How are quarks like puppets?

The nature of the strong nuclear force is such that the energy of its field is concentrated in long thin tubes, or strings, that pull on quarks. Thinking of our two goats as a bottom quark-antiquark pair produced in a Tevatron collision, the strong nuclear force strings will pull them in certain specific directions. The other ends of the strings are connected to the outgoing fragments of the colliding proton and antiproton (not shown). Image courtesy of Megan Brain

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Today’s article involves strings and theory, but not string theory. Rather, today’s result involves string fragmentation, an approximate description of the strong nuclear force.

Quarks pull on each other as a result of the strong nuclear force. The energy of the force field between the quarks is concentrated in a long thin tube or string-shaped volume that stretches from one quark to the other. The idea behind string fragmentation is that when quarks pull on each other, it is as if there is a string tying them together, with a force that increases with their separation.

In a paper published last summer, Jonathan Rosner of the University of Chicago pondered how these strings — strings governed by quantum chromodynamics, or QCD — might pull on bottom quark-antiquark pairs. In particular, he looked at the case where the bottom quark joins with an up and a down quark to make a particle known as the Λb (pronounced “lambda sub b”) in proton-antiproton collisions. He realized that QCD strings would pull on the post-collision bottom quark in the direction of the incoming proton. The other end of the string is tied to the fragments of the proton after the collision; they travel mostly in the same direction as the incoming proton, and so they would drag the bottom quark along in that same direction. From this scenario, it is straightforward to estimate how much the string’s pull would change the direction of the motion of the bottom quark and hence the direction of the Λb.

DZero has recently measured the directions of Λb particles produced at the Tevatron. The interesting thing to measure is how often the Λb is produced going in the direction that the QCD string would have pulled it (that is, in the initial proton direction) versus how often it is produced going in the opposite (antiproton) direction. It turns out that the data match the simple string-pulling prediction quite well!

Leo Bellantoni

These scientists are the primary analysts for this measurement. Top row, from left: Avdhesh Chandra (Rice University), Eduard de la Cruz-Burelo, Jose Andres Garcia-Gonzalez (both of CINVESTAV, Mexico). Bottom row, from left: Ivan Heredia (CINVESTAV, Mexico) and Daria Ziemianska (Indiana University).
The DZero collaboration thanks Rick Van Kooten (left) for his service in ensuring the highest quality of physics output in his role as physics convener. Rick has taken on the new responsibility of vice provost for research at Indiana University. We welcome Boris Tuchming (center, CEA/Irfu/SPP, Saclay, France) as he joins Robert Hirosky (right, University of Virginia) in this crucially important task.