The march to higher and higher precision

The figure shows the agreement between the data and the CDF simulation for the electron transverse energy distribution. The electrons come from the decay of a Z particle.

Z bosons are produced at the Tevatron via the collision of particles – quarks and gluons – inside the proton and antiproton beams. Interactions within the collision can kick the Z boson in directions transverse to the direction of the incoming particle beamline. These kicks range from the very gentle, where the Z boson is moving along the beamline, to the very hard, where the Z boson emerges at large angles and velocities. The details of the collision physics from the very gentle to the very hard kicks are quite different. The study and understanding of these details require precise measurements. One needs a careful understanding of both the detector acceptance and the event selection efficiency. The top figure shows that the data and our simulation are in excellent agreement.

CDF physicists from the University of Rochester, using a sample of 140,000 Z bosons, have measured the momentum distribution of the Z boson in the direction transverse to the beamline to learn more about the physics of collisions that produce Z bosons. The Standard Model of particle physics is very successful in predicting many aspects of proton and antiproton collisions. However, because of the changing nature of the physics from low transverse momentum (gentle kicks) to high transverse momentum (hard kicks), no single Standard Model prediction is reliable and accurate at all transverse momenta. The prediction at low transverse momentum is very challenging as the physics there is complex. A widely used prediction program is RESBOS.

The Z boson transverse momentum distribution is a benchmark measurement. A precise measurement of this is needed to guide and refine predictions using the Standard Model. In the upper right insert in the second figure, one sees the precision of the data and its agreement with the RESBOS calculation in the low transverse momentum region. This knowledge of understanding the transverse momentum distribution can be applied to the production of other particles at both the Tevatron and LHC. It is important for any physics measurement because particles of interest are predominantly produced at low transverse momentum.

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—edited by Andy Beretvas

The plot is proportional to the probability for the process (Z → e+ e) as a function of the transverse momentum. The black crosses are the data and the red histogram is the RESBOS calculation.
These CDF physicists contributed to this data analysis. From left: Arie Bodek, Jiyeon Han and Willis Sakumoto (all from the University of Rochester).