Gluon spin

The points are the measured angular coefficients, A0 and A2, as a function of Z boson transverse momentum. The black curve is the Standard Model prediction which is approximately 70 percent quark-anti-quark (q-qbar in green) and 30 percent quark-gluon (q-G in brown).

Z bosons are produced at the Tevatron through collisions of the particles (quarks and gluons) inside the proton and antiproton beams. Once Z bosons are produced they decay into lepton–anti-lepton pairs (e.g. electron-positron or muon–anti-muon). These leptons are correlated because they are produced from the same Z boson. There is also a connection between the lepton and the Z boson polarization, the particle’s internal spin. Scientists can infer information about the particle’s polarization by looking at the properties of the lepton.

When CDF detects the decay products of the Z boson, experimenters use the location of the particles in the detector to learn more about the mechanisms by which they are produced.

The Standard Model provides a mathematical framework that allows scientists to make extremely precise predictions of how Z bosons decay and how the decaying leptons and the Z boson polarization are related. The shape of this distribution depends on the particles involved in the collision. Quark–anti-quark collisions have a differently shaped distribution than a quark-gluon collision. By looking at the shape of the collision and comparing it to the shapes that we expect to see, experimenters can test the Standard Model to very fine precision.

For this analysis, CDF physicists have measured the angle of electrons departing from a collision using a sample of 140,000 Z bosons. The physicists then converted the result into angular coefficients, numbers that one can relate directly to the Standard Model calculations. The most significant coefficients, labeled A0 and A2 in the figure above, are equal. This consistency is called the Lam-Tung relation. It implies that the gluon has an intrinsic angular momentum spin of one. The analysis of the measured distribution and the number predicted by the Standard Model agree, which means that the Standard Model calculations accurately predict the observed fraction of quark–anti-quark vs. quark-gluon scatterings. The Standard Model holds up to this stringent test. The results will soon be published in Physical Review Letters.

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Edited by Andrew Beretvas

These physicists are responsible for this analysis. First row from left: Jiyeon Han, Willis Sakumoto and Arie Bodek. Second row from left: Yeon Sei Chung, Howard Budd and Kevin McFarland, all from the U. of Rochester.