Chasing the neutrino to measure W boson direction

The direction of positive (blue) and negative (red) W bosons produced in proton-antiproton collisions gives information about the underlying quark structure of these nucleons. For the best possible measurement, the momentum of the neutrinos (the fuzzy gray objects) is needed. This requires some clever techniques and a detailed understanding of the detector.

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The above figure may look familiar. That’s because I used a very similar one for an article in September, describing a measurement of the muon asymmetry from W boson decays. The idea was to look at the direction of the positive and negative muons, from which some important details of the proton and antiproton structure can be extracted. This week the DZero experiment released a new measurement using a similar technique, though with a different particle and with a very important improvement that is worth highlighting.

The process of interest is W boson production at the Tevatron, when a quark from the proton and an antiquark from the antiproton generate a massive, charged and short-lived boson via the weak interaction. The direction of the W boson tells us how much of the proton momentum is carried by the down and up quarks, information of huge importance for many other measurements, both in particle physics (such as determining the W boson mass) and beyond (for example, in nuclear physics).

To reconstruct the W boson direction, the analyzers search for its decay into a charged lepton, in this case an electron, and a neutrino, which passes all the way through the detector without leaving any signal. The W boson direction is equal to the direction of the combined lepton-neutrino system, and so the momentum of both particles, including the undetected neutrino, is needed to make the measurement — or is it?

All is not lost! While the neutrino is not observed directly, it is conspicuous in its absence. Using the principles of conservation of momentum and energy and measuring the momenta of the other particles in the event, we can constrain the neutrino trajectory with sufficient precision to extract the W boson direction. The neutrino is like a missing piece in a jigsaw puzzle: It’s only by putting all the other pieces together that the missing shape can be known.

The muon asymmetry analysis covered previously uses a different method to sidestep the issue of the missing neutrino, relying on the fact that the lepton and W boson directions are strongly correlated. However, interpreting this lepton asymmetry is reliant on certain assumptions about the decay mechanism. By using the additional detector information to infer the W boson direction directly, and also by using a different type of lepton (electron versus muon) in the W decay, this new measurement provides important and independent information about the proton structure. As an example of the benefit of this new data, using it in the final W boson mass measurements from the Tevatron could yield precision improvements of up to 15 percent.

Mark Williams

Hang Yin of Fermilab was the primary analyzer for this measurement. 
The DZero Monte Carlo production team is charged with the daunting task of generating up to 50 million events of simulated particle collisions per week. This is a global project, with jobs “farmed out” to computing centers all over the world. Such simulated data is vital for all measurements performed at DZero.