Looking for axigluons

This plot shows the forward-backward asymmetry in bottom quark production at the Tevatron as a function of the mass of the bottom-antibottom system compared to predictions with and without axigluons.

When a top-antitop quark pair is produced at the Tevatron, does the top quark go more often in the direction of the collision beam’s proton or antiproton?

Early measures of the forward-backward asymmetry — the difference in direction between an outgoing pair of particles relative to the incoming colliding particles — showed a larger effect than predicted, but after much followup there is now reasonable agreement between observation and prediction.

Recent theoretical developments summarized by Alexander Mitov at the Top at Twenty conference show contributions to the asymmetry not understood earlier. And recent experimental results from the Tevatron, summarized by Ziqing Hong, now include a new CDF measurement of the bottom, or b, quark forward-backward asymmetry.

The bottom quark is a partner of the top. In an early speculative theory, the top quark asymmetry was due to a quantum mechanical interference between a gluon — the carrier of the strong force — and a massive, hypothetical partner called an axigluon. At hadron colliders such as the Tevatron and the LHC, the existence of very heavy axigluons has been ruled out, but low-mass axigluons could be hiding under large backgrounds. CDF probes this low-mass region by measuring the b quark asymmetry.

The key experimental challenge — separating the b quark jet (which has 1/3 the charge of an electron) from the anti-b quark jet (which has -1/3 the charge of an electron) — was achieved using a technique that measures the total charge of each jet. CDF then measured the asymmetry in three different regions of the total system mass. A “smoking gun” for an axigluon would be an asymmetry that reverses direction, with the b swapping its preference for forward or backward at a particular energy.

The above figure shows the measured b quark asymmetry compared to predictions in three different regions. The result shows little asymmetry and somewhat disfavors axigluons with mass less than 200 GeV/c2. But because of the large experimental uncertainties, stealth axigluons with masses greater than 345 GeV/c2 could still be out there.

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These scientists are the primary analysts for this result. Top row, from left: Dante Amidei (Michigan), Sarah Henry (Texas A&M). Bottom row, from left: Jon Wilson (Michigan, now at Texas A&M), Tom Wright (Michigan).