CDF finds evidence for top quark production asymmetry

photo of the day
The figures show the number of top events as a function of delta rapidity. The blue shape is that of the background, the green is the Standard Model prediction for top, and the points are our data. The plot on the left contains events in which the ttbar mass is less than 450 GeV/c2 and is very symmetric. The plot on the right is for a ttbar mass of greater than 450 GeV/c2 and illustrates the discrepancy between expected and observed.

A new analysis that will be presented at a Wine and Cheese lecture at Fermilab today points to an asymmetry in top quark production. This analysis raises the asymmetry of forward and backward top quark production found in a 2008 analysis to a ~3 sigma level.

In nature, symmetry is seen as pleasing and balanced designs, such as the intricate pattern on a tortoise shell or the structure of a snowflake.

In elementary particle physics, symmetry is fundamental to the theories we use to describe the world in which we live. A discrepancy in the symmetry predicted by theories of the Standard Model can point to new types of physics, an anomaly in the data or that the current theories need revision.

CDF researchers have measured the symmetry of how top quarks emerge from collisions, forward or backward, and how they decay. This analysis was performed for the first time in 2006 at CDF. CDF and DZero both published their inclusive analysis of this asymmetry in 2008. These highly cited white papers already pointed to an anomaly that has generated much interest in the theoretical community. This latest result takes the 2008 publications a step further, by adding more data, and looking at the dependence on the mass of the system. It is this dependence that is most discrepant with the Standard Model.

Fermilab’s Tevatron produces collisions that create top quark and anti-top-quark pairs via the strong force. Simple theoretical calculations predict that the Tevatron detectors should observe symmetric distributions of both top and antitop quarks. However, more detailed calculations suggest that these oppositely charged particles should have a slight preference as to how they emerge from the collisions.

The origins of this symmetry are subtle, but CDF has the sensitivity to be able to observe the 6 percent imbalance, which is predicted by the Standard Model. This result shows that nature prefers an imbalance that is even larger than predicted.

It is important to determine whether the top quark we are observing behaves the way we expect this Standard Model object to act. There are a number of Beyond-the-Standard-Model theories such as Z’ (pronounced Z-prime) and large extra dimensions that predicts much higher asymmetries. By measuring this asymmetry in top quark production, CDF physicists can compare it to theoretical expectations and probe for potentially undiscovered new physics.

Utilizing 5.3 inverse femtobarns of data, CDF measured the top forward backward asymmetry and observed significant asymmetries when studying the production and the production as related to the pairs’ center of mass energy, t-tbar.

When considering the pairs’ mass, the asymmetry is dependent on both the mass and the direction of the production. Scientists expect that the same number of top quarks and antitop quarks would be produced along the beam line, but CDF saw that more tops were produced along the proton direction of the beam and fewer tops produced along the antiproton direction of the beam. This effect is magnified when one looks at the mass dependence of the top-antitop system.

For Mttbar > 450 GeV/c2 the asymmetry is measured to be 48 ± 11 percent, three standard deviations from Standard Model expectation (9 ± 1 percent). Some theories suggest that such a mass dependence could be evidence of a massive new particle just out of reach at the Tevatron’s collision energy.

The LHC, a machine with significantly higher energy, cannot easily study this phenomenon, since the LHC does not make protons and antiprotons collide. However a new particle would still be observable in energy spectrum at the LHC. If the result at CDF truly is a sign of new physics, it may be that both machines will be required to understand its nature.

This result may provide the first important clue that there is new physics beyond the Standard Model. There may still be other interesting results as scientists from both Fermilab experiments continue to analyze the Tevatron’s now nearly 10 inverse femtobarns of sample data.

— Rob Roser, CDF co-spokesperson

photo of the day
First row from left: Dan Amidei, Myron Campbell and Andrew Eppig; University of Michigan. Second row from left: David Mietlicki, Glenn Strycker, Alexei Varganov and Tom Wright; University of Michigan. Third row from left: Robin Erbacher and Tom Schwarz, University of California, Davis; and Joey Huston, Michigan State University.