Theoretical predictions at LHC

Cross section for the production of Z pairs at hadron colliders as a function of the collider operating energy. The predictions of MCFM are compared with Tevatron data taken in proton-antiproton collisions at 1.96 TeV and with proton-proton LHC data at 7 and 8 TeV. Image courtesy of ATLAS

At the LHC, a single collision between beams of protons produces an event containing a spray of particles that can be detected by the experiments. Each event may contain any of the particles of the Standard Model, for instance, W or Z bosons, top or bottom quarks, or collimated jets of strongly interacting hadrons. Much of the experimental program is driven by the search for new physics—typically direct searches for conjectured types of particles—that may also be produced in the hadron collisions. Teasing out the hints, or signals, of the new particles usually requires an accurate assessment of the rate of production of Standard Model background events. Providing a good description of both signal and background events is where theorists can play a crucial role.

In the 1990s, theorists began producing the first accurate, or next-to-leading order (NLO), predictions for such background events at the Tevatron. In late 1998, Keith Ellis and I embarked on a project to provide NLO predictions for a wide variety of backgrounds that would be important to understand both at the Tevatron and in the future at the LHC. The project was born with a simple vision: to produce a readily available tool that could provide “one-stop shopping” for accurate predictions at hadron colliders.

We began by providing state-of-the-art predictions for the production of pairs of W and Z bosons, which we made available through a computer code called MCFM. Since then the code has evolved to include the production of W and Z bosons and jets, single top quarks, top quark pairs and more. Testing our understanding of these Standard Model processes is important to much of the ongoing work at the LHC. For instance, the discovery of the Higgs-like boson at the LHC relied on analyses searching for the Higgs boson decay into a pair of Z bosons. Besides being used in this role, MCFM was also used to predict the rate for producing a Higgs boson in association with two jets. This production mode is crucial to detecting the Higgs boson decay into tau pairs. Precise knowledge of the signal rate allows the experiments to test the couplings of the Higgs boson to fermions. Accurate predictions for the properties of signal and background events such as those in MCFM and other similar programs have been essential for exploiting the full potential of the LHC.

John Campbell

The following members of the Fermilab Theory Group contribute to the development of MCFM: From left: John Campbell, Keith Ellis, Ciaran Williams, Raoul Röntsch.