CDF finds more evidence of an asymmetry in top production

The plot shows the rapidity difference between the reconstructed top and anti-top quarks in data, and the predicted signal and background simulations. The observed forward backward asymmetry in the data is Afb = 14 ± 5 percent. The corresponding reconstructed forward-backward asymmetry, after correcting for experimental effects, is Afb = 42 ± 16 percent, which can be compared to the Standard Model prediction of Afb = 6 ± 1 percent

Symmetries have long been an important part of characterizing physical laws. Although some of the most beautiful theories exhibit symmetric properties, startling discoveries in physics have come at the expense of these symmetries, which have been broken to reveal an underlying property of nature.

Indeed, studies of weak nuclear reactions in the 1960s led to the discovery of parity violation, and a deeper understanding of electromagnetism and the weak force. Current theory predicts that the strong force, the force that binds quarks together, should exhibit only symmetric properties. However, physicists at Fermilab will leave no stone unturned. Towards this goal, the CDF experiment has tested discrete symmetries of the strong force by studying top quark events produced by the Tevatron.

Top quarks are produced at the Tevatron dominantly through the strong force. They are the most massive known particle, and have properties that some theories suggest may be related to electroweak symmetry breaking and the Higgs boson. The Tevatron is a highly suitable experiment to study the symmetry of producing top quarks. As it collides protons with anti-protons the collision is asymmetric.

Previous research by the CDF experiment found evidence of a larger asymmetry than the Standard Model leads us to expect, which could be evidence of a new particle just out of reach at the Tevatron’s collision energy. The result presented here measures the forward-backward asymmetry in top production in completely separate data from previous work: data where the signature of top quark events in the detector include two leptons. This new result provides an independent look at the this strange effect. In this new result, CDF measures a corrected Afb = 42 +/- 16 percent, which is 2.6 sigma from zero and 2.3 sigma from the Standard Model expectation. This exciting result provides further evidence of a larger than expected asymmetry in top quark production.

View more about this analysis here.

— Edited by Andy Beretvas

These physicists are responsible for this analysis. First row from left: Yuji Takeuchi, Tsukuba; Yen-Chu Chen, Academia Sinica;  Jaroslav Antos, Slovak Academy of Science. Second row from left: Chang-Seong Moon, Seoul National University, Korea; Youngdo Oh, Kyungpook National University, Korea; Hyunsoo Kim, Chonbuk National University, Korea; and Tom Schwarz, University of California, Davis.