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Quark asymmetries hint at physics beyond the Standard Model

Posted August 27, 2013
Quark asymmetries hint at physics beyond the Standard Model

Quark asymmetries hint at physics beyond the Standard Model
This is the first plot showing the predicted bottom-quark forward-backward asymmetry (in percentage) plotted against the energy (in GeV units) of the bottom quark-antiquark pair produced in the proton-antiproton collision at the Tevatron. Orange is the prediction of the Standard Model. The other colors correspond to predictions from proposed extensions of the Standard Model that add a new particle called an “axigluon,” which was proposed to explain the anomaly in the observed top-quark forward-backward asymmetry. The different colors correspond to different parameters assumed for the axigluon (the parameters are the mass of the axigluon, MG, and the strength of the interaction of the axigluon with the top quark, g). The blue plot deviates strongly from the orange, so experiments should be able to tell the difference. The other colors do not deviate that much from the orange, so more work or a different method would be needed if the axigluon has parameters like those (but the blue line is for parameters that best explain the top-quark case). Credit: Grinstein and Murphy. ©2013 American Physical Society

While scientists have become increasingly convinced that the Standard Model of particle physics is incomplete, it’s still unclear exactly how the Standard Model needs to be extended. Experiments have shown that the Standard Model cannot explain certain top quark observations, but a variety of extensions of the Standard Model have been proposed to explain them, and it’s unclear which extension is correct. In a new paper published in Physical Review Letters, physicists Benjamín Grinstein and Christopher W. Murphy at the University of California, San Diego, have explained how upcoming data on the bottom quark can be used to distinguish between competing new physics explanations of unexpected top quark data.

“There has been much excitement the last couple of years precipitated by reports by the two experimental collaborations working at the Tevatron (at Fermilab, outside Chicago) that a much larger-than-expected top-quark forward-backward asymmetry is seen,” Grinstein told “Several models have been proposed to explain this unexpected result. Our paper suggests a way to distinguish among the various models that have been proposed, since these models give very different bottom-quark forward-backward asymmetries. When a sufficiently precise measurement of the bottom-quark forward-backward asymmetry is performed, we will be able to narrow down significantly the new physics that the Tevatron experiments seem to have uncovered.

“But perhaps more importantly, observations of the bottom-quark forward-backward asymmetry in disagreement with expectations from the Standard Model, when put together with the top-quark forward-backward asymmetry, would demonstrate fairly conclusively that there is new physics in the form of new particles and interactions not included in the Standard Model, and would point the way toward its direct experimental confirmation. So, as you can see, this would go to the heart of the question in particle physics.”

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