On the Flavor Problem in Strongly Coupled Theories - THEP Mainz
On the Flavor Problem in Strongly Coupled Theories - THEP Mainz
On the Flavor Problem in Strongly Coupled Theories - THEP Mainz
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156 Chapter 4. The Asymmetry <strong>in</strong> Top Pair Production<br />
)<br />
2<br />
xf(x,Q<br />
1.2<br />
1<br />
0.8<br />
0.6<br />
0.4<br />
0.2<br />
0<br />
10<br />
-4<br />
10<br />
g/10<br />
c, c<br />
-3<br />
MSTW 2008 NLO PDFs (68% C.L.)<br />
s, s<br />
10<br />
2<br />
2<br />
Q = 10 GeV<br />
-2<br />
u<br />
d<br />
u<br />
d<br />
-1<br />
10 1<br />
x<br />
)<br />
2<br />
xf(x,Q<br />
1.2<br />
1<br />
0.8<br />
0.6<br />
0.4<br />
0.2<br />
0<br />
10<br />
-4<br />
b, b<br />
10<br />
-3<br />
c, c<br />
2 4 2<br />
Q = 10 GeV<br />
10<br />
g/10<br />
s, s<br />
-2<br />
u<br />
d<br />
d<br />
u<br />
-1<br />
10 1<br />
x<br />
Figure 4.4: Figure shows a plot of <strong>the</strong> PDFs of <strong>the</strong> gluon and <strong>the</strong> valence and sea<br />
quarks <strong>in</strong> <strong>the</strong> proton for Q 2 = 10 GeV 2 and Q 2 = 10 4 GeV 2 . They are taken from <strong>the</strong><br />
homepage of <strong>the</strong> MSTW collaboration and were computed us<strong>in</strong>g MSTW2008NLO [228].<br />
from which <strong>the</strong> former has <strong>the</strong> same color flow as <strong>the</strong> t channel diagram and thus leads<br />
to <strong>in</strong>terference [229].<br />
The common characteristic of models featur<strong>in</strong>g large t channel currents is a sizable<br />
flavor off-diagonal coupl<strong>in</strong>g between <strong>the</strong> up (or down) and <strong>the</strong> top quark, while o<strong>the</strong>r<br />
flavor chang<strong>in</strong>g coupl<strong>in</strong>gs have to be under control <strong>in</strong> order to not violate bounds from<br />
FCNCs. This can be realized by a W ′ or Z ′ gauge boson with accord<strong>in</strong>gly adjusted<br />
coupl<strong>in</strong>gs, so that <strong>the</strong> relevant <strong>in</strong>teraction terms read<br />
�<br />
LZ ′ /W ′ ∋ ¯t g Z′<br />
V + g Z′<br />
�<br />
A γ5 γ µ Z ′ �<br />
µu + ¯t<br />
W ′<br />
gV + g<br />
W ′<br />
A γ5<br />
�<br />
γ µ W ′ µd . (4.43)<br />
The correspond<strong>in</strong>g expressions for <strong>the</strong> cross sections will not be explicitly repeated<br />
here and can be found for <strong>the</strong> Z ′ <strong>in</strong> [230, Sec VI] and for <strong>the</strong> W ′ <strong>in</strong> [231, Sec.II]. In <strong>the</strong><br />
case of <strong>in</strong>terference between t and s channel, both vector and axial vector coupl<strong>in</strong>gs<br />
contribute equally to <strong>the</strong> cross section and <strong>the</strong> asymmetry and one has <strong>the</strong>refore more<br />
freedom to choose <strong>the</strong> coupl<strong>in</strong>gs. If <strong>the</strong>re is no flavor diagonal coupl<strong>in</strong>g however, <strong>the</strong><br />
new gauge bosons are forced to decay <strong>in</strong>to top quarks which will yield a clear signal of<br />
like sign leptons. Searches for this f<strong>in</strong>al state already place very strong bounds on <strong>the</strong><br />
parameter space of <strong>the</strong>se resonances, except for a Z ′ , W ′ which is too light to decay<br />
<strong>in</strong>to <strong>the</strong> top [232, 233]. This typically excludes models with resonances <strong>in</strong> <strong>the</strong> few<br />
100 GeV range which is preferred <strong>in</strong> order to expla<strong>in</strong> <strong>the</strong> large asymmetry assum<strong>in</strong>g<br />
perturbative coupl<strong>in</strong>gs.<br />
Analogous considerations apply to a scalar with flavor off-diagonal coupl<strong>in</strong>gs. The