More measurements, but the upsilon spin still mysterious

The muon distribution is predicted to appear as either of these shapes; however, it’s actually more spherical. The line along the bottom shows the three upsilon states.

The first direct evidence for a third generation of quarks in the Standard Model came from a discovery at Fermilab in 1977. The experimental team led by Leon Lederman, found upsilon mesons. Upsilon mesons are bound states of bottom and anti-bottom quarks. The three lightest versions of these bound states decay into muons about three percent of the time. These decays are easy to identify and study in colliders. However, even 35 years since the discovery of upsilon mesons, there is still considerable debate about how high-energy collisions of protons and anti-protons can produce bottom quarks that form these bound states.

The Standard Model provides us with a good understanding of the strong interactions between quarks – the force that binds quarks together. However, when this understanding is applied to the upsilon system, the calculations become difficult. Discrepancies between the calculations of upsilon production rates and experimental results became apparent early in Run I of the Tevatron. CDF collaborators found that upsilon mesons were produced at a rate approximately ten times greater than the predictions of that time. This observation spurred the development of new ways to calculate production rates. These new methods can explain the rates, but they don’t explain all of the observed properties of the upsilon system.

One such property is polarization. However, because upsilon mesons are massive particles, they have three distinct polarization states instead of two.

We can measure the fraction of each component by looking at the direction an upsilon meson emits muons as it decays. Some theories propose that if the muons are emitted in peanut-like shape, they are more likely to be parallel to the direction in which the upsilon is moving. Other theories say that the distribution of emitted muons should make a doughnut shape in relation to the movement of the upsilon meson. We found that the muons didn’t favor either of these shapes.

The new CDF measurement confirms and extends what was suggested by observations made almost a decade ago: muons are emitted uniformly in all directions. This means that all three-polarization states come with equal probability and does not conclusively favor any of the current calculations. The mystery continues…

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—Edited by Andy Beretvas

Matthew Jones, Purdue University, carried out this analysis.