|Claire Lee, a PhD graduate in Physics at the University of Johannesburg (UJ), was recently appointed convenor of a group refining a tool used in many analyses of CERN ATLAS data to ‘make invisible particles visible’.|
The universe we can see is actually just a small part of everything there is. In 2012, researchers on the ATLAS and CMS experiments at CERN finally discovered the Higgs Boson, the particle that endows mass to the fundamental particles. It could also be a “portal” to study the vast majority of stuff in the universe which is invisible. Physicists call the invisible stuff “Dark Matter.”
In 2012, the elusive Higgs Boson was coaxed into view at the Large Hadron Collider (LHC), an enormous particle accelerator at the CERN facility in Switzerland. More than 10000 people from 113 different countries worked on the design, building and operation of the LHC accelerator and its four particle detectors, and the discovery was followed by many more people from all over the world with great interest. In South Africa, the SA-CERN Programme is a collaboration of physicists from South African institutions working on CERN-related experiments and theory, including the ATLAS and ALICE experiments at the LHC.
In April 2015, the LHC was switched on again after two years’ worth of upgrades to higher energy levels.
Now the search is on for other particles besides the Higgs boson. If these are Dark Matter particles, and the associated Dark Force are found, we will have to revise our understanding of the forces shaping the universe.
It would mean a major overhaul to the Standard Model in Physics, which currently explains many aspects of our world, including the electronics used in mobile phones and TV’s.
Finding invisible particles
One of the people looking for the invisible missing pieces of the puzzle at CERN is Dr Claire Lee, who recently received her PhD from theDepartment of Physics at UJ.
“Claire and the ATLAS group developed a tool that can make invisible particles visible, so to speak,” says Prof Simon Connell, lead researcher of the UJ’s ATLAS group.
“The invisible particles do not leave any trace in the detector. But the laws of energy conservation and momentum tell you what total sum the visible and invisible particles should add up to. Then you subtract the contribution from the visible particles from the total sum, which leaves you with the invisible stuff,” says Prof Connell.
Historically, experiments have used energy measurements from parts of the detector called “calorimeters” to make this measurement. But to do this properly, you need to find the one particle collision you are looking for, amongst the signals from all the other collisions detected by ATLAS, says Dr Lee.
“Billions of protons pass each other in the accelerator. From that, you get maybe 20 collisions happening. Out of those 20, only one might produce something interesting like a Higgs boson.
“If a neutrino or Dark Matter particle is there, you may have a problem finding it: there is so much energy being measured by the calorimeter that has nothing to do with the one collision you want to know about,” she says, adding that “And that one interesting collision is the one with ‘Missing Transverse Energy’ that helps you find the invisible particles.”
Finding a dark needle in a haystack
The way around this conundrum is to look out for clusters of ‘stars’ along the the accelerator’s beam line, says Dr Lee.
“We turned to the ATLAS inner tracking detector, which detects the tracks of all particles in the accelerator. With these tracks, you can see the actual point of the individual proton-proton collisions: they look like little hand-drawn stars with lots of little lines corresponding to particle tracks. One of those ‘stars’ will be the interesting one, with the Higgs for example.
“So we use information from the tracking detector to get the Missing Transverse Momentum from that particular proton-proton collision, and ignore the billions of other energy tracks we detected. Our sub-group developed this method and it was used in various ATLAS analyses during the first run.
“What’s happened now for LHC Run 2, is that we have combined the two ideas for the best possible measurement: we use tracks in one part of our calculation tool and we use calorimeter information in the other part. Now we have a very good overall measurement of the Missing Transverse Energy that ignores the non-interesting collisions.”
On 1 April 2015, ATLAS appointed Dr Claire Lee as the convenor of the Missing Transverse Energy sub-group of ATLAS.
Says Prof Connell: “The tool the Missing Transverse Energy group developed is used by a large number of other teams to analyse the ATLAS data. If researchers discover that the Higgs boson or any other process leads ultimately to an invisible Dark Matter particle, one of the critical tools used to show that would be the one developed by Claire and the group.”