The top quark is the heaviest of the six known constituents of protons and neutrons. Discovered in 1995 by the CDF and DZERO experiments at the Tevatron 1.8 TeV proton-antiproton collider, the top quark is an excellent laboratory to study Standard Model physics. Due to its extremely short lifetime the top quark does not "dress up" in a colourless hadron, but lives as a free particle, and thus allows us to study many interesting characteristics without the hassle of the complicated low-energy interactions that coloured particles undergo within time scales ten times longer than its lifetime.
One of the interesting things one can study at the Tevatron is the rate of producing top quarks of positive charge in the direction of the (positive-charge) proton beam. This is a measurement specific to the Tevatron: at the LHC both projectiles have the same electric charge, so there is no easy way to single out one direction along the beamline. Or is there a way?


So it is the quark-antiquark annihilation process the one which can readily exhibit an asymmetry: in the Standard Model this is a 7% effect, which arises from next-to-leading-order effects in quantum chromodynamics. New physics processes could however change this number; for instance, if the top quarks were at least in part produced in the decay of a massive object, one might expect a larger effect. Hence the interest of measuring the asymmetry as precisely as possible.
In order to define an asymmetry one must remember that the center of mass of the produced top-antitop pairs may be moving in either direction along the beam line: this is due to the arbitrary value of the momentum fraction carried by each of the initial state partons which give rise to the production process. So rather than counting the rate of top quarks moving in one or the other direction, one is interested in measuring the polar angle between the top and antitop quark. The polar angle is the angle taken with respect to the beam line: a positive value means that the top quark goes more "forward" than the antitop quark.
So now once one has this signed polar angle (we rather use the "rapidity difference" Dy, but that is a unnecessary detail - rapidity is connected to the polar angle by a simple functional form) the asymmetry can be defined as the difference between the rate of events at positive and negative angles, suitably divided by the sum of the two.
The measurements of the forward-backward asymmetry at the Tevatron have consistently shown a discrepancy with respect to quantum chromodynamics in the past: both CDF and DZERO reported two- to three-standard deviation excesses of the asymmetry. CDF last week published a preprint where they report on a measurement based on the full Run II statistics: that is the measurement also discussed in the presentation by Yuji Takeuchi at HCP today.


The asymmetry is then extracted both as a single integrated number and as a function of Dy; the latter dependence is fit to a line in the graph below. One then finds a slope of the data increasing with Dy, and an effect which is almost three times as large. The difference with the Standard Model is quantified at the level of 2.2 standard deviations.

The whole picture will be discussed in more detail in a presentation by Regina Demina tomorrow, so those of you who want more detail on the matter are encouraged to look at the slides - they are put online in the indico site of the HCP conference quite promptly. Just follow link and instructions from here.
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