On the Flavor Problem in Strongly Coupled Theories - THEP Mainz

168 Chapter 4. The Asymmetry in Top Pair Production
C � V
u u
10
5
0
� 5
� 10
10 pb
9 pb
8 pb
7 pb
6 pb
5 pb � 10 �
99 � CL
95 � CL
68 � CL
0 � 10 � 20 � 30 �
� 30 � 20 � 10 0 10 20 30
Figure 4.7: A combined fit to the latest measurements of the asymmetry At FB , the
total inclusive cross section σt¯t and the last bin of the invariant mass spectrum in
the � CV uū − � CA uū plane. The 99%, 95% and 68% CL ellipses are shown in blue, grayish
blue and yellow respectively. The black cross (dot) represents the best fit point (SM
expectation). Dashed black lines show the regions of parameter space which induce a
large corrections to the total cross section and At FB respectively.
From Table 4.1 we further observe that ˜ CV uū grows with increasing (ctL + ctR ), i.e. if
the top quark is localized more strongly in the IR, which was expected from (4.73) and
(4.63). A similar trend in terms of ctR , though less pronounced, is also visible in the
case of ˜ CA uū. The numbers given in the table furthermore confirm our qualitative findings
from Section 4.4 of strongly suppressed axial-vector couplings, | ˜ CA uū|/| ˜ CV uū| ≪ 1.
As a consequence, upon inserting the numerical values of ˜ CV uū and ˜ CA uū into the numerator
of (4.70), it becomes clear, that in the RS model the NLO contributions to
At FB arising from ˜ CV uū dominate over the LO contributions from ˜ CA uū. Numerically, we
find that the vector-current contributions to the asymmetry are typically larger by
about a factor of 100 than the corrections due to the axial-vector current. This strong
enhancement reflects the fact that the KK gluons couple mainly vectorially with a
small axial component. A closer inspection of (4.70) shows however, that in the ratio
of the asymmetric and symmetric cross sections the effects of ˜ CV uū tend to cancel, as
anticipated in Section 4.3. Since both σa and σs are enhanced for ˜ CV uū > 0, but the
dependence of σs on ˜ CV uū is stronger than the one of σa, positive values of ˜ CV uū will
effectively lead to a reduction and not to an enhancement of the t¯t forward-backward
C � A
u u
�
40 �