New Standard Model Results Push Theorists to the Limit

The Oxford ATLAS group performed analysis central to the recently published W+jets and R-jets measurements. Graduate student, Craig Sawyer, worked with ATLAS collaborators to perform these precision Standard Model measurements which extend such measurements to higher energies and regions than have ever been explored. The measurements provide a massive challenge to the high energy physics theory community as no current theoretical calculation can fully predict the full range of both measurements.

Figure 1: The cross section for production of a W boson in association with jets as a function of the leading jet transverse momentum. Many state-of-the-art theoretical calculations have been compared to the data. No prediction has been found which describes the measurement across the whole measured range.

These measurements will provide an invaluable basis for development of analysis methods, benefiting from a massive enhancement in our understanding of W+jets and Z+jets processes, both of which constitute massive irreducible backgrounds to many new physics searches (also being carried out at Oxford). The measurements can also be included in parton distribution function (PDF) fits which enable us to improve our understanding of how protons are built. The ratio measurement (R-jets) is one of the most accurate vector boson + jet measurements due to the massive cancellation of theoretical and experimental uncertainties which occurs when forming the ratio.

Figure 2: The uncertainty of the W+jets (left) and R-jets (right) measurements as a function of leading jet rapidity. The massive reduction in the experimental uncertainty arising from the ratio measurement is clearly seen.