Muons point toward proton’s inner workings

When protons and antiprotons collide, W bosons are produced asymmetrically, since the up quark carries more momentum than the down. By looking at the directions of positively (blue) and negatively (red) charged muons from W decay, we gain deeper understanding about the quark interactions.

The inside of a proton is a tough place to see, but that doesn’t stop us from trying to learn more about the quarks and gluons that make it up.

A proton contains two up quarks and a down quark, as well as a sea of virtual quark-antiquark pairs that are continually appearing and disappearing. Scientists at the DZero experiment have now performed a study to probe how much momentum each of these quarks carries within the proton. To do this, they look at charged W bosons produced in proton-antiproton collisions.

A W+ boson is produced when an up quark from the proton collides with an anti-down quark from the antiproton. The up quark carries more momentum than the down quark, so the W+ bosons tend to travel in the same direction as the proton. For the same reason, any W bosons produced tend to travel in the same direction as the antiproton. By looking at the distributions of the bosons, we are actually learning about the structure of the proton.

However, it’s not as easy as it sounds. In order to observe the W bosons, the analyzers look for their decays into muons, more massive relatives of the electron. In these decays, an unknown amount of energy is lost in the form of an undetectable neutrino, meaning that the W direction cannot be measured directly. Instead, scientists use the muon direction, and the main challenge of the measurement is in understanding how the detector influences the final results, and then making corrections for these effects.

A small fraction of the time, the muon charge is mis-measured, and there can also be different probabilities of detecting positive and negative muons. Thanks to the design of the DZero experiment, these effects are very small, less than 1 percent, and this allows the final muon asymmetry measurement to be made very precisely. The results show that the down quark carries slightly more of the proton momentum than was previously thought, and this helps us to better understand how the quarks inside the proton behave and how often important events, such as Higgs boson production, occur at the Tevatron and the LHC.

Mark Williams

These physicists, all from Florida State University, made major contributions to this analysis.
This week marks a transition in the DZero Frontier Science Result authors. We thank Mike Cooke for his insightful and fun contributions over the past two years and wish him the best of luck at his fellowship with the American Association for the Advancement of Science. Mark Williams will do his best to fill his shoes as DZero’s resident columnist.