If the top is lopsided, will the bottom be also?

A proton and antiproton collision may produce a top or bottom quark in the forward direction, as shown. The situation in which the directions of the produced quark and antiquark are swapped is known as production in the backwards direction.

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Some of the most attention-drawing results from the Tevatron have been the recent ones about a certain asymmetry in the production of top quark pairs. When a proton collides with an antiproton, they can produce a top quark and a top antiquark, and the direction of the top quark tends to be along the direction of the proton. This result, depicted above, should happen more often than the case where the top quark is produced moving in the direction of the initial antiproton. How much more often? The so-called forward case is predicted to happen somewhere between 5 and 13 percent more often than the backward case.

Initial measurements of this asymmetry in 2008 came out somewhat higher than this expectation. Later measurements, taken using more data, are closer to the prediction, but we want to be quite sure that we understand what is happening in this kind of collision. In particular, because a top quark is so massive, there is some possibility that it has new interactions with as-yet-unseen particles that might change this asymmetry. So an obvious thing to do is to look at the forward-backward asymmetry with another quark that is thought not to have new physics associated with its production.

That other quark would be the bottom quark, the first of the two quarks discovered here at Fermilab. The bottom quark is much less massive than the top quark and has been very extensively studied.

The DZero collaboration has measured the forward-backward asymmetry of the production of bottom quark-antiquark pairs. In order to make the measurement easier, we looked at a very specific case of what happens to these quarks after they are produced. We looked at the case in which the bottom quark paired with an up antiquark to make a particle called B- that then decayed into two specific particles: a J/Ψ and a K-. The reason to look for this particular decay pattern is that the J/Ψ itself is easy to see; it decays to a very distinctive pair of muons.

The result is that the forward-backward production asymmetry for the production of bottom quark-antiquark pairs is -0.27 ± 0.45 percent. That is essentially zero. The prediction is fairly close to zero too: The prediction was 1.12 ± 0.77 percent. The agreement between these numbers supports the original assertion that there might not be new physics in bottom quark production. And so it seems that the bottom isn’t lopsided after all!

Leo Bellantoni

These DZero members all made significant contributions to this result.
Every member of DZero contributes to the success of the collaboration. Here are (many) of the members of the DZero collaboration, gathered for their last summer workshop here at Fermilab in June.