Watching the strong force run

The running of the strong coupling constant describes the way that the strong force loses strength at short distances.

The strong force changes its strength at different distance scales, a phenomenon known as the running of the strong force coupling constant. More like pulling on a rubber band than pulling apart two magnets, its strength diminishes at short distances between quarks but increases rapidly when trying to pull two quarks apart. At some point it takes less energy to produce a new quark-antiquark pair to partner with those being separated than to keep pulling those quarks apart. This process happens repeatedly when a quark or gluon is produced in a collision of protons with antiprotons and leads to a spray of particles called a jet.

The running of the strong force coupling constant can be investigated by studying the production of jets. Information about the changing strength of the strong force is encoded in the energy and spatial distributions of the jets. But these jet distributions also depend on the properties of the quarks and gluons that constitute the colliding proton and antiproton. Each constituent of a proton carries some fraction of the proton’s momentum at any given time and the breakdown of this distribution must be experimentally measured. Studying the strong force at short distances is difficult because the inner workings of the proton are not well measured at high energy scales.

A recent DZero analysis (from the May 18 Joint Experimental-Theoretical Seminar) used a new strategy to study the strong force coupling constant at short distances. The analyzers minimized the effect that the proton’s inner workings had on their result by measuring a specific property of each event: the average number of neighboring jets around each jet. This special average was determined for various neighborhood sizes and with different minimum energy thresholds for neighboring jets. Decoding jet information in this manner allowed the analyzers to probe the strong force directly at shorter distance scales than ever before. This constitutes the first minimally biased observation of the expected running of the strong force coupling constant at energy scales above 208 GeV, and up to 400 GeV.

—Mike Cooke

These physicists made major contributions to this analysis.

The software infrastructure group performs a critical service to the DZero collaboration, organizing and building the major releases of software essential for reconstructing raw data, producing simulated events, and performing physics analyses.