by describing the top quark by
a mainly composite particle, which gets its mass from the compositeness scale similar
as the proton from ΛQCD. In this way it differs from the light flavors which are bound
to have small admixtures with their composite partners (provided one does not want
to sabotage the RS-GIM mechanism and give up the explanation of the flavor hierarchies).
Therefore, besides the Higgs, the top quark is the messenger of the composite sector
for strongly coupled theories featuring partial compositeness in the fermionic sector,
even more so as discovering a composite top would reject models with only a composite
Higgs, e.g. CTC. Therefore, observables sensitive to top couplings should receive
special attention.
Interestingly, the only observable in which both CDF and DØ find a substantial deviation
from the SM, and which is not completely overshadowed by LHC measurements,
is the forward backward asymmetry in top pair production. In Sections 4.2 and 4.3
this observable will be introduced and the general implications for new physics which
try to resolve the tension with the SM value, as well as for LHC measurements, are
discussed. Based on the paper [205], in Section 4.4 the contributions from KK gluon
exchange at tree-level and including next to leading order (NLO) SM diagrams will be
examined and numerically evaluated in Section 4.5. Since the asymmetry is sensitive
to the axial vector current, one expects large effects from an RS axigluon and Section
4.6 is dedicated to the analysis on how the effects change if the extended color gauge
group is assumed.
4.1 Top-Antitop Pair Production and Observables at
Tevatron and the LHC
Since the top quark was discovered in 1995 by the Tevatron experiments CDF and DØ,
thousands of top-antitop pairs have been produced which allowed for a measurement
of the top mass mt with an accuracy of below 1% [206],
m Tevatron
t = 173.2 ± 0.9stat+syst GeV (4.1)