Based on Phys.Rev.Lett. 105 (2010) 022001 (arXiv:1001.5027) by Jason Gallicchio and Matthew D. Schwartz.

The idea behind pull is to exploit differences in the way particles are charged under the the strong force (Quantum Chromodynamics, QCD). The charge is not a single number, but a representation. This can be visualized as a set of color strings.


 

Color connections -- for signal (Higgs boson is a color singlet) versus background (gluon -> b bbar is color octet)

 

Signal

Color connections

Background

Color connections

These strings generate characteristic radiation patterns.

Radiation patterns of color strings for signal and background are different

before

We then measure this radiation by calculating the linear moment of the radiation in the jet

.

Here are the distributions of the pull vector for signal and background, in polar coordinates

Since the angle seems a better discriminant than the length of the vector, we use pull angles for discriminants. Here is a simulation of the pull angle for signal and background, comparing two event generators, Pythia and Herwig.

Comparison of Pythia and Herwig

 

Pull has been measured by D0 1101.0648, where it was calibrated on top samples. In a top decay, there is a color singlet (the W boson) as well as jets conected to the beam (the b's). They used this data to measure that the W in top decays was a color signlet, to 3 sigma accuracy.

Measured by D0 (1101.0648) to determinine color of W bosons in top decays

 

 

 

Work is ongoing to measure and use pull at the LHC...