On the Flavor Problem in Strongly Coupled Theories - THEP Mainz

148 Chapter 4. The Asymmetry in Top Pair Production
Even though it cannot be transformed in a forward-backward asymmetry1 , the charge
asymmetry at the LHC has the same physical origin and possible new physics effects
showing up in At FB will also contribute to Ay
C . It has been measured at the LHC by
both ATLAS [217, 218] and CMS [216] and was found to be in good agreement with
the SM [219],
(A y
C )SM = (1.15 ± 0.6) % , (4.14)
(A y
C )exp = (0.4 ± 1stat ± 1.1syst) % . (4.15)
Here, the quoted experimental value corresponds to the latest CMS result, which is
based on 5fb −1 of data. The fact that the asymmetry at the LHC is considerably
smaller than at the Tevatron is rooted in the different center of mass energies (CM)
of both machines. Since the asymmetries are normalized to the total inclusive cross
section, the number of top pairs produced by gluon-gluon fusion contributes to the
denominator but not to the numerator as it is a charge symmetric initial state and at
LHC energies gluon-gluon fusion is by far the dominating process.
We can conclude, that whatever new physics might be responsible for the discrepancy
seen at the Tevatron, its contribution to the LHC charge asymmetry should stay within
the (still significant) errors. It should further not contribute to the total inclusive cross
section which all experiments see in very good agreement with SM predictions, and it
must be heavy or broad enough to have escaped direct detection by dijet searches or
in the t¯t invariant mass spectrum so far.
4.2 The Forward-Backward Asymmetry in the SM
We will concentrate on the Tevatron observables at which t¯t pairs are produced in collisions
of protons and antiprotons, p¯p → t¯tX. At the partonic Born-level contributions
from quark-antiquark annihilation and gluon fusion arise within the SM
q
¯q ¯t
q(p1) + ¯q(p2) → t(p3) + ¯t(p4) ,
t
g
g ¯t
g(p1) + g(p2) → t(p3) + ¯t(p4) ,
t
(4.16)
in which the four-momenta p1,2 of the initial state partons can be expressed as the
fractions x1,2 of the four-momenta P1,2 of the colliding hadrons, p1,2 = x1,2P1,2. We
denote the square of the hadronic CM energy by s = (P1 + P2) 2 and introduce the
1 For more details concerning the different asymmetries and their comparability, see e.g. [220].