Looking for rare events

This event display of the observed ΥZ candidate shows the muon candidates identified from the Upsilon (μ3 and μ4) and Z (μ1 and μ2) decays.

In 1977, Fermilab scientists discovered the bottom quark — one of six quarks in the Standard Model — through the production of upsilon mesons. An upsilon meson (Υ) is a bound state of a bottom quark and an antibottom quark.

In a new analysis using the full CDF data set, scientists conducted a search for the two mediators of the weak interaction — the W and Z boson — produced in association with an upsilon meson. This is a rare process in the Standard Model, one with a probability that is predicted to be below the sensitivity of the Tevatron.

For this result, we look for a particular Υ called the Υ(1S), which decays almost immediately. Its lifetime is 1.2 x 10-20 seconds.

One way the Υ(1S) can decay is into two muons. This decay mode is also rare, occurring in only 
2.5 percent of Υ(1S) decays.

When we look for the decay of the Υ(1S) in association with a W boson, we look for two muons from the Υ plus an additional muon or electron from the W, as well as so-called missing energy — particles that go undetected but that we know must be present, given the initial energy of the interaction. When we look for the decay of the Υ(1S) in association with a Z boson, we look for two muons from the Υ(1S) plus two additional muons or electrons from the Z.

The CDF team (pictured below) searched for these two decay patterns. For the full data set, which is about a million detected Υ(1S) decays, we expected an ΥW signal of less than one: only 0.03 ± 0.01 events. The background is expected to be much larger: 1.2 ± 0.5 events. The result of one observed event is in good agreement with the expected background.

Scientists observed one very nice candidate for Υ and associated Z production (pictured above). The ΥZ signal is expected to be extremely rare, only 0.008 ± 0.002 events from the entire data set. Here the background is expected to be about 0.1 events. This is the first such event of its kind seen.

The cross section limits resulting from the analysis are currently the world’s best and place restrictions on some new physics scenarios.

edited by Andy Beretvas

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From left: Mark Kruse (Duke University), Antonio Limosani (University of Sydney, visitor at Duke University) and Chen Zhou (Duke University) are the primary analysts for this result.