A precision measurement of the
four-lepton invariant mass spectrum

Testing the predictions of the Standard Model

The standard model (SM) of particle physics is undoubtedly a triumph of modern science. To date, almost all collider measurements of particle production cross-sections, across fourteen orders of magnitude, have agreed with its predictions. The SM was completed in 2012 by the discovery of the Higgs boson. And so far, everything we’ve measured agrees with the predictions…

A summary plot of a decade of LHC measurements of particle production cross-sections at ATLAS. The plot spans fourteen orders of magnitude in cross-section, and a whole range of final states. No significant deviations from the SM’s predictions have been reported. That’s quite something.

But we know that the SM is not the end of the story of fundamental physics. For me, its main deficiencies are that it contains no viable Dark Matter (DM) candidate, despite clear cosmological evidence for its existence; it cannot account for neutrino oscillations; and it does not contain a mechanism to explain the large mass differences between its component particles (there so-called hierarchy problem).

Although we’ve been busy testing the SM, there is plenty of room for new physics to be hidden within the uncertainties of our measurements. For example, the best measurements we have of the Higgs boson only constrain it to decay to SM particles at least 90% or so of the time… that means it could still be decaying to new particles 10% of the time, and we would not have seen it!

Detector-corrected measurements for re-interpretation

When we measure something in particle physics (a rate of particle production or decay, for example), we have to do it through the prism of our detectors. Although our detectors are great, they are not perfect. They have finite resolution, and they have gaps through which particles can escape undetected.

To get around these effects, we’ve developed a technique called unfolding. This is just a fancy word to mean “correcting for detector effects”. In a nutshell, if we assume the response of the detector in our simulations to be well modelled, we can work backwards to apply the simulation-derived response in reverse to our data. So we can measure what was going on at the collision point, before the particles interacted with the detector…

This is important because detector simulations are expensive and slow. With unfolded measurements, we can test new predictions of the SM and it’s extensions directly against our data, without the need for this detector simulations step. These measurements can then be fed into the CONTUR re-interpretation software that I helped to create.

An animation showing how our four-lepton mass measurement could be used to set constraints Effective Field Theory operators

What’s in the pipeline ?

In addition to the Higgs boson measurements I made during my PhD, I have been involved in two innovative unfolded measurements on ATLAS. One of the four-lepton final state, and one in dilepton-dijet final states. I’m currently leading a team of a dozen researchers making a new and challenging measurements of the production of jets with missing energy (which could be due to neutrinos… or new physics).