Advancing theory through precision measurements

This figure shows the photon pair-production probability (or cross section) as a function of the angle between the two photons in the plane normal to the proton beam. The bars show the statistical uncertainty and the shaded areas the systematic uncertainty of the measurement. The data points are the CDF measurements and the curves are predictions of various theories.

The detection of photons, produced in proton-antiproton collisions, plays an important role in particle physics by providing clean information about particle interactions. This is because photons produce an easily identifiable signal in the detector. In particular, events with two photons are of special interest because particles such as the boson discovered last year at the LHC or the graviton, the hypothetical particle responsible for the gravitational force in quantum theories of gravity, decay into two photons. To make the identification of such decays more efficient or possible, in case of hypothetical particles like the graviton, physicists need to understand and model all of the ways photon pairs can be produced.

A team of Fermilab physicists identified all events with photon pairs detected in the CDF detector and measured some of their properties in two stages. In the first stage, announced from this column three years ago, the team compared the results with the best available theoretical calculations. The comparisons showed that none of the calculations could adequately describe all aspects of the measurements. This conclusion called for further investigation into the ways the photon pairs can be produced.

In the second stage, using the full data set of 9.5 inverse femtobarns, the results of all pairs of photons are compared with more recent calculations, motivated by the results of the previous stage. An example of the comparisons is shown in the figure. The blue curve (MCFM) is derived from a calculation representing the best theoretical precision available three years ago. The red (NNLO) and green (SHERPA) curves are derived from two new calculations representing current theoretical precision. There is a clear improvement in the description of the measurements by the recent calculations. This improvement is a characteristic example of how high-precision measurements conducted at CDF have advanced our knowledge of particle interactions.

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edited by Andy Beretvas

These Fermilab scientists are responsible for this analysis. From left: Ray Culbertson and Costas Vellidis.