ATLAS Experiment finds evidence of charge asymmetry in top-quark pairs | 9/27/2017 | Staff
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Among the most intriguing particles studied by the ATLAS Experiment is the top quark. As the heaviest known fundamental particle, it plays a unique role in the Standard Model of particle physics, and perhaps in physics beyond the Standard Model.

During Run 2 of the Large Hadron Collider (LHC) at CERN, proton beams were collided with high luminosity at a centre-of-mass energy of 13 TeV. This allowed ATLAS to detect and measure an unprecedented number of events involving top-antitop quark pairs, providing ATLAS physicists with a unique opportunity to gain insight into the top quark's properties.

Interference - Particles - Production - Top - Quarks

Due to sneaky interference between particles involved in the production, top and antitop quarks are not produced equally with respect to the proton beam direction in the ATLAS detector. Instead, top quarks are produced preferentially in the centre of the LHC's collisions, while antitop quarks are produced preferentially at larger angles. This is known as a "charge asymmetry."

Charge asymmetry is similar to a phenomenon measured at the Tevatron collider at Fermilab, known as a "forward-backward" asymmetry. At Tevatron, colliding beams were made of protons and anti-protons, respectively, which led to top and antitop quarks each being produced at non-central angles, but in opposite directions. A forward-backward asymmetry, compatible with improved Standard Model predictions, was observed.

Effect - Charge - Asymmetry - LHC - %

The effect of charge asymmetry at the LHC is predicted to be extremely small (< 1%), as the dominant production mode of top-quark pairs via the scattering of gluons (the carriers of the strong force) emerging from the protons does not exhibit a charge asymmetry. A residual asymmetry can only be generated by more complicated scattering processes involving also quarks. However, new physics processes interfering with the known production modes can lead to much...
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