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 suggest that the current theories need revision.
In proton-antiproton collisions at the Fermilab Tevatron, top-antitop quark pairs are created via gluons, the quantum carriers of the strong force. Theoretical calculations in the Standard Model predict that the direction of the outgoing top quarks should be approximately symmetric with respect to the direction of the incoming proton. The top quarks are measured using the CDF detector, a large general purpose apparatus surrounding one of the proton-antiproton collision points at the Tevatron (see http://www-cdf.fnal.gov). U-M is a long-standing member of the multi-national CDF Collaboration that operates the detector and shares the scientific returns from the data.
The first measurement of an asymmetry in top quark production, performed by U-M Ph.D. student Tom Schwarz in his thesis (2006), and a first publication (2008), hinted at an anomaly. The new result, by U-M Ph.D. students Glenn Strycker and Andrew Eppig, Research Scientist Monica Tecchio, U-M faculty members Dan Amidei and Myron Campbell, MSU Professor Joey Huston, and Schwarz and Professor Robin Erbacher of UC Davis, takes the 2008 publication a step further, by adding more data, and looking at the dependence on the mass of the system.
In the new measurement, as seen in the Figure below, the asymmetry is found to be most discrepant with the Standard Model at large values of the combined invariant mass of the top and antitop quarks. For Mt-tbar > 450 GeV/c2 the asymmetry is measured to be 48 ± 11 percent, three standard deviations higher than the Standard Model expectation (9 ± 1 percent). More detail can be found in the paper posted in the online archive http://arxiv.org/abs/1101.1028.
What could it mean? Some theories suggest that the mass dependent asymmetry could be evidence of a new particle just out of reach at the Tevatron’s collision energy. One very speculative possibility suggests that the new massive particle is an excitation of the gluon into hitherto unseen extra-dimensions. In that case, the top quark asymmetry could be the first evidence for additional dimensions of space-time. Much more work is needed to understand the data and confirm any such hypothesis, and the Michigan CDF team is digging in to see what more can be gleaned from the rich Tevatron data set.
The figures show the number of top events as a function of Delta-Y, an experimental variable that is proportional to the top quark direction. Positive Delta-Y is the same direction as the incoming proton (forward), whereas negative Delta-Y is the opposite direction (backward). The green shape is the Standard Model prediction for top quarks, and the points are our data. The plot on the left contains events in which the t-tbar mass is less than 450 GeV/c2 and is very symmetric. The plot on the right is for a t-tbar mass of greater than 450 GeV/c2 and illustrates the discrepancy between expected and observed: the top quarks tend to go forward.